Introduction
Although this site contains factual knowledge about string theory, we thought that we should expose you to what others – more specifically, actual string theorists and physicists- have to say about string theory.
Dr. Sunil Mukhi
What have been the most exciting recent developments in string theory and M-theory?
(i) The discovery of "D-branes", dynamical objects that are extended like membranes, and that necessarily exist in string theory. D-branes are a powerful tool to probe string theory beyond the perturbative approximation.
(ii) The discovery of "gauge-gravity correspondence" or "holography", according to which a theory of gravity in a given spacetime is in every way equivalent to a theory of gauge fields localised on the boundary of spacetime. This means that at a deep level, all information about a given volume of space can be encoded in data placed on the surface bounding that volume - totally unexpected, and intimately related to the nature of gravitation itself.
(iii) Noncommutativity, the notion that different spatial directions do not commute when multiplied with each other, unlike ordinary numbers. This challenges our very notion of the physical meaning of space.
(iv) Tachyon condensation - the fact that unstable objects in string theory (for example, certain types of D-branes) support a "tachyonic" particle excitation which can be used to describe the decay process.
Why are you so interested in string theory?
The beauty and elegance of its structure, and its closeness to all the basic facts of physics that we know, suggests it could be the greatest discovery ever made about the laws of nature.
What work are you doing on string theory currently?
I am investigating the role of nonperturbative effects in some simplified string models.
What do you see for the future of string theory?
It will take a long time and go through several ups and downs in popularity before it can be regarded as verified in any way. Advances in experiments, for example at new accelerators or in probes of the cosmos, will play an important role.
Do you think string theory is considered legitimate in many physics circles?
I suppose it is considered "legitimate", but many people outside the field feel there is too much hype about it and believe that it is still far away from being a serious part of physics, because of its lack of experimental confirmation.
How can one become involved with string theory or study string theory at the university and professional level?
This is a linear process - there is no way I know of to "jump in" to a subject of this complexity. One needs to get an undergraduate degree in physics, and then enroll in a Master's and Ph.D. programme. On the way, a thorough education in conventional elementary particle physics and quantum field theory is essential. Some advanced mathematical training also helps. By the time one gets a Ph.D., one would have grasped much (though not all) of the basic picture.
String theory works in a ten dimensional space. How can we imagine these extra dimensions? Are there any indications that we can actually detect or “feel” these extra dimensions, like the four dimensions we currently experience?
Extra dimensions, because they would be curled up into a very small space, would not be accessible to the senses of human beings. But then, elementary particles are not directly accessible to our senses either. In both cases, there are indirect ways to detect them. Detection (of a particle, or of a dimension) is done by probing with a test particle. If there are extra dimensions, a sufficiently energetic test particle will traverse them and we will know this by reconstructing its history.
How was it determined that string theory required ten to twenty-six dimensions- are three dimensions not sufficient?
String theory is inconsistent in less than ten flat dimensions because of a "quantum anomaly", a mathematical effect that renders a quantum theory inconsistent even if the corresponding classical theory was consistent. Such anomalies arise because of extreme subtleties in applying the laws of quantum mechanics to extended objects.
Why has combining general relativity and quantum mechanics been such a problem for physicists?
This is a problem with quantum field theory, again it is a subtle inconsistency which develops when we try to take the well-known classical theory of general relativity and convert it to a quantum theory. It is intimately tied up with the fact that the quantum of general relativity, the "graviton", is a particle of spin 2 units unlike the quanta of other basic interactions, such as the photon, which has spin 1. But the root of the problem cannot be described in lay language, it is essentially technical.
Do you believe that string theory is a philosophy or a science due to the controversial fact that string theory cannot be confirmed experimentally?
It's not a philosophy -- it seeks to empirically describe the real world.
How can we prove string theory? Will this happen in the next century?
I hope it happens earlier. But of course we will never "prove" string theory, we can at best confirm more and more of its predictions and become more and more convinced that it is right.
Besides possibly being the Theory of Everything, where does the “beauty” lie in string theory? How do we know that this world of strings is not in fact very dull and uninteresting?
What makes string theory beautiful is that it uses elegant mathematics and describes a world that is (in a very gross, not detailed, sense) like ours. In this sense it possesses both mathematical and physical beauty. The world of strings has already proved to be full of strikingly pretty and unexpected phenomena, even though we don't know which of them are realised in nature. It feels like watching a movie where by the first ten minutes you know it's going to be fantastic, and every additional minute proves to be more fantastic, but this movie may last a century and so we may never see the end of it.
