ask a physicist

[freyar]

Yes, I understand this. There are however regions of space, presumably, which are 'causally disconnected' from us. They are FOREVER outside our light cone. If I travelled instantly to the Sun then in 8 minutes light from that location would arrive and there would be different inertial frames of reference in which causality, conservation of angular momentum, etc would be violated. If I travel instantly a googleplex lightyears from here, that will NEVER HAPPEN. So is such travel disallowed for the same reasons that travel 8 light minutes from here seems to be? I question that! Again, its a question, not a statement of some fact that I think I've uncovered or some crackpot theory that I insist must be true. It is really at this point more of a philosophical question almost than a science question, until some unforeseen time when we invent a way to do it.

Two quick points:
(1) it's not at all clear that our universe has areas forever out of causal contact with earth. For example, in a universe with a cosmological constant, people on earth will eventually be able to see the entire universe, even though it will take infinite time to see an infinite distance away. That's just saying that we need to wait 8 minutes to see the sun, but we need to wait umpteen-gazillion years to see something umpteen-gazillion lightyears away, and we need to wait longer to see farther. Off the top of my head, I am not thinking of an easy way to change that, either, but I admit I'd have to look at a few papers to say more than that. (Unless you are maybe talking about disconnected components of the universe, where there is literally no space between them.)

(2) In a relativistic theory, if you can travel FTL to anywhere, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.

 

[tomBitonti]

I think that, given the assumptions of Special and General Relativity (in which context we are talking about light cones), that the difference isn't relevant. So long as the space between is continuous, smooth, and the same rules of physics apply in all of it, then "forever outside" and "temporarily outside" are not fundamentally different. If those assumptions do not hold, then we have to hld the whole idea of the light cone as suspect.

Specifically, I believe you can construct the following scenario: Point A and Z in space are so far apart, that they never fall into each other's future light cone. There is a point M, between them, that is, at the time of your travel, is not in the light cone of either A or Z, but will eventually lie in the light cones of both. A stepping stone, so to speak, that allows the piecewise measurement. I think that you can always generate a string of such stepping stones, so long as the distance between A and Z is not *actually* infinite.

My apologies for any mangling of terminology:

I *think* we are getting in trouble here, mixing notions of points in space-time with points in space. Reaching a point seems to be shorthand for reaching a point on the geodesic of that point at time zero, with given spatial coordinates, subject to some selection of coordinate systems. That shorthand works pretty well for everyday stuff, but not on a cosmological scale. In flat spacetime, the light cone of a given point always intersects the geodesic of an arbitrary point (from simple geometry), but, that is not true for an expanding universe (nor, for example, for light cones originating from points inside black holes).

(Although, that is only true of geodesics; a photon which is emitted from a given point may be unreachable from another point, for example, if the photon were emitted pointing away from the second point.)

Thx!
TomB

 

[AbdulAlhazred]

In the normal universe, yes.

But you're positing a universe where going there is possible. That possibility throws the principle out the window! We cannot measure the distance only because FTL travel is not possible - as soon as you allow FTL to the spacial location, then the measurement becomes possible in principle.

Which is to say, you cannot stand by the principle, and break the principle, in the same breath and remain consistent.

Eh, I think you'd have to take into account the mechanism by which you undertook this travel. Given that we have no such mechanisms and don't know what they would actually entail its hard to speculate, but suppose you opened a wormhole to such a remote place, you'd never be able to measure the distance there.

I think that, given the assumptions of Special and General Relativity (in which context we are talking about light cones), that the difference isn't relevant. So long as the space between is continuous, smooth, and the same rules of physics apply in all of it, then "forever outside" and "temporarily outside" are not fundamentally different. If those assumptions do not hold, then we have to hld the whole idea of the light cone as suspect.

Well, this may be true, but I think its not really established. Again, questions of definitions like 'space between'. Science can only work with what is observable, what isn't observable in some fashion doesn't exist in some fashion either. Perhaps the effects of this 'continuous, smooth space' ARE significant and it is exactly that, real. I don't know. When one starts talking about places you can never go or see and things you can't ever measure the philosophical ice gets real thin for everyone .

Specifically, I believe you can construct the following scenario: Point A and Z in space are so far apart, that they never fall into each other's future light cone. There is a point M, between them, that is, at the time of your travel, is not in the light cone of either A or Z, but will eventually lie in the light cones of both. A stepping stone, so to speak, that allows the piecewise measurement. I think that you can always generate a string of such stepping stones, so long as the distance between A and Z is not *actually* infinite.

