Originally posted to 13.7 Cosmos & Culture on 10/3/2017
In 1915, Albert Einstein concluded his General Theory of Relativity, a theory that would revise our understanding of gravity in radical ways.
Before Einstein, the dominant description of gravitational phenomena was based on Isaac Newton's theory, proposed in 1687. According to Newton, every two objects with mass attract one another with a force proportional to their masses and inversely proportional to the square of their distance: double the distance, the attraction falls by a factor of four.
Newton knew that his theory had a fundamental flaw, a mysterious "action at a distance." Somehow, and he wouldn't speculate how, the force of gravity propagated instantaneously across space like a sort of omnipresent ghost. Despite this, the theory was so successful at describing so many phenomena that Newton rested his case: "I feign no hypotheses," he famously wrote.
Einstein would have none of that. In 1905, in his Special Theory of Relativity, he established that nothing could travel faster than the speed of light. If something happened to the sun right now, we would only find out in 8.3 minutes, the time it takes for light to travel from there to here. How could gravity act instantaneously then? Einstein knew that Newton's action-at-a-distance had to go.
For 10 years, he worked to figure how to do that. The result, his General Theory of Relativity, was an intellectual triumph rarely equaled in history. Einstein's theory transformed Newton's notions of space and time. To Newton, space was an inert stage where things happened. Time just flowed solemnly forward like a river. In Einstein's theory, space and time became plastic, deformable due to the presence of mass and energy. An image we use, even though it's only in two dimensions, is that of a trampoline. If you place a heavy ball in the middle of the trampoline it will sag down, its geometry deformed. Something similar happens to space in the presence of a mass; and the bigger the mass, the more dramatic the deformation. Einstein's theory thus predicted that space is bent by mass (and energy). Time is also affected, as clocks tick slower near a big mass.
Strange as it may seem, Einstein's theory works like a charm; it has passed every experimental test over the past 100 years. GPS uses corrections from it to increase accuracy.
The one test that was missing was the waving of space itself. Einstein realized that if masses moved about, the deformations in space would also move about, propagating like waves, somewhat like what happens when you throw a rock on a pond. Gravitational waves are a bit more complicated and require a bit of asymmetry: slightly deformed spheres or, more dramatically, two bodies orbiting one another — like the Earth around the sun. However, gravity being such a weak force, the effect is truly tiny and needs something very dramatic to create a signal we can detect here. Ideally, two big black holes spiraling around each other and finally colliding. (See my piece from last week about this here.) This is exactly what was found by the Laser Interferometer Gravitational-Wave Observatory, or LIGO.
In the fall of 2015, physicists were stunned by the signal they detected: the spiraling and final collision of two huge black holes, with 36 and 29 solar masses, merging into one.
The road from prediction to detection was very long, taking more than 25 years. Three Nobel Prize winners, announced October 3, were key to the discovery, each in his own way. Janna Levin's book Black Hole Blues and Other Songs from Outer Space, tells the story in detail.
Rainer Weiss, an emeritus professor from the Massachusetts Institute of Technology, was awarded half the prize for his work in the concept, design and construction of LIGO. Kip Thorne, from the California Institute of Technology, a gentle man and wonderful mentor — whom I have known since my postdoctoral days — made key theoretical predictions of what a detection of gravitational waves would look like and how to distinguish the signal from the data. Barry Barish, also from the California Institute of Technology, used his formidable knowledge of physics and administrative prowess to get the experiment off the ground, rescuing it from almost certain demise in 1994 and spearheading its construction with funds from the National Science Foundation.
The waving of space is very subtle. LIGO, with its L-shaped arms, is capable of detecting distortions in space (essentially in the distances between two points) one thousandth the diameter of an atomic nucleus across a 2.5-mile laser beam. (Maybe you should read this again; it's truly amazing.) To reach this kind of spectacular precision, all sorts of interferences must be eliminated. A truck driving by the road could affect the experiment, as could seismic noise, and thermal vibrations.
As the two black holes circled one another faster and faster, the disturbances in the space around them intensified. By the time they were orbiting each other 30 times a second, LIGO picked up the signal. They had only 20 millisecond of data (20 thousandths of a second), when the two accelerated to 250 orbits a second before the violent final collision that led to a single black hole. An important detail: This happened about 1.3 billion light years away, in a galaxy far away. In other words, the signal traveled through space for more than 1 billion years before it reached Earth and the LIGO detector. If that doesn't amaze you, you must be, as Einstein once wrote "as good as dead, a snuffed-out candle."
Einstein didn't mean it in a nasty way. The full quote reads:
"The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and science. He who does not know it and can no longer wonder, is as good as dead, a snuffed-out candle."
I am sure the three physicists who deservedly received the Nobel Prize would definitely agree. Engaging with the mysterious is not always easy, and the pay-off may take a long time. But how sweet it is to push ideas to the limit and beyond to open a new window into reality.