What is Photons: 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. N

    Coherence time of parametrically downconverted entangled photons

    What is the coherence time for a pair of entangled photons produced in a nonlinear crystal? Is it related to coherence time of the pump photon? Also, if we say that the coherence time of the two photons is T, then does it mean that the two photons can interfere with each other even if their...
  2. H

    Proof of E=h*f: Energy of Photons

    E=h*f (h constant of Planck,f frequency) What is the proof of that equation?
  3. R

    Does the uncertainty principle apply at all to photons?

    I know this may sound strange, given that we cannot really work out where in space a photon is because it cannot be completely stopped. But here's a thought. Let us assume that a photon has been emitted in vacuum going in a straight line. At any given moment in time, this photon will have...
  4. F

    Mobile Phone Tower Question - Wave Equation and No. of Photons Per Sec

    Homework Statement A mobile phone signal with a frequency of 1945Mhz is being broadcast from a transmitter with a peak output of 3kW. A: What part of the EM spectrum is the signal. Classify it in terms of its orientation of oscillation and propagation. B: Write a general equation for the...
  5. Y

    MHB How Many Photons Does a Laser Emit in a Single Pulse?

    A laser emits light of wavelength 463 nm during a brief pulse that lasts for 25 ms and has a total energy of 1.2 J. How many photons are emitted in that single pulse? (c=3.00E8 m/s, h=6.626E-34 j.s)
  6. U

    WaveLengths of Light Waves and their respective photons

    Suppose you have a source of light that emits light with a wavelength of 2 meters, and you set the device to be turned on and switched off alternately. You also set it so that each interval the device is turned on is only long enough for 1 meter to be emitted (1/2 a wavelength). Do you ever...
  7. Y

    MHB Physics: AM Radio Station Emits Photons/sec at 1000-kHz

    Physics: An 84-kW AM radio station broadcasts at 1000-kHz. How many photons are emitted each second by the transmitting antenna? (h=6.626E-32 Js)
  8. Dadface

    Do you know any properties of photons?

    If there are real photons it might be considered reasonable to assume that each photon has certain properties. Here I am interested in finding out all known photon properties, initially by means of making a list. I have started a list and would appreciate if any additions are made to it. I would...
  9. B

    Atom Absorbing and Emitting Photons

    Homework Statement Consider a pretend atom for which the electrons can be in anyone of the following energy states: Label Energy A -1.400×10-18J B -8.000×10-19J C -5.000×10-19J D -3.000×10-19J Start by arranging the four levels on a graph, then answer the following questions...
  10. E

    Photon Interactions with Matter: Explained

    Hi, Why do photons not pass through matter like neutrinos since they have no charge. What are they interacting with when they are stopped by matter? Thanks, Elliott
  11. H

    Why is there little decoherence for photons?

    I read it some where that there is very small decoherence for photons. The reason being that photons do not interact with each other (Is that because photons are chargeless, colorless and flavourless particles?) and hence the information that they contain tends to stay with them. They have a...
  12. Spinnor

    Photons from strings, from expert to layman.

    Don't know how I bumped into this but thought some of you might like this, https://groups.google.com/forum/#!msg/sci.physics.research/5w2S-j0Vyfw/A95XAYUGaWgJ By, Urs Schreiber 4/11/03 photon as strings for peasants (was: Meaning of dilaton field) Oz schrieb: > I presume...
  13. J

    The Effect of Gravity on Photons

    I am not well versed in complex physics (I've taken honors physics in High school) so any help I can get with simpler vocabulary would be appreciated. I stumbled upon this problem when just thinking and have not been able to find an answer. We all know that gravity affects photons, as...
  14. G

    Understanding Photon Thermalization and Its Impact on CMB Anisotropies

    Quick question: what is meant by thermalization of photons? And how could this effect distort CMB spectrum (anisotropies)? Any references are also welcome.
  15. F

    [Statistical Physics] Probability of finding # photons in the mode

    Homework Statement A cavity contains black body radiation at temperature T = 500 K. Consider an optical mode in the cavity with frequency ω=2.5x10^{13} Hz. Calculate a) the probability of finding 0 photons in the mode b) the probability of finding 1 photon in the mode c) the mean number...
  16. Spinnor

    Closed, collapsing universe+only photons at first -> matter when hot?

    Closed, collapsing universe+only photons at first --> matter when hot? Suppose we had closed, collapsing universe with a uniform thermal distribution of low energy photons like that of the CMB and no other matter (I suppose we must pick the initial conditions right for collapse to occur) . As...
  17. 0

    Entangled photons in double slit experiment

    Hi, imagine setup with source of entangled photons (A and B). Photon A travels to double slit so that either wave or particle pattern can be observed, photon B hits detector either before (setup 1) or after (setup 2) photon A has hit screen. Question: Will setup 1 show interference pattern...
  18. Spinnor

    Adjust parameters of Standard Model so only photons, e-, e+ remain?

