The horizon problem for faster than light travel
eriklentzphd.blogspot.com
eriklentzphd.blogspot.com
Granted this article is largely above my reading/comprehensive level, but after describing multiple types of horizons and then saying a wrap drive horizon largely resembles a black hole horizon, I can't see where the author actually defines "the horizon problem".
It looks like the author was referring to this: “In relativity theory, a horizon is also a boundary on the region which an observer can see. Horizons pose an issue for warp drives as one cannot control the warp bubble if one see or communicate with it.”
And for more context, the author seems to note that faster than light travel is possible under Einstein’s physics: https://www.sciencealert.com/faster-than-light-travel-is-pos...
https://hal.archives-ouvertes.fr/hal-00981257/document#:~:te....
Also, even if the claim about the Casimir Effect is correct, since the Casimir Effect occurs between a pair of uncharged plates, the "warp bubble" would have to be between the plates--but that means the warp bubble can't move the plates, so using it as a "warp drive" would not be possible.
That said I think you still need such huge energies that it's impractical.
The geometries in question don't generate negative energy, they are generated by negative energy (which actually isn't a very good term for the kind of stress-energy that is involved). This paper is just talking about different possible geometries that could exist between Casimir plates, based on different possible configurations of "vacuum energy" that could exist between the plates. Even if one of these geometries turned out to be a "warp bubble", it still wouldn't change the issue that I described before, that all of this is taking place between uncharged plates, and the plates are outside the "warp bubble" so the bubble can't move them, so this is not a viable option for an actual "warp drive".
Reworded: "Horizons pose an issue for warp drives as one cannot control the warp bubble if one can't see or communicate with it."
The problem, then, is that warp drives as posited create KVF horizons that would seem to make them impossible to control, enter, or leave. That would mean warp drive is not permitted in the universe.
Some KVF horizons, like the outer ergosphere of a black hole can be detected, entered, and exited. Conclusion, warp drives are not ruled out on the grounds that they involve horizons because we don't know what kind of horizon they might have.
tl;dr Warp drives aren't ruled out... yet.
He's talking about a highly speculative form of "warp drive" he has proposed that supposedly does not require negative energy, as all previous "warp drive" solutions (such as the Alcubierre drive) did. His speculations have not been accepted as valid by other physicists working in the field. (Also, calling such travel "faster than light" is problematic, since a light beam emitted from inside a "warp bubble" would still outrun the bubble itself. These are really just highly counterintuitive spacetime geometries that are most likely not physically possible anyway because the kinds of materials that would be needed to create them don't exist and can't be manufactured.)
Edit: See https://news.ycombinator.com/item?id=27816225 for a discussion of the paper that launched this whole topic.
Edit 2: The actual paper you want is this one by the author of OP, the first one is referring back to this: https://arxiv.org/abs/2006.07125
PS Yes, I am fun at parties, if I ever get invited to one again!
What we can say is our current model of the universe does not allow for any practical travel beyond the speed of light.
It's just as hubristic to assume that science is as ignorant now as it always has been.
Why does FTL produce time travel?
Why go back and build Dyson spheres?
> To answer your question: any FTL drive can be used to create closed time-like loops. With these you can travel back in time, but you get stuck in a loop.
Thanks!
My understanding as a lay person of the physics that allow for things like this, is that they require starting conditions that wouldn't produce a universe anything like what we actually live in, or require energy levels around the output of galaxy superclusters to be focused into planck lengths or such. "Technically not impossible" is different for physicists than to normal people ;-)
If you switch from relativity to regular physics with medium instead of pure vacuum (10 years passed since 2012 already, so relativity is busted at this point), then c is the speed of light in medium, so we don't need to bend anything or use negative t to calculate outcomes.
Excuse me?
Short answer to the first, because the speed of light is the top speed at which information can propagate in the Universe.
Edit: Great filter hypothesis to explain Fermis paradox. The pyramid scheme needed to enslave the galaxy is also powerful enough to destroy a planet before it develops space faring technology.
Who knows what they are like and what they value.
I think you've got the sign on your skepticism reversed. The burden of proof is very much on those who claim that FTL travel is possible.
From Wikipedia:
The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.
As of the 21st century, according to current scientific theories, matter is required to travel at slower-than-light (also STL or subluminal) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative.
