Einstein’s revolution enters second century

A virtual infinity of answers
Currently, the leading candidates in the quest for the unification of physics are in the realm of string theory — which describes particles as vibrating strings or multidimensional surfaces dubbed "branes." Such theories can be spun to explain quantum mechanics as well as the general-relativity view of gravity, if the equations play out in 10 or 11 spatial dimensions instead of the three we can perceive.

In fact, the existence of dark energy appears to allow for a virtually infinite number of possible solutions to the equations, Carroll says. Some might see that as a virtually hopeless task, but in Carroll's view, having too many possible solutions is far better than having no solutions.

"Since there are 10⁵⁰⁰ possibilities, you could hope to search through them and find the one that fits our world," he said. "Now, maybe that's a little bit ambitious. The number of particles in the observable universe is only 10⁹⁰ — but people have started."

And there are yet more possibilities: Perhaps dark energy isn't constant at all, but represents a variable "fifth force" or "quintessence" that has gone unnoticed until now. Perhaps another class of theories, known as loop quantum gravity, will trump string theory. Perhaps we'll have to throw out all of Einstein's ideas and build a new theory from the ground up. Or perhaps, as Dyson believes, the universe just wasn't made to fit within a single "theory of everything."

Narrowing down the possibilities
To find out which possibility is the right one, some scientists are sifting through high-resolution sky surveys to see how the universe's rate of expansion has changed over the course of billions of years. Others hope that particle accelerators yet to begin operations, such as Europe's Large Hadron Collider or the International Linear Collider, will turn up weird phenomena on the subatomic scale.

"If we discovered supersymmetric particles or extra dimensions, or ruled them out, that would have a lot to say about why the value of dark energy is what it is," Carroll says.

But, just as in 1904, no one can predict exactly where the solution will come from.

"As usual, the solution will probably come from an unexpected direction," Dyson says. "So I think it's very foolish to predict how it's going to go."

Why does it matter?
If it's impossible to say where the solution will come from, it's doubly impossible to predict what the real-world consequences of that solution might be. But if the next century plays out like the last one, advances in our understanding of physics could turn today's science fiction into tomorrow's everyday technologies.

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In a science-fiction world, dark energy could be harnessed as a power source or for interstellar travel. Weird quantum effects could serve as the basis for ultra-secure communications,  ultra-miniaturized nanocomputer chips or new ways of storing data as holograms.

But Carroll believes cosmological research is more important for answering cosmic questions — for example, whether the universe will someday spread out to near-nothingness in a "Big Chill" (currently the most widely accepted scenario); tear itself apart in a "Big Rip"; fall back on itself in a "Big Crunch"; begin yet another Big Bang cycle; or even spawn another generation of "daughter universes."

"It might be the case that investigations into quantum gravity or dark energy might end up with tangible benefits on technology in the next 100 years," Carroll admits, "but mostly I emphasize the fact that people just want to know the answer. ... It's part of innate human curiosity to figure out how the universe around us works."

Many things may have changed in the century since Einstein's miracle year, but that innate curiosity is constant.

"I want to know God's thoughts," Einstein once said. "The rest are details."

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