No amount of experimentation can ever prove me right; a
single experiment can prove me wrong.
Albert Einstein
Ripples in the Fabric of Space Time
In the section on
Relativity, we've seen how space and time can
curve. Near the Earth, this curvature is fairly
constant. The "pull of gravity" we experience is basically
the same at any two points in time. There is a gentle
variation in the pull, however. This fact is made clear in
the existence of ocean tides—changing gravity from
the pull of the Sun and the Moon changes the level of
water in different parts of the oceans. But we know that
the gravitational "pull" is really just curvature of
spacetime, so the curvature is changing slightly in
time. One of the most interesting predictions of the
General Theory of Relativity is that this changing
curvature can travel through space, much like a wave
across water. If we spin a paddle in water, waves travel
out across the surface of the water. The Earth orbiting
the Sun is just like a paddle spinning and stirring up
spacetime so
that gravitational
waves travel out
across spacetime.
We can better understand what a gravitational wave is by
looking at how it affects anything it passes
through. Imagine a ship on water. As a water wave passes
from right to left, the ship will rock up and down. If a
gravitational wave passes in the same direction, on the
other hand, the ship grows taller, then shorter—it
waves.
These waves have some important features. We are familiar with the idea of the speed of a wave on water—this is just how far the crest of a wave moves per unit time (miles per hour, or meters per second, for example). The same idea applies to a gravitational wave. Here, the crest is the point where the ship is most stretched out. In the picture above, you can watch as this point moves along the ship from right to left. For a real gravitational wave, this speed is always equal to the speed of light. (The gravitational wave is slowed down in these pictures, because it would be very hard to see otherwise.) Another important aspect of these waves is their wavelength. This is just the distance between wave crests.
We can turn our boat a little to see the waves from a
second perspective. Imagine the waves broadsiding the
boat. For a water wave, this will just raise and lower the
boat, as it rides on the crest and then in the trough of
the wave. For a gravitational wave, the ship will be
pulled and then pinched, as we see in the pictures.
These pictures show one more interesting feature of the waves—their amplitude, or size. For
the water wave, this is the distance the ship moves up or
down from its center position. For the gravitational wave,
this is the percentage of squeeze or stretch by which the
wave distorts the ship.
Nothing is pushing it
back and forth; spacetime is simply warped in such a way
that the path it follows is a "straight" line. Notice
especially that the distance between the flag and the
center of the ship is changing. If that gravitational wave
were passing through your head, the same thing would be
happening. In particular, the distances between parts of
your ears would change. The eardrum would be moving back
and forth, for example. But this is just what causes you
to experience sound. Could you hear a gravitational
wave?
In principle, yes! If the gravitational wave were strong enough, and the sound it made were a pitch humans could hear, then you would hear it passing through you. To be at the right pitch, the wave would have to move the ship's flag up and down at least twenty times per second. Of course, if it were strong enough to be heard, you would probably have other problems. Nonetheless, you could amplify the sounds if you had a microphone that was sensitive enough.
One question is whether anything in Nature makes these sounds at the right pitch, and loud enough, for humans to hear. Astrophysicists have found many sources for sounds that we actually could hear, with a good enough microphone. The next sections will take us through a set of possible sources of gravitational waves which may be heard in coming years. This will provide the strongest test yet for Einstein's ideas about spacetime.
A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose.
Reshaping Space
These waves have some important features. We are familiar with the idea of the speed of a wave on water—this is just how far the crest of a wave moves per unit time (miles per hour, or meters per second, for example). The same idea applies to a gravitational wave. Here, the crest is the point where the ship is most stretched out. In the picture above, you can watch as this point moves along the ship from right to left. For a real gravitational wave, this speed is always equal to the speed of light. (The gravitational wave is slowed down in these pictures, because it would be very hard to see otherwise.) Another important aspect of these waves is their wavelength. This is just the distance between wave crests.
These pictures show one more interesting feature of the waves—their amplitude,
The height of the peak of a wave, measured relative to its center. Equivalently, the depth of the trough of a wave.
The Sound of a Gravitational Wave
Watch the flag at the top of the ship's bow in the first gravitational wave picture above. As it moves back and forth, it is following a geodesic. Essentially the "straightest path" in a curved space or curved spacetime. This is the path followed by an object with no forces acting on it. In the curved spacetime of General Relativity, these paths may seem to be very curved—even appearing as circles or ellipses, for example. A geodesic is easily understood by looking at a very small region around the object. Even in highly curved spacetime, a small enough region will seem flat, so there is a natural idea of a "straight path". By following short segments, the whole geodesic is built up into one long path.
In principle, yes! If the gravitational wave were strong enough, and the sound it made were a pitch humans could hear, then you would hear it passing through you. To be at the right pitch, the wave would have to move the ship's flag up and down at least twenty times per second. Of course, if it were strong enough to be heard, you would probably have other problems. Nonetheless, you could amplify the sounds if you had a microphone that was sensitive enough.
One question is whether anything in Nature makes these sounds at the right pitch, and loud enough, for humans to hear. Astrophysicists have found many sources for sounds that we actually could hear, with a good enough microphone. The next sections will take us through a set of possible sources of gravitational waves which may be heard in coming years. This will provide the strongest test yet for Einstein's ideas about spacetime.
