Astrophysics is as Dramatic as Ever

A panel of some of the brightest minds in the field of physics explore the deep philosophical questions in the one universe versus multiverse debate.

Published April 1, 2006

By Adelle Caravanos

Question: How many physicists does it take to change a light bulb?

Answer: Five. One to think about how it should be done, one to tell you how it’s been done in the past, one to troubleshoot, and two to point out that the whole process might be unnecessary, because in at least one other universe, the light bulb is still on.

When five of the brightest minds in physics get together, whether they’re discussing light bulbs or parallel universes, you’re guaranteed a heated discussion. And that’s what was delivered at the annual Isaac Asimov Memorial Debate on March 29 at the American Museum of Natural History.

Moderated by Neil deGrasse Tyson, an astrophysicist and director of the Hayden Planetarium, this year’s debate, “Universe: One or Many?” brought together string theorists, inflationary cosmologists and astrophysicists to argue about the possibility that our universe might have companions, be they at unimaginably far distances from us, or hovering imperceptibly closer in alternate dimensions.

The panel featured five renowned scientists: Andrei Linde of Stanford University; Michio Kaku of the City University of New York; Lisa Randall of Harvard University; Lawrence Krauss of Case Western Reserve University; and Virginia Trimble of the University of California, Irvine.

Trying to Belong

Our “universe” by definition encompasses all that exists. And until fairly recently, modern physicists agreed on that definition. That is, until a few began thinking that the universe might actually belong to something bigger — a multiverse.

The idea of a multiverse, or a collection of multiple universes, has been postulated by two branches of physics: inflationary cosmology and string theory. Neither has evidence or proof for its set of principles, tenets and formulae. Still, some physicists have begun to argue that our universe is just one of many.

Of course, their lack of evidence rubs a few people the wrong way.

Are We Alone?

To be sure, the notion that our universe has company is not new. According to astronomer and science historian Virginia Trimble, debates about the existence of multiple worlds raged even in the time of Aristotle, who argued for just one. In the 16th century, Italian astronomer Giordano Bruno taught not only that there are more universes, but that some are better governed. Bruno, of course, was burned at the stake for these ideas.

Luckily, the panelists at the museum last week weren’t at risk of persecution, though multiverse proponents Linde and Kaku did catch some heat.

Objects in Mirror May Be Much Larger

Inflationary cosmology argues that ours is one of innumerable universes separated by unimaginably large distances. As the argument goes, at some point in the past our universe expanded exponentially in an infinitesimal fraction of a second, and now, at distances farther than physicists have ever considered the universe to reach, there exist other, for lack of a better word, universes that are completely different and separate from our own.

To package this prediction into a theory, inflationary cosmologist Andrei Linde proposed the idea of a “multiverse.” According to Linde, the multiverse is a collection of universes that branch off from each other, not unlike the fractal patterns of tree branches. Imagine one universe being a bubble out of which flows another bubble, then another, ad infinitum. Although the bubble universes are connected, they are effectively separate entities, too distant for any interaction or communication such as the exchange of photons or other electromagnetic radiation. After all, the particles and forms of energy we are familiar with in our universe might not even exist in other universes, he noted.

Trying to determine whether the other universes of inflationary cosmology exist would be akin to trying to find out what your friend in Shanghai is doing by yelling to him out of your Manhattan apartment window. Either the distance is so large that communication is impossible, or the forms of communication are so completely incompatible that a message cannot be transmitted.

But like the old philosophical question about a tree falling in the forest on deaf ears, some physicists wonder whether it’s worth debating the existence of other universes so far away from us that we would never be able to detect them.

So Close, and Yet So Far

Inflationary cosmologists aren’t the only ones proposing multiple universes, of course. Some string theorists, panelist Michio Kaku included, suggest not only that alternate universes exist, but that they are hovering imperceptibly close to us, within millimeters, in a hidden dimension.

As anyone who keeps up with the popular science literature knows, string theory is a model of physics whose starting point is a two-dimensional string or loop of energy. These strings vibrate at certain resonant frequencies to produce the wide variety of subatomic particles that make up the world we see. But string theory requires more dimensions than the ones we’re used to. In fact, some versions call for up to 26 dimensions, all imperceptible to us.

Far beyond our ordinary three dimensions of length, width, and depth, or even a temporal fourth dimension, Kaku and others suggest there are additional dimensions curled up and unable to be detected by human senses or manmade equipment. It’s in these other dimensions that they say alternate universes would exist. And Kaku said experiments can be designed to test for their existence.

Gravity Gives Weight to Things

Panelist Lisa Randall, a particle physicist and cosmologist, was willing to allow that the existence of universes predicted by inflationary cosmology could be tested by gravitational interaction. Although they might not have the same elementary particles as the universe we know, it’s feasible that they could exert some kind of gravitational force on our own universe. And if they do pull on us in some way, physicists could devise experiments to detect their effect.

But Randall said we would be better able to test for the effects of a universe that exists on a different dimension -— again using gravity as a variable.

Gravitational force follows an inverse square law — that is, as the distance between two objects doubles, the gravitational force between them decreases four-fold. Theoretically, if other dimensions exist, then some of the gravity between two objects could leak into other dimensions. This leak could show up as a deviation from the inverse square law.

A recent experiment in Colorado showed no evidence of deviation from the law, but Kaku joked that this only proved there is no parallel universe in Colorado. Experiments at the Large Hadron Collider, now under construction at CERN in Geneva, might also be used to test for this deviation.

When in Doubt, Experiment, Experiment, Experiment

Even though the Large Hadron Collider will be the world’s most advanced particle accelerator when it begins operation next year, Randall stressed that the problem with string theory and other multiverse arguments is that evidence might only be found at extremely small distances and incredibly high energies — 16 orders of magnitude away from where technology has currently reached. So far, there is no evidence to support string theory or its predictions, including multiple universes.

It is this lack of evidence that riles the more conservative physicists. Lawrence Krauss, an astrophysicist and author of such popular science books as The Physics of Star Trek, complained that the lack of hard evidence leaves the creators of these theories to speculate on what might be instead of on what probably is. It’s crucial not to mislead, he said. Physicists all agree that experimenting on these theories is important, but scientists must be careful not to confuse the public, he said.

It remains to be seen if inflationary cosmology and string theory and their respective multiple universes are falsifiable. What concerns Krauss and others is that future evidence and data could be made to fit the predictions: whatever is discovered could be manipulated to support one theory over another. Some are also concerned that all the talk about alternate dimensions and universes could give the public the impression that these theories are on as solid footing as the existence of quarks or the theory of evolution.

In the midst of the relentless search for a “theory of everything,” it can be comforting to think that there might be another place in which the laws are different. “In physics, we want to find one rule” that governs everything, said Linde. “But it may be more democratic. Until you prove that it is absolutely necessary for the universe to follow one rule, the multiverse theory will be alive.”

Also read: New Findings in our Knowledge of the Universe and A New Chapter in the History of the Universe