The Cosmic Landscape: String Theory And

pt_books0606.qxp 5/9/2006 4:26 PM Page 61

nority in both communities, suspects the sible. He argues strongly that the only The Cosmic existence of an enormous landscape of plausible nonsupernatural explanation Landscape: some 10500 possible vacua of an underly- is “a Landscape of possibilities popu- ing superstring theory. Each vacuum lated by a megaverse of actualities.” String Theory and leads to its own laws of physics, and no Except for possibly appealing to the known principle distinguishes one as idea of an intelligent designer, the tradi- the Illusion of preferred over the others. Furthermore, tional view of unique laws and constants Intelligent Design most of those who hold that minority offers little explanation for why we are view speculate that the universe consists fortunate that nature has just the right Leonard Susskind of an infinite multiverse, or megaverse, conditions for life. The multiverse– Little, Brown and Co, New York, landscape paradigm, on the other hand, 2006. $24.95 (403 pp.). of regions, each expanding or contract- provides a plausible framework for the ISBN 0-316-15579-9 ing according to its own laws of physics. New regions are constantly formed by anthropic principle: Life only evolved Reviewed by Paul Langacker quantum tunneling so that all of the in the tiny fraction of regions of the The Great Debate in 1920 at the Smith- vacua of the landscape are sampled. multiverse that had suitable conditions. sonian Museum of Natural History be- Leonard Susskind’s The Cosmic Land- This view is analogous to the paradigm tween Harlow Shapley and Heber Cur- scape: String Theory and the Illusion of shift that resolved the old question of tis concerned the size of the Milky Way Intelligent Design surveys the new de- why we are so lucky that Earth has hos- and whether it con- bate clearly and amusingly for the gen- pitable conditions: If billions of stars in stituted the entire eral reader. Susskind, one of the inven- the galaxy have planets, then some are universe or was just tors of string theory and a leading bound to be just right. one of innumerable advocate of the landscape and multi- Many scientists are strongly op- island universes, or verse ideas, does an excellent job devel- posed to the multiverse–landscape par- separate galaxies. oping the necessary background in adigm. Some objections are technical. 500 The issue was set- quantum mechanics, relativity, particle For example, are there really 10 tled in 1924 when physics, supersymmetry, string theory, vacua, or does the multiverse really Edwin Hubble ob- black holes, cosmology, and inflation. exist? Others are that the ideas are not served Cepheid Underlying the debate between the testable and not really science, or that variables in the An- landscape idea and the more traditional they might seduce researchers into giv- dromeda galaxy, which allowed him to view is the observation that our uni- ing up the traditional goal of finding a show that Andromeda was an enor- verse and its physical principles appear unique and elegant explanation for the mously distant, separate galaxy. And to be remarkably fine-tuned to allow observed laws of nature. Susskind like Shapley’s determination that the the development of life: the anthropic makes no attempt to give an impartial solar system is not at the center of our principle. For example, small variations overview—after all, he is advocating galaxy, and the earlier Copernican rev- in the relative masses of the electron his own ideas. However, he does offer olution, Hubble’s findings helped to de- and proton or in the value of the fine- a reasonable survey of the objections mote the significance of humans. Hub- and his own responses to them. structure constant would preclude the ble summarized it quite well: “The The Cosmic Landscape is a fascinating necessarily rich structure of atoms and history of astronomy is a history of re- introduction to the new great debate, molecules. Similarly, the observed vac- ceding horizons.” which will most likely be argued with uum energy, or cosmological constant, Recently, a new but related great de- passion in the years to come and may is some 120 orders of magnitude bate has been quietly raging within the once again greatly alter our perception of smaller than what is typically expected communities of string theorists and cos- the universe and humanity’s place in it. mologists. One side holds the traditional from string theory. Steven Weinberg view that the laws of physics and the val- has argued that a positive value much ues of the physical constants are the larger than observed would have led to It’s About Time: same throughout the universe—a uni- such rapid expansion of the universe verse presumably determined as the that galaxies and stars could never have Understanding unique solution of some ultimate physi- formed; a large, negative value, on the Einstein’s Relativity cal theory. Another side, a growing mi- other hand, would have led to a cata- strophically rapid collapse. Many other N. David Mermin Paul Langacker is a professor in the examples exist in Big Bang nucleosyn- Princeton U. Press, Princeton, NJ, department of physics and astronomy at thesis, long-lived stars, and the super- 2005. $29.95 (192 pp.). the University of Pennsylvania in Philadel- nova explosions needed to eject ele- ISBN 0-691-12201-6 phia. His research is in theoretical parti- ments into the universe. Susskind I first came across relativity theory in an cle physics, and he is especially interest- describes at great length “The Mother example featured in an instruction book ed in making connections between of All Physics Problems”—the cosmo- for an early electronic calculator. I was 11 fundamental theory, experiment, and logical constant—and the other lucky years old and was amazed to read that astrophysics. accidents that make our existence pos- an astronaut could travel for 50 years