Even though the strange behaviour we observe in the quantum realm isn’t part of our daily lives, simulations suggest it is likely our reality could be one of the many worlds in a quantum multiverse.
Could we live in a quantum multiverse without ever noticing its oddness? A simulation suggests that the answer may be “yes” surprisingly often.
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| If the multiverse exists, our world could fit right in MARK GARLICK/SCIENCE PHOTO LIBRARY/Getty Images |
“We live in a quantum world, as far as the experiments we do can tell. So then why do I end up having all these [non-quantum] classical experiences?” says Joseph Schindler at the Autonomous University of Barcelona in Spain. He and his colleagues explored this question and found that even in the multiverse, if the microscopic world is quantum the emergence of a non-quantum world may be nearly inevitable.
To reach this conclusion, they started with the equations of quantum theory, which don’t offer clear descriptions of objects and their behaviours, but rather describe them as a fuzzy set of possibilities that become something definitive only after you observe the object. At that point, it is also seemingly impossible to say what that object was doing just moments prior. Quantum theory can look like it is constraining us to only ever be in the now, says team member Philipp Strasberg, also at the Autonomous University of Barcelona. But that doesn’t match our experience of the world.
The “many-worlds interpretation”, which asserts that the universe contains infinite parallel worlds, dispels quantum fuzziness by positing that all possibilities are real but happen in separate worlds, one of which may be our classical world.
The team worked with this idea combined with the framework of “decoherent histories”, which states that every physical process can be broken into a sequence of steps that happen at definite times, thus allowing quantum objects to have well-defined records of past behaviour. The researchers used a mathematical model to evaluate how often the multiverse can provide such sensible, unambiguous histories.
They used a simplified system containing two objects that can exchange heat with each other. Strasberg says that despite its apparent simplicity, this model captures properties that more complex systems in the real world must have. The researchers examined how making the two objects bigger and bigger, up to several thousand particles, increases the possibility of them ending up in a world that obeys classical physics. They found that this happens very quickly – and nearly every time.
“It’s very generic, almost inevitable. This is good news for people who want to live in a classical world,” says Strasberg.
Jess Riedel at NTT Research in California says the team’s model is a useful addition to physicists’ toolbox for studying fundamentals of quantum theory, but adds that its simplicity makes the findings not fully unexpected. In his view, the model would need to be more complex to reflect something definitive about our world.
Jonathan Halliwell at Imperial College London says the calculations would be meaningful for whether the emergence of a non-quantum world is inevitable even if there are not many worlds. Halliwell is not putting stock in the multiverse, just the fact that a world that is microscopically quantum must eventually become the one in which quantum effects are not accessible to us.
However, Robert Griffiths at Carnegie Mellon University in Pennsylvania, who first developed the idea of decoherent histories in the 1980s, says his approach is fundamentally incompatible with the mathematics of the many-worlds interpretation. He says the new work raises interesting questions like how exactly decoherence – the process by which quantum objects lose their fuzziness – shapes our world. But he does not believe that combining these two methods can answer them.
“There are a lot of points of open debate here,” says Strasberg. But he and Schindler feel they are making relatively few assumptions while letting “brute-force equation solving” speak for itself.
Journal reference:
Physical Review X, in press