Is string theory in trouble?

Why are physicists taking the idea of multiple universes seriously now?

First, there was the discovery in the past few years that inflation seems right. This theory that the universe expanded spectacularly in the first fraction of a second fits a lot of data. Inflation tells us that the universe is probably extremely big and necessarily diverse. On sufficiently big scales, and if inflation lasts long enough, this diversity will produce every possible universe. The same process that forged our universe in a big bang will happen over and over. The mathematics are rickety, but that's what inflation implies: a huge universe with patches that are very different from one another. The bottom line is that we no longer have any good reason to believe that our tiny patch of universe is representative of the whole thing.

Second was the discovery that the value of the cosmological constant - the energy of empty space which contributes to the expansion rate of the universe - seems absurdly improbable, and nothing in fundamental physics is able to explain why. I remember when Steven Weinberg first suggested that the cosmological constant might be anthropically determined - that it has to be this way otherwise we would not be here to observe it. I was very impressed with the argument, but troubled by it. Like everybody else, I thought the cosmological constant was probably zero - meaning that all the quantum fluctuations that make up the vacuum energy cancel out, and gravity alone affects the expansion of the universe. It would be much easier to explain if they cancelled out to zero, rather than to nearly zero. The discovery that there is a non-zero cosmological constant changed everything. Still, those two things were not enough to tip the balance for me.

What finally convinced you?

The discovery in string theory of this large landscape of solutions, of different vacuums, which describe very different physical environments, tipped the scales for me. At first, string theorists thought there were about a million solutions. Thinking about Weinberg's argument and about the non-zero cosmological constant, I used to go around asking my mathematician friends: are you sure it's only a million? They all assured me it was the best bet.

But a million is not enough for anthropic explanations - the chances of one of the universes being suitable for life are still too small. When Joe Polchinski and Raphael Bousso wrote their paper in 2000 that revealed there are more like 10500 vacuums in string theory, that to me was the tipping point. The three things seemed to be coming together. I felt I couldn't ignore this possibility, so I wrote a paper saying so. The initial reaction was very hostile, but over the past couple of years people are taking it more seriously. They are worried that it might be true.

Steven Weinberg recently said that this is one of the great sea changes in fundamental science since Einstein, that it changes the nature of science itself. Is it such a radical change?

In a way it is very radical but in another way it isn't. The great ambition of physicists like myself was to explain why the laws of nature are just what they are. Why is the proton just about 1800 times heavier than the electron? Why do neutrinos exist? The great hope was that some deep mathematical principle would determine all the constants of nature, like Newton's constant. But it seems increasingly likely that the constants of nature are more like the temperature of the Earth - properties of our local environment that vary from place to place. Like the temperature, many of the constants have to be just so if intelligent life is to exist. So we live where life is possible.

For some physicists this idea is an incredible disappointment. Personally, I don't see it that way. I find it exciting to think that the universe may be much bigger, richer and full of variety than we ever expected. And it doesn't seem so incredibly philosophically radical to think that some things may be environmental.

In order to accept the idea that we live in a hospitable patch of a multiverse, must a physicist trade in that dream of a final theory?

Absolutely not. No more than when physicists discovered that the radii of planetary orbits were not determined by some elegant mathematical equation, or by Kepler's idea of nested Platonic solids. We simply have to reassess which things will be universal consequences of the theory and which will be consequences of cosmic history and local conditions.

So even if you accept the multiverse and the idea that certain local physical laws are anthropically determined, you still need a unique mega-theory to describe the whole multiverse? Surely it just pushs the question back?

Yes, absolutely. The bottom line is that we need to describe the whole thing, the whole universe or multiverse. It's a scientific question: is the universe on the largest scales big and diverse or is it homogeneous? We can hope to get an answer from string theory and we can hope to get some information from cosmology.

There is a philosophical objection called Popperism that people raise against the landscape idea. Popperism [after the philosopher Karl Popper] is the assertion that a scientific hypothesis has to be falsifiable, otherwise it's just metaphysics. Other worlds, alternative universes, things we can't see because they are beyond horizons, are in principle unfalsifiable and therefore metaphysical - that's the objection. But the belief that the universe beyond our causal horizon is homogeneous is just as speculative and just as susceptible to the Popperazzi.

Could there be some kind of selection principle that will emerge and pick out one unique string theory and one unique universe?

Anything is possible. My friend David Gross hopes that no selection principle will be necessary because only one universe will prove to make sense mathematically, or something like that. But so far there is no evidence for this view. Even most of the hard-core adherents to the uniqueness view admit that it looks bad.

Is it premature to invoke anthropic arguments - which assume that the conditions for life are extremely improbable - when we don't know how to define life?

The logic of the anthropic principle requires the strong assumption that our kind of life is the only kind possible. Why should we presume that all life is like us - carbon-based, needs water, and so forth? How do we know that life cannot exist in radically different environments? If life could exist without galaxies, the argument that the cosmological constant seems improbably fine-tuned for life would lose all of its force. And we don't know that life of all kinds can't exist in a wide variety of circumstances, maybe in all circumstances. It a valid objection. But in my heart of hearts, I just don't believe that life could exist in the interior of a star, for instance, or in a black hole.

Is it possible to test the landscape idea through observation?

One idea is to look for signs that space is negatively curved, meaning the geometry of space-time is saddle-shaped as opposed to flat or like the surface of a sphere. It's a long shot but not as unlikely as I previously thought. Inflation tells us that our observable universe likely began in a different vacuum state, that decayed into our current vacuum state. It's hard to believe that's the whole story. It seems more probable that our universe began in some other vacuum state with a much higher cosmological constant, and that the history of the multiverse is a series of quantum tunnelling events from one vacuum to another. If our universe came out of another, it must be negatively curved, and we might see evidence of that today on the largest scales of the cosmic microwave background. So the landscape, at least in principle, is testable.

