7 ms·

This seems like an interesting article, but I found it very difficult to read. For example, from the introductory paragraph: This literature equates the possib

by cogware 11y ago

This seems like an interesting article, but I found it very difficult to read. For example, from the introductory paragraph: This literature equates the possibility of time travel with the existence of closed timelike curves (CTCs) or worldlines for material particles that are smooth, future-directed timelike curves with self-intersections.[3] Since time machines designate devices which bring about the existence of CTCs and thus enable time travel, the paradoxes of time travel are irrelevant for attempted “no-go” results for time machines because these results concern what happens before the emergence of CTCs.[4] This, in our opinion, is fortunate since the paradoxes of time travel are nothing more than a crude way of bringing out the fact that the application of familiar local laws of relativistic physics to a spacetime background which contains CTCs typically requires that consistency constraints on initial data must be met in order for a local solution of the laws to be extendable to a global solution. I'm not sure what the intended audience for this is; combination philosopher physicists? Given that it's on SEP it seems unreasonable to assume that CTCs need no other explanation than that they are smooth, future-directed timelike curves with intersections. I would love it if someone would ELI5 this concept, though.

jacobush 11y ago
I didn't understand anything. :-(
Kristine1975 11y ago
There's always Wikipedia: In mathematical physics, a closed timelike curve (CTC) is a world line of a material particle in spacetime that is "closed", returning to its starting point. So it's essentially like in the movie Groundhog Day I guess.
- p1mrx 11y ago
  In Groundhog Day, there is no material particle that returns to its starting point, only information (Phil's memory in particular.) Although, intuitively it seems that a time machine for information should be easier to build than one for matter, because information is so much easier to teleport.
  - logfromblammo 11y ago
    If you had a tiny wormhole and put one end of it into a high-speed centrifuge for a while, until there was a meaningful time differential between the two ends, you might not be able to send even an entire photon through it, but you might be able to do something like bounce a laser off each end such that if you modulate the input polarization at one end, the polarization of light reflected from the other end changes at the appropriate time. No mass or energy travels through the hole itself, but information is transmitted.
- vog 11y ago
  An even better (although quite contrived) example is given in the movie "Predestination": http://www.imdb.com/title/tt2397535/ http://www.imdb.com/title/tt2397535/ Alternatively, read the short story "All you Zombies" by Robert A. Heinlein, on which most of the movie is based: http://emilkirkegaard.dk/en/wp-content/uploads/Robert-A.-Heinlein-All-You-Zombies.pdf http://emilkirkegaard.dk/en/wp-content/uploads/Robert-A.-Hei...
KenoFischer 11y ago
I'll have a go at defining it, term-by-term, > smooth Meaning no abrupt kinks (for a notion of abrupt which is probably not physically achievable, so don't worry to much about it) or jumps/discontinuities in the path of a particle, applying to both space and time > timelike roughly speaking, traveling slower than the speed of light > future-directed meaning going into the direction of causality > with intersections meeting itself again. In other words, it's a path through space/time on which you experience your own local time as always running forward, but yet somehow you end up in the past. Perhaps an analogy is in order. Suppose we were in 1+1 dimensions, with the time dimension being compact (closing in on itself). Visualize this as an infinite cylinder (the infinite axis being space). Now, let's impose a finite speed of light (for simplicity 1cm/s) and define the direction of causality, say counter-clockwise. Then, this spacetime has CTC, because we can find a path that, is smooth, future-directed (going around counter-clockwise) and timelike (going at angles < 45 degrees compared to the spacial axis). The simplest such would be a simple circle around the cylinder, but any curve that matches the above rules and intersects itself would be considered a CTC. Now, as a final note " a crude way of bringing out the fact that the application of familiar local laws of relativistic physics to a spacetime background which contains CTCs typically requires that consistency constraints on initial data must be met in order for a local solution of the laws to be extendable to a global solution." means the following. When we usually do physics, we know what happens locally (e.g. ball get shot at wall, bounces back), so we simply have to figure out (or decide if we're doing a thought experiment) where everything is in the universe at some particular time, apply our rules everywhere and we essentially know what's happening everywhere, anywhere. This doesn't work any more in the presence of CTC. "Consistency conditions" basically just means that you have to chose your placement of objects in such a way that they don't cause any paradoxes (there's a more technical notion here, but that's essentially what's meant).
- dvt 11y ago
  Awesome explanation, +1.
- studentrob 11y ago
