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Any physicists wanna throw some cold water on our hopes and dreams here? Compared to most papers with far-out results, there didn't seem to be any obvious cavea

Any physicists wanna throw some cold water on our hopes and dreams here? Compared to most papers with far-out results, there didn't seem to be any obvious caveats here. What's up with worldline numerics? Could there be any issues hiding in the numerical methods there? How established are those methods in the field? Also, to be clear, is it correct that they haven't actually built such a device, this result is still just in simulation?

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kibwen 5y ago
Not a physicist, but some friends who are almost-barely physicists have pointed out that the concept of worldline numerics comes from string theory, which itself may or may not be generally bogus depending on who you're asking.
- tinco 5y ago
  As far as I understand the main problem of string theory is that it has been carefully constructed to fit all the physics we know about, and has not actually predicted anything we didn't already know about that we could experimentally verify. So in a practical sense string theory has been quite useless. If indeed it is true that string theory predicts this effect, and the effect can be experimentally verified (afaik this was dubious at best) then that would be a huge boon for string theory.
  - tsimionescu 5y ago
    Well, string theory has actually predicted Supersymmetry, which so far seems not to exist (though it is always possible to say it exists only on some higher energy than we can detect right now).
    - qubex 5y ago
      As I understand it, supersymmetry was taken as a concept and baked into the strings concept, to come up with superstrings. So it’s not really a prediction as much as it is baked-in by the prevailing notions of the time.
  - xvilka 5y ago
    It's possible to test string theory experimentally, just the amount of energy required is far too big for any foreseeable future. With a bruteforce approach, at least.
  - tzs 5y ago
    > As far as I understand the main problem of string theory is that it has been carefully constructed to fit all the physics we know about. Doesn't string theory have a lot fewer tunable parameters than the standard model? > and has not actually predicted anything we didn't already know about that we could experimentally verify I don't like that argument for a couple of reasons. First, it depends on the timing of when we discover things. If the experimentalists had taken longer to discover some things a theory that we would criticize today as not predicting anything we don't already know about would be a theory that would be celebrated for its predictions if it had just been posed earlier. I think a better thing to look at is if a theory can correctly calculate things that weren't built into the theory. I remember reading [1] that string theory correctly calculated some things concerning black holes that none of the people who had developed string theory to that point knew about. Second, I don't like it from a philosophical point of view. It suggests that if say some pre-Newton gravity theorist had just included enough epicycles, science should have rejected Newtonian gravity because it would not have explained anything that the epicycle theory didn't. [1] Probably either in Brian Greene's "The Elegant Universe" or Lawrence Krauss' "The Greatest Story Ever Told--So Far". Possibly Greene's "The Fabric of the Cosmos", but that seems unlikely. I distinctly remember reading it, and my copy of "The Fabric of the Cosmos" is an audiobook. The other two are Kindle books.
    - tinco 5y ago
      > I think a better thing to look at is if a theory can correctly calculate things that weren't built into the theory. I remember reading [1] that string theory correctly calculated some things concerning black holes that none of the people who had developed string theory to that point knew about. Well if that's true, then string theory definitely isn't useless, so I'd be interested in the specifics. This prediction from string theory wasn't explained by any of the accepted physical models? > Second, I don't like it from a philosophical point of view. It suggests that if say some pre-Newton gravity theorist had just included enough epicycles, science should have rejected Newtonian gravity because it would not have explained anything that the epicycle theory didn't. Well yeah, but why not reject Newtonian gravity if everything observable can be predicted by the epicyclical view? The goal is not to generate pretty models, the goal is to make accurate predictions of the physical universe, ideally through the simplest model we can come up with just to guard against overfitting. Anyway, it's not that I (or anyone, I don't have any physics authority certainly :P), am rejecting string theory, it's just that as far as I understand string theory hasn't had any real use to our physical models. I certainly don't have an alternative to string theory.
LegitShady 5y ago
The issue seems to be that the alcubierre drive requires things like external negative energy which are 'theoretically possible' but have never been detected and probably should have been already if they existed. Also, even if they worked, the negative energy requirements is something like the mass energy of jupiter on the high end or the moon on the low end. This is an hour long interview Dr Miguel Alcubierre who wrote the paper describing the Alcubierre drive. https://www.youtube.com/watch?v=JafY92PhgKU https://www.youtube.com/watch?v=JafY92PhgKU For a shorter 10 minute video, see this PBS Spacetime video https://www.youtube.com/watch?v=94ed4v_T6YM https://www.youtube.com/watch?v=94ed4v_T6YM
- eightails 5y ago
  > The issue seems to be that the alcubierre drive requires things like external negative energy Yes that's traditionally been the stumbling block. But the whole point of the linked article is that they have predicted a way to meet this requirement: > a micro/nano-scale structure has been discovered that predicts negative energy density distribution that closely matches requirements for the Alcubierre metric Obviously it's still a very very long way from a practical application re. Alcubierre (if such a thing is possible), but it's certainly an intriguing result if correct. From a quick flick through the paper, the above commenter seems to be correct in saying that they haven't yet completed a practical experiment to confirm. So nothing more than a simulated result at this stage.
  - LegitShady 5y ago
    Yes again it seems like one of those 'theoretically possible but never seen in a lab' kind of requirements, just like negative energy. The PBS spacetime video mentions you can 'do away' with the negative energy requirements at quantum scales, but those aren't really productive scales for useful space flight. Maybe for very tiny micro exploring robots? I'm not sure how such a device would usefully communicate with us though.
    - eightails 5y ago
      I suppose an optimist would say that success, even at a tiny scale, would be a step in the right direction. But you're right, it could well be possible that the effect doesn't scale up. I think gram-scale probes are definitely being considered -- though not with Alcubierre drives obviously lol. Breakthrough Starshot think they can transmit back from Alpha Centauri at 2.6-15 baud per watt by using their light sail as a laser reflector [0]. Pretty crazy. [0] https://en.wikipedia.org/wiki/Breakthrough_Starshot#Laser_data_transmitter https://en.wikipedia.org/wiki/Breakthrough_Starshot#Laser_da...
mr_mitm 5y ago
FTL travel implies violation of causality, no matter how you slice it. How sacred causality is to you depends of course on your personal world view, but to me it is inconceivable how causality could be violated, so I won't be getting my hopes up about this at all, even though I can't comment on the technicalities of this paper.
- threevox 5y ago
  Causality isn't nearly as central to physics as it may be to your worldview. Lawrence Krauss and others who have commented on the matter have argued that FTL travel, would mostly require a re-evaluation of our perspectives rather than a re-evaluation of physics
  - plebianRube 5y ago
    Hmm, similar to how heavier than air flight was thought to be impossible for humans? Interesting how the will of a few can shape humanity. I love it.
    - threevox 5y ago
      Probably a pretty good analogy! Even now, it's not known whether time has directionality in any way that is meaningful in physics. FTL travel would resolve the "Arrow of Time" question (https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time) but this wouldn't invalidate much/any of physics because the arrow is essentially non-observable at the micro level
  - phkahler 5y ago
    >> Causality isn't nearly as central to physics as it may be to your worldview No, but locality seems to be a sacred cow in physics. They'll latch on to just about anything to explain violation of the Bell inequalities, but nobody is willing to part with locality.
    - threevox 5y ago

