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. C

    Why do photons have momentum yet no mass?

    I imagine this gets asked a lot but I'm looking for an in depth explanation since none of the others I've found are detailed enough. I understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc. Assuming that e =...
  2. F

    Energy of photons from Balmer series

    I can see how I would do part b, but I am confused as to why I have got part a wrong. Apparently the answer is 12.09eV, but I cannot see how. It seems to me that using the formula as I did should give the answer. Thanks for any help.
  3. M

    Amount of photons released during decrease of energy of vibration

    An atom has the following energy of vibration: E= nhv. n can only be in the range of whole numbers. So if an vibrating atom changed energy, say from 6hv to 2hv, it means that 4hv of energy is released in light/photons Does that mean that 4 photons are "released" by the atom (?), because...
  4. M

    Photons fired from the back of a moving spacecraft

    Homework Statement If a passenger at the back of a spaceship traveling at 3/5c fires a photon forward how long will it take to hit the front of the spaceship in the reference frame of the spaceship if it is 1 light second long? How long will it take in the stationary frame? Homework...
  5. Spinnor

    Microwave photons, when do they become real?

    A microwave oven produces microwaves that can heat food. Can we consider the electromagnetic energy leaving the magnetron and traveling in the waveguide virtual photons and when the electromagnetic energy leaves the waveguide the electromagnetic energy "becomes" real photons? Or are they...
  6. E

    What happens to emission spectra when photons pass through a medium?

    I understand that light travels at “c” through any medium, and that the apparent slowing results from the absorption and re-emission of the photons by atoms in the medium. If this is the case, why, for example, are astronomers able to see the emission/absorption spectra of distant stars...
  7. L

    Tunneling photons travel faster than light?

    I recently found an interesting read on the mysteries of quantum mechanics, here : Don't know about you, but I am personally confused by this result. How can a photon travel faster than itself? If it can travel faster than itself, than what does that mean for the universal speed limit? My...
  8. F

    Is the Book's Photon Calculation Off by a Factor of 1000?

    Part a) is correct, but the book gives me an answer of 2.3*10^{23} for part b. Is the book wrong or have I missed a factor of 1000 somewhere?
  9. Spinnor

    Quantum field theory, photons, and polarization.

    A circularly polarized electromagnetic wave can be thought of proper combinations of orthogonal linear polarized waves, and a linear polarized wave can be thought of proper combinations of left and right circularly polarized waves. It seems one type of wave is no more fundamental then the other...
  10. J

    Is it Possible for a Photon to Be at Rest?

    These are not facetious questions. Is a photon at rest still a photon? Can a photon be at rest? Thanks
  11. S

    Photons Are 'Wave-like' in What Way Exactly?

    Exactly in what way are photons wave-like? With respect to what property or metric? For example, we often hear the wave-like nature of photons compared to waves in water. Well, wave behavior in water can be measured or observed in respect to mechanical displacement - crests and troughs which can...
  12. P

    What are some different interactions where photons are released?

    Pretty simple; what are some different ways that photons are released? Specifically, I'm looking for a short list of different particles that will release a photon in any given interaction. Tried googling, but its amazing how difficult it is to find an answer to such a simple question --...
  13. M

    Why are only some photons virtual?

    Sorry it was hard to think of a good title. Essentially, why do only virtual photons carry the electromagnetic force? What stops 'normal' photons from carrying the EM force? Is it the fact that we can detect them? What decides whether a photon will be virtual and hence carry the EM force, or...
  14. S

    Photons violate Uncertainty Principle?

    A photon is considered as a quantum particle, right? However since we know the speed of a photon(speed of light) and hence can predict its position, isn't it violating the Heisenberg Uncertainty Principle? Where am I going wrong? Is it false to believe that a photon is a quantum particle?
  15. A

    A particle traveling at a speed V decays to form two photons (no mass)

    Conservation of relativistic momentum and energy, pion decays Homework Statement A small particle (pion), traveling at a velocity V, decays into two rays, γ1 and γ2. Find the Momentum and Energy of γ1 and γ2 if: a) γ1 is in line with V, and b) if γ1 is perpendicular to V. I drew out the...
  16. H

    Modulate the voltage,current and frequency of photons

    Hello, I have come to the understanding the one form of electrical energy is electromagnetic energy. Suppose i have a photon stream (a) How do i find out the voltage and current of the photons in the stream ?? (b)Suppose i have a photon stream whose voltage is V and current is I. If i...
  17. B

    Can an electron absorb multiple photons at once?

