"Speed of gravity" (big news)

[billvon 3,179]

This is actually fairly big news. The propagation speed of gravity has been a mystery for a long time. It was long suspected to propagate at the speed of light since EM waves do, but electromagnetism is just one of the four fundamental forces so it's not a very good basis for such an assumption. This experiment tends to confirm the speed of light theory:

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Scientists Find Evidence for Speed of Gravity
Wed, 01/02/2013 - 7:00am

Scientists have been trying to measure the speed of gravity for years through experiments and observations, but few have found valid methods.

By conducting six observations of total and annular solar eclipses, as well as Earth tides, a team headed by Tang Keyun, a researcher with the Institute of Geology and Geophysics under the Chinese Academy of Sciences (CAS), found that the Newtonian Earth tide formula includes a factor related to the propagation of gravity.

"Earth tide" refers to a small change in the Earth's surface caused by the gravity of the moon and sun.

Based on the data, the team, with the participation of the China Earthquake Administration and the Univ. of the CAS, found that gravitational force released from the sun and gravitational force recorded at ground stations on Earth did not travel at the same speed, with the time difference exactly the same as the time it takes for light to travel from the sun to observation stations on Earth.

The scientists admitted that the observation stations are located near oceans, indicating that the influence of ocean tides might have been strong enough to interfere with the results.

Consequently, the team conducted separate observations of Earth tides from two stations in Tibet and Xinjiang, two inland regions that are far away from all four oceans, as well as took measures to filter out other potential disturbances.

By applying the new data to the propagation equation of gravity, the team found that the speed of gravity is about 0.93 to 1.05 times the speed of light with a relative error of about 5 percent, providing the first set of strong evidence showing that gravity travels at the speed of light.
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[BigMikeH77 0]

This is fascinating. BUT... The article says that gravity "travels". Since it's a force, I think it's more appropriate to say that gravity "acts" at approximately the speed of light. Just a thought.

[quade 4]

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This is fascinating. BUT... The article says that gravity "travels". Since it's a force, I think it's more appropriate to say that gravity "acts" at approximately the speed of light. Just a thought.

Gravity has waves. The waves travel.

quade -
The World's Most Boring Skydiver

[ryoder 1,590]

"We know the speed of light; So what's the speed of dark?"
-- Steven Wright

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[muff528 3]

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This is fascinating. BUT... The article says that gravity "travels". Since it's a force, I think it's more appropriate to say that gravity "acts" at approximately the speed of light. Just a thought.

Gravity has waves. The waves travel.

Unless they're "standing waves" ...then they just stand.

[billvon 3,179]

> Since it's a force, I think it's more appropriate to say that gravity "acts" at
> approximately the speed of light. Just a thought.

True, although the same is true of EM waves - and we regularly speak of photons traveling at the speed of light.

[popsjumper 2]

All well and good...for a theory but,
Gravity is measured in years.
The proof is:
Young boobies are perky, Grandma's sag.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[lawrocket 3]

This is really neat. But Kopeikin and Fomalont measured at 1.06c +/- .21 in 2002. This was controversial at the time. But looking at the new numbers it’s about what Kopeikin and Fomalont said it was.

It also puts Van Flandern’s 1998 letter re: gravity’s Newtonian characteristic on a bit more defense. http://www.metaresearch.org/cosmology/speed_of_gravity.asp[/url]

Next question, though – is the speed of gravity constant?

My wife is hotter than your wife.

[BigMikeH77 0]

Wouldn't it have to be constant? The force itself would increase on an object as its proximity and exposure increase. I mean, there's no way that I know of to reduce gravity other than reducing an object's mass. but even so, doing that only reduces the force of gravity, not the speed.

[lawrocket 3]

Well that’s the thing. The speed of light through a vacuum is constant. But, yeah, the speed of light itself is not constant since it is slowed down by its encounters with objects, etc., which actually allows us to do things like “see.”

It’s why I’m curious about gravity. Let’s take the sun, earth and moon, for example. Does a gravitational wave from the sun get slowed or sped or scattered or concentrated at langrangian point 2 of the earth/moon relationship when the moon (viewed from earth) is full? That is, the gravitational wave from the sun must pass through the earth and the moon to get to the satellite at L2.

Under relativity, the gravity of the earth and the moon will bend spacetime around it. Is a gravity wave also bent by the strength of its own force?

