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As I understand it, if light is traveling at the speed of light, then from it's point of view space is fully compressed in its direction of travel.

Does that mean that from it's point of view, light isn't traveling through space, and has already arrived as soon as it's created?

Does that mean that light knows the future?

Qmechanic
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Rocketmagnet
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1 Answers1

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To address the concern of "knowing", let us assume that Einstein is riding a beam of light (mind you this would be impossible, but whatever).

What is the set of all things Einstein could "know". Well, in a way nothing. Light (I mean Einstein) experiences no time. There is no concept of future past or present. So there is no meaningful sense of "knowing the future" for Einstein.

The only thing that I could entertain could ever be known by Einstein is if, he (the photon) were absorbed; in which case his existence would cease. An existence that lasted all of, well, zero time. So rather than knowing the future or whatever, I would argue that it would be impossible for light (I mean Einstein) to know anything.

InertialObserver
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  • Lightlike paths are null, but they aren't non-existent. You can even use them as part of the basis of a coordinate system: lightcone coordinates, which has 2 lightlike axes instead of a timelike axis. – PM 2Ring Jan 01 '19 at 01:58
  • I'm not saying that lightlike paths aren't existent. What I'm saying is that $\gamma \to \infty$ as $\beta \to 1$ and so if we could put a clock on a photon we would not see a second go by. – InertialObserver Jan 01 '19 at 02:05
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    It is perfectly true that if you could put a clock on a photon it would not tick. It is equally true (and equally relevant) that if you could put a clock on a photon, it would tick exactly 100 times per second. – WillO Jan 01 '19 at 07:29
  • @WillO how can both of those statements be simultaneously true? – InertialObserver Jan 01 '19 at 07:38
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    Assuming the impossible leads to absurd conclusions. There is no inertial reference frame moving at the speed of light with respect to another inertial frame, so it is not possible to compare the passage of time between anything and a photon, the concept of time itself in such an impossible frame is not well defined in special relativity. – Hugo V Jan 01 '19 at 13:01
  • @HugoV I agree, it's slightly heretical to say this. But if one considers the limit as one approaches the speed of light, it would certainly seem weird if there was some discontinuity in the laws of physics that said light would actually experience a nonzero proper time. This conclusion is as ill defined as the concept of infinity, as that's what I am basing my conclusions from. Perhaps we differ on this philosophical stance, but I will continue to draw conclusions given the data we have and make arguments based on that data. That's how things move forward. – InertialObserver Jan 02 '19 at 02:07
  • My newly posted answer here is relevant: https://physics.stackexchange.com/questions/54162/how-does-a-photon-experience-space-and-time/451711#451711 – WillO Jan 02 '19 at 16:16