  If time travel is possible, why is consistency a concern? If one thing changes at one time, couldn't another change at the same time? Could time travel exist without our being aware of it by constantly changing our understanding of events such that there appears to have been no change? Does consistency mean that some change must cause many other changes in order to make everything end up in perfect alignment?
tomlock 11y ago
There is a fair overlap between the analytic-philosophy community and physicists and mathematicians. So the analytic community generally doesn't talk out of its arse about these things. I'm from the other side of philosophy, the continental-philosophy area, which mostly talks about ethics and art and identity, so for me to talk about CTCs I would be talking out my arse. Philosophy is, generally, very hard to read. So much so that reading Descartes' Meditations (considered an alright starting text) in my first philosophy course took me about 30 mins per page. But, it seems to me that this is one of those areas where analytic philosophy seems to be breaching an area where actual experiments can be proposed and tested in a physical sense, and it leads me to think, what's the point? The logical (philosophical) consistency of light being a particle and a wave doesn't come in to the reality of it. Was it worth ancient Greeks wondering whether all matter was made of water? I think similar questions can be asked about continental philosophy. I certainly read a lot of it as a lot of wank that could be answered by real-world surveys (what makes people happy? I dunno, how about we ask them). But I think questions like, how do we determine whether happiness is the thing we should optimize for in life, are still relevant, and philosophical, and more basic! I think the underlying basic question of CTCs might be, does something being a logical paradox actually prevent it from happening? And, if CTCs are a physical phenomena that we can't observe (maybe, yet), what's the point of a philosopher wondering about them? Shouldn't that be a theoretical physicists job?
- maaku 11y ago
  >But, it seems to me that this is one of those areas where analytic philosophy seems to be breaching an area where actual experiments can be proposed and tested in a physical sense, and it leads me to think, what's the point? I think it would be fair to flip that around: if your philosophy doesn't result in testable predictions, then what's the point?
  - tomlock 11y ago
    What I was talking about in that quote is that CTCs are definitely being seriously talked about and explored in relation to tested elements of our understanding of the universe by theoretical physicists, the experts in physics, and so I think its time for philosophers to give up that ground. Similarly I feel like there are philosophy-of-politics discussions that can (through political change) become the realm of actual politicians, sociologists, etc. I apologize, but I'd like to flip your question about again. I think philosophy is the realm of the untestable, so in my mind you're asking "what's the point of philosophy". I think philosophy is the place to explore the full ramifications and implications of untestable ideas. I'd also ask what's the point of your question? If its not testable, I think its a philosophical question, QED. If it does result in a testable prediction, what's the test?
dvt 11y ago
@KenoFischer already did a great job explaining the paragraph, but the audience is professional philosophers, philosophy graduate students, and some philosophy undergrads. I parsed this SEP entry for one of my undergraduate courses (Philosophy of Space & Time). If you're interested in the topic, I suggest reading Time and Space by Barry Dainton[1]. It was suggested reading for our class and primed me for anything from time time travel to general relativity. [1] http://www.amazon.com/Time-Space-Barry-Francis-Dainton/dp/0773537473 http://www.amazon.com/Time-Space-Barry-Francis-Dainton/dp/07...
Retric 11y ago
Suppose electrons would naturally time travel occasionally. But, we could gain no knowledge about the future from those electrons. It's not interesting. CTC means time-travel with structure. If you can send say a book back in time then our ideas of paradox ect have meaning. Next suppose you could time travel, but only far from your own past that light can't reach you. Further you block other time travelers so you can't send messages back to your self. Again not interesting, you need to be able to influence your past (self intersect) to be fun otherwise it's just like moving quickly.
- mrob 11y ago
  If electrons sometimes time traveled on a human time scale we could exploit that, eg. by building a huge assembly of electron guns, phosphors and photomultipliers. We could gain knowledge of the future signal sent to the electron guns by watching when the phosphors lit up while the electron guns were switched off.
  - Retric 11y ago
    Depends on scale. Suppose they are rare and show up in random locations with random velocity's random amounts of time in the past etc. As in 1 electron in the observable universe per day.
    - mrob 11y ago
      If all their properties are randomized then I don't see how they can be said to "travel". That would be indistinguishable from electrons spontaneously appearing and unrelated electrons spontaneously disappearing.
      - Retric 11y ago
        This is one of the major reasons I said time travel without information transfer is not interesting. Even if it actually happens, it's not distinguishable from something else. PS: Quantum entanglement is also less interesting than you might think for similar reasons.

