The Multiverse: Science, Religion, or Pseudoscience?

Why do physicists believe there are other universes besides our own? I get a lot of questions about the idea that we live in this multiverse. Is it science, religion, pseudoscience or just wrong? That's what we'll talk about today. The topic of this video is covered in more detail in my new book Existential Physics. More info at existentialphysics.com. First things first, what's a multiverse? You may guess that it's a new form of poetry and you wouldn't be entirely wrong. A multiverse is a collection of universes, either infinitely many or a number so large no one's even bothered giving it a name. It's an idea that has sprung up in some esoteric corners of theoretical physics and has, not so surprisingly, caught the imagination of science fiction authors, script writers and also of the public. And it is poetic somehow, isn't it, all those universes out there? There isn't just one multiverse but several different ones, so multiple multiverses, if you wish. The multiverse shouldn't be confused with the metaverse, which is what universes evolve into when they've been fed enough Zuckerberg candy. How many different multiverses do we have? Well, Brian Greene has written a book in which he lists nine different ones, but you know how scientists are. The moment the book came out, they jumped up to complain about what wasn't on his list. And I can totally understand that. I mean, everyone knows that a list needs ten items. Nine is just not right. So let me briefly run through the three types of multiverse that you most often hear about. The probably best known and least controversial type of multiverse is the many-worlds interpretation of quantum mechanics. If you remember, in quantum mechanics, we can make predictions only for probabilities. We can say, for example, a particle goes left or right, each with a 50 percent chance. But then, when we measure the particle, we find it either left or right, and then we know where it is with 100 percent confidence. So when we have measured the particle, what happened with the other possible outcome? In the most common interpretation of quantum mechanics, often called the Copenhagen interpretation, the moment you make a measurement, you just update your probabilities because you got new information. The possibilities which you didn't observe disappear because you now know they didn't happen. This is called the measurement update or sometimes the reduction or collapse of the wave function. In the many-worlds interpretation, in contrast, one postulates that all possible outcomes of an experiment happen, each in a separate universe. It's just that we live in only one of those universes and never see the other outcomes. Of course, then you have to explain why we don't spread over all universes like the outcomes of experiments do. Mathematically, this works the same way as the sudden update of the wave function. This means, for what observations are concerned, many-worlds is identical to standard quantum mechanics. The difference is what you believe it means. If you believe in the many-worlds interpretation, then every time a quantum object is measured, the universe splits into as many different universes as there were possible outcomes of the measurement. And this doesn't just happen in laboratories. A measurement in quantum mechanics doesn't require an apparatus. Anything that's large enough can cause a measurement that may be a Geiger counter but also a banana or, well, you. This means that measurements happen all the time and everywhere. They constantly create new universes and more are being created as we speak, which means more bananas and more yous. For example, each time the wave function of a photon spreads into all directions but then it hits your eye, the universe splits. In some, the photon arrived in your eye. In others, it hit the wall next to you. In some, it went right through your head. And this could be happening to all photons. So, in some universes, an elephant is standing in front of you and you don't see it. It's unlikely, but, well, it's possible. And according to the many-worlds interpretation, anything that's possible is also real. I hope you make friends with the invisible elephant. I think that'd be nice. We don't know how our universe began and maybe we'll never know. We just talked about this the other week. But according to a presently popular idea called inflation, our universe was created from a quantum fluctuation of a field called the inflaton. This field supposedly fills an infinitely large space and our universe was created from only a tiny patch of that, the patch where the fluctuation happened. But the field keeps on fluctuating, so there are infinitely many other universes fluctuating into existence. This universe creation goes on forever, which is why it's called eternal inflation. Eternal inflation, by the way, lasts forever into the future, but it still requires a beginning in the past, so it doesn't do away with the Big Bang issue. In eternal inflation, the other universes may contain the same matter as ours, but in slightly different arrangements, so there may be copies of you in them. In some versions, you became a professional ballet dancer. In some, you won a Nobel Prize. In yet another one, you are a professional ballet dancer who won a Nobel Prize and dated Elon Musk. And they're all as real as this one. Where did this inflaton field go that allegedly created our universe? Well, physicists say it has fallen apart into the particles that we observe now, so it's gone and that's why we can't measure it. Yeah, that's a little sketchy. String theory is an approach to unification of gravity with the other forces of nature. Or maybe I should say it was, because it's rapidly declined in popularity in the past decade. Why? It just didn't lead anywhere. String theorists originally hoped that one day it'd be possible to use their theory to calculate the values of the constants of nature, such as the masses of elementary particles and the strength by which they interact and so on. This didn't work, so they gave up and just postulated that any value is possible. And since they couldn't explain why we only observe a specific set of values, they declared that they all exist. And so this gives you another version of the multiverse. This collection of universes with all possible values for the constants of nature is called the string theory landscape. It contains universes with different types of matter or that have other laws of nature. For example, in some of them gravity is much weaker than it is in our universe. In some, radioactive decay happens much faster. And some universes expand so quickly that stars can't form. If you believe in the string theory landscape, this isn't just theoretically possible, it actually happens. You can combine these multiverses in any way you wish. So you can get married to Elon Musk hopping around at half the strength of gravity with elephants in the room which you coincidentally can't see. If you believe in the multiverse, then you have to believe this is