A revised and improved version of this essay is in my book Sci-Ence! Justice Leak! – hardback, paperback, PDF Kindle (US), Kindle (UK), all other ebook formats
I am utterly astounded that I’d never seen this before today – an experiment that may have more profound implications for our worldview than… maybe any experiment since the Michelson-Morley experiment?
I’m going to assume here that everyone knows about the Grandfather Paradox. This is just the simple question “What happens if you have a time machine, and go back and kill your granddad so you can never be born?”, the staple of many TV science fiction shows.
Now the normal answer to that question is “You can’t, so don’t be daft”. But for physicists, that’s not good enough – apart from anything else, General Relativity allows for the existence of ‘closed timelike curves’. These are paths through space-time that act much like paths through space – you can go in at one end and pop out the other – except that the other end is somewhere else in time as well as space. So it’s theoretically possible that you *could* do that, and we’d quite like to know what would happen if you did before everyone’s granddad starts retroactively never-having-existed.
Now, the main hypothesis in physics up to now has been, in effect, that it doesn’t matter. David Deutsch, a quantum computing expert at Oxford University, demonstrated that in quantum-mechanical terms you could have an outcome that makes sense so long as you accepted the many-worlds version of reality. Essentially, the probability that you were ever born, and the probability that you killed your grandfather, would both be 1/2 – or in other words the ‘you’ in a universe where you were born would travel to a universe where you were never born, kill your grandfather there, then come back to one where you’d never killed your grandfather. Nice and simple.
However, Seth Lloyd, a quantum physicist at MIT, never liked the many-worlds hypothesis (for reasons which, I have to say, make no sense at all to me), and he and a team of colleagues came up with another, simpler, idea, which is just that if you go back in time and try to shoot your grandfather, something will stop you. Maybe the gun will misfire, maybe you’ll be arrested, maybe your grandma was having an affair with the milkman and you’re his biological grandchild – something will just make sure that you can’t do that, because it would be cheating.
Now, there are huge, huge, MASSIVE problems with this – it gets rid of causality, it allows information to come from nowhere, and it just seems like a gigantic handwave. It makes no sense at all, and just seems like a desperate attempt to try to get out of the obvious, blatant, truth that the Many-Worlds interpretation is the only one consistent with the experiments and maths. When I first read about it, I thought it was just a neat way of avoiding the truth.
Unfortunately, it appears to be true. What I hadn’t realised was that they’d *actually done the experiment*!
Lloyd and his colleagues came up with an ingenious experiment, which I’m not entirely sure I’m capable of explaining, as it’s not really sunk in yet. This will be a GROSS oversimplification, and is just designed to get the idea across – please don’t kill me for inaccuracies. The full description is in the linked PDF. This is what Pratchett, Stewart and Cohen call lies-to-adults – the story is right, but each individual fact is wrong.
Essentially, photons (light particles) can be polarised a couple of ways, and they’ll only go through stuff that’s polarised the same way. That’s why Polaroid sun-glasses work – they block all the photons that are polarised the wrong way, so only let some light through.
Now, until something detects it, a photon isn’t in any particular polarisation – it’s in all of the possible polarisations at once. But once something has detected what kind of polarisation a photon is in, it’s always been that way – quantum causality works both ways in time. So you can set up an experiment that only detects photons of one polarisation, and that way you can send a message back to the past, to the photon emitter (light source) saying “Only send photons of this type”. If you do this the right way, you can send a photon back in time (but you can’t look at the photon that’s been sent back in time until it’s come back to the time you sent it from, or the experiment can’t work). That might sound mad, but it’s the way things are – accept it for now.
Now, by doing this, you can set up a kind of quantum ‘gun’ – set it up so that the photon going back in time tries to cancel out itself coming forward in time – all you do is put something in the middle that tries to change the polarisation of the backwards-in-time photon to the opposite of the forwards-in-time one. Changing polarisation is easy, and works about 96% of the time.
It never worked on the backwards-in-time photons.
