What is Photon: Definition and 1000 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. 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...
  2. M

    B Exploring the Impact of Photon Polarization on Electron State Changes

    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.
  3. 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
  4. 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...
  5. 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...
  6. 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...
  7. G

    I Can a Path Integral Formulation for Photons Start from a Massless Premise?

    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...
  8. 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...
  9. L

    I What Is the Diameter of a Photon?

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

    I Exploring the Feynman Diagram of γ+γ→γ+γ: What Does It Reveal?

    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?
  11. 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?
  12. 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...
  13. 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(?))
  14. 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...
  15. 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...
  16. B

    The Pressure-Entropy Relationship for a Photon Gas

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

    B Photon Mass: Does Light Have Direction?

    Could someone ell me if photon has mass? How can light have its direction changed by massive bodies?
  18. J

    I Does a photon travel freely through the CMB photon gas?

    A photon often travels billions of years (Gyr) through the CMB photon gas (410 photons per cubic centimeter) to reach us. Does it travel freely? Let’s share our thoughts about this. For discussion purpose, let’s assume the photon has a wavelength of 500 nm, close to the peak of the solar...
  19. I

    B The superposition of a photon

    When a photon travels through space it is spreading out like a fan while in its superposition (except that it is spreading also vertically in addition to horizontally). So, what happens if for instance the right outer edge of the photon's superposition is captured by the gravity of a black hole...
  20. B

    A Single photon avalanche detector (SPAD) difficulties with photometer

    I have been developing a SPAD based photometer having a 10 ns deadtime, near-zero dark counts, near-zero afterpulsing, and that is temperature independent (within a reasonable range). It works well with photon level pulsed optical signals, (10ns to several microsecond pulse widths at rates up...
  21. meher4real

    Solving Photon Energy with Equations: Am I Right?

    Hi Guys ! Previous question was to determine the energy of photon with wavelength = 820nm I found that E = 2.42x10^-19 J = 1.51 ev using the equation E=hc/wavelength - I tried to solve the above problem with various equations but all failed. The closest equation i tried is...
  22. D

    A Time evolution of one photon

    During time evolution of one photon with vacuum state with hamiltonian as a^†b+b^†a, the answer is cos(t/ℏ)|0,1⟩+isin(t/ℏ)|1,0⟩. But i don't know how to do calculation to get this answer. Can someone please help me? I tried to do this calculation: |0⟩|1⟩(t)=e−iHtℏ|0⟩|1⟩...
  23. Ebi Rogha

    I What does it really mean that photons are quanta of light?

    But then I learned a photon can be split into two or even three photons (red-shifted, energy is conserved), and also photon can lose energy and still be a photon (Raman effect, inelastic scattering). Now, I am not sure what it means when it is said photons are quanta of light (smallest unit of...
  24. BvU

    I How big is a photon - Interesting video

    Bumped into this video after admiring the falling cat by @A.T. As an experimental physicist, indeed experienced a little of the 'hole in the brain' phenomenon!
  25. Ebi Rogha

    I Measuring Photon Intrinsic Nature: Is Interference a Factor?

    I would like to know, how can we be sure this is not due to the influence/impact/interference of our measurement, not necessarily the intrinsic nature of photons? In most reference books, it seems it is a given and it is not discussed
  26. Ebi Rogha

    I Can a single photon be reflected by a mirror?

    If the answer is Yes, then I would ask, if the mirror absorbs a portion of the energy of the photon, so the photon should simply stop existing because we cannot have a smaller package of light than a photon. If the answer is No, then I would ask why a beam of light (which is made of a big...
  27. F

    De-excitation of a moving atom with photon emission

    The information I have are the following: ##p^\mu=(E, p, 0, 0)## ##p'^\mu=(E', p'\cos\beta, -p'\sin\beta,0)## ##k^\mu=\tilde{E}(1, \cos\alpha, \sin\alpha, 0)## Where: ##E=\sqrt{M^2+p^2}## ##E'=\sqrt{m^2+p'^2}## Using the conservation of the four-momentum ##p^\mu=p'^\mu+k^\mu##...
  28. deadbeef

    I Interference Patterns & Photon Paths: Exploring the Double Slit Experiment

    Hi. As i understand in double slit experiment photon can arrive at slits through different paths (longer and shorter) and it would still show interference pattern. My question is - if there was some path cutting device in a way of photon (on longest path after length that photon arrive at...
  29. T

    A Photon self-energy in the finite temperature field theory (real -time)

  30. docnet

    I Can an electron in a magnetic field radiate a virtual photon?

    In Theoretical Minimum: Quantum mechanics, Leonard Susskind describes an electron in the higher energy spin state in a magnetic field radiates a photon of energy ##\hbarγ|B_0|## and flips into the lower energy spin state. I am wondering if this photon is related to the "virutal photon" that...
  31. J

    I What is the size and shape of single optical photon?

    Optical photon is produced e.g. during deexcitation of atom, carrying energy, momentum and angular momentum difference. So how is this energy distributed in space - what is the shape and size of single photon? Looking for literature, I have found started by Geoffrey Hunter, here is one of...
  32. J

    How to calculate the four-momentum of a photon in FRW Metric

    I have calculated the Christoffel symbols for the above given metric, but I don't understand how to calculate a photon's four-momentum using this information. I believe it has something to do with the null geodesic equation but I can't understand how to put that information into the problem...
  33. M

    B Are electronic transitions a result of a single photon?

