It took us 100 years to confirm the last big puzzle of Einstein’s theory of relativity. On Feb. 11, scientists announced that they have detected gravitational waves, which are created when two accelerating masses generate “ripples” in the curvature of spacetime—just like frolicking children on a rubber sheet generate waves in the material. Along with electromagnetic radiation, gravitational waves will give us another window through which we can “listen” to the universe.
This is an exciting moment for science. It is a well-deserved culmination of countless hours of toil and creativity. And yet, this discovery fills me with a certain amount of nostalgia and sadness. Some are already calling it one of the most important discoveries in physics of the last few decades. Let me not mince words here: If that is indeed the case, then physics is in bad shape.
An amazing achievement
The experiment took so long because gravity is the weakest among the four fundamental forces. The only gravitational waves that could be detected on Earth are those that involve the collision of two massive black holes, which is exactly what the team of scientists responsible for this discovery found.
The technology, dedication and attention to detail that has been involved in detecting these waves is a testament to the power of tool-driven science. The theory itself has been around for more than 100 years. But the experimental setup that allowed scientists to validate it consists of a panoply of old and new tools.
The biggest experimental achievement in detecting these waves is the construction of laser interferometers known as LIGO, which split a laser beam and send the two resulting beams to a distance of four kilometers, from which point they are reflected back. The idea is that when a gravitational wave arrives at this source, there will be an asymmetric compression of one beam and expansion of the other, which should be manifested in the different times at which they arrive back at the detector. This difference in length was detected by LIGO’s ultra-sensitive electronics.
But the magnitude of this difference is astonishingly tiny. A gravitational wave from a source four light years away will cause a perturbation of no more than a thousandth of the width of an atomic nucleus. Recognizing this perturbation is trying to drill down to the very limits of what human beings can achieve with their inventions, and it’s a truly noteworthy achievement.
This achievement was made possible by progress across several fields of science. Physics, certainly, but also chemistry and materials science, electronics, mechanical and civil engineering, and a boatload of mathematics and statistics. All these fields needed to have matured before gravitational waves could be discovered.
Yet the discovery got me thinking about John Horgan’s “The End of Science.” The book is rightly controversial, and most working scientists say that its proclamations of the demise of various fields of science are highly premature. But the book has a more subtle message which seems to ring true.
Here’s the problem: While the detection of gravitational waves will be a fitting testament to both experimental and theoretical science and the dedication of countless scientists over the years, in one sense it would be utterly unsurprising. That’s because it is the logical prediction of a theory that has been around for 100 years. In fact, it’s not even the first or most important prediction of this theory, nor would it be the first time that experiment and engineering have been able to probe physical phenomena predicted by this theory to an incredible degree of accuracy. (That honor, in my opinion, goes to gravity probe B). If the true progress of science is measured by the number of novel and original phenomena it unearths, then what would have been really surprising is if LIGO conclusively failed to detect gravitational waves.
Einstein put the finishing touches on his general theory of relativity in 1915. Since then, starting with Arthur Eddington’s pioneering detection of the bending of starlight in 1919, one experiment after another has spectacularly validated the predictions of the theory. Relativity has been probed at a vast range of scales, from right outside the Earth to the innards of galaxies and black holes to the large-scale structure of the entire known universe. Even gravitational waves were indirectly detected from a pulsar, and the scientists who did the work won the Nobel Prize.
Thus, in some sense the direct detection of these waves comes not at the forefront but at the tail end of a century of amazing experimental work. The real star here is the theory itself, which was truly a watershed moment for science. If science grows best at its edges, then gravitational waves are at its dead center. General relativity would have been alive and well even if they hadn’t been detected.
The end of physics?
Would the detection of gravitational waves be a technical tour de force and a tribute to the hard work of brilliant physicists and engineers? Absolutely. Would it be a novel discovery, a discovery like relativity itself or fission or Mendelian genetics that overturned or revolutionized science? Sadly, no.
If, as a chronicler of science in 1950, you were to list the most important discoveries in physics of the last few decades, that list would include special and general relativity, the discovery of the neutron, the atomic nucleus and fission, the whole edifice of quantum mechanics as well as pioneering experimental achievements like the Lamb shift and the laser.
In contrast, listing the important achievements in physics of the last few decades in 2016 is a depressing task. There have been no fundamental revolutions in theoretical physics since the evolution of the standard model.
Experimentally, there have been a few important discoveries like the Hall effect and high temperature superconductors, but nothing comparable to Rutherford’s discovery of the nucleus. Even the discovery of the Higgs boson—a supreme achievement of engineering and collaboration if there ever was one—was vindication of a prediction from the 1960s. It is hard to escape the feeling that physics’s main job these days seems to be experimentally verifying everything that people conjectured forty years ago.
The cynical view would have us say that gravitational waves might be one of the last hurrahs of reductionist physics. The only truly unexpected, startling and revolutionary discovery in physics of the last three decades has been the finding that the expansion of the universe is accelerating.
The fact that detection of gravitational waves would be treated as one of the greatest physics discoveries of our lifetimes should give us pause and should make us think about the history and future of the science. At the same time, there is one reason it should make us feel optimistic. If gravitational waves have indeed been detected, there’s no telling what else they would say about our universe. One truth about science is that the most important discoveries are the ones which we cannot predict. If gravitational waves are utterly predictable, then we can at least hope that what they will tell us about our fascinating universe will be utterly unpredictable and new. That is something all of us can look forward to.