# Does gravity have effects through time?

1. Aug 22, 2011

### 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.

2. Aug 22, 2011

### Andrew Mason

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.

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

3. Aug 22, 2011

### guss

4. Aug 22, 2011

### WannabeNewton

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

5. Aug 22, 2011

### atomthick

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.

6. Aug 22, 2011

### guss

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.

7. Aug 22, 2011

### atomthick

Gravity doesn't go through time.

8. Aug 22, 2011

### guss

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

9. Aug 22, 2011

### WannabeNewton

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.

10. Aug 22, 2011

### guss

How do you know this?

11. Aug 22, 2011

### elfmotat

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

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.

12. Aug 22, 2011

### WannabeNewton

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.

13. Aug 23, 2011

### guss

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.

14. Aug 23, 2011

### An Open Mind

15. Aug 23, 2011

### WannabeNewton

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 $U^{0} = c$.

16. Aug 24, 2011

### guss

17. Aug 24, 2011

### elfmotat

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.