Why the Many-Worlds Interpretation Makes Time Travel Possible

I’ve been obsessed with time travel since I watched “Back to the Future” and possibly before that. To a curious mind, not being able to go back to, say, the time of the dinosaurs and actually see them is incredibly frustrating. Despite my fascination and probably everyone else’s, time travel wasn’t seriously entertained as a possibility until the early twentieth century, when our new theories about space-time seemed to allow a new batch of crazy possibilities.

Emmett Brown

Unfortunately, having crazy hair and talking very fast doesn’t make you smart.

The whole “serious science” talk about time travel got started with General Relativity. To be more precise, with Special Relativity it already became demonstrably true that time travel is possible, though only to the future: if you travel at close to the speed of light, you will age less than the people around you and thus will be able to see the future. Unfortunately, you may not be able to go back, which makes the whole thing a lot less attractive.

The idea of backwards time travel, however, turned out to be lot more problematic. In this case and despite what Doc Brown said in Back to the Future, it’s not enough to travel at 88 mph on a Delorean to disappear into the past. However, General Relativity does provide us a way to go back: since space-time is curved, one can imagine different regions of space-time located at different being connected by “bridges,” which are normally referred to as wormholes.

Time travel hypothesis ; using wormholes.

Time travel hypothesis ; using wormholes. (Photo credit: Wikipedia)

So yes, General Relativity allows for time travel, apparently. But combining Relativity with Quantum Mechanics does not. In 1993, Matt Visser proved that the only method for keeping the two mouths of a wormhole open (feeding it exotic matter) will either collapse the wormhole or make the mouths repel. So there went most people’s hopes for time travel, including mine.

Of course, there were already some of very powerful arguments already against the possibility of time travel, of which I will mention a couple. The first is by Stephen Hawking, who uses a modified version of the Fermi Paradox: he argues that the fact that we’re not seeing any travellers from the future means time travel is not possible: otherwise, the place would be packed with people from other epochs! The second is the classic grandfather paradox: if you could go back to the past you could kill your grandfather and thus never be born, which is impossible since you have been born.

English: Stephen Hawking during the press conf...

Stephen Hawking after destroying my childhood dream. (Photo credit: Wikipedia)

You will be amazed, then, to hear that my recipe for time travel solves all of these technical hurdles and skilfully avoids all the paradoxes. It just requires a huge amount of one single thing: luck.

Here’s the idea. There is a non-zero probability that, for example, a pink elephant materializes in the middle of your room. The chances are slim, admittedly, but they are there. Not only that: if you believe in the Many-Worlds Interpretation, then in fact one such elephant has materialized in your room in some parallel universe you’ll never get to see, since you’d need a humongous amount of luck.

Now, that elephant could also have materialized at the time of Julius Caesar’s assassination, for example. In fact, the MWI tells us that one of them did, though sadly we are not in that branch of time, so we don’t get to hear about how Caesar got flattened by a huge pachyderm. And where I say “elephant” I could also say “you:” there is a non-zero chance of you having materialized during Caesar’s murder. This means that you have actually materialized, but you will never get to experience that because it would require a huge amount of on thing: yes, luck.

English: Pink Elephant No, it's not a festive ...

Yup, Caesar was murdered by this in a parallel universe. (Photo credit: Wikipedia)

The main idea is that nothing (except for overwhelming odds) prevents you from disappearing here and magically appearing in the past, thus feeling the continuity a time traveller would feel. Hence, no wormhole is required and therefore we just don’t care what happens to wormholes. So there, Matt Visser.

How does this theory avoid both the grandfather and Hawking’s pseudo-Fermi paradoxes? Well, here’s the thing: in this scenario, it doesn’t matter if you kill your grandpa. In fact, you can kill all of your family for all I care. All this means is there will be a bunch of universes where you will never be born. In fact, there has really been no change: the possibility of your magically appearing and killing a lot of people was there from the start. You haven’t affected the universe in any way.

