What is Photon: 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. entropy1

    Depiction of distribution of photons over time

    I have a question about photons and the Schrödinger equation. Photons behave like particles but also as waves. I understand that this can be described by the Schrödinger equation as a photon having a certain probability to be somewhere. If I understand this correctly, I take it that there are...
  2. fricke

    Isothermal Compressibility of Photon Gas

    I am really stuck at this question. I tried to get the equation of volume with independent variables P and T, but the equation itself does not give a nice form, and thus I cannot get the derivative of V with respect to P. What should I do?
  3. B

    Impact parameter of a photon in Schwarzchild metric

    Hi, I'm having trouble answering Question 9.20 in Hobson's book (Link: http://tinyurl.com/pjsymtd). This asks to prove that a photon will just graze the surface of a massive sphere if the impact parameter is b = r(\frac{r}{r-2\mu})^\frac{1}{2} So far I have used the geodeisic equations...
  4. B

    Could photons be 2D objects in String Theory?

    String Theory speculates that extra dimensions may exist. Obviously, it would be difficult to describe or imagine that, but is it possible that there are objects or particles that exist observing LESS dimensions. For example, photons travel at c meaning that time travels infinitely slow in for...
  5. U

    Redshift effect And photon energy loss

    I've been searching around the web to figure out why photons shift towards the longer wavelengths as they travel from stars and other light sources but I haven't figured out why they loose energy as they travel ( and after reading some web pages I was told that they don't even loose the energy...
  6. J

    Calculating Total Time Dilation in a Falling Photon Experiment

    This is my very first post, so here it goes. I've had this idea for a while but this is my first attempt at putting it into words. We've all heard of the hypothetical demonstration that tries to put gravity into perspective by pitting a fired bullet against a simple falling bullet. The idea...
  7. J

    Photon Polarization Question

    Homework Statement Linearly polarized light of wavelength 5890 A is incident normally on a birefringent crystal that has its optic axis parallel to the face of the crystal, along the x axis. If the incident light is polarized at an angle of 45° to the x and y axes, what is the probability that...
  8. J

    Understanding photon direction

    I have been trying to better understand the concept of a photon (I know...a thousand threads on this alone) and the direction that it propagates. I understand the time-varying field explanation for em waves, but here is where I break down. If an electric field extends in all directions...
  9. einswine

    Is Lorentz Contraction Specious for Photon Traversal Time?

    bcrowell wrote: "Lorentz contraction doesn't describe what we see. When we see things, that's an optical measurement. Relativistic optics is a whole separate subject. Lorentz contraction describes the results of the kind of elaborate surveying process that we have to undertake in order to lay...
  10. G

    What if a photon interacts with a slower photon in front?

    What happens if a photon, photon b, interacts with a slower another photon, photon a, which is in front? Taking in account that these two photons are identical except for their speed, they are on the same path and direction as well.
  11. W

    Power and rate of photon emission with a given temperature

    Assuming that your surface temperature is 99.1 F and that you are an ideal blackbody radiator (you are close), find (a) the wavelength at which your spectral radiancy is maximum,(b) the power at which you emit thermal radiation in a wavelength range of 1.0 nm at that wavelength, from a surface...
  12. Summer95

    What does "minimum range of uncertainty" mean in QM?

    Homework Statement [/B] The problem on where the photon will hit a screen, after passing through a single horizontal slit. I know the wavelength, slit width, magnitude of momentum of incoming photon (calculated), and distance between screen and slit. Homework Equations I just don't...
  13. T

    Why Is the Photon One-Point Function Zero in QED?

    In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore: I really cannot justify this equation. Can...
  14. G

    How Fourier components of vector potential becomes operators

    Hello. I'm studying quantization of electromagnetic field (to see photon!) and on the way to reach harmonic oscillator Hamiltonian as a final stage, sudden transition that the Fourier components of vector potential A become quantum operators is observed. (See...
  15. F

    Creating a positron and a electron with a photon

    Homework Statement Hi everybody. I have to demostrate that a photon, no matter it's energy, will never be able to create a positron and an electron on it's own. Homework Equations E=Sqrt(m^2+p^2) as long as c=1 Conservation of energy Ei=Ef; initial energy is equal to final energy...
  16. E

    Questions on Photon Excitations: Eigen-Modes & Boundary Conditions

    I’ve seen references on here to photons being “excitations of the EM field”. I have a few questions about those “excitations”. I assume they are eigen-solutions of the 1D wave equation?What are the boundary conditions on the solutions?Is a single photon a single eigen-mode, or a supper-position...
  17. J

    Thrust imparted from a photon?

    If I produced a material that neither absorbed nor reflected light. Or at least did so to an extremely miniscule amount and set it in space and hit it with light. Would there be any thrust imparted unto it? If so would it be significant? My initial thoughts would be that sense it is massless...
  18. I

    Special Relativity: Lenght contraction and a photon.

