photon travel through space

A Journey of Light through Space and Time

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Physics library

Course: physics library   >   unit 14, light: electromagnetic waves, the electromagnetic spectrum and photons.

• Electromagnetic waves and the electromagnetic spectrum
• Polarization of light, linear and circular

Introduction to electromagnetic waves

Basic properties of waves: amplitude, wavelength, and frequency, example: calculating the wavelength of a light wave, the electromagnetic spectrum, quantization of energy and the dual nature of light, example: calculating the energy of a photon, attributions.

• “ Electromagnetic Radiation ” from UC Davis ChemWiki, CC BY-NC-SA 3.0

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How Does Light Travel Through Space? Facts & FAQ

Last Updated on Mar 15 2024

Light is such a fundamental part of our lives. From the moment we’re born, we are showered with all kinds of electromagnetic radiation, both colorful, and invisible. Light travels through the vacuum of space at 186,828 miles per second as transverse waves , outside of any material or medium, because photons—the particles that make up light—also behave as waves. This is referred to as the wave-particle duality of light.  

• What Is Light?

The wave-particle duality of light simply means that light behaves as both waves and particles . Although this has been long accepted as fact, scientists only managed to observe both these properties of light ¹ simultaneously for the first time in 2015.

As a wave, light is electromagnetic radiation—vibrations, or oscillations, of the electric and magnetic fields. As particles, light is made up of little massless packets of energy called photons ¹ .

• What Are Light Waves?

Waves are the transference of energy from one point to another. If we dropped a pebble into a small pond, the energy that the impact creates would transfer as a ripple, or a wave, that travels through the surface of the water, from one water particle to another, until eventually reaching the edge of the pond.

This is also how sound waves work—except that, with sound, it’s the pressure or vibrations of particles in the air that eventually reach our ears.

Unlike water and sound, light itself is electromagnetic radiation—or light waves—so it doesn’t need a medium to travel through.

• What Are Transverse Waves?

Light propagates through transverse waves. Transverse waves refer to a way in which energy is transferred.

Transverse waves oscillate at a 90-degree angle (or right angle) to the direction the energy is traveling in. An easy way to picture this is to imagine an S shape flipped onto its side. The waves would be going up and down, while the energy would be moving either left or right.

With light waves ¹ , there are 2 oscillations to consider. If the light wave is traveling on the X axis, then the oscillations of the electric field would be at a right angle, either along the Y or Z axes, and the oscillations of the magnetic field would be on the other.

• Can Anything Travel Faster Than Light?

The simple answer to this question is no, as far as we know at this time, nothing can go faster than the speed of light ¹ . Albert Einstein’s special theory of relativity states that “no known object can travel faster than the speed of light in a vacuum.”

Space and time don’t yet exist beyond the speed of light—if we were to travel that fast, the closer we get to the speed of light, the more our spatial dimension would shrink, until eventually collapsing.

Beyond this, the laws of physics state that as an object approaches the speed of light , its mass would become infinite, and so would the energy it would need to propel it. Since it’s probably impossible to create an infinite amount of energy, it would be difficult for anything to travel faster than light.

Tachyon, a hypothetical particle, is said to travel faster than the speed of light. However, because its speed would not be consistent with the known laws of physics, physicists believe that tachyon particles do not exist.

• Final Thoughts

Light travels through space as transverse, electromagnetic waves. Its wave-particle duality means that it behaves as both particles and waves. As far as we know, nothing in the world travels as fast as light.

• https://phys.org/news/2015-03-particle.html
• https://www.nature.com/articles/ncomms7407
• https://www.wtamu.edu/~cbaird/sq/2017/07/20/is-the-reason-that-nothing-can-go-faster-than-light-because-we-have-not-tried-hard-enough/
• https://www.physics.brocku.ca/PPLATO/h-flap/phys6_1f_1.png
• https://science.nasa.gov/ems/02_anatomy

Featured Image Credit: NASA, Unsplash

Table of Contents

About the Author Cheryl Regan

Cheryl is a freelance content and copywriter from the United Kingdom. Her interests include hiking and amateur astronomy but focuses her writing on gardening and photography. If she isn't writing she can be found curled up with a coffee and her pet cat.

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A Photon's Million-Year Journey From the Center of the Sun

It's a well-known fact that the light we see from stars has taken hundreds, even thousands, of years to reach us. The photons made in the centers of these distant stars have traveled across enormous expanses of space and time in order to enter our retinas. But what about the light made in our own Sun? What do those photons experience on their way to the Earth?

Let's chart the journey of a single photon:

First Stage: The Core

Our photon was produced in the core of the Sun:  where the densities and temperatures—which can reach 15 million °C (27 million °F) —are high enough to sustain nuclear fusion. The hydrogen atoms that make up most of the Sun's mass have too much energy to stay together, so they split into their component parts: protons and electrons—creating plasma.

When two protons collide inside this high-energy plasma, they bond, ultimately creating a deuterium atom , as well as releasing a neutrino and a positron . If another proton collides with a deuterium atom, they form a helium-3 nucleus and release a gamma ray. This gamma ray is our photon, albeit an extremely energized one. After the photon is created, two helium-3 nuclei can combine to create a helium-4 nucleus, and two protons.

This process, which is called the proton-proton chain, is the backbone of nuclear fusion. But it doesn't end there.

The neutrino, being a weakly-interacting particle, will fly off into space, while the positron will interact with an electron, annihilating both of them and creating another gamma ray. In a later stage of the Sun's life-cycle, the helium atoms produced by the fusion reaction can themselves combine, in different reactions, to make heavier and heavier elements, releasing even more energy.

Next Stage: Radiative Zone

The radiative zone is just beyond the core of the Sun. It gets its name from its primary method of heat transfer: the radiation of light. As our photon leaves the core and enters the radiative zone, it encounters an obstacle: densely packed protons. They are so crammed together, photons can't travel more than a few millimeters without hitting another one. Each time one does, it loses some of its energy and is scattered in a random direction.

As a result, its forward progress is slowed to a crawl. It can take anywhere from a few thousand to a few million years for one photon to escape. It's not just the light from distant stars that takes millions of years to reach us; the light from our own Sun does too!

Phase 3: The Convection Zone

The convection zone is the final interior layer of the Sun; It ends at the Sun's surface. Here, heat transfer is dominated by convection: moving currents of plasma from bounce between the hot interior and cooler exterior. The Sun's plasma behaves much like a boiling pot of water, with hot bubbles forming deep inside the Sun rising to the surface—creating granules and supergranules .

The density of the solar plasma is much less than in the radiative zone. At the surface, the density is ten thousand times less than the density of air, so our photon wouldn't hit many atoms as it makes its way through. After bouncing around in the radiative zone, our photon has lost much of its energy, and has shifted into the visible spectrum. Millions of years after it was created in a fusion reaction, our photon finally breaks free from the Sun's interior.

Emergence: Solar Atmosphere

Leaving the surface of the Sun, our photon then enters the Sun's atmosphere, which (like Earth) has multiple layers.  Our photon passes through the thin photosphere first, before entering the thicker chromosphere. Something interesting happens at this point: the temperatures start to increase as the photon makes its way through the corona (the outermost layer of the Sun's atmosphere). The corona—a white-hot plume of plasma that extends millions of kilometers away from the Sun—is hundreds of times hotter than the surface.

Although the corona is very sparsely populated with atoms, photons can still be scattered by dust or free electrons. However, they usually pass through mostly unimpeded, and head toward the direction of Earth. Many trillions of other photons join it, with trillions more travelling in other directions. Their combined energy can exert a force on any object large enough to get in the way.

This force is called radiation pressure, and it could be used in the future to power solar sails.

Finally, Destination: Earth

It takes approximately 8 minutes for our photon to travel the 93,000,000 miles (150 million kilometers) from the Sun to Earth. Once there, it dodges space debris and satellites and enters the atmosphere. If our photon were a gamma ray, an x-ray, or an ultraviolet ray, it would be absorbed or reflected by the upper atmosphere, and it would never reach the ground.

If it were a visible-light photon with a blue or violet wavelength, it would be scattered by the lower atmosphere, and reach the ground at more extreme angles. However, our photon has a red wavelength, and it passes straight through the atmosphere with minimal interaction.

Near the surface, our photon makes contact with the lens of an eye, where it is focused on a small spot on the retina. There, it is finally absorbed by a protein inside a cone cell, which sends an electrical signal to the brain and marks the end of the million year journey of a photon. 

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How does a photon know to travel at the speed of light?

25 August 2021

Pobytov/Getty Images

How does a photon “know” to travel at the speed of light?

Elaine Patrick Cyffylliog, Denbighshire, UK

I don’t know, ask Erwin Schrödinger. He was a relative of mine on my mother’s side. He told her mum, but she couldn’t understand it either. We’ve been in the dark ever since.

Yang Guijen Balik Pulau, Penang, Malaysia

The laws of the universe require that all the energy and matter particles occupying its space must abide by its rules – so as to maintain a viable home and playground for all. One of these rules is that if you are a massless particle of electromagnetic origin, and you want to play in vacuum space, then you must move at the speed of light, 299,792,458 metres per second, consistently.

If you are a particle with mass, however, then there are other rules that you can follow.

@kbachmann, via Twitter

Wouldn’t any speed travelled by photons be, by definition, the speed of light?

Ian Glendinning Vienna, Austria

All massless particles always travel at a speed represented by the letter c , whereas massive particles can travel at any speed between zero and c . Since photons are massless, they travel at c , which is called the speed of light because the photon was the first known example of a massless particle.

