Thread by @duetosymmetry

Exactly 100 years ago today, the Moon's shadow passed directly over the Earth: a total solar eclipse. On that fateful day, several astronomers were working to photograph the stars by the edge of the sun at the moment of totality. This observation would rocket Einstein to fame. 1/

In Newton's theory, there is no reason for light to be bent by gravity. You can, however, derive some amount of bending by appealing to conservation of energy. But that amount of bending is also wrong. The right answer comes from Einstein's theory of general relativity. 2/

By May 1919, Einstein's theory (GR) had already been published for two and a half years. It had seen an early astronomical success in Einstein's calculation of how fast Mercury's orbit precessed around the Sun: 43 arcseconds per century more than in Newton's theory. 3/

But, the Mercury calculation was not a prediction — it was a confirmation of an observation by Le Verrier (1859). In contrast, the bending of starlight near the Sun was a *prediction*. Now this was a real test of Einstein's theory! 4/

The most famous astronomer observing the eclipse was Sir Arthur Eddington (he was definitely not the only one — science is a team effort). Eddington and his team announced that their observations were in agreement with Einstein's prediction. 5/

By Nov. 1919, word got around to the general public. One (sexist) headline, from the @NYTArchives:
LIGHTS ALL ASKEW IN THE HEAVENS
timesmachine.nytimes.com/timesmachine/1…
6/

These headlines followed the communication by Dyson, Eddington, and Davidson to the @royalsociety, received October 30, read November 6: "A determination of the deflection of light by the sun's gravitational field, from observations made at the total eclipse of May 29, 1919" 7/

You can read the original @royalsociety article here: royalsocietypublishing.org/doi/abs/10.109… 8/

It includes a reproduction of the now-famous photo taken from Sobral: (appearing at en.wikipedia.org/wiki/Solar_ecl…) 9/

The history of the expeditions to Sobral and Principe, and the other observers trying the same work, is fascinating. Here is a lovely piece from @lizlandau on the history: smithsonianmag.com/science-nature… 10/

If you want more scientific details, you can check out this piece by Cliff Will in CQG: iopscience.iop.org/article/10.108… (free preprint version: arxiv.org/abs/1409.7812) 11/

It's also worth noting that the 1919 eclipse observation was not the first time the observation was attempted, but rather the first time it succeeded. Argentine astronomers tried it first: cambridge.org/core/journals/… (h/t @jorgepullin) 12/

Now 100 years later, general relativity is in very good shape. Einstein would probably be happy about it, but not content: 13/

Today @LIGO and @ego_virgo are detecting gravitational waves from black holes crashing into each other every week (Einstein famously flip-flopped about whether GWs were a physical reality or a mathematical artifact, see Kennefick's history: press.princeton.edu/titles/8387.ht…) 14/

And the @ehtelescope performed the most incredible observation that's the modern descendant of the 1919 deflection observation: using the whole Earth as a radio telescope, observing the shadow of a supermassive black hole! 15/

Why is this a deflection observation? Well, the edge of the shadow is associated with the black hole's "light ring": a place where gravity can deflect light so much that it can go in *complete orbits* around the BH. 16/

If you want to play around with these photon orbits, check out my web toy: duetosymmetry.com/tool/kerr-circ… 17/

So, with all these confirmations of general relativity, why would Einstein not be content? For the same reason so many physicists today: general relativity does not play nicely with quantum mechanics, including the rest of the standard model of particle physics! 18/

In general relativity, singularities are not an accident of symmetry. Rather, they are a generic prediction, proved by Hawking and Penrose. So the math tells us that it's unavoidable for curvature to approach the Planck scale, where gravity and quantum mechanics are both key 19/

What's more, even far from the Planck scale, gravity is important for quantum mechanics. Black holes famously radiate (also a contribution from Hawking, but Bekenstein deserves some credit). What's wrong with Hawking radiation? Well, it leads to information being destroyed. 20/

What's wrong with information being destroyed? It means that probabilities stop adding up to 100%, and quantum mechanics stops making predictions or any sense! 21/

I hope that in the next century, we'll figure out a quantum theory of gravity. Maybe we'll get some observational guideposts from gravitational waves created by merging black holes, or some observation nobody's thought of yet. Here's to another 100 years of gravity! 22/22

^ Footnote about this image: this is a rendering I created from a simulation described in this publication inspirehep.net/record/1600952, where Okounkova, collaborators, and I were performing the first BBH merger simulations of a theory of gravity beyond GR. Next papers in the works!

^ Footnote about this graphic: I created this for a "fact sheet" about black holes relevant for the @ehtelescope. You can get the fact sheet PDF (or PNG) here: github.com/duetosymmetry/…

^ Reference for Le Verrier's publication on the Mercury observations/calculations: ui.adsabs.harvard.edu/abs/1859AnPar.… (there's a PDF link at right). It's 106 pages (in French), followed by 89 pages of data tables

^ Footnote: if you want to see the latest gravitational-wave *candidates* from LIGO and Virgo, head to gracedb.ligo.org/latest/ (and probably follow @cplberry, if you don't already!)

^ The EHT's results can be found in this Focus Issue in ApJL: iopscience.iop.org/journal/2041-8…