Are you picturing empty space?
Guess what? Space is not nothing!
As explained in another Space Place Amazing Fact, the great scientist Albert Einstein said that space curves around mass. So if space can be curved, it must be "something," rather than nothing. How weird is that?
According to Einstein, the curvature of space by mass creates gravity.
For example, the Sun is a huge mass. It curves the space around it for billions of miles. The planets that orbit the Sun are really just following the curvature in space created by the mass of the Sun.
Many experiments have supported this idea.
Going a step further . . .
When a big boat moves through water, the water has to get out of the way to make space for the boat. The water has to curve around the boat! When the water moves out of the way, it makes ripples or waves on the surface.
Like a boat moving through water, massive objects moving through space make ripples or waves in space. A massive star or black hole moving through space causes waves in space. We call these waves gravitational waves.
A passing gravitational wave squishes and stretches, squishes and stretches space itself and anything in it, like this balloon.
Credit: National Science Foundation
As we gaze at a starry sky on a quiet night, we cannot imagine all the violence taking place out there. We do not hear even a whisper of the massive black holes that may be orbiting closer and closer to each other in a violent death spiral. We do not hear a massive star exploding in a supernova at the end of its life.
But these violent events create intense gravitational waves.
This video, made by a computer, shows how the gravitational waves might appear, if they were visible—which they certainly are not.
Credit: National Science Foundation
And these waves can be interpreted as sounds of a sort too—the kinds of "sounds" carried through space itself by the gravitational waves.
In the video below, a computer draws the waves produced when a star spirals into a massive black hole. The video squeezes one year into one minute. At the end, the black hole finally swallows up the star in one quick "hoo-it"! The gravitational waves stop. It is suddenly "quiet."
Credit: Steve Drasco, Albert Einstein Institute
A mission to "hear" gravitational waves
No one has yet detected a gravitational wave. Gravitational waves are very, very weak by the time they reach us from some far distant event. But scientists are very close to having instruments precise enough to sense these faint ripples in space.
LIGO is a mission to detect gravitational waves. LIGO stands for Laser Interferometer Gravitational-wave Observatory. Unlike the LISA (the Laser Interferometer Space Antenna) mission to detect gravitational waves from space, LIGO will detect the waves from Earth's surface.
LIGO has two observatory locations: one in Washington state and one in Louisiana.
Each observatory has two long arms at right angles. Each arm is exactly the same length. Inside each arm is a long tube with a laser beam shining from one end to the other. Very precise instruments detect the slightest change in the length of the arms. A gravitational wave passing by will make one arm longer and one shorter, then the other way around, and so on. The difference could be as small as 1/1000th the width of a proton—one of the tiny particles inside an atom!
Scientists need both LIGO and LISA to study the objects and events that create gravitational waves. If gravitational waves were sound waves, LIGO hears the high notes and LISA hears the low notes.
LIGO and LISA each "hear" a different size of gravitational wave, which are created by different types of violent space events. LIGO will study pairs of very dense neutron stars that orbit each other and smaller black holes. LISA will study much more massive black holes found in the centers of galaxies like our Milky Way.