Question for Physics Nerds

[the_scalp]

My interest in physics in only recreational, but I'm often struck by a question regarding gravity.

How fast does the force of gravity work?

I know that every particle in the universe exerts a very weak gravitic pull on every other particle in the universe -- but at what speed is this pull sensed? Is it the speed of light? Is it instant?

If it is instant, wouldn't that open the way for a theoretical method of communicating information at faster than the speed of light (if you could measure the changing gravitic influence of a distant particle on a particle next to you faster than light can transverse the gulf)?

Do the other forces (strong and weak nuclear and whatever else, exert their influence over a distance instantaneously or at some other speed?

[PairTheBoard]

I'm pretty sure the magnetic force works at the speed of light, since light is an electro-magnetic wave.

PairTheBoard

[Duke]

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My interest in physics in only recreational, but I'm often struck by a question regarding gravity.

How fast does the force of gravity work?

I know that every particle in the universe exerts a very weak gravitic pull on every other particle in the universe -- but at what speed is this pull sensed? Is it the speed of light? Is it instant?

If it is instant, wouldn't that open the way for a theoretical method of communicating information at faster than the speed of light (if you could measure the changing gravitic influence of a distant particle on a particle next to you faster than light can transverse the gulf)?

Do the other forces (strong and weak nuclear and whatever else, exert their influence over a distance instantaneously or at some other speed?

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It acts at the speed of light. Look into general relativity.

[PairTheBoard]

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It acts at the speed of light. Look into general relativity.

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Have they actually been able to show that through observations? Is there any reason why it couldn't act at slower than the speed of light?

PairTheBoard

[MusashiStyle]

i think the main question they are asking right now is why is gravity such a weak force.

[Metric]

The short answer is "at the speed of light."

The longer answer is that the gravitational field itself actually defines what you mean by the speed of light. So the question is actually somewhat more subtle than it seems at first blush.

Suffice it to say for now, though, that if you're working on approximately flat spacetime, any perturbations to the gravitational field that you create by moving small masses around will propagate out at the ordinary speed of light.

[evank15]

This problem can be illustrated well using a gedankenexperiment.

What would happen if the Sun were to just "disappear", totally removed from existence, replaced by empty space?

The answer is that we would see it disappear 8 minutes (8.3 or whatever it is, 8 is close enough) after it actually happened, at which time we would exit our orbit tangentially in the direction we were moving 8 minutes after the event.

So for 8 minutes, we would see light from an object that didn't exist and we would feel the gravitational attraction of an object that didn't exist.

[Metric]

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This problem can be illustrated well using a gedankenexperiment.

What would happen if the Sun were to just "disappear", totally removed from existence, replaced by empty space?

The answer is that we would see it disappear 8 minutes (8.3 or whatever it is, 8 is close enough) after it actually happened, at which time we would exit our orbit tangentially in the direction we were moving 8 minutes after the event.

So for 8 minutes, we would see light from an object that didn't exist and we would feel the gravitational attraction of an object that didn't exist.

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Again, it's slightly more complicated than that. "Disappearing suns" don't solve the Einstein equation for the gravitational field, so this gedankenexperiment is highly non-physical and not something that physics is set up to talk about. You might be able to modify it with some kind of external force that pulls the sun away from its current position, but then you're back to talking about "accelerating masses" and the speed of gravitational waves that eminate from them, which again is only valid in a weak-field approximation.

This is a difficult thing about gravitational physics -- if you want to take some system and affect it some way with some external influence, then you really can't ignore the energy/momentum of the external influence -- in that sense it's not really treated as an "external" influence any more. It's just an altogether different solution to the Einstein equation.

[durrrr]

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[ QUOTE ]
My interest in physics in only recreational, but I'm often struck by a question regarding gravity.

How fast does the force of gravity work?

I know that every particle in the universe exerts a very weak gravitic pull on every other particle in the universe -- but at what speed is this pull sensed? Is it the speed of light? Is it instant?

If it is instant, wouldn't that open the way for a theoretical method of communicating information at faster than the speed of light (if you could measure the changing gravitic influence of a distant particle on a particle next to you faster than light can transverse the gulf)?

Do the other forces (strong and weak nuclear and whatever else, exert their influence over a distance instantaneously or at some other speed?

[/ QUOTE ]

It acts at the speed of light. Look into general relativity.

[/ QUOTE ]

are some quantum properties instant?

[Metric]

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are some quantum properties instant?

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Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[arahant]

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are some quantum properties instant?

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Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[/ QUOTE ]

Seriously?
Hmm....good luck with your defense!

[thylacine]

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[ QUOTE ]
are some quantum properties instant?

[/ QUOTE ]
Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[/ QUOTE ]

What do you think of Penrose's `quanglement'?

[Metric]

[ QUOTE ]
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[ QUOTE ]
are some quantum properties instant?

[/ QUOTE ]
Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[/ QUOTE ]

Seriously?
Hmm....good luck with your defense!

[/ QUOTE ]
Quite seriously. However, it may or may not even get mentioned in the defense -- basically a formalism I've developed for generally covariant systems can also be used to study EPR-type situations. And when you do use it for such things (providing a nice little written example of how to use the formalism), you no longer need to think about the "instant collapse" B.S. that already gets way too much attention. There also is no disagreement on the state between different reference frames... It's all very cute.

[the_scalp]

Thanks for all the responses everybody.

The "missing sun" thought experiement makes things a little clearer to me -- speed of light stuff always pisses me off. I'm sure that I'm still missing a ton of the subtlety.

