Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 24/06/2009 09:10:20
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I've read that there is no time delay in when electric charges apply a force to one another. Supposedly this does not break the speed of light constant because the charges will "anticipate" the other charges location, granted they are traveling linearly.
My question is, if charges are anticipating the others exact locations, what would happen if a distant charged particle just changed direction? For example, lets say there was a lone proton traveling at 10,000 miles per hour, 10 light hours away from a charged observer. And then all of a sudden it just stopped. Would the observer "anticipate" the charge going 100,000 miles of course? And if it doesn't anticipate it going somewhere it hasn't, how would that not violate the "You can't send information faster than light" rule?
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I've read that there is no time delay in when electric charges apply a force to one another.
Hello thebrain.
Where did you read it? Electric force cannot be transmitted instantly. If you read the electrodynamic equations in a good physics book, you'll find that fields (electric and magnetic) are functions of ρ(x,y,z,t-r/c) and j(x,y,z,t-r/c) where ρ is the charge density and j is the current density (this one is a vector), evaluated in the source point (x,y,z) at the *previous* instant of time t-r/c where r = distance from the source-point to the field-point, that is from the point where the source (charge or current) is located and the point where you want to evaluate the fields. This implies that the effect of the sources propagates at the speed c.
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Its not really that electric charge is transmitted instantaneously, its that when charges are moving linearly, not accelerating, they "anticipate" each others locations. This results in electric charge acting on their location at the very moment, not where it was however long it takes for light to arrive.
A similar thing happens with gravity, the earth is attracted to the sun where it stands right now, not where it was 8.5 minutes ago. If the earth was attracted to the sun where it "was" and not where it "is" then the result would be an increase in the speed in which the earth orbits the sun, causing the orbits to be unstable. (due to the fact that the sun is not in a fixed location) Calculations of orbits are always done with instantaneous values, not delayed ones.
Likewise if you set two objects in orbit around one another via electric charge. The charges will anticipate each others locations, and maintain a perfect orbit. However I haven't been able to find much in the nature of these "anticipations"
Also this deals with electric fields, not electromagnetic fields.
I know its a tricky subject, I was just wonderin if you guys know anything about this.
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Short answer: it's complicated.
Long answer: it's really complicated.
IRC in quantum mechanics, quantum particles like photons and electrons are allowed to do a certain amount of cheating, provided they make the books balance afterwards. What this means is that particles are able to travel at faster than the speed of light, provided they sort out all the conservation laws at the end.
For electrons this means that photons can jump infinite distances in no time; and this effectively gives you an electric field. However, it turns out (for reasons I'm not sure that I understand, although I can see to be true) that if an electron *accelerates* then the changes that are different from its inertial movement inevitably travel at the speed of light.
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A similar thing happens with gravity, the earth is attracted to the sun where it stands right now, not where it was 8.5 minutes ago.
Do you mean that gravitational force travels at infinite speed?
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Not infinite speed, the curvature of space is there before the object reaches it because it was set up as the mass that caused it moved into place.
Note that when things move at constant speed, the curvature moves with the object, so there's no delay. If things accelerate though, then you do get a delay in the curvature, and it's thought that gravitational waves are made that propagate away when that happens.
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A similar thing happens with gravity, the earth is attracted to the sun where it stands right now, not where it was 8.5 minutes ago.
Do you mean that gravitational force travels at infinite speed?
As I understood it, gravity travels at the speed of light - we are attracted to the sun where it stood 8 minutes ago.
From http://www.thenakedscientists.com/HTML/content/questions/question/1977/:
There was a recent experiment to look at just this sort of thing, which looked at how light was bent by the gravity of Jupiter. Our recent observations are yes, we think the gravity travels at exactly the same speed. If you plucked the sun from out of its orbit, just removed it completely - we would still orbit around the sun for eight minutes before suddenly the Earth realised that it wasn't there anymore and disappeared off into space.
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Not infinite speed, the curvature of space is there before the object reaches it because it was set up as the mass that caused it moved into place.