So the short answer to the question is that a photon knows to travel at the speed of light because it is massless.

Ken Appleby Ledbury, Herefordshire, UK

What we call photons are actually interactions of electromagnetic fields. Between interactions, photons don’t exist. You can’t watch a photon in transit, only detect an excitation of the electromagnetic field when it happens.

Photons don’t exist as particles. There are no particles, just interactions of quantum fields. Maxwell’s equations embody and explain in elegant mathematics the empirical results of Faraday’s experiments into electrostatic and magnetic fields – fields being a novel concept of Faraday’s invention. The equations reveal the existence of electromagnetic waves, which are always observed to travel at the constant speed c , regardless of the motion of emitter, receiver or observer. It was this apparent paradox that Einstein’s special relativity paper resolved, by dispensing with the notions of simultaneity, absolute space and time.

So at root, the answer to your question is just simply that that is reality. That is what we observe. The reasons are illuminated by the equations of electrodynamics, but ultimately it is an empirical observation. At least, so far.

James Bailey Southampton, Hampshire, UK

This question is the wrong way round. A photon is a packet of electromagnetic radiation. A very small part of the spectrum of that radiation (wavelengths of around 400 to 750 nanometres) is detectable with our eyes and we call this light. It is like asking why light takes 1/299,792,458 of a second to travel 1 metre, when in fact we just find it more convenient to define it as that, rather than use the old definition of a metre as a ten-millionth of the distance from the equator to the North Pole.

The really interesting question for me is why does electromagnetic radiation travel at 300,000 kilometres per second, and that brings us back to the question of time that has been raised before. Does light travel through time? If so, what exactly is it that it is travelling through? Or does time itself do the moving and is constantly sweeping past us like the wind while everything else stands still?

Does time actually exist as anything or is it just a convenient invention to allow us to talk about how things are moving?

To answer this question – or ask a new one – email [email protected] .

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What are photons?

Photons carry the electromagnetic force, and act as both particles and waves.

Photon discovery

Are photons particles, do photons have mass and momentum, do photons experience time, are photons affected by gravity, additional resources, bibliography.

Photons are fundamental subatomic particles that carry the electromagnetic force — or, in simpler terms, they are light particles (and so much more). The photon is also the "quantum," or fundamental unit, of electromagnetic radiation . Everyone is surrounded by photons: The light coming from the screen you're viewing is composed of photons, the X-rays doctor use to see bones are made of photons, the radio in a car receives its signal from photons, and the magnets on a fridge use photons to hold themselves up.

Like all other subatomic particles, photons exhibit wave-particle duality, meaning that sometimes they behave as tiny particles and sometimes they act as waves. Photons are massless, allowing them to travel at the speed of light in a vacuum (299,792,458 meters per second) and can travel an infinite distance.

Although physicists have studied the nature of light for centuries, arguments went back and forth as to whether light was made of tiny particles or was wave-like in nature. In the late 1800s, however, the pioneering work of German physicist Max Planck changed the entire picture.

Planck was studying something called blackbody radiation, or light from a special device that emitted light at all frequencies as efficiently as possible. Until Planck, nobody could explain the spectrum of light coming from these devices, so Planck added a "fix" to the equations. By assuming that light could be emitted only in discrete chunks of energy, known as quanta, he was able to develop a formula that perfectly explained the blackbody spectra, according to HyperPhysics .

Physicists weren't exactly sure what to make of Planck's result, but several years later, Albert Einstein took it one step further. To explain the photoelectric effect , which is the release of electrons from a metal when light shines on it, Einstein proposed that light itself is composed of discrete little chunks, according to the American Physical Society . Over time, those little chunks became known as photons.

The work of Planck, Einstein and others to study the nature of light kick-started the development of quantum mechanics .

Strictly speaking, photons are neither particles nor waves; they are a combination of both. In some situations, their particle-like nature comes out more, and in others, their wave-like nature is more apparent.

For example, a detector can register the arrival of a single photon, which appears as a point-like particle. The process known as Compton scattering involves a photon striking an electron, and in that situation, the photon acts as a particle.

However, it's impossible to predict exactly where or when a photon will strike a detector. In quantum mechanics, one can only assign probabilities to events. Those events are modeled by equations for waves, with peaks in the waves corresponding to regions of high probability of receiving a photon and troughs corresponding to regions of low probability, according to AccessScience by McGraw Hill .

This concept is best exemplified by the famous double-slit experiment, which solidified the dual wave-particle nature of light (and, eventually, other subatomic particles). When light passes through a screen with two slits cut into it, it forms an interference pattern on the detector on the other side of the screen, where the peaks of waves line up with each other in some places, and where the peaks and troughs cancel each other out in others. Even though only one photon passes through the screen at a time — with each individual photon acting like a particle — the interference pattern that emerges on the detector is the exact same pattern that would occur if waves were passing through the slits instead.

Photons have zero mass, which allows them to travel at the fastest possible speed in the universe, the speed of light. However, they do have energy and momentum. The energy of a photon is given by Planck's constant times the frequency of the light, and the momentum of a photon is given by Planck's constant times the frequency of the light times the speed of light, according to the University of Calgary's Energy Education website .

The fact that photons have momentum enables a broad array of applications. For example, solar sails are experimental propulsion devices that use sunlight to push a spacecraft. According to NASA , the photons from the sun bounce off of the reflecting sail, thus imparting their momentum on the sail and moving the spacecraft.

Our understanding of the rate of the passage of time comes from Einstein's theory of special relativity , which states that objects traveling closer and closer to the speed of light will experience slower and slower rates of the passage of time. In other words, moving clocks run slowly, according to John D. Horton of the University of Pittsburgh .

However, the mathematics of special relativity apply only to objects traveling more slowly than the speed of light and don't apply directly to photons, which do travel at the speed of light. Thus, it's impossible to say what a photon "experiences" in terms of the flow of time, because scientists have no mathematical language to support it. Another way to put this is that the concept of the flow of time is meaningless to photons.

— What is electromagnetic radiation?

— 7 ways Einstein changed the world

— 8 ways you can see Einstein's theory of relativity in real life  

Because photons have both energy and momentum, they are influenced by gravity . Under Einstein's theory of general relativity, which is our modern understanding of gravity, anything with any form of energy (including mass, momentum and torsion) is influenced by gravity. Specifically, massless particles, such as photons, follow "geodesics," which are paths of minimum distance from one point to another, according to EarthSky .

In general relativity, space-time is curved due to the influence of massive objects. This can make the "minimum distance" path a curved line, just as jets have to follow a curved path to go straight from one city to another, because Earth itself is curved.

The curvature of space-time affects photons in several ways. When photons are moving from a region of strong gravity to a region of weaker gravity, they will lose energy, which lowers their frequencies to the redder end of the spectrum. When photons pass near massive objects, their direction of motion will change.

• You can dig deeper into the relationship between light and time in this YouTube video hosted by the author of this article, astrophysicist Paul M. Sutter.
• For a fun exploration of the nature of quantum mechanics (which, of course, also discusses photons), check out "How to Teach Quantum Physics to Your Dog" (Scribner, 2010) by physicist Chad Orzel.
• The Physics Asylum also hosts a great video explainer on the nature of the photon, which you can watch here .

Afework, B., Boechler, E., Campbell, A., Hanania, J., Heffernan, B., Jenden, J., Street, K., & Donev, J. (2021, October 22). Photon . Energy Education. https://energyeducation.ca/encyclopedia/Photon  

American Physical Society. (2005, January). This month in physics history: Einstein and the photoelectric effect . APS News. https://www.aps.org/publications/apsnews/200501/history.cfm#:~:text=Light%2C%20Einstein%20said%2C%20is%20a,collision%20produces%20the%20photoelectric%20effect 

Hall, L. (2021, October 6). Advanced composite solar sail system: Using sunlight to power deep space exploration . NASA. https://www.nasa.gov/directorates/spacetech/small_spacecraft/ACS3  

Kleppner, D. (2019). Photon . AccessScience. https://www.accessscience.com/content/511100 

Nave, R. (n.d.). Blackbody radiation . HyperPhysics. Retrieved March 8, 2022, from http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html  

Norton, J. D. (2018, October 10). General relativity . Einstein for Everyone. https://sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity/  

Whitt, K. K. (2021, September 8). What is gravitational lensing? EarthSky. https://earthsky.org/space/what-is-gravitational-lensing-einstein-ring/#:~:text=Gravitational%20lensing%20occurs%20when%20massive,bending%20and%20magnifying%20the%20light  

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Paul M. Sutter is a research professor in astrophysics at  SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including  "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy. 

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Three ways to travel at (nearly) the speed of light.

Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that’s immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, called wave-particle interactions. When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

Universe Today

Space and astronomy news

How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity . However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton .

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “ Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light “. According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens . . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his “ Traité de la lumière “ (“ Treatise on Light “). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment , which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations ) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c ).

In 1905, Albert Einstein published “ On the Electrodynamics of Moving Bodies ”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity , which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect , Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes ).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function , a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “ Schrödinger Cat” paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation ). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

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56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

Comments are closed.

Does Light Lose Energy as it Travels?