    Could you have a hypothetical universe with only photons, electrons, and positrons by adjusting the parameters of the Standard Model to eliminate all the "other" stuff? If so is String Theory flexible enough to model a hypothetical universe with only photons, electrons, positrons, and...
  19. tom.stoer

    Photons Trapped on Expanding Event Horizons

    Suppose we have an event horizon H which is a light-like, closed 2-surface. Photons radiated outwards at the horizon along a light-like normal u of the horizon stay on the horizon (this is trivial b/c this statement is nothing else but the definition of the light-like 2-surface). Now suppose we...
  20. PhotonicBoom

    Are photons the antiparticle of itself?

    Hey guys, I was thinking about this for a while now and I seem to be on a dead end. So here it is, this is my speculation. I would love some feedback, tell me which parts are correct/false and if they are false, guide me towards the right path! :) 1. Photons can technically be their own...
  21. Manraj singh

    Exploring Photons and Lightwaves: Particles or Waves?

    I have been wondering, how do photons make up light? Are they particles that travel in transverse waves? Or do they travel in longitudinal waves? Or do they travel in waves at all?
  22. kmm

    Electromagnetic waves with regard to photons

    The electromagnetic spectrum contains wavelengths that are on the scale of macroscopic objects. What I'm not sure about then is, does this mean that photons corresponding to these "macroscopic" wavelengths are actually that size? My guess is that these photons have a corresponding electric field...
  23. D

    Why must a charged particle be accelerated to produced photons?

    Suppose you have an electron moving at constant velocity. This new area that it is moving into initially had 0 electric flux through it but now that the electron moves to that location the electric flux is changing which in turn cause a magnetic flux ad infinitum. So why must a charge be...
  24. Choisai

    Can Single Photons Bend When They Hit a Material?

    As a physics student, I was taught that refraction happens because when light approaches a material with a refraction index that is different than the index of the medium it is traveling through at that moment, the light that hits it first, as shown here: This raises to me two questions...
  25. xortdsc

    Do single photons disperse like a wave?

    Hi, if an electron and a positron collide and annihilate they will produce two gamma photons departing from the collision point in opposite directions at the speed of light, right ? Now since there is this wave/particle duality problem I was wondering if in a vacuum in this particular scenario...
  26. R

    What is the energy of photons in pion decay at rest?

    Apologies, this is going to be a bit disjointed, I don't want to write the full question down as I don't want anyone to give me a solution as its an assignment question. 1. am i correct in assuming when a pion decays at rest its energy will be given by: E^2 = p^2 c^2 + m^2 c^4 which...
  27. C

    Localized absorption of photons and carrier generation

    Perturbation theory predicts rates of transitions between eigenstates of the unperturbed Hamiltonian, which in the independent electron model for a crystal are nonlocal Bloch wave functions or linear combinations of them that extend throughout the crystal. However, photon absorption is...
  28. Dadface

    Entangled photons and polarisation.

    I have been looking up the practical details of certain quantum eraser type experiments and got stuck on a couple of points. I refer to a particular experiment which can be found by googling: "A DOUBLE SLIT QUANTUM ERASER EXPERIMENT" (Sorry I don't know how to include the address) In...
  29. Safinaz

    Maximizing Particle Detection at the LHC: The Role of Photons, Jets, and Leptons

    Hi all, Is it not preferable to detect a particle at the LHC with final states of photons, that when I read in many papers I find for example for a particle beyond SM like an octet scalars if they could decay into jets and photons+jets ( S^+ -> S^- A -> t b A), only jets channels take a...
  30. R

    Understanding Photons: Frequency, Energy & Wave Behaviour

    Hello I am a physics student at the university of Athens (1st year) . Recently i have been studying special relativity by myself and i have question that i can't understand by myself When we speak about photons and their energy we use this formula : E=hf where h : planks constant and...
  31. Q

    A problem in deriving the interaction field equation for photons

    Well, I was doing a problem the other day, inserting the complete interaction QED Lagrangian into the Euler-Lagrange equations, in order to obtain the field equation governing the interacting electromagnetic field, with the Dirac field. The problem is that, by doing so, I got the equation, but...
  32. V

    Special relativity and photons

    Hello everyone! There is a postulate of special relativity that says that the speed of light in vacuum is the same for all observers. This seems to imply (to me) that all the results of special relativity must only be used when dealing with observers who conform to the condition set by the...
  33. J

    Do photons interact with gravitons?

    Does QED predict that the hypothetical graviton particle will interact with light? I mean it would have to interact in some way, right? We already have evidence that it does. (Eddington and such). I was just under the impression that the graviton needs some sort of mass to interact with.
  34. J

    Can the energy flux of an EM wave be represented by a single photon?