    Hello all Is that possible a electron absorbs two photons at the same time to jump to a higher energy level and then emission one higher energy photon and back to before level ? example : electron absorbs 2 photon by energy 10 and 20 eV and jump up from 2 to 3 energy level and emission a...
  18. W

    Medical Seeing "Photon & Floater" Orbs: 23yo with 16/20 Vision

    Since I was a kid i was interested in bright lights they never hurt my eyes to look straight at them such as the sun or any other light. Since about 8 y.o. to current i had better then 20/20 vision. My latest was 16/20. i am 23 now but ever since i was young i seen these weird spheres in light...
  19. J

    Pi meson with velocity v decays into two photons

    Homework Statement A ∏° meson moving with a velocity v in the z direction decays into two photons. One of the photons travels in the z direction and the other travels in the minus z direction. a) If one photon has an energy that is nine times greater than the other photon, calculate the speed...
  20. L

    Why don't photons experience time?

    I am asking this question in order to clarify something which I thought I had understood. First of all, let me say that I understand talking about the perspective (or frame of reference) of a photon doesn't make sense. Yet, after many searches in the web, I feel like there is a consensus among...
  21. R

    Do properties associated with photons also apply to other bosons?

    A photon has an energy correleated with its frequency. Photons can interfere. Can the same be said for other bosons, like the Higgs?
  22. Greg Bernhardt

    Atomic Atoms, Molecules and Photons: An Introduction by Wolfgang Demtröder

    Author: Wolfgang Demtröder Title: Atoms, Molecules and Photons: An Introduction Amazon Link: https://www.amazon.com/dp/3642102972/?tag=pfamazon01-20 Prerequisities: Contents:
  23. F

    Velocity of the centre of mass of two photons

    Homework Statement Find the velocity of the centre of mass of two photons with different energies (they happen to be approaching). Homework Equations E = hf p(photon) = E/C E=gamma(mc^2) The Attempt at a Solution I have calculated the energy of the two photons in the centre of mass...
  24. C

    Measuring the polariztion of a pair of entangled photons

    Hi, I'm quite new to quantum mechanics, learning about it in my free time in a life-long learning fashion :) I've been trying to find a solution to a problem for some time, and the results I included below appear to be OK, but I have doubts about the method I used, so any help or guidance...
  25. Spinnor

    QED; electrons and photons, different fields, modes of a common field?

    In the Standard Model of particle physics are we to think of the electron(positron) field and the photon field as separate fields? Is it possible to think of the electron(positron) field and the photon field as just different "modes of vibration" of some more basic field? Can this line of...
  26. D

    The Physics of Quartz: Laser, Photons, & More

    Hi all, I have been looking at quartz and trying to work out the physics at a atomic level, how it reacts with photons/laser is of particular interest. I will leave this open as any knowledge you have is welcome, there is the obvious it converts electrical to mechanical and vice versa but...
  27. D

    How can I handle an infinite upper limit in an integral for a photovoltaic cell?

    Homework Statement Hi, So I have this problem regarding a photovoltaic cell I found part 1 easy and part 2 wasn't actually too bad either but I couldn't finish the calculation. I managed to get to the stage where I have an integral , the problem is that the upper limit is infinity and I'm...
  28. S

    Are the photons we interact with actually anti photons?

    I know that there are no anti photons since a photon is its own anti particle but if the theory of antimatter is matter moving in the opposite direction of the 4th dimension (moving back in time), then are the photons we interact with actually photons that move in the opposite direction in time...
  29. Cthugha

    Peculiarities of photons in quantum optics

    I would like to open this new topic to continue discussions on single photons which turned up in several other topics, but were a bit out of place there. So here the discussion continues. The first two quotes were basically about coincidence counting, photon statistics and the HBT-experiment...
  30. J

    So when i see white light, is it really just seven different photons?

    Im a little confused as to why the eye detects white light as white instead of all the separate colors that compose white light. I mean it isn't a single photon right (multiple single photons of course)? So how do the different photons converge into one color? Also, which may seem like a very...
  31. E

    Thus, matter can be created out of two photons.

    "Thus, matter can be created out of two photons." Hello, and thank you for opening my thread! I have read over the FAQ on this forum, looked over a few similair topics, and still feel that I lack understanding with this concept. To me, the wiki stated below sounds like it is saying light...
  32. S

    Do photons obey the 1/r^2 gravity law?

    Do photons obey the 1/r^2 gravity law? This was a question that came up in our recent PhySoc meeting and no-one present seemed to know the answer (lecturers included!) Does anyone have a unequivocal answer (and preferably citing a source)? I have done a quick search through old posts...
  33. DrChinese

    Fair Sampling Loophole now closed for Photons

    A team including Anton Zeilinger has performed an experiment closing the so-called "fair sampling loophole" for photons. http://arxiv.org/abs/1212.0533 Bell violation with entangled photons, free of the fair-sampling assumption Marissa Giustina, Alexandra Mech, Sven Ramelow, Bernhard...
  34. S

    Rest frame angular distribution of meson decay into two photons

    Homework Statement consider a pion decaying into 2 photons. In the rest frame of the pion, the two photons must emerge back-to-back photons are equally likely to emerge in any direction. determine the rest frame angular distribution of the emerging photons. Homework Equations ΔP=f(θ)Δθ is...
  35. S

    What are the common fates of photons in matter?

