The fact that the universe has a maximum speed limit is very weird in itself. Think about it. It means that whenever you try to move faster something prevents you from doing it even when there are no other forces acting on you. It’s like a limit built into the very fabric of space and time. And it is happening all the time whenever and wherever there is motion, even at lower speeds. That’s when you are walking running or cycling there is something trying to stopping you from going faster. This effect works even on lower speeds. This means that relativity is not something that kicks in only when you get to the neighbourhood of the speed of light. It happens all the time.
Who cares anyways?
At this point you are like Okay so what if I can’t go more than 300 million metres per second. I wasn’t planning to go over 200 anyways.
While it is true that the speed of light doesn’t affect us in our day to day life, it has a profound impact on how we perceive things. More specifically we have to give up the idea that time and the order in which things happen is not universal. What this means is that time and the order in which things happen can be different for different people.
At this moment your brain is like What the @#&*?! Maybe your nonsense alarms went off. But bear with me, I will explain. Sequences of events can totally different to different people. And it’s all because of the speed of light. To understand how the speed of light can lead to such weird things happening, let us imagine a situation where we have a person(let’s call him George) standing in T- pose and holding lasers in both of his arms and shinning them . If we now ask him to measure the speed of the photons from the laser, he will say both are travelling at the speed of light. So far so good. Now imagine you are in space and watching earth go by to your right with George and the rest of the setup on it. Now how fast do you(the person in space) see those two photons moving? If the photons are travelling at the speed of light relative to George and you are seeing George go to your right, your intuition might tell you that the photon on the right is going at speed of light + speed of earth and the one on the left is going at speed of light – speed of the earth. But that is impossible! Nothing, not even light can go faster than the speed of light. Nor can light go slower than the speed of light. So because of the speed limit you and George will both see the photons moving at the same speed relative to yourselves even though both of you are travelling at different speeds. That’s one of the mind-bending things about the speed limit of the universe. It applies to relative velocities not absolute velocities. But wait, it gets even more bizarre. Lets put two target placed exactly at the same distance from George and ask a simple question : The photon from which laser will hit the target first ?
If we ask George, he will say that both the photons hit their targets at the exact same time. But you, in space will see things differently. since the earth is moving to the right the photon in the left will appear to be faster than the one on the right. In other words you will see the photon on the left hit it’s target before the one on the right. Both of you will see a totally different sequence of events. Here’s the bizzare part : both of you are correct. And it works the other way too. Imagine your best friend Alex returning in his space ship in the direction of the earth i.e. to the right, but he is moving faster than the earth is moving. Thus he will see earth moving to his left relative to him. Thus he will see the right photon hit first and then the left one. Now we have three conflicting reports by you, George and Alex.
All of this will probably make your crazy alarms explode. First of all it will mean that there is no single agreed upon description of what happens in our universe. Think of it in another way. We can change the order of thing just by watching them at different speeds. This means we have to give up on the idea that there is a single common history of the universe.
Join us next time as we push this experiment to it’s limits.