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HAPPY 100TH—GENERAL RELATIVITY!

THIS YEAR is the 100th anniversary of Albert Einstein’s theory of general relativity, so what better time than to describe its intricacies here in 600 words.

Now you tell one.

Yet, according to Science magazine, March 6, 2015, Einstein published his fundamental theory of gravity “in four short pages.”

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In the magazine’s Special Section introduction titled “Einstein’s Vision,” Margaret Moerchen and Robert Coontz observe how general relativity governs the universe on its largest scale, just as quantum theory describes its smallest scale.

Yet, the authors note, “whereas quantum theory was the achievement of many—de Broglie, Bohr, Heisenberg, Schrödinger, Born, Dirac—general relativity leapt fully formed from Einstein’s mind.”

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Albert Einstein, 1879 – 1955, German-born theoretical physicist, 1921 Nobelist in Physics.

Einstein’s theory of special relativity (1905) refined views of space and time. His theory of general relativity (1915) revolutionized science in its understanding of gravitation. The 27-page Special Section in Science includes six Milestones, three of which provide tidbits here: “1914, Relativity and the Great War;” “1915, Mercury delivers good news about a newborn theory;” and “1959, Bringing general relativity down to Earth.”

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GRMan, aka Albert Einstein. Illustration for Science by Nguyen Nguyen.]

An animated toon from Science, March 6, 2015, sets the stage with the main point of general relativity: Gravity can be described as a warpage of spacetime, of our four-dimensional universe, caused by the objects in it.

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Gravity as a warpage of spacetime. Image by User:Johnstone.

Newtonian physics works fine in a straightforward mechanistic world. Force equaling mass times acceleration, F = MA, is still a useful concept for many things. However, it’s not good at explaining extremes, the subtleties of minute electromagnetics or immense astronomical phenomena, occurring at or near the speed of light.

As described in the Science Special Section, World War I had its effect on emerging physics. As one example, German astronomer Erwin Freundlich led an expedition to the Crimea to study a total solar eclipse (this, to observe the behavior of planet Mercury). Alas, the celestial event occurred in August 1914. Russian officials detained the scientists and seized their equipment.

In 1915, physicist Karl Schwarzschild corresponded with Einstein about an implication of spacetime curvature: the theoretical possibility of black holes. Unfortunately, at the time Schwarzschild was also serving as a German artillery officer on the Russian front. Not long afterward, he succumbed to disease.

The Science article’s good news from Mercury in 1915 describes general relativity’s first experimental success (an essential step in verifying any theoretical model). Newtonian physics exhibited errors in predicting planet Mercury’s precession, its gradual shift in orbiting the sun. The physics of general relativity explained this precession exactly. Wrote Einstein later, “For a few days, I was beside myself with joyous excitement.”

As observed in Science, “General relativity mostly reveals itself on cosmological scales, but its effects also show up closer to home—even in our pockets.” Ubiquitous GPS devices profit from general relativity theory.

One of the theory’s oddest implications is time dialation: In warped portions of the spacetime continuum (that is, in gravitational fields), time slows down.

Scientists confirmed time dialation in 1959 through an experiment using a 75-ft tower with a source of light of a known frequency at the tower’s bottom and a detector at its top. A measured difference in frequency agreed with general relativity theory.

In 1977, researchers doing preliminary work leading to GPS confirmed the theory again by launching a satellite containing a highly precise cesium clock. Cites Science, “Sure enough, the clock quickly went out of sync with its Earth-bound counterparts, in agreement with Albert Einstein’s theory.”

Note, this has nothing to do with malfunctioning timepieces or lags in signal transmission. It is a fundamental characteristic of spacetime and its warpage.

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A GPS receiver’s location (green) is the intersection of three spheres. (The fourth satellite gives time.) Image from http://xenon.colorado.edu.

Briefly, GPS works by trilateration, by calculating distances between the Earth-bound device and selected satellites. These distances are based on signal transmission times of precisely synched clocks in orbit and on Earth. Without general relativity, errors arising from time dialation would make these calculations worthless.

Hurrah for Einstein’s general relativity—and happy birthday! ds

© Dennis Simanaitis, SimanaitisSays.com, 2015

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