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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Philip Ball, translator: one, two, three

Multi-world interpretation is one of the interpretations of quantum mechanics, which was first proposed by Hugh Everett, an American physicist, in the 1950s, assuming that there are countless worlds to avoid the so-called wave function collapse. But this theory now appears in popular books more because of its science fiction color. Why are physicists not interested in it today? One of the key problems is that people's probabilistic interpretation of quantum behavior cannot be introduced into multi-world interpretation.

The idea that the universe is constantly divided into different real worlds has been inconceivably embraced by science, philosophy and popular literature. But this treacherous solution to unique problems in quantum mechanics is not an elegant science at all. Not only should we reject its strange request, but it's time to find another solution, Philip Ball wrote.

The idea of "another world" is irresistibly tempting. Whether it's Dickens' Christmas Carol, Frank Capra's Life, or the parallel universe of quantum computing in the recent Alex Garland mini-series Devs, we can imagine a world that might be real but doesn't become a reality. It provides us with a stage to release our fears and fantasies. Like the forked paths encountered by travelers in the forest by the American poet Robert Frost, we often wonder where the missed path will take us.

Then it is not difficult to understand why the multi-world interpretation of quantum mechanics (Many Worlds interpretation, MWI) is so attractive. Although most physicists dismiss or even ridicule this idea, it is always eagerly accepted by physics science people and their audiences. However, it is difficult to know whether some supporters really accept the idea. I believe that some physicists think from the bottom of their heart that it is a beautiful solution to the basic problems of quantum mechanics known as brain-burning, and I sympathize with some of these reasons. But when they start to talk about "quantum brothers" (there must be sisters, although there are only a few female supporters in many worlds), or use "quantum applications" to make difficult choices by triggering some kind of quantum measurement. and convinced that the whole world split in two in the process, and they made another choice in the other world. At this time, I can't help but ask, putting aside the complex philosophical discussion, whether they are just getting high in this fantasy.

Why would anyone think that quantum theory reveals a picture of an unimaginably large number of other worlds, perhaps even infinitely many, constantly separated from our own real world? Daniel Nolan, a professor of philosophy at the University of Notre Dame, explained in a recent article that the above picture stems from the probabilistic nature of quantum theory. As the German physicist Marx born (whose substantial contributions have not received due praise) pointed out in the 1920s, quantum mechanics seems to have a strange property different from any other scientific theory. Generally speaking, it does not seem to be able to predict the observations of a quantum event, but can only get the probabilities of all possible results. For a classic object, such as a tennis ball or a spaceship, given some specific conditions, Newton's equation of motion can tell us exactly which path the object will follow. But when photons are emitted into a screen with two slits, quantum mechanics can only provide the probability that photons take a certain path.

Only when we measure the results of a quantum event can we get a definite answer. For example, the probability that an event will result in An or B is 50:50, and once we observe it, it will somehow become one of the results (100% probability). It's a bit like flipping a coin, there's a chance to get it face up or back up at 50:50 before throwing it, but as long as we toss it and look at it, it turns out to be either 100% face up or 100% back up. We can confirm that the probability of 50:50 at the beginning is correct by repeating the experiment-throwing it many times, and we will find that, on average, we get the same number of heads or tails.

In most scientific theories, probabilistic predictions show that we lack information about certain details. If we knew from the beginning the position and speed of each atom in the coin, the movement of the coin toss, the flow of air, etc., we should be able to predict the result with complete certainty (impossible in practice, but possible in theory). In quantum mechanics, however, probability is basic: the theory seems to insist that no matter what we measure, the best we can do is to predict the probability of a different outcome.

So the question becomes what turns the probability before the event into the certainty after the event. It seems that there are some mutations in this, which quantum mechanics itself cannot explain. The equation given by Schrodinger in 1924 to describe the behavior of quantum particles does not include this transformation. This equation uses a mathematical construct called a wave function to describe particles and their behavior (because it is similar to the equation that describes classical waves). As born showed, the wave function can be used to calculate probability, that is, the probability that we measure the properties of a particle, such as its position in space, and solve the equation to get a particular result. But the Schrodinger equation shows that these properties always change continuously, and the measurements always seem to cause a sudden jump to a certain value: this is what Nolan calls the "collapse process" of the wave function.

Quantum mechanics itself does not provide an explanation for the collapse of wave functions-it needs to be artificially added in a certain way, as the Hungarian mathematician von Neumann first did in the 1930s. This is awkward and unsatisfactory-of course, there is no problem with math. However, we still have no idea how smooth and continuous probability waves turn into unique observations in the real world. To avoid this problem, Hugh Everett (Hugh Everett) proposed a "multi-world" interpretation in his doctoral thesis at Princeton University in 1957. What if the wave function that chooses a certain result from all the possibilities doesn't collapse at all, Everett says? On the contrary, they may all have happened, just in different parallel worlds? In this image, when a photon reaches a double slit, it passes through one slit in one universe and another in another.

