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.
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).
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?
Larry Niven speculated that if time machines were possible, time travellers continually going into the past would change the circumstances that lead to them inventing their time machines and using them, resulting in an unstable timeline. The only steady state for this timeline would be one in which no time machine was ever invented.
So we may well live in a universe in which time travel is possible, and time travellers from alternate future timelines may even have arrived in it in the past (or may arrive in it in the future from more distant futures), but the 'final value' of the timeline would exclude the events that lead to their actual invention.
Consistency here is a mathematical statement. Think about the cylinder example, and suppose you want to assign a number of each location. Then, naturally, going once around the cylinder puts you back at the same spot on the cylinder, so it needs to have the same number. At this point it is useful to remember that the numbers on the cylinder are fixed. We're already representing time as the compact dimension of the cylinder, so there's no extra time dimension.
Now, with this setup, let's imagine living in this 1D world, just sitting still, reading off numbers. Our life would be cyclic with period cm/((2pi r)s).
Let me try to phrase this a different way. Suppose the universe is a database, where (t,x,y,z) are the unique keys. For every key there is some value of "physical reality" at that point in spacetime. All the original statement is saying that if you want to generate such a database by starting with only some of the entries and applying the laws of physics, then in the presence of CTC, there are some very strict conditions on what those initial rows can be if you don't want to run into a situation where you'd want to assign two different values to the same key (e.g. by getting to the same point forwards in time and backwards in time).
Perhaps the database administrator chose the unique keys (observer_id, subjective_observer_time).
Then, the (t,x,y,z) coordinates could all be parametric functions of observer_id and subjective_observer_time. If you follow a CTC and meet your younger self, then convince your younger self to not follow the CTC, you may simply be reassigning your younger self to a new observer_id, rather than creating a paradox that destroys the database integrity.
As long as all observers obey the rules of causality locally, your elder time-twin will never remember meeting their own elder time-twin, and your younger time-twin will never remember not meeting their own elder time-twin. The CTC has no way of knowing whether anyone ever followed it or not, because it isn't an observer. So if you ever meet yourself, you're no longer meeting yourself, because the person you meet is from that moment no longer you. If you do anything at all within the past half of your own light cone that interrupts causality, you are just creating a new observer.
So the photograph doesn't change itself, like in Back to the Future. Marty can't erase himself, but he could erase all references to himself from the historical record, except for whatever he carried through the CTC. He could then take a DNA test that proves him without a doubt to be the child of two people who cannot recall ever even touching one another. Even if he mostly restores causality, upon reversing through another CTC to a time before he entered the first CTC, there will be a different third child of his parents living with them when he gets back--someone who might not ever traverse a CTC.
If that's how it works, time travelers could do no worse than completely erase themselves from all historical records and the memories of every living person. They would be considered people who never previously existed, who spontaneously appeared, complete with memories of future events that might never come to pass. In that case, almost anything even remotely plausible could come out of either end of a CTC.
But this is not the same as a time machine. The article doesn't explain the difference between a machine you can use to travel back in time, and a contrived spacetime in which events run in a loop.
While it's dismissive of "paradox mongers" it doesn't deal with any of the very real issues created by very obvious potential paradoxes - not least of which is that physics fundamentals like conservations of energy stop working.
(If you can move anything or anyone back in time you have a universe with two of something instead of one of something - which means you can get something for nothing, over and over. If you can't do this because the CTC forms a time loop, you don't have a paradox, but you don't have much else of interest either.)
A contrived spacetime with a CTC is fundamentally useless as a practical time machine. And the whole point of a Thornian wormhole is that it offers practical engineering benefits - such as being able to travel in a way that effectively bypasses the speed of light limit.
In any case - relativistic speculations are metaphysical, because it's a fair bet that spacetime isn't a smooth continuous manifold at all. It just looks like one from a distance under normal conditions - normal meaning anything outside a singularity or black hole.
No one has much of a clue what spacetime is made of, and that makes it hard to say much about its properties under stress. For all anyone knows CTCs may be impossible because if you stress spacetime enough it may not act like a manifold any more, and a clean description of what happens at that point needs new completely physics.
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?
No brag, but I read meditations in a few lunch breaks when working a manual job power washing ride on lawn mowers at uni.
It's a simple read.
If you'd have said Kant, I might have agreed with you.
Most of the time complicated philosophy (or science) are hard to read because either they're talking crap (Hegel) or they can't write for shit (Kant). Descartes was pretty good in my book (although obviously the 2nd half complete failed).
I found Kant to be easier than Descartes and Heidegger to be by far the hardest to read of all. Hegel has for me been somewhere between Descartes and Heidegger, but I think its no surprise that Zizek was a Hegelian and also read a bunch of Lacan :D
I think Heidegger is actually deliberately obscure!
>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?
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?
If it's not testable, what's the point? If it is not testable you are literally saying it has no effect on the world.
Note that something can be political or sociological and still be testable. You can have a utilitarian moral philosophy where you assign utilities and sum them up, then do surveys or focus groups to to measure agreement with the results.
But if a theory literally has no testable predictions? That is the definition of useless.
>>If it is not testable you are literally saying it has no effect on the world.
It's provable that things that are true are not demonstrable(see Gödel's incompleteness theorem). It follows that things that are not testable are true, and for some d
definition of truth, it means they have an effect in the word. So we absolutely can, in a meaningful way, wonder about the posible consequences of something that while not testable may be true. Just because something it solely potentially meaningful does not mean it it's entirely meaningless.
Let me stop you right there. You can have a utilitarian moral philosophy, sure. But the question of if you ought to have such a philosophy is a philosophical one. To get to the point of having tests of optimized utility there is an underlying question that has no tests, which is are tests for utility really the best tests to run?
Its either turtles all the way down: tests testing the effectiveness of tests, or, you get into philosophy.
Hi, I'm just some dude, but wouldn't it be best for everyone to work on the problem until the problem is solved? Also, what about philosophers who are also physicists? I had a physics professor in college who had a degree in philosophy, and she didn't seem to mind talking with non-physics philosophers about physics-related topics.
Well, yeah, in my initial comment I said that there are a lot of analytical philosophers that have an understanding of physics. But, I think that CTCs are better suited as an undergraduate physics topic than an undergraduate philosophy one. We did talk at length on them in one of my undergrad philosophy classes.
I think its a fair question if everyone should work on the problem, but I'm wondering if that same question could be applied to every problem?
I don't think many would agree that a mathematics degree should be made entirely of pottery electives. I'm not saying we shouldn't encourage mathematicians to dabble in pottery, I'm just saying pottery is not mathematics, like CTCs don't seem to me to be philosophy.
Oh, it's definitely not cool to force people to do things. I didn't know it was like that. Someone probably thought that it's a really important problem, though. Like, if it were up to me, I'd be okay with forcing students to take a course on global warming, since I think global warming is a really important problem. But that's just me though. Anyway, I'm sorry you had to spend time doing stuff you weren't all that interested in.
> I think it would be fair to flip that around: if your philosophy doesn't result in testable predictions, then what's the point?
"Untestable philosophies are pointless" is an untestable philosophy, and therefore pointless by its own standards. This is a well-known problem with logical positivism, which is roughly the position you are advocating.
@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.
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.
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.
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.
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.
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.
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.
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.
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.
In the case of entanglement though it is actually distinguishable from other, less "interesting" phenomena though, right? That's what the loophole-free Bell tests are about.
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.
> 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.
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.
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.
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?
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.
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).
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)
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?'.
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.