A photon is pure energy, zero mass. Different photons have different wave length and carry a different amount of energy.   Now, it seems logical that a high energy photon can be split into 2 low energy photons. I don't know if this could happen via some natural decay or under an external influence or not at all.  Please explain, thank you.  P.s. this is not a duplicate of the previous similar question about splitting a photon. The other question presents a photon as a particle, where as this one considers a photon as energy. I think this leads to different lines of thought.
120728
I was wondering if a photon is divisible.   If you look at a photon as a particle, then you may be able to split it (in theory).  Is it possible and how do you split it?
101936
As a first approximation, I don't see how a wavelength of less than 2  could exist. The question is: Are there any other limits that would come into play before that?   For example:   Would the energy density cause the photon to turn into a black hole or something like that? Would the energy of the photon exceed the total mass+energy of the universe?
94177
I'm thoroughly confused about the nature of electromagnetic radiation. Light is supposed to exhibit both wave and particle characteristics. But does that mean that it is both a wave and a particle or neither a wave nor a particle? If the latter, then it is not a "real" particle at all, but only occasionally behaves as one (hence, a "virtual" particle). And if the former, then it is, depending on the experimental setup being used to detect it, a "real" particle, but one that only exists (as a particle) for a limited time, which again, corresponds to the definition of a virtual particle ("a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time" - Wikipedia).  So, if photons invariably appear to exhibit the characteristics of virtual particles, why are they not actually virtual particles (even in a vacuum)?
79481
There are a lot of questions about this topic on this site, none of them answer my question specifically. I have read this question:   I think of the emitted photon as a point particle (but with a polarization vector) traveling in a straight line from the atom to the measuring device.    For some people, a photon is a dimensionless point traveling on a world line (Eugene Wigner's definition of a particle).    The photons do not have a well defined trajectory. The diagram shows them as if they were little balls travelling along a well defined path, however the photons are delocalised and don't have a specific position or direction of motion.    because photons take all paths, but since their underlying physics is &quot;wavey&quot; different paths can &quot;interfere.    This calculation assumes that light simultaneously travels over all possible paths. To what extent this is just a calculational device and to what extent it reflects an underlying physical reality is a matter of opinion.    Indeed, nothing in nature moves around on all possible paths, in reality it's a field that permeates the vacuum which has quantized solutions.  There are mainly two thoughts:  the photon travels in a straight line, and that explains why it only interacts with a certain atom, meaning, that the photon can only be detected once, and the photon will not interact with the other atoms because they are out of its trajectory  the photon takes all paths, and that is why it truly goes through all possible avenues, explaining experiments like interference     Now these lead to two different propagation pictures. On the top, you can see the photon (EM wavepacket) spreading spherically in all directions. On the bottom, you can see a photon traveling from A to B, taking all possible paths. But which picture could be experimentally proven to be correct? The picture on the top cannot explain why the photon misses all the other atoms, that is, why it is not interacting with other atoms, because all atoms are in its way basically (it spreads spherically). The picture on the bottom cannot explain diffraction, because the photons is shown not to spread like a wave (cannot interfere), but just like a billiard ball on different paths. After the question was closed, I am editing to clarify (to reopen), that (as I understand), the question was closed due to the word &quot;really&quot; and what it physically means for the photon to take all paths, or whether the picture on the top (spherical spread) physically describes what is happening. I now revise these words, and the &quot;physically&quot; is meant here to say &quot;experimentally provable&quot;. Question:  Do photons take all paths or not?
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.
47641
I'm just starting to learn physics and I have a question (that is probably stupid.)  I learned that energy levels that the bound electron can have are discrete. I also learned that when an electron transitions from one level to another, a photon with a specific wavelength (energy) is created and released into the wild.  My question is this: are there holes in the electromagnetic spectrum, values of frequency that the created photon can't possibly have?
19580
I'm an engineer and have been reading about PET scanners and how they rely on the fact that a positron-electron annihilation will cause two photons to be emitted at 180 degrees from each other. After a bit of research I am left wondering on a couple of points.  From :     Conservation of energy and linear momentum forbid the creation of only one photon.   Is this simply because emitting one photon would produce a force in one direction without an equal and opposite force?     In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron.   Does it follow then that there is a (lesser) probability that 3 photons can be emitted at equal energy and equal angle of 120 degrees?
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
102114
I have a little trouble with the . I know of the  which states that it is not possible to duplicate any state. One the other hand, I know about the stimulated emission which out of a photon produce exactly the same (wavelength, polarisation, etc...). Maybe the fact is that the excited atom do a measure. (Let's say it can stimulate emission with only one direction of polarisation.) But now if I have a statistical number of atoms with random &quot;polarisation&quot; direction, I should be able to copy any incident photon. I've made a cloning machine. This cannot be true because of the no-cloning theorem. But I can't figure why.
