Introduction
At 11:53 am of September 14 2015, Marco Drago, an Italian postdoctoral scientist at the Max Planck Institute for Gravitational Physics (AKA Albert Einstein Institute) in Hannover, Germany, was the first person to see it: the sophisticated instruments at the Laser Interferometer Gravitational-Wave Observatory (LIGO) had detected a very likely candidate signal from gravitational waves. The signal, caused by the collision and merger of two massive black holes, proved to be real; the discovery, announced to the public a few months later, demonstrated one of Albert Einstein’s predictions exactly 100 years after he formulated his general theory of relativity. How exciting!
I was aware that Python was used in both LIGO’s control room and for some of the scientific work and data analysis, through a Python LIGO thread on Reddit. Also, one of the main figures in the discovery paper was made entirely in Python, and used a Matplotlib perceptual colormap (Viridis, the new default in mpl 2.0).
It was not, however, until I decided to attend (virtually, on Youtube) this year’s PyCon Italia that I realized how big a role Python had played. In this post, I will briefly summarize what Franco Carbognani, keynote speaker for day 2 (Python and the dawn of gravitational-wave astronomy), and Tito dal Canton (Python’s role in the detection of gravitational waves) presented on the science and technology of gravitational waves detection, and on Python’s contributions.
The science
Most of us have studied that gravity produces a curvature in spacetime. A less generally known, but major prediction of general relativity is that a perturbation of spacetime produces gravitational waves, which are detected because they stretch and squeeze space producing a measurable strain; this however requires a violent cosmological phenomenon involving large masses, relativistic speeds (approaching the speed of light), and asymmetrical acceleration (an abrupt change in those speeds, such as in a collision).
One such event is that which generated the signal detected in September of last year. It occurred about 1300 million years ago when two large black holes spiralled towards one another and then merged into a single, stationary black hole, losing energy by way of radiating gravitational waves (ripples in space-time) exactly as predicted by Einstein; the whole process took only a few tenths of a second, but involved a total of 65 solar masses (36 + 29), 3 of which were converted, mostly in the final instant of the merger, to gravitational wave energy according to the famous relationship E = mc2. This was so much energy that, had it been visible (electromagnetic), it would’ve been 50 times brighter than the entire universe. Yet, even a huge event like that created very small waves (gravity is the weakest of the four fundamental interactions of nature), which merely displaced a pair of free-falling masses placed 3 km apart by a length 10,000 smaller than the diameter of a proton. Measuring this displacement requires very complex arrays of laser beams, mirrors, and detectors measuring interference (interferometers), such as the 2 LIGO in the United States, and VIRGO in Italy.
This video illustrates in detail how VIRGO (and LIGO) is able to detects the very weak waves; it was produced by Marco Kraan of the National Institute for Subatomic Physics in Amsterdam, and shown during Carbognani’s talk:
The figure below shows an illustration of the event’s 3 main phases and the corresponding, matched signals from the 2 LIGO interferometers.
LIGO detects gravitational waves from merging black holes. Illustration credit: LIGO, NSF, Aurore Simonnet (Sonoma State University).
Python’s many contributions
Franco Carbognani is VIRGO integration manager with the European Gravitational Observatory in Pisa. The portion of his talk focusing on Python and VIRGO’s control systems starts here. He told the audience that where Python played a major role was in the building of a complete automation layer on top of real-time interferometer control, with analysis of data online and warning to the operator in case of anomalies, and also in GUI development and unification. Python was the obvious choice for these tasks because it is a compact, clear language, easy for beginners to learn, and yet allowing very complex programming (functional, object oriented, imperative, exceptions, etc.); its Numpy and Scipy libraries allow handling of the complex math required, without sacrificing speed (thanks to the optimized Fortran and C under the hood); a large collection of other libraries allows for almost any task to be carried out, including (as mentioned above) Matplotlib for publication quality graphs; finally, it is open-source, and many in the community already used it (commissioning, computing, and data analytics groups).
Tito dal Canton is a postdoctoral scientist at the Max Planck Institute in Hannover. The data analysis part of his talk starts here. The workflow he outlined, involving the use of several separate pipelines, consists of retrieving the data, deciding which data can be analyzed, decide if it contains a signal, estimate its statistical significance, and estimating the signal parameters (e.g. masses of stars, spin velocity, and distance) by comparison with a model. A lot of this work is run entirely in Python using either the GWpy and PyCBC packages. For the keener reader, one of the Jupyter Notebooks on the LIGO Python tutorials page, replicates some of the signal processing and data analysis shown during his talk.