Looking for simple answers gravity and time

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Main Question or Discussion Point

- is it a fair enough statement to say that gravity is both infite in range and instantaneous in effect?

- if you have two objects experiencing relative time dilation is it reasonable to consider the two objects to be in different dimensions of time? although they both continue to exist in 'normal' time their relationship to that original timeline is now different, perhaps a y axis of time. or is this looking at relativity too simply?
 

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  • #2
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Gravity is infinite in range as far as we know, but it's effects do not propagate faster than the speed of light.
 
  • #3
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As for your second question, I am far from the person to answer fully, but I think your choice of terminology is not great. They do experience time differently, but I would refrain from calling it different "dimensions" of time, mostly due to the more than well established definitions of dimensions in physics. Perhaps their views are different, or their worldline-slices are different, but different dimensions? I'd say no. But again, I'm not a professional. :)
 
  • #4
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- is it a fair enough statement to say that gravity is both infite in range and instantaneous in effect?
As per mjacobsca's reply.
- if you have two objects experiencing relative time dilation is it reasonable to consider the two objects to be in different dimensions of time? although they both continue to exist in 'normal' time their relationship to that original timeline is now different, perhaps a y axis of time. or is this looking at relativity too simply?
They are under different reference frames, not dimensions.
 
  • #5
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As per mjacobsca's reply.


They are under different reference frames, not dimensions.
I read from a special relativity book that they are under different coordinate systems in the space time. The Lorentz transformation between them is something like a 'rotation' of the coordinate axis so that different observers has different simultaneity of events in the space time continuum.
 
  • #6
Chalnoth
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- is it a fair enough statement to say that gravity is both infite in range and instantaneous in effect?
No. This was Newtonian gravity. It does not apply to General Relativity, as others have mentioned.

P.S. If you got this idea from Van Flandern, he's a crackpot. I can provide a detailed paper describing how the speed of gravity in General Relativity counters his arguments, if you'd like.

- if you have two objects experiencing relative time dilation is it reasonable to consider the two objects to be in different dimensions of time? although they both continue to exist in 'normal' time their relationship to that original timeline is now different, perhaps a y axis of time. or is this looking at relativity too simply?
A dimension is a direction something can move. Up/down, east/west, north/south are all dimensions. Moving northeast doesn't create a different one.
 
  • #7
jambaugh
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One note on the appearance of "instant effect". Both gravitation and electromagnetism have the property that when you solve the field equations for two objects in uniform motion, the force on each is directed at the current center of the other even though that force is the result of each object's earlier position via the speed of c propagation.

This is true from the perspective of any observer moving at any (sub-light) velocity relative to the original set up even though they each have a different definition of "current center" of each object. The force is only apparently instantaneous but in fact propagates at speed c.

You can see this in that if you perturb the motion of one of these objects the other will be attracted to the center of where the one would have been if it had continued without interference. This continues until the effect of this perturbation has propagated at speed c to the other object.

Empirically, the fact that the gravity is not instantaneous is supported by the slowing of rapidly co-orbiting binary stars. Because they are not moving with constant velocity the gravitational field they emit is no longer apparently instantaneous and the result is outwardly propagating waves which bleed off the rotational energy. The observed slowing matches that of the theoretical calculation which uses speed of c propagation while it would not match a theory of instantaneous effect which implies no slowing.

But I think the stronger argument is that this apparent instantaneous effect occurs in both gravity and electromagnetism and we can confirm directly that electromagnetic effects propagate at speed c.
 
  • #8
Jonathan Scott
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P.S. If you got this idea from Van Flandern, he's a crackpot. I can provide a detailed paper describing how the speed of gravity in General Relativity counters his arguments, if you'd like.
Tom Van Flandern was indeed a crackpot, but he's finished arguing, as he died in 2009.
 
  • #9
Chalnoth
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Tom Van Flandern was indeed a crackpot, but he's finished arguing, as he died in 2009.
Ahhh, I seem to remember that now. I'd forgotten.
 
  • #10
Chalnoth
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But I think the stronger argument is that this apparent instantaneous effect occurs in both gravity and electromagnetism and we can confirm directly that electromagnetic effects propagate at speed c.
There's also the matter that the apparent instantaneous effect comes from the field equations in General Relativity, the same field equations that predict light-speed propagation of gravity.
 
  • #11
thanks for the answers everyone. was helpful.
 
  • #12
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Tom Van Flandern was indeed a crackpot, but he's finished arguing, as he died in 2009.
What a shame, although he was a crackpot I learned a lot from reading his arguments and the refutations.
 
  • #13
how about this?

from chalnoth...
"A dimension is a direction something can move. Up/down, east/west, north/south are all dimensions. Moving northeast doesn't create a different one."

normally we/i think of time going forward, and that's the commonly mentioned 4d (is this thought in itself wrong).
i believe, in common terms, that time travels in one direction, though the speed can vary relative to an observer.
given that there is alway a base forward movement of time, that will effect everything within the universe, it still exists in the universe, in that 'universal' time.

from jaradnjames, you mentioned...
"different reference frames"

which i get. the observed time for the individuals envolved is what is different from their own perspective.

if someone travels fast or is near a gravity well then their time slows, but only for them. the effect for them is, like time, a one direction 'movement' or change.

hasn't something changed (like the forward passage of time) for those involved?