Most recent answer: 10/13/2013

Hi Zacki, Let me turn your question around: in a vacuum, why would a photon lose energy? Even a baseball in space won't lose much energy, since there isn't any air resistance, friction, etc. The baseball will interact with radiation pressure, however, so it might lose energy slowly over thousands of years. Photons, however, don't interact strongly with anything except charged particles. When they travel through empty space, one might expect that there is no mechanism by which they can lose energy.

Actually, however, there is one way that photons do lose energy as they travel through space. Because the universe is expanding, the photon's wavelength increases very slightly over time, and in so doing loses a bit of energy.

For the record, the source of a photon's energy is the "flashlight." For example, accelerating charges, hot objects, and particle decays can all lose energy by radiating photons. These photons are simply packets of electromagnetic energy. David

(published on 04/16/2013)

Follow-Up #1: why is light speed constant?

It's true that we have some choices about what sorts of coordinates to use, so we don't absolutely have to say that the speed of light is constant. Still, there are some natural choices. Say that you make a whole lot of meter-sticks in some factory, all just the same. Put little mirrors on each end. Then you make a whole batch of identical clocks, and put one one each meter-stick. Then these  timer-sticks get distributed around to different places and set in various state of motion. Each one can measure a speed of light by seeing how many clock ticks happen as the light bounces back an forth one meter-stick length. They all get the same result. That's what we mean by the constancy of the speed of light. It's a physical fact, regardless of how you express it.

(published on 10/13/2013)

Follow-up on this answer

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Gamma rays: Everything you need to know about these powerful packets of energy

Gamma rays can only be detected by sensors made of dense metals and takes over six feet (1.8 meters) of concrete to block.

How were gamma rays discovered?

How to stop gamma rays, are gamma rays dangerous, gamma-ray astronomy, gamma-ray bursts, additional resources.

Gamma rays are high-energy photons produced by some of the most violent events in the universe.

Photons of light are massless particles that are essentially packets of energy. Because of a quantum-mechanical phenomenon known as wave-particle duality , particles can behave like waves, and photons are no different. Photons have wavelengths, and the amplitude of their wavelength determines where they sit on the electromagnetic spectrum . Radio and microwave photons sit at the lower energy, longer wavelength end of the spectrum, while in the shorter wavelength, higher-energy regime are photons of ultraviolet, X-rays and the most energetic of them all with the shortest wavelengths: gamma rays.

Gamma rays have wavelengths shorter than 10^-11 meters and frequencies above 30 x 10^18 hertz. The European Space Agency describes how gamma-ray photons have energies in excess of 100,000 electronvolts (eV). We can compare this to X-rays, which NASA describes as having energies between 100 eV and 100,000 eV , and optical photons that we can see with our eyes, which are about 1 eV.

Related: What is the cosmic microwave background? 

On Earth, gamma rays are produced by radioactive decay, nuclear weapons and lightning, while in space they are produced by violent, high-energy sources such as solar flares , quasars , black holes tearing stars apart , black-hole accretion disks, exploding stars and the strong gravitational environments of neutron stars .

At the turn of the twentieth century, two forms of radiation emitted by decaying atoms were known, namely alpha particles (which are helium nuclei) and beta particles (which are electrons and positrons). 

However, when the French chemist Paul Villard began experimenting with the radioactive element radium, which had been discovered two years prior by Marie and Pierre Curie , he noticed that the ionizing radiation produced by the decay of radium packed a harder punch than either alpha or beta particles. 

This radiation received its name — gamma-rays — simply because gamma is the third letter in the Greek alphabet after alpha and beta. Unbeknownst to Villard and his cohorts in the early 1900s, the key difference between gamma rays and alpha/beta particles is that gamma rays are a form of light, whereas alpha and beta particles are made of matter. 

To block gamma rays requires a dense material, and the thickness of that material depends on the substance. To reduce the strength of incoming gamma rays by a billion, you need 13.8 feet (4.2 meters) of water, 6.6 feet (2 m) of concrete or 1.3 feet (0.39 m) of lead, according to the radiation protection solution website StemRad . 

This poses a problem for gamma-ray telescopes, such as NASA's Fermi Space Telescope . Ordinary telescopes like the Hubble Space Telescope use mirrors and lenses to collect and focus light, but gamma rays will simply pass straight through an ordinary telescope. Instead, gamma-ray telescopes have to employ other means. 

On the Fermi Space Telescope, a gamma-ray photon will pass through a device called the Anti-coincidence Detector, which blocks cosmic rays that might give a false signal, according to NASA . The gamma-ray is then absorbed by one of 16 sheets of tungsten, a material that is dense enough to stop gamma rays. 

By interacting with the tungsten, the gamma-ray is converted into an electron and a positron (the antimatter or antiparticle counterpart of an electron), the paths of which are read by a tracker, which is a module of silicon strips interweaved by tungsten foil that can determine the direction that the gamma-ray came from in space, based on the trajectory of the electron and the positron.

Finally, the electron and then positron have their energies measured by a calorimeter — a device that measures the energy of a particle by absorbing it — made from cesium iodide, and therefore the energy of the gamma-ray can be determined. 

Because of their high energy, gamma rays are ionizing, meaning they can dislodge electrons from atoms , ultimately damaging living cells and causing a hazard to health. However, as with all radiation, it depends upon the dose that you receive. 

In small doses, very carefully targeted to limit exposure, they can be used safely as a medical diagnostic tool, or even to kill cancerous cells (ironic since exposure to radiation including gamma rays can cause cancer). In particular, one tool used by doctors is the ' Gamma Knife ', which is an ultra-precise form of surgery in which a beam of gamma rays cuts away diseased brain cells and can even penetrate deep into the brain without damaging the exterior lobes.

Given their ionizing power, it's fortunate that Earth's atmosphere is able to block gamma rays from space. For astronomers, however, that's unfortunate, because it means that to conduct gamma-ray astronomy observatories have to either be built on mountaintops where the atmosphere is thinner or sent into space.

The first gamma-ray space telescope was launched in 1961 on the NASA Explorer 11 satellite, but things didn't really begin to kick off until the late 1960s and early 1970s with a major finding, and it wasn't even an astronomical telescope that made the discovery.

Over the years there have been many observatories, both on the ground and in space, that have been designed to observe cosmic gamma-ray radiations. In 1990, NASA launched the Compton Gamma-Ray Observatory as the gamma-ray counterpart to the Hubble Space Telescope. The Compton Gamma-Ray Observatory explored the cosmos from 1991 until 2000.   The aforementioned BeppoSAX was a joint Italian–Dutch mission that operated between 1996 and 2003, while NASA launched HETE-2 (the High-Energy Transient Explorer; HETE-1 had previously failed in orbit) that tracked down many GRBs between 2000 and 2008. 

Currently, as of the end of 2022, several satellites , observatories and telescopes continue to conduct gamma-ray astronomy both on Earth and in space.  NASA's Swift satellite , launched in 2004,  combines both X-ray and gamma-ray observations, as does Italy's AGILE satellite launched in 2007. In 2002, the European Space Agency launched INTEGRAL , the International Gamma-Ray Astrophysics Laboratory.. The current most sophisticated gamma-ray space telescope is Fermi, which NASA launched in 2008. 

Meanwhile, on the ground, there are several gamma-ray observatories including VERITAS (Very Energetic Radiation Imaging Telescope Array System) at the Fred Lawrence Whipple Observatory in Arizona and HESS (High Energy Stereoscopic System) in Namibia.

In 1963, the Soviet Union, the United Kingdom and the United States signed a nuclear test ban treaty that prohibited the world's superpowers from testing any nuclear devices in the atmosphere or in space. However, the U.S. was suspicious that the Soviet Union wouldn't adhere to the treaty, so they launched the Vela series of satellites to watch for any pulses of gamma-ray radiation that could be coming from secretive nuclear detonations. As it happened, gamma rays were detected, but from space: random blasts of powerful gamma-ray energy that seemed to be coming from all around the Earth. But how far away were these gamma-ray bursts ?

Related: Most powerful gamma-ray burst ever seen could help reveal how black holes are born

If these gamma-ray bursts, which are abbreviated to GRBs for short, were coming from our galaxy, then astronomers would detect them mostly in the plane of the Milky Way . Instead, they were spread all over the sky, it could mean only one of two things. Either they were very close, within our solar system , or they were very far away, beyond our galaxy. A special debate was even convened in 1995, echoing a similar ' Great Debate ' in 1920 between Harlow Shapley and Heber D. Curtis that discussed the size of our galaxy based on the distribution of globular clusters . 

In the 1995 debate , chaired by Martin Rees, astronomer Bohdan Paczynski of Princeton University argued that GRBs came from very far away, while Donald Lamb of the University of Chicago reasoned that GRBs must be from close by because the energy required for them to be billions of light-years away would contravene the laws of physics.

Just two years later astronomers had their answer when the BeppoSAX satellite detected a gamma-ray burst that the William Herschel Telescope in the Canary Islands was able to quickly follow up on, in the process detecting the faint afterglow of whatever explosion had created the GRB. Measuring the redshift of the afterglow's light revealed it to have come from six billion light-years away. Bohdan Paczynski was right!

There are two main types of GRB. One type is called short GRBs which last just fractions of a second, while the other kind is known as the long GRBs, and can last many seconds up to an hour. Short GRBS are emitted during the merger of two neutron stars , while long GRBs are the death cries of rare, massive stars . 

