9 ms·

Time Machines

186 points by Hooke 11y ago

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.
cjslep 11y ago
The important question is: are we on the alpha or beta world line? El Psy Kongroo.
- TeMPOraL 11y ago
  I don't know, but I'll keep drinking Dr Pepper until someone finds out. :).
  - Kristine1975 11y ago
    I don't think it's wise to wait until the 2030s to see if CERN takes over the world. We must start operation Mímisbrunnr now! whooshing lab coat
joe_the_user 11y ago
Hmm, The thing about human beings - or goal-seeking subjects, is that they imagine a series of possible future lines and choose those lines they want to have happen. If you put a human being on one of these closed timelike curves (CTCs), then the human does their usual human-thing, the human analyzes several lines and acts to cause the one they want to occur to actually happen. Only their action could causing a timeline that they are already a part of to change. Which is where the grandfather paradox might come in - the situation of the grandfather paradox being not just that a person might kill their own grandfather but rather that they might want to kill Hitler or whoever's grandfather to keep something they don't like from being true about their present world (and humans generally have something they don't like about the world). Of course various devices could be evoked to deal with this - the circuit could be too long to influence or by some magic the things you do to stop the undesirable event actually cause the event (or oppositely for the desired event) - over and over again in the time loop. Science Fiction authors have dealt with along with other scenarios. But the problem is the "magic", (the coincidence or force that stymies human intentions) is required for every instance of comprehensible time travel, a situation that intuitively seems very unstable as a state-of-the-world. Thus I think the element that's incompatible with closed timelike curves is intentional thinking itself.
- studentrob 11y ago
  > Thus I think the element that's incompatible with closed timelike curves is intentional thinking itself. Interesting. I think I followed you.. Expanding on that, We do not know for certain whether intentional thinking exists or if everything is fated. We do not know if true randomness exists. Our intuition says both intentional thinking and randomness do exist, yet there may or may not ever be a science that proves this.
  - joe_the_user 11y ago
    Actually, I think science, quantum mechanic in particular, points to hard randomness being an inherent part of the physical laws. On the other hand, I don't think you need any really specific physical laws to have intentional thinking - I would say that any computer program that works with tree search, alphoGo or even much simpler alpha-beta pruners, is going to be engaging in intentional thinking on a level where they act in the present to attain their most desirable outcome in the future - and if that future somehow loops back to the present then the only stable situation is one where the intentional-thinker's efforts are thwarted or otherwise stabilized - ie, the only stable solution is artificially constructed for the instance of the closed causation loop.
dclowd9901 11y ago
It's cool to read something that feels like a genuine and academic stab at time travel. At the same time, it's so brutally academic I feel like I need a doctorate just to consume it.
nxzero 11y ago
Theory trumps reality in this example, since the assumption is that the timeline would center around humanity, which it might or might not. For example, say a time machine was created on a planet for which interplanetary travel was impossible. Time would be in constrained to that planet and would likely appear as a black hole to any observer outside its sphere of influence; meaning any matter entering that space would be transformed to such a degree it would be relatively speaking unreconizable and unobservable due to the constant flux of time relative to the speed of light.
dmh2000 11y ago
i heard a discussion once on time machines that had an interesting slant. (on the Art Bell show). The guest proposed 'what if you could be frozen in a capsule, then woken up 1000 years later, with no ill affects. Isn't that effectively a (future only) time machine? How is it different from a machine that somehow teleported you to the future? In that case where are you in the intervening 1000 years?'.
- chillacy 11y ago
  You can get something similar by going at near the speed of light. I'm sure future spacefaring generations will figure that one out.
poelzi 11y ago
Every model that does not have a proper arrow of time, and therefor prevents time-travel backwards, is fundamentally broken. It either breaks causality or energy conservation and therefor simply does not relate to our reality. I'm so happy that the model I adopted disallows time travel (this does not mean that your time can't be slowed down, but you simply can't travel backwards).
- nuncanada 11y ago
  Agreed. But such a reasonable assumption would inviabilize philosophers writing so many papers and books... What model did you adopt?
  - poelzi 11y ago
    Basic Structures of Matter - Supergravitation Unified Theory by Dr. Stoyan Sarg. http://www.amazon.com/Basic-Structures-Matter-Supergravitation-Unified/dp/1412083877 http://www.amazon.com/Basic-Structures-Matter-Supergravitati... Uses the most minimal basic assumptions and fits experimental data very well. Of course, the book analyzes many parts simplified, therefor small differences to the observed values. We have a long term plan to implement a numeric simulator to get better values and check the model in more details. Also, there are some questions that can't be answered otherwise - but those are corner cases in the universe (merger of anti-matter galaxy with matter one - the result is different in this model)
Natanael_L 11y ago
Most of these timeline variants assume that the past state of all particles exists in the past (making up something of a trace/ribbon to today's particle state), along the time dimension up to the current point. Kind of like a movie tape, except that they often assume a single-timeline self-consistent causality must exist, such that any version of events created in any timeline must be possible to arrive at without the time travel event to have happened. In other words, the trace can't be broken for any particle in the time traveler's past, or must be restored before the point of the time travel. Obviously that excludes time travel within your "light cone" from even being possible, as going back will sever at least one particle's "causality trace" within your own past. More reasonable assumptions are for example that you don't need to fold timelines together, so those branched time travel graphs made for just about every time travel movie would be what the actual time would look like, so only the time traveler's local causality matters (any change he makes in his past in his light cone does not affect him, it happens in a timeline parallel to the one he came from). Another is one where you essentially have a data tape where the the current time is like a writer head moving along the tape, writing down what happens. Time travel is to jump to another point and rewriting what happened. Multiple write heads may or may not be possible (likely impossible if you assume locally reversing the flow of time is possible, otherwise you'll get a real big mess). Then there's the one where time is a dimension like usual, but particles do not leave a trace in their past - going back or forth by jumping with a time machine puts you in empty vacuum with no end, not even with any cosmic background radiation (that too moves forward in time with all other matter). Perhaps gravity waves from the past would still be visible?

