It’s
sometimes said that supersymmetry would be a kind of thumbs-up for
string theory, which has been impossible to test in any direct way. If
the LHC finds no evidence for supersymmetry, what happens to string
theory?
Damned if I know! Unfortunately, string theory doesn’t make very
specific predictions about physics at the energies that are accessible
to us. The kind of energies of the structures that string theory deals
with are so high, we’ll probably never be able to reproduce them in the
lab. But those energies were common in the very early universe. So by
making cosmological observations, we may get a handle on the physics of
those incredibly high energies. For example, if the matter-energy
density at the time of inflation was
of the order of magnitude that is characteristic of string theory, then
a great deal of gravitational radiation would have been produced at
that time, and it would have left an imprint on the cosmic microwave
background. Last year, scientists working with the BICEP2 telescope announced that they had found these gravitational waves; now it seems they were actually measuring interstellar dust. Further observations with the Planck satellite
may be able to settle this question. I think that’s one of the most
exciting things going on in all of physical science right now.
For theorists, is the ultimate goal a set of equations we could put on a T-shirt?
That’s the aim. The Standard Model is so complex that it would be hard
to put it on a T-shirt—though not impossible; you’d just have to write
kind of small. Now, it wouldn’t take gravity into account, so it
wouldn’t be a “theory of everything.” But it would be a theory of all
the other things we study in our physics laboratories. The Standard
Model is sufficiently complicated, and has so many arbitrary features,
that we know it’s not the final answer. The goal would be to have a much
simpler theory with fewer arbitrary features—maybe even none at
all—that would fit on a T-shirt. We’re not there yet.
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array of different theories, perhaps representing different solutions
to string theory’s equations. Maybe each solution represents a different
universe—part of some larger “multiverse.”
I am not a proponent of the idea that our Big Bang universe is just part
of a larger multiverse. It has to be taken seriously as a possibility,
though. And it does lead to interesting consequences. For example, it
would explain why some constants of nature, particularly the dark energy,
have values that seem to be very favorable to the appearance of life.
Suppose you have a multiverse in which constants like dark energy vary
from one big bang to another. Then, if you ask why it takes the value it
does in our Big Bang, you have to take into account that there’s a
selection effect: It’s only in big bangs where the dark energy takes a
value favorable to the appearance of life that there’s anybody around to
ask the question.
This is very closely analogous to a question that astronomers have
discussed for thousands of years, concerning the Earth and the sun. Why
is the sun the distance that it is from us? If it were closer, the Earth
would be too hot to harbor life; if it were further away, the Earth
would be too cold. Why is it at just the right distance? Most people,
like Galen, the Roman physician, thought that it was due to the
benevolence of the gods, that it was all arranged for our benefit. A
much better answer—the answer we would give today—is that there are
billions of planets in our galaxy, and billions of galaxies in the
universe. And it’s not surprising that a few of them, out of all those
billions, are positioned in a way that’s favorable for life.
But at least we can see some of those other planets.
That’s not the case with the universes that are said to make up the
multiverse.
It’s not part of the requirement of a successful physical theory that
everything it describes be observable, or that all possible predictions
of the theory be verifiable. For example, we have a very successful
theory of the strong nuclear forces, called quantum chromodynamics
[QCD], which is based on the idea that quarks are bound together by
forces that increase with distance, so that we will never, even in
principle, be able to observe a quark in isolation. All we can observe
are other successful predictions of QCD. We can’t actually detect
quarks, but it doesn’t matter; we know QCD is correct, because it makes
predictions that we can verify.
Similarly, string theory, which predicts a multiverse, can’t be
verified by detecting the other parts of the multiverse. But it might
make other predictions that can be verified. For example, it may say
that in all of the big bangs within the multiverse, certain things will
always be true, and those things may be verifiable. It may say that
certain symmetries will always be observed, or that they’ll always be
broken according to a certain pattern that we can observe. If it made
enough predictions like that, then we would say that string theory is
correct. And if the theory predicted a multiverse, then we’d say that
that’s correct too. You don’t have to verify every prediction to know
that a theory is correct.
When we talk about the multiverse, it seems as though
physics is brushing up against philosophy. A number of physicists,
including Stephen Hawking and Lawrence Krauss, have angered philosophers
by describing philosophy as useless. In your new book, it sounds as if
you agree with them. Is that right?
I think academic philosophy is helpful only in a negative sense—that is,
sometimes physicists get impressed with philosophical ideas, so that it
can be helpful to hear from experts that those ideas have been
challenged within the philosophical community. One example is positivism,
which decrees that you should only talk about things that are directly
detectable or observable. I think philosophers themselves have
challenged that, and it’s good to know that.
On the other hand, a kind of philosophical discussion does go on
among physicists themselves. For example, the discussion we were having
earlier about the multiverse raised the issue of what we expect from a
scientific theory—when do we reject it as being outside of science; when
do we accept it as being confirmed. Those are meta-scientific
questions; they’re philosophical questions. The scientists never seem to
reach an agreement about those things—like in the case of the
multiverse—but then, neither do the professional philosophers.
And sometimes, as with the example of positivism, the work of
professional philosophers actually stands in the way of progress. That’s
also the case with the approach known as constructivism—the idea that
every society’s scientific theories are a social construct, like its
political institutions, and have to be understood as coming out of a
particular cultural milieu. I don’t know whether you’d call it a
philosophical theory or a historical theory, but at any rate, I think
that view is wrong, and I also think it could impede the work of
science, because it takes away one of science’s great motivations, which
is to discover something that, in an absolute sense, divorced from any
cultural milieu, is actually true.
You’re 81. Many people would be thinking about retirement, but you’re very active. What are you working on now?
There’s something I’ve been working on for more than a year—maybe it’s
just an old man’s obsession, but I’m trying to find an approach to
quantum mechanics that makes more sense than existing approaches. I’ve
just finished editing the second edition of my book, Lectures on Quantum Mechanics,
in which I think I strengthen the argument that none of the existing
interpretations of quantum mechanics are entirely satisfactory.
I don’t intend to retire, because I enjoy doing what I’m doing. I
enjoy teaching; I enjoy following research; and I enjoy doing a little
research on my own. The year before last, before I got onto this quantum
mechanics kick, I was writing papers about down-to-earth problems in
elementary particle theory; I was also working on cosmology. I hope I go
back to that.
