What is Photons: Definition and 998 Discussions

The photon (Greek: φῶς, phōs, light) is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s (or about 186,282 mi/s). The photon belongs to the class of bosons.
Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles. The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, Planck proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric effect, Einstein introduced the idea that light itself is made of discrete units of energy. In 1926, Gilbert N. Lewis popularized the term photon for these energy units. Subsequently, many other experiments validated Einstein's approach.In the Standard Model of particle physics, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass, and spin, are determined by this gauge symmetry. The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers, and for applications in optical imaging and optical communication such as quantum cryptography.

View More On Wikipedia.org
  1. J

    I How to distinguish between diagonal vs horizontal/vertical polarization?

    I have been working for some time on designing an experiment and have gotten stuck on one particular aspect. I would greatly appreciate any advice that can be offered. I'm using SPDC to produce two polarization-entangled photons. Through the course of the experiment I know that one of the...
  2. L

    I Are photons definitely massless?

    Recently saw this video. "Why No One Knows If Photons Really Are Massless: What if they Aren't?" Arvin Ash He says photons need not be massless, but they must be quite light nonetheless. He separates speed of light from speed of causality. Is it true that we can't know its mass below a...
  3. cemtu

    I Kramer's Equation at max Energy for photons is giving Intensity = 0

    IE = KZ(Em – E) where IE is the intensity of photons with energy E, Z is the atomic number of the target, Em is the maximum photon energy, and K is a constant. As pointed out earlier, the maximum possible energy that a bremsstrahlung photon can have is equal to the energy of the incident...
  4. A

    B Gravitrons vs Photons: how come light and gravity exist?

    if matter and anti-matter meet, they annihilate each other. Gravitons are anti-photons and photons are anti-gravitons. They MUST meet in immesurable quantities in our universe. Yet gravity exists, and light exists in our known universe. This denies the annihilation necessity. And annihilation...
  5. N

    Doping a material in MCNP 6.1

    I am a new user of MCNP and I am trying to generate photons in ZnS:Ag through electrons as my source particle. My simulation as it is now creates photons however they are not right. For example ZnS:Ag should create a lot of photons with energy of around 3.1eV. However I see a spike around 4.8eV...
  6. RobbyQ

    I Photon Entanglement: Identifying & Using an Entangled Pair

    In this video how are the entangled photons later used and actually identified as an entangled pair amongst billions of others. Also does he really mean the photon is split or is the quantised energy split with half frequencies?
  7. B

    B Interferometry: Photons Needed for Interference Pattern

    If you have, say, a two-mirror interferometer that is producing an interference pattern, how many photons are interfering? Two, or just one?
  8. jaketodd

    B Photons on a perpendicular bisector path from a black hole's core

    Not directly from the core, but a trajectory that goes to the event horizon, and gets corrected to a perfect, perpendicular bisector path by the gravity of the core, when it reaches the event horizon. Would they escape the event horizon, since they have to always move at c? On this trajectory...
  9. P

    I Is there a generator of entangled photons with fixed polarization?

    Hello everyone! I don't know where to look for information - maybe here it will work :) Is there a generator of entangled photons with fixed polarization? If not, is it theoretically possible to build or is it against the laws of physics. I need this knowledge for further computer...
  10. J

    I Mass Density of Photons in Refractive Medium

    The effective mass density of photons in a vacuum ##\rho^{vac}_M## is related to the photon energy density ##\rho^{vac}_E## by $$\rho^{vac}_M=\frac{\rho^{vac}_E}{c^2}.$$ Is it true that the mass density of photons inside a medium of refractive index ##n##, ##\rho^n_M##, with phase velocity...
  11. S

    B Passing photons

    Now if two different time durations are measured for one and the same event by two different observers, for example T+1 and T-1 seconds. Is the speed of passage then (T x c)/(T+1) and (T x c)/(T-1) respectively? So not c? You may be wondering…, and yes, there is an example of it!
  12. D

    Question related to completeness relation for photons

    Hi Would you explain to me what is the q^ and how they are related to completeness.How can i solve this exercise?It is from "Quarks and leptons An Introductory course in Modern Particle Physics" of Halzen and Alan D.Martin.Also, can you point me to a useful bibliography?
  13. eneacasucci

    I Understanding Scattered Radiation in Photon Beams

    Consider a source emitting a beam of photons. These photons pass through x thickness of material. The attenuation coefficient of the beam \mu is known. We can write this formula If I'm not wrong, this formula tells us the number of photons that passed through the material of thickness x...
  14. Grelbr42

    I Unlocking the Potential of Back-to-Back Photons: an Experiment

    In some cases, photons can be produced in "back to back" (BTB) conditions. For example, electron-positron annihilation produces two photons, each at 0.511 MeV, with equal and opposite momentum. Or pretty close, up to the original velocities of the electron and positron. Start with a source of...
  15. L

    B Are Photons Real? New Study Challenges Assumptions

    Recently I've stumbled across a preprint in which the author describes a photon is a wave packet and even suggests a transverse extent. I find it strange, as my understanding so far has been that a photon (and the EM field as such) is a construct used to model certain observed interactions...
  16. N

