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Reported by
Sheri Fink
posted
Jun 9, 2006
According to Leonard Susskind, the universe we know might be just one crude but carefully balanced case among a host of different universes, each with its own physical laws.

Sponsored by: New York Academy of Sciences and Little, Brown & Co.

Stanford University professor Leonard Susskind has had an illustrious career in theoretical physics. He is known as a "father of string theory"—the idea that everything, at its most minute scale, is made of combinations of vibrating strings. String theory began as a search for a unified theory capable of reconciling quantum field theory with general relativity, but has expanded in recent years and has caused a major shift in theoretical and experimental physics. In his recent popular science book, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design, Susskind addresses some startling recent developments in string theory, and on April 10, 2006 he took the podium as a part of the Academy's Readers & Writers series to discuss why these ideas are making such waves in the physics community.

Susskind's book deals with the meeting of two controversial ideas. One is the anthropic principle, which suggests that our corner of the universe is perfectly tailored to our existence—otherwise we would not be here to observe it. The other is string theory's prediction of the "multiverse," a giant, diverse universe with a rich landscape of "pocket universes," each governed by its own laws of physics. The expansive possibilities of the multiverse provide a plausible explanation for the unlikely perfection of our own, relatively small, universe.

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The not-so-elegant universe

The array of elementary particles that determine the properties of atoms has grown in recent years. Electrons, photons, quarks, gluons, Z bosons, and neutrinos are just a few of the many elementary particles thought to exist. "It's a rather large list," said Susskind. "It's hardly the kind of list that a minimalist would have invented."

There is no particular reason known for the existence of these particles. Some of them, however, are requisites for life. For example, atoms need to contain electrons, which are held in the nucleus by the force of photons jumping back and forth from the electron to the nucleus. The nucleus, in turn, is held together by gluons jumping back and forth between quarks.

The universe "looks like something that was designed by a rather poor engineer."

"To me the whole thing does not look like the product of an elegant mathematical theory," said Susskind. "It doesn't look like beautiful numbers like e or pi or √2; it looks like a Rube Goldberg machine! It looks like something that was designed by a rather poor engineer for some purpose. It works, but it's hardly elegant."

Aside from particles, the existence of certain forces has allowed life to evolve. Some seem finely tuned such that if the values were slightly bigger, life could not exist. Take gravity, for example—a force 42 orders of magnitude weaker than the electrical force. If it were even one order of magnitude stronger, "the universe would expand and recontract in a much shorter time than it would take for evolution," said Susskind. "Instead of being filled with galaxies, the universe would be filled with black holes. Even if an earth did form, it would not last very long. It would just have been sucked right into a black hole."

The weakness of gravity, the existence of just the right motley set of particles to form the building blocks of life—are these facts enough to cause physicists to abandon their quest for mathematical elegance and shift to embrace the anthropic principle? No, said Susskind, there is still the possibility that they arose by chance. "But there is one fine-tuning of nature, one accident, one conspiracy we might call it, which is so extraordinary that nobody thinks it's an accident."

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The puzzle of the cosmological constant

Even the greatest of scientists have been prone to second-guessing. Einstein was not immune. He posited the existence of the "cosmological constant"—the energy density of empty space, which, if positive, gives rise to a repulsive pressure that counteracts gravity. While he later abandoned the concept, it did not disappear completely. "This is a case of Pandora's Box," said Susskind—once the lid had been raised on the idea, scientists could never explain it away.

The cosmological constant is also known as vacuum energy. In quantum theory, the continuous agitation of a vacuum creates energy, leading to the outward pressure that the cosmological constant describes. However, when physicists combine the theory of elementary particles with the theory of gravity and use quantum field theory to calculate the cosmological constant, they derive a gigantic value; if it existed, such a large amount of energy would conflict with astronomical observations and would be disastrous. "It would be enough not only to shatter the earth, it would be enough to shatter every atom and molecule," said Susskind. "Every nucleus, every quark would go flying apart."

Nothing in known physics explains why the cosmological constant is not the size that quantum field theory predicts it to be. Physicists at first surmised that other particles and constants contributing to the calculation of vacuum energy must cancel out the large value, leading to a cosmological constant that is exactly zero.

"Some features of our own existence determine certain things about the laws of nature."

