IRA FLATOW: This is Science Friday. I’m Ira Flatow. Did you catch that 2017 total solar eclipse? I saw it up there in Casper, Wyoming. And I know many others who pilgrimaged far and wide to watch it.

But 100 years ago, a total solar eclipse was much more than just an amazing sight to see. It was a chance to test one of the most controversial concepts of the day– Einstein’s idea of warped space– if you could actually observe a total eclipse, of course. Many early scientific expeditions were foiled by war or bad luck or bad weather.

But finally, in May 1919, scientists were blessed with a clear view of the blackened disk of the sun and the stars around it. And on November 6, 1919, 100 years ago this week, they presented their observations to the world. And that forever changed our view of Einstein and his theories. And Albert Einstein became an international celebrity literally overnight.

Joining me now to tell a story is science writer Ron Cowen. He’s author of Gravity’s Century– From Einstein’s Eclipse to Images of Black Holes. He joins me from London. Welcome back to Science Friday, Ron.

RON COWEN: Thanks, Ira.

IRA FLATOW: Take us back to those early days of May of 1919. Sir Arthur Eddington has gone out to observe the eclipse. What’s he looking for? What’s happening?

RON COWEN: Right. So he and a colleague went to Principe, which is off the west coast of Africa. Another team went to Northern Brazil. I mean, they were looking to see when the sun is present, will stars get deflected by the mass of the sun.

And you can only see that during a solar eclipse, at least at that point they didn’t have the technology. Because it would be folly otherwise. The bright light of the sun would swamp the faint light of the stars. You needed an eclipse.

But there was a lot of drama because, first of all, this, was just after the end of World War I. This was British astronomers daring to test a theory of a German-born scientist when there was still a lot of hostility.

And there was drama at the two sites because there were clouds. In fact, and in Principe, Eddington could just have three stars that he could look at on photographs to see the deflection. In Northern Brazil, at Sobral, one of the instruments just did not work properly, probably due to the heat of the sun before the eclipse. So they had to scramble.

And a lot was at stake. Was Newton right? My god, I mean, the planets revolved around the sun because of the way Newton explained it. Or was this upstart Einstein with his crazy theory right?

IRA FLATOW: And then they finally published the result 100 years ago this week. And it confirmed what Einstein had predicted about just where those stars should be, right?

RON COWEN: They announced it in November 6, 1919, in London at the Royal Astronomical Society. That’s right. And some people were skeptical, but most people were welcoming.

And then the next morning on November 7, the front page of the Times of London, it was interesting. On one side of the front page, there was a note about King George IV issuing an invitation for all workers to take two minutes out of the day to remember and honor the glorious dead, as he put it, for World War I.

But to the right of those articles, also on the front page, a triple-decker headline. The normally staid Times wrote, “Revolution in Space– New Theory of the Universe– Newtonian Ideas Overthrown.”

It sets off a chain reaction around the globe. New York Times followed suit with a front page story on November 10– “Lights All Askew in the Heavens– Einstein Theory Triumphs.”

IRA FLATOW: And how did Albert react to all of this?

RON COWEN: Well, it was interesting. There were two ways. He had learned that most likely the measurements were going to confirm his work. And he sent a note in the fall of 1919 to his mother, who was dying of stomach cancer. And basically said, mother, joyous news today because they have demonstrated the deflection of light. And he quickly sent a note to a notable German journal, very brief note.

On the other hand, his assistant Ilse Rosenthal-Schneider asked him, what would you have done if they had found the deflection of light that was not correct with your theory. And he said, well, then I would feel sorry for the dear Lord. The theory is correct.

IRA FLATOW: And they’ve been testing this out for 100 years, haven’t they? Any chance they get that a body can warp the space around it is just constantly being looked at.

RON COWEN: Absolutely. And of course, the Nobel Prize was won in 2017 for the discovery of gravitational waves, these ripples in space time that Einstein predicted. We have the iconic image of the region right around a black hole from the Event Horizon Telescope, which is actually an array of hundreds of radio telescopes working on concert to get this image. And the size and shape of the halo of light around the black shadow of the black hole indicated Einstein, once again, was right.

IRA FLATOW: One of the great ironies of this issue was that there was an expedition that went out at in 1914 as the war was broken out. And they got arrested, didn’t they?

RON COWEN: Yes, yes, because they went to the Crimea in August 1914. And the team was promptly arrested as spies. Their equipment confiscated. That was Erwin Finlay-Freundlich.

And he never had good luck. He tried several times. Other times, it was too cloudy. So that’s true.

But it was lucky for Einstein that those earlier expeditions failed because Einstein, when he first calculated how much light would be bent, he made a mistake. He didn’t fully perfect his theory yet.

And by late 1915, he realized it was twice the amount that he’d originally calculated. So if these measurements were done before then, they would have–

IRA FLATOW: He would’ve been wrong.

RON COWEN: Einstein wrong. Yes.

IRA FLATOW: Just quickly, one other point I want to ask you about, some recent confusion over how fast the expansion of the universe is happening. The Hubble constant controversy. You know about that?

RON COWEN: Yes, a little bit. Now first of all, the fact that the universe is expanding, and even expanding at an accelerated rate, is something that Einstein’s equations predict.

But yeah, it’s interesting. It’s not clear if it’s really a crisis or not. I happen to think that they will work it out and figure out one value for the Hubble constant, which tells you about the expansion of the universe. But again, this goes back to Einstein.

IRA FLATOW: Interesting. Ron Cowen, science writer and author of Gravity’s Century– from Einstein’s Eclipse to Images of Black Holes. Great book. We have an excerpt of it on our website at sciencefriday.com/gravity.

Thank you, Ron.

RON COWEN: Thank you, Ira.

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