BUT there will demonstrably be no way to communicate any information from A to Z, even using these intermediary points, as the expansion of space will require that you establish new intermediary points at a rate that is impossible, the number of them will grow to infinity before any information passes along the chain. Any two points which are NOW not every going to be in contact again will fall into this category, you can't get around the speed of light. The distance between A and Z is not actually 'infinite', but it is growing at greater than C.

Now, necessarily there was some point in the past where all these points A, Z, and all the ones in between WERE in communication with each other, around 10^-34 seconds after the big bang as I recall, when the Universe was in thermal equilibrium before inflation blew it up. Some points may have been in contact long after that, indeed some fall over our horizon all the time even now. Maybe that matters, maybe not so much, particularly with points we've not been in contact with since the start of inflation.

 

[freyar]

My apologies for any mangling of terminology:

I *think* we are getting in trouble here, mixing notions of points in space-time with points in space. Reaching a point seems to be shorthand for reaching a point on the geodesic of that point at time zero, with given spatial coordinates, subject to some selection of coordinate systems. That shorthand works pretty well for everyday stuff, but not on a cosmological scale. In flat spacetime, the light cone of a given point always intersects the geodesic of an arbitrary point (from simple geometry), but, that is not true for an expanding universe (nor, for example, for light cones originating from points inside black holes).

(Although, that is only true of geodesics; a photon which is emitted from a given point may be unreachable from another point, for example, if the photon were emitted pointing away from the second point.)

From what I read of Umbran's and AbdulAlhazred's posts, I'm pretty sure we haven't run into that confusion. The conversation seems pretty clear to me. And I know I've been careful on that issue (I work on this all the time, after all). Of course, with FTL travel, there's less of this kind of distinction. But, anyway, talking about causal connection always requires talking about the space and time coordinates of a point.

 

[AbdulAlhazred]

Two quick points:
(1) it's not at all clear that our universe has areas forever out of causal contact with earth. For example, in a universe with a cosmological constant, people on earth will eventually be able to see the entire universe, even though it will take infinite time to see an infinite distance away. That's just saying that we need to wait 8 minutes to see the sun, but we need to wait umpteen-gazillion years to see something umpteen-gazillion lightyears away, and we need to wait longer to see farther. Off the top of my head, I am not thinking of an easy way to change that, either, but I admit I'd have to look at a few papers to say more than that. (Unless you are maybe talking about disconnected components of the universe, where there is literally no space between them.)

(2) In a relativistic theory, if you can travel FTL to anywhere, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.

Oh yeah, I quite understand that FTL and time travel are effectively the same thing, and that ANY FTL travel results in violations of causality, conservation of momentum, and conservation of angular momentum (at least, maybe others too, I'm not sure). In fact IMHO I don't see how even things like Alcubierre Drive schemes would avoid this fact (they might work out OK for observers in the warp bubble and SOME observers outside it, but I think inevitably someone somewhere will see an inconsistency).

The question then is "if you build a wormhole to a causally disconnected location in space-time, will there be a meaningful violation of causality, etc?" In SOME sense the two points are 'in the same space-time continuum' and yet they are utterly out of touch with each other and thus any violations are effectively 'cloaked' forever by relativistic limitations of information transfer. Presumably if a wormhole works at all, the 'local channel' of information transfer doesn't really matter, since our nice closed time-like curve means we have essentially normal connectivity between the points at either end which isn't notionally different from any other such connectivity.

What gets you in trouble if you were to say build a wormhole to Alpha Centauri is that an observer somewhere observing each end of that wormhole WILL, as you point out, see something inconsistent (IE something will happen before its cause, or angular momentum won't be conserved, etc).

EDIT: Oh, yes, your first point about the future history of the Universe. This is a very good point of course. As far as we can determine today nothing will stop the Universe from expanding forever, and the rate of expansion will increase forever as well. We don't really know all that much though, and the interesting question then becomes "does forever count?" If, when you extend your light cone to eternity it overlaps with all the others, but only after literally infinite time, does that mean anything? Does that create constraints on what can happen here and now today? That's an interesting question!

 

[Umbran]

Eh, I think you'd have to take into account the mechanism by which you undertook this travel. Given that we have no such mechanisms and don't know what they would actually entail its hard to speculate, but suppose you opened a wormhole to such a remote place, you'd never be able to measure the distance there.

"Never" is a bit broad, especially when you have already noted that you have to take into account the mechanism. You are positing that you can open a wormhole, but that wormholes so opened do so to *random* places in spacetime? That sounds plausible to you? You wouldn't instead presume that the mechanism for opening a wormhole and keeping it open would have to be known, and thus controlled? Or that you could not infer something from measurements of the wormhole itself?