Now I’m thinking of a binary black hole system. Picture again a satellite at L2 of one of the black holes. Is the L2 satellite subjected to any gravity from the distant black hole since the escape velocity from within the schwarzchild radius exceeds the speed of light? Or, is the L2 satellite subjected to tangential gravity from the distant black hole due to lensing of the proximal black hole? Does it take longer for gravity from the distal black hole to reach the L2 satellite?

Fuck. Now my mind is going everywhere.

My wife is hotter than your wife.

[BigMikeH77 0]

Hmm, excellent questions. The only practical application for gravitational theories that I really have experience with lately however involve a twin otter - and I'm usually preoccupied with dive flows at the time.

[kallend 2,231]

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Well that’s the thing. The speed of light through a vacuum is constant. But, yeah, the speed of light itself is not constant since it is slowed down by its encounters with objects, etc., which actually allows us to do things like “see.”

It’s why I’m curious about gravity. Let’s take the sun, earth and moon, for example. Does a gravitational wave from the sun get slowed or sped or scattered or concentrated at langrangian point 2 of the earth/moon relationship when the moon (viewed from earth) is full? That is, the gravitational wave from the sun must pass through the earth and the moon to get to the satellite at L2.

Under relativity, the gravity of the earth and the moon will bend spacetime around it. Is a gravity wave also bent by the strength of its own force?

Now I’m thinking of a binary black hole system. Picture again a satellite at L2 of one of the black holes. Is the L2 satellite subjected to any gravity from the distant black hole since the escape velocity from within the schwarzchild radius exceeds the speed of light? Or, is the L2 satellite subjected to tangential gravity from the distant black hole due to lensing of the proximal black hole? Does it take longer for gravity from the distal black hole to reach the L2 satellite?

Fuck. Now my mind is going everywhere.

Why the F. did you become a lawyer?

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The only sure way to survive a canopy collision is not to have one.

[lawrocket 3]

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Why the F. did you become a lawyer?

I've been asking myself that since day 2 of law school.

I know they are probably dumb questions.

My wife is hotter than your wife.

[muff528 3]

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.......
Now I’m thinking of a binary black hole system. Picture again a satellite at L2 of one of the black holes. Is the L2 satellite subjected to any gravity from the distant black hole since the escape velocity from within the schwarzchild radius exceeds the speed of light? Or, is the L2 satellite subjected to tangential gravity from the distant black hole due to lensing of the proximal black hole? Does it take longer for gravity from the distal black hole to reach the L2 satellite?

Fuck. Now my mind is going everywhere.

Wouldn't the satellite have to feel the gravitational force from the distant BH for L2 to have any relevance at all. IOW- wouldn't the Lagrangians exist only if they and the satellite are interacting gravitationally as 3 distinct objects? If not then no L2 exists and the satellite "sees" a single rotating BH emitting pulsating grav waves. I'm thinking of 2 orbiting black holes that have not yet merged event horizons. But, once their event horizons have merged, wouldn't they be orbiting too fast for the satellite to keep up with L2 (probably long before merge) and maybe be ejected from the system?

[lawrocket 3]

Right. That's why I'm all weirded out by this. Would there even BE an L2 in a binary black hole system? If Gravity is constant and instantaneous then theoretically yes (assuming you could have a stable orbit). What if the black holes are rotating? What if only one of them is?

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If not then no L2 exists and the satellite "sees" a single rotating BH emitting pulsating grav waves

Do the gravity waves pulsate at the same frequency? Could you imagine some kinda gravity wave harmonic where an object gets blasted at some interval by two gravity waves intersecting?

This is some weird and seriously cool stuff. I owe you a .

My wife is hotter than your wife.

[kallend 2,231]

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Right. That's why I'm all weirded out by this. Would there even BE an L2 in a binary black hole system? If Gravity is constant and instantaneous then theoretically yes (assuming you could have a stable orbit). What if the black holes are rotating? What if only one of them is?

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If not then no L2 exists and the satellite "sees" a single rotating BH emitting pulsating grav waves

Do the gravity waves pulsate at the same frequency? Could you imagine some kinda gravity wave harmonic where an object gets blasted at some interval by two gravity waves intersecting?

This is some weird and seriously cool stuff. I owe you a .

Fascinating stuff. Light cannot escape a black hole event horizon. What about gravity?

One of the reasons I went into solid state physics. Mostly it makes sense.

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The only sure way to survive a canopy collision is not to have one.

