What is Photon: Definition and 999 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.

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  1. N

    I Does the Photon Exist? Exploring Its Necessity

    I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the...
  2. Grelbr42

    I How Big Is a Photon? - Exploring Interference Experiments

    This YouTube video, with the same title I have given this thread, reports on an experiment. He sets up an interference experiment. The light source is a HeNe laser. The light passes through a beam splitter then through two paths. The shorter path being about 0.26 meters, the long one about...
  3. 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...
  4. S

    B The Difference Between Wavefunction & Superposition: Exploring Photon Behaviour

    (For me to understand, please be mindful to avoid a bunch of jargon) I'm not sure if the proper word is wavefunction or superposition, and didn't find anything in a search of the difference between the two. So will elaborate on the question in my own words. To begin, as far as I undestand...
  5. E

    I What is the velocity of a photon through the four dimensions?

    What is the velocity of a photon through the four dimensions? What is the velocity of a photon through the three spatial dimensions x1, x2, x3? What is the velocity of a photon through the fourth dimension x4?
  6. Pradyuman

    A Why can't we say that photon has very very small mass?

    Any wave mass term decays, similarly if I want to explain redshift by considering massive photon, how much should be the mass? Is it less than today's upper limit. Solution of wave equation ##□ϕ=0## gives a wave that doesn't disperse over time. But wave solution of the form ##(□+m^2)ϕ=0## has...
  7. T

    I Fourier Transform of Photon Emission Hamiltonian

    Hey all, I just wanted to double check my logic behind getting the Fourier Transform of the following Hamiltonian: $$H(x) = \frac{ie\hbar}{mc}A(x)\cdot\nabla_{x}$$ where $$A(x) = \sqrt{\frac{2\pi\hbar c^2}{\omega L^3}}\left(a_{p}\epsilon_{p} e^{i(p\cdot x)} + a_{p}^{\dagger}\epsilon_{p}...
  8. M

    A Photon dilemma for MOND-like dark matter

    "Superfluid dark matter in tension with weak gravitational lensing data" (Mistele, McGaugh, Hossenfelder) Regarding this paper, Sabine Hossenfelder tweets One comment summarizes this as
  9. Lotto

    B What do these wavenumbers correspond to in Raman spectroscopy?

    I don't know whether it is an energy of a photon emitted by a deexciting molecule, or if it is an energy of laser's photons. Here is an example of such spectrum: For example, that value of wavenumber ##3000\, \mathrm {cm^{-1}}## is an energy of an emitted photon or a photon from laser? And that...
  10. ribella

    A What is the energy dependence of the Equivalent photon approximation?

    Hi, What is the energy dependence of the Equivalent photon approximation? For this approach to be valid, what is the maximum center of mass-energy. As know, this approach is an energy-dependent approach. Can this approach be used to calculate, for example, at a center of mass energy of 100...
  11. DrChinese

    A How Many Hidden Variables Would It Take to Model Photon Polarization?

    Suppose the EPR* concept were true as it would be applied to entangled photon polarization. (Please note: I am not saying it is.) They thought QM was incomplete, because there must exist "elements of reality" (hidden variables) that supplied the highly correlated results on entangled particle...
  12. sol47739

    I Do atoms recoil when emitting a photon?

    Hello! I have some questions regarding the photon emission and whether the atom recoils or not. When an electron in an atom emits a photon. One can calculate the energy of the photon by the difference between the energy levels from where it left to which it returned. Let’s say it jumped...
  13. MichPod

    B Is the photon a gauge boson for neutrinos?

    If I put this in technically correct terms, to enable a local symmetry related to electron phase change, we need to introduce a spin 1 field which is identified as electromagnetic field. Yet there are other spin 1/2 particles. Say, what about neutrino? As it does not couple with electromagnetic...
  14. Marioweee

    Compton scattering with off-shell photon

    How is it treated or what Feymann's rule applies to a virtual photon in an external leg? I would like to calculate the modulus of squared amplitude for the process e-γ*→e-γ where the * indicates that the photon is virtual. I've never dealt with virtual particles on a external leg and would...
  15. A

    I Is the mass of a photon stationary in space?

    I was reading a recent physics article on the google home page that stated that the current theory of mass is that it is a photon moving at light speed but stationary in space. My analogy of this was like a photon moving in a circle but it is not moving. I wish I could sight the article but...
  16. Green dwarf

    I Is a photon emitted in a particular direction?

