Six questions that keep physicists up at night

It's not your average confession show: a panel of leading physicists spilling the beans about what keeps them tossing and turning in the wee hours.

That was the scene a few days ago in front of a packed auditorium at the Perimeter Institute, in Waterloo, Canada, when a panel of physicists was asked to respond to a single question: "What keeps you awake at night?"

The discussion was part of "Quantum to Cosmos", a 10-day physics extravaganza, which ends on Sunday.

While most panelists professed to sleep very soundly, here are six key conundrums that emerged during the session, which can be viewed here.

Why this universe?

In their pursuit of nature's fundamental laws, physicists have essentially been working under a long standing paradigm: demonstrating why the universe must be as we see it. But if other laws can be thought of, why can't the universes they describe exist in some other place? "Maybe we'll find there's no other alternative to the universe we know," says Sean Carroll of Caltech. "But I suspect that's not right." Carroll finds it easy to imagine that nature allows for different kinds of universes with different laws. "So in our universe, the question becomes why these laws and not some other laws?"

What is everything made of?

It's now clear that ordinary matter – atoms, stars and galaxies – accounts for a paltry 4 per cent of the universe's total energy budget. It's the other 96 per cent that keeps University of Michigan physicist Katherine Freese engaged. Freese is excited that one part of the problem, the nature of dark matter, may be nearing resolution. She points to new data from experiments like NASA's Fermi satellite that are consistent with the notion that dark matter particles in our own galaxy are annihilating with one another at a measurable rate, which in turn could reveal their properties. But the discovery of dark energy, which appears to be speeding up the expansion of the universe, has created a vast new set of puzzles for which there are no immediate answers in sight. This includes the nature of the dark energy itself and the question of why it has a value that is so extraordinarily small, allowing for the formation of galaxies, stars and the emergence of life.

How does complexity happen?

From the unpredictable behaviour of financial markets to the rise of life from inert matter, Leo Kadananoff, physicist and applied mathematician at the University of Chicago, finds the most engaging questions deal with the rise of complex systems. Kadanoff worries that particle physicists and cosmologists are missing an important trick if they only focus on the very small and the very large. "We still don't know how ordinary window glass works and keeps it shape," says Kadanoff. "The investigation of familiar things is just as important in the search for understanding." Life itself, he says, will only be truly understood by decoding how simple constituents with simple interactions can lead to complex phenomena.

Will string theory ever be proved correct?

Cambridge physicist David Tong is passionate about the mathematical beauty of string theory – the idea that the fundamental particles we observe are not point-like dots, but rather tiny strings. But he admits it once brought him to a philosophical crisis when he realised he might live his entire life not knowing whether it actually constitutes a description of all reality. Even experiments such as the Large Hadron Collider and the Planck satellite, while well positioned to reveal new physics, are unlikely to say anything definitive about strings. Tong finds solace in knowing that the methods of string theory can be brought to bear on less fundamental problems, such as the behaviour of quarks and exotic metals. "It is a useful theory," he says, "so I'm trying to concentrate on that."

What is the singularity?

For cosmologist and Perimeter Institute director Neil Turok, the biggest mystery is the one that started it all, the big bang. Conventional theory points back to an infinitely hot and dense state at the beginning of the universe, where the known laws of physics break down. "We don't know how to describe it," says Turok. "How can anyone claim to have a theory of everything without that?" Turok is hopeful that string theory and a related development known as the "holographic principle", which shows that a singularity in three dimensions can be translated into a mathematically more manageable entity in two dimensions (which may imply that the third dimension and gravity itself are illusory). "These tools are giving us new ways of thinking about the problem, which are deeply satisfying in a mathematical sense," he says.

What is reality really?

The material world may, at some level, lie beyond comprehension, but Anton Zeilinger, professor of physics at the University of Vienna, is profoundly hopeful that physicists have merely scratched the surface of something much bigger. Zeilinger specialises in quantum experiments that demonstrate the apparent influence of observers in the shaping of reality. "Maybe the real breakthrough will come when we start to realise the connections between reality, knowledge and our actions," he says. The concept is mind-bending, but it is well established in practice. Zeilinger and others have shown that particles that are widely separated can somehow have quantum states that are linked, so that observing one affects the outcome of the other. No one has yet fathomed how the universe seems to know when it is being watched.

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