    I might be asking a question who's answer is way beyond my level but ill give it a shot. In QED, an electric or magnetic field is described in terms of exchanged photons, right? But if a photon itself is made of electric and magnetic fields... a paradox to my ill-informed brain. :mad:
  35. TrickyDicky

    Double-slit experiment with photons vs electrons

    Both photons and electrons give the same kind of interference pattern in the double-slit experiment, but while in the case of electrons this carachteristic interference pattern is due to the probabilistic complex wavefunction of the Schrodinger equation within NRQM, for photons no such...
  36. A

    Why are photons released when electron jump states?

    I've always taken this for granted. Now I am looking for an answer. When electron jumps from a higher orbit to lower orbit it releases energy. Why is the energy in the form of photon? I will take another example which will make my question easy to understand. When two electrons are kept...
  37. B

    What produces the photons in light bulb filament?

    What produces the photons in light bulb filament? I know that electrons are emmitted by thermionic emmission from the filament . . . do they then fall back into the filament and emit EM radiation as they de-accellerate into the filament?
  38. D

    Photons as virtual particles in High energy

    Hi, So i can accept that photons are virtual particles for the electromagnetic force but i have a question. Considering two stationary point charges. There are photon-like particles exchanged between them to produce the force? If so then placing a double slit between them should create some...
  39. N

    Emission of photons by vibrational relaxation of molecules

    I have read that relaxation of a rotational or vibrational mode in gases occurs through collisions of molecules , transferring energy of an excited vibrational mode into heat. But isn't it possible that an excited vibrational mode in molecules relaxes directly by emission of radiation...? I...
  40. E

    Converting Lumens to Photons and Energy at Varying Wavelengths

    Hello everyone, My issue is regard a conversion from lumens to ~photons or energy at varying wavelengths. I am trying to find a way to determine the amount of energy an led can emit at different wavelengths. I figured because E = (h*c)/λ all I would need is to determine an photon emission...
  41. medgalis

    Information carrued by photons.

    Ok, so I don't understand, what information is a photon carrying? It's mass, velocity or something like that?
  42. J

    Do photons carry more than energy, momentum and angular momentum?

    Noether theorem says that with symmetries come conservation laws, and so because of time, translation and rotation symmetry, EM field itself guards energy, momentum and angular momentum conservation. While atom deexcitation there clearly appears energy and angular momentum difference, so there...
  43. G

    Why does a BBO crystal create entangled photons?

    Hi, If we pump a beta barium borate (BBO) crystal, we get one circle of vertically polarized photons ##\left|V\right\rangle## and an intersecting circle of horizontally polarized ones ##\left|H\right\rangle##: http://quantum.ustc.edu.cn/old/img/image002.gif At the intersection points of the...
  44. 1

    Photons, QFT and electric generators

    hi, friends. i am 15 and have made a device using electric motors and generators which is able to produce more "electrical output" than the "electrical input". i have won the national science fair with this but the problem is that the judges say i am braking the 2nd law of thermodynamics and...
  45. T

    Calculating Photon Momentum and Speed for a 1205 kg Spaceship with a Square Sail

    Homework Statement In space near earth, about 3.84*10^21 photons are incident per square meter. On average, the momentum of a photon is 1.3*10^-27. Assume we have a 1205 kg spaceship, and a square sail that is 26.3m wide. How fast could the ship be traveling after 21 days? Homework...
  46. S

    Non-interfered vs interfered photons

    It seems that - I am getting conflicting information from the forum posts about the above topic. Is there any difference in the properties (such as energy, intensity, spin, momentum etc): of a photon that emerges from a double slit without which-way Vs which-way? or in other words does a...
  47. M

    Earliest Photons: Quark Era & Neutrino Decoupling

    Where there photons around in the electroweak era? I read that The CMB photons travel through the universe relatively unimpeded until recombination when the atoms formed and before this they were being scattered. Were they around in the quark anti quark gluon plasma? I am also interested in...
  48. K

    Gravitational waves analogous to photons and EM radiation?

    Are gravitons and gravitational waves analogous to photons and EM radiation?
  49. Dadface

    Detecting Entangled Photons: Principles and Experimental Methods

    Hello all, I have been searching and trying to find out how entanglement is detected experimentally, but I have a few sticking points. Initially I just want to know the principles behind the experiments. In summary this is how I understand it so far. 1. Entangled photons produced eg in pairs...
  50. MalachiK

    Does the photon have direct interactions with the weak force?

    Hi guys, Could someone with a better understanding of the standard model answer a question about the weak force? In a class I'm teaching, a question in the textbook asks if photons feel the weak force. My first thought was that it's fermions that feel the weak force and therefore the...
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