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. V

    How Can I Calculate the Number of Photons in a Laser Cavity?

    Homework Statement The laser cavity is formed by two mirrors separated by 15 cm. One of the mirrors has an ultra-high reflectivity and the output mirror has the much lower reflectivity of 99.5 %. How many photons are there in the cavity?[/B] The power of the laser is 1mW and the wavelength is...
  2. B

    I Phase shift after double-slit for entangled photons

    Hi all, I'm trying to understand how to describe the quantum state of entangled photons, including their phase, if one of them encounters a double-slit. Here's a simple example: Suppose you have two polarization-entangled photons A and B in the following Bell state: \begin{equation}...
  3. B

    Quantum state of entangled photons

    Homework Statement Suppose two polarization-entangled photons A and B in the following Bell state: \begin{equation} \Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr) \end{equation} 1. What is the state if the photon A passes through a...
  4. Auto-Didact

    I Human eyes can detect single photons?

    I'm not sure where this thread belongs; it is essentially a topic at the intersection between quantum optics, human physiology and even psychology, but I will focus more or less on the implications for quantum physics, so I just opted for this subforum. There is a recent article over Scientific...
  5. A

    I How does stimulated emission work in the process of light amplification?

    I understand the concept of stimulated emission and how it works as light amplification, but a certain technicality in its process eludes me. How is the inciting photon actually interacting with the electron that falls to a lower energy level? In every physical interaction that I know of there...
  6. J

    B Are all particles and photons entangled?

    1) are all particles/photons theoretically entangled at birth but we just never or will know? Or is it under specific conditions? 2) how much of the photons that the sun emits entangled particles/photons?
  7. Louis Nardozi

    I What is the mass of all photons emitted in the universe?

    What is the mass of all photons emitted in the universe since the Big Bang?
  8. F

    A Photons per unit of Energy in Cherenkov radiation?

    If I have a beam of protons that go into a block of water, how can I know how many photons will be produced along the beam path?. I'm assuming all the photons have the same energy. I know that the energy deposition will follow a Bragg Curve, and I think that energy deposition is probably...
  9. A

    I What is the significance of coupling in surface plasmons and plasmon polaritons?

    Hi, I have come across the word coupling a few times looking at surface plasmons and surface plasmon polaritons. I was wondering if anyone could give me a better understanding of what it means for something like a photon to couple to a plasmons to creating a plasmon polariton. From what I'm...
  10. S

    Photons on a Super Cooled Area

    I was wondering what would happen if you shun light onto a super cooled area. Would it mean that the photon's energy would be absorbed and would not be re-emitted? Or put otherwise, can you cool an area to a point that it would absorb all light?
  11. S

    B Bose-Einstein Condensate Photons

    https://www.livescience.com/10288-kind-light-created-physics-breakthrough.html I was reading here that you can freeze photons. What does it mean to freeze up a photon, are you slowing down it's motion, changing it's energy levels. Or are you changing the state of the particles around it and...
  12. J

    Understanding Light Reflection: Exploring Photons and Reflective Surfaces

    Simple question. Are photons reflected as is from a surface like a mirror, or is the reflecting surface atoms capturing the photons and re-emitting them?
  13. zehkari

    Relativistic momentum of two photons from a decay

    Homework Statement Homework Equations (1) E2 = p2c2 + m02c4 (2) E = γm0c2 (3) E = Eγ1 - Eγ2 (4) p = E / c (5) E = hf (6) λ = c / f The Attempt at a Solution a) Using eqn (1), rearranged p = (E - m0c2) / c , I obtained 2.9 MeV c-1. Not sure if I have the right answer here as I...
  14. T

    High-Energy Photon: What Happens?

    Ok guys, so let's suppose we have a mass of 1kg. We can calculate the energy that matter could deliver. E=mc², roughly 10^8c. Supposing this energy was delivered from a photon (supposing it exists such a processus that could delivery such energy in a single photon), we can calculate the...
  15. F

    Electromagnetic waves: Photons generated by two charges attracted to each other?

    Hello all, after thinking about properties of electromagnetic waves, especially concerning the electric field of them, I ran into some serious understanding problems: 1) photons are electromagnetic waves and vice versa and they convey electrical force. Let's now do a thought experiment: let's...
  16. M

    I Photons and Heisenberg's Uncertainty Principle

    To summarize, my current understanding of how Heisenberg's uncertainty principal works suggests that there would be a contradiction (somewhere down the line) with any way that it applies to (or doesn't apply to) photons, due to the fact that they must always travel the speed of light. I...
  17. Mathfan7

    B How to determine the wavefunction of photons?

