Monday, June 20, 2016

A second gravitational wave event.

A few months ago, on February 11, there was an announcement from LIGO. It had detected a signal from a pair of a black holes - the first direct detection of gravitational waves. The signal was actually detected the previous September, but announced in February.

However there were other events. The process works this way: when an event occurs, computer algorithms very quickly sort events into “probably noise from a terrestrial event such as a truck passing nearby” and “probably a real event from black holes or neutron stars.” If the event is a real event, the algorithm estimates the mass of the black holes or neutron stars involved. For some background information on how these algorithms work see

When there are “probably real” events, additional analysis is done on the data and this analysis takes time. This is the reason for the delay between the event detection and the public announcement.

Shortly after the February announcement, there were already rumors of additional events. You'll have to forgive me, I was aware of these rumors, but did not report on it until now – nevertheless I had good reason to believe they were almost certainly true. We now have official confirmation they were true. A second event had been detected in December and was announced last week. See this article from New York Times science writer Dennis Overbye:

What happens now? Almost certainly there are and/or will be additional events beyond the two we know about, and we should see announcements of these events over time. Also in the near future, additional facilities similar to LIGO will come on line. These facilities will allow researchers to make more precise determinations of the direction the gravitational waves are coming from, possibly allowing the source of the waves to be located.

Unfortunately ground based gravitational wave detectors are limited in the frequencies they are able to detect. For this reason, there have been plans to place detectors in space. The first such plan was LISA. A set of spacecraft in space would perform measurements similar to what LIGO does, but because they are not attached to a solid object (namely the earth), they can respond to a wider range of frequencies, allowing a wider range of gravitational waves to be detected. LISA is now defunct, but the idea was resurrected in a new project called eLisa. For information see...

The first phase of this project called eLisa Pathfinder was launched this past December. It is a proof of concept which will not be able to detect gravitational waves, but will develop some of the technologies needed to detect gravitational waves. The current plan is to launch eLisa in the year 2034.