    I Exploring Particle/Wave Duality in Photons

    (Beginner) - W.o. going into particle/wave duality, we know the resultant image came ONLY through something going through this nano opening we left uncovered. We also see that the resultant image still neatly shows diff. colors. But ALL the objects reflecting are sending different colors...
  17. H

    I Global evolution of a Bell pair of photons

    Hi Pfs, consider a pair of maximally entangled photons where the total momentum is null and the same thing for the total angular momentum. I suppose that this pair is like an universe: nothing outside the pair acts on it except maybe a device for the measurement of these two properties (no local...
  18. M

    How could you see an image without photons? (imaging systems)

    What kind of imaging system could be used to see without photons? I ask because I was watching a video and the furthest we can see back is Redshift Z~1090 which is the CMB. We can’t see the universe before the first stars formed or the Big Bang itself. My first guess would be some kind of dark...
  19. NODARman

    I Time Dilation for Photons: Explained

    So, I have a question. The time dilation formula is: t = t₀ • 1 / √(1 - v²/c²) Let's take a photon that travels at c. In my opinion, for a photon "clock doesn't tick" and its life is just a moment. But when we calculate time dilation by this formula, then c over c is 1 and the root of 1 minus...
  20. S

    B Photon Emission: Classical vs Nuclear Fusion

    In classical physics we know photon emits when electron move from higher to lower state but in nuclear fusion photon emits when neutron turn into proton. Is both correct?
  21. J

    B Idle Photons (motionless relative to the Earth's surface?)

    Can a photon ever be motionless relative to the earth's surface? Did it accelerate from 0 mps to 186,000 mps?
  22. S

    I Sachs-Wolfe effect in a Dark Energy universe?

    The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion (presumably caused by dark energy). For that reason, a photon going through a gravitational well would gain energy (blueshift) when entering and it would lose...
  23. Simon Peach

    B Question about photons and neutrinos

    while the photon travels at light speed and a neutrino travel at just below light speed why then are photons stopped by an object and the neutrino can past through?
  24. Christian Thom

    I Double-slit experiment with momentum entangled pair of photons

    In Kaur, M., Singh, M. Quantum double-double-slit experiment with momentum entangled photons. Sci Rep 10, 11427 (2020). https://doi.org/10.1038/s41598-020-68181-1 and in C. K. Hong and T. G. Noh, "Two-photon double-slit interference experiment," J. Opt. Soc. Am. B 15, 1192-1197 (1998) it is...
  25. sol47739

    I Polarization of photons quantum mechanically

    What is it of the photon that gets polarized from a quantum mechanical perspective? In the classical perspective it is often thought that it is the oscillating electric field that gets polarized. But in the quantum case: Is it the de Broglie wave function? Or is it the spin and in case it is the...
  26. N

    B Electrons & Photons: Are They Tiny Gaseous Planets?

    I am not sure if this is the way to ask questions here but having nobody to ask and little time, i hope i can get a fast reply here. So since the scaler the universe above us i.e. bigger than us is so huge and we aren't even sure about it further than sight (acc to my book), isn't it just...
  27. M

    B High Frequency Photons: More Mass, More Bend?

    Since high frequency photons have more relativistic mass, should we expect them to bend more than lower frequency lights when traveling through a gravitational field, thus produce a rainbow effect? But we don't seem to experience rainbow effects with star light.
  28. Marilyn67

    I Young's slit experiment with single photons

    Hello, I have a little problem understanding Young's slit experiment with single photons : I have understood for a long time that each photon impact on the screen corresponds to a photon sent by the source, and that, if we don't try to find out by which path the photon has passed, of course...
  29. N

    B A cone of photons from one (of many seen objects) hits the retina....

    why is the result not like a movie screen where you are projecting 300 films at the same time, over each other ? (I would get it, if each object only sent one discrete beam, the next object another one, and so on, but it is a cone, of equally strong photons, being projected everywhere, into your...
  30. R

    I Interpreting ##A^{\mu}(x)|0\rangle## and ##\psi (x) |0\rangle##

    I can understand how ##\phi (x)|0\rangle## represents the wavefunction of a single boson localised near ##x##.I don't understand how the same logic appies to ##A^{\mu}(x)|0\rangle## and ##\psi |0\rangle##. Both of these operators return a four component wavefunction when operated on the vaccuum...
  31. atyy

    I Relativistic Bohmian trajectories of photons via weak measurements

    I found out about this interesting paper through a Tweet by Steven Thomson. https://arxiv.org/abs/2108.05169 https://www.nature.com/articles/s41467-022-31608-6 Relativistic Bohmian trajectories of photons via weak measurements Joshua Foo, Estelle Asmodelle, Austin P. Lund, Timothy C. Ralph...
  32. LarryS

    I Speed of individual photons in a vacuum?

