Our universe may one day be obliterated or assimilated by a larger universe, according to a controversial new analysis. The work suggests the parallel universes proposed by some quantum theorists may not actually be parallel but could interact – and with disastrous consequences.

Random quantum fluctuations mean the behaviour of particles and photons of light cannot be predicted exactly. The quantum equations that describe them contain a variety of different - and opposing - outcomes in their solution, such as a particular particle causing a bell to both ring and not ring in an experimental setup. Physicists then have to use an equation called the Born rule to calculate the probability of the bell ringing, and countless experiments have shown the rule works.

But researchers have long struggled to understand why a bell will ring – or not ring – in any given run of an experiment, since in theory it has the option of doing both. This conundrum, known as the quantum measurement problem, has led a small subset of physicists to argue that in fact the bell does do both - but that each possible outcome takes place in a different, parallel universe that pops into existence during the experiment.

"This is what the math suggests if you take it literally," says Robin Hanson of George Mason University in Fairfax, Virginia, US. But the idea that "every microsecond, the universe splits into a bunch more universes boggles the mind."

Destructive interaction

And this idea, called the "many worlds" interpretation, raises other problems. Some theorists say it suggests that physicists doing a quantum experiment would find themselves in a random world, such that they would have an equal chance of seeing the bell ring or not ring. But this does not match the well-tested Born rule, which may predict that the bell should ring 70% of the time, for example.

Physicists have attacked this problem in a number of ways. Now Hanson, an economist who also studies physics, is taking a new approach. He argues that these multiple universes are not actually independent, as was thought, but interacting and sometimes destructive.

Quantum theory states that all universes are not created equal - each "parent" universe is much larger according to a particular quantum measure than its later descendants.

Quantum interactions between the universes were thought to be too small to really affect them, but Hanson says the interactions can be significant between universes of vastly different size.

Boiled worlds

The interactions can "smash or mangle the small worlds", says Hanson. He has not worked out exactly what happens, but he believes the small universes would be either destroyed or assimilated by the large universes, like specks of dust colliding with a planet.

"It could act like a big random fluctuation, like suddenly making the temperature of the universe become really high and boiling everything," he told New Scientist. "Or it could be more peaceful, where you're simply converted into somebody who remembers stuff from the large world, so the statistics would be those of the large world."

In this scenario, Born rule predictions that a bell should ring 70% of the time in an experiment work out because small worlds – in which bells ring less or more often – are too mangled to be observed. Hanson says there is a cut-off between small worlds that become mangled and large worlds that do not, and that most universes are near or below this line.

That suggests that the universe we live in now could be mangled at any moment by a larger universe, he says. "It could be there's a moment of pain before the end," Hanson says. "But you could be comforted by the fact that versions of you will go on, even if you don't."

Finely tuned

Physicists who have studied Hanson's idea say it is interesting, though preliminary and probably flawed. Michael Weissman of the University of Illinois at Urbana-Champaign, US, says his biggest concern with the work is the notion of a cut-off.

He points out that the range of universe sizes is constantly growing. So the cut-off for what makes a universe observable must be perfectly balanced with that growth to produce the probabilities seen with the Born rule. "We don't have any real reason to think this fine-tuning will actually work out in practice," Weissman told New Scientist.

"It's interesting work and might feed into part of the Born rule problem," says David Wallace, a philosopher of quantum mechanics at Oxford University, UK. But he criticises Hanson's approach because "it doesn’t seem to handle one-off probabilities, only long-term sequences of probabilities," he told New Scientist. "It doesn’t tell us why right now we’d be better off betting on the bell ringing than not ringing."

Journal reference: Proceedings of the Royal Society A (DOI: 10.1098/rspa.2005.1640)

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