Gravitational wave observations are set to expand the frontiers of our knowledge of the cosmos. But there was a time when physicists had doubted their existence, until the maverick physicist Richard Feynman settled the debate with an ingenious thought experiment.
Gravitational waves are ripples in the gravitational field, just as light waves are ripples in the electromagnetic field. They interact with matter very weakly, making them both famously hard to detect and invaluable sources of information about the universe. Gravitational waves produced billions of years ago are virtually undistorted when they reach us, giving us perfect snapshots of the cataclysmic events that created them. We will peer much further and deeper into the cosmic past when we start ‘seeing’ the universe in gravitational waves.
Scientists are currently setting up a network of gravitational-wave observatories around the world, including one in India that recently received the Union Cabinet’s approval.
Einstein’s solution
Gravitational waves are clearly the future but the field itself has a humble history. Scientists originally ignored the idea in its infancy, including its progenitor Albert Einstein.
In 1915, Einstein discovered the theory of general relativity to describe the effects of gravity. The centrepiece of the theory is a set of equations now known as Einstein’s equations. They are notoriously difficult to solve. Solutions to these equations describe the gravitational fields possible in nature. In 1916, Einstein showed that there was a solution corresponding to a gravitational field with ripples streaming through. He had in effect shown that his theory could describe gravitational waves.
But Einstein himself was not convinced. There was a catch: Einstein had used some approximations to solve the equations. So he suspected that gravitational waves were not so much a feature of his theory as a bug that the approximations had smuggled in. His scepticism rubbed off on the community of gravitational physicists, and not much happened in the next 20 years.
In 1936, Einstein returned to the question with a young collaborator named Nathan Rosen. They wrote a paper solving the Einstein equations without using any approximations and claimed that their results showed gravitational waves do not exist. They sent their paper to a journal, where it was rejected by an anonymous referee. Unhappy with the decision, Einstein wrote an indignant letter to the journal’s editor.
A raging debate
But the referee was right, Einstein came to realise. Einstein had been tripped up by a tricky aspect of general relativity that had confused many others in its early years – the issue of coordinate systems. A coordinate system is like a map of some part of space-time. One has to choose a coordinate system first to understand the gravitational field there. But like our standard world maps may lead us to believe that Greenland is as big as Africa, coordinate systems too can be misleading. A faulty coordinate system may show phenomena that are artefacts of that coordinate system and don’t appear in reality. Einstein and Rosen had made that mistake and ended up concluding that gravitational waves could not exist.
The debate on whether gravitational waves were real or figments of mathematics raged on for decades. In the 1950s, it centred around a question: can gravitational waves transmit energy? People on both sides of the debate produced long, intricate calculations that they each claimed would settle the question in their favour. The confusion stemmed chiefly from the knotty issue of coordinate systems, and the field was tangled up in confusion.
The gravity conference
Enter Richard Feynman. One of the leading figures in 20th century physics, Feynman was as famous for his ability to cut through tangles and grasp the essence of a problem as his cultivated irreverence towards authority. In 1957, he decided to attend a conference on gravity in Chapel Hill, North Carolina. It was his first gravity conference, and he was not impressed by what he saw. “There are hosts of dopes here,” he wrote to his wife about his fellow participants.
The debate on whether gravitational waves could transmit energy was at the centre of the conference. Listening to several experts in the field drone on on the issue and produce complicated but useless calculations, Feynman realised there was a simple solution to their problem. The murky math issues that experts were lost in, Feynman simply bypassed with an ingenious thought experiment of his making.
Forget all the coordinate confusion and think simply, the thought experiment said. Focus on the following question: can we get gravitational waves to burn some energy?
Feynman considered a situation involving two tiny beads lying on a stick. A gravitational wave passes through this set up at right angles to the stick. The ebb and flow of the passing wave would cause the beads to oscillate along the stick, coming close and then moving away, periodically. Now, suppose the stick is just a little sticky, that there is some friction between the beads and the stick. So when the beads move, they must generate some heat as they rub against the stick.
But heat is a form of energy and energy cannot be created, only transferred. Where did it come from here? The only possible answer is that it came from the gravitational wave! It is the energy carried by the gravitational wave that gets converted to heat.
Think on your own
Feynman’s argument was an instant success, cutting through the Gordian knot of confusion and converting almost all gravitational-wave sceptics.
After the Chapel Hill conference, many physicists were encouraged to work on gravitational waves. One of them was Joseph Weber, who participated and was influenced by Feynman’s argument. Weber became the first to attempt to detect gravitational waves using an experiment. While his decades-long efforts failed, they inspired and guided others who came after.
Eventually, scientists announced the first direct detection of gravitational waves in 2016, 30 years after Feynman’s passing. He would have been pleased, and might have reminded us to not be dopes who blindly follow authorities but to think on our own.
Nirmalya Kajuri is an assistant professor of physics in IIT Mandi.