deleted 5y ago

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deleted 5y ago

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kibwen 5y ago

Not a physicist, but some friends who are almost-barely physicists have pointed out that the concept of worldline numerics comes from string theory, which itself may or may not be generally bogus depending on who you're asking.

tinco 5y ago
As far as I understand the main problem of string theory is that it has been carefully constructed to fit all the physics we know about, and has not actually predicted anything we didn't already know about that we could experimentally verify. So in a practical sense string theory has been quite useless. If indeed it is true that string theory predicts this effect, and the effect can be experimentally verified (afaik this was dubious at best) then that would be a huge boon for string theory.
- tsimionescu 5y ago
  Well, string theory has actually predicted Supersymmetry, which so far seems not to exist (though it is always possible to say it exists only on some higher energy than we can detect right now).
  - qubex 5y ago
    As I understand it, supersymmetry was taken as a concept and baked into the strings concept, to come up with superstrings. So it’s not really a prediction as much as it is baked-in by the prevailing notions of the time.
- xvilka 5y ago
  It's possible to test string theory experimentally, just the amount of energy required is far too big for any foreseeable future. With a bruteforce approach, at least.
- tzs 5y ago
  > As far as I understand the main problem of string theory is that it has been carefully constructed to fit all the physics we know about. Doesn't string theory have a lot fewer tunable parameters than the standard model? > and has not actually predicted anything we didn't already know about that we could experimentally verify I don't like that argument for a couple of reasons. First, it depends on the timing of when we discover things. If the experimentalists had taken longer to discover some things a theory that we would criticize today as not predicting anything we don't already know about would be a theory that would be celebrated for its predictions if it had just been posed earlier. I think a better thing to look at is if a theory can correctly calculate things that weren't built into the theory. I remember reading [1] that string theory correctly calculated some things concerning black holes that none of the people who had developed string theory to that point knew about. Second, I don't like it from a philosophical point of view. It suggests that if say some pre-Newton gravity theorist had just included enough epicycles, science should have rejected Newtonian gravity because it would not have explained anything that the epicycle theory didn't. [1] Probably either in Brian Greene's "The Elegant Universe" or Lawrence Krauss' "The Greatest Story Ever Told--So Far". Possibly Greene's "The Fabric of the Cosmos", but that seems unlikely. I distinctly remember reading it, and my copy of "The Fabric of the Cosmos" is an audiobook. The other two are Kindle books.
  - tinco 5y ago
    > I think a better thing to look at is if a theory can correctly calculate things that weren't built into the theory. I remember reading [1] that string theory correctly calculated some things concerning black holes that none of the people who had developed string theory to that point knew about. Well if that's true, then string theory definitely isn't useless, so I'd be interested in the specifics. This prediction from string theory wasn't explained by any of the accepted physical models? > Second, I don't like it from a philosophical point of view. It suggests that if say some pre-Newton gravity theorist had just included enough epicycles, science should have rejected Newtonian gravity because it would not have explained anything that the epicycle theory didn't. Well yeah, but why not reject Newtonian gravity if everything observable can be predicted by the epicyclical view? The goal is not to generate pretty models, the goal is to make accurate predictions of the physical universe, ideally through the simplest model we can come up with just to guard against overfitting. Anyway, it's not that I (or anyone, I don't have any physics authority certainly :P), am rejecting string theory, it's just that as far as I understand string theory hasn't had any real use to our physical models. I certainly don't have an alternative to string theory.