    Hi, I'm sorry if this question has already been answered somewhere and I'm just too incompetent to find it, buuut: As the title already says, I really do not get that part of quantum physics (if you can even say I'm getting ANY part at all...). As I searched all Google for an answer I just...
  18. Hugh de Launay

    I What happens to the energy lost by photons in gravity?

    After I read Martin_K's post of 4:14 Oct. 30 on the frozen image of an object just before it fell through a black hole's event horizon, the next few minutes I was jumped by a handful of related ideas. First, the frozen image scenario is illustrative because when photons are frozen in place...
  19. A

    B What tells us photons were ever in a superposition in EPR

    Hi, I know that entanglement is real and that it tells us something profound about the nature of quantum objects like electrons and photons. I can't explain to a family member how we know that two twin photons in the EPR experiment started off in a superposition. In other words, how do we know...
  20. M

    How Black Holes Absorb Light Despite Photons Being Massless

    We know light made up of photons which is massless, but why it can absorbed by black hole? Is it becuz the Einstein's relativity about every object can curve time space
  21. Amanuel

    How to calculate the energy in gamma photons

    how do I calculate the energy of to photons moving in the opposite direction. starting from the same origin.
  22. C

    I Do gravitons have frequency, like photons?

    Something I've been thinking about lately. If so, how'd we detect high frequency gravitons? I have Bachelors in physics so you may get a little technical ;)
  23. B

    I Photons in the photoelectric efect

    What happens to the UV photons that strike the photovoltaic cell but do not take part in the photo electric efect, do thay reflect? apparently the best comercial solar cells are only 24% eficiant.
  24. LarryS

    I How can laser photons have the same precise energy?

    The photons generated by a conventional quantum laser are all in the same quantum state. Doesn't that mean that they all have the same exact energy? Yet, because of energy-time uncertainty, the exact energy of any particle can never be measured. Also, the Copenhagen Interpretation says that...
  25. MeAndMyLucidLife

    Relative speed of two photons

    Homework Statement What is the speed of a photon with respect to another photon if: the two photons are going in the same direction. they are going in opposite direction? 2. The attempt at a solution I think the answer to the first question should be zero and to the second one be 2xC; C⇒speed...
  26. M

    Creation and Annihilation operators on photons

    Homework Statement The possible (normalized) eigenstates of a photon in a given system are written as: $$|\psi_1>,|\psi_2>,...|\psi_m>$$ Let another state be $$|\phi> = \frac{|\psi_1>+|\psi_2>+...+|\psi_m>}{\sqrt{m}}$$ and denote: $$|n>=|\psi_1>|\psi_1>...|\psi_1>$$ which represent a state...
  27. M

    Write the state of the system after r photons leave the |nϕ> state

    Homework Statement (Note: this is the last part of a longer problem, so I hope I won't miss anything important) We have a laser with 2 energy levels, so the emitted photons will have the energy ##\omega##. For this given energy there are ##m## states in which the photon can be, denoted...
  28. C

    I Entangled Photons in Opposite-Configured Mach–Zehnder Interferometers

    So, I have a pretty basic (for this forum, not for me) question: how will two entangled photons behave in the opposite-configured Mach–Zehnder interferometer systems? One of the photon goes through the closed system while the other goes through the open one. Do they simply lose their...
  29. B

    Average number of photons in a atom at a given temprature

    Can anny body tell me how do I find the average number of photons in an atom at a given temprature for say lead.
  30. AlvisPrabhu

    How are Photons massless? How can a particle have no Mass?

    I did look it up , but couldn't find anything that explains this in a simple manner.
  31. YoungPhysicist

    B Is Time Experienced by Photons at the Speed of Light?

    As far as I know, a object will experience time slower when its speed is close to the speed of light. But photons themselves moves at the speed of light, does that mean that they experience no time?
  32. A

    Can Colder Bodies Send Photons to Hotter Ones?

    Assuming no conduction and no convection will a colder body be able to send photons to a hotter body while the hotter body is sending photons to t. We all know that an equilibrium will result with a temperature somewhere between the two bodies but do the photons emitted by the cooler body...
  33. J

    I Difference between an electromagnetic field and a photon?

    So I'm kind of confused. The way I understand it, an electromagnetic field is just a regular electric field viewed from a relativistic point of view, meaning that since we see the charges moving relative to us, we feel like the particles and the fields created by them come closer together (I...
  34. P

    Why is a state with large number of photons not classical?

