ask a physicist

[freyar]

To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.

In relativity (or any theory based on it), yes, FTL travel implies the ability to travel back in time. There are also "time machine" spacetimes that don't require FTL travel, but most physicists in the field believe something prevents them from forming in a complete theory of quantum gravity (this is the chronology protection conjecture).

 

[Umbran]

There are also "time machine" spacetimes that don't require FTL travel, but most physicists in the field believe something prevents them from forming in a complete theory of quantum gravity (this is the chronology protection conjecture).

Yes, well, the alternative can make a physicist lose many hours of sleep. A little belief instead of a lot of insomnia.

I am not even sure it is really necessary to have the chronology protection conjecture, to be honest. The constructions typically necessary for them are such that, really, you don't have to worry about them happening in reality. As one of my professors long ago mentioned about Tipler's solution: "Infinite cylinders of neutronium don't just happen, and it isn't like anyone is ever going to be able to *make* one if they can't travel faster than light anyway."

Okay, if someone ever finds a cosmic string, that could be an issue. But maybe even that's okay.

 

[AbdulAlhazred]

To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.

And, I apologize for having caused this long of a discussion on this thread with my question. That's twice I've caused such.

Yes, ANY possible FTL travel produces the same paradoxes as (and effectively IS) time travel. If you can travel at FTL then from the perspective of some observer in some inertial frame of reference an effect will precede its cause. Beyond that you will violate the various basic conservation laws. These laws are unlikely to be just bypassable this way, as again Noether's Theorum's successful use in constructing symmetry laws from them suggests. IMHO we have effectively ruled out all forms of FTL travel to the very highest levels of confidence attainable in science.

The only exception MIGHT be what I was suggesting, instantaneous travel to a causally disconnected location in space-time without traversing the intervening space. But of course any way of doing so is the purest fantasy at this point, requiring unobtainium of an extreme nature (negative energy mainly).

 

[AbdulAlhazred]

Well, now you're getting poetic.

You're also... quite possibly wrong. The possibility that we live in a simulation is not one we can actually reject at this time.

https://en.wikipedia.org/wiki/Simulation_hypothesis

And yet there are incalculable results. Lets understand what 'incalculable' means. It means "no calculating system which could be constructed within the physical universe could ever calculate this result" and obviously the universe itself can't host a machine that can simulate the same universe with perfect fidelity, so there's no proof in your statement that it can even calculate the state of a cloud next week.

Trivially most multi-body problems are not solvable. You cannot, even in principle, know where the Earth will be exactly a billion years from now, not even if you knew exactly every single gravitational interaction. Even if you had a universe that consisted of nothing but the Sun, the Moon, and the Earth, even then it would be impossible. You could, with infinite computational power and infinite time, drive the calculation to arbitrary levels of accuracy, but you cannot attain a perfect solution.

Things like the cloud are MUCH MUCH more complex than that. Its doubtful that, given relativistic and quantum limitations on basic computational power, that any conceivable machine can even calculate the shape of that cloud.

I'm not saying these things are non-deterministic, just that they can't be calculated except by a complete full run of the entire universe from 0 to now.

 

[AbdulAlhazred]

As an example, we can imagine a universe with those conservation laws, in which time/FTL travel is possible, in a limited sense. It turns out that so long as your time travel never violates causality, no conservation rules get broken. This is equivalent to a universe in which no logical paradoxes occur as a result of time travel. Robert L. Forward wrote a novel, Timemaster, that takes place in such a universe, that has a form of cosmic sensorship. In the book, this manifests as a sort of predestination for the time traveller - he feels at every particular moment like he has free will, but once he starts time travelling, he really doesn't.

Forward was not the best at characterization, but for finding things to write about that seemed really wacky, but were technically allowed (as of the time of his writing, at least), he was pretty awesome.

The real question there is, could you change ANYTHING AT ALL, or is effectively such censorship going to exclude all time travel (think for instance about the evolution of the Universal quantum wave function, you can't change anything without affecting that, and it is UNIVERSAL). The interconnections between things are vast, almost ubiquitous. Every photon is entangled with something else somewhere, etc. At best you wouldn't be able to change anything significant. Maybe you could use such a system as a 'spying device' to inform you of what happened in the past (and there was a story about doing that, but the hitch is even 1 millisecond is 'the past', so basically you can spy on anyone anywhere with such a device).

We can, less easily, imagine a world in which the conservation laws do not *strictly* hold - that there are ways they may break. Those ways must be pretty arcane, but they may be in the back corners of how things work, such that we haven't ever seen them. But, maybe the ban on FTL still holds, for other reasons.

Well, the Universe, AS A WHOLE, can't be meaningfully said to obey the various conservation laws, so its definitely not clear exactly how restrictive they are. This may simply be a matter of our limited perspective or an incomplete formulation though. We just don't know.

 

[Umbran]

The real question there is, could you change ANYTHING AT ALL, or is effectively such censorship going to exclude all time travel

Impossible to tell. While we can say, "every photon is entangled with something" that's a bit glib - with the statistical nature of quantum mechanics that really may not matter all that often.

Well, the Universe, AS A WHOLE, can't be meaningfully said to obey the various conservation laws, so its definitely not clear exactly how restrictive they are. This may simply be a matter of our limited perspective or an incomplete formulation though. We just don't know.

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.

 

[freyar]

Yes, well, the alternative can make a physicist lose many hours of sleep. A little belief instead of a lot of insomnia.

So true...

I am not even sure it is really necessary to have the chronology protection conjecture, to be honest. The constructions typically necessary for them are such that, really, you don't have to worry about them happening in reality. As one of my professors long ago mentioned about Tipler's solution: "Infinite cylinders of neutronium don't just happen, and it isn't like anyone is ever going to be able to *make* one if they can't travel faster than light anyway."

Well, less exotic spacetimes, like Kerr black holes, also have CTCs. But they tend to be tucked behind horizons or where the matter that forms the black hole would be instead.

Okay, if someone ever finds a cosmic string, that could be an issue. But maybe even that's okay.

I think cosmic strings should be safe because they* don't rotate on their axis. Or rather that rotation isn't physical. *I'm assuming the cosmic strings come from a gauge symmetry breaking** since global symmetry breaking tends to make infinite string tensions and global symmetries don't seem to exist in quantum gravity. **And my statement might be more general anyway, but I'd have to go digging through textbooks to check.

 

[AbdulAlhazred]

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.

What I mean is that the total energy and other conserved values are not well-defined for the Universe as a whole. Not that there is some region wherein these rules don't hold, just that you cannot apply them to the whole, at least in their currently understood forms.

 

[Umbran]

Well, less exotic spacetimes, like Kerr black holes, also have CTCs. But they tend to be tucked behind horizons or where the matter that forms the black hole would be instead.

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?

I think cosmic strings should be safe because they* don't rotate on their axis.

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

 

[Umbran]

What I mean is that the total energy and other conserved values are not well-defined for the Universe as a whole. Not that there is some region wherein these rules don't hold, just that you cannot apply them to the whole, at least in their currently understood forms.

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.