What would the observer see if he could travel at the speed of light and shot a photon beam at the same time?


closed as off-topic by WillO, StephenG, G. Smith, Qmechanic Apr 25 at 15:34

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  • $\begingroup$ What would the observer see if 3+3=7? $\endgroup$ – WillO Apr 25 at 12:15
  • $\begingroup$ Also, this has nothing to do with quantum-gravity. $\endgroup$ – infinitezero Apr 25 at 12:27
  • $\begingroup$ It does not make sense to ask what the laws of physics say would happen in situations that the laws of physics say can't happen. $\endgroup$ – Solomon Slow Apr 25 at 13:56
  • $\begingroup$ Also, even if you had said, "travel at half the speed of light," those words would have no meaning in the context of your question. Specifically, "speed" doesn't mean anything unless you are describing the relative motion of two or more objects. In the experiment you described, there's just the observer and a laser, and neither one of them is moving relative to the other. $\endgroup$ – Solomon Slow Apr 25 at 14:03
  • $\begingroup$ What exactly cannot happen, an observer traveling at the speed of light? $\endgroup$ – Solomon Apr 25 at 15:28

Short answer: Nothing

Thoughts experiments like this are always problematic because you violate physics but still want a physical explanation. So we have to make the following assumptions: Somehow, the observer and the light emitting device are all massless.

From special relativity follows, that everything that moves slower than the speed of light will always be slower than the speed of light and everything that moves at the speed of light will always move at the speed of light (unless it ceases to exist).

Imagine that you and your friend are somewhere in empty space and the distance between you is closing. From your perspective, it might very well look like you are standing still and your friend is floating towards you. However, from his perspective it can look the same, i.e. he is standing still, and you are the one floating. There is no way to tell the difference, as long as you both are moving at a constant speed. The perspective where you are standing still is called your rest frame. By means of Lorentz transformations, a slow object can always be transformed into its rest frame. Lorentz transformations do transform not only the spatial coordinates but also time. This leads to an effect called time dilation. The faster an object is going, the slower it experiences time (you might have heard of the https://en.wikipedia.org/wiki/Twin_paradox). One way to understand this, that you always move at the speed of light, but it is divided into time and spatial dimension. The following might be a bit crude, but I hope it helps in illustrating. "If you are standing still in space, all your velocity is used to move you through time. If you now accelerate, you take part of your velocity and move it to the spatial dimension. So you now move less in time and therefore age slower." (You will not notice the difference in your frame though).

That being said: You can not find a rest frame for a light-like particle, such as a photon. All its speed is located in the spatial dimensions. Thus it does not "move through time"), so light like particles do not age.

Thus, the observer would be frozen in time and not able to observe anything.

  • $\begingroup$ That perfectly answers the initial question. The linguistics approach that Im taking from a metaphisical perspective only aims at closing the gap between what is imagined and what is known, in the sense that from a knowledge perspective, solutions are first imagined and only then "materialized" in a corpus cognoscendi frame. Carrying on, please assume that the observers are massless, in a singularity sense and that for light quanta time goes by computationally where each step is performed instantly. Would you then say that instead of nothing, the observers could see everything? $\endgroup$ – Solomon Apr 25 at 13:52

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