History Daily - Astronomers Prove Einstein’s Theory of General Relativity

Episode Date: September 21, 2026

September 21, 1922. Astronomers photograph a total solar eclipse from a barren tract of land along the west coast of Australia, proving that space is curved and confirming Albert Einstein’s groundbreaking theory of general relativity. This episode originally aired in 2023. Support the show! Join Into History for ad-free listening and more. History Daily is a co-production of Airship and Noiser.Go to HistoryDaily.com for more history, daily.

It's May 29, 1919 in Principae, a remote West African island. Arthur Eddington leans out of an open window, cautiously peering up at the overcast sky. As a few stray droplets of rain trickle down the astronomer's cheek, he mutters grimly, hoping that the poor weather won't hamper the experiment he spent months preparing for. Arthur is leading a British expedition to test a theory proposed by budding theoretical physicist Albert Einstein. According to Einstein, gravity, instead of being an invisible force that attracts objects to one another, is actually the curving or warping of space itself. For now, this is just a theory. But Arthur traveled across the world determined to prove its veracity.

A total solar eclipse, where the moon passes exactly between the sun and earth, is slated to take place in Principate today, allowing Arthur to observe stars usually outshone by the sun. If Einstein's theory is correct, then light from stars behind the sun will be bent by the sun's gravity, making them appear slightly displaced from their normal nighttime positions. With the eclipse approaching, Arthur sharply pulls the window shut and crosses the room to his desk. He rifles through the papers and half-open books scattered upon it, taking the necessary documents and gathering the last of his measuring tools, before stepping outside to prepare for the experiment. As the door slams behind him, a strong gust of wind stings Arthur's face.

Looking up, he carefully surveys the clouds with a growing on ease. As other members of the expedition join him and set up their many devices and cameras, they all wear the same nervous expression as they share their concerns. If they can't see the stars today, their experiment is doomed, and they'll never be able to repeat it, at least not on Prince Abbey. The island was carefully chosen because a cluster of stars is, conveniently positioned near the sun here. But after this, a total eclipse will not take place on this island for nearly 400 more years. Arthur glances down at his watch and checks how much more time

they have. With just 20 minutes left, he nervously inspects all the cameras and measuring instruments his team has brought, ensuring everything is in order. As he makes his rounds, his colleagues abruptly halt their conversation. Alarmed, Arthur glances at them wondering what has gone wrong, but they say nothing can simply point at the sky. Even before he looks up, Arthur can feel the warm rays of sunshine on the back of his neck. The sky has begun to clear. Arthur continues preparing for the experiment with renewed enthusiasm, a broad smile spreading across his face.

Arthur's experiment will seem successful. During the eclipse, he'll find that the stars indeed appear slightly displaced from their normal positions, just as Albert Einstein's theory predicted. When the results of Arthur's expedition is made public at the end of 1919, Einstein will be catapulted to overwhelming fame. But many in the scientific community will continue to be skeptical of the physicist's so-called theory of general relativity. They'll question the validity of Arthur's conclusions and the rigor of his experiment, claiming that he didn't measure the positions of enough stars. And it will take several more years before another expedition will lay these doubts to rest with their own experiment on September.

21st, 1922. Hello, History Daily listeners. The live show I've been telling you about for a few weeks has a new date, October 23rd at the Plaza Theater in Garland, Texas. This is my Days That Made America Tour, a thrilling evening of history, storytelling, and music with me and a band live on stage. So if you're in the North Texas area or anywhere close, be sure to come out and see me live. More shows will be announced soon, too.

So for information on tickets and upcoming dates, go to HistoryDailyLive.com. That's HistoryDailyLive.com. Come see my Days at Made America Tour, live on stage. Go to History DailyLive.com. From Noisor in Airship, I'm Lindsay Graham, and this is History Daily. History is made every day. On this podcast, every day, we tell the true stories of the people and events that shaped our world. Today is September 21st, 1922.

Astronomers prove Einstein's theory of general relativity. It's late 1907 inside an office in Burns, Switzerland, nearly 15 years before Arthur Eddington's experiment in Principae. Albert Einstein's lips twitch ever so slightly as he whispers incomprehensibly under his breath. The young physicist is seated behind a wooden desk, his eyes closed, and his hands awkwardly suspended mid-air. A soft breeze blows a strand of Einstein's wispy dark hair across his forehead, but he remains unmoved, deep and thought.

