## Rumors of Gravitational Waves

The Laser Interferometric Gravitational-Wave Observatory or LIGO is designed to detect gravitational waves—ripples of curvature in spacetime moving at the speed of light. It’s recently been upgraded, and it will either find gravitational waves soon or something really strange is going on.

Rumors are swirling that LIGO has seen gravitational waves produced by two black holes, of 29 and 36 solar masses, spiralling towards each other—and then colliding to form a single 62-solar-mass black hole!

You’ll notice that 29 + 36 is more than 62. So, it’s possible that three solar masses were turned into energy, mostly in the form of gravitational waves!

According to these rumors, the statistical significance of the signal is supposedly very high: better than 5 sigma! That means there’s at most a 0.000057% probability this event is a random fluke – assuming nobody made a mistake.

If these rumors are correct, we should soon see an official announcement. If the discovery holds up, someone will win a Nobel prize.

The discovery of gravitational waves is completely unsurprising, since they’re predicted by general relativity, a theory that’s passed many tests already. But it would open up a new window to the universe – and we’re likely to see interesting new things, once gravitational wave astronomy becomes a thing.

Here’s the tweet that launched the latest round of rumors:

For background on this story, try this:

Tale of a doomed galaxy, Azimuth, 8 November 2015.

The first four sections of that long post discuss gravitational waves created by black hole collisions—but the last section is about LIGO and an earlier round of rumors, so I’ll quote it here!

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about the supermassive black holes that live in the centers of galaxies. I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.

### 18 Responses to Rumors of Gravitational Waves

1. Dave Tweed says:

It’s interesting scientists seem to have similar expressions of joy all over:

https://en.m.wikipedia.org/wiki/Wow!_signal

• John Baez says:

Let’s hope this one does better than the ill-fated Wow! signal.

2. Crippa75 says:

If they can see the ripples there should be many more sources than one binary. There should be more interference?

• John Baez says:

A pair of binary neutron stars or black holes emits most of its gravitational radiation at the very end, when they spiral into each other and collide. If the gravitational waves were turned into sound, they would sound like this. Thus, right now we expect to ‘hear’ just one such collision at a time… usually. Later, when we get better at detecting gravitational waves, we may be able to detect several at once, and detect interference.

3. Ciro Villa says:

Now the real question to me is which ones spread faster: the gravitational waves or gossip waves?…;-)

4. Cy Myers says:

Do you have any information on the angular resolution they can detect? Presumably they preferentially detect in some direction (along the line of each arm?) and can’t detect at all in others (perpendicular to the plane of the arms?)

• John Baez says:

Nice question! Of course there are papers on the angular resolution of LIGO, since this experiment is costing over \$600 million—the most ambitious project ever undertaken by the National Science Foundation. Here’s the first paper I found:

• Linqing Wen, Angular resolution of a network of gravitational-wave detectors, 2007.

She says that

50% of the sources detected by the LIGO (L1,H1)-VIRGO (LHV) detector network have $\delta \theta < 0.5^\circ - 1.0^\circ$

In other words, if we use the LIGO facilities at both Livingston and Hanford, together with the VIRGO detector in Cascina, Italy, half the gravitational wave bursts they detect should be resolved to between half a degree or a degree. And this paper was written before the upgrade to Advanced LIGO. So the situation should be even better now. He says that in a while the resolution will be about 10′ (a sixth of a degree).

I have no idea if this first gravitational wave burst was detected by VIRGO, but I imagine it was detected at both Hanford Washington and Livingston Louisiana, or they would not have had the confidence to announce the result (assuming they will, in fact, announce such a result).

• duetosymmetry says:

Hi John,

Some corrections:

Linqing’s pronoun should be ‘she’

That reference is overly optimistic. Back in the day I worked on the topic of LIGO burst localization during my undergrad thesis (2005-2006), and later as grad student working on LIGO. The typical numbers we found were error boxes of about 3° ⨉ 10°, again for the HLV network. The situation for HL is much, much worse since the two detectors are nearly co-aligned
Such a large error region is essentially impossible to follow up on, unless there happened to be an electromagnetic counterpart which is time-coincident with a (hypothetical) GW signal. For black-hole–black-hole, there shouldn’t be any electromagnetic counterpart. For neutron-star–neutron-star, there could potentially be a gamma ray burst (GRB).
Regarding the angular pattern: the most sensitive direction is straight overhead the ‘L’, though with a spin-2 pattern, so if you spin LIGO around the vertical line, an overhead signal would be modulated through two complete cycles, meaning there are four ‘null’ orientations. The least sensitive directions are the four diagonals that lie in the plane of the two arms, 45° apart from the arms themselves. Along those 4 directions, no polarization of GW shows up in the detectors.

• John Baez says:

Thanks for clearing things up!

5. arch1 says:

If I can believe a quick internet search, gravitational waves are subject to gravitational lensing. Does this usefully affect LIGO’s current capabilities?

• John Baez says:

Yes, gravitational waves, like electromagnetic ones, are deflected by gravitational fields.

