Is it possible to decrease space-time density?

[MattRob]

Alright, I'm a bit ignorant when it comes to more advanced subjects (well, very ignorant...), but, from my understanding, according to Einstein's theories, a strong gravitational field, perhaps describable as increased Space-Time density, increases the rate of time of an observer and warps space, creating gravity.
Now; I'm not hopping off the mainstream here, but according to the various mainstream models of physics, is it possible in any way to decrease space-time density?
If the answer is yes, then how?
If it's not proven to be possible, is it proven to be impossible?
_{To be clear, I'm asking if it's mainstream, not really going into abstract speculations.}

Thanks in advance

 

[bcrowell]

There really isn't any concept of spacetime density. There is just spacetime curvature. An object low down in a gravitational potential well experiences a *decreased* rate of time, not an increased one. A good book to start out with would be Relativity Simply Explained, by Martin Gardner.

 

[MattRob]

So, an object in empty space has zero gravitational potential? And I'm guessing the only way to get a negative potential would be with exotic materials that don't exist.

So another question, would it be possible in any way to reverse the effects of special relativity without creating a negative gravitational potential?
i.e., instead of time running faster, mass increase and length contraction, would it be possible somehow to reverse the effects so time runs slower, mass decreases and length expands?

 

[pervect]

So, an object in empty space has zero gravitational potential? And I'm guessing the only way to get a negative potential would be with exotic materials that don't exist.

Yes, a sufficiently large mass of exotic matter could in theory generate a negative potential energy (along with a repulsive gravity), which would speed up time. As far as whether exotic matter exists or not (especially in the required bulk sense), that's unclear, though theory predicts regions of negative energy density (i.e. the Casimir effect).

 

[MattRob]

But Negative energy density doesn't have the same effects as negative energy, does it?
(Which I assume would be proof of negative mass sense e=m, with a unit converter, c^2...)

And is there any way to get a negative potential energy without using negative mass?

Btw; thanks for all of the help and patience so far. Seems I'm missing some pretty simple concepts here, sorry and thanks again.

 

[pervect]

I'm not sure what you mean when you say (or ask) "negative energy density doesn't have the same effects as negative energy".

There's no good way to measure the total energy in a lot of situations in GR, but one can always measure the energy density in some frame - so a careful writer will say "energy density" rather than "energy". But when you have some rest frame with some volume V, the amount of energy in said volume in said rest frame will be a negative number if you have a negative energy density.

However, exotic matter doesn't necessarily require a negative energy density, because pressure as well as energy, can cause gravity in GR. See for instance http://math.ucr.edu/home/baez/einstein/ for a statement of how it can be said that pressure causes gravity in GR.

I'm not sure that this form of exotic matter (one with a positive energy density but negative pressure) will help you to construct a gravitational time accelerator, though, due to the fact that you need a static system in hydrodynamic equilibrium to really do what you want.

 

[bcrowell]

So, an object in empty space has zero gravitational potential?

No, and this isn't true in Newtonian gravity either. There is no well-defined zero of potential energy. Only differences in potential are well defined.

And I'm guessing the only way to get a negative potential would be with exotic materials that don't exist.

No, and this is also false in Newtonian gravity. The potential gets more negative as you get closer to an object.

If you change your references to potential Φ to references to change in potential ΔΦ, and your negative ΔΦ to positive, what you said makes more sense, but it still isn't quite right. The problem is that you're assuming that there is a normal Φ corresponding to a normal rate of the flow of time.