ELIF: how is time relative?
In: Physics
Okay, this is a hard question to answer in a satisfactory way since the idea of time being relative is against all of our intuition. However the conclusion came from some rather intuitive assumptions – those assumptions just lead to some wild conclusions. Let me explain.
Special relativity (the branch of science that explains how time can differ for non accelerating reference frames without strong gravity) was highly motivated by some weird results that physicists got in their study of electricity and magnetism. Today we use 4 equations to describe the laws of electromagnetism. We call these Maxwell’s equations. These equations describe how electric and magnetic fields behave in the presence of charge. If you combine them in a certain way, you get something called a wave equation.
Now a wave equation has certain properties. It showed that electric and magnetic waves can be created, and that they travel at a certain speed that we’ll call C. It turns out that this speed matched closely with light. Thus we concluded that light was an electromagnetic wave. However, and this part is important, one peculiar thing about the equation is a speed popped up without there ever being a reference frame explicitly. That is, the equation said the waves traveled at speed C, but it wasn’t clear who or what was measuring that speed. In other wave equations, it was always obvious from context – such as waves in the surface of water moved relative to the still surface of the water – the median through which the wave propagated. No such median was known to exist for light.
Now scientists resolved this by claiming light traveled through some unseen median called the ether. They tried to look for this ether in experiment but failed. Instead, Einstein came in and claimed something else. He claimed that if you were still, or if you were moving in a car at a constant velocity, the laws of physics would be the same. Now, we intuitively can see this. Throw a ball up in a car, and it will come back down the same way as if you were still. But Einstein extended this idea to light. Measure the speed of a beam of light, then you will get the same answer whether you are in the car or not.
However, that conclusion immediately contradicts are intuition. Say, I’m traveling on a spaceship going half the speed of light and I shine a flashlight in front of me. For me, the light would be going 1C. For someone outside, we would intuitively think they would see the beam of light traveling at 1.5C, since it’s in a spaceship that is travelling at .5C. Einstein says that’s wrong. He says that the beam of light looks like it’s traveling at 1C to both users.
Say for the person in the spaceship it takes one second for the light beam to hit the front. How does that look for someone in the outside? The light moves the same distance in that time, but the front of the spaceship also moves. So to them, the light takes longer to reach the front. IThis is weird to us. Because we insisted that there speed of light is the same for all viewers, something else has to budge. And since velocity is distance over time, the thing that budges is how we measure both distance and time.
It turns out to reconcile the reference frames, things that are moving relative to you appear to slow down. Moving things appear to shrink as well.
This is not an easy concept to grasp, because it is not evident in our day to day lives. Hopefully this gives you motivation to understand why time has to budge.
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