On the evening of Thursday 13th April, the National Museum of Scotland hosted Professor Sheila Rowan, Director of the Institute of Gravitational Research and Chief Scientific Advisor for Scotland, and Professor Ken Strain, principal investigator of the UK Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) project, as they talked about their research into gravitational waves with science broadcaster Marcus Chown as part of the Edinburgh International Science Festival.
Stating that it was a subject which he found “totally fascinating,” Chown opened the event by asking on behalf of the audience for an explanation of what gravitational waves were. Often believed to require a deep understanding of mathematics, Professor Rowan instead said they were easier to understand in terms of geometry, using a simple analogy of imagining space as an empty room with a flexible floor.
Any object on that floor will stretch it, and that curvature represents the effect of gravity; a second smaller object will roll towards the first down that gradient, and any ripples in the surface are the result of stretching and squashing caused by movement of the objects and represent the gravitational waves.
Professor Strain commented that the interpretation of gravity by Sir Isaac Newton and Albert Einstein were very different, Newton conceiving of a pure vacuum unable to transmit energy yet with all actions propagated instantaneously, whereas now it is known that one conceits is wrong, the other not entirely correct. “We do not know how to describe what spacetime is or how its elastic properties work, but we know there is something.”
The effect of gravitational waves only presenting on the smallest scales imaginable or measurable, detection is extremely difficult, Professor Rowan saying that while we think of gravity as strong, holding objects to the Earth, it is in fact the weakest of the four fundamental forces and in fact it is tiny, only discernible around very massive objects such as stars and planets.
For this reason, only a colossal celestial event can generate a significant gravitational ripple such as the one detected by LIGO caused by two black holes, each around thirty times the mass of the Sun, collapsing into each other, “one of the most violent events in the history of the universe,” the detection of which confirmed the theoretical existence of gravitational waves and added further evidence supporting the existence of black holes which cannot be observed directly.
Believed to normally exist either as supermassive black holes at the centres of galaxies millions of times the mass of the Sun or smaller discrete bodies only a few times the mass of the Sun, Professor Strain confirmed that these black holes must have formed from huge, very rare stars, agreeing with Chown that these might have been the endpoint of the evolution of the last remaining first generation stars or suggested that they might be an aggregation of many smaller black holes, though any proposal they might be dark matter was “controversial.”
The LIGO was actually performing trial runs ahead of the official launch date of the facilities when the signal was received on 14th September 2015, a reading so strong, the maximum predicted by theory, that they were immediately suspicious and the team held back publication of the results until February 2016 in order that they could check and recheck everything and eliminate all other possibilities.
Professor Strain recalled the relief of finally being able to speak openly to university press officers after five months of enforced silence, while Professor Rowan said it had been very stressful, particularly for one unfortunate PhD student who was in their group but working on a different project, the result being that every time he walked into the room everyone else would suddenly stop talking.
A huge technical challenge to conceive, design and build, the LIGO is a Michelson Interferometer four kilometres long capable of measuring movements less than the wavelength of light featuring a series of four heavy pendulums suspended from each other in a vacuum to isolate them from vibrations, as demonstrated by Professor Strain with his pocket watch.
Described by Chown as “the most significant development in astronomy since Galileo invented the telescope,” when analysed the data indicated that the larger black hole formed by the collision of the two bodies now had a mass approximately sixty two times that of the Sun, or around twenty million Earths. With the Schwarzschild radius of the event horizon estimated at 183km it is “about the size of Iceland,” spinning around a hundred times a second with a horizontal equatorial velocity estimated at 0.4 of the speed of light.
Professor Rowan describing the LIGO instrument as “a microphone pointing towards space” rather than the camera of traditional optical astronomy, Professor Strain said that new perspective allowed them to deduce the bulk motion of objects rather than just seeing surfaces, Chown speculating that the work could very well win the 2017 Nobel Prize in physics.
Professor Strain modestly responded that only the committee would know that but agreed it was a significant discovery, with optical astronomy “as old as when we first looked up,” gravitational waves have arrived through history but are only now able to be received and interpreted, and Professor Rowan asking that any award recognise the “fundamental contributions” of the late Professor Ron Drever who was involved in the initiation of the project at Caltech, though ultimately over a thousand researchers participated. “No one person or country can solve these problems.”
The first run of the LIGO which captured a second signal on Boxing Day 2015 completed over three months, the second run has generated a further three candidate signals which have been notified to others working in the astronomy field to investigate with other tools, and the project itself is expanding.
With signals coming from any direction and the facilities fixed, the original two are based in Louisiana and Washington State 3,000 km away and can determine the source but three are needed for proper triangulation and four are best to get the maximum information, all operating at the same time to capture the fundamentally unpredictable event, and further facilities are now present in Germany and Italy with one under construction in Japan and a proposal for another in India.
With the caveat that no major investment is ever built on experimental equipment, the two American facilities used established technologies but the more modest Anglo German funding of the GEO600 in Hanover permitted them to be more versatile, successfully creating a second generation instrument more sensitive than the smaller size of the detector should have allowed using standard techniques, improvements which now established can be fed into future instruments.
The Japanese facility “like a James Bond film set, you drive into a mountain,” Rowan explained the ultimate goal was to place satellites in orbit, “vacuum for free” eliminating all vibrations, the separation increased to millions of miles to increase resolution, but with the current project having taken forty five years to come to fruition that is not expected to occur until 2034. “Things happen no faster in space. I’ll be retiring, so we’ll need a new generation of scientists.”
With more powerful and sensitive instruments, gravitational wave detection could move observational ability back closer to the big bang, Professor Strain considering the parallels with Galileo whose first discoveries were utterly unexpected, the moons of Jupiter and sunspots, so who knows what LIGO may actually find? With no current theory of the universe complete, Strain was asked as Einstein rewrote Newton, could this rewrite Einstein? “It’s possible.”