Is time a property of space or the fields within it?

[StringJunky]

 

 

But if the "toy universe" is purely described by GR then there won't be any thing happening on the quantum level.

Yes.

 

Thanks to everyone for all the responses, I'm a bit clearer now on this.

[Pugdaddy]

I just watched a lecture by Nima Arkani Hamed. He says spacetime is doomed and it needs to be replaced. He talks about how it breaks down at small distances(10^-33cm) and times10^-43 s). At those small distances(Black Holes, Big Bang) the Uncertainty Principle makes the Energy so large in a tiny region of space that time ceases to be something meaningful.

[Mordred]

What he is referring to is the singularity problem.

[koti]

 

 

But if the "toy universe" is purely described by GR then there won't be any thing happening on the quantum level.

 

I assumed that our "toy universe" doesn't care what tools we use to describe it. Just like our real universe

[StringJunky]

 

I assumed that our "toy universe" doesn't care what tools we use to describe it. Just like our real universe

Yes, it didn't matter for my question which model was used but Strange is correct if it was just a GR question.

Edited April 21, 2017 by StringJunky

[Strange]

 

I assumed that our "toy universe" doesn't care what tools we use to describe it. Just like our real universe

 

 

The toy universe is defined entirely by the model used (unlike the real universe).

[koti]

 

 

The toy universe is defined entirely by the model used (unlike the real universe).

And its a great thing we can do that for the purposes of thought experiments. I just assumed that our toy universe cares about quantum mechanics too.

[Mordred]

It does but here is the problem, the energy due to QM fluctuations cause no effective action. Action is motion which GR covers. See the crux? Here is a fairly well detailed formula for action.

 

 

[latex]\stackrel{Action}{\overbrace{\mathcal{L}}} \sim \stackrel{relativity}{\overbrace{\mathbb{R}}}- \stackrel{Maxwell}{\overbrace{1/4F_{\mu\nu}F^{\mu\nu}}}+\stackrel{Dirac}{\overbrace{i \overline{\psi}\gamma_\mu\psi}}+\stackrel{Higg's}{\overbrace{\mid D_\mu h\mid-V\mid h\mid}} +\stackrel{Yugawa-coupling}{\overbrace{h\overline{\psi}\psi}}[/latex]

 

We have GR, the Higgs, the electromagnetic and the Yukawa coupling Also particle/antiparticle pairs under the Dirac umbrella. Provided the particles involved have a quanta of energy.

 

Below that threshold we can't even measure the effective action of a fluctuation with the most perfect instrument. The fluctuation individually can cause no action which includes interactive interferance (necessary to detect). As no action is induced how does GR include it ? Recall GR maps freefall motion via geodesics. (Worldlines).

 

(Do not confuse the above with quantum tunnelling ie that used on inflation, different process far different energy levels involved)

Edited April 22, 2017 by Mordred

[koti]

It does but here is the problem, the energy due to QM fluctuations cause no effective action. Action is motion which GR covers. See the crux? Here is a fairly well detailed formula for action.

 

 

[latex]\stackrel{Action}{\overbrace{\mathcal{L}}} \sim \stackrel{relativity}{\overbrace{\mathbb{R}}}- \stackrel{Maxwell}{\overbrace{1/4F_{\mu\nu}F^{\mu\nu}}}+\stackrel{Dirac}{\overbrace{i \overline{\psi}\gamma_\mu\psi}}+\stackrel{Higg's}{\overbrace{\mid D_\mu h\mid-V\mid h\mid}} +\stackrel{Yugawa-coupling}{\overbrace{h\overline{\psi}\psi}}[/latex]

 

We have GR, the Higgs, the electromagnetic and the Yukawa coupling Also particle/antiparticle pairs under the Dirac umbrella. Provided the particles involved have a quanta of energy.

 

Below that threshold we can't even measure the effective action of a fluctuation with the most perfect instrument. The fluctuation individually can cause no action which includes interactive interferance (necessary to detect). As no action is induced how does GR include it ? Recall GR maps freefall motion via geodesics. (Worldlines).

 

(Do not confuse the above with quantum tunnelling ie that used on inflation, different process far different energy levels involved)

 

I think I understand, at least partially. Would it be right to state that what you wrote would not be valid when dealing with singularities ?

[geordief]

It does but here is the problem, the energy due to QM fluctuations cause no effective action. Action is motion which GR covers. See the crux?

I don''t see the crux(never mind ) but that is a point I had never come across before.

 

I like it.

 

But these fluctuations do have an effect on the "macro world" even if there is no "effective action"? They are not simply "self referential" ,surely?

 

Or is my question "behind the curve" ?

[Mordred]

 

I think I understand, at least partially. Would it be right to state that what you wrote would not be valid when dealing with singularities ?

Correct the BB singularity and the singularity of a BH. The energy levels in such a compact finite region and high temperature far exceeds a quanta.

I don''t see the crux(never mind ) but that is a point I had never come across before.

 

I like it.

 

But these fluctuations do have an effect on the "macro world" even if there is no "effective action"? They are not simply "self referential" ,surely?

 

Or is my question "behind the curve" ?

A collection of fluctuations within a tight enough volume ie pointlike that exceed a quanta will have an effect. Or for example between two plates. However you must exceed a quanta to gain action.

[mistermack]

If time is a characteristic of change, then it seems to me that time can have local character, and universal character.

 

Say for example, that it was physically possible for events for a very simple particle to exactly reverse what had gone before, then you could claim that for that particle, time had gone backwards. Or if nothing happens to it, maybe you could say that time stalled.

 

But, if the rest of the Universe had moved on, as normal, then could you really make the case for any of that?

 

Really, you constantly undergo change, relative to the rest of the Universe. Even if you don't undergo any change yourself.

If every gravitational field extends to infinity, then the field that each individual is in is in a constant state of change. Even if it's minimal. So time of the Universe is ticking onwards, even if you "freeze" or reverse.

[KipIngram]

I just read a paper this past weekend that argued for a time-free expression of physics using stationary action principles. The author went to some length to discuss the history of time, beginning with the use of the sidereal day as the "standard," and went from there to Newton's use of the angle swept out in an orbit as a "stand-in" for time. Then he noted that since no body in the solar system is unperturbed by other bodies, that the swept angle of one body wasn't "ideal" in that sense. So the next step was a similar expression that included all bodies in the solar system. Finally he noted that in the strictest sense even that's not ideal, since gravity never "stops." So by the end of the paper he'd stated that the only truly correct standard of time would incorporate the dynamics of the full universe.

 

Now, he was talking about our assignment of numerical values to instants in time, rather than the "raw notion" (if that means anything). And he limited the argument to classical physics. It was an interesting read; I'd post the link but I don't have it open in my browser anymore.

 

I always felt that special relativity treats time as equivalent to the spatial coordinates (except for the i, of course), and the way that space and time can transform into one another in relativity always made me feel that they aren't just "treated together" but rather have a deep connection of some kind. I never felt that spacetime "rose out" of relativity - it seems more like part of the presumed background framework. On the other hand most of the introductory relativity texts go to some trouble to be very careful about the measurement of distance and duration by observers - that does sort of make them something perceived by observers (which could be anything of course - parts of the system "observe" one another in this way).

 

I've pretty much decided that the notion of space doesn't really mean anything without objects in it - space seems to arise from relationships amongst those objects. Given how closely space and time "play together" I guess I feel that way about time too. Especially in general relativity, where the very shape of spacetime emerges from the "stuff" that's in it.