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. F

    Minimum energy of a photon to produce ##\pi^+##

    I have a doubt about the first request: I suppose to find the minimum energy of ##\gamma## in the situation where ##p## is stationary, there is no reason to say that the proton is stationary if I were to calculate it in the CM, right?. So I have to consider che LAB-frame to find ##E_\gamma##...
  2. F

    Electron-positron annihilation, photon emission angle

    I consider the laboratory system. The four momentums in this reference system are respectively: ##p^\mu = \big(\sqrt{|p|^2+m^2}, 0, 0, |p| \big)## ##p'^\mu= \big(m, 0, 0, 0 \big)## ##k^\mu = E\big(1, 0, 1, 0\big)## ##k'^\mu = E'\big(1, 0, -\sin \varphi, \cos \varphi \big)## I used conservation...
  3. D

    A Photon Statistics in QFT & String Theory

    In a given mode with an average number of photons ``##\bar{n}##, the photons are distributed around their average according to the formula $$p_n = e^{-\bar{n}} \frac{\bar{n}^n}{n!}$$ The justification of this formula in quantum field theory involves considering field operators acting on a...
  4. .Scott

    I The Sub Photon Sphere Escape Game

    The Sub Photon Sphere Escape (SPSE) Game Game Board: Vast Empty Space Game Pieces: ##\space## 1) A large perfect Schwarzschild black hole ##\space## 2) A Carrier/Trigger. This is a massless device that sets the Player Device into a selected position and velocity and then triggers it...
  5. .Scott

    I Investigating The Photon Sphere

    I've spent well over two hours searching the web for two functions of the radius of a Schwarzschild BH. One would give me the escape velocity of the BH assuming a perfectly vertical trajectory (so it isn't a normal escape velocity). The second relates to the trajectory of a photon that is...
  6. Martian2020

    B Does the wave function change from the viewpoint of the photon?

    The question arose when watching Sean Carroll video: The Biggest Ideas in the Universe _ Q&A 6 - Spacetime 3:50 - 13:30 Because photons follow null geodesic in spacetime the question arose from viewers: "photons do they really experience no time this is a question" And in the answer: "but if...
  7. M

    I Photon behaviour across EH threshold

    Physics is not my area of expertise. That being said, philosophy of science is, but I'm not here to discuss philosophy. I recently found myself trying to imagine how light behaves once it crosses the event horizon of a black hole. Presumably, between the event horizon and the singularity...
  8. P

    Energy of a photon scattered due to the Compton Effect

    I don't really know how to proceed. Can someone help me out in this?
  9. K

    B What does the energy of a photon convert to with interference?

    I was just reading about the LIGO experiment wherein an interferometer was used to detect gravity waves. This interferometer uses opposed light waves, detecting if there is a shift in their wavelength due to stretching/squeezing of a gravity wave passing through the lasers. (I hope I'm saying...
  10. entropy1

    B Is a photon an excitation of the electromagnetic field?

    Allow me to hijack this thread for a second: a photon is an excitation of the electromagnetic field, right? The photon does not exist until measured. So how can we send a photon in a particular direction, so it has a known position and momentum?
  11. H

    I Does photon self energy contribute to the speed of light?

    Hi Pf I read that in the light propagator there are loops of electrons. What would be the consequences if we could switch them off (or neglect them)? would it modify the speed of the photons?
  12. H

    B Gravitational signature of a photon in a double slit experiment

    I'm trying to think of a how the double slit experiment can detect a photon without interacting with it in theory. In principal (not reality of course) does a photon have a gravitational signature which could be used to detect which slit it traveled through during the double slit experiment...
  13. PainterGuy

    B Planck's equation and upper and lower bounds on the energy of a photon

    Hi, Planck's equation is written as E=hν where "E" is energy of a photon, "h" is Planck's constant having value 6.626 070 15 x 10-34 Js, and "ν", Greek letter nu, is frequency. Violet color has frequency range between 790–666 THz (Tera =10^12). If a violet photon of frequency 7.5 x 10^14 Hz...
  14. P

    I Article on photon wave-particle duality

    We all know that Feynman declared wave-particle duality as the central/only problem of quantum physics. Not sure how to evaluate a recent publication summary on this topic: https://sciencex.com/news/2020-11-wave-particle-duality-entanglement-customary-pitfalls.html Would like others take on...
  15. virgil1612

    B Relativistic Photon Mass: Can It Exist?

