Tomorrow at 10:30 AM Eastern Time the gravitational wave observatory LIGO will have a big announcement to make. LIGO will tell us whether it has finally observed gravitational waves (Update: The discovery has been confirmed). The finding will be the culmination of a two decade old experiment that in turn builds upon a hundred year old prediction. Gravitational waves are a logical prediction of Einstein's general theory of relativity which tells us that accelerating masses will generate "ripples" in the curvature of spacetime just like frolicking children on a rubber sheet generate waves in the material. These ripples will propagate through space and be detectable in the form of gravitational waves. Along with electromagnetic radiation, gravitational waves will give us another window through which we can "listen" to the universe.
The biggest gravitational waves would logically be detected from intense gravitational fields involving the acceleration of truly massive objects, and there are few gravitational fields that are more intense than those generated by the collision of two black holes; waves from these are the ones that are supposed to be announced by the LIGO collaboration tomorrow.
The technology, dedication and attention to detail that has been involved in the detection of these waves is quite amazing and is a testament to the power of tool-driven science. The theory itself has been around for over a hundred years, but the experimental setup allowing us 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 which split a laser beam and send the two resulting beams to a distance of 4 kilometers from where they are reflected back; fittingly enough, it was interferometry that put the stake in the heart of the luminiferous ether at the turn of the 19th century and paved the way for Einstein. 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 (illustrated well in this BBC article) which should be manifested in the different times at which they arrive back at the detector. This difference in length should be detectable by ultra-sensitive electronics.
But the magnitude of this difference is astonishingly tiny; as this article puts it, a gravitational wave from a source 4 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. What we also need to appreciate is that the detection will be the work of 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.
Gravitational wave detection is certainly going to be an exciting moment for science and it would be the well-deserved culmination of countless hours of toil and creativity. And yet this discovery fills me with a certain amount of nostalgia and sadness. One of the reasons is John Horgan's "The End of Science", a book which continues to stick out from the science literature like a sore thumb. The book is rightly controversial, and most working scientists disagree with its proclamations of the demise of various fields of science as highly premature. Yet the book has a more subtle message which seems to ring true, and the many scientific stalwarts in it who validated Horgan's thinking through interviews certainly don't help dismiss its message.
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 a hundred 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 with the publication of his so-called field equations. 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 here outside earth to the innards of galaxies and black holes to the large-scale structure of the entire known universe. A Wikipedia article lists at least ten different tests which confirmed the basic features of the framework: frame dragging, redshift, the equivalence principle...the whole works. 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. 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. Would their detection be a technical tour de force, 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.
The reason I kept thinking of Horgan's book is because some sources are already calling the putative finding 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. To see why it's worth looking at the history of physics and going back to say, 1950. 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 (including the uncertainty principle) 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. In fact that's a good benchmark for comparing discoveries. Unlike the accelerating universe which was wholly unexpected, the discovery of gravitational waves would be completely expected.
The fact that tomorrow's expected announcement 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. The 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.
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