by cogware 11y ago

jacobush 11y ago

I didn't understand anything. :-(

Kristine1975 11y ago

There's always Wikipedia: In mathematical physics, a closed timelike curve (CTC) is a world line of a material particle in spacetime that is "closed", returning to its starting point. So it's essentially like in the movie Groundhog Day I guess.

p1mrx 11y ago
In Groundhog Day, there is no material particle that returns to its starting point, only information (Phil's memory in particular.) Although, intuitively it seems that a time machine for information should be easier to build than one for matter, because information is so much easier to teleport.
- logfromblammo 11y ago
  If you had a tiny wormhole and put one end of it into a high-speed centrifuge for a while, until there was a meaningful time differential between the two ends, you might not be able to send even an entire photon through it, but you might be able to do something like bounce a laser off each end such that if you modulate the input polarization at one end, the polarization of light reflected from the other end changes at the appropriate time. No mass or energy travels through the hole itself, but information is transmitted.
vog 11y ago
An even better (although quite contrived) example is given in the movie "Predestination": http://www.imdb.com/title/tt2397535/ http://www.imdb.com/title/tt2397535/ Alternatively, read the short story "All you Zombies" by Robert A. Heinlein, on which most of the movie is based: http://emilkirkegaard.dk/en/wp-content/uploads/Robert-A.-Heinlein-All-You-Zombies.pdf http://emilkirkegaard.dk/en/wp-content/uploads/Robert-A.-Hei...

KenoFischer 11y ago

I'll have a go at defining it, term-by-term, > smooth Meaning no abrupt kinks (for a notion of abrupt which is probably not physically achievable, so don't worry to much about it) or jumps/discontinuities in the path of a particle, applying to both space and time > timelike roughly speaking, traveling slower than the speed of light > future-directed meaning going into the direction of causality > with intersections meeting itself again. In other words, it's a path through space/time on which you experience your own local time as always running forward, but yet somehow you end up in the past. Perhaps an analogy is in order. Suppose we were in 1+1 dimensions, with the time dimension being compact (closing in on itself). Visualize this as an infinite cylinder (the infinite axis being space). Now, let's impose a finite speed of light (for simplicity 1cm/s) and define the direction of causality, say counter-clockwise. Then, this spacetime has CTC, because we can find a path that, is smooth, future-directed (going around counter-clockwise) and timelike (going at angles < 45 degrees compared to the spacial axis). The simplest such would be a simple circle around the cylinder, but any curve that matches the above rules and intersects itself would be considered a CTC. Now, as a final note " a crude way of bringing out the fact that the application of familiar local laws of relativistic physics to a spacetime background which contains CTCs typically requires that consistency constraints on initial data must be met in order for a local solution of the laws to be extendable to a global solution." means the following. When we usually do physics, we know what happens locally (e.g. ball get shot at wall, bounces back), so we simply have to figure out (or decide if we're doing a thought experiment) where everything is in the universe at some particular time, apply our rules everywhere and we essentially know what's happening everywhere, anywhere. This doesn't work any more in the presence of CTC. "Consistency conditions" basically just means that you have to chose your placement of objects in such a way that they don't cause any paradoxes (there's a more technical notion here, but that's essentially what's meant).

dvt 11y ago
Awesome explanation, +1.
studentrob 11y ago
If time travel is possible, why is consistency a concern? If one thing changes at one time, couldn't another change at the same time? Could time travel exist without our being aware of it by constantly changing our understanding of events such that there appears to have been no change? Does consistency mean that some change must cause many other changes in order to make everything end up in perfect alignment?

