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. A

    I Dark Matter as a condensation of photons in a space

    if we assume each photon of light as a very very light piece of matter (by famous E = mc^2 and then: m = E / c^2) and sum up all photons that have been made from the creation time of a galaxy (also considering limitation of speed of light) and also photons that accidentally passing throw that...
  2. F

    I Can a static EM field consist of photons?

    We can analysis a static EM field into Fourier serie. Then we can consider a static EM field as a superposition of many running EM wave. So why we could not consider static EM field as a superposition of many photons(maybe virtue photons)?
  3. Killtech

    I How indistinguishable are photons?

    Assume we have a photon pair prepared just so they are suited for a two photon interference. Let's call the corresponding beams A and B. Then we run each beam individually through a 50:50 beam splitter such that we get beams A1, A2, B1, B2 such that we can symbolically write the state of the...
  4. U

    I Does entanglement of photons really facilitate telepathy?

    [This](https://youtu.be/9l6VPpDublg) is a lecture by Persinger where he postulates that photon entanglements and the Earth's magnetic field can facilitate telepathy. (See from 20:00 to 30:00) Links to the study he cited...
  5. Andreea007

    I How do photons transfer energy?

    Hi! So I know about the electron-photon interaction but what about photon-photon interaction? I mean, I do know there is a very small chance for them to interact, but how else do they transfer energy in order to get from Sun to Earth, for example? When it comes to sound waves I get it, for...
  6. J

    B How far do photons travel inside common lasers?

    So (wildly simplistically) lasers work by photons being reflected between mirrors gaining energy on each trip through the medium until they break through the mirror on the output end. How many trips does that typically result in? 10s, millions? And thus the total distance is that count * the...
  7. 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...
  8. N

    B Confused About the Source of CMB Photons: Two Possibilities

    I am still so totally confused about the ultimate source of the photons of the CMB. I am getting really confused by online sources, who are either not very clear, or seem to contradict each other. I feel like I have narrowed it down to two sources; 1. The early universe was full of high...
  9. Matty521

    I A quasi-analogy of wave-particle duality?

    One Major question I have about wave-particle duality of say a photon... Could we describe it like a rock falling vertically into a still pond. Around this point of contact we establish a circular wall which detects the contact of the wave. Two things are evident here: the rock keeps on moving...
  10. 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...
  11. L

    Calculate number of photons absorbed

    I've tried to solve this by calculating the number of photons. I've done this by calculating the energy of one photon, taking h*c divided by lambda. h*c is 1240 eV*nm and lambda is 10 nm. This gives me 124 eV. I then divide the total energy by the energy of one photon, 50 keV/ 124 = 403 photons...
  12. E

    I Are Expectation Values Measured for Coupled Photons from SPDC?

    Entangled photons are generated, for example, by parametric down conversion (SPDC). The ordinary photon beam has polarization 0° and the extraordinary photon beam comes with the polarization 90°. Each photon leaves the source in a cone of light. Both cone shells intersect in two beams, which are...
  13. Kyran

    B Is a black hole a massive ball of photons?

    Black holes suck things in and the current explanation is that they bend spacetime. I have my own hypothesis though. If electrons shoot out photons when they switch positions in the atom that would mean that at the very least electronshave photons in them. Atoms always try to have the right...
  14. M

    B Are photons entangled with a component of the atom that emitted them?

    If an atom were made to release a Photon, then a number of the components of the atoms nucleus were theoretically extremely quickly removed. would the previously emitted photon change wave length?
  15. O

    I Bell's Inequality => 4 entangled Photons impossible?

    Correlation between polarization measurements of entangled photons at angles less than 45 are greater than classically statistically possible. No set of hidden variables can be preordained to explain the 75% correlation of photon measurements at 30 degrees and complete anticorrelation of...
  16. J

    I Solar chromosphere: inter-conversion of Ca I, Ca II, role of photons

    My question relates to the physics of the emission line of Ca II which originates in the solar chromosphere. My understanding is that ionized Ca which lacks a single electron in its valence orbital interacts with free electron in the chromosphere milieu to form Ca I (nonionized). The captured...
  17. QuasarBoy543298

    I Can two photons cancel each other?

    in classic electromagnetism, two plane waves with the same frequency and -1 phase difference can cancel each other. due to conservation of energy, I assume that photons (that so far we have treated as sort of EM energy chunk traveling through space) can't truly cancel each other. then, what...
  18. tmanici

    MCNPX , problem of detecting the photons in lattices

    Hello everyone! I hope you all doing well :) I am having a trouble with detection the radiation in lattices. i am adding the input and the result file here for makes everything clear, If someone can help me i would really be appreciate! thank you! ☺
  19. 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?
  20. S

    B Do photons travel instantaneously?