In 1987, physicist Steven Weinberg proposed another idea. Physicists believe that gravity forced the bland early universe to differentiate into planets and galaxies by squeezing and contracting slightly denser regions of matter and sucking mass out of less dense regions. Weinberg showed that the cosmological constant must be extremely small—on the order of 10−120 units (joules/cm3)—to prevent a repulsive force from counteracting this process. "A cosmological constant even ten times bigger than this would have been destructive and deadly to life," says Susskind. "It would have prevented the creation of the home of life—stars, galaxies, and especially planets." Using the anthropic principle, Weinberg made a prediction. While life depends on the cosmological constant being smaller than 10−120 units, the value does not need to be very much smaller than that. So, he predicted, if the value of the cosmological constant is determined by the existence of life, then its 121st digit will be a number other than zero.

Several years ago, the 121st decimal place of the cosmological constant was measured through cosmological observation; its value appears to be 2 instead of 0. To Weinberg and to Susskind, this confirmation of the earlier prediction is the best support for the anthropic contention that "some features of our own existence determine certain things about the laws of nature."

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Explaining the appearance of design

What else, besides an intelligent designer, could have tailored the universe to fit the needs of planets and people, including unlikely features that defy current mathematical prediction? Susskind's answer lies in string theory—a mathematical model of nature to which many, if not most, physicists now subscribe.

String theory makes sense in 10 dimensions of space, not our usual three. The extra six-dimensional spaces are known as Calabi Yau or CY spaces. "These spaces control all the properties of the world in a large scale," said Susskind. "The (elementary) particles have to be able to fit into these spaces. If they fit, then they're allowable particles. If they don't, they're not allowable. All the laws of nature and string theory are controlled by these features of these CY spaces." There are about a million different CY spaces, or "manifolds." Each one can be decorated with "little lines of flux that can wind around them in many, many ways," said Susskind. "When you start counting up all the possible ways the CY manifolds can be decorated with these fluxes, the numbers are humongous."

String theory allows for a landscape of as many as 10500 different environments.

Thus, string theory allows for a landscape of possible universes "so rich that it appears there may be as many as 10500 different environments that can be described." The number of possibilities is so large that it can compensate for the incredible unlikelihood of the cosmological constant being so exceptionally small.

Do these alternate universes actually exist outside of the realm of possibility, or is the universe everywhere the same as it is here, in all the places we can measure it? Nobody knows the answer yet. What is known is that the universe is far wider than the 10 billion light years across that it was once assumed to be.

The school of inflationary cosmology holds that the universe is expanding at an increasing rate. An exponential and perpetual expansion would be possible if, as the universe expanded, new bits of space formed to fill interstitial spaces. The theory of eternal inflation suggests that as the universe grows, bubbles of alternate types of space appear. "If a bubble is too small, it will melt back into the environment," said Susskind. "If it happens to grow a little bit, it will then start to really expand." Within that expanding bubble, more bubbles will form. "It creates this enormous diversity of different properties and in some tiny, tiny fraction of it, perhaps a comfortable little green neighborhood appears where life can exist. That's where we are."

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A threat to established thinking

Because physics has long posited a world controlled by elegant mathematics, the anthropic principle and the multiverse represent a fundamental shift in the way that many physicists and cosmologists view their fields. In fact, Susskind's theories have drawn the ire of some prominent scientists. Stanford professor Burton Richter, winner of the 1976 Nobel Prize in Physics, has accused Susskind of having "given up" on the effort to find a theory that explains all the properties of fundamental particles and forces, bringing to an end the "reductionist voyage that has taken physics so far."

Religious figures, on the other hand, abhor Susskind's views because they contradict the idea that God created the universe. The Roman Catholic cardinal archbishop of Vienna, Cardinal Christof Schonborn, wrote in The New York Times that the multiverse hypothesis was "invented to avoid the overwhelming evidence for purpose and design found in modern science."

Susskind, for his part, seems to relish the controversy. "Paradigm shifts, serious ones, raise people's anger, raise people's passion. They are threatening," he said. "The anger, the passion, the fighting spirit that goes with these questions is extremely intense." The fact that Susskind's ideas have aroused such emotion reflects the great attention that is being paid to this new way of looking at the universe.

Read the introduction to The Cosmic Landscape: String Theory and the Illusion of Intelligent Design by Leonard Susskind, published by Little, Brown. Copyright © 2005 by Leonard Susskind.

