REVIEWS
POPULAR SCIENCE
Orange glowing doughnuts
The image of an orange glowing doughnut shape stirred a global sensation when it was revealed on 10 April 2019. The first visualisation of a black hole seen as a black shadow in the middle, is framed by the spiralling accretion disk of material being sucked into it. The strong deflection of light even from behind the black hole makes this radiation appear as a doughnut-shaped halo. The image was the result of two
years of data analysis based on observations made in April 2017. Back then, radio telescopes around the world were combined in an effort to produce an Earth-sized instrument to pick up radio waves – 1.3mm is the perfect wavelength for this – from the black hole in the centre of the M87 galaxy, which is 55m light years away and 6.5bn times the mass of our Sun. A similar effort was made to see the black hole at the centre of our own galaxy, the Milky Way, which is much closer and much smaller, and casts a shadow on our sky, matching the size of a real doughnut on the surface of the moon. In the end, the M87 data proved more tractable and was fast- tracked to get the first ever picture of a black hole. The image with the dark shadow in
the middle of the orange ring broadly confirms Einstein’s theory of general relativity. Thus, it neatly closes a century of experimental verifications of this theory. The first of these was Arthur Eddington’s observation on 29 May 1919 of the stars surrounding a solar eclipse. Eddington and colleagues’ results showed that the Sun deflects the light from more distant stars roughly as strongly as Einstein predicted, which is twice
Gravity’s century Author Ron Cowen
Publisher Harvard University Press Pages 192 Price £19.95
ISBN 978-0-674- 97496-8
Reviewer Michael Gross is a science writer based in Oxford, UK
as much as the deflection expected based on Newtonian gravitation. In his breathless but very readable whirlwind tour through the first century of observations putting Einstein’s theory to the test, science writer Ron Cowen tells the story of how Einstein came up with the theory and had to adjust it after initial errors, followed by milestones in observing the Universe in the light of special relativity and mostly proving Einstein right. Between the chapters on Eddington’s eclipse expedition and this year’s sensational image of the black hole there are ones on important milestones, including two on relatively recent achievements. The first two advances that Cowen highlights after Eddington’s work are the realisation that the Universe expands and the discovery – through rather indirect evidence – that there is a black hole at the centre of every galaxy. These 20th century developments in cosmology have been widely popularised by Stephen Hawking and many others, but they left black holes as celestial objects that we cannot see although we can measure their influence on their surroundings. The story moves to the 21st
century and the connections between quantum entanglement and the wormholes predicted by Einstein’s theory. But its climax really comes in the last two chapters, where, as the author succinctly summarises, ’it turned out they would hear the evidence before they would see it’. The first measurement of
gravitational waves recorded at the US-based Laser Interferometer Gravitational-Wave Observatory
(LIGO) observatory in September 2015 were the ripples produced by two black holes colliding. As they are vibrations in the space-time continuum with frequencies between 10 and 1000Hz, they can readily be compared with sound waves, which are vibrations in a carrying medium at similar frequencies. For illustrative purposes, researchers can translate the gravitational waves recorded into sound waves, and then we can hear black holes colliding. This work was quickly confirmed by observations of gravitational waves from similar events, including a collision of two neutron stars where light observations from the same event could also be registered. These discoveries led to the award of the Nobel Prize in physics to Rainer Weiss, Barry Barish and Kip Thorne in 2017.
And just two years after the
gravitational waves reporting black holes colliding, researchers started to collect those radio waves leading to the now famous doughnut image. At just over 1mm wavelength, the electromagnetic waves detected are far removed from anything we can see, so the idea of ‘looking at’ a black hole must be read in a largely metaphorical sense. Sceptics could argue that we are no closer to seeing the M87 black hole than to hearing the collision of the black holes detected by LIGO. In any case, the image, like the book, marks an exciting start into the second century of observational verification of special relativity. Further investigations using the same technique may well give rise to improvements to Einstein’s work.
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EHT COLLABORATION / EUROPEAN SOUTHERN OBSERVATORY / SCIENCE PHOTO LIBRARY
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