    E=hf
  34. J

    I How does a particle's acceleration relate to the emitted photon?

    I understand that any accelerating charged particle will emit a photon. But I do not understand how the rate of acceleration relates to the photon that is emitted. For example: If a proton is falling to Earth in a vacuum at 9.8 m/s^2, then what are the characteristics of the photon that will be...
  35. J

    I How Does a CMB Photon Travel Through Expanding Universe?

    Suppose we receive a cosmic microwave background (CMB) photon from space. According to the Big Bang model, this photon became free to travel when the universe was about 0.38 million years (Myr) old. At that time, it was about 42 million light years (Mly) away from our location. Because of the...
  36. Marshall2389

    B How does a photon not "feel" electromagnetism?

    I've attached a picture of a table in Sean Carroll's The Particle at the End of the Universe. It says that photons don't "feel" electromagnetism, but gluons feel the strong force, the W and Z bosons feel the weak force, and gravitons feel gravitation. How is this so? (I have no formal quantum...
  37. J

    B Photon absorption -- What happens to the excess energy?

    Wikipedia: "When a photon has about the right amount of energy to change the energy state of a system (usually an electron changing orbitals), the photon is absorbed." What happens if a somewhat higher energy photon arrives?
  38. R

    I Photon Emitted without Changing Energy Levels

    In Example 41.5, they are implying that, for a hydrogen atom, if the orbital quantum number ##l## goes down the electron will lose energy. However, they said nothing about the principal quantum number ##n## going down, so there should be no loss in energy. As far as this book has presented, the...
  39. J

    B Is there a magnetic interaction between light and charged particles?

    Light is propagating electric and magnetic fields. The electric field interacts with electrically charged particles, e.g. electrons. Is there a corresponding magnetic interaction?
  40. S

    B Why Photon & Gluon are Exceptions from Higgs Field

    The photon and the gluon in the Standard Model do not interact with the Higgs field and are hence massless and travel at the speed of light. Is there a simple explanation why these two elementary particles are the exceptions?
  41. Athenian

    Young's Photon Interference - Question on Angle & Voltage

    After conducting the photon interference experiment, below is a sample data of what we got: Time (s) Angle (V) Two-slit Diode (V) 0 0.988 0.203 0.102 0.984 0.297 0.805 0.976 0.398 1.201 0.974 0.5014 1.31 0.968 0.526 The above list goes on for quite a few columns...
  42. F

    I The Speed of Light: Comparing Photon and EM Wave Velocities

    Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
  43. JD_PM

    I Green's function for massive photon theory

    I am studying the 'toy' Lagrangian (Quantum Field Theory In a Nutshell by A.Zee). $$\mathcal{L} = - \frac{1}{4} F_{\mu \nu}F^{\mu \nu} + \frac{m^2}{2}A_{\mu}A^{\mu}$$ Which assumes a massive photon (which is of course not what it is experimentally observed; photons are massless). The...
  44. S

    I To measure a Planck length would require a black-hole photon?

    I was viewing this video in which the narrator says that the energy of a photon that could discern a Planck length would require a photon of such high energy that it would be a de factor black hole. Is this accurate?
  45. R

    I How does this experimental result show photon emission?

    First I'll explain my understanding, because I'm not very confident in it. The main point is that the electrons are ejected and then accelerated to a very high kinetic energy. Then they start smashing into the anode. Most will go through a series of collisions before completely stopping, so that...
  46. J

    B Understanding Single Photon Detection in the Double-Slit Experiment

    When double-slit experimenters say an interference pattern is obtained even when only one photon at a time is fired at the slits, how do they know it was only one? The same when a photon detector is said to respond to single photons.
  47. F

    Annihilation: calculation of photon energies

    I set up this problem this way: ##p_a^{\mu}=(E, \sqrt{E^2-m^2}, 0, 0)## ##p_b^{\mu}=(m, 0, 0, 0)## ##p_c^{\mu}=(2E_\gamma, 2E_\gamma, 0, 0)## I have chosen to consider the two photons as a single particle of energy equal to ##2E_\gamma##. At this point I applied conservation of the...
  48. M

    B Redshifted Photon Emission vs Transport: Magnitude of Gravitational Redshift

    I am considering the magnitude of the gravitational redshift and I look at the process of a photon leaving an atom from the Sun. I am asking whether the processes in the atom, viewed as a clock, would lead us to conclude that the emitted photon, at the time of emission, would itself be...
  49. K

    I How do two photons affect Rabi oscillations in a 2-level atomic system?

    Hello! Assuming we have a 2 level system (e.g. an atom with 2 energy levels) and the lifetime of the upper level can be neglected, if we make the atom interact with a laser at a fixed frequency, we would get Rabi oscillations (assume the laser is on resonance). Would we still get Rabi...
  50. J

    B Photon detection methods -- do they always involve displacing an electron?

    Am I right in thinking that all photon detection methods depend on a photon displacing an electron, that then displaces other electrons to give a detectable electric current pulse?
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