- 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.
