THE multiverse is dead, long live the multiverse. A radical new way of looking at quantum mechanics suggests that even the multiverse will come to an end.
A popular view of the multiverse says that our universe is just one of an ever-inflating multitude of discrete 鈥渂ubble鈥� universes. These bubbles are eternally budding off new universes even as individual universes age and die.
But a new view of quantum effects 鈥� the brainchild of at the California Institute of Technology and his colleagues 鈥� challenges this picture. It is also potentially very useful to quantum theorists, as it does away with some thorny issues that currently dog cosmology, including a particularly baffling paradox involving disembodied consciousnesses known as 鈥淏oltzmann brains鈥�.
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Carroll鈥檚 insight comes from a fresh way of looking at random motions known as .
Quantum systems baffle our best physical intuition. Current models say that a tiny particle like an electron has no precise position: the best we can do is describe the probability of finding an electron in a particular spot, given by an equation called its wave function. When you attempt to make a measurement, the wave function 鈥渃ollapses鈥� and picks a single value 鈥� but until that instant, the electron鈥檚 position fluctuates. One upshot of such uncertainty is the emergence of quantum fluctuations out of seemingly empty space.
Despite their bizarre qualities, however, quantum fluctuations get the credit for our very existence. Studies of the first light emitted in the universe, about 380,000 years after the big bang, suggest that quantum fluctuations in the early universe made matter denser in some regions than others, resulting in a cosmic web of galaxies, stars, planets and, ultimately, people.
The random jitters also seemed to have another intriguing consequence. In the split second after the big bang, the universe is thought to have gone through an explosive growth spurt known as inflation, driven by quantum particles called inflatons. These would be subject to quantum fluctuations too, and every so often an inflaton would be randomly infused with extra energy, blowing a separate bubble universe into existence. That bubble would itself go through inflation and in turn blow more bubbles, leading to the idea of the bubble multiverse (see diagram). By this thinking, once inflation starts it can never really end, and new universes are always being born 鈥� so this multiverse is infinite and endless.
That is the prevailing view, at least. Carroll and his colleagues decided to take a second look at this theory because it leads to some unresolved questions. In such an infinite multiverse, everything that has even a slight chance of happening is virtually certain to happen 鈥� you just need to wait long enough.
Some theorists have pointed out that, taken to its logical conclusion, that includes the spontaneous aggregation of matter so that it creates self-aware, disembodied brains. It鈥檚 the same kind of logic that says an infinite number of monkeys typing randomly would eventually produce the complete works of Shakespeare. 鈥淚t sounds like something a bunch of college sophomores would discuss while high. It doesn鈥檛 sound like a real scientific problem,鈥� says at the Massachusetts Institute of Technology.
聯It sounds like something a bunch of college sophomores would discuss while high聰
That may be true, but Boltzmann brains create some serious problems for theorists: over the entire history of the universe, such brains would outnumber consciousnesses such as ours. That鈥檚 a big problem, because the starting point for our understanding of the universe and its behaviour is that humans, not disembodied brains, are typical observers. What鈥檚 more, Boltzmann brains are just too bizarre for some people. 鈥淚 believe they fail the Monty Python test: Stop that! That鈥檚 too silly!鈥� says of MIT.
聯I鈥檓 quite sympathetic. I believe Boltzmann brains fail the Monty Python test: Stop that! That鈥檚 too silly!聰
Carroll set out to write a paper showing that Boltzmann brains are a real threat, but in the process he found a way to vanquish them. His starting point was the idea that quantum fluctuations are dependent on interactions with an external system or particle, known as an 鈥渙bserver鈥� 鈥� a familiar concept in quantum mechanics. When he applied this thinking to our view of inflation, it changed everything. The inflaton must have preceded all the other particles in the very early universe. That means it was the only type of particle that existed, so there would have been nothing 鈥渆xternal鈥� for inflatons to interact with, says Carroll. In this case, the inflaton would not have undergone quantum fluctuations.
This 鈥渜uiet鈥� state lasted until the inflatons decayed into different types of ordinary particles, which could then interact with each other. 鈥淭hen those quantum fluctuations finally come to life,鈥� says Carroll 鈥� allowing them to fulfil their crucial role of seeding the cosmic web but removing the need for an infinitely budding multiverse.
Still, his idea doesn鈥檛 do away with the multiverse altogether. That鈥檚 because the mathematics that make fluctuations dependent on an observer rely on the 鈥渕any worlds鈥� theory of quantum mechanics. This says that each time a quantum system is measured, the universe branches into several different versions, one for each possible outcome. Unlike a multiverse in which each discrete bubble universe starts from scratch and evolves independently, a 鈥渕any worlds鈥� multiverse is made of overlapping branches that all started with the same initial conditions. 鈥淢aybe Hitler won the second world war in a different universe, that鈥檚 one outcome,鈥� says Carroll. 鈥淏ut the laws of physics are the same.鈥�
In Carroll鈥檚 theory, even the branching multiverse must come to an end. The universe is expanding at an accelerating rate, so cosmologists think its death will have a lot in common with its birth, with no recognisable matter and only a single quantum field. In that case, there will once again be no observers to bring quantum fluctuations to life.
The simplicity of the theory impresses Aaronson: 鈥淚 think he鈥檚 fundamentally right about it. I鈥檓 basically sold.鈥�
Proponents of eternal inflation are sticking to their story, however. 鈥淚鈥檓 quite sympathetic to Sean鈥檚 desire not to have Boltzmann brains,鈥� says Lloyd. Nevertheless, he and at MIT 鈥� one of the founders of the theory of inflation 鈥� both think it possible that the ever-bubbling multiverse can exist even if all of Carroll鈥檚 mathematics are correct, and they are working on a paper to make that case.
There鈥檚 currently no way to resolve the debate, but at the University of Oxford says Carroll鈥檚 theory may also have practical consequences, for example in helping us better understand the way matter behaves at the quantum level.
This article appeared in print under the headline 鈥淨uantum twist kills the multiverse鈥�
