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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#61

Post by wnoise »

ggchappell wrote: post #48
wnoise wrote: post #44

Earlier quoted context omitted.

> Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predicted" and the direction of the force is adjusted accordingly - as long as a body is not accelerating, a purely attractive or repulsive force will be pointing at its current position, not its position 8 mins ago. Absolutely correct for E&M. This is because it's a vector field, and the velocity o…

> For gravity, it goes one better -- this is a tensor field (hence the notion of gravitons having to be "spin-2" if they were described by a quantum theory), and the acceleration is encoded as well, .... Well, that's mind blowing. Where could I go to learn more about this?

Other than "graduate textbooks", I'm not really sure. Jackson does cover the E&M case pretty well (EDIT: in chapters 11 and 14, note especially sections 10.11 and 14.1), but I'd combine it with the treatment in Taylor & Wheeler's highly readable _Spacetime Physics_.

For E&M, the standard way to develop this is to explain magnetism as Lorentz-transformed static electrical attraction/repulsion. Take two wires, and run current through them. Transforming to a frame where the electrons are at rest, but the atoms (and hence protons) are moving, length contraction ends up with the density being different, meaning a net charge in this frame. You then get E&M united as tensor field F, but antisymmetric, meaning the spin-2 components are 0, leaving effectively two spin-1 (vector) components. This lets you do relativistic corrections for a propagating field, and the velocity of the original source gets turned into magnetic effects that act the same as if the source were moving at a constant velocity.

Extremely similar things happen with gravity, if you look at weak-field linearized versions of the Einstein field equations. The actual math ... well, it's rather ugly.

Re: What is the Speed of Gravity?

#62

Post by achille »

ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

Wait, so you are saying we would feel the effects immediately, but the light would not disappear for eight minutes?

So let's assume once can cause the sun to vanish instantly somehow. If that's the case, would it then be possible to setup a 'gravitational wave' communication system that travels faster than light?

Ie, I would put a whole bunch of stars together, and then have them disappear at certain intervals. My buddy who sits 100 light-minutes away would then detect the changes and decode the message. TADA: Faster-than-light communication.

I feel safe to say that's not possible.

Re: What is the Speed of Gravity?

#63

Post by avar »

thailandstartup wrote: post #55

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself? So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself. There's so…

I'm not sure I understand your question correctly, but photons have no mass. So light can't pull objects towards itself.

In fact it'd to the opposite, which is why solar sails work. Radiation landing on you propels you away from the object emitting the radiation.

The Pioneer anomaly is very interesting. But we don't have sufficient data to determine what it is exactly. We should really send out more probes like that to find out.

Re: What is the Speed of Gravity?

#64

Post by Jun8 »

sliverstorm wrote: post #28

It always seems rather low-class to take shots at Newton as if he was a fool for coming up with Newton's theory of gravity instead of Einstein's.

Maybe not "taking shots", but I also feel a general "ha, he missed that big one" attitude.

Newton was such a great scientist that he made his famous "I don't make hypotheses" statement in regards to the nature of gravity. Knowing the limits of your (current) knowledge is a fundamental trait of being a scientist (and a philosopher, e.g. see Wittgenstein's motto "Whereof one cannot speak, thereof one must be silent." and Socrates' analysis of the Oracle's answer to him.)

Re: What is the Speed of Gravity?

#65

Post by thailandstartup »

avar wrote: post #63
thailandstartup wrote: post #55

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself? So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself. There's so…

I'm not sure I understand your question correctly, but photons have no mass. So light can't pull objects towards itself. In fact it'd to the opposite, which is why solar sails work. Radiation landing on you propels you away from the object emitting the radiation. The Pioneer anomaly is very interesting. But we don't have sufficient data to determine what it is exactly. We should really send out more probes like that…

I'd always understood gravity to be a mutual attraction between two masses. So the thing I have difficulty understanding is how light can only be attracted to mass, but itself cannot attract mass, no matter the quantity present.

Re: What is the Speed of Gravity?