Speaker

Leonard Susskind, PhD
Stanford University
email | web site | publications

Leonard SusskindLeonard Susskind grew up in the South Bronx, where he worked as a plumber and steam fitter during his early adult years. As an engineering student at the City College of New York, he discovered that physics was more to his liking than either plumbing or engineering. He later earned a PhD in theoretical physics at Cornell University.

Susskind has been a professor of physics at the Belfer Graduate School in New York City and at the Tel Aviv University in Israel. He has also been the Felix Bloch Professor in theoretical physics at Stanford University since 1978. During the past forty years he has made contributions to every area of theoretical physics, including quantum optics, elementary-particle physics, condensed-matter physics, cosmology, and gravitation.

In 1969 Susskind and Yoichiro Nambu independently discovered string theory. Later on, Susskind developed the theory of quark confinement (why quarks are stuck inside the nucleus and can never escape), the theory of baryogenesis (why the universe is full of matter but no antimatter), the Principle of Black Hole Complementarity, the Holographic Principle, and numerous other concepts of modern physics. He is a member of the National Academy of Sciences and the American Academy of Arts and Sciences.

Sheri Fink is the author of War Hospital: A True Story of Surgery and Survival (PublicAffairs, 2003). Fink obtained her MD and PhD in neurosciences at Stanford University and now, based in New York, writes about medicine, public health, and science for a range of publications.
Resources

By Leonard Susskind

The Cosmic Landscape: String Theory and the Illusion of Intelligent Design, by Leonard Susskind. 2005. Little Brown, New York.
Amazon | Barnes & Noble

An Introduction to Black Holes, Information and the String Theory Revolution: The Holographic Universe, edited by Leonard Susskind & James Lindesay. 2004. World Scientific Publishing Company, Danvers, MA.
Amazon | Barnes & Noble

Web Sites and Articles

Anthropic Principle
Contains popular overviews and scholarly material on everything related to observation selection effects, the anthropic principle, self-locating belief, and associated applications and paradoxes in science and philosophy.

The Cosmic Landscape
An American Scientist interview with Susskind about his new book.

Edge.org
Features an interview with Leonard Susskind (The Landscape) and a debate between Susskind and physicist Lee Smolin on the viability of the anthropic principle (Smolin vs. Susskind: the Anthropic Principle).

Is String Theory in Trouble?
New Scientist speaks with Susskind about a "landscape" of universes.

Leonard Susskind
Leonard Susskind's lab web site.

Nova: The Elegant Universe
A three-part PBS series on string theory, featuring interviews with Leonard Susskind and other prominent physicists working in the field. A DVD containing the entire series is also available for purchase through Amazon.com.

The Official String Theory Web Site
Discusses the concept, history, and math behind string theory.

String Theory
A Wikipedia article.

Witten, E. 2002. On a string: Can scientists' 'theory of everything' really explain all the weirdness the universe displays? Astronomy (June). FULL TEXT (PDF, 843 KB)

From the Academy

Podcast

Father of string theory muses on the megaverse, featuring Leonard Susskind. 2006. Science & the City podcast. (MP3, 5.57 MB)

Readers & Writers

Reversing time's arrow: how Richard Feynman tamed infinity. 2006. Readers & Writers feature, sponsored by Science & the City.

Big Bang: Simon Singh takes on the cosmos, featuring Simon Singh. 2005. Readers & Writers feature, sponsored by Science & the City.

A tale of two loves: the physicist as novelist, featuring Alan Lightman. 2005. Readers & Writers feature, sponsored by Science & the City.

The prism and the pendulum: an evening with Robert P. Crease, featuring Robert P. Crease. 2003. Readers & Writers feature, sponsored by Science & the City.

eBriefings

Celebrating Einstein: three events on an historic day, featuring Lawrence W. Krauss et al. 2005. New York Academy of Sciences eBriefing, sponsored by CUNY Graduate Center's Science & the Arts Program and the New York Academy of Sciences.

What did Newton prove? How the Principia changed physics, featuring George E. Smith. 2004. New York Academy of Sciences eBriefing, sponsored by the History and Philosophy of Science Section.

String theory: a conversation with Brian Greene, featuring Brian Greene. 2003. New York Academy of Sciences eBriefing, sponsored by NOVA and the New York Academy of Sciences.

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