This means that if you went back in time and tried to kill your grandfather, the gun really *would* misfire! Every time.
Now, assuming their experimental design wasn’t flawed and their maths works – and it looks OK to me, but I’m not a quantum physicist – then that means a lot of things:
Firstly, it means the universe is completely deterministic. There’s no such thing as chance.
Secondly, it’s strong evidence *against* the many-worlds hypothesis – the first such evidence I’ve ever heard of. It almost certainly means there’s a single universe.
Most interestingly, it means we can say goodbye to cause-and-effect. Effects can cause their own cause. For science-fiction fans, we’re living in the universe of Bill & Ted, the Doctor Who story Blink, and By His Bootstraps, (EDIT or of this rather nice short-short story by Simon Bucher-Jones) rather than Back To The Future or Pyramids Of Mars.
This of course means that access to a closed timelike curve (something that has never been observed in the real universe, but is theoretically possible), gives you essentially godlike powers. Got a closed timelike curve and want a million quid? Just put two pence in the bank and say “tomorrow, if my account has two milion pounds or less in it, I’ll take half of the money out and bring it back today and stick it in the account.” So if tomorrow you’ve still got 2p, you’d go back and put an extra penny in, which means that actually tomorrow you’ve got 3p in, which means… and the only stable way that can work out (other than you dying or something over the next day) is for the million pounds just to appear in your bank account.
Want to write a bestselling novel? Decide to print out five hundred pages just covered with the letter “A” and send it to a publisher. If they publish it and it becomes a bestseller, you send that back to yourself. If they don’t, you print out all the letter “A” apart from one “B” at the end and send that back to yourself to try that, and repeat – the only stable outcome is that you have a novel arrive that you never actually wrote but that will be an instant bestseller. And so on.
The possibility of time-travel in a *single, consistent universe* has never been one that’s really been taken seriously before, because it was just so absurd. I’m still 90% sure that there must be a mistake somewhere – the many-worlds hypothesis, as odd as it may sound, is far, FAR less ridiculous than this. But this is one of those things where either in a few months we’ll have a very quiet paper by Lloyd saying “Oops, I was totally wrong about everything because I forgot to carry the one” or in a hundred years’ time we’ll have a totally new understanding of physics based around this paper. I really can’t see a middle ground here…
Sci-Ence! Justice Leak! 
Phew.
Thanks for this – will have to re-read it though!
The single, consistent universe model of time travel has always been my favorite fictional time travel model. Wally West goes back and time and becomes the mysterious stranger who gave him a pep talk when he was a kid and all that. I’d never seriously considered the possibility that it might be the way the universe really WORKS, I’m just always entertained by the plotting necessary to make it all link up.
But if it really IS all true…! Well, I don’t know that I ever WANTED to live in that kind of universe…
About Back to the Future, though (coincidentally, I went to a limited-rerelease showing at a movie theater in town last night), SOME of the time-travel business actually DOES adhere to the self-consistency principle. Marty tells the guy at the diner he’s going to be mayor one day, which appears to be the inspiration for him to run for mayor in the first place. That, and Chuck Berry’s inspiration for writing “Johnny B. Goode” comes from hearing Marty play it at the dance. (I’m quite certain the filmmakers never thought about the problematic implications, but as a result the movie suggests a white guy DID invent rock ‘n’ roll after all…)
Most of the other stuff in BttF, of course, works out so you CAN change the past (knocking down a pine tree in 1955 changes the 1985 Twin Pines Mall to the Lone Pine Mall), and basically screw the grandfather paradox. Very little of the movie ACTUALLY makes any sense, but y’know, I’ve never asked it to.
I don’t think the attached paper describes sending photons back in time; as far as I understand, doing that would violate at least one widely-accepted physical law and certainly earn the researcher a trip to Stockholm.
Note also that the paper is from arxiv, and therefore not properly peer-reviewed.
It describes sending information back in time, which is used to set the polarity of a photon. In quantum physics, any two particles in the same quantum state are the same particle, so yes, it does send particles back in time – in a very specialised way.