    Photons have 0 rest mass. But could I talk about relativistic, or dynamic photon mass, that would be the solution of hf = mc^2 ? The relativistic mass would be m = m0/sqrt(1-v^2/c^2), where m0 is the rest mass, so 0, and v = c, so the denominator is also 0. The previous equations would give 0/0...
  16. Kostik

    A Upper bound for wavelength of a photon inside an infinite square well

    Obviously a particle inside an ISW of width L cannot have arbitrarily precise momentum because ΔP ≥ ℏ/2ΔX ≥ ℏ/2L. Therefore you cannot have a particle with arbitrarily low momentum, since that would require ΔP be arbitrarily small. I need to show that a photon inside an ISW cannot have...
  17. M

    Photon emission in electronic transitions

    How would you explain, on a basic level, why only one photon (as opposed to two, three...) is emitted when an electron in an atom changes its energy level? This is for students with only introductory Physics background.
  18. Buzz Bloom

    I Q re a photon ionizing a hydrogen atom

    This question is a followup to another thread. https://www.physicsforums.com/threads/qs-re-the-behavior-of-atoms-after-decoupling-completed.994581/ I would like to explore the issue raised by @kimbyd. . . . after reionization the temperature of the intergalactic medium is dominated by...
  19. DarkMattrHole

    B Regarding the coherence of a photon pair decaying from Higgs

    Hi all. I have been reading the following article and have a couple of basic questions about the decay of the Higgs into photons process - https://www.physicsforums.com/insights/lhc-part-4-searching-new-particles-decays/ As i understand this decay - the photons will have the same frequency and...
  20. QuasarBoy543298

    A Why does a photon have a defined frequency but is also somewhat localized?

    a photon have defined frequency , but we treat it as a localized particle ,how that can be? if i am looking at second quantisation, photons are modes of the electromagnetic field , and they are not localized at all , but we do know that photons are somewhat localized , like in the...
  21. JD_PM

    Modifying photon states to get a gauge symmetry

    I see that this procedure helps to get rid of the two extra degrees of freedom (due to the scalar and longitudinal photons) one firstly encounters while writing the electromagnetic field theory in a Lorentz-covariant way; it indeed shows that modifying the allowed admixtures of longitudinal and...
  22. ilper

    A Is the nonlocality of a single photon an indisputable fact?

    A 2015 paper in Nature Communications by M.Fuwa et al arxiv [1412.7790] Experimental Proof of Nonlocal Wavefunction Collapse for a Single Particle Using Homodyne Measurement Authors:Maria Fuwa, Shuntaro Takeda, Marcin Zwierz, Howard M. Wiseman, Akira...
  23. A

    B Photon Frequency: How Special Relativity Affects Light

    How can a photon have a frequency? Anything moving at the speed of light is predicted to have a zero time rate, e.g. the frequency of a ticking clock would be zero. So no aspect of the light should change along its path - in the same way that no aspect of the moving clock would change. If...
  24. I

    I Has this photon experiment ever been done?

    In the original experiment a photon 45 degrees polarized goes through two beamsplitters and comes out 45 degrees polarized and if you measure the individual path, you measure vertical or horizontal. I was just wondering, has the experiment below (or something simular) ever been done? Do we...
  25. thaiqi

    I Photon & Wave Mode: Equivalent or Not?

    I heard there is a saying that photon and light in a certain wave mode are equivalent. Is it so ?
  26. entropy1

    B Photon Speed: Does it Need a Reference?

    Suppose we have two spaceships a lightminute apart from each other. Ship A sends a photon to ship B. To my knowledge the photon will arrive at ship B a minute after is was departed from ship A. The photon will travel away from ship A with speed c, and will travel towards ship B with speed c...
  27. S

    I Orbits of Photons Around a Massball: Explained

    I am reading MWT gravitation and on page 676, they are talking about orbits of photon, and I don't understand it very well. Energy and angular momentum of the photon are important as a ratio when calculating the orbit. But not energy alone or angular momentum alone. Why is that, and the energy...
  28. GREENMONKEY

    I Why is the speed of photons different?

    Hi, I read the Feynman's book about the quantum electrodynamics and I realized, that he was talking about the different speed of photons. I know, that the light travel's "slower" in a material, but he is also talking about the different speed of photons. I read on the web, that some photons...
  29. ilper

    B Understanding Photon in a Box: Eigenmodes, Reflection, and Energy Measurement

    The eigen wavelengths λn(WL) of EM radiation in box are 2d/n where d is the size of the box. If I put a photon in a box with WL>2d via an optic cable trough a hole it must reflect on the perfect mirror walls and be a running wave. Maybe it is possible to decompose it as a set of eigenmodes of...
  30. Quarinteen

    Photon wavelengths and frequencies

    Hello guys. I was thinking about solar sails and was wondering if it was possible to instead simply create a sail that is pushed by photons create something that creates an opposite force that pushes off the photons. If you did this in theory would you not be able to double the momentum? An...
  31. P

    Plot a photon on a space-time diagram

    I think that if we plot an inertial frame in the XY axis separated 90º, the photon, which has a velocity of c, should be put on one of the branches of the light cone. The questions are: Which branch, left or right one? Which position along the branch, if I don't know the distance it has...
  32. P

    Plot the space-time diagram of a photon and a spaceship

    Be the red point this spacecraft , the purple line the world line with slope = 2 and the green point a photon thrown towards the Earth from the spacecraft , would this spacetime diagram ok? (distance would be 1.2 billion km, and the time, 1000 times of shown, but scale is badly displayed, even...
  33. PeroK

