Why does gravity travel at the speed of light and not instantly?


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

>Gravity isn’t a particle like light is

Well we don’t know of a particle for gravity, but many (if not most) physicists believe there is something called the “graviton”, a quantisation of gravity the same as the photon being the quantisation of electromagnetism.

The speed of light is the speed at which anything massless travels through space. It can be thought of as the maximum speed at which things can affect each other according to relativity. Changes in gravity are just as subject to Einstein’s relativity as changes in any other field.

It is instant, at least as instant as anything can be in our universe.

The speed of “light” is misleading. Light moves at the speed of *causality*. We call it the speed of light because light was the most prominent phenomenon to move at this speed when the term was coined, and it’s easier to talk about something people can see than not.

Gravity moves at the speed of causality because thats the fastest any phenomenon can propagate in our universe.

The speed of light is actually the speed of causality itself in our universe. It is not just as fast as any particle can go, it is as fast as anything at all can go.

This includes any sort of cause and effect and any interaction things may have with one another.

Instantly is not really a thing. Things can happen in two places at the same time, because “at the same time” is not really a thing that makes sense in our universe.

We can pretend that instantly exist in our every day life where distances are relatively short and the difference between “instantly” and what is really happening are small enough to ignore for practical purposes.

The real problem is that our naive understanding of how speed works at all is wrong. What we normally think of how it works is wrong by a little and close enough under normal circumstances but gets really wrong outside our normal experiences.

Also we aren’t 100% sure how gravity really works.

Laws of physics prevent information from traveling faster than the speed of light. Gravity is information.

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.

This thread was the first I heard of ‘the speed of causality’. Here’s a link to an article and video that I found useful

[Link to video about speed of causality ](https://www.sciencealert.com/watch-why-the-speed-of-light-is-not-about-light)

the speed of light does not have anything to do with light, that is backwards. there is a speed of causality in our universe and nothing can go faster than that. light has no mass so it automatically goes to the maximum speed, but if the laws of our universe were different that speed could be higher or lower.