What's the deepest cosmic puzzle for the next 20 years? After a Seattle talk that touched upon multiverses, branes and 10-dimensional physics, string theorist Brian Greene says his candidate would be figuring out exactly what the underlying fabric of space and time is really made of. In a couple of weeks, Cambridge cosmologist Stephen Hawking will get his own shot at the question from the same Seattle stage. In the meantime, what's your candidate for the deepest question?
There's been a succession of theories about what constitutes reality - starting with the four elements of the ancient Greeks and moving on to atoms and molecules ... then electrons, protons and neutrons ... then a menagerie of subatomic particles, including quarks and leptons ... and now the teeny, tiny vibrating strings that are the focus of Greene's theoretical work.
But Greene says strings by themselves are not the end of the story. In his view, the multidimensional strings resonate in the structure of the space-time continuum, somewhat like musical notes resonate differently in an oboe or a tuba. In fact, the various possibilities for the structure of space and time may dictate how different universes work, with different physical laws in each universe.
"The really big problem in cosmology that I don't think we've cracked is a full understanding of what space and time actually are. ... We don't know what the 'atomic structure' of space and time really is," he told a questioner at Monday night's talk.
The question about what lies beneath our perception of space-time has been debated for millennia, of course, and some scientists say the answer may well be unknowable. Others, such as the Perimeter Institute's Lee Smolin (author of "The Trouble With Physics"), have proposed that the geometry of space-time arises from underlying "spin networks," with particles and fields defined by nodes and lines on the network. Here's a Scientific American article that goes into Smolin's proposed theory, known as loop quantum gravity.
All these ideas are consistent with the view that our universe is following just one of many possible cosmic scenarios, and that there could be other universes just next door that we cannot perceive. Greene noted that this concept has often been compared to "a big cosmic bubble bath," with each universe representing just one bubble in the foam.
Another metaphor compares our universe to a single slice (or "brane") in an expanding loaf of raisin bread. Greene referred to yet another comparison, to a block of Swiss cheese with each hole standing for a universe. "As a vegan, somehow this metaphor is starting to make me feel a little bit sick," he joked.
Could it be that, in a sense, we each create our own universe? In a recent essay, stem-cell researcher Robert Lanza signed onto that idea, which resonates with some interpretations of quantum theory as well as new-age thought. But Greene wasn't buying it. "My own view is that the observer is highly overrated in quantum mechanics," he said.
Many of the seeming puzzles in the relationship between the microscopic quantum world and the macroscopic classical world can be explained by a phenomenon called decoherence. "You can really take the experimenter out of the equation," Greene said.
So what would Hawking list as the deepest unanswered question of the next 20 years? Greene's questioner quoted Hawking as saying there'd be no unanswered questions left by that time, since he intended to "answer them all." But Hawking might have a different take when he's actually on stage on April 9.
Hawking didn't have time to address the deep questions we sent in a couple of months ago, but he did answer five questions from The Seattle Times - including a question about the title of his talk, "The History of the Universe Backwards."
The good doctor said the cosmic tale is best told backwards "because the universe doesn't have a fixed initial state. Instead, the initial state is determined by the final state." This is an encapsulation of his brain-twisting "top-down" approach to cosmology - and I'm looking forward to hearing more about that in April.
This week we found out at least a partial answer to a somewhat less deep question: How much are people willing to pay to fly with Hawking on a zero-gravity adventure? Based on a recently concluded online charity auction, the answer is at least $75,100, as reported by New Scientist's Kelly Young and "Rocketeers" author Michael Belfiore. I suspect, however, that that's not the final word on the charity seats. Stay tuned ... and keep those deep questions coming.