Everything you need to know about gravitational waves

Last year in September, upgrades of the gravitational wave interferometer LIGO were completed. The experiment – now named advanced LIGO – searches for gravitational waves emitted in the merger of two black holes. Such a merger signal should fall straight into advanced LIGOs reach.

Estimated gravitational wave spectrum. [Image Source]

It was thus expected that the upgraded experiment either sees something immediately, or we’ve gotten something terribly wrong. And indeed, rumors about a positive detection started to appear almost immediately after the upgrade. But it wasn’t until this week that the LIGO collaboration announced several press-conferences in the USA and Europe, scheduled for tomorrow, Thursday Feb 11, at 3:30pm GMT. So something big is going to hit the headlines tomorrow, and here are the essentials that you need to know.

Gravitational waves are periodic distortions of space-time. They alter distance ratios for orthogonal directions. An interferometer works by using lasers to measure and compare orthogonal distances very precisely, thus it picks up even the tiniest space-time deformations.

Moving masses produce gravitational waves much like moving charges create electromagnetic waves. The most relevant differences between the two cases are

1. Electromagnetic waves travel in space-time, whereas gravitational waves are a disturbance of space-time itself.
2. Electromagnetic waves have spin 1, gravitational waves have spin two. The spin counts how much you have to rotate the wave for it to come back onto itself. For the electromagnetic fields that’s one full rotation, for the gravitational field it’s only half a rotation.



[Image Credit: David Abergel]

3. The dominant electromagnetic emission comes from the dipole moment (normally used eg for transmitter antennae), but gravitational waves have no dipole moment (a consequence of momentum conservation). It’s instead the quadrupole emission that is leading.

If you keep these differences in mind, you can understand gravitational waves in much the same way as electromagnetic waves. They can exist at any wavelength. They move at the speed of light. How many there are at a given wavelength depends on how many processes there are to produce them. The known processes give rise to the distribution in the graphic above. A gravitational wave detector is basically an antenna tuned in to a particularly promising frequency.

Since all matter gravitates, the motion of matter generically creates gravitational waves. Every time you move, you create gravitational waves, lots of them. These are, however, so weak that they are impossible to measure.

The gravitational waves that LIGO is looking for come from the most violent events in the universe that we know of: black hole mergers. In these events, space-time gets distorted dramatically as the two black holes join to one, leading to significant emission of gravitational waves. This combined system later settles with a characteristic “ringdown” into a new stable state.



Yes, this also means that these gravitational waves go right through you and distort you oh-so-slightly on their way.

The wave-lengths of gravitational waves emitted in such merger events are typically of the same order as the dimension of the system. That is, for black holes with masses between 10 and 100 times the solar mass, wavelengths are typically a hundred to a thousand km – right in the range that LIGO is most sensitive.

If you want to score extra points when discussing the headlines we expect tomorrow, learn how to pronounce Fabry–Pérot. This is a method for bouncing back light-signals in interferometer arms several times before making the measurments, which effectively increases the armlength. This is why LIGO is sensitive in a wavelength regime far longer than its actual arm length of about 2-4 km. And don’t call them gravity waves. A gravity wave is a cloud phenomenon.

Gravitational waves were predicted a hundred years ago as one of the consequences of Einstein’s theory of General Relativity. Their existence has since been indirectly confirmed because gravitational wave emission leads to energy loss, which has the consequence that two stars which orbit around a common center speed up over the course of time. This has been observed and was awarded the Nobel Prize for physics in 1993. If LIGO has detected the sought-after signal, it would not be the first detection, but the first direct detection.

Interestingly, even though it was long known that black hole mergers would emit gravitational waves, it wasn’t until computing power had increased sufficiently that precise predictions became possible. So it’s not like experiment is all that far behind theory on that one. General Relativity, though often praised for its beauty, does leave you with one nasty set of equations that in most cases cannot be solved analytically and computer simulations become necessary.

The existence of gravitational waves is not doubted by anyone in the physics community, or at least not by anybody I have met. This is for good reasons: On the experimental side there is the indirect evidence, and on the theoretical side there is the difficulty of making any theory of gravity work that does not have gravitational waves. But the direct detection of gravitational waves would be tremendously exciting because it opens our eyes to an entirely new view on the universe.

Hundreds of millions of years ago, a primitive form of life crawled out of the water on planet Earth and opened their eyes to see, for the first time, the light of the stars. Detecting gravitational waves is a momentous event just like this – it’s the first time we can receive signals that were previously entirely hidden from us, revealing an entirely new layer of reality.

So bookmark the webcast page and mark your calendar for tomorrow 3:30 GMT  –  it might enter the history books.

Update Feb 11: The rumors were all true. They have a 5.1 σ signal of a binary black hole merger. The paper is published in PRL, here is the abstract.