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Does Gravity Travel at the Speed of Light? (1998)

math.ucr.edu

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Re: Does Gravity Travel at the Speed of Light? (1998)

#101

Post by chinpokomon »

aphyr wrote: post #30
DougWebb wrote: post #23

Earlier quoted context omitted.

Does anyone know what Wolfram would say about the tickrate? I bought the huge book and know that he believes space is quantized, at least, and I assume he believes time is as well, as the rate the universal state machine changes states. But I never read enough to learn what he thinks that rate might be.

Cellular automata violate continuity and isotropy, which means they're inconsistent with the continuous spatial formalism used in special relativity, general relativity, classical electrodynamics, and as far as I know, quantum electrodynamics. We have no experimental evidence inconsistent with continuous models of space. We also have evidence that Lorentz symmetry holds over scales smaller than the Planck length, whi…

Thanks, you've given me some more topics to research.

Re: Does Gravity Travel at the Speed of Light? (1998)

#102

Post by hcrisp »

vehementi wrote: post #72
diminoten wrote: post #68

Earlier 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…

I believe they showed this on PBS in an episode of The Elegant Universe. http://youtu.be/4yyb_RNJWUM

Re: Does Gravity Travel at the Speed of Light? (1998)

#103

Post by aidenn0 »

mistercow wrote: post #97
AlwaysBCoding wrote: post #28

This 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…

>Like you said, what if "suddenly a black hole appeared?" > >Well, the answer of course is that that never happens, ever. Can't it, though? It would be extremely unlikely, but pairs of particles and antiparticles pop into existence constantly. Again, it would be so unlikely as to be of only academic concern, but as I understand, it is technically possible that ~10^57 of antiparticles could pop up all over the univers…

That can't happen without them annihilating very quickly. Otherwise it would be violating conservation of mass.

Re: Does Gravity Travel at the Speed of Light? (1998)

#104

Post by aidenn0 »

peterwwillis wrote: post #20
ssivark wrote: post #3

It's a fairly short and clear article, so no need for a Tl;DR. Note that this is NOT yet another "revolutionary" announcement, but just contemplative reasoning from a respected expert in gravitational physics.

Yeah, well. I don't get it. Could somebody summarize it in terms a 9 year old like myself can understand?

Gravity travels at the speed of light, but it pulls you towards where the object is going to be when it left the object, not where it was when it left the object.

Re: Does Gravity Travel at the Speed of Light? (1998)

#107

Post by mistercow »

aidenn0 wrote: post #103
mistercow wrote: post #97

Earlier quoted context omitted.

>Like you said, what if "suddenly a black hole appeared?" > >Well, the answer of course is that that never happens, ever. Can't it, though? It would be extremely unlikely, but pairs of particles and antiparticles pop into existence constantly. Again, it would be so unlikely as to be of only academic concern, but as I understand, it is technically possible that ~10^57 of antiparticles could pop up all over the univers…

That can't happen without them annihilating very quickly. Otherwise it would be violating conservation of mass.

The particles do annihilate, but they don't have to annihilate with each other. If an electron/positron pair appear, the positron can annihilate a different electron, in which case the electron appears to jump.

So if the pairs appeared light years apart, the antiparticles would annihilate something nearby, leaving their partners in the sun, as if particles were spontaneously jumping from all over the universe into the sun.

Preposterous, yes, but as far as I understand, not actually impossible.

Re: Does Gravity Travel at the Speed of Light? (1998)

#108

Post by acchow »

aidenn0 wrote: post #104
peterwwillis wrote: post #20

Earlier quoted context omitted.

Yeah, well. I don't get it. Could somebody summarize it in terms a 9 year old like myself can understand?

Gravity travels at the speed of light, but it pulls you towards where the object is going to be when it left the object, not where it was when it left the object.

Thanks for this. Tweaking it slightly:

> Gravity travels at the speed of light, but it pulls you towards where the object would be if the object kept moving at constant speed, not where it was when it left the object

Re: Does Gravity Travel at the Speed of Light? (1998)

#109

Post by acchow »

stcredzero wrote: post #79
hmsimha wrote: post #67

Earlier quoted context omitted.

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.

> The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years.

FTFY.

Re: Does Gravity Travel at the Speed of Light? (1998)

#110

Post by gliese1337 »

Ygg2 wrote: post #50
ars wrote: post #43

Here'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…

Wait, haven't they proven that you can make magnetic monopoles and that wormholes are still a possibility?

It is a prediction of some physical theories that magnetic monopoles could exist from the origin of the universe (and it kinda seems like they should, 'cause other than the existence of one kind of charge and the other, there's no good reason to see either half of electromagnetism as more fundamental than the other- both kinds of field can generate the other, and how much of each one you think you have depends on your inertial frame since they're relativistic transformations of each other), but no one has shown that you can make them, pulling the poles of a magnet apart like you can pull electrons away from protons.
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