The multiverse is having a moment. From Rick and Morty to Marvel movies, the idea that our universe is just one of many has inspired countless storylines in recent popular culture.

Why is the multiverse so compelling? To theoretical physicist and philosopher Sean Carroll, one reason is that we’re drawn to wondering how things might have turned out differently. What if you had chosen a different career path? Married someone else? Moved to a different city?

Of course, there’s obviously no guarantee that you’re living out those alternate timelines in a different universe. But there are real scientific reasons to think that the multiverse exists. And as Carroll explains, that possibility comes with some fascinating philosophical implications.

SEAN CARROLL: There's no question the multiverse is having a kind of moment in popular culture. You know, Marvel movies have been leaning into it. "Doctor Strange in the Multiverse of Madness," various Spider-Man movies, but also "Rick and Morty" and "Everything Everywhere All At Once." You might think it's the fault of physics because physicists have been talking about a multiverse for a few decades now, 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 kind of multiverse that is being used in Hollywood is more like the philosophical idea of the set of all possible worlds. It's clear why this would be so interesting to people because we've all made decisions. We've all wondered what life would be like if they were a little bit different: if an election had gone a different way, if our team had gone a different way, if we had not gotten hurt, if we had asked that person out for a date- and this sort of makes us think, well, maybe we're in the wrong universe, we're in the wrong timeline. There's another world out there, that's the one I wanna be in. Now science comes along- physics my own field- and says, "You know, it's possible, there literally are other universes out there, places where things are different." But there's different ways in which science can lead us to the idea of a multiverse. 

I'm 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're thinking about physicists' versions of the multiverse to realize physicists never start out by saying, "Hey, wouldn't it be cool if there were a multiverse?" It is always the place they're dragged to kicking and screaming because they're trying to explain what we do observe. You know, a big criticism of multiverse ideas is that you can't observe the multiverse, you can't falsify it, you can't test it, etc. But what you can do is observe what happens 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, for the data that we can have access to. And some theories, some very, very simple, very easy to write down theories, like the inflationary theory of cosmology or the many world's theory of quantum mechanics, they both explain what we see in our universe, and unambiguously predict the existence of other universes. In cosmology, which is maybe the most famous one up until recently, there's literally just parts of our universe that are so far away, where conditions can be radically different. They can either be kind of like us, but with different details or even different laws of physics, different particles and forces, and the whole shebang. 

There's an entirely different conception called 'The Many Worlds of Quantum Mechanics,' which is both in some sense more realistic and easy to bring into reality, but also more mind-blowing. It just says whenever you have a quantum mechanical system which, spoiler alert, all systems are quantum mechanical at heart, but whenever you measure it in a particular way, you can get different possible measurement outcomes. This is something we've understood for 100 years now. The question is: What happens to the alternative measurement outcomes that you didn't observe? So if you have a particle, an elementary particle like an electron, and you're gonna observe its location or its spin or something like that, the equations will tell you that certain outcomes are possible, certain outcomes are not. An electron is never gonna turn into a proton. They have different electrical charges that will literally never happen. But the electron could be spinning either clockwise or counterclockwise, and the Many-World's version of quantum mechanics says both of those will generically come true in different universes. So this is literally a parallel universe. This isn't someplace very, very far away. This is a simultaneously existing reality where the outcome of a quantum mechanical experiment turned out differently to ours-and if big noticeable things in the world depend on the outcome of that experiment, you could wind up in a very different looking universe. 

Human beings love to put themselves at the center of every story, so when you start talking about the multiverse and different ways things could have gone, they instantly start thinking: "Oh, if I had made a different decision, things would've turned out differently." And that's fine if you're just being philosophical and thinking about the space of all possible ways the world could have gone. But if you're thinking like a physicist, you solve the equations. It has literally nothing to do with human beings making decisions. If you think about 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 mechanics multiverse is a little bit of a different story because it does happen sort of everywhere. There's things going on that create two parallel realities, but the things that are going on are not human beings making decisions. There's subatomic particles being measured in some quantum mechanical way. If anything, it's the quantum measurements that force you to make a decision, not your decisions forcing different universes to come into existence. After all, you are a body made of a whole bunch of quantum mechanical particles, electrons and protons and neutrons. If you choose to describe yourself that way, there are different versions of you branching the universe. 

So then you have to ask: "Well, if I could have seen the electrons spinning clockwise or counterclockwise, and I saw it spinning counterclockwise, what is my relationship to the person who saw it spinning clockwise? Are they me just in a different universe or are they they a separate person?" I think the clear answer here is there's a relationship, but they're a different person. It's very much like identical twins- you have one fertilized egg that's at one point in time a single cell that is one entity there in the universe, but it splits into two different people. I think that's how we should think about different versions of ourselves in the multiverse. They might share a past, but once they've diverged, once they're in a universe of their own, they're now separate people. It's not weird or impossible to contemplate, it's just a slightly more sophisticated version of our notion of personal identity updated for the multiverse. 

I absolutely believe we can imagine thinking about the multiverse as a useful psychological tool or a personal tool, right? Visualization exercises have been part of psychology for a long time, and so when we physically imagine ourselves embodied in different set of circumstances, we think about that possibility differently- and the multiverse is sort of a nudge in that direction, and maybe, you can argue, that technology these days is making that more possible. We see other lives in a more vivid way than maybe we used to with virtual realities, with alternate ways of thinking about ourselves, augmented reality, just wearing a headset in the world that we're in. Apparently, there's a whole genre of 'Zillow porn,' which is not actually pornography, it's just getting kicks out of looking at houses you can't afford on Zillow, and imagining what it would be like to live there. We're sort of peeking into alternate realities that didn't come true. These could be both positive and negative psychological tools. We use them in the right way by imagining the way things could have gone better and then saying, "Okay, what do I need to do to increase the chances that next time it actually will go better?" But it's psychologically not healthy to push that too far because there are some decisions we can't undo. This is why we talked about the serenity prayer, right, the ability to recognize what we can change, what we can't, and accept the things that we can't as well as to be able to tell the difference. 

You can imagine in a multiverse, having made all sorts of different decisions, but in fact, there's no such thing as time travel in the real world. You cannot actually go back and remake the decisions differently. It's fine to imagine all sorts of possibilities, but at the end of the day, we have to live and affect the universe that we're in. I like to think-maybe I'm being a physicist here more than a human being- but I like to think that by contemplating all of these different possibilities- past, present, and future- we can put things in perspective. We can think about how, "Yeah, there was that moment when things went terribly wrong. Either maybe I did something wrong, or there was an unforeseen event that I couldn't have controlled. But you know what? The causal influence I have on the world only extends toward the future. The choices I can make right now will have an impact that I will feel down the road, but I cannot make a choice right now that undoes what happened in the past." I think this is a truth about physics and cosmology and the world, and psychologically, it's a very important principle to keep in mind.