Gravity and Instantaneous Information sharing

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so today i was in a discussion with my physics teacher today about some of the various facets of general and special relativity. when the discussion came around to gravity my teacher said that if the sun were to suddenly disappear, although it would take the light 8 minutes to disappear, the effect on earth due to the lack of gravity would be instantaneous. my teacher cited "instantaneous information transfer" such as with entangled particles as the reason but i remember reading somewhere (possibly in "an elegant universe") that it would take the same amount of time as light, for the gravitational effects to be felt on earth. i am still a little fuzzy because after class i wiki'ed it and came up with my answer but would like a more thorough explanation. thanks
 

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  • #2
bcrowell
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Your teacher is wrong. Other than the material given in the FAQ answer below, it's also important to understand that quantum entanglement can't have anything to do with this, because it's not a question about quantum gravity, it's a question about classical gravity.

FAQ: How fast do changes in the gravitational field propagate?

General relativity predicts that disturbances in the gravitational field propagate as gravitational waves, and that low-amplitude gravitational waves travel at the speed of light. Gravitational waves have never been detected directly, but the loss of energy from the Hulse-Taylor binary pulsar has been checked to high precision against GR's predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity's description of gravitational waves.

It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., Brans-Dicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances propagate at any other speed than c. Without a test theory that predicts a different speed, it becomes essentially impossible to interpret observations so as to extract the speed. In 2003, Fomalont published the results of an exquisitely sensitive test of general relativity using radar astronomy, and these results were consistent with general relativity. Fomalont's co-author, the theorist Kopeikin, interpreted the results as verifying general relativity's prediction of propagation of gravitational disturbances at c. Samuel and Will published refutations showing that Kopeikin's interpretation was mistaken, and that what the experiment really verified was the speed of light, not the speed of gravity.

A kook paper by Van Flandern claiming propagation of gravitational effects at >c has been debunked by Carlip. Van Flandern's analysis also applies to propagation of electromagnetic disturbances, leading to the result that light propagates at >c --- a conclusion that Van Flandern apparently sincerely believes.

Fomalont and Kopeikin - http://arxiv.org/abs/astro-ph/0302294

Samuel - http://arxiv.org/abs/astro-ph/0304006

Will - http://arxiv.org/abs/astro-ph/0301145

Van Flandern - http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

Carlip - http://xxx.lanl.gov/abs/gr-qc/9909087v2
 
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  • #3
JesseM
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I've read somewhere that unlike with Newtonian physics, it's fundamentally impossible to analyze what would happen if the Sun "vanished" in general relativity since no valid GR solution would allow such vanishing (I'm sure this isn't hard to show, but for future reference, does anyone remember what the link to the article explaining this might be? It doesn't seem to be part of Baez's physics FAQ) However, you can analyze some other situation like one where the Sun explodes or is suddenly knocked to one side, and in this case we would not feel the effects instantaneously, gravitational information is limited to the speed of light. Your teacher is also wrong (or at least talking misleadingly) that entangled particles involve any instantaneous information transfer, at least at the observable level--Eberhard's theorem proves that entanglement can never be used to transfer information FTL (whether there might be some 'hidden' FTL effects behind entanglement depends on which interpretation of quantum mechanics you prefer, the many-worlds interpretation is one of the most popular and its supporters argue that it eliminates the need for any nonlocal FTL effects, even behind-the-scenes ones that humans can't use to transfer messages)
 
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bcrowell
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I've read somewhere that unlike with Newtonian physics, it's fundamentally impossible to analyze what would happen if the Sun "vanished" in general relativity since no valid GR solution would allow such vanishing (I'm sure this isn't hard to show, but for future reference, does anyone remember what the link to the article explaining this might be? It doesn't seem to be part of Baez's physics FAQ)
Good point. I don't know of a reference that states this, but it seems straightforward to see that it makes sense. The Einstein field equations imply local conservation of mass-energy, so you can't just make the sun disappear. The best you could do would be to impart a sudden, very large acceleration to the sun in the direction away from the earth. But then the sun would still only be moving away from the earth at some speed less than c, and its gravitational field would also be altered (in the earth's frame) by the sun's motion relative to the earth.

On the other hand, if God just suddenly nudges the sun a few thousand miles to one side, I think that is basically compatible with the field equations, and the change in the gravitational field would propagate toward the earth at c according to GR.
 
  • #5
JesseM
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On the other hand, if God just suddenly nudges the sun a few thousand miles to one side, I think that is basically compatible with the field equations, and the change in the gravitational field would propagate toward the earth at c according to GR.
Yes, or instead of God, imagine detonating a super-powerful bomb next to the sun that bumps it off to the side--the kinetic energy released by the explosion might have formerly been stored as the potential of some nongravitational force (like the strong nuclear force), and I think GR would allow you to include inputs from nongravitational forces (someone correct me if I'm wrong).
 
