Astronomers have been observing the skies in essentially the same way for nearly 400 years. Since Galileo turned his telescopes on the moon in 1609, they’ve used ever more sophisticated means of detecting light emitted by distant objects, gathering up not just visible light but radio waves, x-rays, and other forms of electromagnetic radiation. But some of the most exciting things in space don’t send light our way.
The ability to observe the universe using something other than light could usher in a new era of astronomy–an era that MIT physicists believe will soon be upon us. Their hopes hinge on detecting gravitational waves, a kind of fundamental radiation predicted by Einstein in 1916 but not yet directly observed. Analyzing gravitational waves, they believe, will provide an unprecedented way to study the activity of spiraling neutron stars, black holes, and the cores of collapsing stars.
Not only are gravitational waves predicted by Einstein’s theory of relativity, but the predictions are supported by indirect empirical evidence, such as changes in the orbits of binary neutron stars that scientists have observed for decades. Before astronomers can analyze gravitational waves from distant objects, however, experimental physicists must detect the waves directly.
A team of researchers from MIT and Caltech can’t resist the challenge. They jointly operate the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is funded by the National Science Foundation at sites in Washington state and Louisiana. Built in the 1990s, LIGO is based on designs developed in the 1970s by Rainer Weiss ‘55, PhD ‘62, who is now professor emeritus of physics at MIT. By detecting changes in distance between finely calibrated mirrors, LIGO’s instruments measure tiny distortions in the fabric of space-time. With the first generation of detectors, which cost $230 million, the probability of detecting a gravitational wave was low. But LIGO physicists think they now have the technology to turn the corner. With upgrades in the works, they hope to achieve a tenfold increase in sensitivity–enough, they believe, to detect gravitational waves a few times a month by 2014.
These physicists relish tackling one of the most difficult precision engineering problems in the world: measuring distances smaller than the smallest atomic nuclei. “It’s really hard. And that has a huge attraction,” says Nergis Mavalvala, PhD ‘97, an associate professor of physics who’s designing upgraded instruments for LIGO. But even more exciting than the challenge is the potential payoff. “Gravity-wave physics is going to change the way we see the universe,” says associate professor of physics Erik Katsavounidis, who’s analyzing LIGO data. “There are few things in that class.”
From Newton to Einstein
Newton described gravity as a force of attraction between masses. But even Newton was dissatisfied with his inability to explain what causes gravity. That was left to Einstein, who explained how gravity arises by describing space itself in a new way.
Before Einstein, space was thought to be absolute, existing outside the influence of the masses moving within it. Einstein conceived of space as malleable and proposed that time and space were part of a four-dimensional system. Masses like the sun distort the fabric of space-time, causing what physicists call space-time curvature. This curvature affects the motion of other masses: in other words, it is what we experience as gravity.
As masses accelerate, Einstein proposed, they cause ripples in space-time like the waves that travel through water in the wake of a boat. These space-time wakes are what he called gravitational waves. Like light and sound waves, they’re described in terms of frequency and wavelength. In Newton’s equations, the force of gravity is exerted instantaneously, an approximation that works beautifully for most of what we can observe. But gravitational waves propagate through space-time at the speed of light. So any change in the gravitational pull of a distant object takes time to reach Earth, just as its light does. All this “takes a while to wrap your head around,” admits Scott Hughes, an assistant professor of physics.