At just 28, Einstein has already made his name known in the field of science. But his day job is not that of a physicist. Einstein is currently an examiner at the Swiss Patent Office, where he studies applications, checking their originality, and ensuring patents clearly protect the novel ideas of other inventors. But this job has inadvertently served as a fertile breach. for the development of Einstein's own groundbreaking theories. In between his professional duties, Einstein lets his mind roam, and he's been able to come up with several revolutionary conclusions

inside the Patent Office. Two years ago, he published several scientific papers that will dramatically alter the course of modern physics. Among them was his theory of special relativity, which reinvented the contemporary notions of space and time proposed by Isaac Newton. As part of this theory, Einstein postulated that space and time are not absolute, as Newton believed, but rather are relative to the observer. While the prevailing consensus was that space has three dimensions, and time only one, Einstein put them together in a single four-dimensional system, where space and time are inseparable. This theory captured the interest of theoretical physicists around the globe,

propelling Einstein into prominence within the scientific community. But while special relativity had undoubtedly expanded the horizons of physics, it had also left Einstein with some nagging questions. For one, his new theory didn't align with Newton's definition of gravity as an invisible force that attracts one object to another, with a strength depending on the object's distance and mass. Einstein has spent months engaged in a variety of thought experiments, trying to understand the contradictions in these two theories.

But he hasn't been able to resolve the discrepancies until today. With his eyes still closed and his hands still up, Einstein imagines a man in an elevator in freefall. If he and the elevator are falling at the same rate, he wouldn't be able to feel the pressure of the floor on his feet. So as the man falls, he wouldn't feel his own weight, he would feel weightless. But if the elevator was rising, in the absence of gravity, it would produce the opposite effect. The man would be glued to the floor just as he would if he felt the downward force of gravity. Therefore, if the elevator had no windows, the man wouldn't be able to tell whether it's at rest in the presence of gravity or constantly accelerating outside a gravitational

field. This must mean that there's no way to distinguish between the effects of gravity and the effects of being accelerated. As he stumbles upon this conclusion, Einstein's eyes shoot open. He hastily sifts through the patent application strewn across his desk until he finds a blank sheet to scribble down his realization, which he dubs the equivalence principle. For four years, Einstein does little with his insight, focusing on other ideas instead. But in 1911, he finally returns to his elevator thought experiment. This time, he imagines a light beam entering through a pinhole in one of the elevator's walls. If the elevator is accelerating upward, the light would be slightly closer to the floor by the time it reaches the opposite wall.

In other words, the light beam's path would seem curved to anyone inside the elevator. And using his equivalence principle, Einstein reasons that light should also bend when passing through a gravitational field. For several more years, the physicist digs into this concept. And eventually he concludes that gravity is actually the curving of the four-dimensional space-time he previously proposed, caused by mass and energy. He theorizes that gravity warps space-time the same way, a heavy bowling ball, might warp a mattress. He hypothesizes that the more massive an object, the more it warps the spacetime around it. And if gravity bends space time, this would explain why light might bend when passing through a gravitational field. Einstein's so-called theory of general relativity

will represent a whole new way of regarding reality, and it will send shockwaves through the scientific community and transform Einstein into a household name around the world. But before other scientists can accept the theory as true, it will need to be proven. It's dawn on August 30th, 1922, on an isolated stretch of beach on the west coast of Australia, seven years after Albert Einstein came up with his theory of general relativity. Foamy waves, swirl, and hiss around William Campbell's knees as he wades toward shore. And the arms of the American astronomer is a fragile wooden box. He clutches it close to his chest, carefully holding it high above the surface of the choppy ocean. William's eyes gleam as he scans the sandy coast,

his gay settling upon a small retinue of men waiting for his arrival. Swiveling around, he checks on his team members who follow behind in a rubber lifeboat with their own heavy boxes. Soon enough, they all trudged towards shore. William and his team are one of the international groups who have traveled to Wallal, a secluded settlement on Australia's western coast, to test Einstein's theory of general relativity by observing, stars during another solar eclipse. If Einstein's theory holds true, the sun's gravitational field will bend any starlight that passes through it, creating a disparity between the sun's daytime and nighttime positions.

