When light interacts (non self interaction) , it is perceived as photon (I am assuming this to be so) . So, is photon then a collapsed light wavefunction?
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Related: Do photons truly exist in a physical sense or are they just a useful concept like $i=\sqrt{-1}$? – John Rennie Mar 13 '20 at 18:11
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Also: What exactly is a photon? – John Rennie Mar 13 '20 at 18:13
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Yes it is a good perspective to think of a quanta, example a single blue photo travels with a certain amount of energy and a red one travels with a different amount. An excited atom (depending on its type) will emit only certain energy levels or quanta, some materials will only absorb certain quanta, some will transmit and some will absorb many (like CCD absorbs visible). See the answers below, also the spacetime concept is complex depending on how deeply you want to know. – PhysicsDave Mar 14 '20 at 17:22
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There is no wave function collapse. – my2cts Mar 15 '20 at 10:03
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It is called photon because it has particle like characteristics. It carries for example momentum and spin and can only carry energy in portions. It is also a quantum mechanical vibration of a photon field which exists everywhere in the universe. The excitations of that field we call photons, light or an electromagnetic wave. It is a duality.
J. H
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For photons in vacuum, the answer is quite simple:
The spacetime interval between the place of emission and the place of absorption is zero, that means that both places are adjacent, and there is no place for such thing as a traveling photon.
As an example, we may observe electromagnetic waves of photons which are traveling from Sun to Earth, taking 8 minutes for a space distance of 8 light minutes. The underlying spacetime interval is zero, that means that the particle properties of the photon - in particular the momentum - are transmitted directly from the electron which is emitting the photon to the electron which is absorbing the photon.
For answering your question: only electromagnetic waves of light are propagating, but there are no quantized particles because the quantized momentum is transmitted directly from one electron to the other, without need for a traveling particle.
Moonraker
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You're suggesting that the momentum is transferred instantaneously, faster than the speed of light/causality? – electronpusher Mar 13 '20 at 18:16
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No, the time interval is 8 minutes, the space interval is 8 light minutes, but the spacetime interval of all lightlike phenomena is 0, according to the metric $$ds^2 = dt^2 - \frac{dx^2+dy^2+dz^2}{c^2}$$ – Moonraker Mar 13 '20 at 19:02
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Ok, thanks. Are you saying that the momentum is transferred at the speed of c, but it is not carried by a traveling particle? Rather, the momentum is carried by an EM wave (which must not be thought of as composed of photons in this situation)? The phrase "transmitted directly from one electron to the other" is confusing me. – electronpusher Mar 13 '20 at 20:16
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This answer is not correct, in case there is elastic scattering inbetween emission and absorption, like if we put mirrors inbetween the Sun and Earth, and keep the photons there for a certain amount of time. Every time the photon bounces off a mirror, it puts radiation pressure on the mirrors, that is momentum transfer. So the momentum of the emitted photon might not be transferred directly between emission and absorption. – Árpád Szendrei Mar 13 '20 at 23:45
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Yes, this is correct - for any kind of interaction with matter we need quantum mechanics. – Moonraker Mar 14 '20 at 06:17
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@Javier, your statement seems to require a bit more of information. Two electrons whose spacetime interval is zero should be adjacent in spacetime. – Moonraker Mar 14 '20 at 17:43
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1@Moonraker No, that is not true. The spacetime interval simply does not have this property. Also, even if it were true, it has nothing to do with the question, which is about quantum effects which apply for all particles, no matter the speed. – Javier Mar 14 '20 at 18:13
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This is totally wrong: "there is no place for such thing as a traveling photon". – my2cts Mar 15 '20 at 10:04
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Does light travel “quantised”?
Choose the one you like best from the many linked answers. Or in other words, blame yourself if you are interested in something that is not observable.
Or, is it ... quantized only when it interacts ,i.e, undergoes measurement?
Clear answer, no:
- In the photoelectric effect it is observed or better measured as a particle.
- In the theory of emission of photons from excited subatomic particles it is treated as a quanta, later called photon.
- In the case of deflection (slit and edge experiments, prism) it is treated as a wave.
In all cases an interaction with the measurement apparatus takes place. What happens between the emission and the deflection or absorption is pure speculation.
HolgerFiedler
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