    I Understanding the Relationship Between Photon Spin and Polarization Vectors

    In Griffiths Elementary Particles (2nd, revised edition) there is a footnote on page 241, which states that the photon states with ##m_s = \pm 1## are related to the polarization vector by: $$\epsilon_+ = \frac 1 {\sqrt 2} (-1, -i, 0) \ \text{and} \ \epsilon_- = \frac 1 {\sqrt 2} (1, -i, 0)$$...
  34. Another

    I Photon Interaction with Environment at Long Range

    Why photon interact weakly with their environment over long distances ?
  35. F

    Bohr Model - Absorbing a Photon with Enough Energy to Ionize the Atom

    I just want to confirm something. You need about 13.6 eV of energy to ionize a hydrogen atom in the ground state. Can the atom absorb a photon with 15 eV of energy? I think it can. This would free the electron, and the freed electron would move off with a kinetic energy of 15 minus 13.6 eV...
  36. docnet

    What is the lifetime of a photon in a medium?

    When light travels through a medium like water, it slows down and its speed is no longer equal to c. How does this affect the life time of the photon?
  37. T

    I Single Photon Double Slit Experiment

    We're told that single photons passing through a double slit produce an interference pattern, but the act of observing which slit the photon passes through causes the interference pattern to show a simple ballistic pattern instead. But observing which slit the photon passes through necessitates...
  38. Davephaelon

    I Single photon double slit experiment

    My question is does the photon that is absorbed by an atom on the detection screen have exactly the same energy as the photon that left the 'gun' source? Hence, does the wave packet representing a photon lose some of its energy when it impinges on the double slit barrier, so that when the...
  39. G

    A Bremsstrahlung single photon or spectrum

    Non-relativistic Bremsstrahlung is discussed classically in Rybicki “Radiative Processes in Astrophysics” where Larmor’s formula is used to find the power radiated in a collision between an electron and a Coulomb field. The Fourier transform of the pulse allows for a description of the pulse in...
  40. StevieTNZ

    B Calculating Photon Polarization States with PBS & Wave Plates

    I have hopefully what is regarded as simple and straightforward questions. If we have the attached set up (comprising a source for photons entangled as |H>|V> - |V>|H>), polarizing beam splitters (PBS) and a wave plate that converts |H> to |45> and |V> to |135>. How does one calculate the...
  41. Steve Blandford

    I Thought experiment on photon entanglement

    This is a thought experiment I had with myself a couple of weeks ago. I have never seen it proposed. But please be kind I am not physicist but just a hobbyist. So this experiment is specifically for entangled photons. So here goes. I was thinking of something going faster and faster and time...
  42. arkantos

    I Who Will Win the Race at the Event Horizon?

    Hi guys. Imagine that in the exact instant when a massive particle A crosses the event horizon of a black hole, a Photon does the same,so that they have a race toward the singularity. Who will win the race? Will they have still different velocities?
  43. S

    B From radio signal to corresponding photons

    Let's assume the following sine signal sent by a low frequency (100 kHz) transmitter. I think with the information that the sending power is 1 watt and the starting phase is 0 the signal is fully described. Are the following assumptions correct?: 1. The frequency of all photons leaving the...
  44. S

    B What is the speed of a photon traveling along the sine function?

    On the image you can see a photon starting at point A at t=0. The photons travels along the sine function and arrives point C. I knot that this takes T=λ/c. But this is the time for a object traveling directly from the origin to point C and not along the sine wave! If the photon travels...
  45. BillTre

    I Universal interferometric signatures of a black hole’s photon ring

    This sounds interesting to me. I would appreciate the opinions of more knowledgeable and physics oriented forumists. The paper is in Science (I don't think its open access). Here is the abstract:
  46. hagopbul

    I Photon induced magnetic effects

    Hello all: Reading about HB-11 start up and they mentioned laser induced magnetic field : How is that created and the size of the magnetic field is in kilo Tesla range anyone heard of that Best Hagop
  47. C

    B Exploring the Concept of Time for Photons

    Do photons, I'm not sure how to express this, "experience" time? Since they move at the speed of light does time not exist for them? If time does not exist, how is it possible for anything to happen to them. If interacting with matter requires a change in their condition, how can a change in...
  48. J

    How do atoms emit different photon energies?

    Summary:: Based on this diagram can someone explain how atom of hydrogen gas can be made to emit photons of different energies?
  49. C

    Troubleshooting: Dash Photon Detector for Car Aircon Recirculation Button

    My car aircon recirculation button often turns off by itself. I guess this happens when the aircon is cold enough. Problem is. When it turns off and the indicator light disappears, the pollution outside gets into my car and my pm2.5 monitor registers 20x higher. What kind of photon detector...
  50. C

    B Detecting a photon passing through a slit without destroying it?

    I cannot see how a photon can be detected and yet remain the same photon. I am thinking that the description "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form" is sloppy and in error.
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