    Special Relativity tells us - the faster things travel their time is slower relative to a stationary observer. Do massless particles, like photons traveling at the speed of light, experience zero time and in their frame of reference travel "instantaneously".
  21. F

    I Is there a smallest wavetrain corresponding with photons?

    Is there a smalless wave packet of EM wave corresponding with photons?I read on Internet saying that EM wave consist of many random wavetrains.(Although it seems to me that this wavetrain can be divided into two parts example in double slit Young experiment)
  22. 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...
  23. J

    I What is the four-momentum of a photon?

    I'm trying to understand the Energy-Momentum relativistic relationships for a light particles. It is commonly said that the Energy of a photon depends on the observer by the relationship ## E = - \mathbf{p} \cdot \mathbf{u}## where p is the 4-momentum of the source emitting light particles and u...
  24. F

    B Why do we know all photons have same speed as the speed of light?

    Why do we know the velocity of all photons are the same as the velocity of light?Can we deduce this or we must have experiment test?What is the experiment test?
  25. Dale

    I Number of photons in an arbitrary EM field

    Correct me if I am wrong. I understand that if ##\vec E## and ##\vec B## are solutions to Maxwell’s equations then ##\Psi= \vec E + i \vec B## is a solution to Schrodinger’s equation. Is there an easy way to calculate the statistical distribution of the number of photons, or at least the...
  26. A

    Thomson scattering -- Photons can only scatter from free electrons?

    Hello , Why does an incoming photon can only scatter from an electron if the electron is not bound top an atom ? Because from what I know a bound electron can absorb a photon then be excited to a higher energy level and then re-emit a photon while transitioning back to it's previous energy...
  27. H

    B Is an electron beam affected by photons?

    I am wondering if one of the prerequisites of the double-slit experiment, when done with electrons, is that the beams must be in a dark vacuum tube so as to not destroy the interference pattern. I am trying to learn if the beams will lose their interference pattern because the particles of the...
  28. Byron Forbes

    I Interference in double-slit with single photons

    My question is about how the interference patterns work in a double-slit experiment when firing individual photons through either slit. Classic - there is 1 photon - it has nothing to interfere with. Quantum - even with a photon going through each slit at the same time, the likelihood of them...
  29. Byron Forbes

    Where do photons come from? Exploring the origin of photon emission in a vacuum

    Filament in a vacuum Take a quick look at this! Now, can the glow be explained in any other way than concluding that photons are being emitted from somewhere other than directly from the filament? i.e. is it not obvious that photons are being emitted from points of origin other than the...
  30. R

    Can infrared photons also eject electrons in 'photoelectric effect'?

    Hello, I have this question whether it will be possible to generate electricity by heating up a suitable metal (conductor). We know from the photoelectric effect that high energy photons (of visible light; probably with frequency closer to that of violet) can knock off the outer electrons from...
  31. DuckAmuck

    I EM Field Strength in Curved Spacetime: Is it Unchanged?

    It seems a gravitational field does not alter the electromagnetic field strength. Is this correct? My reasoning: With no gravity, field strength is: F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu Introduce gravity: \partial_\mu A_\nu \rightarrow \nabla_\mu A_\nu = \partial_\mu A_\nu +...
  32. C

    I Photons and wavelength in ultrafast optics

    I am not understanding how to think of photons and wavelength in ultrafast optics. An ultrashort pulse is the summation of many wavelengths. So, if you refract an ultrafast pulse it will actually spread out spatially? Can you define a wavelength as sort of an average wavelength? And most of the...
  33. P

    I What generates black-body photons?

