IDEAS | THOMAS LEVENSON
February 11, 2016 probably should go down in history as Gravitational Wave Day. The actual data doesn’t look like much: Squiggles on a plot that, when read with the right eyes, reveals the collision of two black holes. The traces of that crash rippled through spacetime for 1.6 billion years before reaching two giant detectors at the Laser Interferometer Gravitational-Wave Observatory in the United States.
It’s impossible to overstate the precision of that measurement, the experimental virtuosity (and sheer stubborn persistence through decades of effort) required to capture wave amplitudes much smaller than the diameter of a proton.
But simple skill, amazing though it may be, is not the only reason the scientific community is so thrilled about this result. The ovations have come because of the discovery itself — which marks the beginning of a whole new way to explore the universe — and because the confirmation of the reality of gravity waves is a triumph of science as an end in itself, the pursuit of knowledge about nature conceived as a work of art.
This discovery turns on humanity’s new ability to put an ear to the universe and listen, with an instrument that only achieved the precision required in September 2015. But the idea behind the gravitational wave observatory has deep roots. The sequence of events that led to this result began on Nov. 25, 1915. That day, Albert Einstein rose to give the last in a series of four weekly presentations at the Prussian Academy of Sciences in Berlin. It was to be a brief set of remarks — the published version of the talk runs three pages. They were enough. Those pages contain the final form in which Einstein presented his gravitational field equations, a mathematical description of nature we now call the General Theory of Relativity.
It can be hard, even a century on, to grasp just how radical general relativity was to those who first tried to understand it. We perceive gravity as a force. In the old Newtonian framework, it’s a pull, instantaneously transmitted between two masses — the sun and the earth, for example, or the earth and our bodies, as our feet stick firmly to the floor. But in Einstein’s cosmos, mass (and energy) doesn’t tug on anything. Instead mass-energy defines the shape of spacetime.
The usual image is of a bowling ball on a trampoline: a big mass (the ball, the sun) creates a dent in the space around it (the trampoline, spacetime). Objects have to follow the shortest possible path through curved spacetime — so what we feel as gravity is just the experience of clambering around the actual curves in the shape of our particular patch of the cosmos.
Einstein forced the world to think of gravity as geometry — a genuinely strange idea then and now. Yet, even as Einstein first worked it out, general relativity made a series of clear, unambiguous predictions to check whether his idea accurately describes the universe in which we live.
In the beginning there were three such checkpoints: Could his new theory explain anomalies in the orbit of Mercury? (It could.) Would starlight bend around large masses like the sun? (Yes.) Does the observed color or frequency of light change as it moves from a strong gravitational field to a weaker one or vice versa? Testing that one took a while, but indeed, it does.
Those were the canonical tests of General Relativity, and the fact that the theory passed all of them has been taken — with good cause — as extremely strong support of a relativistic theory of gravity.
But there was one more phenomenon that Einstein himself noticed lurking in the mathematics of his idea, something that happens when mass and energy move in spacetime. The effects thus produced don’t just sit there; rather, in theory, they should produce tiny vibrations in spacetime, ripples in the gravitational field. At first, Einstein himself didn’t believe those waves would ever be detected -- reasonably enough, given that he was thinking at a time before supernovae explosions or black holes were understood, much less observed.
A century into the age of General Relativity, the theory has helped expose a universe far more exciting more varied and inconceivably more dramatic than the one Einstein imagined he lived in. The event LIGO detected — colliding black holes — produce much more dramatic consequences than anyone in 1915 could have imagined. The amount of energy involved is staggering, easy to state yet impossible to imagine. At the press conference announcing the result, scientists pegged it at about fifty times the instantaneous output of every star in the visible universe.
As the news has spread of the discovery of the gravitational waves such a cataclysm generates, the point physicists have made over and over again is that this isn’t simply a detection of a long-predicted natural curiosity. It marks the opening of a wholly new window on the cosmos. The twentieth century saw the rapid expansion of the number of ways to look at the cosmos: from visible light astronomy, as old as human eyes, to the full electromagnetic spectrum from radio observatories to x-ray telescopes. Gravitational astronomy listens to the universe, detecting events through waves that can be directly appreciated as sound.
This opens up previously impenetrable phenomena to human exploration. No telescope could ever have seen the two black holes become one, just identified by LIGO. Astronomers are accustomed to talking of the infrared sky, for example, or the gamma ray one. This one discovery marks the first direct observation of the gravitational sky.
There is one thing about that whole story, though, that points to a human problem, a line of tension wholly earthbound. Hunting gravity waves is, at least from one point of view, a completely useless pursuit. General Relativity does have some practical implications — the most common example is the GPS receiver in your cell phone, which works because the atomic clocks at the heart of the global positioning system are subject to relativity.
But no one will ever surf a gravitational wave. No coffee will get reheated in a gravity-wave oven. An ancient collision between black holes does not affect the price of gas. Gravitational waves are something the universe does, a feature of the cosmos we can now, for the first time, begin to study in detail. And that’s it. The meaning of the LIGO announcement is that we now know our surroundings just a bit more richly than we did yesterday.
For many — for me — that’s enough. But it is not so for everyone.
It’s kind of a coincidence — but not really, given the long standing suspicion science for science’s sake evokes — that the day before the LIGO announcement, the US House of Representatives passed on a near-party line vote a bill that would require the National Science Foundation to justify each grant it makes. LIGO itself has taken more than 20 years to reach the sensitivity needed to make this week’s discovery. The total cost of the system wouldn’t leave much change out of a billion dollars. That’s a lot of years and a lot of money to justify. And yet that’s what it takes to create a whole new way of seeing the universe. It is a measure of our society that we were able to muster the willingness to do so. The next test? Whether we can do the same again.
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