#66

Post by doki_pen »

ck2 wrote: post #40

Personally I think it's easier to tell people to stop thinking of gravity as a particle or wave - it's not like a beam of light or radio wave. Think of the space/time as the blanket the universe is wrapped in. If that blanket is stretched out like a trampoline, every object on it or moving around on it is causing flexing and ripples. That's gravity. When a girl bounces off a trampoline, the specific instance of her g…

Our reality was build on a 299 792 458 m / s processor. Newer realities are much faster.

Re: What is the Speed of Gravity?

#67

Post by philh »

Tycho wrote: post #57
Luc wrote: post #49

Earlier quoted context omitted.

For fun: the moon has a radius of about 1730 km. At full moon we can see about half of the moon, so the distance from one side to the other is about pi * 1730 km. So how fast do you need to move a laser pointer across the moon surface to make the dot attain the speed of light? http://www.google.com/search?q=%28%281730000+*+pi%29+m%29+%2... A bit less than 2/100 of a second. That seems quite feasible really! (of cours…

So say the dot was fairly large and you had a vantage point on the moon near where the dot sweeps by... What would you actually see??

Suppose you're on the midpoint of a straight line 6e8 m long, which the dot traverses at uniform velocity.

If the dot moves at c, you would see nothing for 1s. Then there would be an instant where the first half of the line is entirely illuminated, as light from the dot at each point reaches you simultaneously.

After that, it would look like an ordinary dot moving away from you at c/2: if the dot moves for x seconds, it takes 2x seconds for the light from the dot at that point to reach you. (Note that a dot moving at c/2 would appear to be moving at c/3.)

If it's moving faster than light (say, kc, k>1), then when the dot arrives at you, you'd see a dot appear at your position, then move back to the original position at (k c / (k-1)), and vanish. (k/(k-1) is because you see it reach the start point 1s after it starts moving, and you first see it 1/k s after it starts moving.) If the dot had been stationary at that point before it began moving, you'd still see the dot there until the new one reached it, at which point they would both vanish.

Simultaneously, you'd see a dot move from your position to the end position at (k * c / (k+1)).

I'm not going to try to work out what happens if you're just standing near the path of the dot.

(I'm not sure about this, but it's what I get when I try to work it out. I'm especially not sure that I'm allowed to discount relativistic effects. I think I am because there are no massive bodies undergoing acceleration, but I don't pretend to fully understand relativity.)

Re: What is the Speed of Gravity?

#68

Post by avar »

thailandstartup wrote: post #65
avar wrote: post #63

Earlier quoted context omitted.

I'm not sure I understand your question correctly, but photons have no mass. So light can't pull objects towards itself. In fact it'd to the opposite, which is why solar sails work. Radiation landing on you propels you away from the object emitting the radiation. The Pioneer anomaly is very interesting. But we don't have sufficient data to determine what it is exactly. We should really send out more probes like that…

I'd always understood gravity to be a mutual attraction between two masses. So the thing I have difficulty understanding is how light can only be attracted to mass, but itself cannot attract mass, no matter the quantity present.

I think I'm correct in saying that the light isn't attracted to the mass, but the mass bends spacetime around it, so the path of everything traveling through that medium is bent, including the paths of photons.

This effect causis gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lens

Re: What is the Speed of Gravity?

#69

Post by gnok »

achille wrote: post #62
ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

Wait, so you are saying we would feel the effects immediately, but the light would not disappear for eight minutes? So let's assume once can cause the sun to vanish instantly somehow. If that's the case, would it then be possible to setup a 'gravitational wave' communication system that travels faster than light? Ie, I would put a whole bunch of stars together, and then have them disappear at certain intervals. My bu…

Out of curiosity, why do you think this is not possible?

Re: What is the Speed of Gravity?

#70

Post by [deleted] »

achille wrote: post #62
ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

Wait, so you are saying we would feel the effects immediately, but the light would not disappear for eight minutes? So let's assume once can cause the sun to vanish instantly somehow. If that's the case, would it then be possible to setup a 'gravitational wave' communication system that travels faster than light? Ie, I would put a whole bunch of stars together, and then have them disappear at certain intervals. My bu…

[deleted]
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