this post was submitted on 11 Feb 2025
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?

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[–] Cutecity@hexbear.net -3 points 4 days ago (2 children)

No, gravity is faster than light. If there was this lag, we wouldn't have stable orbits exactly because of the lag you describe. Wave functions of photons also collapse faster than light when they hit absorbent material.

[–] vrighter@discuss.tchncs.de 5 points 4 days ago (1 children)

wave function (something that does not travel) collapses (something that does not move either) faster than light (themselves?)

this word soup does not make sense

[–] Cutecity@hexbear.net 1 points 3 days ago

I used wave function as a bad form of shorthand for the general properties of the photon, such as the theoretically infinitely extending magnetic and electric fields. Those associated fields stop existing when the photon is absorbed onto a screen. They collapse faster than light can travel. This doesn't ruin much of modern theories, because there doesn't seem to be a way to transfer usable information through this phenomenon.

[–] hedgehog@ttrpg.network 4 points 4 days ago (1 children)
[–] Cutecity@hexbear.net 1 points 3 days ago (1 children)

I don't think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels. Similarly, electric attraction between + and - charges is different from electromagnetic waves being transmitted in the field. It's not light that is "communicating" that attraction.

[–] hedgehog@ttrpg.network 1 points 3 days ago (1 children)

I don't think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels.

I don’t know how you would measure gravitational waves without measuring gravitational attraction.

It's not light that is "communicating" that attraction.

Nobody said it was. The “speed of light” isn’t about “light”. Gravity propagates at the same speed, aka “c.”

This Reddit discussion on r/AskPhysics might help clear up your misconceptions. Notably:

Just to clarify: when people talk about the speed of gravity, they mean the speed at which changes propagate. It's the answer to questions like: if I take the Sun and wiggle it around, how long does it take for the Earth to feel the varitation in the force of gravity? And the answer is that changes in gravity travel at the speed of light.

But that's not what you're asking about. Whenever you're close to the Earth, gravity is always acting on you: it's not waiting until you step off a cliff, like in the Coyote and the Roadrunner. The very instant your foot is no longer on the ground, gravity will start to move it downwards. The only detail is that it takes some time for it to build up an appreciable speed, and this is what allows us to do stuff like jump over pits: if you're fast enough, gravity won't be able to accelerate you enough - but gravity is still there.

I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.

[–] Cutecity@hexbear.net 1 points 3 days ago

I was definitely talking about the first scenario, as is mostly everyone else. I know not everyone admits gravity (gravitational attraction) might travel faster than light as in the "sun moving" thought experiment. I'm not confused, I'm discussing like everybody else. You linked an article about gravitational waves which must transmit through some sort of gravitational field and they might transmit at approximately c as predicted in general relativity. What I believe is that gravitational attraction, so the general effect of the field will be felt as if it acts almost instantly, and that does not contradict anything about the waves in that field. Because the waves in that field are not responsible for the attraction. This is similar to how photons do not mediate the magnetic attraction in magnets even though they are electromagnetic waves. The current theories (which you are pulling from) manage to mathematically explain that in our moving sun thought experiment, the gravitational force coming from the sun appears to "update" instantly as if it's acting from it's actual position without the lag, because of (to my understanding) the curvature of space-time. So I personally can't fight that on mathematical grounds because that's above my understanding. But in the end it doesn't change much of anything to our discussion, because the force of gravity still updates "as if" it was mostly instantaneous and that's the standard model. Meanwhile, gravitational waves do travel at c but are kind of unrelated to the continous force. They are merely fluctuations in that force. Please keep poking and challenging me at that, I'm still wrapping my head around it and will need better and better sources while I'm hyper focusing on it until I move on lol