Tuesday, October 10, 2017

Wave like a nice black hole ...

I have posted enthusiastically several times about gravitational-wave detections by LIGO (that's the Laser Interferometer Gravitational-Wave Observatory) beginning with Leggo my LIGO, in February 2016. Gravitational waves being very weak, only cataclysmic events -- such as the collision of two black holes -- are (so far, anyway) detectable by the gravitational waves they emit. Of course black holes colliding are very cool events to be able to detect.

In breaking news, the Nobel Committee also thought this was pretty keen: "Three Americans win Nobel Prize in physics for gravitational wave discovery."

Rainer Weiss, Barry C. Barish and Kip S. Thorne have won the 2017 Nobel Prize in physics. The three Americans are members of the LIGO-Virgo detector collaboration that discovered gravitational waves. The prize was awarded “for decisive contributions to the LIGO detector and the observation of gravitational waves,” the committee said in a news release.

That's one LIGO instrument
LIGO consists of two detectors, placed half a continent apart. To have just one won't do, because it's necessary to distinguish between the tiny jiggle of passing gravitational waves and, say, the rumble of nearby thunderclaps or passing trucks. As a bonus, when both instruments detect a signal, the slight difference in arrival times (the waves traveling at light speed) gives a rough sense for the direction from which the signal originated.

Emphasis on rough.

And another current event, from Sky and Telescope: "Fourth Gravitational Wave Event Detected." What makes this latest detection different from the original three is that, together with LIGO, the Advanced Virgo detector of the European Gravitational Observatory also recorded the event. It's the first for Virgo. And also, a Big Deal. From the S&T article:

With just the two LIGO detections, the uncertainty area measured some 1,160 square degrees on the sky," says Shoemaker. "By adding the Virgo data, this could be brought down to just 60 square degrees."

Mind you, 60 square degrees still covers a lot of sky -- the full Moon covers about a quarter of one square degree. But a reduction from 1160 down to 60 remains a huge improvement. Optical telescopes have not yet seen anything unusual in the target area, but  ... maybe next time.

(Can one see, as by incoming light, two colliding black holes -- the result of which, besides gravitational waves, is one bigger black hole? Unclear. You can never see a black hole itself, of course, because not even light can escape one, but lots of matter might be orbiting around, spiraling into, either (or both) of the doomed smaller black holes -- and what happens to that matter should be visually spectacular. If/when LIGO and its ilk detect/localize two neutron stars colliding? That event certainly should put on a great show -- if we happen to deduce where to look.)

But what if we had four gravitational-wave detectors? Five? A dozen? Well, we'd have a pretty good idea exactly where to look. Alas, LIGO is big (each instrument is like a letter L -- 4 kilometers on a side) and expensive (in round-ish numbers, the pair of LIGO sites cost $1.1 billion). Getting LIGO built was a multi-decade struggle (for the back story, see Black Hole Blues, by Janna Levin.)

True, we now know how to build these immense, extremely sensitive, laser interferometers. New instruments will cost less than the first. Also, India is planning to build one of its own. But monsters like LIGO will never be cheap. What if, instead, the waves could be detected in another way?

Perhaps they can. Physics World has an intriguing piece on, "How to detect gravitational waves using superfluid helium." Theorists say a few kilograms of the superfluid helium would suffice for a detector. Similar claims were once made for detectors based upon small metal bars -- and the irreproducibility of claimed detections gave gravitational-wave detection a terrible reputation for many years (again, see Black Hole Blues). Still, one can hope ...

Okay. That's all the waves I propose to make today ...

No comments: