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I was wondering about photon's interaction with matter, and why photons dont slow down. They seem to always bounce in a straight line at the same speed (I think), as if some force is charging them forward after the bounce. First, what is this? I've heard of experiments where we actually did slow them down, and they can be absorbed and bounced by matter, so I wanted to know if they interact with matter in such a way that we could spin one, like a curve ball thrown by a pitcher, and make them curve their flight path.

Can someone explain this, in English (back it up by math if you need to but I'm not a physicist, I just like to learn about the fundamental ideas)?

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    $\begingroup$ Photons don't move, at all. They are simply quanta of the electromagnetic field and they only exist where you measure them/where an actual interaction occurs. It's the field that "moves". How the field "moves" is very well known. In the classical limit it's given by Maxwell's equations and in the quantum limit we have the equations of quantum electrodynamics, which, of course, can couple to charged matter. $\endgroup$ – CuriousOne Mar 10 '16 at 23:31
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    $\begingroup$ @CuriousOne Hmm, so if you measure the same one in two different places doesn't that mean it has moved? $\endgroup$ – J.Todd Mar 10 '16 at 23:33
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    $\begingroup$ @Viziionary There is no way to even know, the two photons you measured are the same ones, as photons are not individuals but indistinguishable quanta. Being the same is just not a question you can ask about photons (just as you cannot ask whether two units of currency on your bank account are the same). $\endgroup$ – Sebastian Riese Mar 10 '16 at 23:37
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    $\begingroup$ You can't write "Kilroy was here!" on a photon (that is really a quantum state, rather than a particle), so you can't distinguish "them". The entire mental model of little hard balls flying trough the universe is 100% wrong. $\endgroup$ – CuriousOne Mar 10 '16 at 23:38
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    $\begingroup$ @SebastianRiese but couldn't I measure the same location again and check whether the photon is still there? If there is one at one instant, and then at that very precise very small location, in the next very small fraction of a nano second, if there are no photons filling that point of space, can't we assume that photons do move? $\endgroup$ – J.Todd Mar 10 '16 at 23:42
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The lenses in my reading glasses bend the path of photons, as does gravity. In matter photons move at slower speed than $c$. In Bose-Einstein condensates they can even be brought to a halt.

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so I wanted to know if they interact with matter in such a way that we could spin one, like a curve ball thrown by a pitcher, and make them curve their flight path.

Photons interact with matter with the electromagnetic interaction. There can be elastic scattering of a photon of energy E with a charged particle and only the angle will change , but it happens at one point in space and time , not a continuous curve.

So individual photons do not act like a classical ball, which should be expected as they are quantum mechanical entities.

Light , which is composed out of an innumerable number of photons does display a "curve" in space collectively in a lattice, by additions of the behavior of the collective photon ensemble, as in optical fibers.

There also exists gravitational lensing of light :

A gravitational lens refers to a distribution of matter (such as a cluster of galaxies) between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer.

Individual photons are elastically scattering but the light wave displays curvature.

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Adding to Anna's response,

In small scales, photons wont slow down because they are thought of as being relativisitc quantum fields in all their possible interaction channels. However, at large scales (cosmological distances) there is an indication of slowness (delay in arrival time)-- either due to a varying gravitational potential (due to non-flat spacetime fabric as mentioned by Anna) or due to the expansion of Universe itself.

Einstein himself argued that in the absence of gravity and expansion of Universe (which he was reluctant to believe in the latter) light would never slow down or speed up and was able to show it explicitly.But, current frontier research at small-scale, everyday life and large scale are positive after all that they are seeing a varying speed of light. (Related sources: Varying Speed of Light)

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  • $\begingroup$ Yes that is correct. The only thing that distinguishes individual particles is the Pauli exclusion principle so they are only distinguishable by their individual quantum states. $\endgroup$ – Jaywalker Mar 11 '16 at 6:56
  • $\begingroup$ Just because photons are not indistinguishable does not mean they're not individual. Besides how does this answer the ops question? $\endgroup$ – Bill Alsept Mar 11 '16 at 7:15
  • $\begingroup$ Downvote was because you're misunderstanding the way this site is supposed to work. You aren't supposed to present a request for challenge or debate, you need to present your answer purely as a detailed explanation of the answer to the question asked. $\endgroup$ – J.Todd Mar 11 '16 at 8:08
  • $\begingroup$ Dear Jaywalker, Yes any measurement can tell us which one is up and which one is down. But no experiment can tell us which one was No. 1 and which one was No.2 This was meant as indistinguishibility of the particles which is also true for photon. And, apologized for mistaking a comment with an answer. Should have been more careful in submitting it. So, I will be editing my response to better reflect the answer being asked. $\endgroup$ – Benjamin Mar 11 '16 at 18:02

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