How broken is it? Time travel and physics in Avengers: Endgame.
With a record shattering $1.2 billion globally on opening weekend, Avengers: Endgame is definitely the talk of the town, overshadowing even the mind-blowing latest episode of Game of Thrones. Whilst the movie was pretty close to perfect, as indicated by the whopping 9.0 on IMDb, as a physicist I couldn’t help but get distracted and mildly frustrated by the treatment of physics, and in particular time travel, throughout the movie. Perhaps we’ve been spoilt by the masterpiece that is Interstellar, but the name-dropping of high-level physics concepts to confuse the audience into accepting the time travel mechanism was highly obnoxious. Despite my first impressions though, the Russo brothers appear to actually have put some thought into how time travel would work. I’ve read quite a few articles trying to explain how the time travel worked in Endgame, however, they all end up creating arbitrary rules disconnected from reality to make it work. I’ll try to connect the slightly mind-boggling proposals for real world time travel with the plot of Endgame for you here, and explain where the proposed time travelling works and when it fails dismally. Major spoilers ahead!
First a disclaimer. I haven’t seen Ant Man or Ant Man & the Wasp (I don’t really like Paul Rudd at all) so I haven’t poured over the details of the “Quantum Realm”. But if its anything like what was presented in Endgame, then it is definitely fanciful. The concept of another ‘realm’ existing once you get down to quantum levels is pure fiction. Physically, our entire world is quantum mechanical. The problem is that these effects are so small (only being relevant on the scale of atoms) that the world we experience is really well approximated by classical physics. It may seem that such small scales behave like another world, but this is just a painful misconception. As a result, the concept that objects and beings can be shrunk to subatomic sizes and maintain their structural integrity is laughable. The very same law that generates these weird quantum properties, that is the Heisenberg Uncertainty Principle, dictates that objects of such size must ‘spread out’ into wavefunctions, which fundamentally changes the way they would interact. But this isn’t exactly a new criticism of Ant Man’s shenanigans and I was comfortable to ignore it until they tried to use it as a foundation for Endgame’s time travel mechanism.
The Russo brothers made very little attempt to flesh out the mechanics of how Stark’s time machine worked, but you can’t really fault them for brushing over this as the real-world proposals for time travel are so outlandish as to be purely theoretical. It is at this point that we should note that by time travel, we’re mostly talking about backwards time travel. Forwards time travel is relatively easy, we’ve known about time dilation for over a century thanks to the legend himself, Albert Einstein, and it doesn’t cause many real paradoxes because it doesn’t break causality. Being ‘quantum sized’ doesn’t induce time dilation, so Ant Man is wrong again, he definitely didn’t experience the 5 years at 5 hours. However, backwards time travel, which is always where things get interesting in movies, is much more challenging and we are realistically nowhere near achieving it. In Endgame, we’re dumped with a bunch of physics jargon with the aim to placate us. But for those of you who are curious, I’m going to dismantle some of the terms.
Firstly, the quantum realm is talked about on the Planck scale, which is a very real concept. The Planck length, at 1.616×10−35 𝑚, is sometimes referred to as the resolution of the universe. It’s at this length that quantum gravity is hypothesised to become apparent. In other words, our current physical theories breakdown. Measuring anything smaller, due to the Heisenberg Uncertainty principle, would require photon momentums so great as to create a tiny black hole. Interestingly, the uncertainty principle also predicts the appearance of tiny wormholes on the Planck scale, which could potentially give us real life backwards time travel. So, the discussion on the Planck length seems surprisingly relevant here, especially considering the importance of this ‘quantum realm’ (shame it’s absolute garbage).
