ONCE upon a time, there lived a man who was fascinated by the phenomenon of gravity. In his mind he imagined experiments in rocket ships and elevators, eventually concluding that gravity isn’t a conventional “force” at all — it’s a manifestation of the curvature of spacetime. He threw himself into the study of differential geometry, the abstruse mathematics of arbitrarily curved manifolds. At the end of his investigations he had a new way of thinking about space and time, culminating in a marvelous equation that quantified how gravity responds to matter and energy in the universe.
Not being one to rest on his laurels, this man worked out a number of consequences of his new theory. One was that changes in gravity didn’t spread instantly throughout the universe; they traveled at the speed of light, in the form of gravitational waves. In later years he would change his mind about this prediction, only to later change it back. Eventually more and more scientists became convinced that this prediction was valid, and worth testing. They launched a spectacularly ambitious program to build a technological marvel of an observatory that would be sensitive to the faint traces left by a passing gravitational wave. Eventually, a century after the prediction was made — a press conference was called.
Chances are that everyone reading this blog post has heard that LIGO, the Laser Interferometric Gravitational-Wave Observatory, officially announced the first direct detection of gravitational waves. Two black holes, caught in a close orbit, gradually lost energy and spiraled toward each other as they emitted gravitational waves, which zipped through space at the speed of light before eventually being detected by our observatories here on Earth. Plenty of other places will give you details on this specific discovery, or tutorials on the nature of gravitational waves, including in user-friendly comic/video form.
What I want to do here is to make sure, in case there was any danger, that nobody loses sight of the extraordinary magnitude of what has been accomplished here. We’ve become a bit blasé about such things: physics makes a prediction, it comes true, yay. But we shouldn’t take it for granted; successes like this reveal something profound about the core nature of reality.
Some guy scribbles down some symbols in an esoteric mixture of Latin, Greek, and mathematical notation. Scribbles originating in his tiny, squishy human brain. (Here are what some of those those scribbles look like, in my own incredibly sloppy handwriting.) Other people (notably Rainer Weiss, Ronald Drever, and Kip Thorne), on the basis of taking those scribbles extremely seriously, launch a plan to spend hundreds of millions of dollars over the course of decades. They concoct an audacious scheme to shoot laser beams at mirrors to look for modulated displacements of less than a millionth of a billionth of a centimeter — smaller than the diameter of an atomic nucleus. Meanwhile other people looked at the sky and tried to figure out what kind of signals they might be able to see, for example from the death spiral of black holes a billion light-years away. You know, black holes: universal regions of death where, again according to elaborate theoretical calculations, the curvature of spacetime has become so pronounced that anything entering can never possibly escape. And still other people built the lasers and the mirrors and the kilometers-long evacuated tubes and the interferometers and the electronics and the hydraulic actuators and so much more, all because they believed in those equations. And then they ran LIGO (and other related observatories) for several years, then took it apart and upgraded to Advanced LIGO, finally reaching a sensitivity where you would expect to see real gravitational waves if all that fancy theorizing was on the right track.
And there they were. On the frikkin’ money.
Our universe is mind-bogglingly vast, complex, and subtle. It is also fantastically, indisputably knowable.
I got a hard time a few years ago for predicting that we would detect gravitational waves within five years. And indeed, the track record of such predictions has been somewhat spotty. Outside Kip Thorne’s office you can find this record of a lost bet — after he predicted that we would see them before 1988. (!)
But this time around I was pretty confident. The existence of overly-optimistic predictions in the past doesn’t invalidate the much-better predictions we can make with vastly updated knowledge. Advanced LIGO represents the first time when we would have been more surprised not to see gravitational waves than to have seen them. And I believed in those equations.
I don’t want to be complacent about it, however. The fact that Einstein’s prediction has turned out to be right is an enormously strong testimony to the power of science in general, and physics in particular, to describe our natural world. Einstein didn’t know about black holes; he didn’t even know about lasers, although it was his work that laid the theoretical foundations for both ideas. He was working at a level of abstraction that reached as far as he could (at the time) to the fundamental basis of things, how our universe works at the deepest of levels. And his theoretical insights were sufficiently powerful and predictive that we could be confident in testing them a century later. This seemingly effortless insight that physics gives us into the behavior of the universe far away and under utterly unfamiliar conditions should never cease to be a source of wonder.
We’re nowhere near done yet, of course. We have never observed the universe in gravitational waves before, so we can’t tell for sure what we will see, but plausible estimates predict between one-half and several hundred events per year. Hopefully, the success of LIGO will invigorate interest in other ways of looking for gravitational waves, including at very different wavelengths. Here’s a plot focusing on three regimes: LIGO and its cousins on the right, the proposed space-based observatory LISA in the middle, and pulsar-timing arrays (using neutron stars throughout the galaxy as a giant gravitational-wave detector) on the left. Colorful boxes are predicted sources; solid lines are the sensitivities of different experiments. Gravitational-wave astrophysics has just begun; asking us what we will find is like walking up to Galileo and asking him what else you could discover with telescopes other than moons around Jupiter.
For me, the decade of the 2010’s opened with five big targets in particle physics/gravitation/cosmology:
- Discover the Higgs boson.
- Directly detect gravitational waves.
- Directly observe dark matter.
- Find evidence of inflation (e.g. tensor modes) in the CMB.
- Discover a particle not in the Standard Model.
The decade is about half over, and we’ve done two of them! Keep up the good work, observers and experimentalists, and the 2010’s will go down as a truly historic decade in physics.