sorry if this seems like a silly question. i guess there's probably a formula that describes how this stuff works but that's beyond me.

you can just say there's a formula and it will shut me up :)
 
  • #14
Chalnoth
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how about this?

from chalnoth...
"A dimension is a direction something can move. Up/down, east/west, north/south are all dimensions. Moving northeast doesn't create a different one."

normally we/i think of time going forward, and that's the commonly mentioned 4d (is this thought in itself wrong).
i believe, in common terms, that time travels in one direction, though the speed can vary relative to an observer.
given that there is alway a base forward movement of time, that will effect everything within the universe, it still exists in the universe, in that 'universal' time.
Yes, the direction of motion we call time is a fourth dimension. It isn't, however, universal: which direction in space-time we experience as time depends entirely upon our reference frame. Somebody moving past us at high velocity will see a different direction in space-time as being their time coordinate (not a completely different direction, mind you, but it will be different by an angle that depends upon the relative velocity).

This is somewhat akin to the fact that the direction 'left' to me differs based upon my orientation, and is different for other people depending upon their orientation.
 
  • #15
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One note on the appearance of "instant effect". Both gravitation and electromagnetism have the property that when you solve the field equations for two objects in uniform motion, the force on each is directed at the current center of the other even though that force is the result of each object's earlier position via the speed of c propagation.

This is true from the perspective of any observer moving at any (sub-light) velocity relative to the original set up even though they each have a different definition of "current center" of each object. The force is only apparently instantaneous but in fact propagates at speed c.

You can see this in that if you perturb the motion of one of these objects the other will be attracted to the center of where the one would have been if it had continued without interference. This continues until the effect of this perturbation has propagated at speed c to the other object.

What experiment proves this unambiguously

Empirically, the fact that the gravity is not instantaneous is supported by the slowing of rapidly co-orbiting binary stars. Because they are not moving with constant velocity the gravitational field they emit is no longer apparently instantaneous and the result is outwardly propagating waves which bleed off the rotational energy. The observed slowing matches that of the theoretical calculation which uses speed of c propagation while it would not match a theory of instantaneous effect which implies no slowing.

Evidence yes - but not necessarily conclusive


But I think the stronger argument is that this apparent instantaneous effect occurs in both gravity and electromagnetism and we can confirm directly that electromagnetic effects propagate at speed c.
EM waves travel at the speed of light - agreed ...but what experiments show the electrical force between two charged particles travels at "c" EM radiation and photons involve something going from A to B. Force fields are a continuous affect - there is nothing traveling between the two objects as far as is known except the postulated virtual photons and gravitons
 
  • #16
Chalnoth
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EM waves travel at the speed of light - agreed ...but what experiments show the electrical force between two charged particles travels at "c" EM radiation and photons involve something going from A to B. Force fields are a continuous affect - there is nothing traveling between the two objects as far as is known except the postulated virtual photons and gravitons
Even without the quantum effects, classical Electricity and Magnetism is limited by the speed of light. If you look at the way this works mathematically for the electromagnetic field around a moving object, the force components from the velocity portion of the field exactly cancel the impact of the time delay, for an object moving at constant speed.

For gravity, the anomaly cancellation is even greater, and an object moving with constant acceleration has the force pointing exactly at its instantaneous position. Anyway, if you'd like to look at a more detailed treatment, see here:
http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909087v2.pdf
 
  • #17
Yes, the direction of motion we call time is a fourth dimension. It isn't, however, universal: which direction in space-time we experience as time depends entirely upon our reference frame.
This is somewhat akin to the fact that the direction 'left' to me differs based upon my orientation, and is different for other people depending upon their orientation.
good answer. thanks again.
 
  • #18
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Hi Chalnoth, thanks for the reference - I had read an article by Carlip a few years ago - while I don't have a side to take on this - I think the rationale for c velocity field affects depends to some degree upon the bias that this is what it should be - Carlip makes this statement in his summary:

"Finally, let us return to the question asked in Ref. [1]: what do experiments say about
the speed of gravity? The answer, unfortunately, is that so far they say fairly little. In the
absence of direct measurements of propagation speed, observations must be filtered through
theory, and different theoretical assumptions lead to different deductions. In particular,
while the observed absence of aberration is consistent with instantaneous propagation (with
an extra interaction somehow added on to explain the gravitational radiation reaction), it is
also consistent with the speed-of-light propagation predicted by general relativity."
 
  • #19
Chalnoth
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Hi Chalnoth, thanks for the reference - I had read an article by Carlip a few years ago - while I don't have a side to take on this - I think the rationale for c velocity field affects depends to some degree upon the bias that this is what it should be - Carlip makes this statement in his summary:

"Finally, let us return to the question asked in Ref. [1]: what do experiments say about
the speed of gravity? The answer, unfortunately, is that so far they say fairly little. In the
absence of direct measurements of propagation speed, observations must be filtered through
theory, and different theoretical assumptions lead to different deductions. In particular,
while the observed absence of aberration is consistent with instantaneous propagation (with
an extra interaction somehow added on to explain the gravitational radiation reaction), it is
also consistent with the speed-of-light propagation predicted by general relativity."
Yup. I should mention, however, that the observations of energy loss of binary pulsars, which strongly match the predictions of General Relativity, would be difficult to explain with an alternative theory of gravity that had different transmission speed (presumably gravity wave characteristics would be modified by the different speed of gravity).
 
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