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The Universes We Still Don’t Know

The Grand Design

by Stephen Hawking and Leonard Mlodinow
Bantam, 198 pp., $28.00

1.

In 1992 I joined with other physicists in lobbying for the funding of a large elementary particle accelerator, the Superconducting Super Collider. We had the bright idea of holding a seminar for members of the House of Representatives, at which we would explain the importance of this facility for scientific research. Three congressmen showed up. After we had said our piece, a Democratic congressman from Maryland told us that he would support the Super Collider if we could assure him that it would help the work of Stephen Hawking.

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Sara Krulwich/The New York Times/Redux
Stephen Hawking at the World Science Festival’s opening night gala at Lincoln Center, New York City, June 2, 2010

As this little story illustrates, Hawking had by then achieved the sort of celebrity as a scientist that in the twentieth century was exceeded only by Albert Einstein and perhaps Marie Curie and Richard Feynman. Nor is Hawking’s fame undeserved. Already when young he had done brilliant mathematical work (in part with Roger Penrose), proving that, according to the General Theory of Relativity, there are circumstances in which a disaster becomes inevitable—an infinitely compressed energy arises in an infinitely curved spacetime, as in the case, for example, of black holes. Later, he showed that black holes radiate energy, now known as Hawking radiation. He was one of the first to use quantum mechanics to calculate the properties of fluctuations in the distribution of energy in the early universe, tiny fluctuations that eventually triggered the formation of the galaxies we see in the sky today. All this and more Hawking achieved despite a worsening physical disability that would have defeated anyone not possessed of his remarkable courage.

Hawking’s 1988 book, A Brief History of Time, had stunning sales, so much so that for a while publishers were making unrealistic cash advances to authors of other popular books on science (including one of mine), in the deluded hope that they would match his sales. Now Hawking offers another book for general readers, The Grand Design, this time written with the Caltech physicist Leonard Mlodinow.

2.

The reviews of Hawking’s book1 that I have seen have commented chiefly on the absence of God in his view of the universe, as if this was something surprising. This reaction to the book is pretty silly. Hawking’s point is that we do not need God to understand the cosmos. Scientists may disagree about how well we understand the cosmos, but not many feel that it is God that is needed to fill the gaps. In A Brief History of Time, referring to a possible future discovery of a complete theory of nature, Hawking had written that “then we would know the mind of God.” But this was a metaphor, like Einstein’s remark about God not playing dice with the universe. Perhaps to avert misunderstanding, in The Grand Design Hawking avoids any such metaphors.

One of Hawking’s themes in this book does have an impact on religion. This is his adoption of an idea, increasingly popular among physicists, that the expanding cloud of galaxies that we call the universe, extending at least tens of billions of light years in all directions, may be only part of a much grander “multiverse.” The multiverse is supposed to contain a vast number of other parts that might be called universes, in which even what are usually called the laws of nature are very different from what we observe.

If the multiverse idea is correct, it will remove what some have conceived to be evidence for a benevolent creator: the fact that the laws of nature seem to favor the appearance of life. For instance, if one of the two types of quarks that make up atomic nuclei were much heavier or much lighter than the other, there would be only a few stable elements, not the rich menu of actual elements that appears to be necessary for life. Hawking makes more of such examples than I would; no very great fine-tuning of the quark masses is needed to make the chemical elements needed for life sufficiently abundant.

But there is one example cited by Hawking of a really remarkable fine-tuning of physical constants, without which life could never have appeared. It has to do with dark energy, the energy of empty space. In 1998 astronomers discovered that the expansion of the universe is speeding up, an acceleration that is generally attributed to dark energy. The dark energy turned out to be about three times larger than the energy contained in the masses of all types of matter in the universe. But there is something strange about how much of this dark energy there is. We can calculate the contributions of quantum mechanical effects to dark energy—in fact, these calculations were done by several theorists before 1998. But these contributions to the dark energy turn out to be so large that, if they were not almost entirely canceled out by other contributions, the universe would be expanding much more rapidly than is observed—so rapidly that gravitationally bound systems like galaxies and stars and planets could never have formed.

Such a cancellation is possible because there are other contributions to the dark energy that we haven’t been able to calculate, in part because these contributions depend on things we don’t know, and they could cancel out the contributions to the dark energy that we can calculate. (One of the things we don’t know that affects the dark energy is the value of the so-called cosmological constant posited by Einstein in 1917, in a modification of the equations governing the gravitational field in the General Theory of Relativity.) But in order for these so-far incalculable contributions to give a total dark energy small enough to allow for the formation of gravitationally bound systems (and as small as is inferred from measurements of the expansion of the universe), constants of nature like the cosmological constant would have to be fine-tuned to make the cancellation complete to about fifty-six decimal places.