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bcrowell
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Yes, or instead of God, imagine detonating a super-powerful bomb next to the sun that bumps it off to the side--the kinetic energy released by the explosion might have formerly been stored as the potential of some nongravitational force (like the strong nuclear force), and I think GR would allow you to include inputs from nongravitational forces (someone correct me if I'm wrong).
This raises the question of whether God is a solution of the Einstein field equations. I'll have to get back to you on that one.
 
  • #7
JesseM
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This raises the question of whether God is a solution of the Einstein field equations. I'll have to get back to you on that one.
Heh, you kid, but "God is a solution of the Einstein field equations" is pretty much the idée fixe of the crackpot physicist Frank Tipler...he started out with a sort of interesting speculation about how in a closed universe life might use the collapse to the Big Crunch to do an infinite amount of computing in a finite time (sort of analogous to Freeman Dyson's speculation about how life could do an infinite amount of computing in an open universe), but it devolved into identifying the Big Crunch singularity with the Christian God (or more specifically with God the Father, the Big Bang being identified with the Holy Spirit and the Son with everything in between) and claiming that the shroud of Turin is the key to figuring out how to reverse the acceleration of the expanding universe and make sure the Big Crunch happens...for those interested in crackpotology, here is an article in which he expounds his views, and here is a post of mine summarizing and quoting some of his crazier claims!
 
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I wonder if his teacher heard the classic, "if the sun were EXTINGUISHED the Earth wouldn't experience the loss of energy output until the lag of around 7+ minutes had passed.

Maybe this teacher somehow conflated that notion with delusions of understanding gravity?

@JesseM: I LOVE studying sane, but deranged minds. I've read every crazed scrawl about 8th dimensionsal lizards on David Icke's sites, and I love it. It goes to professional and personal curiosity and wonder. On that note... oh thank you for this gift of crackpottery. It is so lovely! The shroud of turin... oh man... the only science to come out of Turin would be stellar observation a la "Turino Units". What a useful scale... :rofl:
 
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  • #10
JesseM
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JesseM: I LOVE studying sane, but deranged minds. I've read every crazed scrawl about 8th dimensionsal lizards on David Icke's sites, and I love it. It goes to professional and personal curiosity and wonder. On that note... oh thank you for this gift of crackpottery. It is so lovely! The shroud of turin... oh man... the only science to come out of Turin would be stellar observation a la "Turino Units". What a useful scale... :rofl:
Yeah, crackpot theories aren't always interesting, but when they come up with whole imaginary cosmologies that try to answer life's big questions in a bunch of different fields at once (including theology!), the deranged creativity that goes into it often makes it a lot of fun to read! (I'm pretty fond of Icke's lizard-people theories tying together all of human history as well...)
 
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Yeah, crackpot theories aren't always interesting, but when they come up with whole imaginary cosmologies that try to answer life's big questions in a bunch of different fields at once (including theology!), the deranged creativity that goes into it often makes it a lot of fun to read! (I'm pretty fond of Icke's lizard-people theories tying together all of human history as well...)
That's what people so often forget; people dedicate their whole being to these intricate fantasies so they can be really interesting. Completely out there and hanging by a thread made from a unicorn's mane, but interesting. The lizards bit... you know we're in a bad way when a basic human persecutorial desulusion becomes an actual theory of history. Then again, it explains a lot about how Dick Clark is still kicking, and Dick Cheney... and... omg... all Dicks are clearly 8th dimensional lizards. *looks in pants* Mine too! It's waking up! It's *sound of reptilian hiss* ARRRGGGHHHH!!fsdg03dsafa121rrrrrrrrrrrrrrrrrrr
 
  • #12
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i remember reading somewhere (possibly in "an elegant universe") that it would take the same amount of time as light,
Right you are, page 74:

..Einstein was able to calculate how fast disturbances of the fabric of the universe travel and he found that they travel at precisely the speed of light...in the case of the demise of the sun...the influence will not be instantaneously communicated....QUOTE]

Besides, anyone who has read anything about relativity knows NOTHING moves faster than light...at least NO INFORMATION does....and gravity does carry information.....nor do any of the standard model forces...And it would be baffling if gravitational waves traveled at a finite speed (as is required by massive detectors) and gravity was instantaneous...

i wiki'ed it and came up with my answer but would like a more thorough explanation.
Nobody knows why nothing can move faster than light....special and general relativity is as good as it gets....

Having posted that, I now wonder if any quantum gravity theories confirm that gravity moves at lightspeed, c?????????????
 

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