**Edward Witten** is an American physicist who specialises in String Theory, Quantum Gravity and Supersymmetric Field Theories. He is the only physicist who has so far received the Fields Medal, one of the highest honours given to mathematicians. In this interview, Prof. Witten, who was in Bengaluru to attend the conference Strings 2015 organised by the International Centre for Theoretical Sciences, Bengaluru, speaks to **Shubashree Desikan.**

**In most of theoretical physics, advances are made in order to match some experiment, or, at least, experiments tell you at some point whether you are on the right track. With String Theory, this sort of guideline does not exist. On what basis do theorists make progress?**

Twentieth century physics consisted of two great theories — quantum mechanics, which describes atoms, molecules, and subatomic particles, and Einstein's theory of gravity (which he called General Relativity), which we use to understand stars, galaxies, and the universe as a whole.

These two theories are in conflict with each other. If one applies to Einstein's theory the textbook recipes of “quantisation,” one runs into contradictions.

Since stars (for example) are ultimately made of atoms and subatomic particles, it does not make sense to have one theory for the stars and one theory for the subatomic particles. String theory is the framework in which physicists have succeeded in reconciling Einstein's theory of gravity with quantum mechanics.

Although there are other circumstantial indications that string theory is on the right track — among other things from the elegance with which one can use string theory to construct unified theories of gravity unified with the other forces — the success of string theory in reconciling gravity with quantum mechanics is definitely the main reason that people are interested in it.

**Could an experimental test of string theory arise in the future? If so, which aspect might be tested?**

One important facet of string theory is supersymmetry. So, if we are lucky, perhaps supersymmetry will show up at the new run of the Large Hadron Collider, the one which has just started. On a more distant timescale — but this requires a large good fortune — Juan Maldacena gave a lecture at this conference [Strings 2015] about how signatures of string theory could appear in cosmology.

**How far are we from seeing these signatures in reality?**

Juan did describe such a proposal (based on work he did with Nima Arkani-Hamed). It would be great if this proposal pans out. However, it is probably a few decades away to develop the technology for the necessary measurements, and even then, they had to make slightly optimistic assumptions about cosmology in order for the signal they described to be detectably large. So one needs patience and some optimism.

Black holes, neutron stars, neutrinos, and gravitational waves were all considered hopelessly undetectable when they were first proposed. In each case, some difficult-to-anticipate developments (both technological developments and discoveries about the universe) went into making these things detectable.

**It has been 100 years since Einstein proposed the General Theory of Relativity. What are the main advances that have used this theory and what are the landmark discoveries in this field itself?**

We really could not understand cosmology without Einstein's theory. Moreover, Einstein's theory is tested in many detailed measurements in the Solar System. We also see it at work in “gravitational lensing” of distant galaxies. We've indirectly confirmed the theory of gravitational waves (in studies of the binary pulsar, leading to the Nobel Prize of Hulse and Taylor), and, of course, black holes, which we could not understand without Einstein's theory, have turned out to be important in astronomy.

**You have won the Fields Medal for a fundamental contribution to mathematics while being a physicist. Your physical insights have contributed to the development of math itself. Does this put the two subjects on par?**

Well, you have to remember, if you go back in history, physicists and mathematicians were the same people. And in the 18th Century and even early 19th Century, one who was educated in one subject was also educated in the other. But then things became more specialised, and in the 20th Century, physicists made discoveries that seemed to take physics far away from math. Math also became more specialised. But in our times, in the last few decades, discoveries involving quantum fields and strings have led physicists to ask new mathematical questions — applying math to physics in new ways but also obtaining new mathematical insights of their own.

**But in a sense math is seen as a framework which doesn’t relate to physical reality, or is that a wrong perception?**

It is true that mathematics is the language in which physical laws are formulated. Ever since physical laws were formulated in a precise modern way by Newton, this was done in a mathematical language. Remember, Newton was one of the inventors of calculus, which he invented because he needed it to understand his equations. So there have been periods when known mathematics was sufficient for what physicists were doing and therefore physicists were not contributing to new mathematics. But there were other periods like Newton’s, or in the 19th century when partial differential equations of physicists led to much new understanding of mathematics when physics stimulated the development of mathematics..

**Do you see string theory as a theory of elementary particles or a framework with wide applicability to various branches of physics and even mathematics?**

Oh, I see it as being both. I think it probably is a framework for understanding elementary particles more deeply than we do, including gravity and perhaps unifying forces. But because it is a deeper theory in physics it enables us to get better insights about many better-established areas of physics. Also understanding it forces one to bring to bear previously unfamiliar areas of math, and sometimes you get new insights into mathematics.

**Do you make time to connect with world news and activism? What are the areas you feel strongly about?**

There are many areas I feel strongly about, but only one which I am personally involved in – this is the Israeli- Palestinian conflict. Since 1991, I have been on the board of Americans for Peace. Now, since 1991, we work with the Israeli Peace movement and we advocate a two-state solution of the Israeli-Palestinian conflict.

**A message for readers, especially students…**

For the students I would like to say that in physics and math, and even in the other sciences, there are horizons just as wide as there have ever been in the past.

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