Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Riaan Engelbrecht on 20/01/2011 11:30:03
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Riaan Engelbrecht asked the Naked Scientists:
Dear Chris,
I love your show (http://www.thenakedscientists.com/HTML/podcasts/) with Redi every Friday on Radio 702.
I have a question in multiple parts so please bear with me.
Does Gravity propagate instantaneously? In other words, to use a fancy analogy: I move the sun 10,000,000 km in 1 sec in 1 direction, will the earth move instantaneously or will the effect only propagate at light speed to reach the earth 8 mins later?
My feeling is that the effect will be "felt" by the earth instantaneously, due to the fact that Gravity propagates in a field around any body. Apparently the jury is still out on whether Gravity consists of both field and particle (Gravitons) components (sort of like light) but I may have my facts muddled up on this one.
Therefore, accepting that Gravity propagates instantaneously, even at stellar distances, any change, however minute, is immediately discernable. It is then logical that if it is possible to interact with the gravity field of Earth, the Moon or a very large space station, it should be possible to regulate the gravity fields generated by this body. This regulation can then be molded into communication signals that travels the universe around us instantaneously, to be picked up by our own (or alien) receivers without the wait for signals that only travel at the paltry 300,000km/s speed of light, so to speak.
We can start small by trying to regulate (or oscillate) the gravity of a small craft the size of Sputnik, building a working receiver that reads the signal from this source and then working up from there.
Obviously the ability to manipulate the gravity field of any body may be a few (or a great many) years in the future but the theory seems plausible.
Am I totally off the mark here or is there merit in this hypothesis?
Regards
Riaan from Menlyn
What do you think?
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Even if gravity waves were found to propergate at superluminal velocity it would be quite impossible to use them for communication.
The most sensitive devices built or theoretically possible to detect gravity waves can only hope to receive them from merging black holes there is no way that we could generate waves that strong!
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Oh yes, because the info spreads very fast into very large area, it is very close to impossible to detect any info.
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Gravity according to our current model travels at lightspeed. The deAlembertian, a four dimensional wave equation will be represented as ▼. Not usual notation but box notation cannot be used here.
▼hμν=0
This is the same as saying
∂μ∂μhμν=0
This means it will follow the speed of light since ▼∂t2 - c2∂2x.
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Practical for applications or not, superluminal gravity would still be a very important find in physics. I have a feeling that we would have to rework a lot of physics theories if such were the case.
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Notwithstanding Q C's equations, which unfortunately mean nothing to me, is there any respectable theory that suggests that gravity might propagate faster than light?
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Well, if you want to be pedantic. Inertia do act everywhere in the universe as far as we know, ah, as good as 'instantly' locally.
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Notwithstanding Q C's equations, which unfortunately mean nothing to me, is there any respectable theory that suggests that gravity might propagate faster than light?
I can take you through some quantum field theory which would help you understand. But I won't bother if I believe no one is listening.
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Well, if you want to be pedantic. Inertia do act everywhere in the universe as far as we know, ah, as good as 'instantly' locally.
Yes, because the referential clock of particles are in the same frame as their mass... Gravity and time... (are the same thing)
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Notwithstanding Q C's equations, which unfortunately mean nothing to me, is there any respectable theory that suggests that gravity might propagate faster than light?
Short answer: no.
Long answer: define respectable.
Longer answer: general relativity seems to indicate that gravity moves at the speed of light. It seems to be the best theory we have for gravity. No doubt there are theories that propose gravity moves superluminally. Some of them might even be compatible with what we've observed (which so far seems to support GR). But if it does move superluminally this would mean major problems for our understanding of the universe, since we believe that information can't travel faster than the speed of light. It's probably healthy to be very skeptical about FTL gravity, since it would cause us to re-examine our understanding of the universe if it were true.
Fortunately, there are currently experiments under way to measure gravitational waves which should be able to determine if FTL gravity exists.
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The problem with gravity isn't how fast it may 'propagate'. In a universe controlled by one 'speeding' constant, namely lights invariant speed in a vacuum as measured in, and from, any frame possible it seems very plausible that gravity will behave the same.
The problem is 'inertia'.
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Gravity is instantaneous! how can an object disappear and its once great gravitational force now gone and travel at the speed of light with no substance !
All space travel calculations are done in the Newtonian way ,we on Earths orbit around the Sun project 8.5 minutes ahead of the apparent position of the Sun !