6 ms·

The horizon problem for faster than light travel

44 points by hhs 5y ago

awb 5y ago
> But what is the horizon problem? Is it some single well-defined concept or a catch-all term for many phenomena? 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".
- hhs 5y ago
  > “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-possible-within-einstein-s-physics-astrophysicist-shows https://www.sciencealert.com/faster-than-light-travel-is-pos...
  - simonh 5y ago
    Sure, but only for certain uses of the term ‘possible’. There are solutions to Einstein’s equations that imply faster than light travel or communication, but that doesn’t mean such solutions correspond to physical reality. For example such solutions can include terms in the equations that correspond to negative energy densities, or infinite masses, or trajectories around infinitely extended one dimensional massive objects. Such solutions somewhat stretch the meaning of the word ‘possible’.
    - xupybd 5y ago
      Hasn't the Casimir effect given possible real life negative energy densities?
      - pdonis 5y ago
        No. All that happens in the Casimir effect is that the vacuum state between a pair of uncharged plates is slightly different from the vacuum state in the absence of all matter whatsoever. There is no "negative energy" anywhere in the Casimir effect, despite pop science articles that appear to claim the contrary.
        xupybd 5y ago
        I'm probably wrong but I thought the argument was being made that you could generate the same effect? https://hal.archives-ouvertes.fr/hal-00981257/document#:~:text=Alcubierre%20was%20the%20first%20to,warp%20drives%20needs%20astronomical%20quantities https://hal.archives-ouvertes.fr/hal-00981257/document#:~:te....
        pdonis 5y ago
        This paper is claiming that the Casimir Effect could be used to generate a "Natario warp drive spacetime", but the validity of that claim is questionable. (As far as I can tell, this paper has not been peer-reviewed so its claims have not been checked by other experts in the field.) 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.
        xupybd 5y ago
        Again I suspect there is more that I don't understand but this paper goes into geometries that could possibly lead to being able to generate the required negative energy.https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-021-09484-z https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-02... That said I think you still need such huge energies that it's impractical.
        pdonis 5y ago
  - slowmovintarget 5y ago
    The quoted sentence has dropped the word "can't" I suspect. 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.
  - pdonis 5y ago
    > the author seems to note that faster than light travel is possible under Einstein’s physics 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.)
stevebmark 5y ago
Faster than light travel and time travel are both fundamentally impossible in our universe and can't be achieved. I'm sure we'll learn all sorts of cool things looking for ways in. The reality is these will sadly live only in science fiction. There are plenty of things that are fundamentally impossible in our universe, faster than light travel being one of them.
- ctoth 5y ago
  "Heavier-than-air flying machines are impossible." -- Lord William Thomson Kelvin, British mathematician and physicist, president of the British Royal Society, 1895. Edit: See https://news.ycombinator.com/item?id=27816225 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 https://arxiv.org/abs/2006.07125
  - stevebmark 5y ago
    "Inertia is a property of matter" - fundamental unchangeable property of the universe
    - lolsal 5y ago
      ...so far.
  - heavenlyblue 5y ago
    What about perpetual motion machines though? :)
  - BurningFrog 5y ago
    Scientifically speaking, the fact that someone was wrong in the past, doesn't mean someone else is wrong now, even if they use a similar sentence structure. PS Yes, I am fun at parties, if I ever get invited to one again!
    - 5y ago
woah 5y ago
Nice! Great solution to a common inconvenience.
dandotway 5y ago
While faster-than-light (FTL) travel is impossible through spacetime as PhD physicists know it (i.e. as "Relativity-confirming experiments reveal it"), there are forms of apparent FTL that have mainstream PhD acceptance, because they don't allow information to travel from point A to point B faster than light and thus do not violate causality. 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.
- lumost 5y ago
  There is no restriction on relative velocities between objects from a given observer as long as all objects have velocities less than the speed of light. You can point laser pointers in opposite directions and the "front" of the laser pointer will recede at "2x" C from the opposite pointer. 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.

