Conversation

Notices

1. Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:04:13 UTC Robert McNees

   Deflection of starlight by the Sun’s gravitational field, measured during a solar eclipse and confirming a central prediction of Einstein's general relativity, was announced at a joint meeting of the Royal Society and the Royal Astronomical Society #OTD in 1919.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:06:11 UTC Robert McNees

     in reply to

     The measurements were made by two teams during the total solar eclipse on May 29 of that year. Eddington led an expedition to Principe; Crommelin led an expedition to Sobral.

     Eddington would later recount the five minutes of totality in one of his books. A copy is available here:

     https://archive.org/details/spacetimegravita00eddirich/page/114/mode/2up?view=theater

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:07:08 UTC Robert McNees

     in reply to

     Data from the Sobral expedition suggested that the trajectory of light from distant stars was bent by about 2 seconds of arc as it grazed the limb of the Sun. The Principé data gave about 1.6 seconds of arc.

     https://royalsocietypublishing.org/doi/abs/10.1098/rsta.1920.0009

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:08:53 UTC Robert McNees

     in reply to

     The full moon covers about half a degree on the sky. There are 60 arcminutes per degree, 60 arcseconds per arcminute. So 2 arcseconds is about 1/1000 of the angular width of the moon!

     This is a very small effect, greatly exaggerated in the diagram below.

     Image: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/grel.html

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:10:42 UTC Robert McNees

     in reply to

     To get even this minuscule effect, the light has to pass very close to the Sun. And the Sun, being very bright, makes it pretty hard to see distant stars. This is why they had to wait for a total solar eclipse.

     In 1913, an eager Einstein wrote to astronomer George Hale asking if it might be possible to observe stars passing near the sun without the benefit of an eclipse. Hale patiently explained that, no, the eclipse really was necessary.

     ❌: 🧐 ☀️ ★
     ✅: 😃 🌑 ⭐

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:14:41 UTC Robert McNees

     in reply to

     It's good that Einstein had to wait a few years, because he was predicting the wrong amount of deflection in 1913. He had applied the equivalence principle to Newtonian gravity and obtained a prediction of 0.85 arc-second.

     In his 1915 paper “Explanation of the Perihelion Motion of Mercury from the General Theory of Relativity,” he used the full apparatus of general relativity to predict the correct 1.7 arc-second deflection for a light ray just grazing the sun.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:15:19 UTC Robert McNees

     in reply to

     Most of the meeting's attendees, led by Eddington and Sir Frank Dyson (the Astronomer Royal), viewed these measurements as a striking confirmation of general relativity. A stubborn few, including Oliver Lodge, clung to old theories despite the new evidence.

     In closing remarks at the end of the meeting, JJ Thomson said: "This is the most important result obtained in connection with the theory of gravitation since Newton's day... This result [is] one of the highest achievements of human thought."

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:16:45 UTC Robert McNees

     in reply to

     In the days following the meeting, fantastic headlines began to appear in newspapers all around the world.

     "Lights All Askew in the Heavens," read the New York Times. "Stars Not Where They Seemed or Were Calculated to be, but Nobody Need Worry."

     (Apologies for the very of-its-day gendered language in the subhed.)

     https://www.nytimes.com/1919/11/10/archives/lights-all-askew-in-the-heavens-men-of-science-more-or-less-agog.html

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:18:18 UTC Robert McNees

     in reply to

     The very next day, the Times of London ran with “Revolution in Science. Newtonian Ideas Overthrown.”

     Einstein embarked on a well-deserved victory tour. Scientists in almost every country celebrated the success of his radical theory.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:19:59 UTC Robert McNees

     in reply to

     One exception was Germany. Most scientists there were excited by relativity and supported Einstein's work. But a few, like Philipp Lenard, were envious.

     Seeing Einstein ideas as a threat to the status of experimental physics, Lenard and some of his colleagues exploited anti-semitism and Nationalism to sow distrust of his work.

     Lenard, of course, went on to become a high-ranking Nazi.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:22:59 UTC Robert McNees

     in reply to

     But among non-Nazi scientists, there was little doubt that the results presented #OTD in 1919 marked a turning point in our understanding of the universe.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 18:24:09 UTC Robert McNees

     in reply to

     There is a very funny story that Chandrasekhar told, about an encounter between Eddington and the physicist Ludwik Silberstein. They were chatting at a party after the meeting.

     Silberstein said "Professor Eddington, you must be one of three persons in the world who understands general relativity."

     This made Eddington very quiet.

     "There’s no need to be modest," offered Silberstein.

     “On the contrary,” said Eddington, “I am trying to think who the third person is."

   • The Geoff (_thegeoff@mastodon.social)'s status on Monday, 06-Nov-2023 18:35:02 UTC The Geoff

     in reply to

     @mcnees I've never heard who the (apocryphal?) third was, but I'm guessing Lorentz?

   • Victor Tangent (victang@beige.party)'s status on Monday, 06-Nov-2023 19:03:28 UTC Victor Tangent

     in reply to

     @mcnees

     Professor, I would like your opinion on a question I have about the universe.

     Is the universe an open or closed system?

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 19:03:28 UTC Robert McNees

     in reply to

     • Victor Tangent

     @Victang This is a very tricky question. Usually “open system" and "closed system" is meant in the thermodynamic sense: Can a system exchange matter and energy with its surroundings (open) or just energy (closed)? The *observable* part of our universe – everything we can currently see – surely exchanges both matter and energy with regions we can't currently see. Can it be treated as a thermodynamic system? I believe so, and I think that's a more or less generally accepted answer.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 06-Nov-2023 19:05:05 UTC Robert McNees

     in reply to

     The entirety of our Universe, on the other hand, must by definition be an isolated system which cannot exchange anything (matter or energy) with some other system. If it could, then that other thing would necessarily be part of our universe.