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. Y

    I Photon Timelessness: Does It Exist?

    I have read that photons do not experience time... If that's the case then if a particular photon is emitted by a body then that should exist in every time relative to us i.e that same photon should be there at exactly the same point forever.
  2. G

    I Wave-Particle Duality of Electrons

    The wave-particle duality of light was demonstrated first with Thomas Young's 1801 Interference Experiment...and then more clearly with the Double Slit Experiment. Both of these were done with light (so photons). My question is -- How did we come to understand the same of electrons? Did we...
  3. I

    I Differences in photons from protons vs photons from e-?

    In an effort to understand how an electron and proton attract each other in the QFT picture, I am wondering if there any differences at all between photons (virtual or whatever) emitted by electrons and photons emitted by protons? For instance, do photons from protons have a different spin or...
  4. B

    Favorite ways to trap photons or slow down light

    What are currently the best cost-effective ways to temporarily store a photon and preserve its phase? Or, at least, to slow down light? A fiber optic cable is definitely cost-effective for the latter, but not practical for intervals longer than 1µs due to absorption losses and length.
  5. S

    I Do the vacuum states created by soft photons have vacuum flu

    Putting a soft photon in vacuum will result in a zero energy vacuum state. Despite the zero energy, has this state vacuum fluctuations? Putting more of these photons will result in more vacuum states. Would they have vacuum fluctuations as well resulting in more vacuum flctuations? A total...
  6. S

    How Do You Calculate the Average Number of Photons in a Cavity at Temperature T?

    Homework Statement Calculate the mean number of photons in a cavity at temperature T and the mean energy per photon. Homework Equations In the large volume limit, the log of grand canonical partition function is: ##log(Z_g) = \frac{gV}{h^3}\int log(1-e^{(E-\mu)/kT})dp^3 ##, with g - spin...
  7. W

    B Are vertical lines from squinting a Quantum phenomenon?

    When looking at a night light with almost closed eyes, I notice that the light becomes a vertical line. When tilting my head 90 degrees, the line becomes horizontal. Can this be explained by Heisenberg's Uncertainty principle? This question has been asked in another thread : "Squinting at...
  8. SlowThinker

    B Brownian motion from virtual photons?

    This a really simple question: If I have, say, 2 ions close to one another, and measure their repulsion very precisely, is the force constant, or is it a series of little pushes caused by individual virtual photons? I know there are many misunderstandings about virtual particles, but I'm not...
  9. Sophrosyne

    I Why can't we detect virtual photons?

    I have heard a virtual phton as used in QED defined as being forced carriers between two fermions which last for very short periods of time. A couple of questions about this: 1) how do we know these are photons and not some other force carrier if we cannot detect them directly? 2) can...
  10. T

    Why is deBroglie λ for electrons the same as λ for photons?

    Hi, I got the following question in my textbook: [translated]"Compare the wavelength of a photon and an electron where the photon and the electron have the same momentum". My thinking is the following: Firstly, pp (photon) = pe (electron). My textbook briefly mentions the extention of the...
  11. E

    Why do photons pass through particles with a delay

    hi everyone. i'm only 16 so please beer that in mind. I've been doing some reading around and have come across the idea that light slows down in an dense medium as a result of the photons moving tough other particles with a slight delay however they retain all factors about them selves apron...
  12. S

    B Double slit experiment - doesn't measurement affect photons?

    All these videos and articles about the Double Slit Experiment say that if we "look" where the single photons go, they act like particles and if we don't "look" they act like waves, creating the interference pattern... BUT What does it mean to "look"? We're not using our eyes or any camera...
  13. R

    B Photons -- do you know what they are?

    Does anybody know what a photon is? Light used to be known as electromagnetic wave till 1900, but then it's particle was made justified by photoelectric effect, Compton effect. And we know about diffraction, interference, etc. And it has no mass.
  14. S

    I Do slits absorb photons at the edges?

    Xiao et al say the following here: https://arxiv.org/pdf/1703.03818.pdf Is this correct? I'm finding it hard to believe that there is a probability that some irreversible events will occur that would ultimately result in heating up the mask (screen with slits). I mean, we could always make the...
  15. clamatoman

    How many Photons per second are entering the pupil?