Stark also name drops the EPR paradox, Deutsch propositions, eigenvalues, and inverted Mӧbius strips. Whilst the Deutsch proposition has extremely interesting and relevant implications for Endgame, the rest of these terms are just a dose of jargon. The EPR paradox refers to the Einstein-Padolsky-Rosen paradox, which simply presents the apparent contradiction between quantum entanglement and a constant speed of light. When two quantum states are entangled, measurements of otherwise uncertain properties associated with that state are always the same for these two particles. When one quantum state is made certain by measurement, the other state also becomes instantaneously certain irrespective of spatial separation, seeming to contradict the fact that nothing travels faster than the speed of light. This paradox is typically solved these days by invoking the concept of a global wave function, a feature of Everett’s Many Worlds interpretation of quantum mechanics. Whilst many worlds will be crucial to the discussion later, the EPR paradox is only relevant in the sense that all quantum mechanics is relevant here, the link to the mechanism for time travel developed in Endgame is tenuous at best. Similarly, the concept of an eigenvalue is relatively basic in linear algebra (which some of you will have come across in year 12) and really is irrelevant to a time travel mechanism. But it does sound cool. Finally, a Mӧbius strip is quite an interesting two-dimensional manifold with only one side and one boundary. You can easily make one by stuffing up your event band by sticking the glue to white side instead of the fluoro side. This will allow you to trace both sides of the band with a pen, without taking your pen off the band. While it’s a cool mathematical object, I can’t really see any relevance to time travel or quantum mechanics, and you can’t really invert a Mӧbius strip, that just flat out doesn’t make sense. But as some fans have pointed out, the reference to the circular nature of time travel and the homage to Iron Man at the end of the film by mentioning a Mӧbius strip certainly makes for a cool Easter egg.
To understand the Deutsch proposition, we need to revisit Everett’s many worlds interpretation of quantum mechanics. The theory is as mind blowing as it is nuanced, so you might want to check out the Wikipedia page for more detail. Quantum mechanics induces many weird results, but one of the most concerning is the collapse of the wavefunction in response to measurement. According to the Copenhagen interpretation, this is just a feature of quantum mechanics and the value of the measurement is determined probabilistically. But this is exactly why the Copenhagen interpretation has disillusioned many famous physicists, as the collapse introduces a probabilistic factor into the otherwise deterministic quantum equations. The many worlds interpretation, however, proposes that the act of measurement causes a branching point, where all possible results branch off to become their own equally real universe. This means that each event generates multiple, if not infinite, universes that are all encompassed by a universal wavefunction but are functionally separate in that they cannot interact. The implications are mind-boggling to say the least. What British physicist David Deutsch proposed, was that the act of backwards time travel would take you to another branched universe. The beauty of this proposition is that it solves the grandfather paradox, by which you go back in time to kill your grandfather, or in the case of Back to the Future, distract your mother from meeting your father. By traveling to another universe, you can actually kill your grandfather without interrupting your causal timeline since the murder would prevent your birth in the branch you travelled to but not affect the branch you came from in any way. This is precisely how Banner and Romanov dismiss killing baby Thanos; whilst you stopped the Decimation in that universe, you haven’t done anything for the universe you originally came from.
You can start to see now that the Deutsch proposition actually has some very real implications for how the plot of Endgame panned out, but unfortunately the sheer enormity of the infinite universes generated by the many worlds proposition complicates things to say the least. As you’ve hopefully realised by now, every minute little event that occurs across the universe creates a branching point under the many worlds interpretation, so the number of universes generated every second is not just infinite, it’s a really large infinity. However, many of these universes are effectively the same, especially when you’re only considering macroscopic events relevant to you, or in this case, our heroes. A random atom decaying 50 billion lightyears away is not going to affect my perception of the universe in any meaningful way. Thus, when I’m talking about a universe in the following discussion, I’m actually collating the infinite universes that are effectively the same; the real interesting branching points come when big decisions are made.