[muff528 3]

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Right. That's why I'm all weirded out by this. Would there even BE an L2 in a binary black hole system? If Gravity is constant and instantaneous then theoretically yes (assuming you could have a stable orbit). What if the black holes are rotating? What if only one of them is?

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If not then no L2 exists and the satellite "sees" a single rotating BH emitting pulsating grav waves

Do the gravity waves pulsate at the same frequency? Could you imagine some kinda gravity wave harmonic where an object gets blasted at some interval by two gravity waves intersecting?

This is some weird and seriously cool stuff. I owe you a .

By "sees a single rotating black hole emitting pulsating waves", I meant the orbiting pair is seen as a single object emitting waves at the frequency determined by the orbital rotation of the pair. Now add to the mix your reference to the two black holes spinning at different rates while orbiting. What weirdness would be seen then!

[muff528 3]

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.....
One of the reasons I went into solid state physics. Mostly it makes sense.

Yeah, right! Quantum tunneling makes perfect sense.

[kallend 2,231]

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.....
One of the reasons I went into solid state physics. Mostly it makes sense.

Yeah, right! Quantum tunneling makes perfect sense.

Yep. More so than the physics of rotating black holes.

I knew Stephen Hawking in college. He was weird even then.

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The only sure way to survive a canopy collision is not to have one.

[billvon 3,179]

> Light cannot escape a black hole event horizon. What about gravity?

Light can't but electric fields can. And from telescope observations gravity does indeed "escape" black holes.

[muff528 3]

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> Light cannot escape a black hole event horizon. What about gravity?

Light can't but electric fields can. And from telescope observations gravity does indeed "escape" black holes.

So, why are they different? Is light (still traveling at c by definition) confined within the event horizon because the "intensity" of the gravitational field has slowed time to the point that there is not enough time (to an outside observer) for the photons to make it out? Or does gravity physically act on the photons sucking them into the hole? Is gravity unaffected by its own effects on time, allowing gravity to escape the event horizon? Or does gravity not act on itself?

Or is gravity really ("the fabric of") space itself and gravitational waves are propagated on the "surface" of gravity (space) at the speed of light? Like a tsunami, the wavefront moves faster in the open ocean and slows as a land mass is approached. (terrible analogy, but that's all I got.)

[JackC1 0]

Light is made of real photons and as such are limited to C but electric fields are mediated by virtual photons (a consequence of Heisenberg uncertainty principle and perturbation theory in QFT) which are not. Hence light cannot escape a black hole but electric fields can.

So when it comes to gravity escaping a black hole, you might think that if gravity cannot exceed the speed of light then how can it escape the event horizon? This should mean that black holes are gravitationally inert but obviously they aren't (or at least don't appear to be). The idea of force carrying particles (like photons) is a result of quantum theory but General Relativity is not a quantum theory so it doesn't predict force carrying particles (virtual or otherwise). According to GR, it turns out that in the same way that light emitted from an object falling into a black hole appears to slow down (red shifted, time dilation etc) to the point where it seems to stand still at the event horizon, so too does gravity. So you can calculate the gravity of a black hole by considering the mass of the star before it collapsed. The gravitational field is then something of a relic of what the star was before it turned into a black hole.

So a finite speed for gravity isn't a problem for black holes according to GR (which actually predicts just that) and any quantum theory of gravity would have to accommodate that property. This is part of the reason why no such quantum theory of gravity has been successfully devised up to now.

So it seems to me that far outside the event horizon, a black hole should behave pretty much like any other massive object. But the question of whether you can diffract gravitational waves (if their speed is dependent on the surrounding gravitational field strength) is an interesting one. This idea sort of implies that gravity behaves like light, which so far appears not to be true so it would seem that assuming light-like properties for gravitational waves may be unwarranted.

I may be wrong.

[lawrocket 3]

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Is light (still traveling at c by definition) confined within the event horizon because the "intensity" of the gravitational field has slowed time to the point that there is not enough time (to an outside observer) for the photons to make it out?

Right. Speed equals distance/time. Problem is that tests have have confirmed time dilation. So measuring speed of something that affects time is inherently uncertain, isn't it?

My wife is hotter than your wife.

[kallend 2,231]

I took "relativity" in college from Otto Frisch.

One of the reasons I ended in doing solid state.

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The only sure way to survive a canopy collision is not to have one.

[lawrocket 3]

My undergrad college (UC Santa Barbara) is one of the fines engineering and theoretical physics institutions in the world. Shame I didn't take advantage of that.

My wife is hotter than your wife.