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.

View More On Wikipedia.org
Recent contents Top users
  1. R

    Energy of the photon emitted in a gamma ray decay

    Homework Statement An Fe nucleus (A=57) decays from an excited stated to the ground state by emitting a gamma ray. The energy of the photon is 14.4 KeV when the nucleus is held fixed. If the nucleus is free to recoil then the energy of the photon emitted will be? Homework Equations ## E =...
  2. Albi Mema

    Consider Compton scattering of a photon by a moving electron

    Homework Statement Consider Compton scattering of a photon by a moving electron. Before the collision the photon has wavelength λ and is moving in the +x-direction, and the electron is moving in the −x-direction with total energy E (including its rest energy mc2). The photon and the electron...
  3. C

    I Ward identity for off shell photon?

    Consider an amplitude for some subprocess involving an off shell external state photon with polarisation ##\epsilon_{\mu}## and momentum ##q_{\mu}##, stripped of the polarisation vectors so that e.g ##T = \epsilon_{\mu} \epsilon_{\nu}^* T^{\mu \nu}## (##\epsilon_{\nu}^*## is polarisation vector...
  4. Daniel Petka

    Exploring Gamma Photon Ionization: Effects on Wavelength and Ion Creation

    Gamma rays scatter and ionize atoms, which stretches out their wavelength, right? How many ions could a single gamma ray photon create before it's absorbed due to the photoelectric effect?
  5. F

    I Entanglement and photon polarization

    Hello, A photon can have various types of polarization states (horizontal, vertical, circular, elliptical, linear at an angle ##\theta##). Any valid polarization basis is two-dimensional and can represent any state of polarization. What are the actual eigenvectors of the polarization...
  6. Daniel Petka

    B What is the best way to imagine a photon?

    What is the best way to imagine a photon? Sometimes I hear that they are simply transverse waves with oscillating EM fields, which just move forward. Other times, though, the photon is said to move along the wave. The third image reminds me of a probability wave. So which one is correct...
  7. Ivanov

    I Exploring Light Absorption: What Happens When a Photon Hits an Electron?

    Hello, I have a presentation tomorrow and in a segment, I talk about light absorption. It's more conceptual than technical. I did quite a bit of research on the topic but because of simplifying information I may have butchered the facts and written something wrong. Could anyone please confirm/...
  8. L

    Find the energy of a photon after this annihilation process

    Homework Statement [/B] The problem is as follows: in a reference frame there is one electron at rest and one incoming positron which annihilates with the electron. The positron energy is E and two gamma rays are produced. Find first the energy of the photons in the center of mass frame as...
  9. Spinnor

    B Photon, string theory string, E&M field

    Is it possible to associate electric and magnetic fields with the vibrating string that is thought to represent a photon in string theory? Thanks!
  10. S

    I Photon number state and Doppler shift

    If a mode of light is in the single photon state in the reference frame of the emitter, what will the state look like in a reference frame where the wavelength is, say, 5% less or more? How about a state with, say, 5 photons? I saw some online discussions and some papers on arXiv (not...
  11. S

    I On-Demand Photon Pairs: Entanglement or Necessity?

    When I search for "on demand photon pairs" I find that most publications are about entangled pairs. I found no papers and articles about on-demand pairs where the photons are identical in all respects except their direction, and whose polarization & frequency are the same across pairs and...
  12. A

    Is the Photon Riding on an Electromagnetic Wave?

    Hi there, I was wondering what the relationship between the photon and the electromagnetic wave is? Is the photon traveling on that wave like a surfer is riding a wave? :-) Thanks in advance!
  13. L

    Finding momentum of a photon/ Finding the mass of a object

    Homework Statement So me and my friend were comparing our homework and we noticed that although we had the same answers, we both had different ways of answer them. So I was just wondering who's work, my friend or mine, is the most correct?Here's the questions. 1. Calculate the momentum of a...
  14. L

    Compare and contrast a 2.2 eV photon with a 2.2 eV electron

    Homework Statement Hey everyone, So my teacher assigned a mini project just before our finals and I was wondering if someone could look over my work and my calculations. I just want to make sure I understood everything since my final exams are coming. Here's the assignment. Compare and...
  15. Conductivity

    Why there isn't a scattered photon in the Pe effect?

    We have studied the weird enough not continuous energy aka photons, And that they are like the way that waves interact with matter by giving them bursts of energy and that these photons have relativistic mass ( Before you give me the FAQ link, I have read it.). Mass is a property of energy. So...
  16. dylanreynolds1

    I Thought Experiment Contradiction: Find the Answer

    Hello all, I have a question that's been bothering me the last few days and wasn't sure where to turn. Recall the original Special Relativity thought experiment: A spaceship travels at constant velocity v, moving in the positive x direction. An observer on the spaceship emits a photon directly...
  17. S

    I Could it be that the photon has a non-zero mass?

