After the holy grail: Prof. Sen delivering the Jagadis Chandra Bose Memorial Lecture (Picture by Sanat K. Sinha)

The celebration of the 20th year of the string theory, physicists? holy grail, almost coincides with the centenary year of Albert Einstein?s Special Theory of Relativity. So it was an appropriate occasion for the Bose Institute to invite eminent string theorist, Prof. Ashoke Sen of the Harish-Chandra Research Institute, Allahabad, to deliver the 66th Jagadis Chandra Bose Memorial Lecture. Prof. Sen spoke on ?The Universe of elementary particles?.

Sen began by painting a simple but detailed picture of the basic constituents of matter. ?There?s an unending search for a unified theory of the universe,? he said. Einstein, the inventor of General Relativity, dreamed of finding a single theory that would embrace all the forces of nature. General Relativity has had many observational successes, including the expanding Universe, existence of dark matter and black holes.

The Standard Model was designed within a framework known as Quantum Field Theory (QFT), which is consistent with both the quantum mechanics and special theory of relativity. Over the years physicists have detected elementary particles like quarks and leptons and their mediators like gluons, along with the W+, W- and Z particles, highlighting the success of the Standard Model.

?However,? said Sen, ?despite its success, it does not take into account the gravitational force in the realm of the small.? Gravity, beautifully described by Einstein?s General Relativity (GR), does not fit into its scheme. ?Between two electrons, the effects of gravitational force are there, but the quantum theory neglects it, and that was the problem of the Standard Model,? said Sen. ?Can we modify Standard Model so as to include gravity in its fold to get a theory of quantum gravity? The query has lead to the advent of string theory.?

Particle interactions occur at a single point of spacetime, at zero distance between the interacting particles. In string theory, the strings collide over a small but finite distance, Sen explained.

He urged his audience to think of a guitar string that has been stretched. Tiny, one-dimensional loops are created when one plucks the string. String theory visualises each particle as a vibrating, oscillating, dancing filament with an average length of about 0.000,000,000,000,000, 000,000,000, 000,000,001 centimetres. This means that strings are too small to be seen, even by today?s particle smashers. The strings in string theory are floating in spacetime.

Sen is hopeful that string theory will unite all the forces of Nature by holding that strings, not particles, are the ultimate constituents of matter. According to string theory, the different masses and other properties of both the matter particles (quarks) and force particles (leptons) are a reflection of the various ways in which a string can vibrate.

Currently, there are five string theories. But the number has been shrinking in recent years, because experts are discovering that some theories that were thought to be completely different initially later turned out to be merely different ways of looking at the same concept. ?This has given birth to the ?M-theory?, which encompasses all the string theories,? said Sen.

According to the M-theory, strings can have different phases, just as we see water in different states ? liquid, solid and gas. In M theory, strings vibrate in 10 spacetime dimensions. ?But we don?t have any machine to prove that,? said Sen. ?And that?s one grey area of M-theory.? According to him, nobody has seen the right phase of a string that can exactly represent right kind of a particle. Initially, it didn?t explain the dark energy, but now it has found the non-zero dark energy phase. But no phase has yet been discovered to deduce the right value of the dark energy. To sum it up, M-theory explains why life exists in this planet. But it cannot answer the question: Why not in any other place? ?A lot more needs to be done in order to put M-theory on a firm footing,? commented Sen.