Physicists Andrew MacFadyen and Stan Woosley of the University of California, Santa Cruz, developed a model to explain how stars could explode and produce long GRBs without breaking the laws of physics. When a massive star with 50–100 times the mass of the sun reaches the end of its life, the star begins to collapse in on its core, and if the star is rotating fast enough, the energy within the collapsing layers rebounds off the core and is blasted out in two jets that move at almost the speed of light and blow the star apart. Charged particles within these jets spiral around powerful magnetic fields and produce something called synchrotron radiation, in the form of the gamma rays that we observe. Because the gamma rays are only released in the direction of the jets, and not in all directions at once, the total energy released does not contravene the laws of physics.

Learn more about ionizing radiation with the United States Environment Protection Agency (EPA) , and the American Cancer Society . Explore gamma rays in more detail in a tour of the electromagnetic spectrum with NASA Science.

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook .  

Bibliography

Flash! The Hunt for the Biggest Explosions in the Universe by Govert Schilling (Cambridge University Press, 2002)

The Biggest Bangs: The Mystery of Gamma-Ray Bursts, The Most Violent Explosions in the Universe by Jonathan Katz (Oxford University Press, 2002) 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

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Breaking space news, the latest updates on rocket launches, skywatching events and more!

Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

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• larbud Interesting; no mention of 1859. Reply
• billslugg The 1859 Carrington event was caused by a stream of charged particles from a solar flare. Any gamma rays emitted would have been blocked by the Earth's atmosphere. Reply
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Breaking the Scaling Limits: New Ultralow-noise Superconducting Camera for Exoplanet Searches

When imaging faint objects such as distant stars or exoplanets, capturing every last bit of light is crucial to get the most out of a scientific mission. These cameras must be extremely low-noise, and be able to detect the smallest quantities of light—single photons.  Superconducting cameras excel in both of these criteria, but have historically not been widely applicable because their camera sizes have been small, rarely exceeding a few thousand pixels, which limits their ability to capture high-resolution images.  However, a team of researchers has recently shattered that barrier, developing a superconducting camera with 400,000 pixels, which could be used to detect faint astronomical signals in a wide range of wavelengths—from the ultraviolet to the infrared.

While plenty of other camera technologies exist, cameras using superconducting detectors are very appealing for use in astronomical missions due to their extremely low-noise operation.  When imaging faint sources, it is crucial that a camera report the quantity of received light faithfully, and not skew the amount of light received or inject its own false signals.   Superconducting detectors are more than capable of this task, owing to their low-temperature operation and unique composition. As described by project lead Dr. Adam McCaughan, "with these detectors you could take data all day long, capturing billions of photons, and fewer than ten of those photons would be the result of noise."

But while superconducting detectors hold great promise for astronomical applications, their usage in that field has been stymied by small camera sizes that permit relatively few pixels.  Because these detectors are so sensitive, it is difficult to pack a lot of them into a small area without them interfering with each other.  In addition, since these detectors need to be kept cold in a cryogenic refrigerator, only a handful of wires can be used to carry the signals from the camera to the warmer readout electronics.

To overcome these limitations, researchers at the National Institute of Standards and Technology (NIST), the NASA Jet Propulsion Laboratory (JPL), and the University of Colorado Boulder applied time-domain multiplexing technology to the interrogation of two-dimensional superconducting-nanowire single photon detector (SNSPD) arrays. The individual SNSPD nanowires are arranged as intersecting rows and columns. When a photon arrives, the times it takes to trigger a row detector and a column detector are measured to ascertain which pixel sent the signal. This method allows the camera to efficiently encode its many rows and columns onto just a few readout wires instead of thousands of wires. 

SNSPDs are one type of detector in a collection of many such superconducting detector technologies, including microwave kinetic inductance detectors (MKID), transition-edge sensors (TES), and quantum capacitance detectors (QCD).  SNSPDs are unique in that they are able to operate much warmer than the millikelvin temperatures required by those other technologies, and can have extremely good timing resolution, although they are not able to resolve the color of individual photons.  SNSPDs have been collaboratively researched by NIST, JPL, and others in the community for almost two decades, and this most recent work was only possible thanks to the advances generated by the wider superconducting detector community.

Once the team implemented this readout architecture, they found it immediately became straightforward to construct superconducting cameras with extremely large numbers of pixels. As described by technical lead Dr. Bakhrom Oripov, "The big advance here is that the detectors are truly independent, so if you want a camera with more pixels, you just add more detectors to the chip." The researchers note that while their recent project was a 400,000 pixel device, they also have an upcoming demonstration of a device with over a million pixels, and have not found an upper limit yet. 

One of the most exciting things that the researchers think their camera could be useful for is a search for Earth-like planets outside of our solar system. To detect these planets successfully, future space telescopes will observe distant stars and look for tiny portions of reflected or emitted light coming from orbiting planets. Detecting and analyzing these signals is extremely challenging and requires very long exposures, which means that every photon collected by the telescope is very valuable. A reliable, low-noise camera will be critical to detect these incredibly small quantities of light.

SNSPD cameras can also be used on Earth to detect optical communication signals from missions in deep space. In fact, NASA is currently demonstrating this capability via the Deep Space Optical Communications (DSOC) project, which is the first demonstration of free-space optical communication from interplanetary space. DSOC is sending data from a spacecraft called Psyche—which was launched on October 13 and is on its way to the Psyche asteroid—to an SNSPD-based ground terminal at Palomar Observatory. Optical links can transmit data at a much higher rate than radio frequency links from interplanetary distances. The excellent timing resolution of the camera developed for the ground station receiving Psyche data allows it to decode optical data from the spacecraft, which enables much more data to be received in a given time than if radio signals were employed.

These sensors will also be useful for many applications on Earth. Because the operating wavelength of this camera is very flexible, it could be optimized for applications in biomedical imaging to detect faint signals from cells and molecules, which were previously not detectable. Dr. McCaughan noted, “We would love to get these cameras in the hands of neuroscientists. This technology could provide them with a new tool to study our brains, in a completely non-intrusive way.”

Finally, the rapidly growing field of quantum technology, which promises to change the way we secure communications and transactions as well as the way we simulate and optimize complex processes, also stands to gain from this exciting technology. A single photon can be used to transfer or compute a single bit of quantum information. Many companies and governments are currently trying to scale up quantum computers and communication links and access to a single-photon camera that is so easily scalable, could overcome one of the major hurdles to unlocking the full potential of quantum technologies.

According to the research team, the next steps will be to take this initial demonstration and optimize it for space applications.  "Right now, we have a proof-of-concept demonstration," says co-project lead Dr. Boris Korzh, "but we'll need to optimize it to show its full potential." The research team is currently planning ultra-high-efficiency camera demonstrations that will validate the utility of this new technology in both the ultraviolet and the infrared.

PROJECT LEADS

Dr. Adam McCaughan (NIST) and Dr. Boris Korzh (JPL)

SPONSORING ORGANIZATIONS

Astrophysics Research and Analysis (APRA) Program, DARPA Invisible Headlight Program

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• Published: 11 May 2024

Cavity-enhanced photon indistinguishability at room temperature and telecom wavelengths

• Lukas Husel 1   na1 ,
• Julian Trapp 1   na1 ,
• Johannes Scherzer   ORCID: orcid.org/0009-0008-1184-0917 1 ,
• Xiaojian Wu   ORCID: orcid.org/0000-0002-7977-0969 2 ,
• Peng Wang 2 ,
• Jacob Fortner 2 ,
• Manuel Nutz 3 ,
• Thomas Hümmer 3 ,
• Borislav Polovnikov   ORCID: orcid.org/0009-0001-7295-9232 1 ,
• Michael Förg   ORCID: orcid.org/0000-0001-6587-6124 3 ,
• David Hunger   ORCID: orcid.org/0000-0001-6156-6145 4 , 5 ,
• YuHuang Wang   ORCID: orcid.org/0000-0002-5664-1849 2 &
• Alexander Högele   ORCID: orcid.org/0000-0002-0178-9117 1 , 6  

Nature Communications volume  15 , Article number:  3989 ( 2024 ) Cite this article

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• Carbon nanotubes and fullerenes
• Single photons and quantum effects

Indistinguishable single photons in the telecom-bandwidth of optical fibers are indispensable for long-distance quantum communication. Solid-state single photon emitters have achieved excellent performance in key benchmarks, however, the demonstration of indistinguishability at room-temperature remains a major challenge. Here, we report room-temperature photon indistinguishability at telecom wavelengths from individual nanotube defects in a fiber-based microcavity operated in the regime of incoherent good cavity-coupling. The efficiency of the coupled system outperforms spectral or temporal filtering, and the photon indistinguishability is increased by more than two orders of magnitude compared to the free-space limit. Our results highlight a promising strategy to attain optimized non-classical light sources.

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Introduction.

The capability of two indistinguishable single photons to interfere on a balanced beam splitter and exit jointly on either one of its output ports is a premise to quantum photonic applications 1 such as quantum teleportation 2 , quantum computation 3 or quantum optical metrology 4 . Solid-state-based sources of indistinguishable single photons have witnessed tremendous progress in the past decades 5 , and among them semiconductor quantum dots stand out as they enable the generation of pure and indistinguishable single photons 6 , 7 when coupled to optical microcavities 8 , 9 , 10 . However, their operation is so far restricted to cryogenic temperatures and wavelengths in the near-infrared. These limitations motivate alternative platforms operating at ambient conditions and telecom wavelengths to facilitate long-distance quantum communication in optical fibers at reduced loss. Various quantum emitters have proven capable of emitting pure telecom-band single photons at room temperature, including color centers in silicon carbide 11 and gallium nitride 12 . Recently, the realm of such emitters has been expanded by luminescent nanotube defects (NTDs) in sp 3 -functionalized single-wall carbon nanotubes 13 . Unlike other emitters, NTDs allow for precise control over the emission wavelength via covalent side-wall chemistry 14 , 15 , 16 . Moreover, carbon nanotubes are straightforward to integrate with gated structures 17 , microcavities 18 , 19 , 20 , 21 or plasmonic cavities 22 . These properties, combined with high single photon purity 14 , 22 , render NTDs excellent candidates for the development of sources of quantum light.

