If you read my last post about light, you know that light of all kinds (visible and invisible) is produced when things with an electric charge (like electrons) are jiggled.
Today the science blogs and even the ordinary news websites are abuzz with the announcement that researchers with an experiment called BICEP2 (what a great name!) have detected gravitational waves in the microwave radiation left over from the Big Bang. Deep, exciting, but still somewhat preliminary, this is a discovery worth watching. It could settle once and for all whether cosmic inflation really happened and may even give us insight into whether or not we live in a multiverse. So what in the world are gravitational waves, and what, if anything, do they have to do with light?
Jiggling electric charge produces electromagnetic waves (radio light, visible light, x-ray light, and so on). Is there such a thing as “gravitational charge”? Yes there is. We call it mass. But since all matter particles have mass, they all possess this “charge.”
We notice gravity every day when we stand up, jump, or try to keep our meatballs from rolling off our plate. Mmm, meatballs. This is because we live on Earth, and Earth has a lot of mass. But everything, not just things with a lot of mass, produces a gravitational pull. It’s just that, for most things, the pull of gravity is incredibly small.
Yet even this small pull has been measured. In 1797-98, English scientist Henry Cavendish measured the gravitational force between two heavy balls. His experiment was so sensitive that Cavendish had to observe it from far away, ensuring that his own movements wouldn’t disturb the delicate equipment. A telescope focused on the apparatus led out of Cavendish’s basement laboratory and to the enraptured scientist stationed far away. He watched as the two balls oh so delicately pulled toward one another. Cavendish had just measured the strength of gravity.
Cavendish’s experiment helped us know the value of the gravitational constant, G, that appears in Newton’s equation for gravity. G plays the same role in Newton’s gravity equation that ε0 and µ0 play in the equations for electric and magnetic fields.
Just as Maxwell’s manipulations showed that electromagnetic waves (i.e. light, both invisible and visible) moved through space with a particular speed c, Albert Einstein’s work with gravity showed that it, too, moved through space with a particular speed. What is that speed? Amazingly, it is that same value c, the speed of light (and, as it turns out, gravitational waves). Einstein’s work showed that, just as jiggling an electron up and down produced electromagnetic waves, jiggling any massive object up and down produces gravitational waves. The catch is that gravitational waves are so incredibly weak that they are extremely difficult to detect.
While the BICEP2 data is not what most scientists would call a direct detection of gravitational waves, it is (if it holds up) excellent evidence for their existence. However, despite what some web stories are claiming, this is not the first indirect gravitational wave detection in history. In 1974, Joseph Taylor and Russell Hulse, using data collected from the Arecibo radio telescope in Puerto Rico, found two neutron stars traveling about one another in tight, fast orbits. These neutron stars are so massive, and are orbiting so quickly, that they produce copious gravitational waves.
Just like light waves, gravitational waves carry away energy. That energy has to come from somewhere; Einstein’s theory showed that the energy for gravitational waves produced by two such orbiting bodies must come from their orbital energy, causing the bodies to move toward one another and spin even faster (a bit like water getting faster as it spins down a drain). Taylor and Hulse were able to measure this changing rotation rate, and it matched Einstein’s prediction beautifully. While they hadn’t detected gravitational waves directly, Taylor and Hulse had shown that the orbiting neutron stars behaved exactly as if gravitational waves were real.
There’s a beautiful symmetry between the behavior of electromagnetic waves and gravitational waves. Both are, in an important way, properties of the fabric of spacetime – both are something the universe does. Both move at a particular speed, c. Both come in “colors” determined by their frequency. And, crucially, both give us a window to understand the universe.
When new kinds of electromagnetic waves beyond visible light were discovered, they revolutionized our understanding. Radio light, infrared light, ultraviolet light, x-ray light, and gamma ray light all gave us new insights when they were collected from the sky, including discoveries of pulsars (neutron stars), quasars, black holes, gamma ray bursts, and the cosmic microwave radiation itself. If and when we’re able to not just infer but in fact detect and study gravitational waves, we’ll have an entirely new way of “seeing” the universe. Who knows what discoveries await?