Peer-review is overrated as a method of preventing error. In medicine, for example, something like 41% of peer-reviewed published papers are wrong ( source, haven’t looked at the primary source yet), and that’s not counting metaanalyses, which are *ALL* wrong. Remember that, for example, Newton and Darwin’s most famous work wasn’t peer-reviewed either.
Seth Lloyd is one of the most respected people in the field, so I don’t think the peer-review process would have got rid of it…
Well, I can’t be sure about the paper – it’s somewhat outside my field, and it’s a while since I’ve studied this stuff anyway – but I’m pretty sure parts of it are hypothetical, thought-experiments or the like. He talks about sending photons back in time, events from the future affecting the past, information getting out of black holes etc., all of which sounds counter-factual, or at least hypothetical.
As I say, I can’t be sure exactly what the paper signifies, but that’s rather the point – without knowing all the context and actually being able to understand the paper in detail, we can’t really be sure what it’s saying, and chances are it’s something far less exciting than it might appear.
In re peer review, I’d be very interested to see another suggestion for implementing a basic ‘bullshit filter’ for the physical sciences. Medicine is a special instance, of course, because of all the commercial interests tied up in success of trails.
To be clearer: all that papers like this really show are some clever mathematics which may have some bearing on real physics, which in turn may have some bearing on the real world you and I experience. Interesting as quantum mechanics is, laymen reading the literature without proper background are liable to fall into horrible traps, like Capra with his bizarre quantum mysticism.
Except this isn’t ‘just’ mathematics. The stuff you’re talking about is experimentally demonstrated – quantum teleportation has been happening for decades, and that’s FTL (i.e. backwards-in-time) transportation of particles. And Lloyd has *actually carried out the experiment*. Send particles back in time, try to make them stop themselves having been sent, find out you can’t do that.
The only questions are 1) is there some hole in his experimental setup? (unlikely) and 2) is his interpretation the only correct one, or is there some way of reconciling the results with a multiversal interpretation?
I’m confident enough of my understanding to say that this does what Lloyd says – that it shows that any time travel is forced to be consistent with a single deterministic universe. The only question for me is whether that means multiversal interpretations have been disproved, or if there’s another way of looking at the results.
‘quantum teleportation has been happening for decades, and that’s FTL (i.e. backwards-in-time) transportation of particles’
Really? I’m pretty sure it’s not FTL.
And can you point to the bit of teh paper that you’re reading to say that he is actually sending particles back in time? This would be a huge advance, practically and theoretically, so I’d be surprised if it’s been achieved without a great fanfare.
BTW, what are you using ‘deterministic’ to mean here? Devoid of any stochastic processes?
I’ve had a chance to read the paper properly now, and I’m pretty sure that he’s not really sending anything back in time. What he’s doing is using a quantum computer (circuit) to simulate an equation for an hypothetical CTC. I.e. one of the qubits *represents* a particle travelling backwards in time; it’s isn’t actually *travelling* backwards in time.
This is directly analogous to a binary register in a classical computer representing the velocity of a particle as it exceeds the speed of light under certain assumptions; the results may be physically meaningful, but nothing is actually being accelerated past c.
Small point, if I may: any instantaneous transmission is automatically FTL, and any FTL is automatically time-travel, no?
(Though there is no acceleration still, obviously.)
Yup, if you could send information or matter instantaneous ly, then it would be FTL. But we can’t send anything instantaneously.
Quantum teleportation isn’t instantaneous?
By which I mean: I think it is instantaneous.
Quantum teleportation? No. As far as we know, it’s impossible to send information or matter faster than the speed of light. That’s pretty basic.
Bah, damn the formatting on this web page! When comments nest deeply, more than half this input box is hidden :-/
I mean to say ‘that’s a fairly basic limit that we have no good reason to doubt at this time’, or words to that effect.