As common to solid-state quantum emitters, NTDs are subject to strong dephasing at room temperature. As a result, the coherence time T 2 of the emitted photons is orders of magnitude smaller than the population lifetime T 1 . The respective photon indistinguishability, which can be quantified by T 2 /(2 T 1 ) 23 , 24 , is therefore limited to vanishingly small values. This limitation represents a major challenge in the development of single photon sources based on NTDs and other solid-state quantum emitters. The strategy of reducing T 1 to enhance the photon indistinguishability via Purcell enhancement 6 has been successfully applied to quantum dots and Erbium ions in various cavity geometries 8 , 9 , 10 , 25 , 26 , 27 , 28 as well as to NTDs by coupling to a plasmonic nanocavity 22 . However, all these experiments were operated in the regimes of coherent or incoherent bad cavity coupling 29 , where strong dephasing at ambient conditions limits both photon coherence time and Purcell enhancement, and thus all experiments to date crucially relied on operation at cryogenic temperatures with reduced dephasing. Although at ambient conditions spectral or temporal filtering of mainly incoherent photons would increase the photon coherence in principle, it would come at the cost of drastically reduced collection efficiency. Therefore, enhancement of T 2 at efficiencies exceeding those attainable through spectral or temporal filtering has remained elusive for quantum emitters subject to strong dephasing.

Here, we demonstrate enhancement of photon indistinguishability for telecom-band single photons from individual NTDs coupled to an optical microcavity. Motivated by a recent theoretical proposal, we operate the NTD-cavity system in the regime of incoherent good cavity coupling 30 , where the photon coherence time is determined by the cavity linewidth. By choosing a cavity with a spectrally narrow linewidth, we enhance T 2 and thus the photon indistinguishability of the coupled NTD-cavity system. At the same time, the cavity enhances the emission via the Purcell effect, thus yielding simultaneous increase of both indistinguishability and efficiency unattainable by spectral or temporal filtering. As a consequence, the efficiency of our system outperforms spectral or temporal filtering within the same bandwidth by at least a factor of four, with an estimated increase of photon indistinguishability by two orders of magnitude as compared to free-space NTDs. Our results experimentally establish the regime of incoherent good cavity-coupling as a powerful strategy for optimized sources of quantum light.

The NTDs used in this work, shown schematically in the left panel of Fig.  1 a, were obtained by functionalizing (8,6) carbon nanotubes by diazonium reaction 31 , 32 (see the Methods section for details). The photoluminescence (PL) excitation map of an aqueous suspension with covalently functionalized carbon nanotubes is shown in Fig.  1 b, with an excitation resonance at 718 nm corresponding to the E 22 transition and emission via E 11 around 1170 nm, characteristic of (8,6) chiral tubes 33 . The red-shifted emission peak, labeled as \({E}_{11}^{*}\) and centered at 1470 nm, corresponds to the luminescence from excitons localized at nanotube side-wall defects with emission wavelength tuned to the telecom S-band 34 by the choice of the functional group, in this case the 3,4,5-trifluoro-2-chlorosulfonyl-aryl group paired with the hydroxy group 32 . For integration in a fiber-based Fabry-Pérot cavity 35 shown schematically in the right panel of Fig.  1 a, the nanotubes were dispersed onto a planar macroscopic mirror with a polystyrene layer on top (see the Methods section for details) to ensure optimal coupling near the antinode of the intra-cavity field. Both spectral and spatial overlap between individual NTDs and the fundamental Gaussian cavity mode were optimized by lateral displacement of the macro-mirror and vertical tuning of the fiber-based micro-mirror via piezoelectric actuators. Photons emitted by the NTD-cavity system were coupled into a single mode fiber upon transmission through the planar mirror.

a Schematic of luminescent nanotube defects (NTDs) coupled to the fiber-based open micro-cavity system with tunable cavity length L c and lateral displacement degrees of freedom of the macroscopic mirror x and y . b Photoluminescence (PL) excitation of functionalized (8,6) carbon nanotubes with emission band of fundamental excitons (E 11 ) and NTD states ( \({{{{{{{\rm{{E}}}}}}}_{11}^{*}}}\) ). c Schematic spectral weight of strongly dephased free-space NTD luminescence (dark green) subjected to incoherent cavity coupling (orange). d Ensemble PL spectrum (dark green) and cavity finesse in transfer-matrix simulations (orange). The NTD luminescence spectrally close to maximal cavity finesse was excited at the E 11 transition at near-unity transmission of the cavity mirrors.

To implement the regime of incoherent NTD-cavity coupling, we employed a distributed Bragg reflector (DBR) mirror coating for spectrally narrow cavity linewidth at the target wavelength of telecom-band emission. Figure  1 d shows jointly the ensemble PL spectrum and the cavity finesse obtained from a transfer matrix simulation of the DBR coating. In the cavity, the NTD states were excited resonantly through the E 11 transition at near-unity DBR mirror transmission and thus independent of the cavity resonance condition. With finesse values on the order of 1000 at the \({E}_{11}^{*}\) transition wavelength, the cavity mode provided the primary radiative decay channel for the NTD emission. A combination of long-pass filters was used to suppress the excitation laser and other emission at wavelengths below 1400 nm before detection.

The effect of cavity-coupling on the photonic spectral bandwidth is illustrated in Fig.  1 c. At ambient conditions, the spectral width of the NTD emission profile is dominated by pure dephasing at rate γ * , with Γ = 2 γ * on the order of ten nanometers or 10 meV. This is orders of magnitude larger than the experimental cavity linewidth, which was determined as κ  = 35.4 ± 0.1 μeV for the lowest accessible longitudinal mode order, corresponding to 61.7 ± 0.2 pm in the wavelength domain. The small value of κ enables operation of our system in the regime of incoherent good cavity coupling, where 2 g   ≪   γ  +  γ *  +  κ and κ  <  γ  +  γ * holds for the light-matter coupling strength g , the population decay rate γ , κ and γ * (see Supplementary Note  1 for details). In this regime, the cavity is incoherently pumped upon initial (incoherent) excitation of the NTD at a rate R  ≈ 4 g 2 / γ * 30 , which in our system is smaller than the population decay rate. Any photon that is coupled into the resonator will be emitted via the cavity mode on a timescale 1/ κ . Since the emission process from the cavity is coherent 30 , this constitutes a giant increase in the photon coherence time compared to the free-space limit of 1/ γ * . In the spectral domain, the effect corresponds to a drastic spectral purification as illustrated in Fig.  1 c, similar to spectral filtering. This effect is a key feature of the incoherent good cavity coupling regime and is instrumental for enhanced photon indistinguishability.

In addition to the coherence time, the cavity also enhances the emission spectral density, with the enhancement quantified by the Purcell factor F p   ∝   g 2 36 (see Supplementary Note  3 for details). Increasing F p via the light-matter coupling strength g increases the single photon efficiency, i.e. the probability that a photon is emitted into the cavity mode. In the incoherent good cavity regime, this probability is smaller than the free-space quantum yield due to the large mismatch in the spectral bandwidths of the emitter and the cavity. However, as we demonstrate in the following, maximizing g (achieved in our case by minimizing the microcavity mode volume) results in an efficiency which by far exceeds that obtained by filtering at a spectral bandwidth κ or an equivalent temporal bandwidth.

Individual NTDs were identified in the cavity from maps of PL intensity as in Fig.  2 a, recorded upon lateral raster-scan displacement of the macroscopic mirror for a fixed cavity length. The two maps of Fig.  2 a were acquired for two orthogonal linear polarizations in the detection path and feature bright PL spots with lateral extent given by the point spread function of the Gaussian fundamental cavity mode with a waist of 2 μ m. The left (right) map in Fig.  2 a was obtained for parallel (orthogonal) orientation of the polarization axis with respect to the nanotube with NTD 1. The contrast in the brightness between the two maps for most PL hot-spots indicates a large degree of linear polarization at the emission sites, a hallmark of the well-known antenna effect in individual carbon nanotubes 37 , 38 .

a Cavity-enhanced PL raster-scan maps recorded for two orthogonal linear polarizations. The detection basis is chosen parallel (left) and perpendicular (right) to the axis of the emitter NTD 1 marked by the dashed circle. The scalebar is 5 μm. b Normalized PL of NTD 1 as a function of the cavity length, tuned over three longitudinal mode orders (blue circles). The emission spectrum is probed at the resonance wavelengths of the transverse electromagnetic (TEM) cavity modes (yellow, orange and red squares). The solid line was obtained from the fit described in the main text. The colored arrows indicate the respective y-axis. c Maximum PL intensity of a different emitter (NTD 2) as a function of the longitudinal mode order, normalized by the coupling efficiency into the single mode fiber. Cavity-enhancement of the PL intensity is inversely proportional to the mode volume V c , as evident from best-fit (solid line). The error bars give the standard uncertainty, dominated by experimental uncertainty in fiber coupling.