    Reading an old thread (wich is now closed or i would post the question there) there was a discussion about the size of a photon, and if it was an adequate question at all. The discussion on the other thread couldn't agree on a response. Yet there was some postulates that could work with this...
  19. B

    The magnetic field of a photon

    Do different photons with different frequencies/energies have same magnetic field? Does the fact that all photons have same magnetic angular momentum imply a positive answer?
  20. Q

    Further Reading on Mass of Photon

    Hi, Im in my final undergraduate year of a physics major. I need to prepare a short presentation for "The mass of a photon" in my theoretical physics class. I do not do particle physics at a high level, so I don't have much experience in this field. Can anyone recommend me a source or key...
  21. E

    Can Photons Transmit Negative Momentum?

    Is there any experimental evidence that photons can transmit negative momentum? After all, there is also a negative root: E^2 = (pc)^2 + (mc^2)^2
  22. Ryan Reed

    Quantum Entanglement: Measuring Particles & Retaining Entanglement

    When two photons' spins are entangled, measuring one spin gives you the spin of the other. My question is, after one of the particles is measured, does it still retain its entanglement? Could you keep measuring photon A's spin to get photon B's spin?
  23. Finny

    Why is gravity but not EM 'ripples' in spacetime?

    Why do we say gravity [GR] is a theory about ‘spacetime curvature’ and gravitational waves are ‘ripples’ but nobody uses such a description for electromagnetic fields? Don't EM waves 'ripple' spacetime? For example, one might imagine different types of spacetime curvature associated with each...
  24. H Smith 94

    Can Light Orbit a Massive Object?

    According to Hawking [1] it is posited that light photons at the event horizon of a black hole must cease to move, and remain motionless for the entire lifetime of the black hole. It is also observed [http://dls.physics.ucdavis.edu/~scranton/LensedCMB/a2218.gif] (and calculated) that the path...
  25. G

    Do observables for polarizers at different angles commute?

    Hi. We can write a polarised photon as ##\left|\alpha\right\rangle=\cos(\alpha)\left|\updownarrow\right\rangle+\sin(\alpha)\left|\leftrightarrow\right\rangle##. Trigonometry gives us $$\left\langle\alpha | \beta\right\rangle=\cos(\alpha)\cos(\beta)+\sin(\alpha)\sin(\beta)=\cos(\alpha-\beta)$$...
  26. samsara15

    Where does photon energy go in universe expansion?

    [Moderator's note: Spun off from another thread, where it was off topic. Please start a new thread when you have questions on a new topic.] Photons lose energy when they travel long distances. Where does that energy go? What happens to that energy? Is there any theory regarding this?
  27. entropy1

    Does entangled polarized photon get through filter?

    Suppose we have a creation point C where two entangled photons A en B are produced. 1m in the direction of A is a vertically polarized filter V. 2m in the direction of B is a horizontal polarized filter H. Now, I understand that when photon A strikes filter V, it either gets blocked or gets...
  28. DaTario

    Coherence length of a single photon

    Hi All, Is there any experimental evidence that photons may have attached to itself a coherence length? Best Regards, DaTario
  29. G

    Recoil of a hydrogen atom emitting Lyman alpha photon

    I am interested in what the recoil velocity of an initially stationary hydrogen atom in free space would be when it emits a Lyman alpha photon. I tried to do the calc and got about 3 metres per second which seems rather high.
  30. J

    Can We Track a Photon's Path in the Double Slit Experiment?

    The results of the double slit experiment lead to the conclusion that a photon travels as a wave. Question 1: Is it possible to track the journey of the photon? It seems to me (correct me if I'm wrong) that from the moment we release the photon till contact with the detector we don't know what...
  31. J

    Energy of electron-hole pairs in 3D space

    Dear All: I'm trying to use fluctuation dissipation theorem to describe spontaneous photon emission process by electron-hole recombination in semiconductor material. I notice that all the references using such a method considers the dipole's degree of freedom separately, for example in x, y, z...
  32. F

    Find Optic Laser Material That Responds to Perpendicular & Parallel Light

    I am looking for group of material that behaves according to the following: (1) When a certain laser light passes perpendicular to the surface of the material it enters and exits unimpeded (2) However, when another laser light (with direction parallel to the surface) enters the material, the...
  33. P

    Does light slowly spread out longitudinally?