So why isn’t our planet full of visitors from the future? Simple: since there are a myriad of presents, the odds of a visitor from the future landing in ours are astronomically small. In fact, they become astronomically small because of the fact that the probability of someone appearing out of nowhere is very, very close to zero. Remember, it’s the same as for the pink elephant.


  • Time travel is possible.
  • You will never experience it unless you’re very, very lucky.
  • There is actually no travelling involved, since it is already hard-wired into the make-up of our universe.
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16 thoughts on “Why the Many-Worlds Interpretation Makes Time Travel Possible

    1. David Yerle Post author

      Yeah, it looks like a pretty hopeless enterprise. But you never know… I think our best chance at time travel is by plain and simply just simulating the past and putting ourselves in it as an avatar.

  1. Luigi Acerbi

    Time travel aside (which I don’t “believe” in for other reasons) I have issues with MWI and arguments that make use of super-tiny probabilities which are not physically observable and then everything goes.

    The reason why I don’t like them is that they are not, how to say it, laws-of-physics-background-independent or probability-background-independent – that is, I believe that the (probabilistic) laws of physics should not depend on your reference frame or where you are in the multiverse. Or, in other words, you can think of it as a “no evil genius” principle (which is equivalent to the unreasonable effectiveness of math in describing the natural universe).

    If MWI and the tiny probability argument is true, then there is a world (infinite worlds) in which all sorts of incredibly “unlikely” events keep happening, supposedly because the famous evil genius was constantly deceiving the poor physicists in those universes. But then, in those worlds, those rare (for us) events would become likely, and so if you tried to derive the physical laws there, you would get something completely different in terms of both “laws” and “probabilities”.
    So a world filled with very unlikely events is indistinguishable from a world with different laws of physics, and probabilities are redefined.
    Hence, things get weird and I think that basic MWI needs to be modified to handle that.

    In other words, I do not take for granted that there is a world in which a pink elephant appears out of thin air…

    1. David Yerle Post author

      I actually like the MWI, since I feel the Copenhagen interpretation has a huge problem defining “measurement” properly. In fact, I’d probably go as far as saying that decoherence pretty much implies MWI, unless you include some “ad-hoc” collapse of the wave function.
      That said, it’s the first time I see your argument against it and I quite like it. I’ve actually been thinking about elevating it to a first principle. That is, just like relativity says all reference frames should see the same laws of physics, we could say something like “all possible worlds should observe the same laws of physics” and see where it takes us. So I decided to explore a bit.
      Let’s say we have some event with a 50% chance of happening. Now, there is a small but substantial probability that, if I perform this experiment 100 times, I will get a 30% of occurrences. The probability of this happening decreases with the number of samples, but is non-vanishing. If I understood it correctly, your “probability relativity” would deny that getting this consistently is possible: every experiment should find a number that is close to 50% if done enough times.
      The problem now comes from defining what “consistently” means. That is: what probability is small enough to not be viable?
      Put another way: is it impossible for us to observe extremely unlikely occurrences in our universe? If so, what is the threshold?

      1. Luigi Acerbi

        For me, MWI is one of the less problematic interpretations – but far from satisfying!

        In the past few years I have been armchair-thinking about several “informational” relativity principles, although I never went past the level of very shameful hand-waving. For a while a borrowed the term “epiontic” from Zurek (I had a blog that lasted a couple of posts) and I used to call it “the epiontic principle” – I still hold a similar position, although details may vary depending on barometric pressure.

        In reply to your questions, my educated guess would be that the threshold is given by the information capacity of the universe, or something along those lines (assuming that the universe uses some form of Shannon-efficient coding).

        The universe strives for consistency (i.e. short descriptions are the fittest).
        If some event becomes extremely unlikely, maybe the universe itself would provide some alternative explanation which is less costly.
        (By the way, people use a similar line of thinking to support quantum immortality, which I think it’s flawed for other reasons – namely, consciousness is not discrete.)