I’ll have to disagree with you there, Jim…though not about how annoying the comments-nesting gets! But QT is “spooky action at a distance” in the flesh. That’s why the question’s always been about “why doesn’t this violate SR?” rather than “why would anyone think this violates SR?”
If you know about some other type of QT with which I’m unfamiliar, please do tell :-)
Regular QT, the thing that we’ve been doing for years and is in all the textbooks etc., is emphatically not instantaneous.
Would you care to elaborate on that? You seem very certain.
Well, it just isn’t… Any textbook
on QM or QIP will tell you all
about it. A quick google even…
Thank you, but I already know quite a bit about it.
http://www.its.caltech.edu/~qoptics/teleport.html
Thank you, but I don’t require that link.
I only asked you if you’d care to elaborate. If you don’t care to, that’s totally fine.
I’m sorry if I’ve upset you, but
if you think that QT involves
instantaneous transfer of information,
you do indeed need links like that one.
You haven’t upset me at all. You’re quite wrong about me needing that link, though.
Again, it is totally fine with me if you’d prefer not to elaborate. But I’m not interested in getting a link in lieu of elaboration. We could easily leave this to one side now, I think.
And no hard feelings, of course.
For my money, too, the “why doesn’t this violate SR” question hasn’t yet been answered adequately…we’re still stuck in what I think should be called “portmanteau thinking” about this stuff. Just like the question “but why doesn’t your attempt to kill your grandfather work” opens up (or should open up) a whole new field of questioning. You could argue (I think) that not being able to kill your grandfather represents a change in the conditions of the past…after all, what’s “consistency” when it’s at home anyway?
I also sort of wish I could stop thinking of c as a “limit”, myself — and start thinking about it as what it really is: a constant. Though that might not be as useful as I hope. After all, is symmetry itself an adequate explanation for inertia?
Hmm, maybe it is better to think of it as a “limit”…
Nice article. Will think about the implications.
For what it’s worth, I always felt that diffraction of the electron already disproved the many-worlds interpretation.
(Experimentally if you fire an electron at a narrow gate then it will follow a curved path as though diffracting like a light wave. Essentially the electron must be causing interference with itself. In the many-worlds model, each possible electron path would represent a particle in a different universe: so what would be interfering with it to cause the diffraction? Only in a superposition universe, where different versions of the same electron exist in the same universe is the observed outcome possible.)
Oh and a closed time-like curve is a path though spacetime that allows you to arrive back at your starting point (in time as well as space) without violating any of the laws of motion. Hence “closed”. (In practice, that’s any path that can get to your own reverse timecone – since the laws of physics already say that your reverse timecone is all the points from which there are paths that can reach you.) But you probably know that.
That’s verrrry interesting…
I’ll disagree with you about it making the universe entirely deterministic, though: if it’s only the past that can’t be changed, and not the future! Actually at first glance this seems to me to put back the element of randomness that Many-Worlds removes: we don’t see something happened in a certain way because in another universe it happened the other way, but we see something happen in a certain way because sometimes things just happen in ways.
I think maybe the question of why is more unavoidable in this experiment than in your garden-variety uncertainty situation, though, don’t you? Why does your gun jam as you’re firing at your grandfather, that’s what’s going to keep me awake in bed tonight…
Er…I don’t think that reads as clear on the screen as it did in my head…
Looking over the thread above, I realize that I had allowed the
steadily-narrowing comment box to make me extremely curt, a fault from
which I do not normally suffer. I’d also completely lost track of the
overall question. Please find below a comment displaying neither of
these vices, but perhaps many worse ones.
One of the steps in the QT algorithm is for Alice to transmit two
classical bits of information – the result of her measurement of her
entangled qubit – to Bob. With this information, he can transform his
entangled qubit and get a copy of the state of Alice’s other qubit. As
you can see, no information nor any matter is transmitted
instantaneously, or at any extraordinary speed at all.
The fact that the paper is using quantum teleportation to discuss time
travel is entirely accidental. What the authors are doing is presenting
some hypothetical mathematics which could describe theoretical CTCs.