    If a lone hydrogen atom in space emits a photon, is it emitted in a particular direction or in a superposition of all directions?
  17. Marilyn67

    I Construction of an Entangled Photon Source

    Hello, Happy New Year 2023 ! Interested in spontaneous parametric down-conversion (SPDC), and the possibility of creating entangled photon pairs, I studied several possible configurations with type 1 and type 2 nonlinear crystals. When using these crystals, the dominant phenomenon is second...
  18. LittleSchwinger

    I Single Photon States: Definition & Real-Life Applications

    A photon in QFT is defined in the same way as all particles. That is they denote a set of quantum states that transform in the simplest possible way under Poincaré transformations. Properly this is known as an irreducible representation (irrep) of the Poincaré group. You can classify these...
  19. F

    I Is photon a real object?

    Photon is real object, a gamma photon can create a pair electron and positron. But if photon is a reality object then why itself is not a inertial frame so in this frame its velocity is equal 0?
  20. James1238765

    I What is the probability of an electron emitting a photon?

    I have seen many tutorials that provide steps how to transcribe a Feynman diagram into algebra, for instance [here]: However, I have never seen the final line of the calculation converted into a real number. What are the steps to get from the algebra equations transcribed using the Feynman...
  21. Mr Fallspring

    I Double-Slit Experiment: Does rate of photon emission matter?

    Hi there! High school physics teacher hoping to pick the brains of people who know more than I do here. I'm curious whether the rate of photon emission has any noticeable effect on the diffraction pattern generated by the double-slit experiment. To be clear: I understand a diffraction pattern...
  22. Ahmed1029

    I How is photon momentum compatible with special relativity?

    In relativity, momentum of a body is given by ##p=mv/\sqrt{1-v^2/c^2}##, but if mass is exactly zero and velocity is exactly ##c##, how is the photon momentum even defined? I don't think this problem can be resolved by simply stating the other formula relating energy to momentum, since it was...
  23. T

    I The pattern of one photon at a time passing through large-distance setups?

    Let's say that we're sending a collimated beam of EM radiation through a slit that's 3 cm wide, and it diffracts and forms a clear interference pattern on a screen, with the bands of brightness and darkness, or of high and low intensities. And next, we reduce the intensity of the beam until it...
  24. S

    I Can there be some kind of photon emission caused by space expansion?

    Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?
  25. C

    B How is energy transferred from a wave to a photon?

    This is just a curiosity to me. My interest is from the position of a layman (as you will see from my description below). In the double slit experiment it shows a wave passing through both slits and interfering with itself to create an interference pattern. This is how I understand it... From...
  26. J

    Chemistry Electron Transitions after Absorption of Photon

    When a photon is absorbed, electrons move from a lower energy level to a higher energy level, so my answer is a, b, c, and f. I don't understand why the solution is b, c, d, e. Can electrons in this case move from a higher to lower energy level? Thank you.
  27. entropy1

    I Does the photon in a double slit experiment create many worlds?

    I can't find the anwer on this anywhere, so I hope I may ask it here. My question: In a standard double slit experiment, according to the Many Worlds Interpretation, does the photon create different worlds with different impact locations of the photon? So, without measuring which way...
  28. CPW

    B QM concept of photon.... still a bit of a mystery today?

    Hi PF. I desire deeper understanding of fundamental physics, and quantum mechanics can be a challenge for me (compared to other topics in physics). I read in the intro of the physics textbook chapter that QM explains many phenomena, e.g. why copper conducts electricity and glass does not. In...
  29. Daniel Hendriks

    I When and why does an electron emit a photon?

    Is this known in QFT or is this "spontaneous".
  30. A

    I Photon interference and beamforming

    In a double slit experiment with one photon sent at a time, the wave function Ψ of the photon reaching the screen is a superposition of Ψ1 (photon going through first slit) and Ψ2 (photon going through second slit). At the screen, Ψ1 and Ψ2 interfere (i.e. the photon interferes with itself)...
  31. P

    I Ultra-high energy photon interactions with matter

    While this was inspired by another thread, I think the question is different enough that it can be asked separately. It's also more suited to this forum than the forum where the question that inspired this one was asked. Wiki gives four possible interactions for interactions of gamma rays with...
  32. ThiagoMNobrega

    B Low magnification, high resolution, experiments on photon sieves?