tomlock 11y ago

There is a fair overlap between the analytic-philosophy community and physicists and mathematicians. So the analytic community generally doesn't talk out of its arse about these things. I'm from the other side of philosophy, the continental-philosophy area, which mostly talks about ethics and art and identity, so for me to talk about CTCs I would be talking out my arse. Philosophy is, generally, very hard to read. So much so that reading Descartes' Meditations (considered an alright starting text) in my first philosophy course took me about 30 mins per page. But, it seems to me that this is one of those areas where analytic philosophy seems to be breaching an area where actual experiments can be proposed and tested in a physical sense, and it leads me to think, what's the point? The logical (philosophical) consistency of light being a particle and a wave doesn't come in to the reality of it. Was it worth ancient Greeks wondering whether all matter was made of water? I think similar questions can be asked about continental philosophy. I certainly read a lot of it as a lot of wank that could be answered by real-world surveys (what makes people happy? I dunno, how about we ask them). But I think questions like, how do we determine whether happiness is the thing we should optimize for in life, are still relevant, and philosophical, and more basic! I think the underlying basic question of CTCs might be, does something being a logical paradox actually prevent it from happening? And, if CTCs are a physical phenomena that we can't observe (maybe, yet), what's the point of a philosopher wondering about them? Shouldn't that be a theoretical physicists job?

maaku 11y ago
>But, it seems to me that this is one of those areas where analytic philosophy seems to be breaching an area where actual experiments can be proposed and tested in a physical sense, and it leads me to think, what's the point? I think it would be fair to flip that around: if your philosophy doesn't result in testable predictions, then what's the point?
- tomlock 11y ago
  What I was talking about in that quote is that CTCs are definitely being seriously talked about and explored in relation to tested elements of our understanding of the universe by theoretical physicists, the experts in physics, and so I think its time for philosophers to give up that ground. Similarly I feel like there are philosophy-of-politics discussions that can (through political change) become the realm of actual politicians, sociologists, etc. I apologize, but I'd like to flip your question about again. I think philosophy is the realm of the untestable, so in my mind you're asking "what's the point of philosophy". I think philosophy is the place to explore the full ramifications and implications of untestable ideas. I'd also ask what's the point of your question? If its not testable, I think its a philosophical question, QED. If it does result in a testable prediction, what's the test?

dvt 11y ago

@KenoFischer already did a great job explaining the paragraph, but the audience is professional philosophers, philosophy graduate students, and some philosophy undergrads. I parsed this SEP entry for one of my undergraduate courses (Philosophy of Space & Time). If you're interested in the topic, I suggest reading Time and Space by Barry Dainton[1]. It was suggested reading for our class and primed me for anything from time time travel to general relativity. [1] http://www.amazon.com/Time-Space-Barry-Francis-Dainton/dp/0773537473 http://www.amazon.com/Time-Space-Barry-Francis-Dainton/dp/07...

Retric 11y ago

Suppose electrons would naturally time travel occasionally. But, we could gain no knowledge about the future from those electrons. It's not interesting. CTC means time-travel with structure. If you can send say a book back in time then our ideas of paradox ect have meaning. Next suppose you could time travel, but only far from your own past that light can't reach you. Further you block other time travelers so you can't send messages back to your self. Again not interesting, you need to be able to influence your past (self intersect) to be fun otherwise it's just like moving quickly.

mrob 11y ago
If electrons sometimes time traveled on a human time scale we could exploit that, eg. by building a huge assembly of electron guns, phosphors and photomultipliers. We could gain knowledge of the future signal sent to the electron guns by watching when the phosphors lit up while the electron guns were switched off.
- Retric 11y ago
  Depends on scale. Suppose they are rare and show up in random locations with random velocity's random amounts of time in the past etc. As in 1 electron in the observable universe per day.
  - mrob 11y ago
    If all their properties are randomized then I don't see how they can be said to "travel". That would be indistinguishable from electrons spontaneously appearing and unrelated electrons spontaneously disappearing.
    - Retric 11y ago
      This is one of the major reasons I said time travel without information transfer is not interesting. Even if it actually happens, it's not distinguishable from something else. PS: Quantum entanglement is also less interesting than you might think for similar reasons.