ask a physicist

[freyar]

Rotation isn't necessary. Richard Gott showed there's a solution in which two moving cosmic strings have a close encounter, and you can get a closed timelike curve around the pair. Which makes sense, as the *pair* can have angular momentum, and that means rotational frame dragging.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.66.1126

Interesting, I either didn't know or forgot about that.

 

[AbdulAlhazred]

If the rules hold *strictly* locally in all places (in no interaction do you ever violate them), then they pretty much have to sum up to conservation globally, as a result. In order for them to not hold globally, some individual interaction would have to violate the rules.

That is, unless you invoke some form of violation from *outside* the universe, but that's a non-falsifiable posit, and thus not really appropriate for a science discussion.

No, not really. The problem is that in GR 'total energy' of a system is not a well-defined concept, and thus the conservation of energy in the system as a whole is not defined. Even though no one observer will see a failure of conservation of energy within their reference frame the total energy of the universe as a whole need not remain the same.

 

[Umbran]

No, not really. The problem is that in GR 'total energy' of a system is not a well-defined concept, and thus the conservation of energy in the system as a whole is not defined.

Yes, I know that total energy is not easily defined in all systems in GR. But, that's not actually relevant. I don't need to know the total value to know it if is conserved.

Consider a large tank of water. I don't need to know what volume of water is in the tank to know whether water is entering or leaving the tank. I don't need to know how how much energy there is in a system to speak about the *change* in energy of the system. Actually knowing the total is useful, and often makes it easier - calculate the total energy at time X, and at time Y, and compare.

But, wait - time X and time Y are not absolutes in GR! So, I wouldn't expect this to be a useful way to find the change in energy anyway!

This is why I brought up, in effect, the differential form, rather than the integral form.

 

[tomBitonti]

Technically, we only have a visible universe to work with, yes. But, there's no particular reason to say, "beyond that edge, we know *nothing*, and anything goes!" Occam's Razor tells us that, unless we have a good reason to think the rules change out there, they probably don't.

It comes down to a point to consider:
Do we expect that the laws of physics are *absolutely* unbreakable, or is it enough that they be such that there is no real chance of them breaking?

CTCs behind event horizons? Who cares? CTCs around constructs that cannot ever be built? Again, so what? Do we really need to make presumptions about the mechanisms that will prevent such violations?

Why treat the interior of a black hole different than regions beyond the observational limit due to cosmic expansion? Wouldn't we expect the normal laws of physics to apply in either case, or to be outside of our interest and only a philosophical concern, again, in either case?

Thx!
TomB

 

[Umbran]

Why treat the interior of a black hole different than regions beyond the observational limit due to cosmic expansion? Wouldn't we expect the normal laws of physics to apply in either case, or to be outside of our interest and only a philosophical concern, again, in either case?

I'm not treating them differently.

For the Kerr black hole, or Tipler time machine, someone said, "Hm. Look at this weirdness that the math said is possible." Some folks in the science community react with, "Well, we should look for fundamental laws that will prevent that situation." I question how hard you have to look for solutions to the problem that may never happen.

If someone said, "Hey, look, the math says that you can do this thing, that will only dump you outside the observable universe," I might respond similarly. This is not sweating some particular details of a theory that otherwise works pretty darned well and has many verified predictions, because they are edge cases that may never actually occur.

But that's not what's happening here. Someone instead went, "Well, if I claim it is outside the observable universe, I can posit anything and consider it viable, even if it goes contrary to current observation and theory." This is basically leaning on non-falsifiability as an excuse to make stuff up.

 

[tomBitonti]

Sorry, have been away dealing with events in my extended family.

My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable? Considerations of both invoke an assumption of universality.

A question which arose when thinking about the above: Is there an analogue to Hawkin's radiation which is implied by the disappearance of matter and light across the observational limit? How is that different from the disappearance of matter and light into a black hole. (Using "disappearance" to mean "placement outside of our observation".) In both cases, entropy is carried outside of our observation. Except that not really, for black holes. Is there a similar "not really" for the other case?

Thx!

TomB

 

[Umbran]

My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable? Considerations of both invoke an assumption of universality.

One of the basic assumptions of GR is that the rules are the same everywhere.

So, the basic difference is this:

I was talking about some places where it looks like the rules can break - but the math tells us they only break in unobservable conditions, near unbuildable artificial constructs, or near objects whose existence is merely theorized that must be moving near each other in a very particular way while a sentient creature with a spacecraft is nearby. The math that breaks *also* requires the situation that may never happen.

Upthread, AA posited rule breaking that does not come out of the math, but would be cool, and suggested we pull a veil of the un-observable between here and there as an afterthought because otherwise it is problematic. Nothing in the math suggests what he wanted was possible - there's nothing mathematically or physically special about the border between the observable and non-observable universe, no real discontinuity is predicted. So then tagging on, "But it happens outside the observable universe," doesn't somehow make it plausible.

 

[freyar]

Sorry, have been away dealing with events in my extended family.

My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable? Considerations of both invoke an assumption of universality.

A question which arose when thinking about the above: Is there an analogue to Hawkin's radiation which is implied by the disappearance of matter and light across the observational limit? How is that different from the disappearance of matter and light into a black hole. (Using "disappearance" to mean "placement outside of our observation".) In both cases, entropy is carried outside of our observation. Except that not really, for black holes. Is there a similar "not really" for the other case?

Re: your first question, I agree whole-heartedly with Umbran's answer. Regarding your second, the answer is yes, there is an analog of Hawking radiation for cosmological horizons. For example, in the very far future, it may be that our universe looks like a spacetime that's empty except for a cosmological constant. If you were sitting in such a universe, there is a distance beyond which you can never see, which is a horizon (for you) much like the horizon of a black hole. And you would indeed detect Hawking radiation from that horizon. There are some subtle differences in comparison to the black hole, and this type of universe is actually a lot less well-understood than a black hole is, but there are some definite similarities.

I should say that this isn't something we see in our universe. Specifically, if we look far away, we can see the opaque age of the universe in the form of the cosmic microwave background as closer (in time, really) than the horizon distance. The universe will have to get a lot older before we can't see the CMB.

 

[Landifarne]

Somewhat related to what you chaps have been discussing:

Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.

So, why don't we see a great deal of gravitational lensing due to it? Or, do we, and we just don't hear about it?

I can see how the lensing of objects within our own galaxy would be minimized if the dark matter is spread so uniformally within it but, if dark matter is so ubiquitous in the visible universe, shouldn't nearly all of the furthest galaxies be lensing in our scopes?

 

[Umbran]

Somewhat related to what you chaps have been discussing:

Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.

So, why don't we see a great deal of gravitational lensing due to it? Or, do we, and we just don't hear about it?

The thing to remember is that since dark matter interacts through gravity, clumps of dark matter will correspond to clumps of normal visible matter drawn to it - so, those clumps of dark matter are where galaxies are. So, when they talk about lensing around a galaxy, they are implicitly talking about lensing around dark matter, too.