In Einstein’s theory of general relativity, he put forth the radical idea that space is not just a static, empty arena in which celestial objects go about their business. Instead, he described our universe as a sort of fabric that is curved and distorted by the objects within it. Sound crazy?
Maybe. But researchers at Northwestern and their colleagues within a world-wide scientific collaboration have developed ways to detect and measure these disturbances in the fabric of space.Vicky Kalogera, professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences, explains.
How can we think of space as “fabric?” And what causes these disturbances?
First, imagine space as an elastic membrane, spread flat. Now, set a big ball on the membrane. It will cause the membrane to sag. This is similar to how an object with mass, like a planet, affects the geometry of space. Although we cannot perceive this distortion in everyday life, aside from strong gravitational fields on Earth, it is measurable in the motion of planets due to our Sun’s gravity.
Now, picture another object moving in a circle—an orbit—in this distorted, sagged membrane. This creates regular disturbances that propagate through space like a wave. We call them gravitational waves, because it is gravity that keeps objects in orbit and disturb the “membrane” of space. It’s like the ripples that follow when you throw a rock in a lake.
In order for these waves to be large enough for us to detect them on Earth, they have to be caused by very massive objects orbiting around one another at speeds approaching the speed of light, like two black holes.
A black hole is an object in space with a very high mass in relation to its size, making it extremely dense. For example, a black hole with the mass of the Earth would only be an inch across—very, very dense! This makes the gravitational pull of black holes incredibly strong—so strong that even light cannot escape.
We know black holes exist—we’ve observed 20 of them so far, but only visually and through X-rays. Measuring the gravitational waves they produce will give us a new way to study them.
So how do you detect gravitational waves?
We use special L-shaped detectors, called laser interferometers, with two arms each a few kilometers long. Each arm houses a pair of mirrors that are two and a half miles apart. The entire system is carefully constructed to block out as many vibrations as technologically possible, from minute seismic shifts to cars driving on the street outside.
This allows an extremely sensitive laser, which repeatedly measures the distance between the two mirrors in each arm, to detect shifts in their position at the atomic level. When this distance varies between arms of the detector at regular intervals, and the variation is confirmed by both detectors (one in Washington D.C. and the other in Louisiana), we can infer this is due to gravitational waves. Keep in mind that these are very, very small changes—we liken it to the width of a human hair as viewed from as far away as the Sun—10-21, to be exact, or one-thousandth the diameter of a proton.
These detectors, which took ten years to construct, were built by the Laser Interferometer Gravitational–wave Observatory (LIGO), a collaboration funded by the National Science Foundation that includes Northwestern. Other similar detectors are operating in Europe, and both groups work together in a large-scale collaboration.
What can we learn from this research?
Using these detectors, we can study the force of gravity itself, as well as the matter in space that creates gravitational waves. Right now, we only know about 10% of the matter that makes up our universe—this research may provide a new way of learning about matter we don’t yet understand.
For example, my research group is interested in the fundamental properties of black holes—their masses, if they are spinning and, if so, how fast. The data that come from LIGO, compared against models we’ve already created, will help us determine answers to these questions.
>Want to learn more? A special traveling exhibit that further explores gravitational waves and the LIGO detectors is on display at the Adler Planetarium through August 10th.