CTCs have never been observed in the field, but they are consistent with
known physics. The authors don’t have a CTC, so they can’t actually set
up a grandfather paradox. Instead they calculate the outcome under their
hypothesis using a quantum circuit. The quantum teleportation is just
part of the algorithm they’re running on their quantum computer.
Don’t give it a second thought, Jim! Nested comments doth make fools of us all; I think I got a little curt as well.
But I still hold to an interpretation different from yours: observation of one half of an entangled pair affects what can be observed in the other, instantly. The only trick is, until the observations are compared you can’t tell. This saves SR, because it means the total system of signalling ineliminably contains a slower-than-light component. But it doesn’t mean there’s nothing “spooky” about it.
I think I can make an analogy here, for our difference of opinion. STAR TREK! Suppose the folks in Star Trek develop a Transporter that can “beam” Captain Kirk anywhere at all, to any distance, at instantaneous speed. So: great! First thing we do is beam a Transporter machine out to, let’s say, Andromeda. Now he’s got something to reassemble him from energy into matter. So we send him out there too. But unfortunately, without anyone to work the remote console, the instantaneous signal is received but can’t be rematerialized. Sigh. So we send Mr. Spock after him in the Enterprise, it takes a hundred years but he finally gets there, fiddles with the controls, and voila — William Shatner steps off the pad saying “Mr. Spock, what are you doing here, and how come there’s so much grey in your hair?”
Is this an acceptable analogy? It’s a bit rough-and-ready, I admit, but I think it’s good enough for horseshoes and hand-grenades. You would say “it took a hundred years to transport Captain Kirk from where he was to where he is.” I, on the other hand, would say he’s been there for a hundred years already, but he couldn’t be “made sense of”. You might respond: “well, what difference does it make, travel time is travel time.” But I would disagree, and say that if Mr. Spock could’ve somehow gotten there twice as fast, Kirk’s “travel time” would’ve been halved.
You might fairly reasonably reply: “well, but he can’t go any faster than he does, because the laws of physics forbid it.” And then maybe I would say something like “what if he was halfway there already?”
And you might say: “well, the travel time is STILL THE TRAVEL TIME, it still took him a hundred years to get there, it doesn’t matter if he left early, that hundred years can’t be done away with, it EXISTS!”
And I would counter that the Transporter beam is not travelling with Mr. Spock, but is only meeting up with him at some point, so his speed and its speed don’t have to be the same thing. But I think we would each have a valid point, even if I think I’d be righter about mine.
In one way at least, though, it really is a lousy analogy: because in the end nothing can be yielded from that instantaneous-transport scenario that isn’t yielded equally from Mr. Spock just knocking Captain Kirk out with the Vulcan neck-pinch, stuffing him in a freezer, travelling to Andromeda with him frozen beside him in the passenger seat, and then reviving him when he gets there. “Mr. Spock, what are you doing here?” And that seems to favour your point of view over mine. But in the “real world”, if a quantum system yields something other than just “exactly what a classical system does anyway”, then that would favour my interpretation. If it didn’t, you could say “well, it’s all a distinction without a difference, isn’t it?” You could accuse me of being rhetorical, and rightly so. You could say “QT might be good for cryptography, but it doesn’t have any sort of time-saving component to it.”
But if it could be shown to have a time-saving component to it…well, what then? You seem like a reasonable person; if something like that could be shown, wouldn’t you say “hmm, maybe there’s something to this instantaneity business after all”? It may seem a bit, ah…epicyclical, to say that if we can just get get it so instantaneity doesn’t violate SR then there’s nothing wrong with it (I think it smacks of epicycles just a tiny bit myself!), but there is precedent for such a tactic…after all, QED itself tells us it isn’t that things can’t travel faster than c, it’s just that they don’t travel faster than c…and there are a small number of processes that do indeed exhibit a superluminal speed in nature, it’s just that by the time the process is all done it hasn’t mattered in terms of outcomes, so SR’s descriptive ability hasn’t been affected by them.
