Who is faster: You or light?

There wasa young lady named Miss Bright, who travelled much faster than light. She departed one day, in an Einsteinian way, and came back the previous night.

This limerick by Arthur Buller, published in Punch in 1923, reflects the interest of the man had in time travel when the concepts behind Einstein’s theory of relativity began to percolate down to the lay world.

Indeed a decade before Einstein proposed his remarkable ideas, the science-fiction novelist H.G. Wells had written his absorbing long story about time machine in his novella The Time Machine. This contraption enabled its rider to travel into the past or future, with, of course, remarkable consequences.
But what does relativity have to say about a time machine? Let us try to see what is possible and what is not, given that the relativity theory is correct. Imagine two twin brothers, A and B. A stays put on the earth while B undertakes a long space travel at a speed, say, 80 per cent of the speed of light. By the time keeping devices on the earth, B travels for 20 years before coming to a halt and then reversing his path back to the earth at the same speed. So A has aged by 40 years during B’s adventure. But how old is B as he steps out of his spaceship? Einstein’s theory tells us that he would be aged by only 24 years.
What does this mean? Effectively B has time-travelled into the future and has come to a world that lies 16 years in the future. The time machine has worked so far. However, if A now wishes to go back into the past, he can’t! There is no device that will take him 16 years back in time to be at par with his twin brother.
Then, what about Miss Bright of the limerick? It seems that she managed to do what we have just said can’t be done. She set out on her journey today and came back yesterday. She apparently did the impossible by breaking a sacrosanct rule of relativity: she travelled faster than light. Thus, she broke a speed limit that is rigidly imposed on all material objects in the universe. They cannot travel faster than light.
Why not? Did not good old Isaac Newton tell us that we can accelerate, i.e. increase our speed by applying a force? Yes, he did, and that law still applies under the relativistic regime. So B’s spaceship can be accelerated by supplying rocket fuel. Additional boost will make the spaceship go faster. But can B thus attain the speed of light? Alas, no. For relativity has ensured that the speed limit can never be attained by B, no matter how much fuel he uses. Because as the ship’s speed increases so does its mass and in such a way that the mass becomes infinitely large as the light barrier is approached. And Newton’s law warns us that the acceleration produced by a force is less and less as the mass of the spaceship increases. Literally, to attain the speed of light the spaceship will require infinite quantity of fuel. So, says Einstein, that light speed barrier will remain unattainable, let alone someone emulating Miss Bright to travel faster than light.
There is one known exception to this restriction, namely light itself! Light is made up of particles called photons, which, naturally, travel at the speed of light. The photons have one restriction to submit to: they always travel at the speed of light. Thus we have photons that travel at the speed of light and other particles of matter that travel with speed less than the speed of light. Scientists, by and large, do not believe that there exists a third category of particles that always travel at speed greater than the speed of light.
Nevertheless, in 1960, India-born physicist George Sudarshan introduced the idea of a special class of particles called tachyons, which always travel faster than light. The existence of such a species of particles does not violate any of the tenets of relativity. For, the tachyons need force to slow them down, and this force is infinite if the tachyon speed touches the light barrier! So just as we mortals are restricted to remain on the subluminal side of the light speed barrier, the tachyons are required to be on the superluminal side. Thus, if Miss Bright were made of tachyons, she would naturally travel faster than light. For tachyons many of the effects are counter-intuitive and one is the travel into the past. But while Miss Bright can return before she set off, she cannot claim to be from the human species.
Do tachyons exist? They can exist but so far they have not been found. They have not appeared in any laboratory experiment, such as the accelerators like the Large Hadron Collider (LCH) at European Organisation for Nuclear Research (Cern). Nor have they been spotted in the showers of cosmic ray particles coming from energetic sources far away from the earth. Finding them will not disprove relativity. Rather, it will provide the physicists with the satisfaction that a possibly missing species has been found.
In this context, the announcement in September by a team of Cern scientists that they have found a bunch of neutrinos travelling faster than light was totally unexpected. Neutrinos were thought to travel at the speed of light unless they have a tiny mass, in which case they would travel at subluminal speed. In 1987, a star exploded as a supernova, thus releasing a flux of neutrinos. Fortunately three detectors of neutrinos, in Japan, Russia and the US, were operating and so the arrival of the neutrino flux could be timed. These neutrinos were estimated to travel at speed very close to the speed of light. Since they had come from a distance as large as 170,000 light years, the fact that their travel time did not differ much from that of light coming from the supernova, scientists had assumed that neutrinos travel at speed very close to that of light.
So scientists are puzzled by the recent finding that neutrinos travel faster than light. No doubt they will want to verify the result by further experiments. Only then they will decide if the theory of relativity stays or goes.

The writer, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy and Astrophysics, Pune University Campus