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Wednesday, November 25, 2015

EINSTEIN | Nov. 25–General Theory Finished

Albert Einstein
Made a timeline.
He had the nerve
To make it curve.
(Clerihew by JT Marlin.)
Nov. 25 – This date in 1915, 100 years ago, Einstein finished his general theory and submitted for publication his article, "The Field Equations of Gravitation."

It included ten equations that comprise his Theory of General Relativity. Through his equations, Einstein propounds the origins of his theory of gravity and how it interacts with "space-time."

Einstein had ten years before developed his theory of special relativity which said that the speed of light is a constant and a limit in our universe. [See Comment below, where this summary is contested by a man with a D.Phil. in Physics from Oxford.]

The New Yorker has a useful article that tells the "big idea" of the "space doctor"using only the "ten hundred" words in most common use in the English language ("thousand" is not one of them). Einstein's new general theory viewed space and time as interwoven – changing one produces an effect on the other. The basic idea came from his former professor, Hermann Minkowski, and he generalized it.

Speed depends on the observer's frame of reference:
  • If you are on an airplane, you don't feel yourself moving.
  • But an observer on the ground will report that the plane is moving. 
Einstein realized that if space and time are on a single continuum, then as the rate of speed goes up, the rate of time must go down and vice versa:
  • For an object moving slowly through space, time passes quickly. 
  • For an object moving quickly, time slows down. 
  • The closer an object gets to attaining the speed of light, the bigger the effect.
Since then, scientists have proved his theory by sending atomic clocks up in high-speed rockets. When they return, the clocks on the rockets are slightly behind their earthly counterparts. 

According to Einstein's general theory, matter bends the "fabric" of spacetime. If space-time is a rubber sheet stretched above the ground, and you put a heavy object like a bowling ball in the middle of the sheet, the bowling ball will pull down the center of the sheet. Any other lighter-weight balls you place on the blanket will be pulled toward the bowling ball, because the bowling ball is keeping the blanket from being flat.

That's where gravity comes from. For us on earth, the sun is the bowling ball, and the planets are other, smaller balls rolling around the sun. The planets are moving so fast in their orbits that they just keep circling the sun; their speed keeps them from falling into the sun, and gravity keeps them from flying off into space.

Einstein theorized that light curves as a result of gravity's effects on the fabric of space-time.

Willem van Stockum,
Like William of Ockam,
Did what he must –
He invented his Dust.
He said that curve ought to be provable by carefully photographing an eclipse. In 1919, astronomers went to an island off the coast of Africa to get the best possible photo of a solar eclipse. Sure enough,  they showed a deflection of the sunlight matching Einstein's prediction.

Willem J. van Stockum is the first person to write in English about the implication of Einstein's general theory for the possibility of time travel.

His 1937 article showed that Einstein's equations generate closed time-like curves, following on his dissertation for his Edinburgh Ph.D. in physics.

His "van Stockum dust" invention is viewed as an ingenious interpretation of the most difficult parts of Einstein's general theory.

Comment

I sent the above to my nephew Chris Oakley (D.Phil. Physics Oxon.) to make sure I got it as right as can be and still be talking English, with or without the ten-hundred-word limit. He has responded only to the bit about the special theory of relativity, which he says is not correctly expressed above. Here is his proposed new language for that bit:
Einstein's Special Theory of Relativity (1905): Motion can only be detected with reference to another object; when you say you are travelling at 50 mph it is 50 mph relative to the ground – relative to the surface of the Moon (for example) it would be a lot faster. The notion that the laws of physics do not depend on relative motion is called Galilean Invariance (neither word being in the 1,000-word list). Thus if you are conducting a physics experiment in a train travelling smoothly and levelly on a completely straight track at constant speed & it makes no reference to what is going on outside, you will get the same results whatever the speed of the train – if Galilean Invariance is true. Newton’s laws of motion are Galilean-invariant. The problem was that the new-fangled electromagnetism, discovered in the 19th century, which also governs the propagation of light, as embodied by Maxwell’s equations, is not Galilean-invariant. It looked as though there was a preferred reference frame where light would propagate in spherical waves much in the way that if you dip your finger in a lake from a stationary rowing boat, the ripples will radiate outwards in circles. That is the “preferred” reference frame; if the boat is moving the ripples will bunch in the direction of motion of the boat, and spread out downstream. Ripple propagation, at least as seen from the boat, is faster downstream (you add the speed of the boat), but slower upstream (you subtract the speed of the boat), and you can figure out the speed of the boat from this difference. That was the idea of the Michelson-Morley experiment c. 1900, only the boat was planet Earth, the lake they called the Lumeniferous Ether and the ripples were light waves. They found, and continue to find, nothing. All sorts of explanations for the null result were advanced, e.g., that the ether is dragged by heavy objects like the Earth, but Einstein came up with the simple but mind-warping notion that the Ether does not exist, rather space and time themselves conspire to ensure if you measure the speed of light you will always get the same result (about 186,000 miles per second) regardless of the motion of your measuring instruments. He was rightly dismissed as a lunatic… well, initially, at least. 
I am promised a subsequent bit about the general theory (1915).