A long time ago in a galaxy far, far away…
Two black holes collided and merged, releasing unimaginable quantities of energy in the form of gravitational waves. Last year these waves were detected on Earth by an exquisitely sensitive pair of instruments near Hanford, Washington and Livingston, Louisiana, and last week that detection was announced to the world. Unless you have given up all media for Lent, you have surely heard of it by now.
So what’s the big deal? Why are scientists so excited? Why is there talk of a Nobel Prize? What are gravitational waves, anyway?
Einstein and Gravity
We’ll start the story at a time much more recent than the ancient event whose signals only reached Earth after more than a billion years of travel: almost exactly a century ago, in fact. The name of Albert Einstein was known to the world of physicists, but had not yet become synonymous with scientific genius to the public at large. After publishing his Special Theory of Relativity in 1905—this is the one that predicts clocks running at different rates as they approach the speed of light, along with other non-intuitive results—Einstein worked for years to generalize his theory. The 1905 work was “special” because it didn’t really incorporate the effects of gravity into its equations. This is not much of a problem except in very strong gravitational fields.
The General Theory of Relativity published in 1915 took full account of the effects of gravity. The Special Theory had already shown that space and time could no longer be considered as independent of each other, that they merged into a single entity best described as four-dimensional space-time.
I’ve never met anyone who could visualize four dimensions. To help us understand what is going on, it’s helpful to drop back to our familiar three dimensions, and visualize a stretchable rubber sheet. For the sake of convenience, we’ll put a grid on it. You’ll see why in a minute.
Here you go. Kind of boring, isn’t it? This is “flat” space-time, with no gravitational effects to be seen. Light traveling from one side to the other will follow a straight path along one of the grid lines. And what causes gravitational effects? Well, matter will do it—the more the better. And what matter does to space-time is to change its shape. In the presence of matter, space-time is curved.
The greater the mass, the greater the curvature of space-time. Einstein had described gravity in an entirely new way. Instead of its being some mysterious force that magically reached out from the sun to influence the planets, it was simply our description of how space-time was curved by the massive sun, and how the planets naturally responded to that curved space-time. In the classic words of John Wheeler: “Mass tells space-time how to curve, and space-time tells mass how to move.”
Gravity Is Really Weak
What? How can you say that? Look at what we had to do to overcome it and send people to the moon!
Here’s another way to look at it.
The world record for the high jump is 2.45 meters (over 8 feet), set by Javier Sotomayor of Cuba in 1993. He weighed 82 kilograms (181 pounds). It took the entire mass of Earth—5,972,200,000,000,000,000,000,000 kg (13,166,000,000,000,000,000,000,000 lb)—to drag a man back after he had elevated himself over 8 feet! That’s pretty weak.
Another way to look at it is to compare it to another familiar force, electromagnetism. Just think about the comparison between the relatively small amount of electrical energy being expended here versus the gravitational force exerted by the entire mass of the Earth.
Gravitational waves are a prediction of General Relativity that Einstein himself thought would never actually be detected. Why not? Because gravity is such a weak force, the only way to generate waves strong enough to be detected is to move a LOT of mass, and to move it a LOT. In this supercomputer simulation of what happened, the two black holes circle each other ever and ever faster, generating gravitational waves until they finally merge.
How Did We Detect This?
When a gravitational wave passes through the Earth, it compresses and stretches space-time like this.
But unless we were VERY close to this black hole merger—not a good idea—the effect is so weak as to require instruments that can detect movements much less than the diameter of a subatomic particle. Needless to say, it is only in recent years that we have had the ability—and frankly, the funds—to build such devices. One is located near Hanford, Washington and the other near Livingston, Louisiana; two widely separated instruments allowed scientists to eliminate local effects as a cause of the very weak signals detected. Each consists of two 4 km long arms at right angles to each other. If a gravitational wave passes through, it would affect the two arms differently.
Why Is This Such a Big Deal?
I really can’t improve on a few quotes that I will pull out of this article.
“The discovery is profound in 3 ways… First, we now know that gravitational waves exist and we know how to detect them. Second, the signal detected by the LIGO stations on Sept. 14, 2015, is the strongest evidence yet of the existence of a binary black hole system — each black hole “weighing in” at a few tens of solar masses. The signal is exactly what we’d expect to see during the violent merger of two black holes, one 29 times the mass of our sun and the other 36 solar masses. Thirdly, and possibly even more important, “short of sending someone to a black hole,” this is the strongest direct evidence of the existence of black holes.”
And even better:
“It was a clear signal from the universe that Einstein got it right and his gravitational waves were real…”
And now you know why I used another quote, from a friend who informed me of this discovery, as the title of this blog.