6 ms·

FTL always implies closed timelike curves, and that means paradoxes. Until whatever mechanism people come up to travel FTL with has a reasonable way of prevent

FTL always implies closed timelike curves, and that means paradoxes. Until whatever mechanism people come up to travel FTL with has a reasonable way of preventing paradoxes right there also in the paper, it isn't even worth looking at. We would like to do FTL, but the universe isn't obliged to humor us.

ben_w 4y ago
One thing I’ve been wondering about that, but I can’t reliably keep track of the frames and their transformations and I know I need to do that to get the answer myself: The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?
- at_a_remove 4y ago
  No. There's a good physics stackexchange on it.
  - remarkEon 4y ago
    Link? Or can you give us the tldr?
  - ben_w 4y ago
    My experience of physics stackexchange is getting three mutually exclusive answers to one question, and another question of mine getting a dismissive non-answer reply that outranked the actual answer. I don't think it's meant for people like me.
sterlind 4y ago
I thought the "closed" part of closed timelike curves takes care of that. Like in a Feynman diagram where a photon splits and recombines into an electron/positron pair, isn't that equivalent to an electron looping through time and annihilating itself? You could argue it's just a formalism, but mathematically it still seems like (trivial) time travel. Why can't you extend that from a single particle to a system of particles? Where's the math break down?
- at_a_remove 4y ago
  No, in a closed timeline curve, the closed portion means "You can arrive where you started, eventually, if you turn around or whatever." The timelike means that you have, effectively, gone into the past. Wherein you can shoot yourself before you step into your FTL ship.
  - sterlind 4y ago
    thanks, I had gotten things mixed up in my head. though, I still don't see how FTL has to cause paradoxes. What if a wormhole connects you to your destination's past, specifically the 4.3 years ago? There's no causality paradox because both points are in each other's light cones. Earthicans can watch you live by telescope. ...oh. that would imply that you'd see Earth 4.3 years in the future by telescope, while stepping back into the wormhole would either take you into the future (not closed timelike) or to the past (where a Centaurian could shoot themself.) I see.
Communitivity 4y ago
Let's say we have three broad possibilities: time is point-like or doesn't exist, time is linear, time is a branching tree where a different universe exists for every possible outcome (this is a bastardization of the Copenhagen theorem, but is discussed). Closed time-like curves is implied if time is linear in the way most people think of it (B happens after A). If time is a point-like construct, or if time is emergent from observation, or if time is a branching tree, then the problem of closed time-like curves goes away. For example, physicist Carlo Rovelli's theories are based on the concepts that "time itself disappears at the most fundamental level", and "that time is merely a function of our “blurred” human perception".[1] There are also several ways of time being linear, if that's the case, that are different than current approached. For example, Nicholas Gisin [2] applied intuitionist mathematics and showed that time might be linear but also have an inherent fluidity similar to quantum mechanics. The above is just the start of a conversation. There's also contractible vs non-contractible closed time-like curves [3], and the Novikov self-consistency principle [4]. The main point is that 'always' in physics is rarely accurate, over sufficient time lengths. The secondary point is that when we start adjusting the metrics of an observer, we start changing what we define as travel (if I stand still and the space around me is deformed such that my observed location is now 10m away, have I travelled?). The tertiary point is that we are still actively theorizing and experimenting about time, with many ongoing competing theories - we just don't know enough yet to say that FTL travel is or is not possible (look at Novikov's experiments proving Bell theorem 'spooky action at a distance'). [1] https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-exploding-commonsense-notions-order-of-time-interview https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-... [2] https://en.wikipedia.org/wiki/Nicolas_Gisin https://en.wikipedia.org/wiki/Nicolas_Gisin [3] https://en.wikipedia.org/wiki/Closed_timelike_curve#Contractible_versus_noncontractible https://en.wikipedia.org/wiki/Closed_timelike_curve#Contract... [4] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
monocasa 4y ago
The point of an Alcubierre drive is that there aren't any reference frames that are moving FTL. There's cases where spacetime itself is stretching FTL but we know that's possible because we can already see that in the expansion of the universe. Pretty much all of the galaxies outside of the local group are going to slowly fade away from our perspective because of the stretching of space crossing FTL.
- at_a_remove 4y ago
  This is wrong. The method of travel is irrelevant. Visit here https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_violation_and_semiclassical_instability https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_vio... and, after that, read further down to the section: "Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[44] where he writes: 'beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)'." Draw your light cones. Finish the diagram. Any FTL gives CTC, so long as your propulsion method allows for you to steer, or stop and build one that goes in any other direction.
  - monocasa 4y ago
    I mean, we don't know what he said when those slides were up, but in context with the other slide cited, it seems to be a rejection of the base point. What that slide says is true. But the whole point of the drive is that you aren't traveling FTL from the viewpoint of GR. It's no different than traveling through a wormhole for the purpose of this discussion; it's that you dynamically create a shorter path and don't exceed the speed of light along that shorter path.
mcswell 4y ago
If God had intended for us to travel faster than the speed of light in a vacuum, He would have slowed down light.
- SkyMarshal 4y ago
  Are we sure he didn't already do that?
upwardbound 4y ago
Paradoxes aren't necessarily a deal-breaker. It's possible that reality accepts paradoxes and has some reconciliation method where e.g. new changes overwrite older ones. The best deep dive I've read on how this would work is the twist ending of Isaac Asimov's excellent story The End of Eternity. https://www.amazon.com/The-End-of-Eternity/dp/B08CFWPQW9/ https://www.amazon.com/The-End-of-Eternity/dp/B08CFWPQW9/
hansvm 4y ago
Closed timelike curves are only a problem if you can act differently to change the outcome. The whole loop could just as easily be deterministic, playing the same scene forever.