    Spectrally generated photons I get: electrons "fall down to a closer orbit" and a photon is created. I don't understand however how the photons from black body are created. I have searched extensively for this without finding answer. Can someone enlighten me?
  34. T

    How do you derive the units for momentum for photons

    So if i have a photon of some energy and i want to find the magnitude of the momentum, i can get the right answer but the units don't make sense. So i derive p = E/c since i know the energy of the photon and i used f=E/h and substituted this into p=hf/c This means for units of the equation p...
  35. L

    B Basic Question: What are energy levels, of photons, for example?

    I often read of photons manifesting different levels of energy. I know that energy increases as wavelength decreases and frequency increases. Are there other ways particles gain or lose energy? As water boils heat energy is transferred to the water causing water molecules to move faster and to...
  36. danielhaish

    B Photon Energy Change: Speed of Light

    photos are in the speed of light which means that the fasts change in photons energy would take infinite time for the outside observer so does it means that the photons can't spin or interact with each other or have any kind of change
  37. Hiero

    B A few basic questions about the absorption/emission of photons

    1) Photons have momentum. So if an atom emits a photon it ought to recoil to conserve momentum. This recoil will change the kinetic energy of the atom (by an amount dependent on the initial momentum). Does this mean the energy of the emitted photon is the difference in energy levels (of the...
  38. thaiqi

    Quantum How to grasp Cohen-Tannoudji's Photons and Atoms?

    I got one book by Cohen-Tannoudji, that is, Photons and Atoms. It is hard to understand for me now. What books are the prerequesites to read for understanding Cohen's this book?
  39. E

    I Photon Momentum: The Impact of Light on Movement in Space

    We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if: 1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's...
  40. Leonardo Muzzi

    I Quantum Eraser: do entangled photons arrive at different moments?

    Considering a Delayed Choice Quantum Eraser experiment such as the one in the picture above: It is said that paired photons arriving at the detectors A,B,C,or D, reach the detectors after the entangled pair has reached the interference screen. Are we actually able to measure the time interval...
  41. f95toli

    A Number of thermal photons in a real (finite Q) cavity

    This is probably a common questions, but I can't find it answered anywhere... It is of course well known that the average number of thermal photons in a mode can be calculate from from the B-E distribution: <n>=\frac{1}{e^{hf/k_BT}-1} The usual understanding of this is that what we are...
  42. Haorong Wu

    I What is the nature of the photons?

    Hi. I am reading quantum optics by Marlan Scully currently. Although the book has devoted a whole section to explain the photon concept, I am still confused about some nature of photons. First, how do photons distribute in space? For example, there are ##n## photons for a Fock state ##\left | n...
  43. P

    I Jonsson’s apparatus for photons rather than electrons

    In the prompt 4c in this problem set: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/assignments/MIT8_04S13_ps1.pdf. We are asked to find out how d, D and w will change (where d is the slit width, D is the distance from the slits to the screen and w is the distance...
  44. 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...
  45. S

    Do photons, phonons and electrons have mass?

    These articles have energy but do they have mass?
  46. Haorong Wu

    I Can atoms absorb photons with energy higher than transition energy?

    Suppose a atom has two energy levels ##\hbar \omega _ a##, and ##\hbar \omega _b##, and let ##\hbar \omega _0 =\hbar \omega _b -\hbar \omega _a ##. Also, there is a phton with enerigy of ##\hbar \omega##. If ##\hbar \omega =\hbar \omega _0##, then a atom in level ##\hbar \omega _ a## can absorb...
  47. 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...
  48. Marcin

    I Time measurement in a double slit experiment with single photons

    Assumption: Screen detector is much closer to the slits than in "standard experiment" and the small angle approximation can't be used to determine the interference fringe maxima, but the interference pattern still occurs. Is it possible to measure the time of detection in such setup accurately...
  49. J

    How can hydrogen gas can be made to emit photons of different energies

    This above is the diagram I'm not too sure about the solution to this problem as to why I came here. Is it something to do with photons having different frequencies i.e emitting different amounts of energy based on its frequency
  50. K

    I Steering the polarization of entangled photons

    The following is a diagram of an EPR thought experiment using photons that uses the same type of components as described in this paper http://www2.optics.rochester.edu/~stroud/cqi/rochester/UR19.pdf Would you detect any photons at the right detector? I want to be convinced one way or another...
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