In Fig.  2 b, we show the normalized PL intensity of NTD 1 as the cavity length is tuned over three longitudinal mode orders q  = 7, 8 and 9. For each mode order, we observe an asymmetric emission profile, stemming from higher order transverse electromagnetic (TEM) modes. Since the cavity linewidth κ is much smaller than the emitter PL linewidth Γ, the NTD emission spectrum is probed at the resonance wavelength of each TEM-mode 39 with resonance wavelengths given explicitly on the right axis of Fig.  2 b. We fitted the data by the sum of three Lorentzians for each longitudinal mode order, with the result shown as the solid line in Fig.  2 b (TEM m n mode orders with n  +  m  > 2 were neglected due to vanishing contributions). From the fit, we obtained the emission wavelength 1465 ± 3 nm, and a FWHM linewidth of 28 ± 5 nm, corresponding to γ *  = 8 ± 2 meV.

Figure  2 c shows the effect of the cavity mode volume on the photon emission efficiency. We measured the collected PL intensity of a different NTD with comparable brightness for ten consecutive longitudinal mode orders, normalized to the largest value and corrected for the variation of the measured fiber coupling. The fiber coupling efficiency depends on the mode waist, which in turn changes with cavity length. We observed an increase in the PL intensity by a factor of six as the cavity was tuned to the lowest accessible longitudinal mode order q  = 4. This mode order corresponds to an intermirror separation of 2.6 μm, mainly limited by the profile depth of the fiber mirror of 2 μm. The increase in the PL intensity stems from an enhancement in light-matter coupling strength g as the cavity length and hence the mode volume V c is decreased. For our regime of low Purcell enhancement, where the NTD population lifetime is mainly unaffected by the cavity, the emission intensity is proportional to g 2 , which in turn is inversely proportional to V c (see Supplementary Note  3 for details). A fit of \(\alpha {V}_{{{{{{{{\rm{c}}}}}}}}}^{-1}\) , with V c calculated from the cavity length L c  =  q λ /2 35 and the amplitude α as a free fit parameter, yields a good correspondence with the data (solid line in Fig.  2 c).

Operating the coupled NTD-cavity system at maximum cavity-enhancement of the PL intensity, we determined second-order correlations in photon emission events with a fiber-based Hanbury–Brown–Twiss (HBT) setup shown schematically in Fig.  3 a. Photons generated via pulsed laser excitation were coupled into a fiber beamsplitter, and detection events at the output ports were time-correlated to obtain the normalized second-order autocorrelation function \({g}_{{{{{{{{\rm{HBT}}}}}}}}}^{(2)}(\tau )\) . The shot-noise limited results of the HBT experiment on two distinct NTDs are shown in Fig.  3 b, with the corresponding antibunching values \({g}_{{{{{{{{\rm{HBT}}}}}}}}}^{(2)}(0)=0.31\pm 0.09\) and 0.09 ± 0.07 as measures of the single photon purity.

a Schematic of a Hanbury–Brown–Twiss (HBT) setup based on a fiber beamsplitter (BS). b HBT autocorrelation function of cavity-coupled NTD 1 (light green) and NTD 3 (dark green), with second order coherence at zero time delay \({g}_{{{{{{{{\rm{HBT}}}}}}}}}^{(2)}(0)=0.31\pm 0.09\) and 0.09 ± 0.07, respectively.

The photon indistinguishability was quantified in Houng-Ou-Mandel (HOM) type experiments using an imbalanced Mach–Zehnder interferometer shown schematically in Fig.  4 a. The train of single photon pulses generated by the source was first split in a fiber beamsplitter. The time delay Δ t in the interferometer was tuned by the path difference Δ z with an adjustable delay stage to enable two-photon interference between consecutively emitted photons at the second beam splitter. In this setting, a delay of zero implies a separation by one excitation pulse. The relative polarization between the interferometer arms was set by fiber polarization controllers, and the detection events at the output ports were time-correlated to obtain the HOM-autocorrelation function \({g}_{{{{{{{{\rm{HOM}}}}}}}}}^{(2)}(\tau )\) (see the Methods section for details). First, we initialized the interferometer at zero delay and performed a two-photon interference experiment for co- and cross-polarized interferometer arms on NTD 3. The shot-noise limited results are shown in Fig.  4 b. For the co-polarized configuration, we observe a reduction of the measured correlations at zero time delay. This is a hallmark of quantum coherent two-photon interference: the (partially) indistinguishable single photons arriving simultaneously at different input ports of the beamsplitter are likely to exit at the same output port, resulting in reduced correlations at zero time delay 8 , 9 , 40 . We quantify the respective degree of the photon indistinguishability by the two-photon interference visibility v that one would detect in an interferometer with balanced beamsplitters and unity classical visibility 10 . We obtain v  = 0.51 ± 0.21 for the data in Fig.  4 b, taking into account non-identical reflection and transmission of the beamsplitters and finite single photon purity of NTD 3 (see Supplementary Note  4 for details).

a Schematic of the imbalanced Mach–Zehnder interferometer to probe the photon indistinguishability in Hong-Ou-Mandel (HOM) type experiments based on fiber beamsplitters (BS). The time delay between the interferometer arms was tuned via the displacement Δ z , and their relative polarization by the fiber polarization controller (FPC) in one arm. b HOM autocorrelation function of NTD 3 for co-polarized (dark green) and cross-polarized (orange) interferometer arms with delay of one excitation pulse. The difference in the coincidence probabilities at zero-delay is a hallmark of two-photon interference with visibility v  = 0.51 ± 0.21. c HOM autocorrelation function of NTD 1, measured in co-polarized interferometer configuration for interferometer delays 0 ps (dark green) and 5 ps (orange). Zero interferometer delay again corresponds to delay by one excitation pulse separation. d HOM autocorrelation function at time delay τ  = 0 for NTD 1 as a function of the interferometer delay, with visibility v  = 0.65 ± 0.24. The solid line is an empirical fit to the HOM dip described in the main text. The horizontal error bars correspond to the standard uncertainty in the interferometer delay; the vertical error bars correspond to the standard uncertainty determined as described in the Methods section. e Temporal PL decay of NTD 1 (dark green data) and instrument response (light green data). The orange line shows the result of a biexponential decay model.

Successively, we performed the HOM interference experiment for varying interferometer delays on NTD 1, with autocorrelation histograms for interferometer delays of 0 and 5 ps shown in Fig.  4 c. The observed reduction in correlations at zero time delay is again a hallmark of two-photon interference, where tuning between the two interferometer delay settings is approximately equivalent to switching the polarization configuration as in Fig.  4 b. In Fig.  4 d, we show the measured value of the HOM autocorrelation function at zero time delay for varying interferometer delay. Upon transition through zero-delay, we observed the characteristic HOM dip due to reduced cross-channel correlations by two-photon interference, described by the empirical formula \(c[1-a\exp (-| {{\Delta }}t| /{\tau }_{{{{{{{{\rm{HOM}}}}}}}}})]\) , where a is an amplitude, c is an offset at large interferometer delays Δ t , and τ HOM is the characteristic timescale of the HOM interference 40 . From the best fit to the data shown by the solid line in Fig.  4 d, we determined τ HOM  = 2 ± 2 ps, and a visibility of 0.65 ± 0.24 (see Supplementary Note  4 for details), consistent with the value of 0.51 ± 0.21 for NTD 3.

The characteristic two-photon interference time scale τ HOM is given by the jitter in the photon arrival time at the beamsplitter, which in turn is determined by the population lifetime 23 (see Supplementary Note  5 for details). For the emitter NTD 1, the fit to the data in Fig.  4 d thus implies a population decay within a few picoseconds. This time scale can be associated with the short decay component of the biexponential PL decay characteristic for NTDs 14 , 41 . The fast and slow decay channels with time constants τ fast and τ slow arise from an interplay of bright and dark exciton reservoirs, with τ fast as short as a few picoseconds and relative decay amplitudes close to unity in larger-diameter nanotubes 41 . In Fig.  4 e, we show by the solid line the result of a cavity-coupled biexponential model decay with τ fast  = 2 ps and τ slow  = 91 ps, convoluted with the instrument response function, together with the measured PL decay for NTD 1.

Although the short decay component is not resolved directly in the instrument-response limited data of Fig.  4 e, the identification of τ fast with τ HOM is plausible. In the framework of the incoherent good cavity regime, the feeding of the cavity through the fast decay channel generates photons with near-unity visibility 30 . The actual visibility in Fig.  4 d is lower than unity (0.65 ± 0.24), most probably due to photons generated via the slow process with lifetimes exceeding the cavity coherence time of 20 ps, which renders them partly distinguishable. A reduction in visibility is also backed by our model for time-dependent NTD-cavity coupling, which predicts v  = 0.3 for the NTD 1 in Fig.  4 d (see Supplementary Note  4 for details). The deviation between measured and estimated value is consistent with operation of our experiment at wavelengths on the edge of the DBR stopband (see Fig.  1 c). In this regime, small shifts towards larger resonance wavelength caused by cavity length drifts can decrease the cavity linewidth by a factor of up to two and in turn result in increased visibility, which is inversely proportional to κ 30 .