On the other hand, a multiverse would have so many parts that quantities like the quark masses and Einstein’s cosmological constant and other constants of nature would have a wide range of possible values. It is likely that in the great majority of these parts of the multiverse, constants like the quark masses and the cosmological constant and even perhaps the dimensionality of space would take values unsuitable for life. But with a wide enough range of these constants in different parts of the multiverse, there would be some parts where life could appear. Obviously, it should be no surprise and no sign of cosmic benevolence that we are in one of these favored parts of the multiverse, just as it is no sign of a benevolent creator that in a galaxy with billions of planets, we evolved on one of the minority of planets that are suitable for life. Where else could we be, except on a planet that can sustain life?

Hawking quotes a notorious 2003 statement by the cardinal archbishop of Vienna, which attacked the multiverse idea as something “invented to avoid the overwhelming evidence for purpose and design found in modern science.” Not so. As Hawking says, the multiverse idea is not a notion invented to account for the miracle of fine- tuning. He discusses two different lines of thought that led physicists to the idea of a multiverse, neither having anything to do with the conditions necessary for life.

3.

One such line of thought arose in the theory of chaotic inflation, developed by Andrei Linde. Inflation is an early period of exponentially rapid cosmic growth, like the growth of a bank account that pays 100 percent interest every tiny fraction of a second.2 This exponential expansion is now believed to have preceded the present, more stately phase of cosmic expansion. As originally conceived by Alan Guth (and still assumed in most calculations), inflation was supposed to have occurred uniformly everywhere in space. But no theory accounts for such uniformity. It seems more natural to suppose that the universe, on very large scales, is chaotic, pervaded by wildly fluctuating fields, and that purely by chance now and then conditions in a patch of space allow that patch to begin an exponential inflation. In a small minority of cases these patches would grow into something like our present universe, in which life is possible.

The other suggestion of a multiverse comes from quantum mechanics, the mathematical framework for all physics. The weirdest thing about quantum mechanics is what is called the superposition of states. It is possible (even common) for a particle to be in a quantum state in which it cannot be said to be at any single one of its possible positions. Instead, it is in a combination of all its possible positions—a superposition—so that any single observation of its position can give any of a number of possible results, with different probabilities depending on the nature of the superposition. In principle, as Erwin Schrödinger famously pointed out, even a cat can be in a superposition of states, in some of which it is alive and in others dead. Similarly the whole universe can be in a superposition of many different states, in which the constants of nature like quark masses take different values, with a small minority of these states favorable for life.

All this is highly speculative, but not idiosyncratic. These ideas are widely discussed by physicists. Hawking does take a somewhat unusual position in his suggestion that the multiverse came into being as a quantum mechanical superposition of states because in the very early universe all four dimensions behaved like space, with no time. I won’t try to explain how this works, because I don’t find it convincing. True, Hawking has shown that it is useful to carry out calculations of processes in the early universe by mathematically distorting the dimension of time so that it becomes effectively one of space. But this does not mean that time actually was space in the early universe. After all, other theorists, going back to Julian Schwinger in the 1950s, have calculated subtle effects in atomic and particle physics by just such a distortion of the time dimension into one of space, but the usefulness of this mathematical trick does not change the fact that today we live in three spatial dimensions and one temporal dimension.

4.

The idea of a multiverse received a big boost in recent years from developments in what used to be called string theory. It is now thought that the various known versions of string theory and a vast number of other theories all represent approximate solutions to an unknown fundamental theory, which Hawking calls M-theory. These different approximate solutions describe different sets of particles or strings or membranes in spacetimes of various different dimensionalities, with different values of physical constants. Supposedly, these various solutions of the fundamental theory that Hawking calls M-theory are realized in different parts of the multiverse.

  1. 1

    For brevity I will refer here to the book by the name of its senior author, Hawking, rather than Hawking and Mlodinow. 

  2. 2

    To be specific, on the basis of observations of microwave radiation left over from the early universe, the time in which the universe doubled in size during inflation is estimated as having been roughly ten to the minus thirty-seven seconds. (Ten to the minus thirty-seven is a decimal point followed by thirty-six zeroes and a one.) 

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