Do you think that string theory is the Theory of Everything? Is it reasonable to think that we will forever continue to discover smaller structures, more dimensions, and more accurate theories?
There is no reason why there should be ever smaller structures, just because there have been in the past. Nature avoids using the same device too many times, since she hates to be boring. A more serious argument is that physics at the scale of quantum gravity (the "Planck scale") is largely un-understood, but via string theory it will probably be understood, and then there might be no more surprises at a fundamental level since we have no reason to suspect there is another energy scale in nature beyond the Planck scale.
This would not mean the end of physics, only the end of discovering the fundamental structures in it. Physics has such complexity that knowing the fundamental structures hardly solves any practical problem - for example, we know little about how the brain works, though we are quite sure it is governed by fundamental laws of physics that we already understand.
Sunil Mukhi
26 August 2004
Dr. Michael Peskin
When did you first become interested in string theory?
As you may have learned from Brian Greene's book, string theory started out
as a theory of the strong interactions. The "Veneziano amplitude", the first
result in string theory, was written down in 1967 by Veneziano as the solution
to a problem in the strong interactions. By 1973, when I started graduate
school, it was understood that the strong interactions were based on
quarks and gluon, but string theory was an interesting paradigm for the
confinement of quarks into hadrons. Because I was interested in the
mechanism of quark confinement, I spent a lot of time studying string
theory, in particular, from the review paper of Joel Scherk
(Reviews of Modern Physics, vol. 47, p. 123 (1975)).
It was around this time that Scherk and John Schwarz realized that string
theory made more sense as a theory of gravity (better, as a theory of
everything). But it took a lot of time for this to be clear to the rest
of the physics community. Certainly I did not accept this, or the idea
that string theory necessarily required extra dimensions of space, until
after the 1981 work of Polyakov. Polyakov's paper was a real breakthrough
and suggested an very interesting program of research, which I then
devoted a lot of time to.
What has been the focus of your study with string theory and quantum field theory?
As you can see from my Web site, my main interest is in the question of
what is the next set of fundamental interactions to be discovered in
high-energy physics experiments. At the moment, we have the gravitational
and electromagnetic interactions, which have very long range, the strong
interactions, which have a range of about 10^{-13} cm, and the weak
interactions, which have a range of about 10^{-16} cm. The strong and
weak interactions both play important roles in the structure of atomic
nuclei. From our understanding of the weak interaction, it is clear that
there must be another set of fundamental forces with a range of about
10^{-17} cm.
These forces do not act strongly on quarks and leptons (at
least, not on the electron and the up and down quarks that comprise ordinary
matter), but they act strongly on the W and Z bosons and are responsible for
the fact that these particles have large masses. Some people think that
the masses of W and Z bosons come from the action of one new particle,
the Higgs boson. But I feel that the model with one Higgs boson is much
too simplistic and what we really need is a new system of particles and
forces. Various approaches to this problem involve a new set of strong
interactions, new symmetries such as "supersymmetry", or extra space
dimensions beyond the usual 3 space dimensions.
In addition, our current theory of quarks and leptons has many mysteries: Why do we have 6 quarks and 6 leptons? Why do these particles have the mass spectrum that that do? Why do these particles act through vector bosons with precisely the symmetry groups of the strong and weak interactions?
Ordinary quantum field theory does not give insight into this latter class
of problems. In quantum field theory, you can have any number of quarks
and leptons, any spectrum of masses, etc. However, string theory is a
much more restrictive structure that, in principle, could predict the
number of particles and their fundamental symmetry groups. I would like
very much to know what kind of elementary particles theories can be built
from string theory.
In addition, string theory naturally contains supersymmetry and extra
dimensions. It can give us insights into how these elements behave and
how they can play a role in physical theories.
Finally, I am interested in what would be the "ultimate" formalism underlying
string theory. From Brian Greene's book, you learned that space-time
geometry is a derived notion in string theory. Similarly, the electromagnetic,
strong, and weak interactions are derived quantities that follow from
solving the string equations of motion. So, what is the fundamental
idea on which string theory is built? We don't know; this is an open
problem.
Does string theory really require mathematics that has [not] been developed yet? If so, how long do you believe it will take for physicists and mathematicians to develop the appropriate mathematics?
In the science-fiction book "Green Mars", by Kim Stanley Robinson, scientists
in the mid-22nd century finally have the mathematics to do string theory
correctly. I am not so pessimistic that are present tools are inadequate.
