The Day Einstein Was Proven Right
Imagine a single experiment that could topple centuries of scientific understanding. In 1919, this dramatic scenario unfolded not in a lab, but across the vast expanse of the sky, during a rare solar eclipse.
For over 200 years, Isaac Newton's law of universal gravitation had reigned supreme, describing a universe with a rigid, static space. Then, a young Albert Einstein proposed a radical new vision: space and time were woven into a flexible fabric called spacetime, which could be warped and curved by mass. This was his general theory of relativity. But a beautiful theory needed proof. The 1919 Eddington Eclipse Expedition became the experimentum crucis—the crucial experiment—that decisively showed Einstein's vision was correct, forever altering our understanding of the cosmos 4 .
To grasp the expedition's significance, we must first understand the clash of ideas it sought to resolve.
Isaac Newton envisioned gravity as an instantaneous, attractive force between two masses. His laws were spectacularly successful at predicting the orbits of planets, but they described the "what" of gravity, not the "how." What was the mechanism that allowed the Sun to pull on the Earth across 93 million miles of empty space? Newton himself found this action-at-a-distance deeply unsatisfying.
In 1915, Albert Einstein proposed a revolutionary answer. He suggested that mass and energy do not exert a force across space, but instead, they warp the very fabric of spacetime around them. Einstein argued that starlight passing near a massive object like the Sun would have its path bent, an effect known as gravitational lensing 4 .
Instantaneous force acting at a distance
Mass curves the fabric of spacetime
This theoretical prediction set the stage for a definitive test. Einstein's theory predicted a specific, measurable amount of bending—exactly twice the value that Newtonian physics would suggest if light had mass. The only time stars near the Sun become visible from Earth is during a total solar eclipse 4 .
In March 1919, two teams led by the British astronomer Arthur Eddington set out from England. One team traveled to Sobral in northern Brazil, and the other, led by Eddington himself, went to the island of Príncipe off the west coast of Africa. Their goal was to use the brief moments of totality during the solar eclipse on May 29, 1919, to photograph the stars in the Hyades cluster that appeared right next to the obscured Sun 4 .
Sobral, Brazil & Príncipe, Africa
Months earlier, astronomers had taken reference photographs of the same star field at night, when the Sun was nowhere near them. This established the stars' "true" apparent positions.
During the eclipse, the teams used specialized telescopes equipped with photographic plates to capture new images of the same stars, now visible in the darkened sky around the Sun.
After the expedition, the eclipse photographs were painstakingly compared to the reference photographs. The scientists measured the tiny shifts in the positions of the stars that appeared near the Sun's location.
The measured shifts were then compared to the predictions of both Newtonian physics and Einstein's general relativity.
| Observation Site | Date | Number of Useful Photographic Plates | Measured Deflection (arc-seconds) |
|---|---|---|---|
| Sobral, Brazil | May 29, 1919 | 7 | ~1.98 |
| Príncipe, Africa | May 29, 1919 | 2 | ~1.61 |
| Combined Result | ~1.64 |
The data told a compelling story. The combined measured deflection was 1.64 arc-seconds. To put this in perspective, an arc-second is 1/3600 of a degree—an almost imperceptibly small angle. Yet, this tiny measurement had universe-shattering implications.
Newtonian Prediction
0.87"
Measured Value
1.64"
Einstein's Prediction
1.75"
It was far closer to Einstein's prediction of 1.75 arc-seconds than to the Newtonian prediction of 0.87 arc-seconds. Eddington's results provided the first solid experimental evidence for general relativity 4 .
When asked how he would have reacted if the results had not supported his theory, Einstein famously quipped, "I would have felt sorry for the dear Lord. The theory is correct." The data, however, required no such sympathy.
Modern astrophysics relies on a sophisticated toolkit to study gravitational lensing and other phenomena predicted by general relativity.
Far more sensitive than the photographic plates used in 1919, these digital sensors in telescopes detect and record light from distant stars and galaxies with extreme precision.
Splits the light from cosmic objects into a spectrum, allowing scientists to determine their composition, temperature, mass, and velocity.
Allow observations of objects that are invisible in optical light and can achieve extremely high resolution by combining data from multiple telescopes spread across a wide area.
Powerful supercomputers run complex simulations to model the warping of spacetime around massive objects, helping to test theoretical predictions against observational data.
The confirmation of Einstein's theory was front-page news around the world, catapulting him to international fame. But its impact went far beyond headlines.
The success of the 1919 expedition cemented the concept of the experimentum crucis in the history of science—a single, well-designed test capable of decisively choosing between competing theories 4 .
The story of the 1919 eclipse is a powerful reminder that science advances not just through brilliant ideas, but through the courageous efforts to test them against the fabric of reality. It was a triumph of human curiosity, one that stretched our understanding of the universe to its very limits.
Adjust the mass to see how it affects spacetime curvature:
Medium curvature: Light deflection ~1.64"