    Hi, Can someone tell me the reason why it is impossible for the photon to have a rest-mass? Without referring to the theories which are actually build on the fact that the photon is massless. Is there any possibility that the speed of light is not about light and that there is some...
  18. C

    Momentum in a photon -> cathode collision

    Homework Statement A photon with the length of lambda hits a cathode perpendicularly to its surface. As a result, an electron leaves its surface perpendicularly to the direction of the photon. How much momentum was transferred from the photon to the cathode? The work function of the cathode is...
  19. CassiopeiaA

    I Path of Photon in Free Falling Lift: General Theory of Relativity Q&A

    I just started reading general theory of relativity. I have some elementary questions. Not an english speaker so bear with me. I am reading the thought experiment which describes path of a photon in a free falling lift. For an observer inside the lift, the photon path is a straight line. But...
  20. Simon Peach

    B How can gravity act on a photon?

    Black hole or more correctly the event horizon will not let anything past through it, from inside to outside, ok. But then we get to a particle with zero mass eg. photon. How can gravity act on that particle, surely for gravity to act there must be mass? Or does it act on the electromagnetic...
  21. B

    Relativistic Momentum of photon

    Homework Statement How much work is required to accelerate a proton from rest up to a speed of 0.999c? What would be the momentum of this proton? Homework Equations p=γmv The Attempt at a Solution I got part A, which was the momentum. I found that to be 20.1 GeV. Now for part B I have to find...
  22. LarryS

    I Why no position operator for photon?

    Apparently, in QM, the photon does not have a position operator. Why is this so? As usual, thanks in advance.
  23. donavi

    I The spin of the photon -- a weird experiment with a laser

    Hello everybody, I'm playing with a ne -ne laser And i conducted an interesting experiment I did colliding beams the type of interferometer Fabry Pero between the mirrors I put the cuvette with different substances I tried to detect changes of polarization of the laser beam with the help of a...
  24. LarryS

    I When can the Klein-Gordon Equation be used for a photon?

    Consider the double-slit experiment done with photons from a laser. If one was interested only in computing position (vertical) probability amplitudes and did not care about spin/helicity, could the Klein-Gordon Equation (with mass set to zero) be used? Thanks in advance.
  25. Talisman

    I What is Photon Phase? Explaining Confusions

    It occurs to me that I've probably been confusing "phase" in a number of contexts. In particular: 1) If we write a photon's polarization as \psi = |x\rangle + e^{i\theta}|y\rangle, then we can call \theta a "phase." 2) When a photon bounces off a mirror, it picks up a relative phase of i. If I...
  26. I

    I Can a Z Boson Decay to a Photon Affect Its Weak Charge?

    If a Z can decay to a photon, what happens to the Z's weak charge?
  27. D

    Can a photon's energy be so high that it creates a black hole?

    Is it possible for a photon to be so energetic that it forms a black hole? If so, how fast would that black hole be moving?
  28. EnumaElish

    Can a space vessel generate its own photon wind?

    In a nutshell, does Newton's "action = -reaction" law apply to massless particles? If a spaceship directs a condensed light beam on its own heat-resistant photon sail, what would happen?
  29. Grinkle

    I Question about expansion and photon fatigue

    @phinds noted in a recent thread that energy is not conserved on a cosmological scale, which hadn't occurred to me before, so I did some reading and happily the concepts were easy to digest and things I already understood but just hadn't connected the dots myself on regarding the implications...
  30. Docdan6

    Why an electron at rest cannot emit a photon?

    Hi! Could someone explain to me why an electron at rest without any influence from a magnetic or electric field cannot emit a photon ? Could you explain it mathematically too ? Thanks in advance...
  31. D

    Stargazing А neutron star and a photon sphere

    Wikipedia(https://en.m.wikipedia.org/wiki/Photon_sphere): 'This equation entails that photon spheres can only exist in the space surrounding an extremely compact object (a black hole or possibly a neutron star)' But how can a neutron star? I have a doubt because 1. When а mass of а star...
  32. S

    I Photon Propagator: Exploring Mass in Feynman Diagrams

    Hello! I have a question about the photon propagator in Feynman diagrams. I am looking over a brief derivation (probably there are some details missing), so basically it starts from Proca equation for a mass 0 particle, then it assumes Lorentz condition and in the end it obtains...
  33. S

    I Photon Polarization: What are indices i and j?

    Hello! I am a bit confused about the completeness relation for photon polarization. So it says ## \sum_{s=1,2}\epsilon_i^s\epsilon_j^{s*}=\delta_{ij}-p_ip_j##, with ##p_i, p_j## - unit vectors. So I am not sure what are the indices i and j. I though that they represent the component of the...
  34. SunRay-dvsh

    A Why is the Matter wave analogous to photon and derived from it?

    The electromagnetic wave derivation uses the fact that charge enclosed is zero and it goes to obey plane wave equations. Lets say we were deriving a wave equation from maxwell's equations for electron wave motion, but we assume that charge enclosed is not zero, and come up with some...
  35. shihab-kol

    B What is the relationship between time, speed, and mass of a photon?

    Someone asked me the other day that a photon is traveling at c and he is also traveling at c (suppose) then to him the photon is at rest and so it must have a mass I could not answer him and so I need some help
  36. A

    B Need Listing of Photon and Particle Quantum States

    Hi, I am learning quantum entanglement. I am interested to create an up to date list of all known : - Photon Quantum States - Particle Quantum States - Classically entagled photon states I guess that there is an organization out there that already have this info. If someone can point me into...
  37. Leyzorek

    B Can a photon be made energetic enough to escape a black hole

    why can't a photon escape a black hole? i think it is because the photon is red shifted away to nothing, if this is true, it would be possible to create a photon that would be energetic enough that a black hole would not have enough time to red shift it away to nothing, unless there is some...
  38. C

    B Has anyone tested a polarization filter sending through photon by photon?

    To properly test a filter, I understand we can use a source of vertically polarized light. We send photon by photon say 1000 though a filter at, for example, 45 degrees the count the ones on the other side. We should get roughly 500, depending of the quality of the setup. I was interested in a...
  39. FallenApple

    A photon vs an electron. Wave or particle?

    So we know for a fact that an electron is a particle. The "wave" like properties are not waves at all, its just the wavefunction that is a mathematical wave which is used for getting probabilities for where the electron will end up. But what about a photon? When a charge oscillates, its gives...
  40. kiwaho

    A Photon number needless conservation, consolidation possible?

    We know lepton conservation law, that means multiple neutrinos can not be consolidate to big single neutrino. But photon is boson not lepton, no need of conservation, does that mean it is possible to combine or fuse a bunch of photons into ONE big photon, or say, more energetic photon, i.e...
  41. O

    B Untangled Photon - Commonplace or Fictional?

    Everyone speaks of entangled photons and entangled states as if entangled photons were rare in nature, products only of our investigations into the quantum weirdness. With the intent of confirming whether or not this is true according to modern physics, can someone explain how truly "untangled"...
  42. Mridul Tripathi

    I Can Photons Be Accelerated by Altering Their Path?

    I am a student . My question is that in what way we can accelerate a photon , is this possible ? I am a student . My question is that in what way we can accelerate a photon , is this possible ? what if i pass the light to a medium of varying refractive index,containing lots of biprism at...
  43. J

    I Is the Massless Photon Explanation for E=mc2 Valid?

    I have wondered how a photon could be massless, given E=mc2. I have seen explanations involving treatment of the situation through consideration of momentum. It seems to me my own explanation is valid, and I ask for comments. Actually E= m’c2 =gmc2, where g=1/sq rt{(1/[1-(v2/c2)]}. Photons...
  44. C

    I Accelerating Internal OAM Photon Wavefronts Under Gravity

    It is known that wavefronts of internal OAM photons travel slower than light but I wonder what happens if you accelerate such a beam. This should be possible under gravity.
  45. Bolhuso

    I Use of polarization to experimentally check photon mass?

    Hi all! I've checked Wikipedia and a related thread regarding experimentally measuring an upper limit for the photon rest mass: https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass https://www.physicsforums.com/threads/photon-rest-mass-0-wasnt-proven-experimentally.792583/ I...
  46. Albert Thomas

    Is the Photon Truly Massless? Examining Einstein's Law of Relative Mass

    We know that the photons are massless particles. The traveling speed of the photon is equal to the speed of the light(C). The Einstein's law of relative mass is says when a particle travels at the speed of light, then it's mass becomes infinity (i.e. highly increases). The photons are also...
  47. texasnano

    A Does increasing photon flux density increase attenuation?

    follow up from responses in old thread. https://www.physicsforums.com/threads/why-does-attenuation-coefficient-decrease-with-increased-energy.569981/ I have seen in my experiment using a saline media ( some oxygen bubbles) that an increase in Incident PFD is showing an slight increase in the...
  48. learis

    B Trajectory of photon or electron in double slit

    Hello, first post. I recently became interested in quantum physics and its mysteries. To my understanding, In the double slit experiment, the photon or electron will scatter and behave like a wave when both slits are open and neither slit is measured. Over time their cumulative scattering mimics...
  49. akvadrako

    I Single photon entanglement

    I've recently been reading about entanglement between two spatial modes of a single photon. It's a little over my head and there is one aspect about it that I'm particularly unclear on, which I was hoping someone here might be willing to shed some light on it. The basic setup is described with...
  50. D

    I I can determine the photon path in delayed erasure

    Dear, I am going into quantum physics the past days specifically the dual slit because it boggles my mind. You can find the Wikipedia here https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser And this is the picture...
Back
Top