Time travel doesn't exist because time travel wiped out the timelines where it did

[Umbran]

Well, this breaking news from The Great White North should put the entire debate to rest!


(to be fair, he seems like a pretty awesome grandpa!)

If only he had said that he built it in 2028...

 

[tomBitonti]

So … this was an interesting read. (What parts that I followed.)

https://arxiv.org/pdf/0808.1081

(It seems) there is a lot more nuance than is being presented here.
Can some more practiced in the field say if this paper is legitimate?
TomB

 

[Paul Farquhar]

This idea is really put well together in an explanation in Event Horizon!

AKA the WH:40K movie.

 

[Corinnguard]

They go back in time, but go back to the wrong location!

The Earth's movement is not static in space. It moves at a very fast speed. It moves thousands (if not faster, it's hard to calculate for me, but it's not just it's rotation, it's a movement around the Sun, then the Solar System's movement around the Galactic Center, then the Galaxy's movement through the Galactic Cluster and the Galactic Cluster's movement through space itself which may be up to millions of miles (Kilometers) per day/week/month/year).

Without knowing the size and mapping the universe, it may be impossible to know the exact location the Earth has been in the past (you would need a coordinate map for the Universe...for starters). So...the time traveler's go back in time, but as this would be breaking the time/space relationship to a degree, they go back to where the Earth is currently...except...there is no Earth in that location at that time period.

In Avengers: Endgame, Iron Man had to invent a Space/Time GPS to get around this particular problem. Plus they also had to use the Quantum Realm, a place that sits outside of normal space/time that connects to all points in space/time.

 

[Umbran]

So … this was an interesting read. (What parts that I followed.)

https://arxiv.org/pdf/0808.1081

(It seems) there is a lot more nuance than is being presented here.
Can some more practiced in the field say if this paper is legitimate?
TomB

So, to start with, what do you mean by "legitimate"? Noting that "legitimate" does not mean "correct in its conclusions".

Articles in arXiv are not peer reviewed by that organization, if that's what you mean. Unless this paper was also published by someone else who does such review, we don't have confidence that it has been given a lot of scrutiny.

Moreover, this is in the "popular science" section of arXiv, and doesn't claim to be presenting new results or new science, but an interpretation. Interpretations do not question data or established science. They present ways of thinking about the established results. Sometimes, this is to suggest what is physically happening, and sometimes it is just a source of inspiration for further investigation, "If we think of this as X, then that implies Y, which may be interesting," kind of stuff.

And, at the end of the paper, they even say, "There is no “fact of the matter” about the interpretation of the cosmological redshift: what one concludes depends on one’s coordinate system or method of calculation." So, they aren't even claiming to present a "fact" or "truth".

One element of the paper I find odd is "Redshifts of nearby galaxies are Doppler shifts". To which I say, "Yeah, but we already knew that." Cosmological expansion does not occur within gravitationally bound (or, more strongly bound) systems. All galaxies near us are part of the Local Group, which is gravitationally bound. I suppose this discussion is useful in establishing the first step of their logic, but it isn't telling us anything interesting in and of itself.

From there, they seem to take the position that since you can think of cosmological expansion in terms of covering the distance from A to B piecewise with (nigh infinite) observers that can each think of it as local Doppler effects, then the whole thing is a Doppler effect.

And I suppose that isn't... wrong. But it also has the ring of... thinking of the issue as a collection of individual trees rather than a forest, which risks missing important larger-scale aspects of the phenomenon. For example, in my admittedly quick skimming of the paper, it looks like they seem to manage to avoid talking about objects so distant that we'd have to say their relative velocity is greater than the speed of light, which makes no sense as a physical interpretation.

 

[briggart]

Not really. This is most readily seen in how the observable universe has a measurable radius (some 46.5 billion light years)

Anything beyond the edge of the observable universe is receding from us faster than light, such that light from it will never reach Earth.

This is correct, but it is mostly just a coincidence. There are few things that are mixed together here, which are clearly related but I do not think they apply to the point you are trying to make.

When we speak of observable universe, we usually refer to the farthest distance from which a signal could have reached us by now, which gives that 45 something billion years.

The Hubble radius is the distance in which "current" galaxy recession velocity would be equal to c according to Hubble law. It is in general not the same as the radius of the observable universe, but it is typically close to it.

