Physics in Layman's terms #1 - Einstein's relativity theory part 1

Do you understand Einstein?

I think everyone agrees that Einstein is one of the biggest physicists of human history. His greatest achievement was the idea of relativity and the theory behind it. But what is this all about? In layman's terms without gigantic formulas and a terminology that barely no one understands? Is it possible to explain in 'relatively' easy words? For this post, we will stick to the first part of the 'special' relativity theory ('time dilatation') which for instance is the easiest one to explain. I will continue with space warp in part 2. So... Easy to explain?

Well, let's give it a shot. I am pretty sure you will understand everything and be 'enlightened'. We need one important assumption to mention before we take off:

Light is always moving with the exact same speed. It does not depend on any observer position.

Light, space and time - and relativity. This is what everything is about in Einsteins theory of relativity. In order to understand the first of the two basic principles of Einstein's theory we do some mind experiments. Nothing special at all.

Here we go.

Experiment (1):

Imagine you are standing in a moving train with a basketball in your hands. The actual experiment is nothing more than throwing the ball to the ground and catching it once it has bounced off the ground in exact the same position where it was released. Let us keep it simple and say 1 meter above the ground. Pretty intuitive. While you do this, a friend is stands next to the rails and watches you.

You see:
How does the flight path look like for you? The answer is easy. A straight line. Just like it would if you did the same experiment on a train which stays inside a station i.e. a train which is not moving at all. You and the train have the same speed and it will always be that straight line. What about the total length of the flight path? 2 meters.

Your friend sees:
The flight path that your friend looks at is different. Like on the picture below, he would describe it as something like a 'V'. The faster the train moves the more compressed is this 'V'. The total length that he observes is apparently greater than 2 meters. Let's assume it is 3 meters.

Experiment (2):
Now we change one thing: You do not throw a ball right now but we are doing the experiment with a laser pointer and a mirror. Let's assume that both you and your friend can observe the light moving.
This will lead to analogous results like in experiment (1).

Important:
For you, the distance the light travelled is 2 meters, for your friend more than that (let's say 2.1 meters). Both you and your friend see the light arriving where it started at the exact same point of time.

Let us think about this.

• the timeframe is constant
• the speed of light is constant
• the distance is 2 meters for you and 2.1 meters for your friend

This does not work in the world you and I know. We know from Newton that

• speed = distance / time

How can the speed of light always be constant when it travels 2 meter for you and 2.1 meter in the exact same time?! Well, here we are. The only valid answer is that the time for you goes by slower than for your friend. That way everything works finely together. Crazy thing, huh? Time does not seem to be a constant moving forward thing. It's more like

'the faster you move the slower is time for you compared to an observer who doesn't move.'

This phenomenon of time relativity is called 'time dilatation'. Congratulations. You got the first part.

Your face expression right now:

(The very interesting part is that this was actually tested. If it was true that time goes by slower when something is moving, some physicist thought: Let's compare 2 (atomic) clocks: One clock on the ground and we fly the other one around the world in a plane. If Einstein's theory was right, the 2 clocks should not be synchronous any more. On the 'ground clock' it is later than on the plane clock. And surprise - this is true and was observed. We though do not recognize this < 0.000000001 sec time difference because of the relatively low speed of the train compared to the speed of light.)

The next part will be about space warp and crazy mind experiments that led to pioneering results.

Hope you liked it. Thank you for reading!