The aim of the Avengers in Endgame is to reverse the decimation in all universes, not just the ones they’re time travelling to. This is why they need to use the infinity stones, which for fantasy reasons, seem to be able to affect the entire universal wavefunction (that is all branched universes). According to the Ancient One, the infinity stones have a stabilising effect on the universes and removal of the stones will generate new, and much more evil, versions of these universes. This is where the MCU seems to depart from the real Deutsch proposition because the plot relies heavily on being able to travel back to the exact universes you came from, both to return all four teams to the same universe to unite the infinity stones, and to allow Cap to return the infinity stones once they’re done. But this somewhat ruins the beauty of the Deutsch proposition because if you can choose the universe you return to, you invoke a meta level version of the grandfather paradox again. The act of time travelling to get the stones also brings up the question of whether they could retrieve multiples of the same infinity stone. What’s to stop the team from traveling to many different universes and retrieving multiple time stones say? For that matter, what’s to stop Thanos from other universes time travelling to collect the stones and trying again? Typically, this time travel mechanism appears to create more problems than it solves.
Thus, the time travel in Endgame looks to be a hybrid between the realistic Deutsch proposition and the more traditional single timeline approach, the crucial factor being the ‘stabilising effect’ of the infinity stones. The time travel machine combined with Stark’s ‘spacetime GPS locators’ allow the Avengers to return to specific universes, something prohibited by the very action of time travel in the real Deutsch proposition. The meta grandfather paradox is avoided in the film either by ignorance or the self-consistency proposal. The self-consistency proposal simply solves the grandfather paradox by saying that a universe where such causal inconsistency is possible cannot exist, and therefore doesn’t! You can’t go back and kill your grandfather because no such universe exists and you exist in a universe. Rather dissatisfying I know. Self-consistency also doesn’t solve the bootstrap paradox. Suppose I travel to 2020, find a design for a light sabre and return with it to 2019 and patent it. Suppose further that the patent is developed into the design that I find in 2020. The obvious question is who would have invented the light sabre? Apparently no one would have invented it, the design would have been generated quite literally from nothing courtesy of a time machine. So with the Endgame time travel, we’re basically back where we started from and have discarded the much of the beauty of the Deutsch proposition.
But that doesn’t mean it was all for nought! The introduction of a Deutsch like time travel mechanism made for some quite interesting plot devices. Banner is able to reverse the Decimation across all universes, and when past Thanos is killed, he is actually simply removed from that universe he came from (lucky for the Loki in that universe). It appears we are saved from other universe Thanos’ trying the same thing by their own ignorance. Similarly, there’s nothing wrong with Nebula killing herself because she’s just killing off the version from a separate universe, not her actual self. Cap’s decision to spend the rest of his life with Peggy was probably also done in a different universe that doesn’t impact the one he came from. There’s also a new Gamora on the loose in the universe we care about, but she’s a fundamentally different character from the Gamora who died in Infinity War, so it looks like Star Lord has got his work cut out for him again. Sadly, no version of Romanov makes it through, she’s properly dead, and so is Vision. This time travel mechanism appears to open a whole new array of alternate universes to use, but at the end of the day, the one we care about and the one that is focused on in Endgame is unique, and all of the infinite other MCU’s are different, even if only subtly. Lets hope that’s enough to keep the audience morally engaged.
In conclusion, we’ve looked at what fails and what holds up in Endgame when compared to reality. Most of the physics is purely name dropping and the only concept they try to flesh out, the mysterious ‘quantum realm’, is just dumb. The saving grace is how they seem to implement the Deutsch proposition into their concept of time travel. Other writers, like the ambitious attempt by Alex Leadbeater on Screen Rant, try to frame the time line in a linear fashion. I really don’t buy this, not only for the awkward implications on the original MCU timeline (like Cap being in on Hydra), but also for how the grandfather paradoxes would be handled with the death of Thanos’ entire retinue. In this particular scenario, the Deutsch proposition really saves the day but the necessity to return to the same timeline inevitably ruins the consistency and generates many logical problems. Conveniently, these aren’t realised in the film and to be honest are actually somewhat negligible compared to massive plot holes like how they only have enough Pym Particles for one round trip each (no stuff ups, no do-overs remember?), and yet Thanos can bring the entire Sanctuary II through! Seems to me like they could’ve all jumped on a few ships together and gotten many more chances at it (not that it stopped Cap and Stark anyway). Maybe I should have just shut up and accepted it!