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An interview with Leonard Susskind

Greg Ross

It's often noted that the universe seems strangely tailored to support human existence. The cosmological constant, for example, is tiny but not quite zero, producing a delicate cosmic balance without which life could not exist. This unlikely hospitality has given rise to the "anthropic principle," a controversial concept that invokes the requirement for human existence in seeking to determine the rules of our universe.

The principle is unpopular among physicists, who would prefer to reach a single elegant solution that prescribes values for all the constants of nature without appealing to our own existence. The difficulty is that string theory currently gives rise to an unmanageable number of possible solutions.Leonard SusskindClick to Enlarge Image

In The Cosmic Landscape (Little, Brown, 2005), Leonard Susskind offers a different conception. The Stanford physicist champions the idea of a "megaverse," a sea of pocket universes whose local environments correspond to the myriad solutions offered by string theory. Rather than seek a unique theory that somehow allows for our existence, he argues, physicists should consider a landscape of parallel universes in which the "local weather" is, here and there, hospitable to life.

The landscape idea has sparked an ongoing debate. American Scientist Online managing editor Greg Ross interviewed Susskind by e-mail in December 2005.

Briefly, what is the anthropic principle?

Here is what I mean by it. It is also what physicists/cosmologists like Steven Weinberg, Andrei Linde, Sir Martin Rees, Alex Vilenkin and Alan Guth mean:

1. The universe is tremendously big—much bigger than the 10 billion light-years that we are directly aware of by astronomical observation. The evidence for this is extremely strong and comes from the recent quantitative success of inflationary cosmology. In fact the universe is almost certainly so big that the observable portion of it is an infinitesimal fraction of the whole.

2. Inflation also tends to make the universe diverse. I mean that on enormous scales, it tends to make the universe more like a patchwork crazy quilt of diverse environments than a uniform homogeneous blanket. Of course it cannot make the universe more diverse in its properties than what is allowed by the equations of the theory. For example, in a smaller context, the Big Bang created diversity—hot stars, cold voids, giant gas clouds, a wide variety of planets, black holes, etc. All of these environments are solutions of the equations of physics. It did not create planets with antigravity or places where 1+1=3. So there are limits. In the bigger context diversity means variety in the properties of elementary particles and the constants of nature.

3. The basic equations that control that spectrum of particles and constants do admit a very large number of solutions. The space of these possibilities is what I have called "the landscape."

4. Given 1, 2 and 3, it is certain that some features of what we ordinarily call the laws of physics will turn out to be local environmental facts contingent on our particular region. If this is so, then the explanation of why a certain constant—the cosmological constant, for example—has its value, will be the following: The CC has one value in this patch, some other value in that patch and yet another value in some other patch. Our kind of life can only form in a narrow range of values, and so we find ourselves in such a region.

That's it. That's all it means. I hope that's brief enough.

So it's not that the universe is somehow contorting itself to accommodate us; it's just a diverse place and we find ourselves in a friendly corner. Is that right?

Right! But it's not a done deal. The question of whether the universe is a crazy quilt or a mono-colored blanket is still not definitively answered, although things are pointing toward the crazy quilt. I doubt that it will be settled for a good long time.

How does your idea of a cosmic landscape address these questions?

First let me say that the landscape is not only my idea. It's true that I named it, but many physicists, especially Raphael Bousso, Joe Polchinsky, most of my fellow theorists at Stanford and many more have contributed to it.

The notion of a landscape is an old one used both by biologists and by molecular physicists/chemists. Thus "the landscape of biological designs" or "the landscape of molecular configurations." It simply means the space of possibilities. In both these examples the landscape is huge. The number of real species is large, but the number of possible species is immeasurably larger. Similarly, the number of arrangements of 1,000 atoms to form metastable molecules is also tremendously large. In a similar vein, the number of solutions of string theory appears to be similarly large. By solutions, I mean the number of possible environments it can describe. Thus string theory is a natural candidate to address point 3.

You've noted that cosmologists are more receptive to this idea than physicists are. Why is this?

