Stars and Stuff (Part 3) ? Golden Gravitational Waves *Updated

Artist’s depiction of neutron stars colliding after inspiralling.
(Credit: NASA/Swift/Dana Berry)

In previous posts, we firstly discussed the determination of the nature of white dwarf stars which are very hot and extremely dense and, in the second post, we reviewed Chandrasekhar’s conceptual breakthrough by which he determined the limiting core mass of a white dwarf, namely the Chandrasekhar Limit and we discussed the resulting scientific controversy.

Chandrasekhar had built on Einstein’s theory of relativity to show that there was a limit to the electron degeneracy pressure countering gravitational forces in a white dwarf and therefore a star core of more than 1.44 solar masses would collapse to form a black hole. Stellar cores of more than approximately 1.2 but less than 1.44 solar masses would form a neutron star. Neutron stars are extremely dense; a single 1mm3 grain of (neutron star) sugar would weigh about 500,000 tonnes ? more than the largest oil tanker.

Einstein had not only postulated that gravity would bend light (confirmed by observation of the solar eclipse in 1919) but that merging massive objects would emit gravity waves as their orbits decayed. What exactly are gravity waves? How are they detected? Brian Clegg’s book Gravitational Waves* explains the theory of gravity waves in everyday terms which do not require a doctorate in tensor calculus to understand and he also provides an excellent detailed description of the detection apparatus now in service (also summarised here).

A Laser Interferometer Gravitational-wave Observatory (LIGO) consists of a laser source, a beam splitter and two 4 km long evacuated 1.2 m diameter tubes set at right angles. The laser beam is reflected back and forth many times from mirrors at the ends of the tubes so that the total beam path length is more than 1,100 km. The mirrors are mounted on suspended ‘test masses’ with active and passive damping systems to counter the effects of seismic and environmental vibrations. The apparatus is so sensitive that it can detect movements 100 times smaller than the diameter of a nucleus. That’s the expected displacement a gravitational wave originating from a remote supermassive merger would cause. There are three main LIGO installations, two in the USA and one in Italy which together, by comparing differences in the time that a signal is detected, provide a celestial triangulation to locate the source (this is very similar to how a GPS works).

Processing the LIGO signals requires huge computational power to separate a gravitational wave signal from random noise caused by non-related events.

Because of the substantial investment and high running costs of LIGO, strict scientific protocols were set in place to ensure that false or sensationalised claims of detection could not happen and compromise the integrity of the observing team. Firstly, a processed signal had to meet statistical quality criteria so that there would be less than a one in 3? million chance of it being a spurious random event (see our previous post on this topic). Secondly, a team independent of the LIGO observing team would, at random intervals, inject a false positive signal into the data being processed; this was to ensure that all required protocols were strictly followed and there was no massaging or fudging of data. Thirdly, no announcements or leaks to the media were to occur unless the event had been proven to be real and was not random or caused by a test (false) signal.

This is how real science is done: through painstaking observations and confirmatory evidence testing a falsifiable hypothesis; not flawed models, partisan politics and sensationalist journalism.

LIGO had a small number of confirmed detections in 2015 and again in 2017 all of which resulted from inspiralling black hole mergers. These result in a signal of short duration, less than a second; dubbed a ‘chirp’. In August 2017, shortly after Gravitational Waves went to print, LIGO struck gold (in more than one sense) ? the inspiral merger of two neutron stars; merger events such as this one are now thought to account for most of the precious metals in the universe ? iridium, platinum and gold ?plus all the naturally-occurring radioactive elements.

Gravitational waves preceding event and ?-rays after event
Reproduced under Creative Commons Attribution 3.0 licence.

The ‘chirp’ lasted more than 100 seconds and was followed by the brief gamma-ray burst which is characteristic of a neutron star merger. The astronomical community worldwide was notified and confirmatory evidence from multiple observatory sources, X-ray, ultraviolet, optical, infrared and radio poured in.

Sky & Telescope (February 2018) summarised this in a ‘discovery timeline’ illustration:

Reproduced by kind permission of Sky & Telescope.

It is a scientific principle that observation trumps theory and, often, making that observation requires painstaking research, heavy investments, patience and sometimes a bit of luck too. The great physicists and mathematicians of the early 20th century had the ability to freely question and argue the merits of their theoretical hypotheses (in spite of the negative effects of individual disputes) and this has led to great advancements in mankind’s knowledge.

In the century since then, Chandrasekhar’s postulates of the existence of neutron stars and black holes have been proved correct by observation as has Einstein’s theory of relativity and gravitational waves. When we shut down arguments and disputes in favour of a single perceived preferred viewpoint only, this is to the detriment of our progress.

*Clegg, B. (2018). Gravitational Waves – How Einstein’s Spacetime Ripples Reveal the Secrets of the Universe. Omnibus Business Centre, London, N7 9DP, United Kingdom: Icon Books Ltd.

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