Relativity Theory for Non-Physicists

(Original date: September 15, 2014)

The best part about science is, anybody can do thought experiments. I thought I would share my understanding of relativity theory — as a non-physicist. I promise you there will be no equations. Not even the one you (probably) have in mind right now.

Relativity existed centuries before Einstein

This might come as a surprise, but the concept of relativity was brought up, way before Albert Einstein, by Galileo in the first half of the 17th century. Like many breakthroughs done in science, it can be very simply stated: Have you ever experienced being in a train, full stop at a station, and you are looking at another train through the window? At some point, you feel like your train is departing, when suddenly, looking at the opposite window, you notice that the platform is inert and it is actually the other train that is moving? This is relativity.

So, imagine you are on a train. This train moves with constant speed and in a straight line. All windows in your compartment are closed, curtains are down. The train has such good suspension that you do not feel the train rolling.

Then, whatever you try out, you will not be able to tell the speed of the train, and you will not even be able to tell whether it is moving or not. You can release a ball, and it will fall down vertically. You can throw a ball, jump in squares, yell and measure the echo — everything will be as if the train were not moving.

Relativity states that whatever train you take that fulfils the criteria above, the laws of physics will be the exact same. Except Galileo used ships instead of trains.

Fiat Lux

There are moments, in many scientific disciplines, when it feels like there is not much more to discover. I think it was pretty much how physicists felt at the end of the 19th century. Usually though, there is always something that “doesn’t feel right”. And in this case, it was light (more generally, electromagnetic waves).

So, let us go back to the train and switch on a light torch. Light has a speed. For example, it takes 8 minutes for a sunlight beam to reach you. So, normally, if you orientate the light torch towards the front of the train, its speed would add to that of the train. It has to — if it did not, then you would notice that the train is moving.

The problem is, this is not what happens. If you open the curtains and somebody outside the train watches the light, they will see it moving at the same speed as if it were their own, immobile light torch.

Einstein’s discovery

As always when there is a contradiction, something has to be wrong. And it turned out that, as Einstein discovered, what was wrong is that speeds do not add up. On the other hand, the speed of light is constant and will be the same regardless of the constant-speed train you are taking. In other words, the relativity principle still holds: only the laws of physics got a 2.0 upgrade.

The consequences of this are impressive, though:

Imagine one person in a (very-high-speed) train sees the lady at the station café swallowing her tea, and then the station master whistling another train’s departure.

Another person, in a different (very-high-speed train), might actually first see the station master whistling and only after that, the lady swallowing her tea. So the order in which things happen may differ depending on the observer.

Causality

The order in which things happen may differ depending on the observer… or not. Sometimes, the order in which things happen is really independent on the observer. If you put sugar in your coffee and then drink it, nobody will see you first drink your coffee and then put sugar in it. In this case, one can say that the order in which you interact with your favourite coffee is absolute. In a way, relativity theory gives a theoretical framework (light cones) to support causality.

Time Travel

So, two events (each at a given position, at a given time) can either have a relative order or an absolute order. There is a criteria to tell: if starting at the position and time of the first event, you can reach the other event at a speed less than that of light, then the ordering is absolute.

If navigating from an event to the other requires a speed faster than that of light, then ordering is relative. You might remember that, a while ago, at the same place the world wide web was invented (on the other side of the Röschtigraben), neutrinos were (thought to be) detected at a speed faster than light . Why was it such a big deal?

There are two events here: the departure of the neutrino, and the arrival of the neutrino in Italy. Put simply: since the ordering of the departure event and the arrival event is relative (because faster than light), then there has to be an observer (say, the lady sipping her tea at the station café) for whom the arrival takes place before the departure.

So, if you manage to travel faster than light some day, please give a warm hello to the dinosaurs from me.

Credits

Galileo Galilei, Albert Einstein, Hendrik Lorentz and many others. Who would not like to drop by and have some coffee with them once we get faster-than-light travel?