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Today in the world of astrophysics we celebrate the 100th anniversary of an eclipse. But not just any eclipse. This one provided the first observational evidence to the theory we've been using for a century to make sense of the Universe. So, thread #1919Eclipse #onceacosmologist https://twitter.com/RMGreenwich/status/1133737273990811653
The theory is Albert Einstein's general relativity, presented in 1915. Why an eclipse, though? Within general relativity, we understand gravity as the way objects that have mass interact with their environment, aka spacetime. Which by the way isn't flat but curved & flexible. Ha!
Massive objects bend spacetime, and with it the paths of everything that moves in it. Including light particles (which by the way have no mass). To test this, you need a massive object in the foreground and sources of light in the background. Like the Sun and the stars, right?
Indeed, the Sun and the stars would do: just look at some stars in the night sky and then again after a few months during the day, when the Sun comes in between you and the stars. But wait, how can we see the stars during the day? The eclipse, ha!
The idea that massive bodies may bend light is older than Einstein's theory. Newton and others had pondered it before, and Einstein himself tried to calculate by how much the Sun would deflect the light from distant stars in 1911, before he had completed general relativity...
Pre-general relativity calculations (even Einstein's one from 1911) predict that the position of a star would appear as shifted by the 'gravitational lensing' effect of the Sun by 0.85 arc seconds – a tiny amount (the full Moon spans 30 arc minutes, more than 1800x as much)
Within general relativity, there is an extra term because spacetime is curved (Einstein's novelty, remember?) so the deflection should be 2x as much: 1.74 arc seconds – still pretty tiny though.
When Einstein presented general relativity in 1915, he could explain the one thing that was not understood within Newton's theory of gravity – a tiny shift in the orbit of Mercury. The prediction that mass bends light by that amount provided an opportunity to test the theory...
Of course things were more complicated than that because science doesn't happen in a vacuum. In particular, a century ago, science was happening within the world-order-reshuffling horrors of World War I.
A summary of events courtesy of @telescoper's paper https://arxiv.org/pdf/astro-ph/0102462.pdf
A summary of events courtesy of @telescoper's paper https://arxiv.org/pdf/astro-ph/0102462.pdf
Finally, on 29 May 1919, two expeditions to observe a solar eclipse measured the gravitational bending, roughly in the amount predicted by Einstein. A few months later, the results announced the triumph of general relativity – splashy headlines included https://twitter.com/HdAstro/status/1133684924463165440
Do you also wonder where were the women of science while the men were busy being 'more or less agog'? Surely counting stars on astronomical plates, classifying spectra, typing papers for the male professors, fighting their way to become the first to graduate at some university...
More on the eclipse expeditions, the measurements and their relevance – at the time and in the years and decades to come – in today's blog post by Prof @telescoper https://twitter.com/telescoper/status/1133658137687408640
To some, the results of the #1919Eclipse were not convincing. Still, they were confirmed and with increasing precision eclipse after eclipse. I'm not an observational astronomer but I wish I could try once during an eclipse, just for fun – and also to finally see a solar eclipse!
Besides eclipses though, it would take 60 years, almost to the day, to observe another instance of gravitational lensing. The paper by D. Walsh et al. on 0957+561, aka the 'twin quasar', would appear in Nature on 31 May 1979 https://www.spacetelescope.org/images/potw1403a/
Ever since, gravitational lensing has become a whole little discipline in astronomy and cosmology, with measured deflections by objects as huge as galaxy clusters and as small as planets (yes, @ESAGaia has to take that into account to measure correctly the positions of stars!)
... but it all started with the #1919Eclipse! I like to refer to @telescoper's account because, to the best of my recollection, that's how I first learnt about gravitational lensing – as part of a cosmology course at U. Bologna based on the Coles & Lucchin textbook. Thanks btw :)
For some reason though, the geographical aspect of the #1919Eclipse expeditions didn't register into my head at the time, so I (re-)discovered the locations of the expeditions during the International Year of Astronomy 2009 and the celebrations of the 90th eclipse anniversary
And because again, science doesn't happen in a vacuum, do you wonder why the eclipse path crossed two Portuguese-speaking lands (one of them still a colony at the time) either side of the Atlantic? Yes, the other link between the coasts of West Africa and North-Eastern Brazil...
Still, my first approach to this story - and most of the coverage to the day - focussed around Eddington's observations. I would find out the details of Andrew Crommelin's expedition to Sobral (Brazil) only later, in the beautiful novel 'The falling sky' by @goldipipschmidt
Which takes me to today – or actually, to four years ago, when I first visited Brazil with the partner. Besides São Paulo, we also went to Fortaleza, where his father was originally from, in the north-eastern state of Ceará. The same state of Sobral, one of the eclipse locations
Not far in Brazilian terms (about 200km) turned out to be too far for us to visit Sobral and the eclipse museum @Luz_de_Sobral back then. We thought we would be back – the latest by the centennial of the eclipse, it seemed there was still so much time (we haven't so far)
I'm happy to read (especially in today's climate) the #1919Eclipse museum in Sobral underwent renovation and it was inaugurated today. I hope I can visit soon – though it seems not this year. Perhaps at another anniversary – perhaps a prime number... https://twitter.com/SobralEcoa/status/1133530918428258304
Shout out to the colleagues celebrating in Sobral today https://twitter.com/jorgegrivero/status/1133770825813889029
And those in Sundy (Príncipe island) too! Glad to see a more diverse crowd being 'more or less agog' today than the men of science of a century ago https://twitter.com/marksubbarao/status/1133162146639097856