possible. There are some other multiverses which I didn't talk about, like Max Tegmark's mathematical universe, in which all mathematics supposedly exists, or the simulation hypothesis, according to which our universe is a computer simulation. Because if you can simulate our laws of nature, why not simulate some others too? I don't want to go through all the different multiverses because they all have the same problem. The issue with all those different multiverses is that they postulate the existence of something you can't observe, which is those other universes. Not only can you not see them, you can't interact with them in any way. They are entirely disconnected from ours. There is no possible observation that you could make to infer their presence, not even in principle. For this reason, postulating that the other universes exist is unnecessary to explain what we do observe, and therefore something that a scientist shouldn't do. Making an unnecessary assumption is logically equivalent to postulating the existence of an unobservable god, or a flying spaghetti monster, or an omniscient dwarf who lives in your wardrobe. Fine if you do it in private, not so fine if you publish papers about it. But this does not mean that other universes do not exist. It merely means that science doesn't say anything about whether or not they exist. If you postulate that they do not exist, that's also unnecessary to explain what we observe, and therefore equally unscientific. So now what is the multiverse unscientific or pseudoscience or religion? Well, depends on what you do with it. If you assume that unobservable universes exist and write papers about them, then that's pseudoscience. Because this is exactly what we mean by pseudoscience – pretends to be science, but isn't. If you accept that science doesn't say anything about the existence of those other universes one way or another, and you just decide to believe in them, then that's religion. Either way, multiverses are not science. They're like Tinkerbell, basically. They exist if you believe in them. You might find this whole multiverse idea rather silly, and I wouldn't blame you, but some physicists are quite serious about it. They believe these other universes exist because they show up in their mathematics. You see, they have mathematics, and some of that describes what we observe. And then they claim therefore everything else that their mathematics describes must also exist. They're confusing mathematics with reality. There are some standard objections that physicists always try on me. You've probably heard some of them too, so here's how you can deal with them. The first point that multiverse fans always bring up is that we also say the inside of a black hole exists even though we can't observe it. But that's just wrong. You can observe the inside of a black hole, you just can't come back to tell us about it. Besides, we know that black holes evaporate, so they eventually reveal their inside. Second objection that I hear is that we can only observe a patch of our own universe because light needs time to travel and it's got only so far since the Big Bang. But certainly, no one would say that therefore the universe stops existing outside of the part we can observe. No, of course not. No one says if you can't observe it, it doesn't exist. The point is, if you can't observe it, science says nothing about whether it exists or not. The third standard objection is that some physicists have tried to come up with cases in which the presence of other universes would be observable. For example, there's been the idea that another universe could have collided with ours in the past, leaving a specific pattern in the cosmic microwave background. Or our universe could have been entangled with another one. So the Nobel Prize winning ballet dancer isn't married to this Elon Musk but has a quantum connection to an Elon Musk in another universe. Again, this would leave a specific pattern in the CMB. The answer to this objection is that people have looked for these patterns in the CMB and they are just not there. But to be fair, the testable multiverse models are a different problem than the one that I named above. The big problem with multiverse ideas is that physicists mistake mathematics for reality. The problem with the testable multiverse ideas is that they think just because a hypothesis is testable, it is also scientific. This is not what Popper meant. He said if it isn't testable, it isn't science. Not if it's testable, then it is science. The fourth and final objection is that the multiverse is good because it's a simple theory. You see, multiverse fans argue that if you don't make assumptions about what the values of the constants of nature are, but just say they all exist, then you have fewer assumptions in your theory. And a simpler theory is better because Occam's razor and all that. But look, if that argument was correct, then the best theory would be one with no assumptions at all. There's just a little problem with that, which is that such a theory doesn't explain anything. I mean, it literally isn't a Just saying that it's simple doesn't make a scientific theory a good one. For a theory to be good, it still has to describe what we observe. It's like just telling my hair please stay put may be simple, but it doesn't make a good hair day. And that's exactly what happens in those multiverse theories. They're too simple to be good for anything. If you don't specify the values of the constants of nature, then you just can't make predictions. To be fair, I would agree it's simpler to not make predictions than making them. But even in physics, you can't publish predictions you didn't make, at least not yet, which is why multiverse physicists always end up making assumptions for the values of those constants. They don't always do this directly. Sometimes they instead postulate probability distributions from which they derive likely values of the constants. But that's more difficult than just using the and certainly not simple. Same issue with the many-words interpretation. Those who work on it claim that their theory is simpler than standard quantum mechanics, because it just doesn't use the measurement update. But if you don't update the wave function upon measurement, then that just doesn't describe what we observe. We don't observe dead and alive cats. And that was Schrödinger's whole point. Therefore, you have to add other assumptions to many-words about what a detector is and how the universe is split and so on, which for practical purposes amounts to the same as updating the wave function. In most cases, these prescriptions are actually more complicated than the measurement update. So multiverse theories are either simple but don't make predictions, or they make predictions but are more complicated than the generally accepted theories. Let me finish by saying I'm not against the multiverse or poetry. I would like to apologize to all the poets watching this. It's not like I think science is the only thing that matters. 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