[deleted]
Does Gravity Travel at the Speed of Light? (1998)
71–80 of 139 posts
Re: Does Gravity Travel at the Speed of Light? (1998)
#72Earlier quoted context omitted.
That... doesn't contradict anything.
> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. That's not true. Our ability to measure the "speed of gravity" gives us measurements that say "the speed of light": > the actual measurement confirms that the speed of gravity is e…
This:
>when an object is dropped, it begins falling instantaneously.
Is answering a different question than this:
>the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%.
It is true that things drop immediately when you let go of them. Because they're already in the gravitational field. It is not true that when I drop something, a gravity particle from the surface of the earth (say 1km away) comes up and gets my apple, and until that gravity particle reaches it, the apple magically is suspended in the air. RRC covered this in the post.
It is the changes in the gravitational field that propagate at the speed of light. So if the sun vanished, which it can't, we wouldn't be flung away for 8 minutes.
Re: Does Gravity Travel at the Speed of Light? (1998)
#73This is the winning comment from the reddit discussion from /u/RobotRollCall * This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe. But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move…
They seem to not understand the issue or concepts very well or are pretty bad at explaining. The concept people might think you walk off a cliff and suddenly you're waiting for gravity from the ground to come up is bizarre. No one thinks that. Most people would think the gravity is the same as when you were one foot the other way on dirt. There's no instantaneous since nothing has changed (Except a bit of dirt below…
It's starting from basic ideas. This was posted in ELI5 or askscience. Talking about what "everyone thinks" about gravity, a complicated science thing, is silly.
Your post boils down to, you don't like how cute it is. OK.
Re: Does Gravity Travel at the Speed of Light? (1998)
#74Earlier quoted context omitted.
> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. That's not true. Our ability to measure the "speed of gravity" gives us measurements that say "the speed of light": > the actual measurement confirms that the speed of gravity is e…
This: >when an object is dropped, it begins falling instantaneously. Is answering a different question than this: >the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. It is true that things drop immediately when you let go of them. Because they're already in the gravitational field. It is not true that when I drop something, a gravity particle from the surface of the…
That's all true, but that's not what I quoted. RobotRollCall's comment said we can't measure the "speed" of gravity, but in the article linked, we do precisely that.
Re: Does Gravity Travel at the Speed of Light? (1998)
#75This is the winning comment from the reddit discussion from /u/RobotRollCall * This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe. But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move…
> Yes, it's the Wile E. Coyote problem. Wile E. Coyote runs off a cliff, floats in mid-air long enough to hold up a sign that says "Help," then begins to fall.
Clearly that's an exaggeration. But just how much time does elapse, in real life, between stepping off a cliff and beginning to fall?
Meh, not so much. One is already in a gravitational field before one falls off a cliff. The time it takes you to start falling has more to do with propagation of forces inside your body, which would happen at around the speed of sound.
https://www.youtube.com/watch?v=wGIZKETKKdw
The speed of propagation of gravitational waves would be more applicable to something like blowing up a planet. What if someone could blow up the earth in such a way that all the debris flew away from Wile E. Coyote at the speed of light? (And also that he has the powers of Superman and would be unaffected by radiation and vacuum.) How long would it take for him to notice that his weight has started to change?
Re: Does Gravity Travel at the Speed of Light? (1998)
#76Here's another way to understand it. The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted". The explanation: Just like the object itself is moving, the gravitational field (or electric/magnetic field) is ALSO moving! It moves at exactly the same speed, and in the same direction as the object which created it. It's "d…
> The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted".
[citation needed]
My understanding is that it points to where the object was, and that the change propagates at the speed of light as gravitational waves.
Re: Does Gravity Travel at the Speed of Light? (1998)
#77Earlier quoted context omitted.
How about "Yes, gravity travels at the speed of light"?
I like this better: "All of our theories say that yes, gravity travels at the speed of light, but nobody was ever able to test it on practice."
> All of our theories...
Not so much.
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_rad...
Re: Does Gravity Travel at the Speed of Light? (1998)
#78I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?
How would that modulate a gravity field? If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.
You could only modulate gravity as fast as you could move energy close to you or far away from you. An implausibly powerful gamma ray flashlight flicking on and off would let you do this. Get the beam powerful enough, and it will have a gravitational field.
Re: Does Gravity Travel at the Speed of Light? (1998)
#79Earlier quoted context omitted.
Now, here's the thing that confuses me. Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll just call it "the big mass". They can, at will, fly it back and forth between two distinct positions we'll call 0 and 1. And let's say some distance away - a light-year, say - they have an facility where they can measure with excruciating precision the force of…
I think this is actually a brilliant exercise in though. I have no idea what the answer is, and it's likely impossible to ever cancel out the effects of every other source of gravity within a 1 light-year radius, but it seems like in theory, if the force due to gravity propagates faster than the speed of light, your suggestion may hint at a method of communication that exceeds the speed of light.
No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.
Re: Does Gravity Travel at the Speed of Light? (1998)
#80This is the winning comment from the reddit discussion from /u/RobotRollCall * This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe. But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move…
> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. Not after a very small interval of time, but absolutely instantaneously. As in zero time elapses between dropping and falling.
> This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.
I don't know a lot about physics and relativity but this actually seems to me how I'd expect it to be. I don't understand why people expect that there has to be an interaction before gravity starts, or that gravity is directed linearly between two masses (you and the earth). It's already being emitted, and you're stepping out off the cliff into a gravity wave that is already there. You're always in that wave. Now the thing beneath you preventing you from being pulled in is gone. No initial setup was required.
Imagine the same example with a light beam instead of gravity. At the foot of the cliff is a ball as bright as the surface of the sun, radiating light in every which way. If you step off the cliff, it doesn't take a few nanoseconds for the light to hit you, because you're stepping into a lightbeam that is already there. No interaction is necessary to set it up.
I dunno. This seems like the intuitive way to think about it to me. Anything else would seem strange. Maybe I'm misunderstanding?