I have always been interested in Physics, and lots of people say that light is a particle and a wave.  How is it possible? How can a photon (a light particle) be a wave as well, when its a particle?  I have tried looking this up, but I do not really understand the answers, as they are quite complicated to me. If you could answer this, please could you explain it in quite a simple way!
24597
I often hear about the wave-particle duality, and how particles exhibit properties of both particles and waves. However, I wonder, is this actually a duality? At the most fundamental level, we 'know' that everything is made up out of particles, whether those are photons, electrons, or maybe even strings. That light for example, also shows wave-like properties, why does that even matter? Don't we know that everything is made up of particles? In other words, wasn't Young wrong and Newton right, instead of them both being right?
73549
I understand how one associates the spin of a quantum particle, e.g. of a photon, with intrinsic angular momentum. And in electromagnetism I have always understood the polarization of an EM wave as the oscillations of the E and M field, not necessarily being  aligned with the direction of propagation of the wave.     Questions:   But when one talks about the polarization of a photon in Quantum Mechanics, how does it really differ from its spin?  Are they somehow related and what's the physical idea behind photon polarization in contrast to photon-spin? Feel free to use mathematical reasonings as well if you see it fit!
22168
How does light travel, does this not contradict the idea that going the speed of light stops time? Because if going the speed of light stops time and light goes the speed of light shouldn't it be dark?
23955
Do we have any experimental evidence to confirm that light is an electromagnetic wave? Or is it confirmed simply by Maxwell's equations showing a similarity in speed?
51053
Why does light have the speed it does? why is it not considerably faster or slower than it is?  I can't imagine science, being what it is, not pursuing a rational scientific explanation for the speed of light.  Just saying "it is what it is" or being satisfied saying it is 1 ($c=1$), does not sound like science.
96930
Since virtual particles are disturbances in a field, and not particles in any sense, as explained , how is it that true photons arise from them when excited with kinetic energy via the dynamical casimir effect?  While the effect is described , I was hoping to get an answer that touches more with the virtual particle's role in the effect, if it is of sigificance.
85305
In many pieces of literature, light is said to travel like a "wave". Does this mean the light literally propagates through space like a wave as in up and down and so on or does light move linearly through space? Also, if it move linearly, why do we say photons are a wave then?
94921
Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the momentum direction). I know this results from the transverse nature of EM waves, but how to derive this from the internal symmetry of photons? I read that the internal spacetime symmetry of massive particles are $O(3)$, and massless particles $E(2)$. But I can't find any references describing how $E(2)$ precludes the existence of photons with helicity 0.
75699
Is there any mathematical relationship between matter wave (or de Broglie wave) and wave function?  Also, does each type of particle (e.g. photon, electron, positron etc.) have its own unique wave function?
107721
We know that any particle moving at speed of light will have infinite mass.  Since even Light has particles. Then what is the size of the Particle? How to imagine this with the mass of light particles.
63732
I am reading Berkeley Physics Course vol. 4 (Quantum Mechanics) , chapter 4 (photons). (1)  Section 46: book says: consider a typical photon emitted by  the source. It can be regarded as a a wave train of finite duration, spreading out in all directions in space and carrying a total energy $\hbar\omega$. If this is true, a photon emitted by a star which is few light years away, would have spread over a sphere of radius of few light years. Then it is not possible to &quot;detect&quot; that photon using a detector.  Then  why does the book say like this?  In section 47, the book gives same example of photon spreading over light years. But in this section, it says that how the people can use this as a paradox; and clarifies that paradox by saying that classical expression for &quot;energy density&quot; refers to a large number of photons, and not for single photon.  So does photon really spread in all directions?   (2)  In Section 38 book says: almost monochromatic photons can not be split into 2 photons of the same frequency which carry only a fraction of the energy of the original photon. While in section 48, it says (with reference to double-slit diffraction experiment) photon came through BOTH the slits.  Does a photon REALLY goes through both the slits? Doesn't it indicate that a single photon can be split into two photons of same frequency ?   Any help will be appreciated. Thanks
123184
In basic quantum mechanics courses, one describes the evolution of quantum mechanics chronologically. Interference experiments with particles showed that particles should have a wave character; on the other hand, the photo-electric effect showed that electromagnetic waves should have a particle character. So there's a particle-wave duality.  The course continues with postulating the Schrödinger equation, and solves the problems for the wave character of classical particles (e.g. with respect to harmonic oscillators to solve the UV-catastrophe in the description of blackbody radiation). On the other hand, one describes the photo-electric effect extensively with perturbation theory, as the electromagnetic wave acts as a periodical force on an electron. But the electromagnetic wave is still described with the Maxwell equations, and not by Schrödinger.  Suppose history went a little differently: the corpuscular character would still be the generally accepted behaviour of light (as Newton preferred), and one discovered the wave character of photons and electrons simultaneously by doing interference experiments. The laws of Maxwell, Gauss, Faraday and Ampère weren't discovered yet. Schrödinger comes up with his equation, and one describes the wave character of electrons as well as photons with this equation.   Would this equation give a exhaustive description of the electromagnetic (and vector like) behaviour of photons? In other words: is the Schrödinger theory, in some way, equivalent to the Maxwell theory for the description of photons? Or are the Maxwell equations some kind of limit for greater dimensions (like the Newton equations for mechanics)? What is the link between these two "wave character" descriptions of photons?
21899
The question itself is pretty self explanatory. I asked this to my chemistry teacher when he was doing the photoelectric effect while teaching atomic structure, and he just shrugged it off.  One possible mechanism that I thought up, is an atom with a unpaired electron (say hydrogen) to absorb one photon, and jump to a higher energy level, and then just immediately be hit by another photon with a different energy which raises it to an even higher level. Then finally the electron falls back into its proper non exited state releasing not two but only one photon which has the energy of both the initial photons, ideally combining photons.  Is this mechanism correct? Does it occur in nature? Do photons have other ways in which they merge? or do they not merge at all?
73600
I know what anti-matter is and how when it collides with matter both are annihilated. However, what about anti-photons? Are there such things as anti-photons?  I initially thought the idea preposterous. However I am curious because, if anti-photons don't exist, then anti-matter could theoretically transfer its energy to normal matter - through the mechanism of light. Is it right?
69377
I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a  as a quantum of light, which it further explains as some kind of a .     What exactly is a quantum of light?   More precisely, is a quantum of light meant to be just a certain number of wavelengths of light (something like "1 quantum = a single period of a sine wave" perhaps?), or is the concept completely unrelated to wavelengths? In other words, how much is a single quantum?
69880
As a student in a highschool physics class, my teacher has repeatedly told me that photons are massless. Yet, I have also heard from other sources that photons have momentum. If photons were to have momementum, that would mean that they have mass as according to p = mv.  Do photons really have mass?  Also, how would this mass be calculated?
44010
For electromagnetic waves there exists a wave/particle duality: light sometimes behaves as a wave, and other times as a particle (photons).  Does such a duality exist for gravitational waves? In other words would we expect gravitational waves to sometimes behave has particles (gravitons)?