# Looking for simple answers gravity and time

• pw_jamison
In summary, gravity is infinite in range but its effects do not propagate faster than the speed of light. As for the second question, the two objects experiencing relative time dilation are under different reference frames, not different dimensions of time. This can be seen in the Lorentz transformation between them, which is like a "rotation" of the coordinate axis. The apparent instantaneous effect of gravity is due to the field equations in General Relativity, which also predict light-speed propagation of gravity. It has been empirically confirmed through the slowing of co-orbiting binary stars. Tom Van Flandern, a known crackpot, argued for an instantaneous effect of gravity, but his arguments have been refuted. Time is commonly thought of as moving in

#### pw_jamison

- 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?

Gravity is infinite in range as far as we know, but it's effects do not propagate faster than the speed of light.

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. :)

pw_jamison said:
- 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?

They are under different reference frames, not dimensions.

jarednjames said:

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.

pw_jamison said:
- 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.

pw_jamison said:
- 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.

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.

Chalnoth said:
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.

Jonathan Scott said:
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.

jambaugh said:
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.

Jonathan Scott said:
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.

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.

"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 :)

pw_jamison said:

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.

jambaugh said:
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

yogi said:
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

Chalnoth said:
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.

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:

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."

yogi said:
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:

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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## 1. What is gravity?

Gravity is a force that pulls objects towards each other. It is responsible for keeping planets in orbit around the sun and for keeping us grounded on Earth. It is also the reason why objects fall towards the ground when dropped.

## 2. How does gravity work?

Gravity is caused by the mass of an object. The more mass an object has, the stronger its gravitational pull. This pull is also affected by the distance between objects - the closer two objects are, the stronger the pull will be.

## 3. Why do objects fall at the same rate?

Galileo was the first scientist to observe that objects of different mass fall at the same rate in a vacuum. This is because the force of gravity is proportional to the mass of the object, but the acceleration due to gravity is constant. Therefore, the acceleration of an object is the same regardless of its mass, resulting in objects falling at the same rate.

## 4. Can gravity be explained by Einstein's theory of relativity?

Yes, according to Einstein's theory of relativity, gravity is not a force but rather a curvature of spacetime caused by the presence of mass or energy. This curvature causes objects to move towards each other, giving the appearance of a gravitational force.

## 5. Does gravity affect time?

Yes, according to Einstein's theory of relativity, gravity also affects time. Time passes slower in stronger gravitational fields, meaning that clocks on Earth will run slightly slower than clocks in space. This effect has been observed and confirmed through experiments with atomic clocks on Earth and in space.