Actually, this generalizes, so that the mechanism actually doesn't really matter.

Science can only work with what is observable, what isn't observable in some fashion doesn't exist in some fashion either.

More like, what isn't observable isn't science - if we cannot even in principle observe it, we are in the realm of the non-falsifiable.

So, in order to question the science, you bring in something unobservable - a mechanism for travel where you don't know where you are going?

In reality, science deals with a *lot* of things that are not directly observable, but can be inferred from observations. Nobody, for example, has ever observed a lone quark. It is expected by many that seeing them, even in principle, is not possible.

BUT there will demonstrably be no way to communicate any information from A to Z

If we are all limited to normal sub-light speed travel, you are correct. There is never any way to communicate from A to Z.

But *you* are then positing what happens when you go to Z!

I then say, any method you can use to go to Z, I can in principle use to measure the distance. The ability to travel to some arbitrary point in spacetime implies the ability to measure the distance.

 

[AbdulAlhazred]

"Never" is a bit broad, especially when you have already noted that you have to take into account the mechanism. You are positing that you can open a wormhole, but that wormholes so opened do so to *random* places in spacetime? That sounds plausible to you? You wouldn't instead presume that the mechanism for opening a wormhole and keeping it open would have to be known, and thus controlled? Or that you could not infer something from measurements of the wormhole itself?

Actually, this generalizes, so that the mechanism actually doesn't really matter.



More like, what isn't observable isn't science - if we cannot even in principle observe it, we are in the realm of the non-falsifiable.

So, in order to question the science, you bring in something unobservable - a mechanism for travel where you don't know where you are going?

In reality, science deals with a *lot* of things that are not directly observable, but can be inferred from observations. Nobody, for example, has ever observed a lone quark. It is expected by many that seeing them, even in principle, is not possible.




If we are all limited to normal sub-light speed travel, you are correct. There is never any way to communicate from A to Z.

But *you* are then positing what happens when you go to Z!

I then say, any method you can use to go to Z, I can in principle use to measure the distance. The ability to travel to some arbitrary point in spacetime implies the ability to measure the distance.

Well, of course these are all perfectly good points . I think there are processes which are fundamentally incalculable though, like the exact shape of a cloud which will appear over a given point on the surface of the Earth a week from now, no amount of detailed information about the current state of the Earth will let you calculate that, not even in principle (maybe a week is too little, its hard to say, but I know enough about non-linear dynamical systems to know that there are FUNDAMENTALLY incalculable results). So maybe a 'wormhole to anywhere' really is perfectly feasible. It goes SOMEWHERE, but you won't know where until after you open it, and if the only openings have to be outside causally connected space... Its sort of like the weak anthropic principle, I'm just saying "the opening is where the opening is, and that place meets the criteria for where an opening can be".

Things like quarks of course ARE 'observable', in the same way that a book is observable, they have effects that can be sensed by our senses and thus affect changes in our mental state which represent our knowledge of their existence. The TRULY unobservable is different, and of course making statements about it is in some sense 'non-scientific', but there's that area of 'speculation' in which we use the rules we have derived from the process of science, as opposed to just say painting a dragon over there and talking about how cool the space dragon is. I think there's a meaningful difference there. So I would side with people who speculate about other Universes and etc calling those 'scientific' speculation. Not science precisely perhaps, and the difference is meaningful, but still not simple blind invention without reason.

 

[CaptainGemini]

(2) In a relativistic theory, if you can travel FTL to anywhere, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.

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.

 

[Umbran]

Well, of course these are all perfectly good points . I think there are processes which are fundamentally incalculable though, like the exact shape of a cloud which will appear over a given point on the surface of the Earth a week from now, no amount of detailed information about the current state of the Earth will let you calculate that, not even in principle

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

 

[Umbran]

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.

Er. We should be careful about how we say such things, really. Sometimes I'm sloppy.

Technically, at the moment, we expect that those conservation laws hold strictly, and that FTL is not possible.

We do not know for absolute certain they are both strictly and totally true. If they are both strictly true, we do not know that one *causes* the other (and which way that causality goes), or if both are merely axioms of the universe (such that both must be true in order to have a consistent universe, and neither one really has priority), or if both are merely logical results of some even higher set of principles.

In reality, when we say, "You can't do time travel, because that violates causality/conservation, etc," what we are really saying is, if you plug in numbers faster than light, the rules as we understand them give results that do not make sense.

But, there are ways...

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.

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.