    Has there ever been any detection of a pulse of light spreading out longitudinally even by an infinitesimal amount? I'm aware of the expansion of space (~74 km/sec/Mpc), but talking about something else. So for example if our light pulse is say one second in duration, then after traveling...
  34. E

    Lagrangian of a Photon: Understanding the Fundamental Particle in Light

    I can't find this in any textbook, so I must not understand something about it. What is the Lagrangian of a photon? Would it be just h*nu?
  35. M

    Concept of Light: The Photon

    Homework Statement If an electron in metallic cesium absorbs a photon of red light (6.6x10^-7) m in vacuum), all the energy is used up in escaping from the attractive forces in the metal. Suppose light with 5x10^-7 is used, what percentage of the photon's energy does the electron retain after...
  36. P

    Why is black hole photon sphere outside the event horizon?

    Homework Statement I am preparing a report on black holes and I recently learned about a phenomenon I was previously unaware of: the photon sphere of a black hole. While reading an article on said occurrence (I have now confirmed this on multiple sources) the photon sphere which is the minimum...
  37. F

    Understanding Photon Polarization with Dirac's Principles of Quantum Mechanics

    Is this statement correct: ? "The effect of making this observation is to force the photon entirely into the state of parallel or entirely into the state of perpendicular polarization." * I don't see how you can talk about how the polarization of a photon changes if the photon gets absorbed...
  38. Christian Grey

    Which theory explains behavior of photon?

    Which theory explains the complete behavior of photon, Quantum mechanics, Quantum field theory, String theory or Quantum Gravity? Like in young's double slit experiment, photon will go through 1st slit or 2nd slit.
  39. P

    What is the collision probability for an energetic photon in free space?

    Hi. Does anyone know the probability of an energetic photon interacting with ions in free space if we know the ion gas density and the photons wavelength? I'm trying to find out if the energetic photon is treated like a particle where its size is relative to it's wavelength. In other words, is...
  40. Y

    Parity of photon in nuclear transitions

    Let's say we have a transition from ##J^P = \dfrac{1}{2}^+## to something like ##J^P = \dfrac{5}{2}^+##. It radiates a photon with some energy ##E_\gamma##. How does one know the parity of said photon? How does conservation of parity work here?
  41. meyol99

    What is the longest wave-lenght of a photon?

    What is the longest wave-lenght of a photon particle in the nature and can it be longer?
  42. P

    Energy Levels in Hydrogen: Calculating Photon Energy & Wavelength

    This school page goes over the basic math of calculating the photon energy and wavelength emitted by an electron in a hydrogen atom. It comes to one photon near 490nm. Is one photon always emitted by atoms? Why can't it be two or more photons at lower energy? Has a fraction of photons ever been...
  43. The Un-Observer

    Photon Experiencing Infinity Over Time: A Relativity Question

    According to relativity, from a photon's frame of reference time is instantaneous, correct? So in an instant a photon would, to its frame of reference, experience being absorbed immediately after its creation, as well as hundreds of years of travel through space in the same instant. Say...
  44. S

    Excitation of two-atom state by single photon : FTL hope?

    This concept is rather interesting : "Excitation of two atoms by a propagating single photon pulse" -- http://arxiv.org/abs/1411.3445 . They say that, in principle, one can tailor an optical pulse so that it will excite two atoms from the ground state (i.e., the |gg> state) so it ends up in any...
  45. S

    Photon Displacement in EM Waves (Amplitude)

    I am curious on what is meant by amplitude in an EM wave. From all the searching I have done the amplitude has something to do with the density of photons in the EM wave, and the frequency with energy of the photons. The part I am confused about is the fact that the wave is oscillating. I...
  46. Julian Blair

    Understanding the Energy of Photon Pulses in the James-Cummings Model

    I'm confused because the James-Cummings model of an EM field interacting with a 2 state atom uses fields of "n" photons of frequency "omega" But, a pulse of time , t = pi/(2*coupling) creates an equally superimposed state, |superimposed states> = 1/(sqrt2) (|1> - i*|2>) . My questions are...
  47. P

    Can a beam of photons accelerate an atom or object?

    Not quite sure how to ask this, but here goes: I think I understand how a photon impacting an atom can increase the energy level of an electron in the atom. When I read about "light pressure", I thought, is there a way for, say, a stream of photons to accelerate an atom by continually impacting...
  48. $H3K

    Does a Photon Experience Time or Space During Travel?

    Hi there, I have some simple questions concerning a thought experiment about photons. Set-up: There are two molecules in the universe and in between them there is empty space. A photon of light is emitted from molecule A and is absorbed by molecule B. From the perspective of the photon, does...
  49. D

    Measuring Electron Recoil from Photon Emission: Experiments & Methods

    Has electron recoil due to photon emission ever been confirmed by experiment? cause I can't find any reference to electron recoil being measured anywhere I look. If it has been measured, what methods do they use?
  50. DiracPool

    Can Electrons Bounce Off Voltage Potentials?

    From my understanding, photons travel at the speed of c in the straightest line possible. Say I take a photon and send it off to the right toward a mirror. It hits the mirror and is deflected in the opposite direction in relation to the angle of incidence. My question is is the photon that is...
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