Archive for LIGO

Questioning LIGO

Posted in The Universe and Stuff with tags , on June 17, 2017 by telescoper

Well. Cat, meet pigeons..

A paper appeared on the arXiv this week with the following abstract:

To date, the LIGO collaboration has detected three gravitational wave (GW) events appearing in both its Hanford and Livingston detectors. In this article we reexamine the LIGO data with regard to correlations between the two detectors. With special focus on GW150914, we report correlations in the detector noise which, at the time of the event, happen to be maximized for the same time lag as that found for the event itself. Specifically, we analyze correlations in the calibration lines in the vicinity of 35 Hz as well as the residual noise in the data after subtraction of the best-fit theoretical templates. The residual noise for the two more recent events, GW151226 and GW170104, exhibits equivalent behavior with respect to each of their time lags. A clear distinction between signal and noise therefore remains to be established in order to determine the contribution of gravitational waves to the detected signals.

I’m going to tread carefully here because (a) I have a number of colleagues at Cardiff who are directly involved in the analysis of LIGO data; (b) one of the authors of the new paper (Panel Naselsky) is a longstanding collaborator of mine; and (c) the new paper has not yet been refereed.

In fact I’m planning to visit Copenhagen in July/August and will catch up with Panel and the other authors then.

Whether or not the points raised in the new paper are correct – and I am firmly agnostic, having not done the analysis myself – I think it’s entirely reasonable of the authors to subject the LIGO data to independent analysis. That’s how science is supposed to work; the relevant data are in the public domain now. 

No doubt the LIGO consortium will respond officially in due course. Of course, if anyone would like to comment unofficially then they are free to do so through the box below.
Update: Here is a fairly detailed rebuttal post.

Simulation of the binary black-hole coalescence GW170104

Posted in The Universe and Stuff with tags , , , on June 2, 2017 by telescoper

Via the Cardiff University news website, I found this video of a computer simulation of the binary black-hole coalescence that gave rise to the gravitational wave event GW170104 whose detection was announced yesterday, so I thought I’d share it here.

Here’s the  blurb accompanying the video:

The video shows a numerical simulation of a binary black-hole coalescence with masses and spins consistent with the GW170104 observation. The strength of the gravitational wave is indicated by elevation as well as color, with blue indicating weak fields and yellow indicating strong fields. We rescale the amplitude of the gravitational wave during the simulation to show the signal during the entire animation not only close to merger, where it is strongest. The sizes of the black holes are increased by a factor of two to improve visibility. The bottom panel in the video shows the gravitational waveform starting at frequency of 25Hz. The fade in of the video corresponds to a frequency of about 30Hz.

© Numerical-relativistic simulation: S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics) and the Simulating eXtreme Spacetimes project; scientific visualization: T. Dietrich (Max Planck Institute for Gravitational Physics), R. Haas (NCSA).

The colour scheme gives me a headache, and there’s no sountrack, but it’s quite instructive nonetheless.

 

New Astronomy at the New Year (GW170104)

Posted in The Universe and Stuff with tags , on June 1, 2017 by telescoper

The inside story of the detection of gravitational wave event announced earlier today..

Write Science

by Shane L. Larson

Newton’s portrait.

January 4 holds a special place in the hearts of scientists — it is Isaac Newton’s birthday (*). Newton stood at the crossroads that led to modern science, and astronomy in particular. He was the first person to build a workable reflecting telescope, a design that now bears his name and for the past 4 centuries has been the dominant type of telescope used by amateurs and professionals alike. Newtonian telescopes have revealed much about the Cosmos to our wondering minds. Newton was also responsible for the first formulation of a physical law that describes the working of gravity, called the Universal Law of Gravitation. Today we use the Universal Law to launch satellites, send astronauts into orbit, convert the force of your feet on the bathroom scale into your “weight“, and a thousand other applications.  There is much to celebrate…

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LIGO Echoes, P-values and the False Discovery Rate

Posted in Astrohype, Bad Statistics, The Universe and Stuff with tags , , , , on December 12, 2016 by telescoper

Today is our staff Christmas lunch so I thought I’d get into the spirit by posting a grumbly article about a paper I found on the arXiv. In fact I came to this piece via a News item in Nature. Anyway, here is the abstract of the paper – which hasn’t been refereed yet:

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8MlogM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the “look elsewhere” effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at 2.9σ significance level (corresponding to false detection probability of 1 in 270). Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.

I’ve highlighted some of the text in bold. I’ve highlighted this because as written its wrong.

I’ve blogged many times before about this type of thing. The “significance level” quoted corresponds to a “p-value” of 0.0037 (or about 1/270). If I had my way we’d ban p-values and significance levels altogether because they are so often presented in a misleading fashion, as it is here.

What is wrong is that the significance level is not the same as the false detection probability.  While it is usually the case that the false detection probability (which is often called the false discovery rate) will decrease the lower your p-value is, these two quantities are not the same thing at all. Usually the false detection probability is much higher than the p-value. The physicist John Bahcall summed this up when he said, based on his experience, “about half of all 3σ  detections are false”. You can find a nice (and relatively simple) explanation of why this is the case here (which includes various references that are worth reading), but basically it’s because the p-value relates to the probability of seeing a signal at least as large as that observed under a null hypothesis (e.g.  detector noise) but says nothing directly about the probability of it being produced by an actual signal. To answer this latter question properly one really needs to use a Bayesian approach, but if you’re not keen on that I refer you to this (from David Colquhoun’s blog):

One problem with all of the approaches mentioned above was the need to guess at the prevalence of real effects (that’s what a Bayesian would call the prior probability). James Berger and colleagues (Sellke et al., 2001) have proposed a way round this problem by looking at all possible prior distributions and so coming up with a minimum false discovery rate that holds universally. The conclusions are much the same as before. If you claim to have found an effects whenever you observe a P value just less than 0.05, you will come to the wrong conclusion in at least 29% of the tests that you do. If, on the other hand, you use P = 0.001, you’ll be wrong in only 1.8% of cases.

