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Is Nature Unnatural?
Decades of confounding experiments have physicists considering a startling possibility: The universe might not make sense.
by: Natalie Wolchover
May 24, 2013



Is the universe natural or do we live in an atypical bubble in a multiverse? Recent results at the Large Hadron Collider have forced many physicists to confront the latter possibility. (Illustration: Giovanni Villadoro)


On an overcast afternoon in late April, physics professors and students crowded into a wood-paneled lecture hall at Columbia University for a talk by Nima Arkani-Hamed, a high-profile theorist visiting from the Institute for Advanced Study in nearby Princeton, N.J. With his dark, shoulder-length hair shoved behind his ears, Arkani-Hamed laid out the dual, seemingly contradictory implications of recent experimental results at the Large Hadron Collider in Europe.

“The universe is inevitable,” he declared. “The universe is impossible.”

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“The universe is impossible,” said Nima Arkani-Hamed, 41, of the Institute for Advanced Study, during a recent talk at Columbia University. (Photo: Natalie Wolchover/Simons Science News)


With the discovery of only one particle, the LHC experiments deepened a profound problem in physics that had been brewing for decades. Modern equations seem to capture reality with breathtaking accuracy, correctly predicting the values of many constants of nature and the existence of particles like the Higgs. Yet a few constants — including the mass of the Higgs boson — are exponentially different from what these trusted laws indicate they should be, in ways that would rule out any chance of life, unless the universe is shaped by inexplicable fine-tunings and cancellations.

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 â€œThere are frustrating theoretical problems in quantum field theory that demand solutions, but the string theory ‘landscape’ of 10500 solutions does not make sense to me. Neither does the multiverse concept or the anthropic principle,” said David J. Gross, a Nobel Prize-winning physicist.

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In such a picture, not everything about this universe is inevitable, rendering it unpredictable. Edward Witten, a string theorist at the Institute, said by email, “I would be happy personally if the multiverse interpretation is not correct, in part because it potentially limits our ability to understand the laws of physics. But none of us were consulted when the universe was created.”

“Some people hate it,” said Raphael Bousso, a physicist at the University of California at Berkeley who helped develop the multiverse scenario. “But I just don’t think we can analyze it on an emotional basis. It’s a logical possibility that is increasingly favored in the absence of naturalness at the LHC.”

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“Naturalness has a track record,” Arkani-Hamed said in an interview. In practice, it is the requirement that the physical constants (particle masses and other fixed properties of the universe) emerge directly from the laws of physics, rather than resulting from improbable cancellations. Time and again, whenever a constant appeared fine-tuned, as if its initial value had been magically dialed to offset other effects, physicists suspected they were missing something. They would seek and inevitably find some particle or feature that materially dialed the constant, obviating a fine-tuned cancellation.

This time, the self-healing powers of the universe seem to be failing. The Higgs boson has a mass of 126 giga-electron-volts, but interactions with the other known particles should add about 10,000,000,000,000,000,000 giga-electron-volts to its mass. This implies that the Higgs’ “bare mass,” or starting value before other particles affect it, just so happens to be the negative of that astronomical number, resulting in a near-perfect cancellation that leaves just a hint of Higgs behind: 126 giga-electron-volts.

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If no new particles appear and the Higgs remains astronomically fine-tuned, then the multiverse hypothesis will stride into the limelight. “It doesn’t mean it’s right,” said Bousso, a longtime supporter of the multiverse picture, “but it does mean it’s the only game in town.”



Brookhaven Forum 2013 David Curtin, left, a postdoctoral researcher at Stony Brook University, and Alessandro Strumia, a physicist at the National Institute for Nuclear Physics in Italy, discussing Strumia’s “modified naturalness” idea, which questions longstanding assumptions about how to calculate the natural value of the Higgs boson mass. (Photo: Thomas Lin/Simons Science News)

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Dark Dilemma

The energy built into the vacuum of space (known as vacuum energy, dark energy or the cosmological constant) is a baffling trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion times smaller than what is calculated to be its natural, albeit self-destructive, value. No theory exists about what could naturally fix this gargantuan disparity. But it’s clear that the cosmological constant has to be enormously fine-tuned to prevent the universe from rapidly exploding or collapsing to a point. It has to be fine-tuned in order for life to have a chance.

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Most particle physicists hoped that a more testable explanation for the cosmological constant problem would be found. None has. Now, physicists say, the unnaturalness of the Higgs makes the unnaturalness of the cosmological constant more significant. Arkani-Hamed thinks the issues may even be related. “We don’t have an understanding of a basic extraordinary fact about our universe,” he said. “It is big and has big things in it.”

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“It was definitely an aha-moment for me,” Bousso said. But the paper sparked outrage.

“Particle physicists, especially string theorists, had this dream of predicting uniquely all the constants of nature,” Bousso explained. “Everything would just come out of math and pi and twos. And we came in and said, ‘Look, it’s not going to happen, and there’s a reason it’s not going to happen. We’re thinking about this in totally the wrong way.’ ”

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In the multiverse scenario a vast and diverse array of bubble universes fluctuate into existence inside a larger vacuum. A small fraction of the universes have physical properties conducive to life.

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Despite these seemingly successful explanations, many physicists worry that there is little to be gained by adopting the multiverse worldview. Parallel universes cannot be tested for; worse, an unnatural universe resists understanding. “Without naturalness, we will lose the motivation to look for new physics,” said Kfir Blum, a physicist at the Institute for Advanced Study. “We know it’s there, but there is no robust argument for why we should find it.” That sentiment is echoed again and again: “I would prefer the universe to be natural,” Randall said.

But theories can grow on physicists. After spending more than a decade acclimating himself to the multiverse, Arkani-Hamed now finds it plausible — and a viable route to understanding the ways of our world. “The wonderful point, as far as I’m concerned, is basically any result at the LHC will steer us with different degrees of force down one of these divergent paths,” he said. “This kind of choice is a very, very big deal.”

Naturalness could pull through. Or it could be a false hope in a strange but comfortable pocket of the multiverse.

As Arkani-Hamed told the audience at Columbia, “stay tuned.”




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