One universe or many? Panel holds unusual debate March 30, 2006 Special to World Science

Scientific debates are as old as science. But in science, "debate" usually means a battle of ideas in general, not an actual, politician-style duel in front of an audience.

Occasionally, though, the latter also happens. And when the topic is as esoteric as the existence of multiple universes, sparks can fly.

According to one proposal, new universes could sprout like bubbles off a spacetime "foam" that's not unlike soap bubbles. (Courtesy Lawrence Berkeley National Laboratory)

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Such was the scene Wednesday evening at the American Museum of Natural History in New York.

Museum staff put together five top physicists and astronomers to debate whether universes beyond our own exist, then watched as the experts clashed over a question that's nearly unanswerable, yet very much alive in modern physics.

New universes may appear constantly in a "continual genesis," declared Michio Kaku, a theoretical physicist at City College of New York and key supporter of the idea that there exist multiple universes, or a "multiverse."

"The multiverse is like a bubble bath," with a bubble representing each universe, he added. There are "multiple universes bubbling, colliding and budding off each other" all the time.

Another panelist backed the multiverse idea, but three more insisted there's virtually no evidence for the highly speculative concept.

A brief history of other universes

Some versions of the many-universes concept date back to ancient Greece, said panelist and science historian Virginia Trimble of the University of California, Irvine. But scientific justifications for the idea began to appear in the second half of the 20th century, when U.S. physicist Hugh Everett proposed it as a solution to a puzzle of quantum mechanics.

Physicists in this field found that a system of subatomic particles can exist in many possible states at once, until someone measures its state. The system then "collapses" to one state, the measured one.

This didn't explain very satisfactorily why the measurement forces the system into that particular state. Everett proposed that there are enough universes so that one state can be measured in each one. Each time someone makes a measurement, the act creates a new universe that branches off the pre-existing ones.

The "multiverse" theory later reappeared as a consequence of another theory of physics, that of "inflation," developed by various physicists in the late 1970s and early 1980s.

The theory solved several gnawing problems in the Big Bang theory, the idea that the universe was created from an explosion of a single point of extremely compact matter, by postulating that this expansion was stupendously fast in the first infinitesimal fraction of a second, then slowed down.

As part of this initial superheated expansion, known as the inflationary period, the universe could have sprouted legions of "baby universes," said Andrei Linde of Stanford University in Stanford, Calif., a panelist at Wednesday's event and a developer of the inflation theory.

A third argument for the multiverse theory comes from string theory, seen by some physicists as the best hope for a "theory of everything" because it shows an underlying unity of nature's forces and solves conflicts between Einstein's relativity theory and quantum mechanics.

String theory proposes that the many different types of subatomic particles are really just different vibrations of tiny strings that are like minuscule rubber bands. The catch is that it only works if the strings have several extra dimensions in which to vibrate beyond the dimensions we see.

Why don't we see the extra dimensions? A proposal dating to 1998 claims we're trapped in a three-dimensional zone within a space of higher dimensions. Other three-dimensional zones, called "branes," could also exist, less than an atoms' width away yet untouchable. The branes are sometimes called different universes, though some theorists say they should be considered part of our own because they can weakly interact with our brane in some ways.

In part the question rests on definitions, noted Lisa Randall, a Harvard University physicist who was one of the panelists on Wednesday night. Different universes can be defined as zones of spacetime that interact with each other weakly or not at all, she said.

Where's the evidence?

Marshalling their best evidence for extra universes, Kaku and Linde-the two panelists who back the notion-presented a variety of arguments, which all boiled down to two basic points.

One, explained Linde, is that the multiverse solves the problem of why the laws of physics in our universe seem to be fine-tuned to allow for life. "If you change the mass of the proton, the charge on the electron," or any of an array of other constants, "we'd all be dead," he argued.

Why is this so, Linde asked-"did someone create this special universe for us?"

Steering clear of the straightforward answer many religious believers would give, "yes," Linde argued that the multiverse explains the problem without resorting to the supernatural. If there are infinite universes, each one can have different physical laws, and some of them will have those that are just right for us.

The second key argument they presented is the one based on inflation, a theory considered more solidly grounded than the highly speculative string theory and its offshoots. The equations of inflation, Kaku explained, suggest spacetime-the fabric of reality including space and time-was initially a sort of foam, like the bathtub bubbles.

New bubbles could have sprouted constantly, representing new universes, he added. Linde has argued that this occurs because the same process that spawned one inflation can reoccur in the inflating universe, beginning a new round of inflation somewhere else. This would occur when energy fields become locally concentrated in portions of the expanding universe.

Scientists might one day create a "baby universe" in a laboratory by recreating such conditions, Kaku said. This would involve resurrecting the unimaginably high temperatures of the early universe. A spacetime foam can be recreated by literally "boiling space," he said, adding that a sort of advanced microwave oven could do the trick.

Experiments already planned could "test the periphery" of these ideas, he added including a super-powerful particle accelerator to switch on next year, the Large Hadron Collider in Switzerland.

Randall countered that the new accelerator won't bring particles anywhere near the level of energy needed to recreate the spacetime foam envisioned by multiverse proponents. The energies attained will be lower by a factor of 10 followed by 16 zeros.

Lawrence Krauss, a physicist and astronomer at Case Western Reserve University in Cleveland, said the whole multiverse idea is so speculative as to border on nonsense. It's an outcome of an old impulse, which also gave rise to the correct notion that other planets exist, he argued: "We don't want to be alone."

It also caters to our desire for stability, he added: the universe changes, but "the multiverse is always the same." And if there are many universes, you don't have to make any predictions that will subject your pet theory to awkward tests, "because there's always one in which the answers work out."

Krauss allowed that he might buy the multiverse idea if it's a consequence of some new theory that also successfully accounts for many other unexplained phenomena. But otherwise, multiverse concepts "are extending into philosophy" rather than science, he added, "and may not be testable."

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