Gravity isn’t a particle like light is, so why is it bound by a certain speed? Why wouldn’t it be instant?
In: 5
Stuff in our universe can only communicate with the stuff basically right next to it. This is a principle called “locality”.
You may have heard that gravity is spacetime bending. So let’s imagine the sun is in the middle of the solar system, sitting there, bending spacetime around it.
Now imagine the sun disappears. Only the spacetime sitting right where the sun was will realize that the sun is gone now, so that spacetime goes back to being flat. But now the spacetime touching that flat part suddenly notices that it’s no longer being pulled to bend, so it flattens out too.
Then the part right next to **that** flat bit notices, and **it** flattens.
This sort of “chain reaction” continues outwards. You can go through the math, and you find that the speed of propagation of this “chain reaction” happens to be the speed of light.
___
Interestingly enough, the same kind of “chain reaction” idea is also how the speed of light was discovered.
Light is electromagnetic waves. It works because when an electric field changes, it tells the magnetic field right next to it, which makes the magnetic field start to change.
But then this changing magnetic field tells the electric field next to it that it’s changing.
So then the electric field starts to change and…
You have basically the exact same scenario where one bit of the universe influences the bit of the universe right next to it, which influences the bit right next to it, on and on.
Again, if you go through the math of this chain reaction, you find that the reaction moves at… the speed of light.
From these two examples, you can see how the speed of light is a kind of universe speed limit. It’s the speed these “chain reactions” of information travel through the universe, and if we accept locality, that means that these chain reactions of information sharing are the fastest any information at all can travel.
Latest Answers