Ok, but if the Sun moves, even at constant speed, the curvature there is not the same as it was before, so I don't understand what you are saying.
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A similar thing happens with gravity, the earth is attracted to the sun where it stands right now, not where it was 8.5 minutes ago. If the earth was attracted to the sun where it "was" and not where it "is" then the result would be an increase in the speed in which the earth orbits the sun, causing the orbits to be unstable. (due to the fact that the sun is not in a fixed location) Calculations of orbits are always done with instantaneous values, not delayed ones.
What!?
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A similar thing happens with gravity, the earth is attracted to the sun where it stands right now, not where it was 8.5 minutes ago.
Do you mean that gravitational force travels at infinite speed?
Yes that is what I mean. Using gravity measuring devices, the earth appears to be attracted to the sun where it is right now.
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Can you show me some evidence of this?
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http://www.metaresearch.org/cosmology/speed_of_gravity.asp
When I try to research the speed of forces I always get conflicting results. There is the instantaneous side (like my link) versus the speed of light side. Probably more people say gravity moves at the speed of light. However the people on the other side always have way bigger detailed explanations on how the experiments disagree with the speed of light theory. I believe the anti-lightspeed guys theory goes, light speed isnt broken, but charges in a static electric field (not electromagnetic field) and gravitational fields "anticipate" where they are, hence they dont break the speed of light. Whenever I read an article about gravity traveling at light they never reference an experiment in their argument, they only reference that einstein predicted it would travel at light speed. Or they say it travels at light speed, and leave it at that.
Anyways its a very easy experiment to do, we have the ability to directly measure the suns and moons gravitational pull (like viewing the tide) unless the articles I've read are just elaborate lies, gravity does not act on where the sun was, it is attracted to where it is.
BTW BenV that experiment where light passes around jupiter is under a lot of scrutiny. Apparently some well respected physicist came out with a paper before the experiment that said that the experiment would not measure the speed of gravity, but it would measure the speed of light. And there are others that agree with him. Anyways I believe that that experiment is generally considered not accurate.
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http://www.metaresearch.org/cosmology/speed_of_gravity.asp
I have read something.
In my opinion, the reasonings are wrong at least because the relativity principle doesn't hold for non-inertial frames of reference, as in the case of curved spacetimes (that is, in presence of gravitational sources). The 'non aberration' of gravity, in comparison with light or other thing's aberration, is due to the fact that light and other things moves *in* the spacetime, while gravity is... the spacetime itself.
If the source of gravity is in a fixed position, then a moving target will find a spacetime curvature which is fixed and so, already established. If it is the source of gravity to be moving, it's not the same situation (no relativity principle) and in this case there is an 'aberration', that is the target will be subject to a force directed towards the previous position of the source.
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Lightarrow, your point is only valid if you assume two things.
1. That the direction of an approaching graviton has no bearing on the direction of its gravitational force, unlike the way a photons direction changes via motion (aberration). To me it seems like everything would have to obey that principle. But lets say that is feasible, like you said since gravity "is" spacetime and doesnt travel like photons per se. Okay maybe thats right. But you would also have to assume something else.
2. That the sun is actually fixed. That's false, the sun can move over a million kilometers away from the solar system's barycenter. Just because the sun is huge, doesn't make it any less susceptible to gravitational forces. The planets wiggle the sun, the point is that gravity acts in the direction of the sun where it is at the moment. (relative to the rest of the galaxy/universe not just relative to where the earth is). Secondly if you need another example of a non stationary gravitational force you could consider the moon, light that bounces off the moon does not correspond (exactly) to the strength of its gravitation.
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As I understood it, gravity travels at the speed of light - we are attracted to the sun where it stood 8 minutes ago.
I don't think that's right. We're attracted to where the Sun was going to be according to its motion and position as of 8 minutes ago.
If we were attracted to where it was 8 minutes ago then the Earth would spiral in, because the Earth would constantly lose angular momentum.