The visibility in the two-photon interference data in Fig.  4 d corresponds to a 217-fold enhancement of the value estimated for the free-space limit (see Supplementary Note  4 for details). For spectrally filtered free-space emission, the same visibility can be achieved in principle, yet at the cost of very low single photon efficiency. In the incoherent good cavity regime implemented here, the measured lower bound \(\min ({\beta }_{{{{{{{{\rm{c}}}}}}}}})=(4.0\pm 0.1)\cdot 1{0}^{-3}\) and expected value β c  = 6.6  ⋅  10 −3 for the Purcell-enhanced single photon efficiency are a factor of four and seven larger than the estimated upper bound β fs  =  κ /( π γ * ) = 1  ⋅  10 −3 for spectrally filtered free-space decay, whose actual value we expect to be at least one order of magnitude smaller when taking into account the non-unity NTD quantum yield (see Supplementary Note  3 for details). Further benefit arises from the fiber-based design of our cavity, which in principle allows unity in-fiber coupling efficiency in contrast to free-space collection with inherent diffraction losses.

To conclude, we have presented a room-temperature source of telecom-band single photons with emission efficiency and indistinguishability drastically enhanced by incoherent NTD-cavity coupling. To our knowledge, our results represent the first demonstration of cavity-enhanced indistinguishability for a quantum emitter with room-temperature dephasing. We estimate that the current two-photon interference visibility of about 0.5 can be improved to near-unity values by increasing the cavity finesse to 35,000, a feasible value with open fiber cavites 20 . Simultaneously, a further reduction of the mode volume to recently reported values 42 would yield an enhancement in emission efficiency by another order of magnitude. Even without these improvements, our results represent a major step towards room-temperature quantum photonic devices for applications at telecom-wavelength in optical quantum computation 43 or long-distance communication relying on optical quantum repeaters 44 .

Sample preparation

The NTDs were prepared by functionalizing (8,6) carbon nanotubes based on a method we reported previously 32 . Briefly, raw HiPco SWCNT material (NoPo Nanotechnologies, India) was dissolved in chlorosulfonic acid (99%, Sigma-Aldrich) at a concentration of 0.5 mg/mL, followed by adding 2-amino-4,5,6-trifluorobenezen-1-sulfonyl chloride, which was synthesized from 3,4,5-trifluoeoaniline, and NaNO2 (ReagentPlus® >99.0%, Sigma-Aldrich) to concentrations of 0.24 mg/mL and 0.2 mg/mL, respectively. After fully mixed, the acid mixture was then added drop-by-drop to Nanopure® water with vigorous stirring, resulting in the formation of NTD functionalized carbon nanotubes that precipitated from the solution as black precipitates. The precipitates were filtered and rinsed with an excessive amount of Nanopure® water. The synthesized NTDs were dissolved in 2% (wt/v) sodium deoxycholate (DOC, Sigma-Aldrich, ≥97%) solution and centrifuged at 16400 rpm for 1 h to remove any bundles. The nanotubes with NTDs were then sorted by aqueous two-phase extraction 32 , 1 nm diameter single-wall carbon nanotube species using aqueous two-phase extraction. ACS Nano 9, 5377–5390 (2015)." href="/articles/s41467-024-48119-1#ref-CR45" id="ref-link-section-d9628939e2018">45  in a solution of 2% (w/v) DOC in deuterium oxide (D2O, Cambridge Isotope Laboratories, Inc. 99.8%) to obtain NTDs on (8,6) chirality enriched nanotubes.

Next, a macroscopic planar mirror was spin-coated with a 10 μL solution of 3% (wt/v) polystyrene/toluene, at 2000 RPM for 1 min, resulting in the formation of a polystyrene spacer layer estimated to be 150-nm thick. The coated mirror was then vacuum-dried at room temperature for 24 h before being deposited with 5 μL of the NTDs containing solution by spin-coating at 3000 RPM for 1 min.

Fiber-based cavity

The experiments were conducted in an ultra-stable fiber-based open-cavity platform ( Qlibri Quantum , Qlibri GmbH). The cavity is formed by a microscopic concave fiber mirror with a radius of curvature of 25 μm, fabricated by CO 2 laser ablation 35 , 46 , and a macroscopic planar mirror with a 150-nm thick polystyrene spacer layer and functionalized carbon nanotubes on top. The spacer layer was included to place NTDs close to an antinode of the intra-cavity field. Three translational degrees of freedom are accessible through piezoelectric positioners, allowing for lateral scans and length-tuning of the cavity with sub-nanometer precision. Fiber and sample mirror have identical DBR coatings, designed for high reflectivity at telecom wavelengths (minimum transmission T  = 95.2 ppm at wavelength of 1535.4 nm) and fabricated by ion beam sputtering (Laseroptik GmbH). At a wavelength of 1468 nm, close to the \({E}_{11}^{*}\) peak maximum, the largest measured finesse was 3010 ± 10 for the lowest accessible longitudinal mode order q  = 4. For this mode order, corresponding to a mirror distance of L c  = 2.6 μm, we calculated a mode waist of ω 0  = 2 μm and a cavity mode volume of V c  = 8.2 μm 3 35 .

Photoluminescence and photon correlation experiments

PL measurements were performed under resonant excitation of the E 11 transition using a pulsed supercontinuum white light source (NKT SuperK Extreme) at a repetition rate of 78 MHz that was spectrally filtered in a home-built monochromator to a linewidth of 2 nm. The cavity was tuned on resonance with the \({E}_{11}^{*}\) transition by changing the mirror distance. The PL emitted through the planar mirror of the cavity was collimated by an achromatic doublet lens (Thorlabs AC127-019-C-ML), filtered with two longpass filters (Thorlabs FEL1400, band edge 1400 nm, and Semrock BLP02-1319R-25, band edge 1320 nm) and coupled into a single mode fiber. Detection was performed with a pair of superconducting nanowire single photon detectors (Scontel TCOPRS-CCR-SW-85) and time-correlated with a TCSPC module (Swabian Instruments Time Tagger Ultra and PicoQuant PicoHarp300). Second-order photon correlation measurements were performed in a standard Hanbury–Brown–Twiss configuration. For Hong-Ou-Mandel type two-photon interference experiments, a home-built fiber-based imbalanced Mach–Zehnder interferometer was employed. A mechanical delay stage was used to tune the interferometer delay on sub-picosecond scale. Polarization was set by fiber-polarization controllers (Thorlabs FPC562).

Photon correlation histograms were obtained by integrating detection events in 2.5 ns wide windows. The resulting histograms feature prominent peaks separated by the delay between the excitation pulses. To obtain the correlation functions \({g}_{{{{{{{{\rm{HBT}}}}}}}}}^{(2)}\) and \({g}_{{{{{{{{\rm{HOM}}}}}}}}}^{(2)}\) , we normalized the histograms with respect to the average height of histogram peaks N ∞ at large time delays. The standard uncertainty of the measured peak height N 0 at τ  = 0 is given by \(\sqrt{{N}_{0}}\) 47 and is the dominant uncertainty in the measurement of N 0 . The standard uncertainty in quantities derived from measured peak heights was obtained by Gaussian error propagation, considering the uncertainties in all input parameters. The normalized second-order correlation at zero time delay g (2) (0) was obtained from the measured histograms as \({g}^{(2)}(0)={N}_{0}/{N}_{\infty }(1\pm 1/\sqrt{{N}_{0}})\) 47 including dark count and background correction 8 . The uncertainties in N ∞ and background were found to have negligible influence on this measurement, whose uncertainty is dominated by the uncertainty in N 0 .

Data availability

The source data generated in this study have been deposited in the LMU Open Data database under accession code https://doi.org/10.5282/ubm/data.460 .

Code availability

The codes that support the findings of this study are available from the corresponding authors upon request.

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Acknowledgements

We gratefully acknowledge helpful discussions with Lukas Knips and support by Max Huber for manufacturing of the cavity. This research was funded by the European Research Council (ERC) under the Grant Agreement No. 772195 as well as the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the Germany’s Excellence Strategy EXC-2111-390814868. L.H. and A.H. acknowledge funding by the Bavarian Hightech Agenda within the EQAP project. B.P. acknowledges funding by IMPRS-QST. D.H. acknowledges support by the Karlsruhe School of Optics & Photonics (KSOP). Y.W. gratefully acknowledges the U.S. National Science Foundation for funding support (grant no. PHY1839165 and CHE2204202).

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These authors contributed equally: Lukas Husel, Julian Trapp.

Authors and Affiliations

Fakultät für Physik, Munich Quantum Center, and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, 80539, München, Germany

Lukas Husel, Julian Trapp, Johannes Scherzer, Borislav Polovnikov & Alexander Högele

Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, USA

Xiaojian Wu, Peng Wang, Jacob Fortner & YuHuang Wang

Qlibri GmbH, Maistr. 67, 80337, München, Germany

Manuel Nutz, Thomas Hümmer & Michael Förg

Physikalisches Institut, Karlsruhe Institute of Technology, Karlsruhe, Germany

David Hunger

Institute for Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology (KIT), Herrmann-von-Helmholtz Platz 1, 76344, Eggenstein-Leopoldshafen, Germany

Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799, München, Germany

Alexander Högele

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Contributions

D.H., Y.W. and A.H. conceived the project. L.H. and J.T. set up and performed the experiments with contributions by J. S., evaluated the data, and carried out theoretical analysis and modeling. X.W. led the sample preparation of defect-tailored carbon nanotubes synthesized by P.W. with contributions from J.F. and supervision by Y.W. B.P. contributed to the initial sample characterization by optical spectroscopy. M.N., T.H. and M.F. designed and manufactured the cavity and provided support for its operation. L.H., J.T., D.H. and A.H. analyzed the data. L.H., J.T. and A.H. wrote the manuscript. All authors commented on the manuscript.