ben_w 4y ago

One thing I’ve been wondering about that, but I can’t reliably keep track of the frames and their transformations and I know I need to do that to get the answer myself: The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?

at_a_remove 4y ago
No. There's a good physics stackexchange on it.
- remarkEon 4y ago
  Link? Or can you give us the tldr?
- ben_w 4y ago
  My experience of physics stackexchange is getting three mutually exclusive answers to one question, and another question of mine getting a dismissive non-answer reply that outranked the actual answer. I don't think it's meant for people like me.

sterlind 4y ago

I thought the "closed" part of closed timelike curves takes care of that. Like in a Feynman diagram where a photon splits and recombines into an electron/positron pair, isn't that equivalent to an electron looping through time and annihilating itself? You could argue it's just a formalism, but mathematically it still seems like (trivial) time travel. Why can't you extend that from a single particle to a system of particles? Where's the math break down?

at_a_remove 4y ago
No, in a closed timeline curve, the closed portion means "You can arrive where you started, eventually, if you turn around or whatever." The timelike means that you have, effectively, gone into the past. Wherein you can shoot yourself before you step into your FTL ship.
- sterlind 4y ago
  thanks, I had gotten things mixed up in my head. though, I still don't see how FTL has to cause paradoxes. What if a wormhole connects you to your destination's past, specifically the 4.3 years ago? There's no causality paradox because both points are in each other's light cones. Earthicans can watch you live by telescope. ...oh. that would imply that you'd see Earth 4.3 years in the future by telescope, while stepping back into the wormhole would either take you into the future (not closed timelike) or to the past (where a Centaurian could shoot themself.) I see.

Communitivity 4y ago

Let's say we have three broad possibilities: time is point-like or doesn't exist, time is linear, time is a branching tree where a different universe exists for every possible outcome (this is a bastardization of the Copenhagen theorem, but is discussed). Closed time-like curves is implied if time is linear in the way most people think of it (B happens after A). If time is a point-like construct, or if time is emergent from observation, or if time is a branching tree, then the problem of closed time-like curves goes away. For example, physicist Carlo Rovelli's theories are based on the concepts that "time itself disappears at the most fundamental level", and "that time is merely a function of our “blurred” human perception".[1] There are also several ways of time being linear, if that's the case, that are different than current approached. For example, Nicholas Gisin [2] applied intuitionist mathematics and showed that time might be linear but also have an inherent fluidity similar to quantum mechanics. The above is just the start of a conversation. There's also contractible vs non-contractible closed time-like curves [3], and the Novikov self-consistency principle [4]. The main point is that 'always' in physics is rarely accurate, over sufficient time lengths. The secondary point is that when we start adjusting the metrics of an observer, we start changing what we define as travel (if I stand still and the space around me is deformed such that my observed location is now 10m away, have I travelled?). The tertiary point is that we are still actively theorizing and experimenting about time, with many ongoing competing theories - we just don't know enough yet to say that FTL travel is or is not possible (look at Novikov's experiments proving Bell theorem 'spooky action at a distance'). [1] https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-exploding-commonsense-notions-order-of-time-interview https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-... [2] https://en.wikipedia.org/wiki/Nicolas_Gisin https://en.wikipedia.org/wiki/Nicolas_Gisin [3] https://en.wikipedia.org/wiki/Closed_timelike_curve#Contractible_versus_noncontractible https://en.wikipedia.org/wiki/Closed_timelike_curve#Contract... [4] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

monocasa 4y ago

The point of an Alcubierre drive is that there aren't any reference frames that are moving FTL. There's cases where spacetime itself is stretching FTL but we know that's possible because we can already see that in the expansion of the universe. Pretty much all of the galaxies outside of the local group are going to slowly fade away from our perspective because of the stretching of space crossing FTL.

at_a_remove 4y ago
This is wrong. The method of travel is irrelevant. Visit here https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_violation_and_semiclassical_instability https://en.wikipedia.org/wiki/Alcubierre_drive#Causality_vio... and, after that, read further down to the section: "Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[44] where he writes: 'beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)'." Draw your light cones. Finish the diagram. Any FTL gives CTC, so long as your propulsion method allows for you to steer, or stop and build one that goes in any other direction.
- monocasa 4y ago
  I mean, we don't know what he said when those slides were up, but in context with the other slide cited, it seems to be a rejection of the base point. What that slide says is true. But the whole point of the drive is that you aren't traveling FTL from the viewpoint of GR. It's no different than traveling through a wormhole for the purpose of this discussion; it's that you dynamically create a shorter path and don't exceed the speed of light along that shorter path.

mcswell 4y ago

If God had intended for us to travel faster than the speed of light in a vacuum, He would have slowed down light.

SkyMarshal 4y ago
Are we sure he didn't already do that?

upwardbound 4y ago

Paradoxes aren't necessarily a deal-breaker. It's possible that reality accepts paradoxes and has some reconciliation method where e.g. new changes overwrite older ones. The best deep dive I've read on how this would work is the twist ending of Isaac Asimov's excellent story The End of Eternity. https://www.amazon.com/The-End-of-Eternity/dp/B08CFWPQW9/ https://www.amazon.com/The-End-of-Eternity/dp/B08CFWPQW9/

hansvm 4y ago

Closed timelike curves are only a problem if you can act differently to change the outcome. The whole loop could just as easily be deterministic, playing the same scene forever.