It’s finally out, the Snyder cut from Justice League. So what about that part with Flash? This isn’t a spoiler, since Flash does it in other situations – it has to run faster than the speed of light to travel back in time to warn the Justice League of something.
Of course, there are a lot of physics questions to answer, so let’s get right to the point.
What is so special about the speed of light?
It is easy to understand that speed is relative. If you were walking at 1 m / s in a train that is moving at 10 m / s, then someone on the stationary ground would see you moving with a speed between 9 and 11 m / s (depending on how you were walking) . But our ideas about relative speeds are based on our own experiences with things in motion. And here’s the important part – virtually every example of a moving object moves slowly. Yes, this supersonic plane is slow. Even a rocket that goes to the moon is slow. Everything is slow – slow in relation to the speed of light, which has a value of around 3 x 108 Ms. We often represent this speed of light as the constant vs.
And at faster speeds things are a little different. It turns out that no matter what frame of reference you are in, you will measure the same value for the speed of light. OK, let me give you an extreme example so you can see how it works.
Suppose you are sitting on Earth with a flashlight. In your frame of reference (let’s call it Frame A), the Earth is stationary, and when you turn on the light, you measure its speed as vs. Sounds reasonable, right? Now there is another person in a spaceship moving towards Earth at half the speed of light (0.5vs). Let’s call this spacecraft Reference Frame B. From a Frame B perspective, it is also stationary, but Earth is heading towards it at 0.5vs.
But what about the measured speed of light from frame B? Since the light was coming from the Earth, and the Earth seems to be moving at 0.5vs, wouldn’t that make the light seem like it is moving at 1.5vs? Nope. It doesn’t work that way. It turns out that frame B ALSO measures the speed of light at normal vs. This is the key idea of Einstein’s special theory of relativity.
Time dilation and speed of light
Do you know what happens when two different people in different frames of reference both measure the speed of light? Strange things are happening with our perceptions of time. We call this time dilation. Let me explain this to you with a classic example: a luminous clock. Imagine you have a clock and “ticks” are light bouncing between two mirrors. If you are in the same frame of reference (speed) as this light clock, then the time for 1 “tick” will be the distance between the mirrors divided by the speed of light (vs).
Now suppose you see another glowing clock, but this one is in a spaceship (with windows so you can see inside). The spaceship goes very fast, like half the speed of light (0.5vs). You can see the light in the luminous clock moving just vs, since everyone sees light at this speed. But with each “tick”, this light not only goes back and forth between the mirrors, but it must also move forward as the mirrors move with the spaceship.