With our current understanding of the universe, yes, indeed, FTL travel is not possible. But it’s naïve to think that our understanding of universe won’t change.
For all we know, our understanding of the universe could be at a stage of analogous to “there are four elements: earth, air, fire, and water”. Many things that we enjoy daily were once “fundamentally impossible”.
Not so. They may have been conceptually impossible but did not run into the "fundamental" laws of physics. FTL travel breaks physics. A lot abiut our observable universe would be different if it were possible.
The evidence that the universe obeys these laws is significant and, currently, not counter indicated.
Like quantum mechanics broke physics?
Those "fundamental" laws of physics didn't exist several centuries ago.
That's what it means to be contradicting fundamental physics. It implies all of our other knowledge is wrong, but we are just lucky enough that our machines work anyway. Which, unless we see a phenomenon that we can't fit into our existing theories, seems unlikely
So yes, perpetual motion could start working tomorrow, or could already exist for all we know. That doesn't mean it's wrong to say physics _currently_ says it's impossible and any claim better have some pretty good evidence to dispute it (which none currently do). But that also doesn't mean that if we're around centuries/millennia from now we won't have found a special case where our current understanding breaks down.
Sure, it might be wrong, but as far as claims go it has a lot going for it, both experimentally and theoretically.
It can't be ruled out, but neither can Russell's Teapot.
Personally, I think that the “speed of light” is a misleading misnomer. It’s the speed of information propagation in flat space - and even that’s a misnomer, as speed is a very human concept, as is time, as is space.
For all we know we’re waddling around in flatland, thinking that what we perceive is what is. More likely than not, most everything we know isn’t even wrong.
To borrow Haldane - “Nature is not only queerer than we suppose, it is queerer than we can suppose.”
Only if you ignore the enormous acceleration/deceleration forces needed for this.
I definitely agree that such travel would require enormous amounts of energy.
[0]: https://en.wikipedia.org/wiki/Space_travel_under_constant_ac...
Time dilation doesn't matter here.
Near-light-speed travel to the Moon would take a few seconds. Near-light-speed travel to Mars would take longer than my drive to the grocery store. These are not "instantaneously."
Not literally instantaneous, but still practically instantaneous at the scale of the solar system.
That may very well be, and true in our lifetime, but given how little we know about a lot of things, and that in many cases we don't even know what we don't know, I would hesitate in making an absolute statement.
c0 is the speed of light only incidentally. It is the speed of causality first. You can warp spacetime but you can't change the speed limit.
You might think there would be a hypothetical way to cheat it from some unknown phenomenon, but then any occurrence of that phenomenon would destroy itself in zero time.
You're essentially relocating from one location to another by bypassing the distance... Leaving this universe long enough to travel a comparatively smaller distance/time in another then pop back into our universe a long long way but short amount of local time later.
Sure you dont travel FTL but you do arrive in the location Faster than light would take to travel in mundane space.
I mean its all just sci-fi at the moment. But lots of things we take advantage of today was at one time in the past as well.
I prefer to be optimistic about potential breakthroughs.
And to say that backwards time travel isn't possible at this point doesn't make much sense. We don't even have a clear understanding of what time is, or why it goes forwards.
And the singularity should be around the corner, and that might present a computerized solution to emulate reverse time travel.
The expansion of the observable universe itself being apparently FTL is perhaps the most interesting. If we point the Hubble telescope (and soon JWST) towards opposite ends of the observable universe, we observe extreme redshift galaxies receding away from each other in opposite directions greater than twice lightspeed if we measure according to "distance in light years" from earth. But no photons or information from a galaxy at one side of our observable universe can be sent to a galaxy at the opposite end: they are receding away from each other too rapidly, these two galaxies exist Beyond the Cosmological Event Horizon relative to each other and are forever unknowable to each other, and thus causality is not violated.
An advanced alien race in galaxy A might be able to beam a message to earth before galaxy A vanishes Beyond the Cosmological Event Horizon within the next 100 million years earth-time, and likewise galaxy B might beam a message to earth within the next 100 million years earth-time, but earth can't relay the message from A to B or B to A because they have already forever receded from each other and will soon both likewise forever recede away from us.
In the case of the expanding universe, our standard "flat" space time represented by minkowski space is warped into de-sitter space. As distances increase apparent acceleration increases up to and beyond the speed of light due to the expansion of space time.