    Homework Statement (a)Estimate the number of photons per second emitted by a 100-W lightbulb, assuming a photon wavelength of 550nm.(b) A person can just see this bulb for a distance of 800m, with the pupil dilated to 7.5mm. How many photons per second are entering the pupil? Homework...
  16. tom.stoer

    I Interpretation of the photoelectric effect

    The photoelectric effect is usually presented as an example disproving classical electromagnetism as viable model for interaction of light with matter and as evidence of quantization of energy in the electromagnetic field, i.e. the existence of photons. I would like to discuss a thought based on...
  17. B

    I Observation of photons from CMB by different observers

    If two observers on Earth in different locations around the globe, were both viewing the CMB with their equipment pointing at the same point in the sky, and charting the fluctuations in it, would they correspond or vary from each other greatly. In other words, if you made a graph of the CMB and...
  18. S

    Rotate only H-Pol Component of Light beam by 90 degrees

    Is there a single optical component that will do the following? Do nothing to the horizontally polarized input, but rotate the vertically polarized input by 90 degrees, so that both beams finally emerge with the same polarization? I am looking for a single element that will replace the...
  19. A

    Can photons act as medium for sound?

    can light collectively or individual photons act as a medium for propagation of sound waves?
  20. I

    B Questions about the highest-energy photons detected

    https://en.wikipedia.org/wiki/Very-high-energy_gamma_ray Wikipedia says this about Very-High-Energy Gamma Rays: This is approximately equal to wavelengths between 1.24 × 10−17 and 1.24 × 10−20 meters, or frequencies of 2.42 × 1025 to 2.42 × 1028 Hz. Such energy levels have been detected from...
  21. Marrrrrrr

    I Coherent Photons: Uncertainty and Locality

    I understand that laser is based on the phenomenon of coherence. But I wonder how, say, two photons could be said to be located at the exact same spot when their locations could not be precisely defined due to the principle of uncertainty. To specify, which between the following two would be...
  22. C

    Can entangled photons be used to explore black holes?

    Gabriela Lemos and her team successfully entangled photons. Would it be possible to explore the interior of a black hole by letting one of the entangled photons enter beyond the event horizon and observe the impact on the other?
  23. Daniel Petka

    B Quantum Tunneling and Heat Distribution in Laser Cavities?

    What if I replaced the half transparent mirror in a laser cavity with a 100% reflective mirror? -would all photons escape due to the quantum tunneling effect? -would the mirrors melt? Thanks :biggrin:
  24. Daniel Petka

    B What is the amplitude of light?

    Like every wave, light has a frequency and an amplitude. So far, I know that the frequency equals to the photon energy and the amplitude to the amount of photons. My question is: how big is that amplitude and does it matter? Thanks for every reply :wink:
  25. N

    I Are two entangled photons described by the same wavefunction?

    Are two entangled photons described by the same wave function or wave function shape? Heres an example... Say for example, we have a laser in TEM01 mode that is shooting individual photons (this mode as two distinct maxima). Then the individual photons are going through a BBO crystal to become a...
  26. J

    Do photons carry magnetic force?

    I understand that in the standard model, photons are the carriers of the EM force. Does that apply to a normal bar magnet? I.e., when you sprinkle iron filings near a bar magnet and the filings align along the field lines, does that mean that photons are being exchanged between the magnet and...
  27. B

    Calculating Minimum Energy of Photons in Ionization and Relativistic Corrections

    Hi at all! I need one more help from you. 1. Homework Statement 123.4 eV photons ionize further a rarefied gas of ##B^{2+}## ions. A small fraction of electrons emitted in this process is immediately captured by ##B^{3+}## ion, going to occupy the states 2p, 3p, and 3d. Calculate the minimum...
  28. C

    I Do photons that carry orbital angular momentum have mass?

    It is known that particles with rest mass cannot travel at the speed of light. Can we also say that particles that travel at subliminal velocity, like these OAM photons do, have mass? It has been demonstrated [1] that these beams can be thought as made of photons that posses intrinsic OAM, and...
  29. B

    Neutral Pion Mass from Its Decay into Two Photons

    Homework Statement A ##\pi^0## meson decays predominantly to two photons. Suppose the energies (E1, E2) and angle (##\theta##) between the emitted photons are measured. Find an expression for the ##\pi^0## mass in terms of E1, E2, and ##\theta##. Homework EquationsThe Attempt at a Solution...
  30. J

    I What is the Waveform of Virtual Photons?

    My understanding is that the wavelength or frequency of an individual photon cannot be reliably measured. For example, if a beam of monochromatic light is filtered so that only one photon at a time enters a spectroscope, each photon may land anywhere on the screen. Only when a statistically...
  31. C

    I Photon Energy Increase Under Gravity: Explained

    Why does the energy of a photon increase when falling into a gravitational field ? If we use the equation E=hf, then the energy of the photon increases, but I understand that we also need to add the potential energy to find the total energy. Et=Ep+hf. The potential energy decreases by the same...
  32. edguy99

    I Are fixup operations required for entanglement in photon splitting?

    Photons are prepared in a vertical state (90º), split in a crystal and sent to Bob and Alice. Both Bob and Alice will measure 100% vertical and 0% horizontal. Photons are prepared in a 45º state, split in a crystal and sent to Bob and Alice. Both Bob and Alice will measure 50% vertical and 50%...
  33. P

    A Do all photons really follow all available paths?