Well, it isn’t affected by Cerenkov radiation in the swimming pool either!
Why should this be different?
> But if it could be shown to have a time-saving component to it…well, what then?
Well, not QT itself, but some so-far unknown quantum effect might be shown to have such an effect.
However, as you say earlier, it would contradict SR. Possibly a future synthesis of GR and QED might show a way to FTL travel, by explaining all the things those two theories do, without the no-FTL constraint we currently have. But such a synthesis could lead to all kinds of amazing consequences; all we can do is speculate.
The same goes for many-worlds and its ilk: although the speculation might be very interesting, it is only speculation. We can’t distinguish experimentally between the different interpretations of QM, because they all make the same predictions about what we can actually observe. They only differ in the philosophical explanation they give for the superficially odd results of QM.
In some interpretations, something ‘happens’ at a distance when Alice measures her entangled qubit, but in others nothing really happens to Bob’s qubit at all.
The two qubits are together at first, with the third qubit (the one whose state is being copied). Then one qubit travels a distance, and later it can be measured to get the value of the third qubit. This measurement is no use without the information given by Alice’s measurement of her entangled qubit, but nothing in QM says that Alice’s act of measuring her qubit *does* anything to Bob’s qubit.
One of the important things about QM is that it’s a completely abstract theory – qubits are best manipulated using linear algebra and lots of e^i*pi stuff, which gives very neat results for quantum circuit calculations,but really doesn’t mean anything physically. QM doesn’t give nice concrete explanations that you can imagine the mechanics of – the only way you can visualize the operations of QM is with abstract mathematics.
You’ve no doubt heard of Shor’s algorithm for factoring large numbers on a quantum computer. Thinking about a simple algorithm like that is impossible in terms of photons flying around – you can get understand the physical implementation, but to really *think* about it, in computational terms, you have to see it as complex operations on vectors. Once you start using bra-ket notation and linear algebra to manipulate qubits, QM starts looking a lot less bizarre – if you want a philosophical message from QM, it’s that the God made the fabric of the universe from complex vectors.
Actually, it is only QM that insists something must happen to Bob’s qubit, eh? The substance of Einstein’s complaint at Solvay in a nutshell.
If my memory’s not faulty, I think we do find that the quantum system behaves in ways that identify it as a different cat from classical systems…exactly in a “time-saving” sense, and in a way that doesn’t disturb SR’s descriptions. But, hmm, I’d have to look that up, I think…
I should probably not add, but will anyway since you’ve got me thinking about it, that as for me I don’t think QM is as abstractified as all that…I just think it tends to accumulate abstract adornments over time. There’s always going to be funky math in physical theories, but I don’t think we’re progressed to the point where non-mathematical imagining is passe…! But that’s sort of an argument about elegance, isn’t it? As in: how important is elegance to a physical understanding? I think it’s a slightly more complex question than it perhaps appears to be, if only because we are constantly attempting to achieve elegance, and the best kind of elegance is the “realistic” kind…the “simple” kind, if you will.
The problem being, that just because something’s elegant doesn’t make it true…and yet we’ve never yet made a better physical theory, that wasn’t in some way simpler and easier to grasp than the theories it replaced. Even QM isn’t difficult to visualize one’s way around, once one stops being “blown away” by its joyful embrace of the paradoxical…and, it must be said, once one allows in just the tiniest bit of more-precise language, i.e. math. So how much weight should we be comfortable placing on “elegance”? The worst lies are elegant too…there’s nothing wronger than a dumb elegant theory, and so rather like Descartes I think we’re constantly in pursuit of a way to justify our bias towards beauty by finding classes of it…an aesthetics of physics, if you like, or a physics of aesthetics. Einstein had a pretty good one: logic is in what you can see. QM puts logic in what you can talk about. But neither of these will get the total job done, it seems.
Sorry, a bit of pre-coffee burbling there!