Of course the actual false detection probability can be much higher than these limits, but they provide a useful rule of thumb,

To be fair the Nature item puts it more accurately:

The echoes could be a statistical fluke, and if random noise is behind the patterns, says Afshordi, then the chance of seeing such echoes is about 1 in 270, or 2.9 sigma. To be sure that they are not noise, such echoes will have to be spotted in future black-hole mergers. “The good thing is that new LIGO data with improved sensitivity will be coming in, so we should be able to confirm this or rule it out within the next two years.

Unfortunately, however, the LIGO background noise is rather complicated so it’s not even clear to me that this calculation based on “random noise”  is meaningful anyway.

The idea that the authors are trying to test is of course interesting, but it needs a more rigorous approach before any evidence (even “tentative” can be claimed). This is rather reminiscent of the problems interpreting apparent “anomalies” in the Cosmic Microwave Background, which is something I’ve been interested in over the years.

In summary, I’m not convinced. Merry Christmas.

 

 

Infinite LIGO Dreams

Posted in Art, The Universe and Stuff with tags , , , , , on November 28, 2016 by telescoper

There was a special event in the School of Physics & Astronomy here at Cardiff University on Friday afternoon – the unveiling of a new work of art in our coffee area. The work, a large oil painting, called Infinite LIGO Dreams by local artist Penelope Rose Cowley was inspired by the detection of gravitational waves earlier this year:

 

gravitational-wave-artwork-copyright-penelope-cowley-16x9

You can read more about this work, and the circumstances behind its creation, at the Cardiff University website and via the Physics World blog. If you like the piece you can order a poster-sized print from Penelope Cowleys’s own website here.

The unveiling of this artwork was preceded by a drinks reception, which probably accounts for the errors that crept into the blog post I wrote on Friday after the party!

 

!Happy Birthday GW150914!

Posted in The Universe and Stuff with tags , , on September 14, 2016 by telescoper

A birthday message to the first gravitational wave source to be detected, from my new office mate, Bernard Schutz!

The Rumbling Universe

Just a year ago today, after travelling some 1.4 billion years, the gravitational wave chirp we christened GW150914 passed through Earth. It disturbed the two gravitational wave detectors of the LIGO observatory enough for us to notice it, to get excited about it, and to get a large fraction of the general public excited about it! But GW150914 just kept on going and is now one further year along in its journey through the Universe. And it will keep going, spreading out and getting weaker but not otherwise being much disturbed, forever. Literally forever.

And GW150914 hardly noticed us! When we observe the Universe with our telescopes, detecting light or radio waves or gamma rays from the enormous variety of luminous objects out there, we capture the energy that enters our telescopes. The photons from a distant star terminate their journeys in our telescopes, leaving a tiny hole in the…

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Jobs in Gravitational Waves at Cardiff University

Posted in The Universe and Stuff with tags , , , , on September 5, 2016 by telescoper

Gradually settling back in here to the School of Physics & Astronomy at Cardiff University, I thought I’d indulge  in a bit of promotional activity and point out that, following on from the recent detection of gravitational waves by the Advanced LIGO Consortium, of which  Cardiff University is a member, there are two opportunities open for jobs in gravitational physics.

One is in the area of Gravitational Wave Astronomy. Here is the blurb:

The current Cardiff Gravitational Physics group has expertise in gravitational-wave data analysis, numerical relativity and source modelling, and astrophysical interpretation, and consists of four full-time and two part-time academic staff, two research fellows, five postdoctoral researchers and nine PhD students. Our research is supported by the UK Science and Technology Facilities Council (STFC), the Royal Society, and the European Horizon 2020 programme. The group is a founding member of GEO600, a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration and has played a leading role in these collaborations from their inception through to the recent first direct detection of gravitational waves, and is also active in planning and development of future detectors, such as LIGO-India, Einstein Telescope and Laser Interferometer Space Antenna (LISA).

This new appointment is part of a long-term expansion of the group, to broaden and strengthen our current research in gravitational-wave astronomy, and to build a world-leading group in gravitational-wave experimentation.

For the full advertisement, links to further particulars etc, see here.

The other is the area of Gravitational Wave Experimentation:

The current Cardiff Gravitational Physics group has expertise in gravitational-wave data analysis, numerical relativity and source modelling, and astrophysical interpretation, and consists of four full-time and two part-time academic staff, two research fellows, five postdoctoral researchers and nine PhD students. Our research is supported by the UK Science and Technology Facilities Council (STFC), the Royal Society, and the European Horizon 2020 programme. The group is a founding member of GEO600, a member of the Laser Interferometer Gravitational-Wave Observatory ( LIGO) Scientific Collaboration and has played a leading role in these collaborations from their inception through to the recent first direct detection of gravitational waves, and is also active in planning and development of future detectors, such as LIGO-India, Einstein Telescope and Laser Interferometer Space Antenna ( LISA).

This new appointment is part of a long-term expansion of the group, to broaden and strengthen our current research in gravitational-wave astronomy, and to build a world-leading group in gravitational-wave experimentation, with additional appointments expected in the near future.

For full details on this one see here.

The second appointment is intended to build on existing strengths by adding a more experimental dimension to Cardiff’s research in Gravitational Waves.