    Is there experimental evidence that confirms that the speed of individual photons in a vacuum never varies, even slightly? Thanks in advance.
  33. D

    I What determines the time between atomic absorption and emission of photons?

    What determines the time between atomic absorption and emission of photons? Is there a correlation to blackbody radiation?
  34. samy4408

    I Question about electromagnetics (waves and particles)

    I saw that we can talk about the light as particles (photons ) or as an electromagnetic wave , the question is that do we represent other electromagnetic waves (like microwaves or radio waves ) as particles (like we do with light ) ?
  35. vanhees71

    A What do physicists mean when they say photons have a "path"?

    [Moderator's note: thread spun off from previous thread due to topic/level change.] This [Ed.: the claim that photons have a "path"] is a misconception of quantum theory already for massive particles. It's even more severely misleading for massless quanta of spin ##\geq 1##, which do not even...
  36. E

    B Virtual photons as force carriers

    (My multipart question is from a very naive perspective, so sorry if it is rife with misunderstandings. Please answer conceptually, with as few & as simple equations as possible. I think that all of the answers to these questions should be understandable to a high schooler, though maybe the...
  37. I

    B Time and Space Distortions of Photons: What Am I Misunderstanding?

    As I understand it, photons are subject to the same time and space distortions under SR as anything else, which is why they don't perceive time or space, since they travel at the speed of light. To an outside observer, then, they should appear stationary, immobile at their moment of creation...
  38. kevincb672

    Calculating attenuation necessary to reduce number of photons

    Hi there. I am attempting to do calculations for my own project, the question being what is the attenuation necessary to reduce the number of photons in a beam to single-photon levels. N approximately 1 or 2. The laser in question is a 650nm 5mW laser. I have solved the energy per photon...
  39. M

    A QED Formulation with Massive Photon Fields

    I was reading Diagrammatica by Veltman and he treats the photon field as a massive vector boson in which gauge invariance is disappeared and the propagator has a different expression than in massless photon. After some googling, I found that this is one way to formulate QED which has the...
  40. spacecadet11

    I General physics question -- How can massless photons have momentum?

    P=mv *momentum equals mass X velocity. Light particles or "photons" are said to be "massless". And yet they have momentum. How is that possible? (p.s. I used to know the answer)
  41. elua0105

    How can I model both photons and neutrons with MCNP?

    Hello, I am a student who started studying MCNP. I'm not used to writing in English, so I'd appreciate it if you could understand even if there were grammatical errors in my thread. I want to check the energy of gamma rays from neutrons reacting with matter. So, I wrote this in the content of...
  42. G

    I X-ray fluorescence photons question

    Tentatively, I ask in this forum for a qualitative pointer to what end effects one might expect when a gamma energy photon energizes an atom of a substance, and causes fluorescence. It relates to a practical endeavour about using a PIN diode as an X-ray detector, where the device considered...
  43. D

    B Quarks, electrons, neutrinos, and photons?

    Can someone please explain the four classes of fundamental particles? (Just the basics) I came here because I never learned any chem or physics in school so please explain like I’m five :)
  44. Green dwarf

    I Virtual Photons: The Long-Distance Repulsion of Electrons

    My understanding is that the repulsion between two electrons is mediated by the exchange of virtual photons – virtual meaning not lasting long enough to be detected. But suppose one of the electrons is in the sun and the other on Earth. There should still be a repulsion because the...
  45. S

    I Interference of signal photons entangled with idlers

    Consider the following experiment: A photon hits a beam splitter, then a non-linear crystal (nichtlinearer Kristall - sorry, prepared the image in German) on each path that does parametric down conversion, splitting the photon into a signal and an idler. The idlers proceed to two detectors (D1...
  46. A

    I Dark Matter as a condensation of photons in a space

    if we assume each photon of light as a very very light piece of matter (by famous E = mc^2 and then: m = E / c^2) and sum up all photons that have been made from the creation time of a galaxy (also considering limitation of speed of light) and also photons that accidentally passing throw that...
  47. F

    I Can a static EM field consist of photons?

    We can analysis a static EM field into Fourier serie. Then we can consider a static EM field as a superposition of many running EM wave. So why we could not consider static EM field as a superposition of many photons(maybe virtue photons)?
  48. Killtech

    I How indistinguishable are photons?

    Assume we have a photon pair prepared just so they are suited for a two photon interference. Let's call the corresponding beams A and B. Then we run each beam individually through a 50:50 beam splitter such that we get beams A1, A2, B1, B2 such that we can symbolically write the state of the...
  49. U

    I Does entanglement of photons really facilitate telepathy?

    [This](https://youtu.be/9l6VPpDublg) is a lecture by Persinger where he postulates that photon entanglements and the Earth's magnetic field can facilitate telepathy. (See from 20:00 to 30:00) Links to the study he cited...
  50. Andreea007

    I How do photons transfer energy?

    Hi! So I know about the electron-photon interaction but what about photon-photon interaction? I mean, I do know there is a very small chance for them to interact, but how else do they transfer energy in order to get from Sun to Earth, for example? When it comes to sound waves I get it, for...