The team at LIGO (the Laser Interferometry Gravitational-wave Observatory) annouced they directly measured the gravitational waves emitted by two black holes merging into one. What are they talking about? Here’s the answer to 4 of the most common questions (plus 2 extra-credit, if you feel up to it).

The merging of the two black holes, and the resulting gravitational wave. NASA

What are gravitational waves?

Gravitational waves are ripples in space-time predicted by Einstein’s General Relativity theory. If you heard anything about this theory, it probably is that, by their mass, objects warp space-time around them.

If mass is moving in the right way, it should (in theory) create gravitational waves. These propagate in space like ripples on a pond. They periodically make it longer and narrower, then shorter and broader, and so on.

Credit: MOBle/English Wikipedia

Credit: MOBle/English Wikipedia

Edit: Confused by the “stretching space” thing? I unpacked this (using puppies!) in a later post.

How big is the effect?

Tiny. Less than tiny: unfathomable catastrophes (crashes between black holes, supernovae exploding, stuff like that), relatively close (which means within our galaxy) change the distance between Earth and the Moon by about a thousandth the width of an atom.

As you can figure, an effect this tiny is pretty hard to measure.

How did we find them?

LIGO is a laser interferometer. It works by splitting a laser beam in two: one goes on its original way, the other bends 90 degrees away. Each of them bounces back and forth a few times a few hundred times along a tube. Then the two laser lights meet again, so that their lightwaves interfere with each other, perfectly canceling each other.

If a gravitational wave is to pass through the device, it stretches and shortens each leg, alternatively. This breaks the perfect synchronism between the two laser waves. The scientists looked for this sort of signal, where the two laser branches were out of sync.

To be absolutely sure they eliminated every disturbance, then, they looked for identical traces to appear in both a detector in Northwestern US and one in the Southeast. An additional detector, the Italian VIRGO, will join soon.

If the effect is so tiny, why bother?

Because gravitational waves provide a completely new way to perceive and study the universe. As Catherine Man, of the French Observatoire de la Côte d’Azur, said

Now we are no longer observing the universe with telescopes using ultraviolet light or visible light but we are listening to the noises produced by the effects of the gravitation of celestial bodies on the fabric of space-time

Among the things we could “listen” to there’s the echo of our universe’s youth. Until it became 380 thousand years old, the universe was opaque: it didn’t let light through. So we couldn’t observe anything earlier than that time using electromagnetic waves. But gravitational waves already existed, if we could hear them, we could learn much.

Extra credit for confident readers

Didn’t someone find gravitational waves a few years ago?

Yes and no: gravitational waves carry energy, and in 1993 Russel Hulse and Joseph Taylor did indeed win a Nobel prize in physics for observing that very energy. But, until now, nobody measured gravitational waves directly.

Also, last year, the BICEP2 project announced they found them. They were wrong. They didn’t look good that time.

So what changes with eLISA now?

Likely, not much. eLISA is a space observatory for gravitational waves planned by the European Space Agency, slated to launch in 2024. Even if gravitational waves won’t be new by then, eLISA is much much more sensitive than LIGO.

Basically, it will be a larger ear, capable of measuring different, fainter, farther away sources. And, being in space, it will experience much less disturbance.

If you want more
  • For the first anniversary of the discovery, we dug deeper: what does it mean to stretch space? how do you measure the stretch at all? And explained it with puppies!!

Cover photo: CC0 Austin Schmid, via unsplash.

3 thoughts on “Four fundamental things about gravitational waves

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