Multi-world interpretation and multiverse - can they be the same idea?

Original author: Sean Carroll
  • Transfer
Answering questions about parallel worlds, physicists need to carefully distinguish between the interpretations of this idea. In inflationary cosmology, there is the idea of ​​a “multiverse”, in quantum mechanics, a “multiplicity of worlds” or “branches of the wave function,” in string theory, “parallel branes”. But lately, people are increasingly thinking about whether the first two ideas can come from the same basic one. (Branas, from my point of view, is still a completely separate concept).

At first glance, this is crazy - or, at least, I thought so at first. When cosmologists talk about the multiverse, they use a somewhat poetic term. Actually, we mean different regions of space-time that are so far away that we cannot observe them, but still belong to what we would like to call the “universe”. In inflationary cosmology, these remote regions may be relatively self-sufficient — as Alan Guth calls them, “pocket universes.” If we combine this with string theory, then the emerging local laws of physics may differ from one pocket universe to another. They may have different particles, different forces, and even a different number of measurements.. Therefore, it is quite reasonable to consider them as separate universes, even if they are all part of the same space-time.

With a cursory glance at quantum mechanics, the situation there seems to be completely different. Remember the cat Schrödinger. Quantum mechanics describes reality through wave functions that assign values ​​(amplitudes) to all possible probabilities of what we can observe. The cat is neither alive nor dead - it is in the superposition of life + death. At least, until we make an observation. In a simplified Copenhagen interpretation, at the moment of observation, the wave function “collapses” into one real possibility. We see either a live or dead cat. Another possibility has disappeared. In interpreting the multiplicity of Everett's worlds, both possibilities continue to exist, but we, the macroscopic observer, are divided in two — one of us is observing a living cat, and the other is a dead cat. And now there are already two of us, both absolutely real, and we cannot converge with each other.

These two ideas seem completely different. In the cosmological multiverse, other universes are just very far away. In quantum mechanics they exist right here, but in different spaces of possibilities (in different parts of the Hilbert space , if you want to go into details). But some physicists have long thought about whether these ideas could not be the same. A couple of new scientific papers published by courageous thinkers from the San Francisco Bay area are developing this hypothesis in detail.

1. Physical Theories, Eternal Inflation, and Quantum Universe , Yasunori Nomura
2. The Multiverse Interpretation of Quantum Mechanics , Raphael Bousso and Leonard Susskind

The ideas related to this hypothesis were recently discussed under the heading “How to work on quantum mechanics in an infinitely large universe” - these are works by Don Page and Anthony Aguire (and others) . But the previously mentioned works are devoted directly to the hypothesis of "multiverse = multiplicity of worlds."

After reading these two works, I turned from a doubting skeptic into a careful follower. It happened for a simple reason: I realized that these ideas are well combined with others, which I myself thought about! So I will try to explain what is happening. However, my interpretation of these works was influenced by my own ideas. Therefore, I will explain what, in my opinion, may be true. I think that the explanation will be close enough to the one described in these two works, but one should not blame their authors for any stupidity coming from me.

There are two ideas that together bring this insane assumption to something meaningful. The first is the weakening of the quantum vacuum.

image

When particle physicists say "vacuum", they do not mean "empty space", they talk about "the state with the lowest energy of all similar states." Suppose you have a scalar field that fills a universe that can take on different values, and with each of them is associated some potential energy different from the others. In normal events, the field tends to reach a minimum of potential energy - this is the “vacuum”. But at the same time there is a “true vacuum”, in which the energy is really the smallest possible, and there are “false vacuums” where you have reached a local minimum, but not a global one.

The fate of the false vacuumwas worked out in several famous works of Sydney Coleman and his colleagues in the 1970s. In short, the fields are subject to quantum fluctuations. Therefore, the scalar field is not in a quiet vacuum state. If you watch him, you can see how it deviates slightly. Sometimes it deviates so strongly that it even moves over the barrier in the direction of a true vacuum. It does not occur throughout the space at the same time; it happens in a small region, in a “bubble”. But when this happens, the field is already striving to remain in a state of true vacuum, rather than a false one — the first is energetically preferable. Therefore, the bubble grows. Other bubbles in other places also grow. As a result, the bubbles collide, and the transition from a false vacuum to a true vacuum is successfully completed. (Unless the Universe expands so fast that the bubbles do not reach each other). This is very similar to how water turns into steam, forming bubbles.

It is in this vein that everyone talks about the fate of a false vacuum, but in reality everything is going wrong. Quantum fields do not experience "fluctuations"; it is a poetic language used to facilitate communication with our classical intuition. Our observations are fluctuated - we look at the same field many times, and each time we observe different values.