But it's too hard to see stars when there's daylight, so the only chance to accurately chart their daytime positions is during a total solar eclipse. Several expeditions have already attempted to chart stars during eclipses, but technical difficulties or poor weather conditions have foiled their attempts. In 1919, Arthur Eddington's expedition seemed successful, but scientists have been unwilling to treat it as proper evidence due to the low number of stars that he measured. Now, William and his team hope they can satisfy Arthur's critics and prove one of the most groundbreaking scientific theories ever proposed. A total eclipse is scheduled for the last week of September, and it's predicted that

Wallal, a largely uninhabited and arid land, is the most optically advantageous location to conduct the experiment. But there are some drawbacks. Most notably, the region is inaccessible, and its only existing infrastructure is an old telegraph station. Concerns about transporting equipment made many in the scientific community plainly dismissed the idea of coming here. But Alexander Ross, a math and physics professor at a leading Australian university, resolved to show the international community that Wallall was viable venue. After finding a source of fresh water in the settlement and noticing that it was possible to bring equipment to Wallal by boat, Alexander wrote in passion letters to leading astronomers across the world, urging them to conduct the experiment in Australia. William, the director of

California's Lick Observatory, was one of the scientists who agreed, along with the smattering of other astronomers from Australia, Canada, and India. Now, the international expedition has just completed the last leg of their taxing journeys. William is among the first to arrive to shore. He's greeted by a group of officials from Wallal and representatives of the indigenous Nian Gimarta people. The Nian Gimarta play an instrumental role in helping the scientists carry nearly 35 tons of equipment across the tumultuous surf. The precious cargo, much of which was designed specifically for this experiment, is then loaded onto wading donkey carts, which transported to a campsite about a mile and a half inland. As William helps unpack, he inspects the area, squinting to shield his eyes from the plumes of dust that fill the dry air.

Clusters of robust acacia trees and unrelenting flies populate the land, but there are no other signs of life, so the scientists have plenty of room for their equipment. The Australian Navy assists the group in setting up observation stations, sleeping tents, storage areas, and a cook's galley. Then the scientists start assembling the equipment they will use to conduct the experiment, including two 15-foot cameras and a 40-foot camera inside a specially constructed tower. The task will occupy most of three weeks they have leading up to the total eclipse. The scientists will work from dawn to dusk every day, and in between they will eat their meals in a makeshift cafeteria

that the team will playfully nickname Cafe Einstein. All the while, their anticipation will build as their chance of making scientific history inches closer and closer, but nothing will fully be. prepare them for the all-inspiring celestial spectacle that awaits. It's 1.40 p.m. on September 21st, 1922 in Wallall. William Campbell draws a deep breath as the shrill clang of a bell echoes across the campsite, signaling that the total solar eclipse will take place in 10 minutes. His pulse racing, William joins the other members of the expedition in last-minute preparations. While the scientists position their cameras and test their shutters, the moon's silhouette.

can be seen approaching the sun, gradually enveloping the world in a darkening twilight. And then suddenly there's a chill in the air. Standing at his post, William wraps his arms around himself, his skin tingling from suspense and the dropping temperature. An overwhelming stillness envelopes Wollall until the moment of totality arrives. Each member of the expedition jumps into action, meticulously performing the role assigned to them. William, who is in charge of one of the cameras, snaps photograph after photograph over the five minutes and 15 and a half seconds that the eclipse lasts for. Not a single moment of the spectacular natural experiment is wasted. Though teams of astronomers around the world will conduct the same experiment,

heavy cloud cover will ruin attempts elsewhere. Experimental proof of Einstein's theory of general relativity will rest squarely on the evidence gathered at Wallall, and the world will wait with bated breath as their data is analyzed by scientists around the world. Over six months after the experiment, in April 1923, William Campbell will finally send the cable to Albert Einstein, confirming that the experiment at Wallall aligned with the physicist's theoretical calculations, providing conclusive proof of general relativity. The expedition's findings will be widely reported in the media and will help establish Einstein as one of the most famous and respected scientists of his time.

The theory of general relativity will irreversibly change modern physics, And it will go on to be regarded as a remarkably accurate model of gravitation and cosmology, surviving every test thrown its way since astronomers helped verify it on September 21, 1920. Next on History Daily, September 22nd, 1980. Seeking a quick victory over a weakened neighbor, Iraqi President Saddam Hussein orders his military to invade Iran. From Noisor and Airship, this is History Daily. hosted, edited, and executive produced by me, Lindsay Graham. Audio editing by Mohamed Shadi, sound designed by Katrina Zemrad.

Music by Lindsay Graham. This episode is written and researched by Ria Perohit. Executive producers are Alexandra Curry-Buckner for airship and Pascal Hughes for noisering.