Well, perhaps it's because physicists tend to look down at their equations, whereas cosmologists look up at the universe. Cosmologists are like naturalists who look at the details of nature and try to find patterns in their observations. Theoretical physicists have a bit of the aesthete in them. They are preoccupied with the form and beauty of their mathematical formulas. The two complement each other, but this time I think the cosmologists have the advantage. By now almost every theoretical physicist (of the kind that studies gravity, elementary particles, string theory and cosmology) that I know has reluctantly acknowledged the likelihood (not the certainty) that some aspects of the usual laws of physics may be contingent on our local place in the universe.

But I might add that I am not sorry that theoretical physicists are unready to throw in the towel. More than anything I think we need a good hard battle of ideas to help clarify things. I only wish the elegant-math enthusiasts had an idea.

How do you respond to critics who see the anthropic approach as quasi-religious or unscientific?

I cannot put it better than Steven Weinberg did in a recent paper:

Finally, I have heard the objection that, in trying to explain why the laws of nature are so well suited for the appearance and evolution of life, anthropic arguments take on some of the flavor of religion. I think that just the opposite is the case. Just as Darwin and Wallace explained how the wonderful adaptations of living forms could arise without supernatural intervention, so the string landscape may explain how the constants of nature that we observe can take values suitable for life without being fine-tuned by a benevolent creator. I found this parallel well understood in a surprising place, a New York Times op-ed article by Christoph Schönborn, Cardinal Archbishop of Vienna. His article concludes as follows:

Now, at the beginning of the 21st century, faced with scientific claims like neo-Darwinism and the multiverse hypothesis in cosmology invented to avoid the overwhelming evidence for purpose and design found in modern science, the Catholic Church will again defend human nature by proclaiming that the immanent design evident in nature is real. Scientific theories that try to explain away the appearance of design as the result of "chance and necessity" are not scientific at all, but, as John Paul put it, an abdication of human intelligence.

There is evident irony in the fact that the cardinal seems to understand the issue much better than some physicists.

David Gross of UC Santa Barbara says, "Science has managed to explain lots of other weird numbers—so why shouldn't we expect eventually to explain the cosmological constant and other key parameters?"

David is entirely correct in one respect. The views that I have expressed are far from rigorous scientific facts. The observational evidence for a cosmological constant, for inflation, and the mathematical evidence for a string theory landscape could all evaporate. So far they show no signs of doing so, but surprises happen. It is certainly premature to declare victory and close the question. I would be very worried if all theoretical physicists "gave up" (as David puts it) looking for a mathematical explanation for the "weird" value of the cosmological constant. But I think David exaggerates when he claims that science has explained anything like the fine-tuning of the cosmological constant.

Some physicists say the landscape idea lacks "beauty" or "elegance." Is that a fair criticism?

It's a silly criticism. One should not decide the truth of a scientific proposition by appeal to someone's aesthetic sensibilities. Einstein said, "If you are out to describe the truth, leave elegance to the tailor." Similarly Thomas Huxley referred to "the great tragedy of science—the slaying of a beautiful hypothesis by an ugly fact."

Without experimental data, how will we ever decide these questions?

Ah, now we come to the heart of the matter. How do we find out? Definitive evidence of the rest of the multiverse is out of the question. My guess is that over time we will learn more about the physical basis for inflation, and much of it will come from observational data. We may find deviations from the simplest inflation models that might tell us about how inflation began. We may be able to use statistical properties of the landscape to make predictions about quantities that we have not yet measured. Anyway, I don’t grant you the conclusion that there will be no more experimental data.

But let's keep our focus on the question: Is the universe very large (apparently so) and diverse, or is it everywhere the same, with particles and constants uniquely determined by elegant mathematics? No one knows for sure. Both are hypotheses. It is just as hard to confirm or falsify one as the other. Recent developments have favored the former, but the question is still open. Honestly, I don't know how it will be definitively decided. But quite frankly, I find the unique/elegant solution to be far more faith-based at the present time than the diversity solution.

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