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Correspondence to David Hunger , YuHuang Wang or Alexander Högele .

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Husel, L., Trapp, J., Scherzer, J. et al. Cavity-enhanced photon indistinguishability at room temperature and telecom wavelengths. Nat Commun 15 , 3989 (2024). https://doi.org/10.1038/s41467-024-48119-1

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DOI : https://doi.org/10.1038/s41467-024-48119-1

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Like wavelength and intensity, polarization is a fundamental property of light and represents which direction the electric component of a light wave is oriented with respect to the wave's general direction of travel. Most light, including sunlight, is unpolarized, meaning that its electromagnetic waves move and travel in all directions.

However, filters called polarizers can be used to create light beams with one specific polarization. A vertical polarizer, for example, only allows photons with vertical polarization to pass through. Those with horizontal polarization (meaning that the electric component of the light wave is oriented horizontally with respect to the direction of travel) will be blocked. Any light with other polarization angles (between vertical and horizontal), will partially pass through. The outcome is a stream of vertically polarized light.

This is how polarized sunglasses reduce glare. They use a vertically polarizing chemical coating to block sunlight that has become horizontally polarized by reflecting off a horizontal surface, such as a lake or snowy field. This means that the wearer only observes vertically polarized light.

When changes in light intensity or color are not enough to give scientists quality images of certain objects, controlling the polarization of the light in an imaging system can sometimes provide more information about the sample and offer a different way to identify contrast between a sample and its background. Detecting the changes in polarization caused by certain samples can also give researchers information about the internal structure and behavior of those materials.

Wang's newest microscopy technique, dubbed quantum imaging by coincidence from entanglement (ICE), takes advantage of entangled photon pairs to obtain higher-resolution images of biological materials, including thicker samples, and to make measurements of materials that have what scientists call birefringent properties.

Rather than consistently bending incoming light waves in the same way, as most materials do, birefringent materials bend those waves to different degrees depending on the light's polarization and the direction in which it is traveling. The most common birefringent materials studied by scientists are calcite crystals. But biological materials, such as cellulose, starch, and many types of animal tissue, including collagen and cartilage, are also birefringent.

If a sample with birefringent properties is placed between two polarizers oriented at 90-degree angles to each other, some of the light going through the sample will be altered in its polarization and will therefore make it through to the detector, even though all the other incoming light should be blocked by the two polarizers. The detected light can then provide information about the structure of the sample. In materials science, for example, scientists use birefringence measurements to get a better understanding of the areas where mechanical stress builds up in plastics.

In Wang's ICE setup, light is passed first through a polarizer and then through a pair of special barium borate crystals, which will occasionally create an entangled photon pair; about one pair is produced for every million photons that pass through the crystals. From there, the two entangled photons will branch off and follow one of the system's two arms: one will travel straight ahead, following what is called the idler arm, while the other traces a more circuitous path called the signal arm that causes the photon to pass through the object of interest.

Finally, both photons go through an additional polarizer before reaching two detectors, which record the time of arrival of the detected photons. Here, though, occurs a "spooky" quantum effect because of the entangled nature of the photons: the detector in the idler arm can act as a virtual "pinhole" and "polarization selector" on the signal arm, instantly affecting the location and polarization of the photon incident on the object in the signal arm.

"In the ICE setup, the detectors in the signal and idler arms function as 'real' and 'virtual' pinholes, respectively," says Yide Zhang, lead author of the new paper published in Science Advances and a postdoctoral scholar fellowship trainee in medical engineering at Caltech. "This dual pinhole configuration enhances the spatial resolution of the object imaged in the signal arm. Consequently, ICE achieves higher spatial resolution than conventional imaging that utilizes a single pinhole in the signal arm."

"Since each entangled photon pair always arrives at the detectors at the same time, we can suppress noises in the image caused by random photons," adds Xin Tong, co-author of the study and a graduate student in medical and electrical engineering at Caltech.

To determine the birefringent properties of a material with a classical microscopy setup, scientists typically switch through different input states, illuminating an object separately with horizontally, vertically, and diagonally polarized light, and then measuring the corresponding output states with a detector. The goal is to measure how the birefringence of the sample alters the image that the detector receives in each of those states. This information informs scientists about the structure of the sample and can provide images that would not otherwise be possible.

Since quantum entanglement allows paired photons to be linked no matter how far apart they might be, Wang is already imagining how his new system could be used to make birefringence measurements in space.

Consider a situation where something of interest, perhaps an interstellar medium, is located light years away from Earth. A satellite in space might be positioned such that it could emit entangled photon pairs using the ICE technique, with two ground stations acting as detectors.

The large distance to the satellite would make it impractical to send any kind of signal to adjust the device's source polarization. However, due to entanglement, changing the polarization state in the idler arm would be equivalent to changing the polarization of the source light before the beam hits the object.

"Using quantum technology, nearly instantaneously, we can make changes to the polarization state of the photons no matter where they are," Wang says. "Quantum technologies are the future. Out of scientific curiosity, we need to explore this direction."

More information: Yide Zhang et al, Quantum imaging of biological organisms through spatial and polarization entanglement, Science Advances (2024). DOI: 10.1126/sciadv.adk1495

Provided by California Institute of Technology

What happens if you fall into a black hole? NASA simulations provide an answer.

Available on youtube, the four visualizations include explanations to guide viewers on what they're witnessing and include 360-versions to allow viewers to look around during the virtual trip..

Anyone who has watched Matthew McConaughey plunge into a supermassive black hole in "Interstellar" may think they have a rough idea of what it'd be like to encounter one of these terrifying cosmic formations .

But a Hollywood blockbuster set decades in the future is no comparison to the real thing – even if it was directed by Christopher Nolan . Ten years after "Interstellar" hit theaters, NASA is now giving us a more personal experience of what would happen if we were to fall into a black hole.

No, not even the most intrepid spacefarers are yet able to get anywhere near these massive behemoths, where the pull of gravity is so intense that even light doesn't have enough energy to escape their grasp.

In the meantime, simulations released Monday instead simply imagine what a person may see while plummeting toward a black hole's event horizon to their inevitable death. Yet another simulation released by NASA shows the imagined point of view of an astronaut flying past a black hole as space appears to bend and morph.

"I simulated two different scenarios, one where a camera – a stand-in for a daring astronaut – just misses the event horizon and slingshots back out, and one where it crosses the boundary, sealing its fate," said Jeremy Schnittman, an astrophysicist at  NASA’s Goddard Space Flight Center  in Greenbelt, Maryland who produced the visualizations.

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NASA simulations show plunge into black hole

While humanity has learned much more about black holes in recent years since the first one was identified in 1964 , the objects remain notoriously mysterious.

NASA's new visualizations, available on Goddard's YouTube page , erase some of that enigma. The two visualizations are divided into one-minute trips rendered as 360-degree videos that allow viewers to look around during the trip, and extended versions with explanations to guide viewers on what they're witnessing.

The destination of the simulation is a virtual supermassive black hole with a mass 4.3 million times that of Earth's sun, a size equivalent to the monster Sagittarius A* located at the center of our Milky Way galaxy.

The first simulation shows the viewer approaching the black hole from around 400 million miles away and rapidly falling toward the event horizon – a theoretical boundary known as the "point of no return" where light and other radiation can no longer escape. Like Sagittarius A*, the event horizon of the simulation spans about 16 million miles.

Cloud structures called photon rings and a flat, swirling cloud of hot, glowing gas called an accretion disk surrounding the black hole serve as a visual reference during the fall. As the camera reaches the speed of light, the accretion disc becomes more distorted as space-time warps.

Once inside the black hole itself, the viewer rushes toward the black hole's one-dimensional center called a  singularity , where the laws of physics as we know them cease to exist.

The simulations were made using the Discover supercomputer at the  NASA Center for Climate Simulation , and generated around 10 terabytes of data, which is about half of the estimated text content in the  Library of Congress .

Second simulation shows viewer narrowly escaping black hole

Astronomers divide black holes into three general categories based on mass: stellar-mass, supermassive, and intermediate-mass.

Stellar-mass black holes , which form when a star with more than eight times the sun’s mass runs out of fuel and its core explodes as a supernova, are even less ideal to find yourself falling into than its supermassive counterpart, Schnittman explained.

“If you have the choice, you want to fall into a supermassive black hole,” Schnittman said in a statement. “Stellar-mass black holes, which contain up to about 30 solar masses, possess much smaller event horizons and stronger tidal forces, which can rip apart approaching objects before they get to the horizon.”

This occurs because the gravitational pull on the end of an object nearer the black hole is much stronger than that on the other end. Falling objects stretch out like noodles, a process astrophysicists call  spaghettification . For this simulated black hole, it would only take about 12.8 seconds for the viewer to meet their end by spaghettification.

The alternate simulation  shows a viewer orbiting close to the event horizon but escaping to safety before ever crossing it.

If an astronaut flew a spacecraft on this 6-hour round trip, the explorer would return 36 minutes younger than those who remained on a mothership far away, NASA explained . This is another concept that will be familiar to "Interstellar" fans and is due to time passing more slowly near a strong gravitational source.

"This situation can be even more extreme," Schnittman said. "If the black hole were rapidly rotating, like the one shown in the 2014 movie 'Interstellar,' (the astronaut) would return many years younger than her shipmates."

Eric Lagatta covers breaking and trending news for USA TODAY. Reach him at [email protected]