5 October 2006
The Questionable Science of String Theory
Among the most exciting scientific developments of the last twenty years is string theory,
a physical model for the universe that promises to explain the universe. Every popular science
magazine has writers professing its beauty and every university physics department has graduate
students banging away on its details. Why all this interest? Well, it could be the holy grail of
theoretical physics, the so-called Theory of Everything, the one central concept that explains all
physical phenomena. But there has been a steadily growing chorus of dissent among the ranks.
Many physicists don’t believe that it is a valuable use of time. Steven Weinberg sums up the
conflict perfectly: “there has been a division among physicists, not so much as to whether or not
string theory will ultimately be proved to be right or not, but as to whether it's worth working on
something that's so far removed from experimental reality.”
I was fortunate to come across a series of interviews with several physicists involved with
string theory, as well as some physicists not in the field. PBS carried out the interviews in
preparation for their three hour television program titled “The Elegant Universe.” The program
was inspired by Brian Greene’s popular book on the subject with identical title. NOVA is a
television series which discusses scientific topics. The show is intended for a general audience,
therefore no technical expertise is assumed and technical language is used only when needed.
Popular science as a whole may not necessarily be very reliable because it is often written
by those with no formal education in the sciences. One need not worry about NOVA, though.
The series has a high standard of quality and has won multiple Emmys and multiple Peabody
Awards (“Broadcast Awards”). In fact, the episodes on string theory have won both an Emmy
and a Peabody, as well as other awards. The interviews that I paraphrase and quote from are
published on the companion website. The website contains supplemental material and links to
further reading on the subject.
As for the specific scientists interviewed, Edward Witten is a mathematical physicist at
the Institute for Advanced Study in Princeton and has made significant contributions to string
theory. David Gross is a 2004 Nobel Prize winner and an important contributor to the theory.
Steven Weinberg was awarded the 1979 Nobel Prize in Physics. He is not working in string
theory but has optimism about the endeavor. Sheldon Glashow shared the 1979 Nobel Prize with
Weinberg. He is an outspoken critic of the theory.
When asked what string theory is, Edward Witten replies, “String theory is an attempt at
a deeper description of nature by thinking of an elementary particle not as a little point but as a
little loop of vibrating string.” To help illustrate, he talks about strings in musical instruments.
A piano and a violin can play the same note, but have distinct sounds because of the two
different methods the instruments use for causing vibrations in their strings. The unique timbre
of a stringed instrument is caused by higher frequencies above the fundamental frequency.
Analogously, different-looking particles could be represented as strings vibrating with different
frequencies. Witten's words are “Unity of the different forces and particles is achieved because
they all come from different kinds of vibrations of the same basic string.” The special properties
that make an electron different from a proton would be explained using the differing vibration
frequencies of that particle's strings.
Dr. Witten goes on to explain why this theory is better than any existing theories. This is
an evaluation argument in favor of studying string theory; it's better than anything else.
According to Einstein's general relativity, it is possible for space to bend, but there cannot be any
holes in space. Quantum mechanics, however, says that in rare cases, it is possible for holes in
space to appear for a brief amount of time. Dr. Witten says it has been shown that there are
examples that illustrate that string theory supports a more malleable topology than relativity
does. That is to say, string theory is more general than relativity, but has behavior similar to
quantum mechanics. The ability to bridge the gap between quantum mechanics and relativity
makes the theory very promising.
A counterargument to this sentiment is the fact that string theory has no experimental
evidence to date. The theory predicts that strings are extremely tiny, tinier even, than the Planck
length. (The Planck length is the smallest length that quantum mechanics says is possible to
observe reliably.) It would take an enormous amount of energy to probe distances that small
using particle accelerators. The amount of energy required would be “orders of magnitude
beyond the capability of any particle accelerator that will ever be built on Earth.” (Overbye)
Why would a person study something that might be just a bunch of hot air? When asked
whether string theory could possibly be wrong, Dr. Witten replies
I guess it's possible that string theory could be wrong. But if it is in fact wrong, it's
amazing that it's been so rich and has survived so many brushes with catastrophe and has
linked up with the established physical theories in so many ways, providing so many new
insights about them.
He feels that the fact that string theory has helped physicists better understand less exotic
theories, like quantum field theory, is an indication that it will eventually be proven correct.
When asked about experimental evidence for string theory, Dr. Witten insists that it does
make testable predictions. Supersymmetry is a very testable prediction made by string theory.
