Does gravity have effects through time?

[guss]

Let's say we have a blank universe and have a 1kg mass. At some point in time, this is near-instantaneously replaced by a heavier mass. Both masses are stationary. Will the lighter mass, before it is replaced, see effects due to the heavier mass existing in the future? In turn, will the heavier mass see effects due to the lighter mass existing in the past? More importantly, why or why not?

Since, according to Einstein, a stationary object travels through time at c, the distance between these two masses in time would rapidly increase, likely making the effects very small. Also, I'm not sure about what this would cause, and I don't want to get into too much speculation right now, but it could lead to an increase or decrease in mass of some objects at certain points in time.

 

[Andrew Mason]

Let's say we have a blank universe and have a 1kg mass. At some point in time, this is near-instantaneously replaced by a heavier mass. Both masses are stationary. Will the lighter mass, before it is replaced, see effects due to the heavier mass existing in the future? In turn, will the heavier mass see effects due to the lighter mass existing in the past? More importantly, why or why not?

This is an odd question. You state a hypothetical scenario that defies the laws of physics and you ask for an explanation of how the laws of physics would apply. Physics cannot provide an answer to your question.

Since, according to Einstein, a stationary object travels through time at c,

Where do you get this from? This is not correct. Actually it does not make any sense to me. Can you explain what you mean?

AM

 

[guss]

Where do you get this from? This is not correct. Actually it does not make any sense to me. Can you explain what you mean?

Here's one source:
http://en.wikipedia.org/wiki/Spacetime#Spacetime_intervals

 

[WannabeNewton]

Where do you get this from? This is not correct. Actually it does not make any sense to me. Can you explain what you mean?

When you lorentz boost to the frame of a massive particle, in that frame U^{i} = 0 and U^{t} = \frac{\mathrm{d} x^{t}}{\mathrm{d} t} = \frac{\mathrm{d} (ct)}{\mathrm{d} t} = c.

 

[atomthick]

Let's say we have a blank universe and have a 1kg mass. At some point in time, this is near-instantaneously replaced by a heavier mass. Both masses are stationary. Will the lighter mass, before it is replaced, see effects due to the heavier mass existing in the future? In turn, will the heavier mass see effects due to the lighter mass existing in the past? More importantly, why or why not?

Let's assume this is possible, the lighter mass will not see this coming. After it's replaced there will probably be some gravitational waves.

 

[guss]

Let's assume this is possible, the lighter mass will not see this coming. After it's replaced there will probably be some gravitational waves.

If gravity does go through time, and if you observe the lighter mass from a future reference frame, then the the lighter mass in the past should be pulled by the heavier mass in your current reference frame.

 

[atomthick]

If gravity does go through time, and if you observe the lighter mass from a future reference frame, then the the lighter mass in the past should be pulled by the heavier mass in your current reference frame.

Gravity doesn't go through time.

 

[guss]

Gravity doesn't go through time.

Sorry, I must have misunderstood what you were saying before. Anyway, how do you know it doesn't?

 

[WannabeNewton]

Sorry, I must have misunderstood what you were saying before. Anyway, how do you know it doesn't?

When you talk about something "going through space - time" you are talking about a particle's path on a time - like geodesic or null - like geodesic. Given initial conditions, you can determine the past and future of this particle's wordline. The metric (which describes the gravitational field) can evolve with time but it doesn't "go through time". The metric, instead, determines the behavior of the geodesics.

 

[guss]

When you talk about something "going through space - time" you are talking about a particle's path on a time - like geodesic or null - like geodesic. Given initial conditions, you can determine the past and future of this particle's wordline. The metric (which describes the gravitational field) can evolve with time but it doesn't "go through time". The metric, instead, determines the behavior of the geodesics.

How do you know this?

 

[elfmotat]

Where do you get this from? This is not correct. Actually it does not make any sense to me. Can you explain what you mean?

\mathbf{U}=(\gamma c,\gamma \mathbf{u})

Boosting to a frame where \mathbf{u}=0 (i.e. the object is at rest) gives \gamma=1, therefore the temporal component of \mathbf{U} is just c. guss was correct in this regard.

 

[WannabeNewton]

How do you know this?

That is how things are defined. The metric doesn't "go through time" or "go through space - time"; it is what gives space - time structure.

 

[guss]

That is how things are defined. The metric doesn't "go through time" or "go through space - time"; it is what gives space - time structure.

I understand that time is traditionally this medium for change. It's just a "that's just how it is" as an answer isn't really very satisfying. I'm looking for a scenario where this effect would break conservation of energy, or something.

This is obviously a very difficult hypothesis to prove/disprove, so granted I'm not expecting much. But, it would be cool to see.

 

[An Open Mind]

Here's one source:
http://en.wikipedia.org/wiki/Spacetime#Spacetime_intervals

Wikipedia...I can't understand it either that infomation sounds wrong.

 

[WannabeNewton]

Wikipedia...I can't understand it either that infomation sounds wrong.

What is wrong about it? Look at it this way instead: g_{\alpha \beta }U^{\alpha }U^{\beta } = -c ^{2} by definition. In the locally inertial frame of the test particle, \eta _{00}(U^0)^{2}= -(U^{0})^{2} = -c^{2} so U^{0} = c.

 

[guss]

Still looking for an answer.

 

[elfmotat]

Still looking for an answer.

Your original question doesn't make much sense. What do you mean when you say "this is near-instantaneously replaced by a heavier mass" ? Before you said this hypothetical universe contained nothing but the 1 kg mass you described. So where did this "heavier" mass come from?

Also, how exactly was one "replaced" by the other? If you mean that one magically disappeared, only to be replaced by another which magically appeared, then I can't give you an answer. I can't explain the physics of a situation that defies physics.