Space-time created ahead of matter - how far?

[Chalnoth]

By "pre-existing space", do you mean that which existed before spacetime began?

No. I'm purely talking about an expanding universe here. So the pre-existing space is the space that existed slightly earlier in the expansion.

If we run time backwards to the BB doesn't everything converge according to GR?

Yes, that is the case according to GR. Which is one reason why we know GR is wrong. Because GR is wrong here, we can't conclude anything about the very early universe based solely upon this sort of analysis.

And if the total amount of energy is increasing I fail to understand how the law of conservation of energy isn't being violated?

As marcus pointed out, that law doesn't apply here. Conservation laws stem from symmetries. The symmetry which energy conservation stems from is symmetry in time. The expansion of the universe violates that symmetry.

 

[Chalnoth]

Yes, I can see how there is no physical boundary condition with relation to space, but surely we have to consider time as well? And yes, two observers are effectively within their own time frame references but the future and the past still exists for them both, doesn't it?

This is why I talked about the fact that there is no such thing as a global now. If there is no such thing as a global now, then the present cannot be considered any sort of physical boundary.

 

[DaveC426913]

I think the topic has drifted away from Saddlestone's original question, which I'm not sure has been adequately answered in the spirit in which it was asked (i.e. in layperson's terms).

Let me try.

If the Big Bang had created only a whopping two particles, each flying away from the other at .5c which, after 2 years, are now 1 light year apart, what would the shape of spacetime be?

- a 1D line, 1 light year long, one particle wide (only exists where the particles are)
- a 3D sphere, 1 light year in diameter (only expands with particles' speed)
- a 3D sphere 2 light years in diameter (expanding at c, along with any EM)

If the BB had actually made 3 particles, and the first two as before whizzed away at .5 c, but the third bopped around in between the first two, would space time be formed as that particle moved, say perpendicular to the other two?

i.e does space time exist anywhere where matter has not made it yet?

 

[Lost in Space]

This is why I talked about the fact that there is no such thing as a global now. If there is no such thing as a global now, then the present cannot be considered any sort of physical boundary.

It's hard for me to get my head round this but does that mean then that different parts of the universe can be in front of or behind each other in terms of the time that has passed, and if so would there be any kind of limit as to how much? In other words are there areas of the universe that are already existing but as far as we're concerned they're in our future and we likewise are in their past?

We locally share a common history of the past. And as far as I'm aware there's no way for us to determine anything that lies in our future but I think I'm correct in saying that from what you say it seems to indicate that in other areas of the universe our future has already happened?

 

[Chalnoth]

It's hard for me to get my head round this but does that mean then that different parts of the universe can be in front of or behind each other in terms of the time that has passed, and if so would there be any kind of limit as to how much?

No, it means that when you are dealing with points separated by some distance, there is no non-arbitrary way to say whether a particular time at one point is ahead or behind a particular time at another. You always can write down a global "now", but it is always arbitrary: somebody else could come up with a completely different definition that would be every bit as valid.

Fundamentally, this means that the past and the future must exist in the exact same way as the present. Reality cannot be a sort of wave moving from the past into the future, but instead the flow of time is merely a result of our perception of it.

 

[Lost in Space]

No, it means that when you are dealing with points separated by some distance, there is no non-arbitrary way to say whether a particular time at one point is ahead or behind a particular time at another. You always can write down a global "now", but it is always arbitrary: somebody else could come up with a completely different definition that would be every bit as valid.

Fundamentally, this means that the past and the future must exist in the exact same way as the present. Reality cannot be a sort of wave moving from the past into the future, but instead the flow of time is merely a result of our perception of it.

And yet aren't we constrained by the fact that we cannot accurately predict the future? So isn't this a boundary of sorts, even if it is a localised phenomenon? If as you say the flow of time is purely perceptual, shouldn't we be able to receive information from the future as easily as that from the past?

 

[Saddlestone]

I think the topic has drifted away from Saddlestone's original question, which I'm not sure has been adequately answered in the spirit in which it was asked (i.e. in layperson's terms).

Let me try.

If the Big Bang had created only a whopping two particles, each flying away from the other at .5c which, after 2 years, are now 1 light year apart, what would the shape of spacetime be?

- a 1D line, 1 light year long, one particle wide (only exists where the particles are)
- a 3D sphere, 1 light year in diameter (only expands with particles' speed)
- a 3D sphere 2 light years in diameter (expanding at c, along with any EM)

If the BB had actually made 3 particles, and the first two as before whizzed away at .5 c, but the third bopped around in between the first two, would space time be formed as that particle moved, say perpendicular to the other two?

i.e does space time exist anywhere where matter has not made it yet?

Thank you DaveC426913, I think you've captured my question, and brought the thread back on track.

I also had in mind that any particle has an uncertainty in its position and therefore space-time must exist where ever the particle might exist, even though it may never have actually been there. Therefore space-time has to be bigger than the matter that is making it. Or is this uncertainty just a phenonenon of a human trying to measure the particle's position?

 

[Chalnoth]

And yet aren't we constrained by the fact that we cannot accurately predict the future? So isn't this a boundary of sorts, even if it is a localised phenomenon? If as you say the flow of time is purely perceptual, shouldn't we be able to receive information from the future as easily as that from the past?

Well, that sort of constitutes a local boundary after a fashion. But it isn't a physical boundary, and certainly not a global one.

But no, the reason why we remember the past but not the future is because of entropy, which sets the arrow of time.