Supersymmetry implies that there are several unobserved types of particles that have yet to be
seen in particle accelerators. Finding evidence of these particles would lend support to string
theory. Astronomical observation is another option for testing the theory. He says that another
way to confirm the theory is by looking at cosmic background radiation. The cosmic
background radiation is radiation left over from the big bang. Analyzing it could give glimpses
into conditions during the big bang. This in turn could help verify whether the early universe
behaves according to string theory's model.
Sheldon Glashow has a different argument against string theory: it is harmful to the
culture of physicists. He remembers back in the sixties, while he was working at the University
of California in Berkeley, that theoretical physicists worked very closely with experimental
physicists. Theorists constructed theories that told experimentalists what to look for.
Experimentalists provided new data so that theorists could ensure that their theories agreed with
reality. This has changed. String theory deals with energies and sizes that current particle
accelerators cannot probe, so string theorists have become much more distant from the rest of the
physics community. He says, “Let me put it bluntly. There are physicists, and there are string
theorists.” He says that string theorists will not attend lectures on experimental physics. The
mathematics of cutting edge string theory are too complicated to follow unless one dedicates
oneself to studying it full time.
Dr. Glashow says this is a problem because string theory attracts the best and the
brightest minds in the field. But string theory as a culture is not terribly interested in
experiments. Now we are in a situation where the best and the brightest are not looking at real
numbers. Groundbreaking experimental data may not be recognized as groundbreaking. The
people who usually invent new theories to explain the experimental data are instead off behind
closed doors scribbling on a blackboard. Steven Weinberg agrees with Dr. Glashow on this
point. He says
It's really a pity that we have a generation of very bright theoretical physicists who work
on string theory and who, because of that work, become really detached from experiment,
don't follow experiment, don't really understand what's possible experimentally.
He does, however, make the important point that this is really the only way that string theorists
can make any headway. They are doing the best they can with what they have. Dr. Weinberg is
more optimistic than Dr. Glashow about string theory's potential. He is glad someone is studying
string theory, but he's also glad that not everyone is.
In its defense, string theory is almost universally described as beautiful and elegant. For
some, this is reason enough to study it. As Dr. Weinberg puts it, “I don't think it's ever happened
that a theory that has the kind of mathematical appeal that string theory has has turned out to be
entirely wrong.” But how can a mathematical framework be considered “elegant”? In a separate
interview with PBS ("A Conversation with Brian Greene"), physicist and popular author Brian
Greene explains elegance:
You know, when we talk about theories of physics being elegant, what we often
mean is that a theory is able to explain a wide range of phenomena using a very
small number of powerful ideas. The elegance comes from the tremendous reach
of these few simple ideas.
This sense of Spartan elegance is the key to understanding in science and mathematics. For this
reason, many string theorists feel that string theory is just too beautiful to be wrong.
Rather than giving a straight answer, David Gross recounts a little parable to
communicate the importance of beauty in scientific theories.
Einstein was once asked about some experiments that contradicted his theory of
relativity. Someone asked him, "So Professor Einstein, what would you say if these
turned out to be true and your theory of relativity was wrong?" And Einstein said, "Oh, I
would have been very disappointed that God didn't take advantage of this beautiful idea."
This is a narrative argument in support of string theory research, but it also makes an appeal to
authority. When considering whether someone deserves to be called a genius or not, that person
is inevitably compared to Einstein. Einstein is unarguably an authority on developing
groundbreaking scientific theories. He developed one himself, so he should know what's worth
pursuing! Dr. Gross piggybacks on Einstein's credibility in order to support his own viewpoint.
As stated in the beginning of this essay by Dr. Weinberg, the controversy here is not
whether or not string theory is an actual representation of the universe. That question will
eventually be answered by scientific inquiry. The question is whether the theory is worth
studying at all. This is a decision that every theorist must make for himself.
"A Conversation with Brian Greene." PBS. 2003. 3 Oct. 2006.
“Broadcast Awards Listed by Date.” PBS. 2005. 3 Oct. 2006.
Glashow, Sheldon. "Viewpoints on String Theory: Sheldon Glashow." PBS. 2003. 3 Oct. 2006.
Gross, David. "Viewpoints on String Theory: David Gross." PBS. 2003. 3 Oct. 2006.
Overbye, Dennis. "String Theory, at 20, Explains It All (or Not)." New York Times 7 Dec 2004,
late ed.: F1
Weinberg, Steven. "Viewpoints on String Theory: Steven Weinberg." PBS. 2003. 3 Oct. 2006.
Witten, Edward. "Viewpoints on String Theory: Edward Witten." PBS. 2003. 3 Oct. 2006.