If we do not accept the landscape idea are we stuck with intelligent design?

I doubt that physicists will see it that way. If, for some unforeseen reason, the landscape turns out to be inconsistent - maybe for mathematical reasons, or because it disagrees with observation - I am pretty sure that physicists will go on searching for natural explanations of the world. But I have to say that if that happens, as things stand now we will be in a very awkward position. Without any explanation of nature's fine-tunings we will be hard pressed to answer the ID critics. One might argue that the hope that a mathematically unique solution will emerge is as faith-based as ID.
From issue 2530 of New Scientist magazine, 17 December 2005, page 48
Leonard Susskind

Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at Stanford University in California. His book Cosmic Landscape: String theory and the illusion of intelligent design is published this week by Little, Brown ($24.95, £14.33, ISBN 0316155799)

That is a cosmological battle - Big Crunch or Cold Empty Universe. In other words, will the Universe end in fire or ice (metaphorically speaking). Data suggests that (based on our current thinking), the universe will gradually drift away into nothingness.

Which reminds me of the Robert Frost poem:

Some say the world will end in ice,
some say it will end in fire,
And having tasted desire,
I hold with those who say fire.

***I've been doing research for my doctoral on g-string theory. Now, if I can just find a school which offers that as a major...***

Have you checked the University of Phoenix, they seem to have a Major for everything.

Little Rock, AR has a vast amount of information on file with respect to everything related to this subject ... in the Clinton Library.

Why are physicists taking the idea of multiple universes seriously now? ... Without any explanation of nature's fine-tunings we will be hard pressed to answer the ID critics.

Take out all the imaginary scientism between the two quotes and there you go! Not so hard after all.

By the way, the argument that we find the universe improbably friendly to life because we are alive in this universe is neither an explanation nor an argument, but merely begs the question. In these imaginary other universes, does conscious life exist in gasseous form in starless space? How about flying monkeys? In an infinite number of universes, surely there is one with flying monkeys! Cthulu? Wise and patriotic Democracts?

In an infinite number of universes, surely there is one with flying monkeys!

I can't get my head around infinity. I prefer to speculate (and all of this is in the realm of speculation) on the possibility of a large but finite number of other bubbles and also prefer to think of us and our monkeys (flying or otherwise) as unique in all the cosmos. But that's just my particular fancy and means nothing.

What is interesting is that people who "know better" (that is, those who can do the math) are speculating along the lines of this article.

the argument that we find the universe improbably friendly to life because we are alive in this universe is neither an explanation nor an argument, but merely begs the question.

You are right. It's not an argument, or an explanation. It is a speculation. And to my mind, an interesting one. Frankly, more interesting than the supposition that a book written by the scholary members of a nomadic desert tribe a few thousand years ago actually specifies the dynamics of the universe.

To my mind (and I'm not trying to win an argument, merely justify my own speculations), it makes more sense to toy with ideas of alternate big-bangs (in which some get the physical constants "right for life" and others don't), than to believe that a book written at the dawn of mankinds erudition correctly lists the technical specifications of our cosmos.

inflation implies: a huge universe with patches that are very different from one another. The bottom line is that we no longer have any good reason to believe that our tiny patch of universe is representative of the whole thing.

What would differ in those "anomolous" patches? Would they not conform to the same physical laws that govern this "patch?" Are we supposed to believe that Relativity extends to the very fabric of reality, and that Truth itself varies from locale to locale?

Would an intelligent observer from Tau Ceti IX see a universe governed by different laws, arising from completely different origins? Can we no longer rely on the assumption that certain values are immutable and universal?

Intriguing, but I suspect more of a parlor exercise than a physical reality.

The idea is that scientists don't want a universe with a concrete beginning: it implies a creation event, and since nothing creates itself, it can imply a Creator, or another universe creating it, but then, what created that other universe (since nothing creates itself)--another Creator?

Steady State (or any pop theory amounting to such) is an elegant mathematical way to commit the logical fallacy of "begging the question" by not addressing: What was the First Cause?

Remember: Nothing creates itself.

Remember: There is no such thing as infinity.

Q.E.D., Kalam Cosmological Argument, q.v.

Sauron

Sure. And it's really a fool's errand to try to understand the formation of the Universe without understanding what is outside the Universe.

It's like trying to reconcile where the interior of your house came from (if it's all you've ever known) if you have no understanding of anything outside your house.

"We still aren't certain that the universe collapses are we?"

Not certain, but things are looking bad for the Big Crunch. Our expansion rate appears (recent observations) too high,

I thought a couple of years ago scientists were puzzled by measurements that the expansion of the universe was speeding up, accelerating. This flew in the face of gravity which should decelerate the rate. I have not heard any contrary reports since. I'd settle for an answer on the driving force behind this acceleration as more useful than wondering if the universe is antropical.

But the belief that the universe beyond our causal horizon is homogeneous is just as speculative and just as susceptible to the Popperazzi.

Seems true. Nice turn of phrase, too. In my falsificationist persona, I suppose I am one of them.

But does the act of observation change the results of the observation, per Shroedinger? Theoretically, if "reality" changed as we changed location, then the "new" reality would not even be observable to us. Or more precisely, since there would be nothing to contrast it with, the DIFFERENCE would not be noticeable. I can't imagine there's some physical threshold beyond which Reality A becomes Reality B, so the change would have to be gradual as A faded out and B emerged. What do physical laws become in the transition?

I don't know that this is a straightforward Shroedinger case, but it does pose some intriguing questions.

the experimental method still works for events inside our universe, which are the only events we can see or test anyway

like i said, this is speculation, but i personally find it interesting.

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