7 ms·

The problem is, faster than light travel violates causality in some reference frames [1]. This is a big problem. Forget grandpa-killing travellers; even several

The problem is, faster than light travel violates causality in some reference frames [1]. This is a big problem. Forget grandpa-killing travellers; even several bits of time travelling information allows one to easily solve NP-complete problems (it's an even more powerful mode of computation, actually) [2] Any new physics that allows for FTL must be very weird, even in a logical sense. [1]: https://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality https://physics.stackexchange.com/questions/52249/how-does-f... [2]: https://www.scottaaronson.com/democritus/lec19.html https://www.scottaaronson.com/democritus/lec19.html

residualmind 4y ago
I guess then causality will be broken if some external force introduces FTL/time travel to our handful of dimensions. Looking at our 4D hypercube of space time, any point in that x|y|z|t space becomes mutable, whats the problem? ;) (I know you need at least one new "time" dimension then to have mutability, this is difficult to talk about. And then somebody in 6D will find that very cute..)
- simonh 4y ago
  Except that 6D mathematics, and even physics, is routine. Mathematicians and physicists solve problems in more dimensions that that without breaking step these days, and in fact have done for generations at this point. A lot of things like that, which non-specialists think are spooky and weird and unknown, are actually old hat.
  - residualmind 4y ago
    And? Exactly my point, nothing spooky about it. At first you look at it in 4d, then 5, then 6..
    - simonh 4y ago
      Right, it's boring and routine maths, but there's no evidence that sort of mathematics will somehow bypass the rest of mathematics and physics. It's like suggesting that long division might solve quantum non-locality.
      - residualmind 4y ago
        Or it could all match up - aren't several dimensions the very core of e.g. field theory? No idea, really, as you can tell I'm not an expert, just throwing around random words in a thread that is ultimately based on a SF story.
        simonh 4y ago
        Yes it is, like I said it's very well known and understood. It's not a wild poorly understood frontier full of unimaginable possibilities. The actual maths of n-dimensional geometry is dull as ditchwater. There's no magic lurking there.
- wruza 4y ago
  A stupid question. What’s wrong with breaking causality?
  - abrax3141 4y ago
    Primer
  - curtainsforus 4y ago
    We observe causality in our universe and day-to-day lives. If FTL movement is possible, you might expect at least some particles to be doing it, and that there'd be closed timelike curves or causality breaking happening already in nature, and we don't. It's not clear what 'breaking causality' would look like. It's like saying what if you made 0 equal 1?
    - wruza 4y ago
      Well that’s relatively easy. Some object (or particle if you like) appears out of nowhere and causes the events to happen that make it go time travel and appear out of nowhere and cause… you get the idea. Something caused the entire universe, before “time” was even a thing (or there is much more to it). Why couldn’t that something cause few events to cause themselves in an already “deployed” spacetime? We observe causality If we can’t observe something it may just mean it’s rare and/or hard to detect. I understand that probability of me asking this first time is zero and people definitely brought that up and analyzed why this is not the case. I’m interested in reasoning and whether it’s just a rule, like conservation of information (is it so?), or if there is something fundamental preventing it completely.
  - simonh 4y ago
    Everything in the universe is connected to, and influences everything else. Suppose I hit the cue ball on a very low friction billiard table really hard and cause a cascade of collisions, such that some of the balls undergo 20 impacts. That's a lot, but not inconceivable. Even extremely tiny variations in the initial movement of the cue ball or configuration of the balls can cause the final configuration on the table to be completely different. How tiny? Well, even the removal of a single electron from an atom over ten billion lightyears away could cause a measurable change in the position of some of the balls when they come to rest. That's how interconnected everything is, and how small changes are vastly magnified by cascades of events. Breaking causality is basically time travel. It would in theory allow future events to change the conditions that lead to our present. Timelines are extremely unstable. Even the most minuscule change could make our present utterly impossible or unrecognisable. A stable consistent universe isn't possible in the presence of causality violations.
wruza 4y ago
As if we didn’t have weird physics already. You’re basically saying that the problem with magic is that it works weirdly. But isn’t that the whole point of it? I understand that it would be TROUBLE at the mathematical and logical level, but that is exactly a box here we’d like to jump out of. Also, aren’t you bothered by the unexplainable fact that the universe somehow adheres to some rules and not the other. What has set them in the first place? Are they even “rules”, or we just accept them naturally, because our most primitive interactions were based on them for few ages?
abrax3141 4y ago
To be clear, it breaks causality in every reference frame. It is the definition of causality and derived from the definition of reference frames. In fact, c is not really the speed of light, it’s the speed of causality. Light itself could theoretically go slightly slower.
- tnzk 4y ago
  > In fact, c is not really the speed of light, it’s the speed of causality I like this interpretation. I would be happy if you provide pointers to something written from this perspective.
  - Timwi 4y ago
    PBS Space Time did a piece on that. https://youtu.be/msVuCEs8Ydo https://youtu.be/msVuCEs8Ydo
    - morninglight 4y ago
      This could have been an interesting video, but PBS had some nutty production values. Why did they insist on filling half the screen with some guy waving his arms to distract you? I hope the producer moved on to his true calling - gasoline pump videos.
      - synu 4y ago
        They have toned that down a bit over the years since that video was recorded six years ago. The videos are really informative, and you can always close your eyes and listen if you like.
  - abrax3141 4y ago
    Here’s another: https://physics.stackexchange.com/questions/708173/could-the-speed-of-causality-be-significantly-faster-than-c https://physics.stackexchange.com/questions/708173/could-the...
- atwood22 4y ago

residualmind 4y ago

I guess then causality will be broken if some external force introduces FTL/time travel to our handful of dimensions. Looking at our 4D hypercube of space time, any point in that x|y|z|t space becomes mutable, whats the problem? ;) (I know you need at least one new "time" dimension then to have mutability, this is difficult to talk about. And then somebody in 6D will find that very cute..)

