Star Talk: Einstein’s theory of gravity turning 100

Our understanding of the universe changed on Dec. 2, 1915. That was the day when a soft-spoken, wild
This undated file photo shows the famed physicist Albert Einstein, author of the Theory of Relativity. (The Associated Press)
This undated file photo shows the famed physicist Albert Einstein, author of the Theory of Relativity. (The Associated Press)

Our understanding of the universe changed on Dec. 2, 1915.

That was the day when a soft-spoken, wild-haired, German physicist named Albert Einstein presented his new description of gravity to the world. No longer was gravity a mysterious force reaching out from objects and pulling on other objects. Gravity was now geometry.

The background for this new description of gravity can be traced to 1905 when Einstein first achieved notoriety within the scientific community. During that year, while working as a full-time patent clerk in Bern, Switzerland, Einstein published four theories that reset the foundation of physics.

In March 1905, he described light as a particle, called a photon, rather than the prevailing view as a wave.

In May of that year, he proposed tests of the atomic theory of matter that when performed were shown to be true, confirming that matter was made of discrete units called atoms.

In June 1905, his famous Special Theory of Relativity was released explaining that the speed of light was the absolute speed limit and that immutable physical quantities such as length and time were not fixed but contract and dilate as objects race to near light-speed. Special Relativity joined the ordinary three dimensions of space with time creating a four-dimensional, “spacetime” structure as the foundation for the universe.

Finally, in September 1905, Einstein released his iconic equation, energy equals mass times the speed of light squared or “E equals m, c squared” which quantifies the total amount of energy contained within each bit of matter.

Science usually moves ahead incrementally, but thanks to Einstein, it made several leaps forward in 1905. It was his miracle year. Yet, his ability to shift the bedrock of reality had not ended. Gravity needed to be reconciled with his Special Theory.

General Relativity

The seeds for Einstein’s new description of gravity were already in place when he established that nothing could travel faster than the speed of light. That was in conflict with gravity, which presumably exerted its influence instantly. Einstein spent most of the next decade rewriting the almost three-centuries old theory of gravity established by its original authority, Isaac Newton.

Einstein’s moment of revelation came with one of his “Gedanken” or “thought” experiments, a trait he had mastered. He envisioned that a person in free-fall — only being acted on by the force of gravity — and a person “floating” in a region of deep space far from any objects, would both be weightless. Further, if both people were enclosed in a windowless room — for example, an elevator — there would not be any experiment that either person could do to distinguish between those two situations.

Now if the elevator for the person in deep space began to rocket upward and accelerate, the floor would soon press against their feet, the same as if they had dropped to the floor under the force of gravity. It would be impossible to distinguish between the effects of gravity and the effects of being accelerated. Einstein called this the Principle of Equivalence and it is the underlying concept for his General Theory of Relativity.


The implications that the equivalence principle has on reality are astonishing. If a person in an accelerating elevator shines a flashlight at the elevator wall, by the time the light reaches the wall, the wall will have moved upward. To the person in the elevator, the light beam will have curved downward. Since the equivalence principle tells us that the effects of acceleration and gravity are the same, a light beam should bend when passing through a gravitational field.

Matter, responsible for gravity, is just distorting the “fabric” of spacetime, similar to the shape formed when a bowling ball is placed on the fabric of a trampoline. When matter — or light — moves through this gravity well, its path is deflected — just like the path of a billiard ball rolled by the bowling ball on the trampoline.


Since the more massive an object, the more spacetime is distorted, the first test of General Relativity came in 1919 with a total eclipse of the sun. The positions of stars appearing near the sun during the solar eclipse would be changed ever so slightly compared to when the sun did not appear in that part of the sky.

English astronomer Sir Arthur Eddington led an expedition to Africa to view this eclipse and measure the deflection of starlight. It was exactly the amount that Einstein’s General Theory predicted. The next day, the front-page headline of the New York Times read, “Astronomers Agog over Starlight Deflection” and Einstein was instantly catapulted to worldwide fame.

There was more hidden within the General Theory of Relativity: black holes, gravity waves and the expansion of the universe. It took astronomers several decades to observe — and accept as true — some of these phenomena. And currently, the search for gravitational waves — propagating disturbances in spacetime — is underway.

December sky

You almost have to be a member of a breakfast club to see the planets these days, but the early wake-up call is worth the view that you will see in the predawn sky.

By 5 a.m., Jupiter, Mars and Venus have all gained altitude in the southeast. Bright Jupiter has the greatest elevation of the three planets while the brilliant light of Venus is the closest to the eastern horizon; dim Mars is between the two.

On Dec. 4, the moon, a day past third quarter, will appear next to Jupiter. Then the moon shows a waning crescent face near Mars on both Dec. 5 and 6. During the morning of Dec. 7, the moon is a thin crescent, extremely close to Venus.

One of the best meteor showers of the year will be very active during the nights of Dec. 13-15. Unlike the Perseid meteor shower of August that is at its best around 2 a.m., the December Geminid meteors show the most activity by 11 p.m. As in viewing any meteor shower, it is best to be at a location with the clearest view of the sky as well as the least amount of outdoor light. Just be sure to dress warmly.

Richard Monda is an astronomer living in the Capital Region.

Categories: Life and Arts

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