24153
Consider the question, .  The answers say, "an elementary particle" and not much else. They don't actually answer the question. Moreover, the question is flagged as a duplicate of,  &ndash; the answers there don't tell me what a photon is either. Nor do any of the answers to  mentioned in the comments. When I search on , I can't find anything useful. Questions such as,  look promising, but bear no fruit.  say things like, "the photon is an excitation of the photon field." That tells me  nothing. Nor does the , which says:     The photon is the quantum of the electromagnetic four-potential, and therefore the massless bosonic particle associated with the electromagnetic force, commonly also called the 'particle of light'...   I'd say that's less than helpful because it gives the impression that photons are forever popping into existence and flying back and forth exerting force. This same concept is in the  too - but it isn't true. As , "Virtual particles only exist in the mathematics of the model." So, who can tell me what a real photon is, or refer me to some kind of authoritative informative definition that is accepted and trusted by particle physicists? I say all this because I think it's of paramount importance. If we have no clear idea of what a photon actually is, we lack foundation. It's like what :     Photons seem to be one of the foundation ideas of quantum mechanics, so I am concerned that without a clear definition or set of concrete examples, the basis for understanding quantum experiments is a little fuzzy.   I second that, only more so. How can we understand pair production if we don't understand what the photon is? Or the electron? Or the electromagnetic field? Or everything else? It all starts with the photon.   I will give a 400-point bounty to the least-worst answer to the question. One answer will get the bounty, even if I don't like it. And the question is this:    What exactly is a photon?
113642
See the , wherein two  gamma photons are converted into an electron and a positron via a process that's the reverse of . I do not doubt . However I'm less confident about the Wikipedia . It talks about gamma-gamma pair production, and as far as I know it's in line with what some particle physicists say. It says this:  "From  it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes[clarification needed]. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple".  However as far as I know, a 511keV photon does not spend its life magically morphing into a 511keV electron and a 511keV positron. That’s in breach of conservation of energy. In similar vein the electron and the positron cannot then magically morph back into a single 511keV photon. That's in breach of conservation of momentum. Moreover photons travel at the speed of light whilst electrons and positrons do not - a photon cannot spend its life fluctuating into fermion pairs, if it did it couldn't travel at c. Besides, virtual particles are virtual. As in not real. They aren't short-lived real particles that pop in and out of existence like magic. Instead . Which is why hydrogen atoms don’t twinkle, and magnets don’t shine. On top of all that pair production surely does not occur because pair production occurred. Spontaneously. Like worms from mud. All in all, this explanation for pair production is woefully inadequate. A better explanation is required. So:  How does gamma-gamma pair production really work?     I will give a 500-point bounty to the least-worst answer to the question. One answer will get the bounty, even if I don't like it.
10231
Quite a few of the questions given on this site mention a photon in vacuum having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at the speed of light in all frames according to special relativity.  Does a photon in vacuum have a rest frame?
116641
If you have a set amount of space, lets say 10 cubic centimeters, and you would be able to trap photons in there. If you would then add more and more photons to that space, could you then go on infinitely or would you eventually run into a maximum amount of photons that can be in that space?
101619
The wikipedia page "" says:     The electromagnetic force can be described by the exchange of virtual photons.   The virtual photon thing baffles me a little. I get that  are supposed to be short lived, but as photons live for zero units of proper time I can't see how their lifetime can be used to distinguish between virtual and non-virtual.  Another idea I had was that virtual photons are only those associated with the electromagnetic field, non-virtual ones are not. But in this case, I could not see what was wrong with this: If I have a photon detecting instrument it is just detecting the force carrying particles of the electromagnetic interactions between it and the thing I am using it to observe? (even if that thing is a long way away)  Are virtual photons just photons that you don't observe? Or, is there some kind of photon that is not connected with the electromagnetic field? Or something else? Or perhaps there is no concrete distinction to be made?
118354
I was wondering why a laser beam diverges. If all the photons are in the same direction, I would imagine that it would stay that way over a long distance. I am aware that a perfectly collimated beam with no divergence cannot be created due to diffraction, but I am looking for an explanation based on  photons rather than wave physics.
94731
This question can have many answers. For one, I would be enlightened to understand whether two photons can truly create an electron-positron pair, as well I would like to know if a photon is able to change other photon's directional path.  I've read on wikipedia about  and that it is possible to convert two bosons into two fermions, yet I haven't found a single experiment that says this has been done. I as well cannot find anything online that says whether a photon can influence another photon's path.  Finally, I want to just say that my brother has put this idea in my head that photons simply pass through one another without any effect whatsoever, so if you can ingrain a more true to reality understanding of photons affecting one another, that would be phenomenal.
43352
Since electromagnetic energy is carried by photons and moves in forms of waves, does it mean that a single photon when propagating through space doesn't follow the straight path but instead always moves up and down, up and down like a wave. If so another question arises the speed of propagation of light in vacuum is fixed meaning that it will always take the same amount of time for it to travel from point A to point B, but if a photon always moves up and down it will also mean that it travels longer distance than the distance between A and B and so it ill travel faster than light propagates, is it even possible, could you please clarify these concepts for me?