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. Spinnor

    I Photon Double Slit Experiment: Visualizing the Guiding Field

    Does the following picture which I think shows the guiding field for electron in the double slit experiment have a corresponding image when the experiment is done with photons? Thanks for any help.
  2. KeesDeVries

    I Photon Bell Experiment, what happens in a single case?

    I am wondering what happens in the case of 100 entangled photons with a polarization angle of 0 degrees (or 0 and 90) that interact with Alice at 22,5 degrees and Bob at 45 degrees in a Bell Experiment. Do I get a count of 85 for Alice and 50 for Bob, which means a maximum coincidence count...
  3. F

    I How can a photon "stimulate emission" if it really does (Einstein coefficients)

    Einstein coefficients tell us that there is some probability for an atom to go from E_1 to E_2 given by the coefficient of absorption. This is fine, but why is there only one coefficient (absorption) going from E_1 to E_2 and two for the transition E_2 to E_1? Spontaneous emission makes sense...
  4. D

    I Photon absorption for an atomic electron

    Quick question: let's say we have an atomic electron in the ground state which requires, say, one "unit" of energy* to jump up to the next orbital energy state. If a photon arrives with a bit more or less than this, say 1.00003 or 0.99997 units of energy, is there some finite, non-zero...
  5. K

    I Photon interference and path differences

    Most sources I come across on the topic of photon interference focus on the phase differences, but neglect discussion on the wave amplitude. Wave amplitude diminishes with distance; a single photon's energy does not. So in a double slit experiment with photons being emitted one at a time, if...
  6. Physics4Funn

    A Can Photon Have Orbital Angular Momentum?

    This is a very special case. In my 50 years studying physics I have never seen any discussion of photons having orbital angular momentum. Any angular momentum for photons in orbit around a black hole must be a GR question. I have not specialized in GR but I don’t recall any discussion of it. I...
  7. L

    I What is the behavior of photons inside glass?

    I'm aware of the numerous threads on the subject "light inside glass" there have been in the past, in case you will indicate me an appropriate one, but I haven't been able to find a clear answer to the questions: 1) Do photons really exist inside a (homogeneous, linear, transparent, refracting...
  8. A

    I Is the energy of a burst of light the sum of the energy of each photon?

    In A.P. French's Special relativity the author said, The mass and length of the box are irrelevant here. He said the momentum of the radiation is ##E_{radiation}/c##. We know that the momentum of a single photon with energy ##E_{photon}## is ##p_{photon}=E_{photon}/c##. So is...
  9. A

    B Unraveling the Tachyon and Photon Debate: Exploring Faster-Than-Light Particles

    A tachyon or tachyonic particle is a hypothetical particle that always travels faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. - Tachyon definition The photon is a type of elementary...
  10. olgerm

    I Photon occupation numbers in relation to the 4-potential

    I have some questions about this video. I have watched other videos in this series. Otherwise very nice series, but I think there may be mistake. Isn't the video flawed because it forgot forgot 0'th component of 4-fector ##A## aka ##\varphi## in 3-vector representation, I think it because...
  11. M

    A Does Dim Reg really avoid a photon mass?

    In the loop integral for the one-loop correction to the photon propagator in QED, the dominant term, after Wick rotation and angular averaging, has the form (omitting uninteresting factors) $$(1-2/d) e^2 \eta^{\mu\nu}\int_0^\infty \frac{p^{d+1}}{(p^2+\Delta)^2}dp,$$ where ##p## is the absolute...
  12. Saptarshi Sarkar

    I Speed of Light in Inertial Ref Frames: Photon Perspective

    According to the 2nd postulate of Special Relativity, speed of light in vacuum is the same in all inertial reference frames. If I take a beam of photons and see the other photons in the beam from a frame of reference of a single photon, do they look stationary or moving at the speed of light...
  13. J

    B Photon Energy & Wave Amplitude

    A photon's energy is E=hv where v, the frequency, is a wave property. Particles don't have frequencies. But a wave's energy also depends on its amplitude. Where does this come into the energy relation?
  14. Maslova

    B Position of a particle and photons

    Once I have read that we can’t know a actual position of a particle because to see the particle we need to send photons and when we send photons it colides with the particle and change it’s position. Is this true?
  15. F

    I What happens if a low energy photon collides with an atom in the ground state?

    What happen if a small energy photon collide an atom in ground state that the gap between energy levels of atom is greater than energy of photon?It seems that the medium absorbs light and transform to heat?
  16. F

    I QED & In-Ground Electrons: Why No Photon Emission?

