The many worlds interpretation of quantum mechanics was put forth by graduate student Hugh Everett in 1957. It was considered preposterous at the time, but is now going mainstream. It requires us to change our paradigm about our experience of reality, and consider that there may be many worlds where every possible quantum outcome happens. And that we are living in just one of those branches of the universe at any one moment.
the Copenhagen interpretation was championed by Neils Bohr in the 1920’s. It that objective reality doesn’t exist until it is observed or suggests measured. The quantum world is governed by a set of probabilities as described by a wave function that evolves over time in the Schrodinger equation. The act of measuring forces the set of probabilities to randomly assume only one possible value.
But the many worlds interpretation says that the entire universe is in a state of superposition. A measurement may look like a particle has some set of properties, but that is not the overall reality. It posits that the world splits every time we THINK a quantum measurement is made. And that although we may see one thing in our world, there is another world in which another thing has occurred.
Okay, so in the universe where I made this video, I am going to eat this Apple. So that the video that you are watching. But in another universe I am eating a lollipop in my video. That universe is just as real as this one. You may or may not be watching me in that universe, but I’m eating a lollipop there.
So in many worlds, the universe is not random, like it is in the Copenhagen interpretation. It is deterministic according to the Schrodinger equation. The randomness is just our perception because we only experience one of two branches. Now both interpretations are weird but there’s some good points about the many-worlds idea. First it gets rid of the measurement problem. We use the same equations whether we measure something or we don’t measure something. There’s no special role for the observer. Second, it let’s us apply quantum mechanics to the entire universe. This is quite elegant.
How does that occur? Second, how is energy conserved?
And third, the puzzle is why do we see only one world, if the other worlds are equally present?
Why can’t we experience them? Where the heck are these worlds located? I posed these questions to the foremost expert on the many-worlds interpretation, Sean Carroll theoretical physicist at Caltech. His latest book is called Something Deeply Hidden - Quantum worlds and the Emergence of space-time. The link to his book is in the description below. I highly recommend it. So first I asked about why the idea of branching is better than the idea of wave collapse. Take a listen.
It seems to me you’re replacing the mystery of the collapse of the wave function, which is mysterious in Copenhagen interpretation, with the mystery of the branching in the many-worlds interpretation. So, can you talk about how the branching is less mysterious than the collapse would be?
The problem with Copenhagen is that it is manifestly an incomplete theory. Nobody can possibly think it’s the final answer. You say when you observe the quantum system its wave function collapses. And if you ask, well when do you observe?
What counts as an observation?
What about one atom bumping into it - does that count as an observation? And also what is the underlying reality that is happening?
What is the real true mechanism that is going on beneath the surface?
Copenhagen just says, we don’t know. That not our job. We’re not going to answer those questions. Okay we know it when we see it - wave function is collapsing. Whereas many-worlds is a hundred percent, completely specified. There’s an equation, the Schrodinger equation, and that it. There’s nothing that deviates from the Schrodinger equation at any time. So there’s no mystery about when branching happens in the Evert interpretation. You have the Schrodinger equation describing the behavior of physical systems. And those physical systems interact with each other. And they branch, which is to say, there is an environment.
There is a set of things out there in the world that we don’t keep track of - all the photons, all the molecules of air in your room. You might know some course features of them, but you don’t know the position and velocity of every molecule or every photon, right?
So you don’t keep track of that. And therefore, when a quantum system becomes entangled with that environment, that when branching happens. And we can quantify it. We can tell you how quickly it happens. We can tell you when it happens. There’s no loose ends there anymore. So according to Sean Carroll, in many worlds, the Schrodinger equation always applies. It’s the entanglement of two systems that causes a decoherence, leading to the branching.
So the question is, what the heck is decoherence?
It’s not quite like wave collapse. It can be thought of as a loss of information to the environment. When
an isolated quantum system, like say an electron, gets entangled with its environment, which contains things like photons and other molecules that may be present, this has the effect of a transfer of quantum
information. All the photons and atoms that bounce off the electron are agents of decoherence, and can fix the electrons position in space, and give it a sharp outline.
