SEAN CARROLL: There’s no doubt that the multiverse is having something of a moment in popular culture. You know, the Marvel movies leaned into that. “Doctor Strange in the Multiverse of Madness”, various Spider-Man films, but also “Rick and Morty” and “Everything Everywhere All At Once”. You might think it’s physics’ fault because physicists have been talking for a few decades about a multiverse, both in cosmology and quantum mechanics, but in fact, as often happens in Hollywood and elsewhere, you ignore what the scientists say, and you do your own kind of thing. The type of multiverse used in Hollywood is more like the philosophical idea of the set of all possible worlds. It’s definitely going to be so interesting for people because we’ve all made decisions. We’ve all wondered what life would be like if they were a little different: if an election had gone differently, if our team had walked a different path, if we hadn’t been hurt, if we had asked this person to go out on a date – and it makes us think, well, maybe we’re in the wrong universe, we’re in the wrong timeline. There’s another world out there, it’s the one I want to be in. Now science comes in – physics my own field – and says, “You know, it’s possible, there are literally other universes out there, places where things are different.” But science can lead us to the idea of a multiverse in different ways.
My name is Sean Carroll, I’m a theoretical physicist and philosopher at Johns Hopkins University, and I’m the author of “The Biggest Ideas in the Universe: Space, Time, and Motion.”
I think it’s really important when we think about physicists’ versions of the multiverse to realize that physicists never start out by saying, “Hey, wouldn’t it be cool if there was a multiverse?” This is always the place where they are trained to kick and scream because they are trying to explain what we are observing. You know, a big criticism of multiverse ideas is that you can’t observe the multiverse, you can’t tamper with it, you can’t test it, etc. But what you can do is observe what is happening in ours. And what you want to do as a scientist is come up with a theory – which is kind of a story – a theory that accounts for what we see, what data we can access. And some theories, some very, very simple, very easy to write, like the inflationary theory of cosmology or the many worlds theory of quantum mechanics, they both explain what we see in our universe and predict unambiguously the existence of other universes. . In cosmology, which was perhaps most famous until recently, there are literally only parts of our universe that are so far apart, where conditions can be radically different. They can be a lot like us, but with different details or even different laws of physics, different particles and forces, and all that stuff.
There is an entirely different design called “The Many Worlds of Quantum Mechanics”, which is both more realistic and easier to bring to life, but also more breathtaking. It just says that whenever you have a quantum mechanical system which, spoiler alert, all systems are at the heart of quantum mechanics, but whenever you measure it in a particular way, you can get different measurement results possible. It’s something we’ve understood for 100 years now. The question is: what happens to the alternative measurement results that you have not observed? So if you have a particle, an elementary particle like an electron, and you’re going to observe its location or its spin or something like that, the equations will tell you that some outcomes are possible and some aren’t. An electron will never turn into a proton. They have different electrical charges that will literally never happen. But the electron could spin clockwise or counterclockwise, and the quantum mechanics version of the multiple world says both will generically happen in different universes. So it’s literally a parallel universe. It’s not a very, very far place. This is a concurrently existing reality where the outcome of a quantum mechanical experiment turned out to be different from ours – and if great notable things in the world depend on the outcome of that experiment, you could find yourself in a very different universe.
Human beings love to put themselves at the center of every story, so when you start talking about the multiverse and the different ways things could have happened, they instantly start thinking, “Oh, if I had made a different decision, things would have turned out differently.” And that’s fine if you’re just philosophical and think about space in all the possible ways the world could have taken. But if you think like a physicist, you solve equations. It literally has nothing to do with human beings making decisions. If you think of the cosmological multiverse, the other universes are literally billions of light years away. They have nothing to do with you and your choices. throughout the day. The quantum mechanical multiverse is a bit of a different story because it happens all over the place. There are things happening that create two parallel realities, but things that happen nt are not human beings who make decisions. There are subatomic particles measured in a quantum mechanical way. If anything, it’s the quantum measurements that force you to make a decision, not your decisions forcing different universes into existence. After all, you are a body made up of a whole bunch of quantum mechanical particles, electrons, protons and neutrons. If you choose to describe yourself this way, there are different versions of you that plug in the universe.
So you must be wondering, “Well, if I could have seen the electrons spinning clockwise or counter-clockwise, and I saw them spinning clockwise. counterclockwise, what is my relationship to the person who saw them spinning clockwise? Are they just me in a different universe or are they a separate person?” I think the clear answer here is that there is a relationship, but they are a different person. It’s kind of like identical twins – you have a fertilized egg that is at one point a single cell that’s one entity out there in the universe, but it splits into two different people. I think that’s how we should think about the different versions of ourselves in the multiverse. They may share a past, but once they diverged, once they are in their own universe, they are now separate people. It’s not weird or impossible to contemplate, it’s just a slightly more sophisticated version of our updated notion of personal identity for the Multiverse.
I absolutely believe that we can imagine thinking of the multiverse as a useful psychological tool or a personal tool, can’t we? Visualization exercises have been part of psychology for a long time, and so when we imagine ourselves physically embodied in different sets of circumstances, we think differently about this possibility – and the multiverse is kind of a nudge in that direction, and can Maybe you can argue that technology these days makes this more possible. We see other lives in a more vivid way than perhaps we did with virtual realities, with alternative ways of thinking about ourselves, augmented reality, just wearing a headset in the world in which we are. Apparently there’s a whole genre of ‘Zillow porn,’ which isn’t really porn, it’s just having fun looking at houses you can’t afford on Zillow and imagining what it would be to live there. We’re sort of looking at alternate realities that haven’t come to pass. These could be both positive and negative psychological tools. We use them in the right way by imagining how things could have gone better, then saying, “Okay, what should I do to increase the chance that next time everything will be better?” But it’s not psychologically healthy to take it too far because there are decisions you can’t undo. That’s why we talked about the serenity prayer, right, the ability to recognize what we can change, what we can’t, and to accept the things we can’t as well. to be able to make a difference.
Yesou can imagine being in a multiverse, having made all kinds of different decisions, but in fact, there is no time travel in the real world. You can’t actually go back and redo the decisions differently. It’s fine to imagine all sorts of possibilities, but ultimately we have to live and affect the universe we find ourselves in. I like to think – maybe I’m a physicist here more than a human being – but I like to think that by contemplating all these different possibilities – past, present and future – we can put things into perspective. We can think of how, “Yeah, there was that moment when things went horribly wrong. Either I did something wrong or there was some unforeseen event that I couldn’t have controlled. But you know what? The causal influence I have on the world only extends into the future. The choices I can make now will have an impact I will feel later, but I can’t make a choice. now that undoes what happened in the past. I think that’s a truth about physics, cosmology, and the world, and psychologically, it’s a very important principle to keep in mind.