    In ground state electron does not emit photon.Can we use QED to explain why in ground state electron does not emit any photons?
  17. lLehner95

    Find the photon energy in the center of mass frame and vice versa

    I tried to use the Lorentz transformation: ##E^{*}_{\gamma }=\gamma _{cm}E_{\gamma }-\beta _{cm}\gamma _{cm}p_{\gamma }## We have a photon, so it becomes: ##E^{*}_{\gamma }=\gamma _{cm}E_{\gamma }(\beta _{cm}-1)## Unfortunately, the solutions say that the correct way is to use the inverse...
  18. F

    Can a photon with energy greater than energy level difference of atom.

    Can a photon with energy greater than energy level difference of atom be absorbed by atom?Is there any case in that photon share a part of energy for atom and keep other part of energy for itself?
  19. Erik Ayer

    I Two Photon Sagnac Source

    In this paper, Cramer references a Sagnac source and mentions it has a half wave plate to vary the entangement vs. coherence by rotating the plate. Look just below figure 1: https://arxiv.org/pdf/1409.5098.pdf What is this setup? My guess is that the half wave plate is right before the...
  20. S

    I Mach-Zender-Interferometer with polarizers

    I have a question on how exactly polarizing filters would influence interference in a Mach-Zehnder interferometer. To explain, I'll show some configurations and what I would expect to happen - please tell me if I am incorrect anywhere. Here is the standard MZI configuration with no filters and...
  21. gmalcolm77

    MIT Material 10x Blacker Than Any Other: 99.995% Light Absorption

    Summary: MIT researchers have created a material that is 10x blacker than any other material ever recorded. The foil captures at least 99.995% of incoming light. Does this mean that there is practically no EM re-radiated?
  22. Erik Ayer

    I Does downconversion cause pump photon wavefunction collapse?

    I saw a paper on an experiment where a pump beam first went through a double-slit, then was downconverted with BBO. Recently a friend with a PhD in quantum physics said the downconversion will cause the pump's wavefunction to collapse and the implication for this experiment is that the pump...
  23. S

    I "Dumb" question : is there an upper bound on the energy of a photon?

    I was wondering if anybody knew if there was an upper bound on how much energy you can pack into a photon, if such a thing exists. I'm wanting to say no there isn't but it occurred to me that I did not know the answer. Sorry if this is an absurdly easy question but I don't remember reading...
  24. I

    I What happens to an electron's frequency during absorption of a photon?

    The point that I want to know is how two frequencys react with each other and does electron's frequency wait to reach to higher frequency on that fraction of second when they(photo's frequency and electron's frequency)collapse to each other than go to higher energy level or what?
  25. L

    A Timescale of virtual photon interaction in particle scattering

    I'm studying the electron-proton deep inelastic scattering. In the notes that I'm studying from the author states that the time-scale for a virtual photon to interact with a proton is ##τ\sim\frac{1}{Q}##, where ##Q## is the momentum transfer with ##Q\gg M##, which is the mass of the proton. I...
  26. M

    B Question about the rest frame of a photon

    [Mentors’ note: this thread was split from the FAQ at https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/] I am not an expert! As I understand it, Einstein considered, what a guy standing in a photon will experience if he look at his watch. My understanding says, that the...
  27. F

    Poisson distribution and Shot Noise

    My setup: I have the an LED (LED370E) in front of a photodiode (S12915-16R). The photodiode is connected to an ADC (DT5751) which has a counting functionality. The way it works is that it counts how many times the signal goes above a certain threshold and makes a histogram out of it. I know...
  28. F

    Solving the Debate Over Photon Counting With a Scintillation Device

    Hello, I'm working on a scintillation device to detect protons, I have a disagreement with one professor and I would like your opinion. There is one photodiode model we want to use to measure the light intensity from the scintillators, and we want to relate the signal of that photodiode with...
  29. W

    B Is information lost when a photon is absorbed?

    Suppose I fire a photon with an upward spin towards a hydrogen atom. It is absorbed by the hydrogen atom's electron which subsequently emits a photon when it returns to its ground state. I then fire another photon with a downward spin towards the same hydrogen atom and the same thing happens...
  30. bob012345

    I Force on Photon? Interaction with Matter Explained

    When photon interact with matter there is a force. Do we speak in terms of Newtonian action-reaction? When a photon exerts a force on matter -an action- do we speak of a force on the photon - a reaction? Thanks.
  31. P

    Photon Polarization: Questions & Answers

    This text https://en.wikipedia.org/wiki/Photon_polarization starts with: "Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. An individual photon can be described as having right or left circular polarization, or a...
  32. zeronem

    I Photon states and interference in an Interferometer

    Hello. I have a question. In the book I am reading, They derive the Ubs operator applied on a photon state with the beam splitter at a ratio of 50/50. A beam splitter that is used in the Mach-Zehnder interferometer. I'm having a hard time deciphering whether the formula for beam splitter...
  33. Jamister

    I How can the photon wave function be described?