186 points by Hooke 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.

cjslep 11y ago

The important question is: are we on the alpha or beta world line? El Psy Kongroo.

TeMPOraL 11y ago
I don't know, but I'll keep drinking Dr Pepper until someone finds out. :).
- Kristine1975 11y ago
  I don't think it's wise to wait until the 2030s to see if CERN takes over the world. We must start operation Mímisbrunnr now! whooshing lab coat

joe_the_user 11y ago

Hmm, The thing about human beings - or goal-seeking subjects, is that they imagine a series of possible future lines and choose those lines they want to have happen. If you put a human being on one of these closed timelike curves (CTCs), then the human does their usual human-thing, the human analyzes several lines and acts to cause the one they want to occur to actually happen. Only their action could causing a timeline that they are already a part of to change. Which is where the grandfather paradox might come in - the situation of the grandfather paradox being not just that a person might kill their own grandfather but rather that they might want to kill Hitler or whoever's grandfather to keep something they don't like from being true about their present world (and humans generally have something they don't like about the world). Of course various devices could be evoked to deal with this - the circuit could be too long to influence or by some magic the things you do to stop the undesirable event actually cause the event (or oppositely for the desired event) - over and over again in the time loop. Science Fiction authors have dealt with along with other scenarios. But the problem is the "magic", (the coincidence or force that stymies human intentions) is required for every instance of comprehensible time travel, a situation that intuitively seems very unstable as a state-of-the-world. Thus I think the element that's incompatible with closed timelike curves is intentional thinking itself.