Hugh Everett (Hugh Everett,1930-1982) but in this way, every universe must have an observer who can see a result. In other words, everything is divided: instruments, researchers, laboratories, the universe. Everett is reluctant to mention too much in his paper the "doubling" of real people (not just the doubling of the number of particles), but the concept is rooted in theory. It expanded in the 1970s, and now the proliferation and diffusion of the "quantum self" has become a typical eye-catching element in multi-world interpretation of popular science speeches.

At the same time, some of the outstanding issues left by Everett, especially how the "split" took place, have been clarified. Physicists now know that in some macroscopic measurement devices, the process of recording quantum events (such as occurring on particles) as observable results involves a phenomenon called decoherence. the quantum state of the particle is mixed with the quantum state of the particle in its environment ("entanglement"). Proponents of multi-world interpretation believe that the split proposed by Everett is a kind of "disassembly", which is caused by decoherence, which leads to the division of two (or more) possibilities inherent in the universe before the event and no longer affects each other. Then, in all senses, they are in different universes, coexisting "in the same space", but "forgetting" each other.

As a result, proponents of multi-world theory believe that the problem of wave function collapse does not exist. But as a price, we must accept that the universe is constantly disintegrating into a parallel reality.

For many people, the price is too extravagant. First of all, by definition, these parallel worlds can never be observed, because only cause and effect independent of the current world can really be called other worlds. On the contrary, if the two results are not decoherent and can still interact with each other, they can "interfere"-a typical manifestation of quantum behavior. Moreover, some people ask, how can we accept a proposition that can never be proved to be true as a true science? Proponents of the multi-world theory replied that the correctness of their theory was contained in the Schrodinger equation itself, and that other interpretations of quantum theory should be questioned, which were forced to add something to the equation. to explain how the equation "breaks" to give an observation.

In other words, although some people object to "multi-world interpretation" as a bad science because it spends too much on the universe, proponents of multi-world theory say that it is actually the simplest explanation in terms of theoretical premises. But the truth is that neither position has a strong argument.

A more serious objection to the interpretation of many worlds is that it does not explain how probability enters quantum mechanics-the probability satisfies the born rule of extracting the expected result from the wave function. If the fact is that all results always occur with a 100% probability (in this world or other world), how can we say that the probability of result An is 50% (but we can verify it through experiments)? Proponents of many worlds claim that these probabilities should be understood as the "weight" of the branches of quantum splitting, which Nolan calls "intensity". But it doesn't make much sense in itself. For example, if the probability of An and B is 75% and 25% respectively, then this does not mean that universe An is more real, more absolute or more robust than universe B.

Proponents of many worlds argue that these apparent probabilities should be understood as expectations assigned to the end result in every possible world. In other words, they are subjective probabilities that describe personal experiences, even though "a version of me" will appear in every universe with a 100% probability. There are many variations of this idea: it has been asserted that the born rule itself can be derived from the rational observer's subjective idea of how to bet on various quantum results; although the observer knows (if the "multi-world" view is correct) that different versions of her will certainly appear in all possible worlds, each version will be aware of only one of them, and she can bet on what she will observe later before the quantum split.

But this is where the interpretation of many worlds is broken! Because when trying to eliminate the dilemma of wave function collapse, this theory is forced to impose individuals-people who make conscious decisions! As an element that can "explain" how particles behave. So it doesn't look so simple anymore, does it? In fact, the situation is much worse than this. Because MWI even deprives us of our ability to talk about individuals-to talk meaningfully about what "you" or "I" will observe, and few supporters around the world are willing to condescend to consider it. In short, there is no meaningful and clear way to connect the "you" before the quantum "split" with the "you" after the "split". We can imagine what this means, and some physicists even try to defend the idea by talking about Star Trek transporters or human clones. However, in terms of philosophical clarity and fidelity, it is inconsistent for a divided self to bet on the future. To be exact, MWI itself has completely eliminated the things that make it seem meaningful.

MWI advertises itself as an antidote to the wave function collapse fallacy, which is also a "red herring". Because now the whole multi-world concept is superfluous: think carefully about what happens when measuring quantum systems, and we will find that "collapse" is no longer a useful concept. In contrast, there are better images of how the intrinsic possibilities of understanding quantum systems slowly tilt towards a certain result by interacting with the environment, which eventually leads to decoherence and definite results. Supporters of many worlds seem keen to stick to this outdated collapsing scarecrow, but the study of quantum physics continues to move forward. This is not to say that we can explain everything, but now it seems that this interesting, creative and bold suggestion about the spreading universe is unlikely to be of great help. It's time to go the other way.

This article is translated from Philip Bal. The many worlds fantasy

Original address: https://iai.tv/ articles / the-many-worlds-fantasy-auid-1793?_auid=2020

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