    In the video above we can see the wave like patterns. The microscope on the video, used on the experiment, has a low quality and a low magnification. If anyone has a scientific article or a video source for such experiment with low magnification but with a high quality of definition it would be...
  33. C

    Photon Arrival Rate, Chapter 1.5 from Computer Graphics by Folly

    I've been reading Computer Graphics: Principles and Practice by Folly and I've encountered the following paragraph in chapter 1.5. I haven't understood how he got to the result in the following calculation, that if there exists a single 100W light bulb, emitting an order of ## 10^{12} ##...
  34. ThiagoMNobrega

    I Blender Particle Photon Simulation (interesting results w soft bodies)

    (0:00 / 0:42) photon going light-speed blender simulation I have no idea how a mathematician would translate this example into an equation. Every time I've worked with soft bodies I seem to run short of mathematicians buddies. Regardless of the mathematics of continuous object deformation, this...
  35. Vick

    I Formula for redshift of photon decoupling

    I have come across an old formula from my notes and I have no reference for it but it is using truncated digits in its formula to calculate the redshift for decoupling. The formula is nearly as accurate as the observed data from Planck 2018. So I would like to figure out the derivation of those...
  36. M

    B Effect of photon polarization

    How does the polarization of a photon impact the state change of an electron that absorbs it? Presumably the change of an electrons state (including spin) differs based on the polarization of the photon it absorbs.
  37. J

    B Photon Chirality: Left, Right & Beyond

    Are photons left- and right-handed? If so, does the handedness change when a photon is reflected? Is there any other way the handedness can change? -- Jeff, in Minneapolis
  38. F

    I Determination of entanglement by observing only one photon

    Hi everyone, background for my question is here: and https://www.researchgate.net/publication/45424433_Direct_generation_of_photon_triplets_using_cascaded_photon-pair_sources: My question is whether it's possible to determine if two photons are entangled without using a coincidence counter...
  39. K

    B Experience Travel on a Photon: Hitch-Hiker's Guide to the Universe

    We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are...
  40. 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...
  41. G

    I Path integral for photon

    I am aware that one usually starts from the Maxwell equations and then derives the masslessness of a photon. But can one do it the other way round? The action of photon would be ##S = \hbar \int \nu (1 - \dot{x}^2) \mbox{d}t##, where ##\nu## is the frequency acting as a Lagrange multiplier...
  42. S

    A Does the Z boson pole show up in the photon propagator?

    If I look at the photon propagator <A_mu (x) A^nu(0) > in momentum space, as I understand it I am to compute this by summing up all the self-energy diagrams of the photon, which look like: photon -> stuff -> photon In particular, since the photon shares the same quantum numbers as the Z, you...
  43. L

    I What Is the Diameter of a Photon?

    What diameter does a photon have? Does it vary and if so within what range?
  44. G

    I Photon-Photon Feynman Diagram

    If you have γ+γ→γ+γ what would the Feynman diagram look like (time-ordering implied). I think it will be a square with four photons on each vertex but is this all there is to it or am I missing something?
  45. F

    I Why we know average speed of single photon equal speed of EM wave?

    Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
  46. Sophrosyne

    B Polarization of a single photon

    I was just reading on this forum (and other sources) about the relationship between photon spin and the polarization of light. From what I have gathered, photon spin corresponds to circular polarization: +1 and -1 spins correspond to right and left helical polarizations. So I have a few...
  47. F

    I Is there a multicolor single photon?

    We know that we can not cut a single photon into many photons. So that there must be a single color for a single photon?Because if a single photon has many color then it will be dispersed through prism(so a single photon would be cut into many photons(?))
  48. D

    A Faraday effect breaks photon interaction laws

    I was taught that photons ( non-ionizing at least) never interact. So Its really bugging me that most info on faraday effect invokes B field as the cause of ( for example) rotation effects. Since EM-waves (IE Photons) themselves propagate a (oscillating) Magnetic field through infinite space...
  49. G

    Photon and Neutrino detector and photon trawl device

    Summary:: Hello I am a writer and presently working on treatment for a science-fiction story. So I am not a scientist, just a neophyte interested in science and wanting to write a fiction that would not be too far-fetched and that would make some sense for everyone, including the scientific...
  50. B

    Pressure of a Photon Gas

    This is from Problem 7.45 of Thermal Physics by Daniel Schroeder.