Category Archives: Gravitational Waves

Gravitational Waves: One year on

It’s a year since the LIGO Scientific Collaboration, including the Gravitational Physics group here in Cardiff, announced the very first detection of gravitational waves. I’ve been working with the teams in Cardiff and internationally for a little over a year, and it’s been a rollercoaster.

The announcement day itself was, quite frankly, crazy – but a good kind of crazy (radio interviews, live TV interviews on the roof, videos, podcasts, press conferences etc.). Since then I’ve given a number of interviews, public talks and school workshops about the detection and it’s all still as amazing as it was a year ago.

A lot of that is because the numbers are so earth-shattering, for example:

It says something that to express the power you need not one but two non-standard expressions of magnitude (septillion and yotta) – though since “yotta” and septillion both mean 1024, I can’t help thinking that it might sounds better as a “yotta-yotta-Watt”!

I also took this opportunity to update the Gravitational Wave Catalogue I made a while ago (fullscreen version here). You can change the axes on the graph, and show more information about the detections.

As for the future, LIGO is currently in Observing Run 2 (“O2” for short). Nothing much to say at the moment, but an official announcement by the LIGO Scientific Collaboration on 28th January read:

The second Advanced LIGO run began on November 30, 2016 and is currently in progress. As of January 23 approximately 12 days of Hanford-Livingston coincident science data have been collected, with a scheduled break between December 22, 2016 and January 4, 2017. Average reach of the LIGO network for binary merger events have been around 70 Mpc for 1.4+1.4 Msun, 300 Mpc for 10+10 Msun and 700 Mpc for 30+30 Msun mergers, with relative variations in time of the order of 10%.

So far, 2 event candidates, identified by online analysis using a loose false-alarm-rate threshold of one per month, have been identified and shared with astronomers who have signed memoranda of understanding with LIGO and Virgo for observational followup. A thorough investigation of the data and offline analysis are in progress; results will be shared when available.

The “reach” of the LIGO is defined by its sensitivity – because more distant events are fainter and so harder to detect (meaning that a more sensitive detector network can detect more distant events. The reason for giving three distances for different mass pairings is because more massive binaries produce stronger signals so can be “seen” further away – historically the range of gravitational wave detectors has been stated as the 1.4+1.4 Msun mergers (i.e. two neutron stars).

In terms of scale a Mpc is a “megaparsec” (one million parsecs), which is about 3.26 million light years. The Andromeda Galaxy (our nearest large neighbour) is about 2.5 million light years (0.75 Mpc) away. The Virgo cluster of galaxies, the closest large galaxy cluster, is about 50 million light years (15 Mpc) away, so well within range of LIGO. Though do remember that the previous detections were much further away than that – at around 1 billion light years.

What would a gravitational wave sound like up close?

LIGO Sensitivity

Actual sensitivity of the LIGO detectors (H1 = Hanford, L1 = Livingston) at a range of frequencies. Image credit: LIGO Scientific Collaboration

The gravitational waves detected by LIGO tend to be in the frequency range of a few tens of Hertz to a few hundred Hertz. That’s not just a coincidence – it’s the frequency that LIGO is designed to operate at, and that in turn is because that’s the frequency at which it’s expected that some of the most common relatively powerful gravitational wave sources in the sky occur at. It’s also a feature of the design of the instrument, with higher frequencies limited by the laser power (more photons = more sensitive) and lower frequencies limited by things like the seismic isolation, thermal variations in the mirrors and suspension, and a range of other things. If you are interested in finding out more, you should play Space Time Quest.

Because frequency range to which LIGO is sensitive is is within the human hearing range, it’s possible to convert the gravitational wave signals directly into sound. They’re often a bit low, so it’s common to shift them up in frequency – it can be that’s not really any different from shifting infrared data into the visible to make an image we can actually, well, see! The result, in the case of GW150914 (the first gravitational wave detection) is a sound like the one below. Continue reading

GW150914 – birth of a monster

It can be hard to have missed the news last week of the detection of gravitational waves – an event known as GW150914 [GW = Gravitational Wave, followed by the date of the event in the YYMMDD format]. There was, understandably, an awful lot of excitement – it hit pretty much every major news network, and was even mentioned at the BAFTA award ceremony (which is how we know we’ve made it…)!

In the weeks in the run-up to the detection there was a lot of talk about analogies and comparisons of the event, and the black holes involved. But how is that all calculated, and how does that compare to other things in the Universe? Continue reading

Gravitational Wave detection – the technical achievement

The gravitational wave signal detected by LIGO. Image credit: LIGO Scientific Collaboration

Well, what an exciting week it’s been! I’ve been lucky enough to be working with the Gravitational Physics group here in Cardiff, and the wider international LIGO Scientific Collaboration, for the last few months on the plans for the big announcement last week of the first detection of gravitational waves. I should make it clear that I’ve not been involved in any of the scientific analysis of the results, but in the outreach activities associated with them. Continue reading