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The concept of the multiverse, multiple versions of the same world existing side by side, is widespread in movies and television. In the real world, theoretical physicists are still grappling with the mathematics of multiple worlds. A by physicists at �ٺ���Ƶ takes a fresh look at the quantum math of multiple worlds, implying that there may be more possibilities than we imagine. 

Professor , a theoretical physicist and cosmologist at �ٺ���Ƶ, has been thinking about multiple worlds for decades. Physicists tend to avoid the subject and its esoteric implications, he said. 

“Physicists tend to think of themselves as practical people,” he said. “Quantum physics throws us for a loop.” 

Quantum mechanics says that the state of a system depends on an observer. In Ernst Schroedinger’s , a cat in a box may or may not have been killed by a poison released at random. Whether the cat is dead or alive can only be determined by opening the box. So according to quantum mechanics, before the box is opened the cat is neither alive nor dead but in an indeterminate state. 

(We don’t know what Schroedinger had against cats. He could have picked a rat or a cockroach, but here we are.)

Dead, alive, neither or both?

One view of this scenario is that the cat is now either alive or dead, and the other possibility disappears. But another, intriguing possibility is that reality forks when you open the box. In one universe, the cat is alive and in the other, dead. 

This implies that every time we make a decision – or something could go one way, or another – alternate universes pop into existence. So how does one alternative become the ‘real’ one? How does a quantum fluctuation translate into classical physics? 

This matters quite a lot because according to our current understanding of cosmology, our universe is based on quantum fluctuations. In the instant of time after the Big Bang, quantum fluctuations turned into fluctuations of matter and energy in the expanding universe around which the first galaxies would coalesce.

At a cosmic scale, this does indeed throw most of us for a loop. 

Arsalan Adil, a graduate student working with Albrecht at the �ٺ���Ƶ , has led the development a new framework for addressing the problem of how classical physics emerges from the quantum realm. 

Physics from first principles

Rather than a whole universe, or even a whole cat, Adil and Albrecht’s model considers a limited number of atoms in a box that can interact with each other through quantum mechanics. They looked at the rules that emerge from the model under different scenarios. Essentially, they reconstructed physics from first principles. 

It turns out that rather than collapsing into a single outcome, or forking into alternate universes, the model can produce different outcomes for a single scenario depending on the filter you use to look at it. While some results are more likely than others, none has a particular priority, and the math shows that they cannot interact with each other. They are, however, equally ‘real.’ 

“It ended up being much less constrained than we thought,” Albrecht said. “It turns out a lot of things can happen depending on what filter you use.”

The new work could have implications for designing . While digital computers are based on data bits that can be read as “one” or “zero,” quantum computers use qubits that can simultaneously exist as one or zero. These machines have the potential to solve extremely complex problems very quickly, basically using the multiverse as parallel computing cluster

“We’re going to need to think more about these results to fully understand their implications,” Albrecht said.

Coauthors on the paper are Manuel Rudolph and Zoë Holmes, Ecole Polytechnique Fédérale de Lausanne, Switzerland; Andrew Arrasmith and Andrew Sornborger, Los Alamos National Laboratory.