It’s as if the environment is always observing. This is decoherence, and it’s the cause of the branching. And this is how quantum systems can start behaving like classical systems. So for the second item, here’s what Sean Carroll had to say about the important question of energy conservation. Energy energy conservation it feels like, you know, where could all the energy come from for these new worlds? Now in one of your interviews, I saw that you mentioned that we can’t really think of it in terms of new energy being created, but rather the energy of the Schrodinger equation being fractionalized into new worlds as they’re created. I’ll quote you. You said each world gets a little bit thinner, although its inhabitants can’t tell. So what does it mean when you say that a
world gets thinner?
so there’s two things going on if you get this. And this is again, you know, this is not like a question you ask because you don’t get it. This is an important question that we need to sort of think about what it means. It’s a legitimately difficult question that is new to the many-worlds way of looking at things. In many worlds, there’s two different things you might mean by the energy of the universe. One is the energy of all the branches of the wave function, okay, like all the worlds at once. That has an energy. The other thing is, what the energy of each individual branch looks like from inside, okay. To the perspective of the people inside each branch, what is the energy?
And these are two things. They’re different things, but they’re related to each other. The relationship is not that you take the branches - the energy of each branch from the interior point of view, and add them. That not what it is. Sorry, that just not what the math says. What the math says is - take the energy from inside each world, that the observers in that world would say, weight it by the wave function squared, right. Every branch of the wave function comes with an amplitude, a number. This is where the probability comes from, right - that, that amplitude squared is the probability that if you find yourself on that branch, the energy equation works exactly the same way, as the probability - probability of being on one of the branches somewhere is always going to add up to one. The energy of the universe that you get by adding the energies of the individual universes, weighted by their amplitude squared, will always add up to the energy of the total universe. So the total universe, with all the branches, has an energy which is 100% conserved. The energy, within any one branch, almost looks like its conserved. It’s not quite exactly conserved because there’s quantum fluctuations.
But it’s pretty close to being conserved. So when you branch the universe in two, the energy that you perceive in your universe, doesn’t decrease by half. It stays is almost exactly the same. So essentially what Sean Carroll is saying is that the total energy of the universe, and all its branches, is conserved, analogous to the way that all the individual probabilities inherent in the Schrodinger equation, add up to one. And finally, something that bugs me a lot is if there are many worlds, why do we experience just this one world.
Here’s what Sean Carroll had to say about that. Why is it that I experience just this branch?
I’m conscious at this branch. I am not conscious of the other branches. Look, it’s almost exactly like identical twins. At one point, were the same single-cell fertilized egg, okay. But rather than just duplicating itself, and becoming one person, it split into two different people. Nobody says those identical twins are two copies of the same person. That would be a weird thing to say. They’re different people, right. When the branching happens, there are now two different people. They had the same past.
But they’re different people now. So I think Sean Carroll did a great job of demystifying the common questions around the many-worlds interpretation, and he also answered several other questions. If you want to see the interview in its entirety, you can view it on my Patreon, or youtube membership channel.
Now I came in as a skeptic. And I remain a skeptic, although less so now than before. I think the math is there to support everything that Sean Carroll says. I don’t think there’s any disputing that. But should the universe be interpreted based strictly on mathematics, regardless of its intuitive aesthetics. Perhaps it should. Or is there deeper truth that would be more satisfying? No test can tell us the difference between the outcome of the two interpretations - Copenhagen versus Many Worlds, because they both give you the same outcome, and both invoke the Schrodinger equation. Both interpretations seem bizarre. It’s a matter of choosing the one that seems less bizarre to you - a non-deterministic universe, with probabilistic realities -- or, a deterministic universe, but with near infinite realities that you can’t see, except the one that you’re in. The wise advice, according to some scientists is - look it doesn’t matter what you think or subscribe to, because the equations give you the correct answer. so stop thinking and just shut up and calculate. I asked Sean Carroll about that, and he like me, hates that idea. I myself think that physics is more than calculations.
It is a science that tries to get at the truth about what the true nature of reality actually is. That what we should really be after, I feel. And that pursuit, at least on this channel, will always exist. I’d like to thank my generous members on YouTube and Patreon. If you like videos like this, consider joining them. Feel free to share this video with your friends. Post your questions below. I'll try to answer all of them. Thank you for subscribing. And I will see you in the next video my friend!
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