LegitShady 5y ago

The issue seems to be that the alcubierre drive requires things like external negative energy which are 'theoretically possible' but have never been detected and probably should have been already if they existed. Also, even if they worked, the negative energy requirements is something like the mass energy of jupiter on the high end or the moon on the low end. This is an hour long interview Dr Miguel Alcubierre who wrote the paper describing the Alcubierre drive. https://www.youtube.com/watch?v=JafY92PhgKU https://www.youtube.com/watch?v=JafY92PhgKU For a shorter 10 minute video, see this PBS Spacetime video https://www.youtube.com/watch?v=94ed4v_T6YM https://www.youtube.com/watch?v=94ed4v_T6YM

eightails 5y ago
> The issue seems to be that the alcubierre drive requires things like external negative energy Yes that's traditionally been the stumbling block. But the whole point of the linked article is that they have predicted a way to meet this requirement: > a micro/nano-scale structure has been discovered that predicts negative energy density distribution that closely matches requirements for the Alcubierre metric Obviously it's still a very very long way from a practical application re. Alcubierre (if such a thing is possible), but it's certainly an intriguing result if correct. From a quick flick through the paper, the above commenter seems to be correct in saying that they haven't yet completed a practical experiment to confirm. So nothing more than a simulated result at this stage.
- LegitShady 5y ago
  Yes again it seems like one of those 'theoretically possible but never seen in a lab' kind of requirements, just like negative energy. The PBS spacetime video mentions you can 'do away' with the negative energy requirements at quantum scales, but those aren't really productive scales for useful space flight. Maybe for very tiny micro exploring robots? I'm not sure how such a device would usefully communicate with us though.
  - eightails 5y ago
    I suppose an optimist would say that success, even at a tiny scale, would be a step in the right direction. But you're right, it could well be possible that the effect doesn't scale up. I think gram-scale probes are definitely being considered -- though not with Alcubierre drives obviously lol. Breakthrough Starshot think they can transmit back from Alpha Centauri at 2.6-15 baud per watt by using their light sail as a laser reflector [0]. Pretty crazy. [0] https://en.wikipedia.org/wiki/Breakthrough_Starshot#Laser_data_transmitter https://en.wikipedia.org/wiki/Breakthrough_Starshot#Laser_da...

mr_mitm 5y ago

FTL travel implies violation of causality, no matter how you slice it. How sacred causality is to you depends of course on your personal world view, but to me it is inconceivable how causality could be violated, so I won't be getting my hopes up about this at all, even though I can't comment on the technicalities of this paper.

threevox 5y ago
Causality isn't nearly as central to physics as it may be to your worldview. Lawrence Krauss and others who have commented on the matter have argued that FTL travel, would mostly require a re-evaluation of our perspectives rather than a re-evaluation of physics
- plebianRube 5y ago
  Hmm, similar to how heavier than air flight was thought to be impossible for humans? Interesting how the will of a few can shape humanity. I love it.
  - threevox 5y ago
    Probably a pretty good analogy! Even now, it's not known whether time has directionality in any way that is meaningful in physics. FTL travel would resolve the "Arrow of Time" question (https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time) but this wouldn't invalidate much/any of physics because the arrow is essentially non-observable at the micro level
- phkahler 5y ago
  >> Causality isn't nearly as central to physics as it may be to your worldview No, but locality seems to be a sacred cow in physics. They'll latch on to just about anything to explain violation of the Bell inequalities, but nobody is willing to part with locality.
  - threevox 5y ago