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Solar Eclipse Proves Einstein's Theory of General Relativity

In this image, from NASA and ESA (the European Space Agency), we see that "gravity distorts space near a massive object." Image online via NASA's "Testing General Relativity." Click on the image for a full-page view.

 

What’s “relative” about Einstein’s theory of Special Relativity? In short ... nothing in the universe has a fixed frame of reference. That means everything is moving relative to everything else.

It was a very odd thing to think at the time Einstein first proposed this theory. People everywhere—including scientists—pushed back. Everything is moving relative to everything else? How absurd!

Until ... the proof came in.

Where did the earliest proof originate for Einstein’s General Theory of Relativity? Initially ... by way of a total solar eclipse. Then another total eclipse, and another.

Before we look at the evidence, from the total eclipses, let’s first distinguish between the two types of Einsteinian relativity:

  • First came “special relativity,” in 1905;

What’s the difference between special and general relativity?

In short ... “special” applies to special situations, where frames of reference are in constant unchanging motion (such as people traveling in a straight line on a train that is moving at a constant speed with no changes in gravity), while “general” applies to frames of reference that accelerate regarding each other (such as a man falling from a roof who, in Einstein’s words, “would not feel his weight”).

What do the two theories have in common? Both assume that:

  • Nothing travels faster than the speed of light, including gravity; and
  • The speed of light is constant UNLESS an external force causes it to accelerate.

How about an example, where we put those theories to work? One of the most relied-on examples, widely used today, is GPS (Global Positioning System). Without Einstein’s theories, GPS wouldn’t exist.

Before GPS could exist, however, scientists needed to know that Einstein’s seemingly crazy idea of General Relativity was actually true. That’s where 20th-century total solar eclipses played a huge role (and 21st-century total solar eclipses will also play a role).

How?

Einstein suggested that Earth has curved space around it—that’s what keeps our feet on the ground (since this curved space, called gravity, pushes on Earth’s atmosphere and every object on our planet). Applying Einstein's theory, we must conclude that gravity is not a force. Instead, gravity is "a property of space-time geometry." (See "Einstein Online.")

After a year of work, a NASA probe—called Gravity B, depicted in this artistic conception—proved this theory to be true. Earth does, indeed, have curved space around it—and—gravity is not a force which pulls us to the ground.

Einstein also theorized that the Sun’s gravitation distorts the space around Earth (creating a force which pushes the Earth to move around the Sun). That really seemed weird since most people would have said that the Sun's gravitational force pulls the Earth to orbit around it.  

In a way, Einstein turned a Newtonian idea on its head by theorizing that gravity pushes on objects (instead of pulling them).

How could anyone prove this seemingly bizarre theory? Einstein hypothesized that if he (or other scientists) could measure the effects of gravity, on a straight beam of light, he could prove his theory. But how (and where) would that be possible?   

That's when Albert the Genius had another breakthrough idea. Stars produce light. As a light source, stars throw out straight beams of light. What if those straight beams of light came into contact with a massive body that had enough gravity to bend the starlight?

The Sun might be such a massive body with enough gravity to bend light. If the light, from distant stars, bent while passing through the Sun’s gravitational field, that bending would prove Einstein’s theory of General Relativity.

But how could anyone examine starlight passing through the Sun’s gravitational field? Who can see stars, near the Sun, when the Sun is shining? The only time anyone could undertake such an experiment would be during a total solar eclipse.

When the Moon blocks the Sun’s rays, during a total solar eclipse, then—and only then—can we briefly see the stars around it. Then—and only then—can we see where the stars appear, at that time, compared to their normal locations in the night sky.

If those stars seem slightly out-of-place, it could be evidence that their light was being bent as it traveled past the Sun.

It would be up to astrophysicists to prove Einstein’s theory. Not only would these individuals have to make the observations, they would have to photograph them so other scientists could verify their findings. This took a while (made more difficult by the years of World War One).

In 1917, Frank Watson Dyson—then known as the “Astronomer Royal” of Britain—had an idea. An upcoming total eclipse, predicted for the spring of 1919, could provide the platform for testing Einstein’s theory.

Arthur Eddington, an English astronomer, took Dyson’s suggestion seriously. He traveled to Príncipe—an island off the coast of west Africa—where the May 29, 1919 solar eclipse would be total. Eddington’s plan followed Dyson’s recommendation to observe a bright cluster of stars called the Hyades. During the darkness of totality, Eddington could observe the Hyades as the Sun passed in front of them.

What was Eddington trying to find? If Einstein was right—and a massive object like the Sun was actually causing the “spacetime” around it to curve—then the light emitted by the Hyades would bend as the Sun (and its gravitational field) passed by, causing the star cluster to seem dislocated compared to its “true” position in the night sky.

How perceptible would that dislocation have to be (if it actually occurred)? If Einstein was right, the stars in the Hyades cluster would appear to have shifted by about 1/2000th of a degree.

To run his experiment, Eddington needed two points of comparison. One would be a pre-eclipse picture of the Hyades in the night sky; the other would be a picture of the Hyades during the May 29, 1919 total eclipse at Príncipe. To cover his bases, in the event that Príncipe was cloud-covered, Eddington also had observations made at Sobral, Brazil.

We could say, tongue-in-cheek, that the May 1919 eclipse cooperated with Eddington’s efforts. The length of totality, that day, was six minutes—far longer than that predicted for the “Great American Eclipse of 2017" (whose longest stretch of totality is estimated at 2 minutes, 40 seconds and whose closest-to-the-sun star will be Regulus, in the Constellation Leo).

When Eddington compared the plates of his photos, he found that there was, in fact, a slight difference in the Hyades' location that could only be explained by Einstein’s theory of general relativity. NASA's "Cosmic Times," for 1919, shows us the comparison with this illustration:

Eddington published his findings on November 6, 1919. Skeptics, worldwide, disagreed with him. Those skeptics were sure that Eddington’s experiment wasn’t truly objective and had been rigged, somehow, to support Einstein’s theory.

This 1919 total solar eclipse, and its impact on Einstein’s theory of general relativity, turned Albert into an international celebrity. The New York Times proclaimed: “Einstein’s Theory Triumphs.”

Another total eclipse occurred on September 22, 1922. Observations, at Wallal Downs (in Western Australia), again revealed a slight difference in star locations between their "normal" place in the night sky and their encounter with the Sun's gravitational field during a total eclipse.

Einstein, as Eddington had earlier concluded, was right. Starlight bends when it comes into contact with a massive body, such as the Sun.

These findings ultimately led to a new phrase—and a new field of study—for the bending of light around massive objects. It’s called “gravitational lensing.”

If it hadn’t been for a total solar eclipse, who knows when Einstein’s world-altering theory of general relativity would have been proved? The facts are, however, that scientists keep proving Einstein was right (in one experiment and test after another).

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Author: Carole D. Bos, J.D. 5124stories and lessons created

Original Release: Aug 20, 2017

Updated Last Revision: Sep 01, 2017


Media Credits

Top image depicts gravity, distorting space, near a massive object. Image credit: NASA/ESA. Online via NASA's "Testing General Relativity."

 

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"Solar Eclipse Proves Einstein's Theory of General Relativity" AwesomeStories.com. Aug 20, 2017. Oct 22, 2017.
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