simonh 4y ago
Except that 6D mathematics, and even physics, is routine. Mathematicians and physicists solve problems in more dimensions that that without breaking step these days, and in fact have done for generations at this point. A lot of things like that, which non-specialists think are spooky and weird and unknown, are actually old hat.
- residualmind 4y ago
  And? Exactly my point, nothing spooky about it. At first you look at it in 4d, then 5, then 6..
  - simonh 4y ago
    Right, it's boring and routine maths, but there's no evidence that sort of mathematics will somehow bypass the rest of mathematics and physics. It's like suggesting that long division might solve quantum non-locality.
    - residualmind 4y ago
      Or it could all match up - aren't several dimensions the very core of e.g. field theory? No idea, really, as you can tell I'm not an expert, just throwing around random words in a thread that is ultimately based on a SF story.
      - simonh 4y ago
        Yes it is, like I said it's very well known and understood. It's not a wild poorly understood frontier full of unimaginable possibilities. The actual maths of n-dimensional geometry is dull as ditchwater. There's no magic lurking there.
wruza 4y ago
A stupid question. What’s wrong with breaking causality?
- abrax3141 4y ago
  Primer
- curtainsforus 4y ago
  We observe causality in our universe and day-to-day lives. If FTL movement is possible, you might expect at least some particles to be doing it, and that there'd be closed timelike curves or causality breaking happening already in nature, and we don't. It's not clear what 'breaking causality' would look like. It's like saying what if you made 0 equal 1?
  - wruza 4y ago
    Well that’s relatively easy. Some object (or particle if you like) appears out of nowhere and causes the events to happen that make it go time travel and appear out of nowhere and cause… you get the idea. Something caused the entire universe, before “time” was even a thing (or there is much more to it). Why couldn’t that something cause few events to cause themselves in an already “deployed” spacetime? We observe causality If we can’t observe something it may just mean it’s rare and/or hard to detect. I understand that probability of me asking this first time is zero and people definitely brought that up and analyzed why this is not the case. I’m interested in reasoning and whether it’s just a rule, like conservation of information (is it so?), or if there is something fundamental preventing it completely.
- simonh 4y ago
  Everything in the universe is connected to, and influences everything else. Suppose I hit the cue ball on a very low friction billiard table really hard and cause a cascade of collisions, such that some of the balls undergo 20 impacts. That's a lot, but not inconceivable. Even extremely tiny variations in the initial movement of the cue ball or configuration of the balls can cause the final configuration on the table to be completely different. How tiny? Well, even the removal of a single electron from an atom over ten billion lightyears away could cause a measurable change in the position of some of the balls when they come to rest. That's how interconnected everything is, and how small changes are vastly magnified by cascades of events. Breaking causality is basically time travel. It would in theory allow future events to change the conditions that lead to our present. Timelines are extremely unstable. Even the most minuscule change could make our present utterly impossible or unrecognisable. A stable consistent universe isn't possible in the presence of causality violations.

wruza 4y ago

As if we didn’t have weird physics already. You’re basically saying that the problem with magic is that it works weirdly. But isn’t that the whole point of it? I understand that it would be TROUBLE at the mathematical and logical level, but that is exactly a box here we’d like to jump out of. Also, aren’t you bothered by the unexplainable fact that the universe somehow adheres to some rules and not the other. What has set them in the first place? Are they even “rules”, or we just accept them naturally, because our most primitive interactions were based on them for few ages?

abrax3141 4y ago

To be clear, it breaks causality in every reference frame. It is the definition of causality and derived from the definition of reference frames. In fact, c is not really the speed of light, it’s the speed of causality. Light itself could theoretically go slightly slower.

tnzk 4y ago
> In fact, c is not really the speed of light, it’s the speed of causality I like this interpretation. I would be happy if you provide pointers to something written from this perspective.
- Timwi 4y ago
  PBS Space Time did a piece on that. https://youtu.be/msVuCEs8Ydo https://youtu.be/msVuCEs8Ydo
  - morninglight 4y ago
    This could have been an interesting video, but PBS had some nutty production values. Why did they insist on filling half the screen with some guy waving his arms to distract you? I hope the producer moved on to his true calling - gasoline pump videos.
    - synu 4y ago
      They have toned that down a bit over the years since that video was recorded six years ago. The videos are really informative, and you can always close your eyes and listen if you like.
- abrax3141 4y ago
  Here’s another: https://physics.stackexchange.com/questions/708173/could-the-speed-of-causality-be-significantly-faster-than-c https://physics.stackexchange.com/questions/708173/could-the...
atwood22 4y ago