    Fermions such as the electron and proton can be described by wave function in momentum and in position, and it is possible to get the momentum wavefunction from space wave function and vice versa by Fourier Transform. what about photons? can photons be described by position wave function? If...
  34. M

    Calculate the energy release of an atom

    First of, I have no idea what I'm supposed to do with the neutrinos and the photons. Can somebody explain how to handle these? The rest of what I tried is quite straight forward $$\begin{align*}\Delta E &= 4M_p - M_{He} - 2 M_e + E_{\text{Neutrino and Photons}}\\&= 4M_p - (2[M_p+M_n]-E_B) - 2...
  35. Manasan3010

    Reflection of a photon by an atom

    I suppose the reason I am able to see myself in mirror and not in wood is the reflection off a mirror is specular whereas wood is diffuse. In reflective material(e.g: silver) when the photon hits the frontline atom, the atom's electrons absorbs the energy and release it back with tiny energy...
  36. D

    I What is the range of natural photon frequencies?

    Given that electromagnetic waves transport energy via photons, what is the lowest and highest possible frequency of a photon and what physical phenomena do these relate to? It is clear that the larger the wavelength the lower the energy for a photon. so what are the limits of the wavelength?
  37. Haorong Wu

    I Questions about an optical photon quantum computer

    Hi. I'm learning the optical photon quantum computer from Nielsen's QCQI. Since I'm not familiar with quantum optics, I got some questions about it.Q1. In page 288, the book reads: A laser outputs a state known as a coherent state ##\left | \alpha \right > = e^{- \left | \alpha \right | ^2 /2 }...
  38. e2m2a

    Center of mass momentum of a photon and electron system

    Suppose there is a photon with momtum p=h/lambda moving in the positive x-direction. Suppose it collides with an electron at rest and is completely absorbed by the electron, and that after the collision, the electron moves to the right with the same momentum of the photon. This seems...
  39. D

    I Photon Absorption & Electrons: Mass, Energy & Releasing Photons

    I'm not that familiar with the current theoretical standing on how electrons "absorb" photons, as in the sense that electrons in an atom absorb photons and move from lower to higher energy states. But during the absorption of a photon the electron, if you set units of c=1, gains energy and...
  40. merlyn

    The phase of electric and magnetic forces in a photon.

    I'm curious if someone help me understand why the electric and magnetic forces are IN PHASE at right angles to one another? Should they not be 90 degrees out of phase in order to conserve energy? I do understand they are in phase but why? Thank you all for your time. Merlyn.
  41. D

    How does a massless photon impart force?

    I've been watching a lot of physics videos lately and a couple of said that photons are massless. What I don't understand is how a massless photon can impart force? Like the ideas of having a laser propel a deep space probe.
  42. B

    I What is the experience of a photon?

    I’m watching TV. An excited electron in an LED in the screen falls back to its normal energy state, releasing a quantum of electromagnetic energy in the form of a photon. Let’s call this event ‘A’ (x1, y1, z1, t1). The wave packet of this photon fills the universe (quantum mechanics). Across the...
  43. B

    Single photon wave packet autointerference

    I've been troubled by this problem for some time now and have received several answers to it none of which I find compelling, so I am posing it again in hopes of getting something more convincing. Here's the problem. Consider one had a large optical interferometer with two siderostats place...
  44. N

    B Planck Length: Upper Limit on Photon Frequency & Momentum

    Is it known how much momentum a photon possesses if it's wavelength is at the Planck length, and what happens if it's momentum is somehow increased from that?
  45. O

    B Photon exchange in molecular bonds

    So a photon is absorbed into and emitted from electrons, causing the electrons to jump energy levels around an atomic nucleus. And enough energy absorbed into the electron will cause the electron to break from the atom altogether. My question is, where does this energy that enters the...
  46. andrewkirk

    Insights Equations of State for Photon Gas and Relativistic Electron Gas

    Continue reading...
  47. F

    B Does a photon contain a pair of spinning charges?

    A photon with enough energy can create a pair of electron and positron and we call it pair production. And a position and electron meet can create a photon , we call it annihilation of electron and positron. And we know a photon has a spin of 1, when pair production happen , require positron and...
  48. T

    A Why does the effectiveness of low-Z shielding increase with photon energy?

    Concrete for example - you need less concrete to obtain the same lead equivalent for photon energy 500 keV than for 200 keV. What is the reason for this?
  49. R

    Understanding Photon Energy: Help Needed for Chemistry Student

    So I'm a chemistry student here in the UK - and I'm feeling a bit like a fish out of water on a physics forum but... I'm having trouble with a derivation that seems simple but i can't for some reason seem to understand. I know that the energy of a photon is given by E=hc and that c=lambda*v...
  50. C

    I Is a photon simply a vibration of the spacetime lattice?

    Is a photon simply a propagating vibration of the spacetime lattice similar to gravitational waves but at a different wavelength and amplitude, and the electron that creates it plucks a single lattice string rather than a bunch? Therefore it has no mass and travels differently through spacetime...
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