Is this ERP "instantaneous force" thing you're attempting to discredit in your thesis the same thing as the stuff I've read saying that "twin" particles seem to be created where affecting one will instantaneously affect the other? (i.e. inducing one to have a "coutnerclockwise spin" will induce the other to instantly have a "clockwise spin" regardless of how far apart they are)

Is there a good place where I can get a (somewhat smart) layperson's view introducing me to physics. I've read Bill Bennet's "A Brief History of Nearly Everything" and found it too shallow . . . I want a more substantive intro to theoretical physics that doesn't require a lot of math knowledge.

[arahant]

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[ QUOTE ]
are some quantum properties instant?

[/ QUOTE ]
Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[/ QUOTE ]

Seriously?
Hmm....good luck with your defense!

[/ QUOTE ]
Quite seriously. However, it may or may not even get mentioned in the defense -- basically a formalism I've developed for generally covariant systems can also be used to study EPR-type situations. And when you do use it for such things (providing a nice little written example of how to use the formalism), you no longer need to think about the "instant collapse" B.S. that already gets way too much attention. There also is no disagreement on the state between different reference frames... It's all very cute.

[/ QUOTE ]

Does this involve allowing 'reverse causality'? (for lack of a better word...you catch my drift i presume). Anyway...if you publish to a pdf or something, feel free to share...

[Metric]

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Is this ERP "instantaneous force" thing you're attempting to discredit in your thesis the same thing as the stuff I've read saying that "twin" particles seem to be created where affecting one will instantaneously affect the other? (i.e. inducing one to have a "coutnerclockwise spin" will induce the other to instantly have a "clockwise spin" regardless of how far apart they are)

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Yes, that's the EPR "paradox." To make my claim a bit clearer -- people who know their stuff don't really view it as a paradox these days. However, it does tend to promote a somewhat strange view of what a "quantum state" really is -- different observers can disagree on the state of a system, depending on their relative state of motion. This has caused people to take a "relational" point of view of quantum mechanics -- that the "quantum state" of a system should really just be viewed as information gathered by a specific observer.

What I have seen is that this point of view is not necessary -- one can define a state in a way such that all observers can agree upon the state of the system. Measurements made by observers in relative motion correspond to different information-theoretic questions asked of the same state, which can have different (but correlated) answers.

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Is there a good place where I can get a (somewhat smart) layperson's view introducing me to physics. I've read Bill Bennet's "A Brief History of Nearly Everything" and found it too shallow . . . I want a more substantive intro to theoretical physics that doesn't require a lot of math knowledge.

[/ QUOTE ]
If you want a book that is more substantial than the usual pop-physics stuff but not as demanding as a textbook, I recommend "The Road to Reality" by Roger Penrose. There is a huge amount of physics to be learned there, and different chapters need not be read in order.

[Metric]

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Does this involve allowing 'reverse causality'? (for lack of a better word...you catch my drift i presume). Anyway...if you publish to a pdf or something, feel free to share...

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No, there's no retrocausality involved. Part of the motivation for the formalism is to make sense of measurement when the usual notion of time isn't available as happens, for example, in loop quantum gravity and quantum cosmology and is an aspect of what is known as "the problem of time." So it's not too surprising that making sense of things in this general case also has implications for EPR-type situations where the confounding issue can be traced to disagreement between observers on the time-ordering of measurements.

Anyway, yeah, I'll let you know when I have the current paper up online...

[thylacine]

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are some quantum properties instant?

[/ QUOTE ]
Some people would like you to believe this is the case or at least that it looks like something like this is the case even if it should be forbidden by relativity (look up the "EPR paradox").

I intend to personally crush this point of view in my Ph.D. thesis, however -- I'm writing a paper on it right now.

[/ QUOTE ]

What do you think of Penrose's `quanglement'?

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PS It's in section 23.10, pp603-7 "The Road to Reality" by Roger Penrose. It occurred to me that it is related.

[Subfallen]

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If you want a book that is more substantial than the usual pop-physics stuff but not as demanding as a textbook, I recommend "The Road to Reality" by Roger Penrose. There is a huge amount of physics to be learned there, and different chapters need not be read in order.

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Metric -

I just finished this book tonight after working on it for at least a month, and I enjoyed it as much as I could. But the math was so far over my head, so often, that I'm left at a loss at how to even begin addressing my mathematical deficiencies. (Extent of which is a horrible Calc 2 class I took 4-5 years ago.)

Do you have any suggestions for how I could do this outside an academic environment?

[Metric]

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If you want a book that is more substantial than the usual pop-physics stuff but not as demanding as a textbook, I recommend "The Road to Reality" by Roger Penrose. There is a huge amount of physics to be learned there, and different chapters need not be read in order.

[/ QUOTE ]

Metric -

I just finished this book tonight after working on it for at least a month, and I enjoyed it as much as I could. But the math was so far over my head, so often, that I'm left at a loss at how to even begin addressing my mathematical deficiencies. (Extent of which is a horrible Calc 2 class I took 4-5 years ago.)

Do you have any suggestions for how I could do this outside an academic environment?

[/ QUOTE ]
Is it actually the math you are objecting to, or is it more the jargon and notation that gets used? The whole first part of the book, of course, is designed to bring the reader up to speed on important math concepts without actually forcing you to become adept with their use... So if you could be a little more specific I might be able to point you in the right direction. Maybe reference a particular chapter/page to give me an idea of what you find to be over your head -- I have the book right here, so I can look it up.