    Quantum electrodynamics "states that any particle (e.g. a photon or an electron) propagates over all available, unobstructed paths and that the interference, or superposition, of its wavefunction over all those paths at the point of observation gives the probability of detecting the particle at...
  34. B

    How can a pi_0 meson decay into 2 photons?

    Homework Statement Earlier my teacher put this equation on the board, π_0 --> γ+γ It appears to work until you consider spin. Homework Equations a quick google search reveals that the spin of a photon is +1 and the spin of a pi meson is 0 The Attempt at a Solution This appears to work for...
  35. Comeback City

    B Centripetal Acceleration of Photons?

    This question came to mind from the thread... https://www.physicsforums.com/threads/can-a-photon-be-accelerated.901484/ Knowing... 1) Strong gravitational fields create strong curves in spacetime 2) Light traveling through strong gravitational fields get curved along the spacetime 3)...
  36. DoobleD

    B Are photons the wave sources of Huygens-Fresnel principle?

    1 - Huygens-Fresnel principle states that every point in a light wave is itself a wave source, 2 - light is made of photons, 3 - photons have a wavelength, they are QM objects. Hence my question : are photons the actual wave sources in Huygens-Fresnel principle ? Or are those two...
  37. J

    Conservation of energy in refraction

    Hello, This has been bugging me for some time now, so I would be interested to see what I have been missing so far. Imagine a single ray of light (made up of many photons) hitting a perfectly non-absorbing (for this wavelength of light) spherical dielectric object, which has finite mass. The...
  38. T

    I Bose-Einstein distribution for photons

    When computing the probability distribution of bosons, why is A = 1 for photons? Does this not imply that photons will have an increasingly high probability of being present as E approaches 0? What is the significance of such a situation?
  39. B

    Number of photons in a given space

    Because they are massless, could you theoretically fit an infinite number of Photons into a small space? There is probably an equation that proves this wrong but I'm curious.
  40. J

    I Difference between Scattering and Emission of Photons

    Hello, Ok, so I’ve been searching about this for a while and there seems to be a difference in explanations that confuses me. What concerns me is the exact detailed physical mechanism that makes particles cause a scatter of a photon and an emission of a photon. The Wiki page about scattering...
  41. akvadrako

    I Single photon entanglement

    I've recently been reading about entanglement between two spatial modes of a single photon. It's a little over my head and there is one aspect about it that I'm particularly unclear on, which I was hoping someone here might be willing to shed some light on it. The basic setup is described with...
  42. R

    I Estimate the number of CMB photons in 0.25-litre

    i know that the total energy density of a blackbody radiation is E= 4 segma/C * T^4 so how i can use this to find the number of the CMB photons in 0.25 litre?
  43. B

    Physics: Are Photons Considered Objects?

    Are photons(when described as a particle) considered objects? You can't see them on an individual level, but they hit you and interact with gravity. Might be a silly question, but was just debating it with a friend.
  44. J

    A The Mystery of Photoresist Ridges: A Question About Light and its Behavior

    Let me start off with a reference: http://www.lithoguru.com/scientist/lithobasics.html This is an article about the field of lithography and microlithography. Before anyone dismisses the importance of this technology , let me point out that it is used to make EVERY integrated circuit and...
  45. FallenApple

    B What is the Mass of Photons According to QFT?

    So according to QFT, fundamental particles are just excitations in their underlying fields. So the photon would correspond to an oscillation of the EM field. But it takes energy to excite this field. And E=mc^2. So therefore the photon should have some mass. Energy was put into create it. That...
  46. Vibin Narayanan

    I Do photons of all energies exist?

    If a quantum sistem can vibrate only in discreate energies, can there exist photons of all energies (All frequencies in the real number line)?
  47. CassiopeiaA

    I Exchange of photons inside atoms

    I was reading Feynman Diagrams and stumbled upon this query: If the electrons and protons interact by exchange of photons, does the electron inside an atoms also interact with the nucleus with a similar kind of exchange?
  48. T

    I Confused about emission/absorbtion of photons

    Hey! I have classes of physical chemistry (also had physics in high school) and have some misconceptions about how quantum mechanics work. The question I want to ask is: what's the difference between luminescence (fluorescence/phosphorescence) and (not sure about terminology here) concepts of...
  49. David hzf

    Photons, Protons, and Electrons

    Photon is the force carrier for charged particles. Questions: 1. Are photons generated between proton and electron in an atom? 2. If yes who generate the photon? The proton or the electron? 3. If yes, quantum mechanically, the electron is never at a fixed position, what is the photon doing...
  50. X

    A Why are photons described by real field?

    Why are photons described by real field, while electrons by complex field?
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