        1. David Yerle Post author

          I’ll have to think about this more and maybe come up with some kind of mathematical formulation for this. I like the informational approach, maybe something can be put together along those lines.
          I am perfectly aware that quantum immortality can be argued for along those lines, having done this myself! However, I think it is unnecessary to use quantum mechanics to predict immortality. What I mean is that, if the universe is a “frozen” 4D structure (or 11D, anything that includes time) then basically all instants are simultaneous and there is no such thing as being dead, in the sense that we are only aware of those states in which we are alive. So “mortal” in this sense just means “occupying a finite volume in space-time,” nothing more. If the MWI is true, it could be that the states where we are aware are so many that we occupy an arbitrarily high volume of space-time, but that really makes no difference: you will always be aware of being alive and never of being dead.
          What I’m trying to say is that there is a certain amount of states in which you are aware, that they all happen “at the same time” (I know this is a horrible way to put it) and that the feeling that we’re somehow advancing towards our death is an illusion. Quantum immortality just says that there are more of those states, but changes nothing apart from that.

          1. Luigi Acerbi

            Whops, didn’t really mean to deviate the discussion on (quantum) immortality, but since we’re here… 🙂

            I agree that if you believe in any sort of block universe everything simply exists. It’s also a comforting thought, if you happen to live a decent life: every moment is eternal (see Einstein’s words). Although, I am not sure I “believe” in the block universe, I mean, aside of the neat mathematical formulation. But I may get similar results with a “changing” universe assuming infinite time (across the multiverse) and some recurrence theorem.

            Regarding the block universe, I don’t know how to reconcile it with QM, given that I do not trust MWI too much. And also with the intuitive fact that, you know, “time seems to flow”.
            I appreciate that we might be wrong and it could be an “illusion”, but still an explanation for the illusion is needed. My latest position (which I haven’t re-checked recently) is that time as we know it is a byproduct of information exchange, which is something more fundamental. I am still confused by the problem of “information about what”.

            In short, I “believe” in change but not in time-as-a-dimension. Which I know is a kind of heterodox position but there are good reasons for being skeptical of the independent existence e.g. of the past (after all, there is no past, only inferences about the past). There are several caveats with this position – this is another huge topic, let’s leave it for another time. 😉

  2. elkement

    I am just catching up on blog posts published in the last days… and this one is the third post related to time-travel 🙂 Obviously something weird has happened to the fabric of spacetime.
    Thanks to the pointer to the 1993 proof – I wasn’t aware of that!

  3. Kangethe Mbugua

    I’m from 2156 can anyone disprove that?. Your thesis is amusing. When(sic) I come from the question isn’t can we travel back in time it’s how to regulate precisely how far you go. Pre big bang would be cool

    1. David Yerle Post author

      The whole argument was of course made in jest, since this “method” of time travel is of course completely useless. If we had a “luck” machine that could get us to be lucky enough to materialize where we wanted… now that would be something!

  4. Len Arends

    For those skeptical of MWI, I think we need to take a moment to ponder the true meaning of “unlikely” in this context.
    It is similar, but dramatically more intense, than our discomfort with the inarguable mathematical reality of shattered objects reassembling even in classical physics. Entropy in open systems is a statement of what is much more likely to happen, not an exclusion of the unlikely.
    Similarly, a quantum gravity description of all behavior will tell us what is much more likely to happen, as opposed to what MUST happen.
    It’s alarming to consider that every consciousness has a beginning, but from the internal perspective never has an end.
    I’m about halfway through my nascent meat puppet phase. I plan to make it as likely as possible that when this body finally fails, my next conscious moment is awaking in happy cyberland.
    It doesn’t matter if freezing my head after death is unlikely to successfully preserve my being, as long as it is far MORE likely than just about any other possible scenario that extends my awareness.


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