But it is certainly true that string theory depends on mathematics,
in particular, areas of algebraic geometry, that has been developed very
recently and is still under development. Some interesting theorems in
algebraic geometry and algebraic topology have been suggested by string
theory. There are some geometrical problems (e.g. in how many ways
topologically distinct ways can a 2-dimensional complex surface without
boundary be embedded in a fixed 6-dimensional manifold?) that only have
solutions from string theory and are much more difficult with "conventional"
mathematical methods. So there are areas of string theory and mathematics
that are advancing in parallel, with each side getting insight from the
other.
Either string theory is the Theory of Everything or it is the theory of nothing.Do you believe that string theory is truly the Theory of Everything?
Yes, but it is just a belief, without real evidence to support it.
One thing is known---it is not possible to couple outside elements to
string theory. So string theory cannot be a description of a part of
Nature. Either string theory is the Theory of Everything or it is the
theory of nothing.
Do you think that string theory has a legitimate future? How long will it take before string theory appears in physics textbooks?
For string theory to be taken seriously outside the community of mathematical
physicists, some important aspect of string theory---for example, supersymmetry
or extra dimensions---needs to be discovered in experiments. It is
possible that we could see signs of quarks and leptons behaving like
strings. I've written a paper about this: Phys. Rev. D vo. 62, p. 055012
(2000), but I feel that that is not very likely. However, the discovery
of a profound new symmetry in Nature could come as soon as 2008 when
the LHC at CERN begins operation.
How can an interested student or individual learn more about string theory?
Barton Zwiebach, a professor at MIT, has written an introductory textbook
on string theory that has just appeared from Cambridge U. Press.
Here is the listing on amazon.com:
http://www.amazon.com/exec...details
I hope this is useful to you.
On supersymmetry, there is a popular book, at a rather low level, by
Gordon Kane,
http://www.amazon.com/exec...books
For a more detailed discussion, you might look at my review:
http://xxx.lanl.gov/abs/hep-ph/hep-ph/0212204
Dr. Sean Carroll
Why are you so interested in string theory?
In physics right now we have two theories that are impressive and powerful -- the Standard Model of particle physics, which is based on quantum mechanics, and general relativity, which describes gravity.
Unfortunately, these theories are incompatible with each other. The biggest unanswered question in fundamental physics is how we can reconcile quantum mechanics and general relativity. String theory is by far the leading candidate.
How did you first gravitate towards this field?
My primary interest is in cosmology, which studies the beginning of the universe. In the vicinity of the Big Bang, when the universe was very small, both quantum mechanics and general relativity were necessarily important. We will need to understand string theory, or something like it, in order to truly understand the Big Bang.
Do you foresee any monumental breakthroughs in the field of string theory or M-theory in the near future?
You can never predict monumental breakthroughs! Thus far, in the brief history of string theory, we have been fortunate enough to consistently find breakthroughs that have greatly increased our understanding of the theory. What we don't have is any direct, detailed experimental results pertaining to quantum gravity, nor are there any on the horizon.
As long as we are dependent purely on theoretical work, it will be very hard to predict when progress will occur.
What are the implications when string theory and the field of cosmology are studied jointly?
As mentioned above, cosmology and string theory will both be required to understand the beginning of our universe. It works both ways: just as cosmology will require string theory, progress in string theory is spurred by discoveries in cosmology (for example, that the universe is accelerating).
Do you personally believe that string theory is the "theory of everything" or is it simply a stepping-stone in the pursuit of an even more comprehensive theory?
We have no way of knowing. String theory has at least a chance of being the theory of everything, since it can account for gravity as well as all of the other known forces of nature. But it might turn out to be completely wrong, or just the first step in a very long journey.
Is string theory awarded merit in scientific circles (other than those that specifically study string theory)?
Sure. Some physicists are very skeptical of string theory, and some are very optimistic. But it is predominantly respected; otherwise, you wouldn't see large collections of string theorists at all the most prestigious universities.
Is string theory (or M-theory) the greatest idea to be generally hidden from the majority of the public? How do you believe that we can inform people about the potential greatness of this theoretical theory?
I don't know what the greatest ideas are, and I don't think string theory is really hidden. It's true that string theory has not been made as consistently accessible as different topics in biology or cosmology, but that's largely because it is a highly abstract and mathematical subject, not easy to convey to a general audience. Brian Greene has done a great job in popularizing string theory with his books and PBS series, but it will take a concerted effort by many scientists to help the general public understand what is going on.