Finally, we have the distance a photon can travel from now to the end of time.

If our universe were made up by regular stuff (i.e. stuff with "not too big" negative pressure), the radius of the observable universe and the Hubble radius both grow with time faster than than the expansion rate, while the future horizon is infinite. Given enough time, and light from anywhere in the universe will reach us, even if the emitter is currently superluminal with respect to us.

However, we seem to live in a spatially flat universe which has entered a phase of dark energy dominated expansion which, were it to continue indefinitely, would lead to a finite future horizon. As it is now, nothing that is currently emitted farther than a few billion light years will ever reach us, but this is a consequence of the fact that dark energy leads to an accelerated expansion, not to the fact that some stuff is moving in some sense faster than light.

But, Einstein tells us that real motion faster than light is impossible. Therefore, the measured motion due to cosmological expansion isn't real.

He told us that in regard to inertial frames in special relativity, not general relativity which is the relevant framework here. Locally, GR reduces to SR, but on cosmological scales there is no global inertial frame in which to apply SR results.

With cosmological expansion, the motion is an illusion. What is actually happening is that space is added between objects. This is extremely important when we speak about whether expansion is accelerating or decelerating or steady-state. For accelerating expansion, if it were actual motion, you'd need to be exerting forces on objects to accelerate them. But, there is no such force.

I think you are looking at this from a SR point of view where spacetime has an existence of its own. In GR spacetime is nothing more than the 4D-geometric properties of the distribution of stuff, it would not expand if stuff were not moving away from each other. At the very least, GR gives us a loop: stuff is moving away because space is expanding, but space is expanding because stuff is moving away.

Regarding acceleration, if you take the position that dark energy is a pure cosmological constant, then the force you are looking for is gravity. Otherwise you need some dynamical component with large negative pressure, which you can interprete as a force if you like, but in the end it is just the request of high symmetry and the initial conditions of the problem which lead to an accelerated expansion.

 

[tomBitonti]

So, to start with, what do you mean by "legitimate"? Noting that "legitimate" does not mean "correct in its conclusions".

Mostly in the sense of whether the view expressed is a mainstream view or more "fringe".

One of the points that was made (if I understood it correctly), is that defining the velocity of cosmologically distant objects is problematic.

(See: https://arxiv.org/pdf/gr-qc/0506032v2 and Velocities of Distant Objects in General Relativity Revisited - Gravitation and Cosmology. I don't have a subscription to read the second, but it looks very interesting and on point.)

A second matter is whether there are local effects of cosmological expansion. I have read elsewhere that expansion does cause a local "stretching" effect, which appears as a slight outwards force. This would be detectable as a slight change to stable planetary orbits. This does not mean continuous expansion. The referenced article flat out says this doesn't happen.

(See, for example: The influence of the cosmological expansion on local systems, referenced by Why does space expansion not expand matter?.)

Thanks!
TomB

 

[Umbran]

Mostly in the sense of whether the view expressed is a mainstream view or more "fringe".

It isn't the consensus view as I understand it (which is supported by the fact that the paper exists at all - you don't typically write papers reiterating currently accepted views). But it doesn't have implications such that anyone would say it is "fringe" either.

One of the points that was made (if I understood it correctly), is that defining the velocity of cosmologically distant objects is problematic.

Well... I would more say that it is technically difficult, with various potential sources of error, rather than "problematic".

A second matter is whether there are local effects of cosmological expansion. I have read elsewhere that expansion does cause a local "stretching" effect, which appears as a slight outwards force. This would be detectable as a slight change to stable planetary orbits. This does not mean continuous expansion. The referenced article flat out says this doesn't happen.

So, scale matters. Cosmological expansion is a model developed to discuss distance scales so vastly, incredibly large that galaxies, and even galaxy groups, are effectively pinpoint objects of zero size. One should not generally expect effects seen in a model at one scale to be visible on a radically different scale.

And in this case, that seems to hold. We don't see the expansion impact planetary orbits, for example. In general, we find that cosmological expansion does not hold in gravitationally bound systems. One interpretation of that is that expansion physically does not happen in sufficiently curved spacetime. Since the model was developed with such things taking zero space anyway, it doesn't detract from the model.