44 points by hhs 5y ago

awb 5y ago

> But what is the horizon problem? Is it some single well-defined concept or a catch-all term for many phenomena? 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".

hhs 5y ago
> “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-possible-within-einstein-s-physics-astrophysicist-shows https://www.sciencealert.com/faster-than-light-travel-is-pos...
- simonh 5y ago
  Sure, but only for certain uses of the term ‘possible’. There are solutions to Einstein’s equations that imply faster than light travel or communication, but that doesn’t mean such solutions correspond to physical reality. For example such solutions can include terms in the equations that correspond to negative energy densities, or infinite masses, or trajectories around infinitely extended one dimensional massive objects. Such solutions somewhat stretch the meaning of the word ‘possible’.
  - xupybd 5y ago
    Hasn't the Casimir effect given possible real life negative energy densities?
    - pdonis 5y ago
      No. All that happens in the Casimir effect is that the vacuum state between a pair of uncharged plates is slightly different from the vacuum state in the absence of all matter whatsoever. There is no "negative energy" anywhere in the Casimir effect, despite pop science articles that appear to claim the contrary.
      - xupybd 5y ago
        I'm probably wrong but I thought the argument was being made that you could generate the same effect? https://hal.archives-ouvertes.fr/hal-00981257/document#:~:text=Alcubierre%20was%20the%20first%20to,warp%20drives%20needs%20astronomical%20quantities https://hal.archives-ouvertes.fr/hal-00981257/document#:~:te....
        pdonis 5y ago
        This paper is claiming that the Casimir Effect could be used to generate a "Natario warp drive spacetime", but the validity of that claim is questionable. (As far as I can tell, this paper has not been peer-reviewed so its claims have not been checked by other experts in the field.) 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.
        xupybd 5y ago
        Again I suspect there is more that I don't understand but this paper goes into geometries that could possibly lead to being able to generate the required negative energy.https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-021-09484-z https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-02... That said I think you still need such huge energies that it's impractical.
        pdonis 5y ago
- slowmovintarget 5y ago
  The quoted sentence has dropped the word "can't" I suspect. 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.
- pdonis 5y ago
  > the author seems to note that faster than light travel is possible under Einstein’s physics 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.)

stevebmark 5y ago

Faster than light travel and time travel are both fundamentally impossible in our universe and can't be achieved. I'm sure we'll learn all sorts of cool things looking for ways in. The reality is these will sadly live only in science fiction. There are plenty of things that are fundamentally impossible in our universe, faster than light travel being one of them.

ctoth 5y ago
"Heavier-than-air flying machines are impossible." -- Lord William Thomson Kelvin, British mathematician and physicist, president of the British Royal Society, 1895. Edit: See https://news.ycombinator.com/item?id=27816225 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 https://arxiv.org/abs/2006.07125
- stevebmark 5y ago
  "Inertia is a property of matter" - fundamental unchangeable property of the universe
  - lolsal 5y ago
    ...so far.
- heavenlyblue 5y ago
  What about perpetual motion machines though? :)
- BurningFrog 5y ago
  Scientifically speaking, the fact that someone was wrong in the past, doesn't mean someone else is wrong now, even if they use a similar sentence structure. PS Yes, I am fun at parties, if I ever get invited to one again!
  - 5y ago

woah 5y ago

Nice! Great solution to a common inconvenience.

dandotway 5y ago

While faster-than-light (FTL) travel is impossible through spacetime as PhD physicists know it (i.e. as "Relativity-confirming experiments reveal it"), there are forms of apparent FTL that have mainstream PhD acceptance, because they don't allow information to travel from point A to point B faster than light and thus do not violate causality. 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.

lumost 5y ago
There is no restriction on relative velocities between objects from a given observer as long as all objects have velocities less than the speed of light. You can point laser pointers in opposite directions and the "front" of the laser pointer will recede at "2x" C from the opposite pointer. 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.