    Main fate of low energy photons in matter is photoelectric effect. At intermediate energies, it is Compton scattering (presumably mainly off electrons). At high energies, it is said to be "pair production". Pairs of what? At low energies, mainly electrons and positrons, obviously...
  36. H

    Are polaritons simply photons propagating in medium?

    In vacuum, the photon has a 4-momentum (E, p) with E^2 - p^2 = 0, i.e. it's massless. However, upon entering a medium of refractive index n, we expect that the photon retains its energy, while reducing its momentum by a factor n (due to increased wavelength). We then have for the 4-momentum of a...
  37. A

    What is the mechanism behind the weight of a box of photons?

    Supposing two cubic containers with perfectly reflective interior surfaces. In one there is 1 mole of some gas and in the other is an equivalent number NA of photons of some frequency. We put the gas on a scale and the scale registers the total weight (mass) of the enclosed gas. But...
  38. I

    Entangled photons in bell experiment: transfer phase or angular momentum?

    Regarding the polarization correlation studies generated using parametric down conversion. All the studies appear to be done correlating the polarization of linearly polarized photons. Has any experiment been done showing the same effect with circularly polarized light? 1) If this...
  39. O

    Do photons have momentum without mass?

    Is it correct to say that as photons have no mass they are pure energy and have no mass equivalent according to E=mc^2? In a particle collision involving photons, conservation of momentum can not be used as photons are mass-less. Is that also correct? A proton in a linear accelerator can...
  40. A

    Can Low E photons combine into High E photons by themselves

    This post is not related to my E=mc^2 post but is a follow-up on the thread: photon collision by misnoma. The following questions were not addressed in that post but I wanted to ask anyway since that thread is now locked. This is better a high energy question anyway. A) Is it possible for two...
  41. Z

    Coherent state of photons in a laser

    Do photons used to produce the Sagnac effect (interference) have to have their phase in a coherent state or will a collection of random photons have the same property. Along the same lines, if you purchase a simple laser pointer are the photon produced here in a coherent state? Finally, is...
  42. Q

    Exploring the Mystery of Disappearing Photons in a Sealed Box"

    If you had a perfectly sealed box that did not absorb energy, and was a perfect mirror on the inside, and you fired a stream of photons through a photon sized hole in the side of the box such that they would not reflect (bounce) back out through the hole, and you placed a photon detector inside...
  43. L

    Problem with Parity for photons

    I am trying to understand how it can be shown that under parity transformation we have to have \hat{P} \hat{a}_{\mathbf{p},\lambda} \hat{P} = - \hat{a}_{-\mathbf{p},\lambda} , I mean the negative sign (negative intrinsic parity of photon). So I am trying to prove that from the vector nature of...
  44. R

    Question on Photons and perception

    Its my understanding that a photon will travel and exist in space forever. My question is if light, whether it be from a distant galaxy or star, can be perceived after it has passed by our location in space. To me it seems a confusing subject to think about but i assume you can see a beam of...
  45. B

    Can Two Colliding Photons Create a New Particle?

    Homework Statement Imagine that we discover a new particle, called P particle by a head-on collision of two photons of energies 500 Mev and 200 MeV. The photons are annihilated in the process. a) what is the mass of the newly discovered particle P? b) what is the kinetic energy of the P...
  46. Physicist50

    The Mystery of Atoms Releasing Photons: A Scientific Exploration

    I was wondering, after discovering that in particular scenarios atoms can release photons, how they do it? Is some of the energy stored in the nucleus converted into light energy, or is it something else?
  47. S

    Entanglement after the photons no longer exist

    Refer to the experiment below: Trying to understand this phrase/sentence: "Entanglement after the photons no longer exist" What is being entangled if the photons no longer exist? Victor can entangled the two photons but Alice and Bob's photons are destroyed, so how can they be entangled...
  48. N

    Exciting electronic motion via polarized photons

    Hi In introductory atomic physics one usually goes through the classical derivation of the Zeeman effect, where one finds that an electron in a magnetic field can have three different kinds of motion: along the B-field with frequency ω and rotating CW/CCW with ω±Ω. Polarized photons can...
  49. DrChinese

    Entanglement Between Photons that have Never Coexisted

    I have posted many times that entanglement between photons that have never existed at the same time is theoretically possible under QM. Now an experiment has been performed that demonstrates this. The technique uses entanglement swapping. Photons 1 and 2 are created as an entangled pair...
  50. D

    Photoelectric effect with n photons

    So in photoelectric effect, electrons can be ejected from a material when a beam of light shines upon the material, provided that the incident photons have enough energy to overcome the work function, i.e. hbar*omega has to be greater than W. Now, why can't I have n photons simultaneously...
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