November 2019: The Centennial of Einstein's Theory of General Relativity

 November 6, 1919, the Royal Astronomical Society and the Royal Society held a join dinner meeting in London to mark the verification of Albert Einstein's General Theory of Relativity.  

According to Einstein's theory a large mass (in this case the Sun) would warp space-time causing the light from the star cluster to bend as it passed through the warped space-time.  Five months earlier on May 26 a total eclipse of the sun had made it possible for Sir Arthur Eddington and his colleagues to observe that indeed the light from the distant Hyades star cluster was deflected as it passed the sun.  

The dinner was held in November of 1919 because it took that much time for Eddington to ​verify the accuracy of the observation. At the dinner the Astronomer Royal Frank Dyson concluded with “If it is sustained that Einstein’s reasoning holds good...then it is the result of one of the highest achievements of human thought.”

The news was even reported on the front page of The New York Times on November 10, 1919.

What specifically had been achieved by the 40 year old physicist?
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In 1905, Albert Einstein, then a 26 year old level three clerk at the Swiss Patent Office had published a paper “On the Electrodynamics of Moving Bodies” which presented his special theory of relativity. He followed this paper with another entitled “Does the Inertia of a Body Depend upon Its Energy Content”  with its now famous equation that shows that Energy equals mass times the speed of light (c) squared, E=mc^2. These two papers lay present what is called “special relativity.”

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The “special” in the theory’s title means that the theory applies to cases in which objects are in constant motion and where gravity plays not much of a role.

The special theory establishes two understandings in addition to the famous equation.
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First, it establishes the principle of relativity which means that in order to describe an object’s speed or velocity (an object’s speed + its direction) the description must include the who or what is doing the observation. For example, from my perspective I am sitting more or less still at my desk. To an observer on the sun, in contrast I am in motion, being rotated with earth’s twenty-four hour daily cycle as well moving as earth follows its 365 day yearly cycle around the sun.

Second, the speed of light is constant to any and all observers who will find that whether running away from or towards a light source the light always travels at 670 million miles per hour. No matter how fast you run, you will never catch a photon of light.  (Greene, pp. 28-31)

If Einstein had stopped his explorations with special relativity, he would have been famous for contributions to physics but would not have likely become the world-famous, electric-haired icon of science.  Einstein didn’t stop because he continued to ask himself about what happens to space-time when it interacts with Energy-mass. (Bogdanis, 2000, p. 204)

Special relativity’s assertion that the speed of light is constant and and finite is inconsistent with Newton’s gravity, a force that appears to act instantaneously at great distances.

General relativity would address this inconsistency while providing an explanation for gravity’s mechanism, something that Newton’s theory does not include.

Einstein viewed space was not simply as a bare stage on which the cosmic dance took place, but something he called space-time that was also an active part of that dance.

Energy-mass interacts with space-time by tugging it into curves the way a heavy (an accelerating mass) weight bends a flexible fabric that is stretched taut. Gravity is not a force but rather is what we experience as we travel through a particular part of space-time following its curves.
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Einstein needed a way to test this idea. Bogdanis writes “The proving test came from the heart of the theory, that diagram of a warp in the very fabric around us. If empty space really could be tugged and curved, then we’d be able to see distant starlight ‘mysteriously’ swiveled around our sun. It would be like watching a bank shot in billiards suddenly take place, where a ball spins around a pocket and comes out in a different direction Only now it would occur in the sky overhead, where nobody ever suspected a curved corner pocket to reside.” (Bogdanis, p. 204)
 
 
Note on Sources: The story of the Burlington House dinner celebration is found in Bogdanis. The aspects of special and general relativity are found in both Bogdanis’s epilogue and in Greene, chapters 1-5.
 
Fortunately, Eddington established two separate observation sites because the high heat and humidity at the Principe Island site ruined the photographic plates. The plates from the Brazil observation site were used to determine the deflection. Eddington’s report concluded that during the eclipse, the actual observation showed a deflection of  1” 75 +/- .06” arc seconds, almost exactly what Einstein’s equations predicted which was 1”.72 arc second. This was about twice of what Newton’s equations yielded.
Space-time: In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive where and when events occur differently. Wikipedia, “Space-time.” Einstein’s theory transforms what we have traditionally thought of as being “empty space” and simply the background into an actual participant in cosmic events. Space-time is a kind of stretchy fabric that is shaped by Energy-mass.
 
You can find all of Einstein’s relativity papers with their English translations  at this archive.
 
 
Resources:
Bogdanis, David (2000). E=mc2 :The Biography of the World’s Most Famous Equation. Walker & Company, New York.
 
Greene, Brian (2003). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. Vintage Books, New York.
 
Rhodes, Richard (1987) The Making of the Atomic Bomb. Simon & Schuster, New York. This Pulitzer Prize winning book is provides a lucid history of atomic physics as it led to the creation of the atom bomb.
 
Segal, Ethan (2018). The Three Meanings of E=mc2, Einstein’s Most Famous Equation.
Online at Medium https://medium.com/starts-with-a-bang/the-three-meanings-of-e-mc²-einsteins-most-famous-equation-a0ec1549b4cd
 
Image: How Earth’s mass bends the fabric of space. Source: https://www.nasa.gov/mission_pages/gpb/gpb_012.html 
 
Image: New York Times front page on 11/10/1919. Sourch: https://medium.com/starts-with-a-bang/how-a-solar-eclipse-first-proved-einstein-right-d761cba20ac0

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