Very few people in the world understand it well, but the theory of general relativity won Einstein worldwide fame and a person of the century award from Time magazine. At the Strings conference in Bengaluru last week, attended by stars of the physics world, a special session celebrated 100 years of this grand theory of gravity.A scientist at the conference took pride in mentioning that a theoretical physicist became the person of the twentieth century. Even as physicists celebrate Einstein’s extraordinary achievement, they are also trying to stretch the theory to breaking point, testing it at extremities and coming up with extensions and alternatives that apply it to new domains.Among the proposals lying on Prime Minister Modi’s table is a Rs 1,000 crore plan to build the Laser Interferometer Gravitational-Wave Observatory (LIGO), an exquisitely sophisticated piece of equipment to detect gravitational waves predicted to exist by Einstein’s theory.The Indian LIGO will be part of a global network to detect gravitational waves, which can also become a new window to the universe in due course. Although they are known to exist, direct observation of gravitational waves could throw up surprises and kinks in existing physical theories, think some physicists.An upgraded version of two LIGOs in the US will have its first observing round after a few months. “We are at a sensitivity that is well beyond what we did in the first decade of this century,” says Peter Saulson, professor of physics with the Syracuse University Gravitational Wave Group.

The LIGO experiments will continue for a long time. Meanwhile, two more observatories are being upgraded in Europe, and they will join the already upgraded LIGOs — called Advanced LIGOs —in the search for gravitational waves. The European scientists are planning more ambitious missions in the future.



The European Space Agency is now at work developing a probe to be launched in 2020. Called Euclid, this probe will map the structure of the dark universe, a part that we know exists but cannot see. It so happens that what we see is only 4% of the universe.



While these experiments will test the general theory of relativity to the hilt, theorists are trying to extend or modify Einstein’s creation in areas where it does not seem to hold. One of them is in the weird arena of black holes, supermassive collapsed stars that are predicted to exist by the theory. Einstein theory collapses with these stars, as it predicts a point of infinite density called a singularity.

The mathematics of a singularity is not complicated, but no one knows what it means in physical terms. Physicists cannot give an accurate prediction of what will happen at a specific time to an object on the surface of the star as it collapses into oblivion. This may not be a problem with just Einstein’s theory of gravity.“Classical theories of gravity encounter the singularity problem,” says L Sriramkumar, associate professor of physics at IIT Madras. “It has to be resolved through a quantum theory of gravity.”Two years after he published his general theory, Einstein modified his equations a bit by introducing what is now called a cosmological constant. He did so to stabilise the universe against the inward pull of gravity. In those days, scientists thought that the universe was static. Later it turned out that it was expanding, and Einstein called the cosmological constant his greatest blunder.In recent times, the cosmological constant has reappeared through the mysterious dark energy in empty space that is supposed to push the galaxies apart faster and faster. No one knows why. Fixing the problem is one of the greatest endeavours in science at the moment.An even greater problem is the unification of Einstein’s theory of gravity with quantum mechanics, the other pillar of modern physics that describes matter at small scales. Quantum mechanics is extraordinarily successful in explaining the behaviour of matter, and has also been tested many times over. However, general relativity and quantum mechanics are inconsistent with each other.Both are probably part of a more general theory that has not been worked out. Physicists consider this unification of gravity with quantum mechanics the most important problem of theoretical physics. These are important problems, and their solution may revolutionise our understanding of the universe, in the same way Einstein did when he formulated his theories.When he started work on the general theory of relativity, Einstein had already changed the course of physics with some major discoveries. In 1905, he discovered the famous equation E = MC2. He formulated the special theory of relativity, a theory about the relationship between space and time. Einstein explained Brownian motion, the squiggling of particles on the surface of a liquid, which confirmed the existence of atoms and molecules. He discovered the photoelectric effect, which won him a Nobel Prize.But the big job remained to be done. More than two centuries before Einstein, Isaac Newton made a remarkable breakthrough. He found out that a falling apple and revolving planets are both directed by the same law, now known as the inverse square law of gravitation.Newton could calculate the forces between two bodies far apart, but he had no explanation for how this force acted from a distance. After his burst of publications in 1905, Einstein set out to understand gravity. He was still a patent clerk and not famous. He was working on physics in his spare time.In the next decade, Einstein formulated one of the most sophisticated and beautiful scientific theories of all time, without a shred of experimental data to check his ideas, and guided solely by the beauty of his equations. “Even if it has no practical application,” says T Padmanabhan, professor at the Inter-University Centre for Astronomy and Astrophysics in Pune, “I would say that general relativity theory is one of the greatest intellectual achievements of all time.”But it turns out that the theory has applications. Our Global Positioning System (GPS) will not work without using Einstein’s theory. We will not understand the depths of the universe without the general theory of relativity. It contained far more than what Einstein himself realised at that time.Einstein’s insight was to say that gravity is a property of space and time, the two entities he himself had helped to unify. An object anywhere in the universe curves the space-time around it. The more massive the object, the more the curvature. This curvature makes other objects in the vicinity slide towards the first one, just as an object placed at the edge of a round-bottomed vessel slides towards the centre. Since the planets keep moving, they continue sliding permanently towards the centre, and coast along forever in a near-circle, just like a stunt motorcyclist inside the hollow mesh-sphere.In this theory, gravity is not really a force. It is a property of space-time. Einstein’s theory gave some predictions that could be verified. In 1919, when astronomers used an eclipse to verify the bending of starlight, as predicted by Einstein, he became an instant global celebrity.Over the years, his theory had an enormous impact on the course of physics, one of which was to kickstart the field of cosmology. “Before Einstein, cosmology was addressed by religion and philosophy,” says David Gross, professor and former director at the Kavli Institute for Theoretical Physics at Santa Barbara. “After Einstein, it became the subject matter of physics.”In 1929, in what is considered one of the greatest scientific discoveries of all time, Edwin Hubble found out that the universe was expanding. With a bit more courage, Einstein might have predicted it from his theory. “Einstein got cold feet,” says Padmanabhan, “but the general relativity theory contained within it the power to predict the expansion of the universe.” Towards the end of the twentieth century, scientists discovered another startling fact: the expansion of the universe is accelerating. Galaxies are moving apart faster and faster, and could lead one day to fly so fast that all matter could be ripped apart.Many tests are being planned around the world to find out what causes this expansion. The Euclid probe will try to estimate dark energy — and the equally mysterious dark matter — from the distortion in the images of far-away galaxies. It will also look for fluctuations in the clustering of galaxies, which will give hints about the nature of dark energy and the cosmological constant. Looking for gravitational waves is another test. Finding gravity waves by itself may not be a surprise, but there could be variations to the theme that may give hints to a different order in the universe.Gravitons, the particle associated with gravitational waves, are not supposed to have mass. What if they turn out to have mass, just like the elusive neutrinos did some time ago? The discovery that the neutrino has a small mass opened up new fields of enquiry in particle physics, as existing theories proposed that it should not have mass. “Measuring gravitational waves routinely might show up inconsistencies that can lead to modifications of the general theory of relativity,” says Bala Iyer, professor at the International Centre for Theoretical Sciences (ICTS) in Bengaluru.In the next few decades, finishing what Einstein began may turn out to be the most important event in science for a long time. Einstein himself expected such a turn of events. “Einstein considered his theory to be a stepping stone,” says David Gross. “He didn’t consider it as permanent.” Whoever builds the successor to this theory will likely be as famous as Einstein.