Hmmm, actually a closed time loop means that you can’t change the past, or the future as they have both already happened in some sense, ie take a step outside of the 4D space-time and look at it, and it is a loop where everything is causally connected, so there is a path that is closed that things are happening on. If you
could change anything along that path, then the path would no longer be closed, so the closed time loop would always have been there, and always would be there, although “always” in this context doesn’t really make sense, since it is just a loop, you don’t go round lots of times you just go round once, since the time at the beginning is always the “start”. To go round multiple times, you would need some external “time” which continues to increase, so then it wouldn’t be a loop in this “new” time.
Of course, in the standard
“travel back in time” scenario if you did travel back in time, to time X, then if you did not exist at time X in your own history, then the initial conditions for the universe to evolve from time X would have changed, and as such the universe would evolve differently from then on, and so it wouldn’t matter whether you killed your grandfather or not, since the subsequent evolution of the universe would be different anyhow, ie merely traveling back in time would create a separate branch in the time line and so if you traveled forward from then, maybe just by waiting around, the future would *not* be the same as the one you traveled from, and of course, you could never go back to your original future, and no paradoxes occur. Viewed this way, Back to the Future is actually, pretty sound. It does however, mean that in each of the fresh branches created, when Marty leaves to go to another branch, the rest of the “new” universes carry on exactly as they were, so in one, Biff does indeed continue to be very rich.
At the quantum level, if things happen randomly, then at the subatomic level, the entire universe would evolve differently in a microscopic way at any branch in the time line, and that these subatomic differences would gradually diverge as time went on, much like chaos theory. Of course, this is not the same as a time loop, can the timeline branch in this way? This is a bit like the many worlds hypothesis, although not quite the same, and I’ll come back to this later.
In fact, if there were a time loop then your presence in the past would mean that you had always been there in the past, it would be a continuous loop, so you couldn’t have just “popped” into existence out of nowhere, and in fact, if you think about it, a closed loop would need to be disconnected from the rest of the universe, and in fact
everything in that loop would happen only once, but end up with the same initial conditions.
This is a bit of a problem for general relativity, since it means that the quantum fluctuations would have to be the same at the “beginning” and “end” of the loop, for everything in the loop, your “timeline” includes the entire universe (or at least that bit that is looping determined by the distance light can travel during of the loop) so this would seem very unlikely.
Overall, General Relativity is a classical theory, but the universe is quantum mechanical in nature, and we haven’t yet unified GR with QFT, so whether GR allowing such things to happen is actually consistent with the
actual universe will depend on what is allowed *after* unification.
Regarding the “Many Worlds” hypothesis, this is *not* actually how things are, just a useful fiction to “explain” supposition of states, and get round the Schroedinger’s cat situation. At the quantum level, there is a supposition of states, and Many Worlds is a way of explaining how the apparent collapse of this supposition into a specific state can happen. MW says, that the wavefunction doesn’t actually collapse, but the universe branches. Such a branch would have to happen to travel back in time. In fact, the MW scenario isn’t quite correct. The “Wavefunction collapse” isn’t actually what people think is happening. We don’t “observe” a quantum system, and watch it “collapse”; we are part of the universe, and are ourselves quantum systems, made up of wavefunctions. What is believed to happen, is that the wavefunction os the “collapsing” system is evolving in time, as is our own, and the wavefunctions at some point will start to interact, and will then evolve together as part of the same system, so when the wavefunction “collapses” really, it is interacting with additional wave functions, and evolving to a more defined state. The whole system is evolving, including the “spooky action at a distance” of quantum entanglement, but that is what is happening. No wave functions are “collapsing”, no universes are “branching”, just the wavefunctions – which are all connected anyhow – are becoming less localised and evolveing into more defined states.
Anyhow, plenty of food for thought there. If you were a buddhist, you might argue, that because people can’t just “pop” into existence maybe they can still travel backwards in time, but by having your consciousness enter a newly conceived being. Indeed, this would be plausible, but difficult to verify. Who knows, maybe we are all time travellers from the future, but just don’t realise.