studentrob 11y ago
> Thus I think the element that's incompatible with closed timelike curves is intentional thinking itself. Interesting. I think I followed you.. Expanding on that, We do not know for certain whether intentional thinking exists or if everything is fated. We do not know if true randomness exists. Our intuition says both intentional thinking and randomness do exist, yet there may or may not ever be a science that proves this.
- joe_the_user 11y ago
  Actually, I think science, quantum mechanic in particular, points to hard randomness being an inherent part of the physical laws. On the other hand, I don't think you need any really specific physical laws to have intentional thinking - I would say that any computer program that works with tree search, alphoGo or even much simpler alpha-beta pruners, is going to be engaging in intentional thinking on a level where they act in the present to attain their most desirable outcome in the future - and if that future somehow loops back to the present then the only stable situation is one where the intentional-thinker's efforts are thwarted or otherwise stabilized - ie, the only stable solution is artificially constructed for the instance of the closed causation loop.

dclowd9901 11y ago

It's cool to read something that feels like a genuine and academic stab at time travel. At the same time, it's so brutally academic I feel like I need a doctorate just to consume it.

nxzero 11y ago

Theory trumps reality in this example, since the assumption is that the timeline would center around humanity, which it might or might not. For example, say a time machine was created on a planet for which interplanetary travel was impossible. Time would be in constrained to that planet and would likely appear as a black hole to any observer outside its sphere of influence; meaning any matter entering that space would be transformed to such a degree it would be relatively speaking unreconizable and unobservable due to the constant flux of time relative to the speed of light.

dmh2000 11y ago

i heard a discussion once on time machines that had an interesting slant. (on the Art Bell show). The guest proposed 'what if you could be frozen in a capsule, then woken up 1000 years later, with no ill affects. Isn't that effectively a (future only) time machine? How is it different from a machine that somehow teleported you to the future? In that case where are you in the intervening 1000 years?'.

chillacy 11y ago
You can get something similar by going at near the speed of light. I'm sure future spacefaring generations will figure that one out.

poelzi 11y ago

Every model that does not have a proper arrow of time, and therefor prevents time-travel backwards, is fundamentally broken. It either breaks causality or energy conservation and therefor simply does not relate to our reality. I'm so happy that the model I adopted disallows time travel (this does not mean that your time can't be slowed down, but you simply can't travel backwards).

nuncanada 11y ago
Agreed. But such a reasonable assumption would inviabilize philosophers writing so many papers and books... What model did you adopt?
- poelzi 11y ago
  Basic Structures of Matter - Supergravitation Unified Theory by Dr. Stoyan Sarg. http://www.amazon.com/Basic-Structures-Matter-Supergravitation-Unified/dp/1412083877 http://www.amazon.com/Basic-Structures-Matter-Supergravitati... Uses the most minimal basic assumptions and fits experimental data very well. Of course, the book analyzes many parts simplified, therefor small differences to the observed values. We have a long term plan to implement a numeric simulator to get better values and check the model in more details. Also, there are some questions that can't be answered otherwise - but those are corner cases in the universe (merger of anti-matter galaxy with matter one - the result is different in this model)

Natanael_L 11y ago

Most of these timeline variants assume that the past state of all particles exists in the past (making up something of a trace/ribbon to today's particle state), along the time dimension up to the current point. Kind of like a movie tape, except that they often assume a single-timeline self-consistent causality must exist, such that any version of events created in any timeline must be possible to arrive at without the time travel event to have happened. In other words, the trace can't be broken for any particle in the time traveler's past, or must be restored before the point of the time travel. Obviously that excludes time travel within your "light cone" from even being possible, as going back will sever at least one particle's "causality trace" within your own past. More reasonable assumptions are for example that you don't need to fold timelines together, so those branched time travel graphs made for just about every time travel movie would be what the actual time would look like, so only the time traveler's local causality matters (any change he makes in his past in his light cone does not affect him, it happens in a timeline parallel to the one he came from). Another is one where you essentially have a data tape where the the current time is like a writer head moving along the tape, writing down what happens. Time travel is to jump to another point and rewriting what happened. Multiple write heads may or may not be possible (likely impossible if you assume locally reversing the flow of time is possible, otherwise you'll get a real big mess). Then there's the one where time is a dimension like usual, but particles do not leave a trace in their past - going back or forth by jumping with a time machine puts you in empty vacuum with no end, not even with any cosmic background radiation (that too moves forward in time with all other matter). Perhaps gravity waves from the past would still be visible?