If you had read our last post in which we had talked about the universal speed, you will know that the universal speed limit of light can cause events to be reordered in terms of your speed. If did not read that, you can read it here.
Anyways, getting back to this article, how far can you push back this reordering of things. Till now Alex has been our fastest observer. He reported the right photon hitting first. Now lets imagine he is on a ship that can actually break the speed of light. What happens then? As he begins to go faster and faster, he will see the time taken by the photon to reach it’s target get lesser and lesser. Eventually he will be going so fast that he sees the photon hit the target before it leaves the laser.
But that breaks all logic because it violates causality. What is causality ?
Well causality is basically the idea that effects are caused by causes and not the other way round. In a serious universe(i.e. one in which causality is not there), everything will go wee. Water boils before you heat it, your computer shuts down before you start it, etc. In such a crazy and bizarre universe, it is difficult to understand how things happen and it might be impossible to build reasonable laws of physics. Incidentally this is how we know that the speed limit of the universe is well universal. Two scientists, Michelson and Morley performed an experiment similar to ours, in 1887. They shot a beam of light and split it in two perpendicular directions. Then they measured the time the light beams took to return back to the starting point after bouncing of from a mirror. They found that light took the same amount of time to travel in any direction. And because the earth is moving at some unknown speed relative to the rest of the universe, they concluded that the speed of light is always the same no matter how fast or slow your relative motion is. From this we can conclude that nothing can go faster than light because it would result in situations where causality is broken. And breaking causality is not a minor thing even for first time offenders. The universe tends to take it pretty seriously.
So why is there a speed limit ? Why should the universe care how fast our cars and cats go ? What possible purpose could this serve ?
Can we derive this speed limit from any first principles or make sense of it in any way ? The short answer is that we have no hard and fast rule why our universe has a speed limit, but there is a very good excuse to have one. A speed limit is useful in order to have a universe that is local and casual.
We talked about causality and it seem like a reasonable requirement in a universe. By “locality” we mean the idea that the number of things that can effect you is the number of things that are close to you. If the universe had no locality then things that happen anywhere will have instantaneous affect on you on earth. In such a universe, alien versions of the NSA could , in theory read your messages in real time. Alien scientists trillions of light years away could create death rays to instantly kill everyone on earth. Thankfully we have a rule that limits how fast anything can travel, which means only the thing in your locale environment can have casual connections to you.. That is why we can never have true real time system, because information will have to travel at an infinite speed in that scenario.