I just learned that, according to Einstein's relativity theory, time reaches zero for an observer (light) when traveling at the speed of light, so everything is supposed to be at the same place in the universe for light. But why does it take 8 minutes for light to travel from the Sun to Earth? Is it because we are observing it from earth? At 300.000 km/s distances in the universe are hardly zero? I can't seem to grasp this.
[Physics] If traveling at the speed of lights stops time, why does it take light 8 minutes to reach Earth
inertial-framesobserversreference framesspecial-relativitytime
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This kind of question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.
The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.
Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)
What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.
You have two facts.
The observer standing on Earth sees the light ray as taking eight minutes to cover the distance from the Sun.
The ray itself sees the universe as infinitely compressed so that no time elapses on it's travels (over any distance).
Both of the facts are correct. Both frames are equally valid, and physics works in both (though the zero proper-time frame of the light ray is rather boring).
The same phenomena can be observed in a less extreme way in the decay of muons created by cosmic rays.
Best Answer
Light travels at the speed $c$ this speed is finite and with out using any relativity we can calculate the time it takes for something travelling at this speed to reach us: $\text{time} = \frac{\text{Distance}}{\text{speed}}$ or $ t= \frac{d}{c} = \text{8 minutes}$ in this case.
For a person travelling very close to the speed of light with velocity $v$ from the sun towards the earth time does slows down, and he goes past the earth in a matter of seconds. But for us time doesn't slow we see the person with almost the speed of light and the time it takes to reach us is again $ t= \frac{d}{v}$ which will be almost 8 minutes but slightly longer.
Now for light you say time freezes completely this is not really accurate, but for arguments sake I will accept it: Then the same logic applies as before. For light it seems that zero time has passed but for us it is still 8 minutes.
This might seem like a paradox, but time is relative in Einstein's theory of relativiy.
Note that your argumentation is backwards, "time reaches zero for light, because everything is at the same place". While the more "correct" way to say it would be that the photon does not experience time and therefore everything seems to be at the same place.
Hope this helps