Can we detect the existence of parallel worlds in the lab? Quantum physicist Vlatko Vedral checks out the suggestion that a new—and surprisingly simple—experiment could tell us if alternate universes exist, once and for all.

From Vlatko Vedral:

Parallel worlds are a staple of science fiction and fantasy, with characters weaving in and out of alternate universes and interacting with...

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Can we detect the existence of parallel worlds in the lab? Quantum physicist Vlatko Vedral checks out the suggestion that a new—and surprisingly simple—experiment could tell us if alternate universes exist, once and for all.

From Vlatko Vedral:

Parallel worlds are a staple of science fiction and fantasy, with characters weaving in and out of alternate universes and interacting with other versions of themselves. Back in our universe, some physicists, including FQXi’s own Max Tegmark, believe that parallel universes are simply a natural extension of quantum mechanics. In such a scenario, both Obama and McCain will win the upcoming election in different universes, and there’s even room for an ultra-liberal parallel version of Stephen Colbert to comment on it.

Hiros (www.buzzsugar.com)

But can we ever know for sure that such parallel universes exist? There’s no definitive evidence for or against parallel universes—and it’s often debated whether the idea could ever be tested. At the moment, whether you choose to accept their existence or not seems to be more a question of personal taste—how unnerving (or comforting) do you find the suggestion that there are an infinite number of universes, with other “yous” that made different choices? So when I saw a paper by Frank Tipler at Tulane University in New Orleans suggesting that not only is there an experimental test that can tell us if parallel universes exist, it’s so easy that it could conceivably be carried with equipment that you could find in many high-school or college physics labs, I wanted to know more.

Tipler is proposing an experiment that can differentiate between the standard “Copenhagen interpretation” of quantum mechanics and the less conventional Many World’s interpretation, first proposed by Hugh Everett just over fifty years ago, which gives rise to parallel universes. Both alternatives attempt to make sense of one of the paradoxes of quantum mechanics: Before observation, the properties of particles and atoms are undecided; instead, quantum objects are described by a “wavefunction” that simultaneously holds many mutually complementary properties.

In the 1920s, at a meeting of quantum bigwigs in Copenhagen, this weird property was explained away by postulating that when the wavefunction is measured (or observed), the wavefunction “collapses”—settling into one particular classical state. Before any measurement is made, you cannot predict with certainty the outcome you will get—just the probability of finding a particular result. That is the standard interpretation of quantum mechanics. But just how, when, or why this collapse should occur remains a mystery.

Everett took a different tack. He proposed a collapse-free quantum theory, by suggesting that quantum laws apply to everything, not just to atoms and particles on the smallest scales. According to this view, the entire universe could exist as a superposition of many worlds—or parallel universes. The upshot is that if you make a quantum measurement, every possible outcome of that experiment is realized—each one existing in a different parallel universe, with a different parallel-you reading a different result. (See the FQXi article, “Squishy Bedrock” (pdf).)

The whole thing is a bit reminiscent of the discussions of what the perfect form of a society ought to be in order to balance the discrepancy between the needs of individuals and the needs of the society itself. There are two extreme resolutions (just like Many Worlds and Copenhagen). One is to say that there are no individuals and only the society matters (socialism)—or in our case, that everything obeys quantum mechanics, even the entire universes. The other is to say that there is no society, but that we just have a bunch of individuals (this is capitalism and its definition comes from a quote by former British Prime Minister Margaret Thatcher: “There is no such thing as Society. There are individual men and women, and there are families."). Here, that attitude corresponds to the notion that quantum mechanics is more fragmented only applying in certain regimes to certain things on small scales.

The key to testing which extreme is correct, according to Tipler, is to look at the probabilistic outcomes of quantum experiments predicted by both the rival theories. As I mentioned, the experimenter can’t predict beforehand what the outcome of a quantum experiment will be before she makes her measurement, but she can work out the _probability_ of getting a particular outcome. That’s true in both cases. The probability of getting a particular outcome is determined by the so-called Born rule, which has been verified experimentally. But if both alternatives—Copenhagen and Many Worlds—predict probabilities according to the Born rule, how does that help us choose between them?

Tipler’s answer relies on the fact that you need to perform the experiment many times, to verify that you are getting the expected probabilistic outcome. That’s true in normal experiments too. Let’s say you wanted to check that the probability of rolling a die and getting a value of “four” is one out of six. You will need to perform more than one roll. Even six rolls won’t necessarily be enough. To build up the correct pattern of probabilities, you’ll have to roll the die many many times, and slowly you’ll see the expected results emerge—turning up each die value one sixth of the time on average after many throws.

Similarly, Tipler points out that you need to perform quantum experiments thousands of times to see the expected outcome predicted by the Born rule. So while both the Copenhagen interpretation and the Many World’s interpretation predict you’ll end up with results in line with the Born rule _eventually_, just how quickly you will get there is different for the different theories. So just look at how quickly your actual experimental results build up to form the quantum pattern you expect, and you can use this rate to discriminate between Many Worlds and Copenhagen.

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How tough would this be to test in a lab? Tipler proposes looking at the “double slit experiment”. This is a relatively simple experiment using not much more than a lamp and a screen. A beam of light is split apart by two slits, producing a peculiar pattern of light on a screen, showing that light has both a wave and particle nature. It’s a standard experiment, typically carried out in advanced physics classes at school, or in universities, in basic classes in quantum mechanics. So, it would be very simple to carry out indeed.

It's an interesting idea. However, I have an issue with his approach. Tipler states that Copenhagen couldn’t give you the same result for obtaining the probabilities as Many Worlds. But this is not true. In fact, you just need to consider the screen as a quantum object and model the interaction between photons and the screen and then you can work out all the probabilities you need for any number of photons to hit any position at the screen. And this is a purely Copenhagen interpretation—we don’t need to think of the rest of the universe behaving in a quantum manner. Thus, Copenhagen can give you the same result as Many Worlds.

So sadly, I don’t think that this experiment will work. But my final thought is that I like attempts such as Tipler’s a lot. The motivation—which I very much sympathize with—for Everett’s original Many Worlds proposal and for Tipler’s paper is that in the standard formulation of quantum mechanics (Copenhagen) we have two very different processes: continuous evolution when the system is not measured and then an abrupt change due to the measurement. To have this difference between measurement and evolution is not nice and clean. There are now two ways of getting rid of the dichotomy. One is to say that there are no measurements (the Many World’s philosophy) and the other one is to say that there is no evolution (this would be the "ultra" Copenhagen view).

But maybe Tipler is wrong to be preoccupied with these two extremes. Much like the fact that every society lies between extreme socialism and extreme capitalism, it might just be that the correct view of quantum mechanics is somewhere between Many Worlds and Copenhagen...whatever this might be.

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Vlatko Vedral is a professor of quantum information science at the University of Leeds, UK, and a professor of physics at NUS, Singapore.

by Doug Savage (www.savagechickens.com)

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