But right now we’ve never seen a gravitational wave. Or maybe we’ve seen just one: we should know later this week. So, it’s like we’re astronomers who have never yet managed to see stars or galaxies, and you’re asking about the effect of gravitational lensing on the images of stars and galaxies.

There’s an additional problem, too: while stars and galaxies put out light for a long time, the biggest sources of gravitational radiation (at least among the ones we know about) are brief in duration.

I could make all this a bit more quantitative if I knew 1) the angular deflection caused by the strongest known gravitational lenses, 2) the angular resolution of LIGO, or various planned gravitational wave observatories.

Above you’ll see that 1° is a very optimistic answer for question 2): with good gravitational wave detectors only in Louisiana and Washington, our angular resolution is lousy. Getting VIRGO or some other detector from from the US up to speed — making it equally sensitive — might help a lot.

As for question 1), I don’t know the answer, but online I see some examples of gravitational lensing where the deflection is about 3 arcseconds—that is, about 0.001°.

So, it seems we have quite a way to go!

• arch1 says:

Thanks John. I see that this paper estimates that lensing due to the black hole at our galaxy’s center could increase GW intensity by a factor of thousands, but only for sources within a very narrow cone (if I got this right, under their assumptions it would be the region of sky directly above a patch of ground roughly the size of the following circle: o).

6. John Baez says:

On Twitter, LIGO has announced they will hold a press conference on Thursday February 11th at 10:30am EST.

7. John Baez says:

The folks at New Scientist have done some clever investigative journalism to locate the source of the gravitational waves probably detected by LIGO:

• Joshu Sokol, Latest rumour of gravitational waves is probably true this time, New Scientist, 8 February 2016.

They write:

If we had a third detector, we could triangulate the source of the signal. That will be possible later this year, when the VIRGO experiment in Cascina, Italy, comes online. But with only the twin detectors, the best we can do is gesture vaguely toward a region of the sky.

So the LIGO team reaches out to astronomers at 75 observatories around the world, who scan that region for more traditional signals: visible light, gamma rays, even neutrinos. If a gravitational wave event is like the elusive Cheshire cat, then finding these counterparts is like spotting the grin.

Although the results of these searches are not public, whether or not they have taken place is – and so are the coordinates.

New Scientist focused on searches from the European Southern Observatory, a set of telescopes in Chile that will hunt for the bright flash of light that may accompany a gravitational wave signal. By plotting the locations of these searches, we can infer where and when LIGO’s ears may have been pricked.

We found that the first search began on 17 September, starting in the constellation Dorado and veering across other southern sky constellations (see blue spots on map). LIGO’s observing run officially began the very next morning, but the experiment had spent the previous few weeks collecting data in preparation.

The first rumour of a signal appeared just a week later, to the consternation of team members. If borne out, then the first signal ever seen fell into LIGO’s lap before the experiment officially started.

Two other searches – one around the constellation Aries, the other in the rough vicinity of Hydra – both began on 28 December and continued until the end of the run on 14 January (see red spots on map). The three searches together suggest LIGO may have been unbelievably lucky.

8. John Baez says:

The rumors are true: LIGO has seen gravitational waves! Based on the details of the signal detected, the LIGO team estimates that 1.3 billion years ago. two black holes spiralled into each other and collided. One was 29 times the mass of the Sun, the other 36 times. When they merged, 3 times the mass of the Sun was converted directly to energy and released as gravitational waves.

For a very short time, this event produced over 10 times more power than all the stars in the Universe!

We knew these things happened. We just weren’t good enough at detecting gravitational waves to see them – until now.

I’ll open comments on this breaking news item so we can all learn more. LIGO now has a page on this event, which is called GW150914 because it was seen on September 14th, 2015:

You can see the gravitational waveforms here:

At left in blue is the wave detected in Livingston, Louisiana. At right in red is the wave detected in Hanford Washington. The detector in Hanford saw the wave 7 milliseconds later, so it must have come from the sky in the Southern hemisphere.

The signals need to be carefully filtered, since there’s a lot of noise. On the LIGO webpage there’s a windowed Fourier transform of the signals, and this caption:

Figure 1. The gravitational wave event GW150914 observed by the LIGO Hanford (H1, left panel) and LIGO Livingston (L1, right panel) detectors. The two plots show how the gravitational wave strain (see below) produced by the event in each LIGO detector varied as a function of time (in seconds) and frequency (in hertz, or number of wave cycles per second). Both plots show the frequency of GW150914 sweeping sharply upwards, from 35 Hz to about 150 Hz over two tenths of a second. GW150914 arrived first at L1 and then at H1 about seven thousandths of a second later – consistent with the time taken for light, or gravitational waves, to travel between the two detectors.

9. John Baez says:

Here is a nice picture of the waves seen in Livingston and Hanford, separately and together:

Notice the vertical scale: the amount of change in the spacetime metric, roughly one part in a sextillion.

10. John Baez says:

Here’s the most definitive-sounding rumor I’ve heard about other gravitational wave detections at LIGO:

• Dennis Overbye, Gravitational waves detected, confirming Einstein’s theory, New York Times, 11 February 2016.

Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up.

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