When Einstein first published his special theory on relativity people were flabbergasted. It couldn’t be right, they thought. How can the sequence of an event differ if you observe it from a different frame of reference? How is it possible that I see event A happening before event B, but someone else can see it the other way around?

(First watch the video above)

#### Recap:

Observer A stands on a train platform. A train passes by. Lightning strikes simultaneously the front and the back of the train. Because the light reaches her retina at the same time she concludes both strikes happened at the same time. However, her friend observer B sitting in the train is in the frame of reference of the moving train. Since the lightning strike that hit the front of the train reaches her retina earlier than the lightning strike that hit the back of the train, she concludes the strikes did not happen at the same time. The front strike was earlier. So, which perspective is true? What happened, really?

### Going back in time

Einstein proposed a 4th dimension in our reality. Next to our three-dimensional space, he added another one for Time. Only with this dimension, he thought, could we explain what actually happened. How can things happen at the same time, and at the same time, don’t happen at the same time?

According to Einstein, time moves slower for somebody in motion. Of course motion here is always motion relative to something else. So the faster you go, the slower time is going also! And if you can find a way to move faster than the speed of light, something we call now the universal speed limit, you would actually catch up light from the past, and thus actually be in the past!

His solution, then, is that both frame of references are true. Simultaneity is relative to your frame of reference!

## What are the implications of this?

1. You never see something at the moment it happened. Think about it, if light takes time to travel, you don’t see things how they really are, but how they were in the past! We could place a large mirror a lightyear away from the earth, and if we would wait for two years, we could look back into the past! When you look at the stars at night, you don’t see stars how they are right now (whatever that means, see #2) but how they were millions of years ago! For all we know, they could have been wiped out already. Of course this also holds for looking your loved one in the eye, seeing them how they were 667.1 picoseconds ago!

2. Every different frame of reference, this means YOU, experiences something else. This is fundamentally ingrained in our physical reality. While these differences are extremely small in ordinary life they could lead to weird paradoxes like the twin paradox.

3. When you travel in an airplane, you age less, relative to other human beings down below on earth, because time moves slower. If it were possible to go as fast as light you wouldn’t age at all!