What if it all happens twice as fast

According to Einstein, the faster you move, the slower the time goes by. Would time stand still for me at the speed of light?

And could you, if you get even faster, travel back in time?

This is one of those misconceptions that is easy to come up with when dealing with the theory of relativity. Because it does indeed say: How quickly time goes by depends on the speed. This sentence should not be misunderstood, because one of the core statements of the theory of relativity is: I myself do not notice any difference whether I stay in place or move at whatever speed through space. If I have a watch with me, it always ticks at the same speed for me. But those who observe me from the outside can, in principle, tell a difference. Hence the theory of relativity: If I were to move through space at, say, half the speed of light and you had such a great telescope that you could look at my watch the whole time, which I have with me, then from your perspective the clock ticks much more slowly.

How much does that matter - at half the speed of light?

At half the speed of light, not that much, around 13 percent. That means, if a minute has passed on your watch and you then look at my watch, it only shows 53 seconds. But the closer my speed gets to that of light, the more extreme this effect becomes. The curious thing now is that the same also applies the other way round, because if I move relative to you at half the speed of light, then you also move relative to me at half the speed of light. The theory of relativity says: There is no absolute reference point. So it doesn't make sense to say I move and you stay in place. The only thing that matters is how we move relative to one another. So if I could look at your watch from my high-speed spaceship, it would look like it was going slower to me as well.

But that's a contradiction: If we do meet again at some point and compare our watches with each other, you can find out which one actually ran faster.

Yes, but only because someone has to change direction every now and then so that we can meet. If I fly into space at half the speed of light and then come back, I logically have to turn around somewhere, and then different laws apply. Every change of direction is physically an acceleration. And a law of the theory of relativity also reads: Time goes slower for accelerated bodies. If I fly through space at half the speed of light compared to you and then come back to you in a wide arc, I will therefore have aged more slowly than you. This is the famous mind game known as the twin paradox: A twin flies through at almost the speed of light All, the other remains on earth. When the first comes back after 60 years, he has barely aged, while his brother is an old man. If you ignore all practical difficulties - just the fact that a spaceship could never take so much fuel with it to accelerate to just below the speed of light - then this so-called twin paradox is completely in line with the theory of relativity.

The listener wanted to know what happens at the speed of light. Would your clock then run backwards in relation to mine - could you then go back to earth and be in the past?

And then could prevent my own birth ...? Many have already thought about this idea, because that would in principle be the consequence. But that's exactly where the theory of relativity says: Stop! It just doesn't get any faster than the speed of light. Not only because such nonsense then emerges. It doesn't work mathematically either, because then suddenly the square root of a negative number appears in the formulas. And physically it doesn't work because, according to Einstein, only massless particles can reach the speed of light, such as light particles. But if you were to accelerate a body to the speed of light, you would need an infinite amount of energy. That will not do; And beyond that, of course, it is certainly not possible.