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Articles in arXiv are not peer reviewed by that organization, if that's what you mean.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.

 

[briggart]

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

It is mainstream in the sense both authors are working cosmologists and know what they are talking about. The paper was part of a genuine discussion between members of the community, which I'm not sure it found a definite resolution or simply faded away. I wouldn't call it mainstream in the sense that "this is how it is explained in every cosmology textbook" today, partly because I feel some of this is the cosmological version of the "interpretation of quantum mechanics". People generally agree on the math, but have different opinions on the interpretation of the results for things so far removed from our everyday experience.

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

Yes, that is correct. In curved spaces there is no way to uniquely relate quantities measured at vastly different places. Assume that you are on the Equator and have measured a vector quantity and found that it points straight North. Then assume that you want to translate this vector all the way to the North pole in a way that the vector always remains parallel to itself at any point of the translation. The direction of the vector at North pole will be different if you go straight North from the Equator, or if you first move along the Equator by 90 degrees and then move North. This doesn't happen on a flat surface.

In cosmology, if you want to relate a quantity as measured by a far away observer to what you would measure, you face a similar issue. Bunn & Hogg choose to transport the velocity along the path a photon would travel from the distant source to us, and they find that redshift can be fully described by incremental Doppler shifts. They claim this is the "only natural path". I agree that is "natural", not sure about "only" (see now where subjective opinions are entering the discussion?).

This paper (The kinematic component of the cosmological redshift) instead chooses a different path and finds that redshift is a combination of a kinetic contribution (they call it Dopplerian) due to the part of the path connecting the position of the emitter with the one of the observer, and a gravitational contribution due to the change to the local inertial frame at the observer position.

For a somewhat different viewpoint, Nick Kaiser (Astronomical Redshifts and the Expansion of Space) argues that in general redshift cannot be fully reduced to kinematic effects. I think that the introduction of this paper is also a very balanced summary of the previous discussion.

Overall, I think that all papers agree that cosmological redshift is not a unique effect but it can be expressed in terms of the usual Doppler and gravitational redshifts, so the argument that the existence of a third form of redshift proves that expansion of space is a real physical effect distinct from movement of galaxies is not valid. Similarly for the issue of superluminal recession velocities.

(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.)

Haven't read either of these, but here is a public version of the second one: Velocities of distant objects in General Relativity revisited

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

My understanding is that Bunn & Hogg are saying that there is no continuous expansion, so that is not in contradiction with other possible local effects. But I haven't read the paper you linked, so maybe I'm missing something.

 

[tomBitonti]

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.

Sure, but there is a difference between "there is no local effect" and "the expected local effect is too small to be noticed". Uniform enlargement of space seems to imply local stretching. This seems to (obviously?) not occur. The usual answer to that is that space is not stretched in bound systems. However, this is an unsatisfying answer. What I have read is that local systems are affected, very slightly, and not in a way that result in continuous local expansion.
Thanks!
TomB