    In the last paragraph of these notes, https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/MIT8_04S16_LecNotes3.pdf, it says how a state with large number of photons is not classical. Why is that? I thought quantum mechanics' laws were most applicable when we...
  35. N

    On board a laser powered solar sail

    Can an on board laser be used to propel a solar sail spacecraft if the laser is pointed at the sails ? Would Newtons third law affect the laser and maybe prevent the ship from moving ? Thank you for answering my very ignorant questions. NineNinjas911
  36. I

    Can you ionize metal with photons?

    The photoelectric effect occurs when light causes electrons to fly off atoms. An equal number of electrons to protons gives an atom a net charge of 0. If I left a piece of metal in the sun for a long time, that would mean a large number of electrons would fly off. This should, in theory, give...
  37. W

    Writing: Input Wanted Alternative energy sources? How about photons?

    I'm looking to get into the realm of science fiction writing and am currently working out how the main "SciFi energy source" works in this world I'm trying to make. Fusion reactors are the obvious go-to, but I wanted to get creative, that said: how plausible/effective would it be to directly...
  38. Doruk Y

    How can we visualize the movement of Photons on the water?

    We all know that the photons have a wave-like motion,if the waves on the water form because of the motion of the particles inside the air(oxygen+nitrogen gas and %1 another gasses) so in the beginnig of this process,photons hit the air and make particles jiggle and that make air jiggle and air...
  39. Buzz Bloom

    How to calculate the number of photons

    An issue arose in another thread about photons and gas in equilibrium. I made an effort to find an answer searching the internet, but my researching skills are not up to the task. The following is one example of the question for which I would like to learn how to calculate the answer. Given one...
  40. J

    Does Heat Radiation Decrease the Number of Photons Over Time?

    Homework Statement Homework EquationsThe Attempt at a Solution No . The number of photons should decrease . As the rate of heat transfer decreases with time , the net electromagnetic radiation from the hot body to the surroundings decrease . Heat radiations are composed of photons ...
  41. J

    MCQ : Comparing characteristics of two photons

    Homework Statement Homework EquationsThe Attempt at a Solution I am surely making some very silly mistake in this problem . Presuming linear momenta means magnitude of momentum , to me all four options look correct . a) since wavelengths are same , energy is same which means momentum is...
  42. platosuniverse

    B Do photons interact with spacetime?

    The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?
  43. G

    The reality of photons (real vs virtual etc)

    I mean there is this division between virtual (in static situations) and real photons as the quanta of the EM field, but aren't all photons essentially "virtual" in the sense that they are purely made up by us in order to explain why we see discrete impacts of specific energy for a given EM...
  44. P

    I Why do photons interact with e- but not with other photons?

    As far as I understand, two electric fields can interact with each other (for instance, two charges being brought close to each other will either repel or attract). A photon is an EM wave and when it comes in contact with a charged particle, say, an electron, the electric field of the wave...
  45. R

    Do photons carry the electric field?

    In basic electrostatics any charged particle will produce an electric field at every point in space, and will have electric filed lines spreading out radially. E = kQ/r^2 The Standard model of particle physics says that the Photon is the force carrier for the Electromagnetic force, just like...
  46. J

    B The setup of "2012 entanglement between photons that (....)"

    Hi all! Sorry for the bad english! Please, can someone help me understand the setup from "entanglement between photons that never coexisted "? So, we start with a bean from a laser, that hits a bbo crystal, makes two entangled photons (1 and 2), 1 gets measured (lets say its vertically...
  47. S

    Difference between emission due to absorption and reflection

    What is the difference between the photon interaction mechanisms in emission due to absorption and reflection?
  48. K

    B A couple of questions about photons and superposition..

    There are two polarization filters, A and B. Polarization filter A has angle of 0° and B has an angle of 30°. A photon is in superposition, so it doesn't have a definite polarization axis. The likelihood it's passing through a filter is depend on the difference between angle of the...
  49. D

    I Electrons Emitting Photons And Ambient Light

    Ambient for lack of a better term... I'm reading "Beyond The Cosmic Landscape". Perhaps out of date, but a very understandable explanation of QED. Am I right to deduce that the air which surrounds us is jam-packed with electrons emitting photons? Thanks...
  50. K

    B Why 50% of entangled photons pass through the polarizer?

    Hi. If we have 1 million entangled photons separated from their "entangled partner". We send all those photons (without their "entangled partners") through a polarizer. Each photon has has 50% chance of passing through the polarizer. So 50% of the photons will pass through the polarizer and...
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