Similarly, to say that “a bubble is forming and growing” is not quite right. In fact, a quantum amplitude exists for a bubble, and it grows with time. When we look at the field, we either see the bubble, or we don’t see it - just like when we open the Schrödinger box, we see a live cat or a dead cat. But in fact, there is a quantum wave function that describes all the possibilities at once.

We take this into account, and introduce the second key ingredient: complementarity (complementarity) of the horizon. This is a generalization of the idea of complementarity of black holes , which, in turn, grows out of the quantum complementarity principle . (Already confused?). The concept of complementarity was introduced by Niels Bohr, and it means that "you can think of an electron as a particle, or as a wave, but not both at the same time." That is, there are different, equally valid, ways of describing something that cannot be used simultaneously.

The complementarity of black holes, roughly speaking, is that "we can talk about what is happening inside the black hole, or outside, but not simultaneously." This is a way to avoid the paradox of the disappearance of information in a black hole.as it evaporates. If you throw a book into a black hole, and information about it is not lost, then in principle you should be able to recreate its content, having collected all Hawking radiation emitted by the black hole. This rings true even if you don’t understand the mechanism that drives this process. The problem is that you can "cut off" a piece of space-time, which contains both a fallen inside book and outgoing radiation! So where is the information? (In two places it cannot be at the same time - this is prohibited by the theorem on the prohibition of cloning .

Susskind, Torlatius and Aglum, as well as Gerard 't Hooft suggested additionality as a solution: you can either talk about a book falling into a singularity inside a black hole, or you can talk about Hawking radiation outside, but not both of them at once. This is a bit like wishful thinking and an attempt to save physics from the unpleasant prospect of information disappearing along with the radiation of black holes. But the more theorists think about the work of black holes, the more is going to the evidence of the truth of something like complementarity.

According to the principle of complementarity of black holes, an outside observer should not think about what is happening inside. More precisely, everything that happens inside can be encoded with information located on the horizon of events. This idea is well combined with holography, and the fact that the entropy of a black hole is proportional to the area of ​​the horizon, and not to its volume. In fact, you change the “inside of a black hole” to “information that lives on the horizon” (more precisely, on the “stretched horizon” located directly above the real one). In turn, this idea is connected with the membrane paradigm of black holes, but this article has already become bloated.

The event horizon is not the only type of horizon in general relativity. Horizons are in cosmology. The difference is that we can be outside the black hole while being inside the universe. And the cosmological horizon is the sphere around us, beyond which everything is so remote that the light does not have enough time to reach us.

image

And there is the addition of horizons: you can argue about what is inside your cosmological horizon, but not about what is outside. All that, in your opinion, can occur outside the horizon can be encrypted as information on the horizon itself - just like in black holes! This turns into a very clear and plausible statement in empty space with a cosmological constant (de Sitter space), where there is even an exact analogue of Hawking radiation. But the complementarity of horizons asserts that this is true even in a more general sense.

From the point of view of supporters of complementarity, all these pocket universes of cosmologists do not make sense. More precisely, no need to think about them literally. All you need to talk about is what happens inside (and on the surface) of your own horizon. And this is a finite amount of everything, and not an infinitely large multiverse. You can imagine that such a perspective has far-reaching consequences in the field of cosmological predictions. Arguments about how to tie it all together, flare up in the scientific community.

Now we will connect both ideas together: the complementarity of the horizon (“think only about what is happening inside the observable universe”) and the weakening of the quantum vacuum (“at any point in space there is a quantum superposition of various vacuum states”).

The result is a multiverse in a casket. Or, at least, the multiverse inside the horizon. On the one hand, complementarity says that there is no need to argue about what is outside the observable universe. Any reasonable question can be answered in terms of what happens inside the horizon. On the other hand, quantum mechanics says that a complete description of everything that happens inside the observable universe includes an amplitude that is in various possible states. So we replaced the cosmological multiverse, in which different states are located in extremely separated regions of space-time, with a localized multiverse, where different states are in one place, just on different branches of the wave function.

It is difficult to learn right away, but I hope that the main points are clear. But is all this true? And if so, what should we do about it?

Obviously, we have no answers to these questions, but it is very interesting to talk about it. I tend to believe that this may well be true. And if so, then I would like to ask what consequences this has for the initial cosmological conditions and for the arrow of time. I do not think that such an approach gives simple answers to these questions, but it can offer a relatively reliable platform from which to start developing certain answers. The universe is very large, and we can expect that its understanding will be a serious challenge for us.

Also popular now: