Einstein and Relativity

Albert Einstein, also, and

·         1879-1953

 

In 1905 Einstein published papers on:

·         Brownian motion. This is an animation of Einstein’s explanation.

·         The Photo-Electric Effect – An oversimplified cartoon-like explanation, but it gets the main idea across.

·         light knocking electrons out of matter This is a bit more staid and informative, but still not too technical.

·         He received the Nobel Prize for this

·         His doctoral thesis

·         and most importantly, “On the Electrodynamics of Moving Bodies” – the paper that launched Special Relativity. This is the actual paper, translated into English. In case you are interested.

 

Special Relativity. This site is actually called Special Relativity Made Simple.

The twins paradox

What’s so special about Special Relativity?

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Energy

·         Mass and energy are not independent concepts.

·         When a body radiates energy of amount e it loses mass by an amount e/c2

·         Therefore, in principle m = e/c2

·         Or more familiarly, e = mc2

 

GENERAL RELATIVITY

·         Special Relativity concerns frames of reference that move inertially with respect to each other.

·         This is a special case.

·         All motion that is not inertial is accelerated.

 

Acceleration and Gravity

·         In Newton’s physics, inertial motion is not perceived as different from rest.

·         Acceleration is perceived as an effect on inertial mass due to a force impressed.

·         Viz., Newton’s second law, F = ma

·         Inertial mass is measured as resistance to change of motion (acceleration).

·         Gravitational mass is measured as attraction between bodies, causing acceleration.

·         But inertial mass = gravitational mass.

·         Inertial and gravitational mass are equal in value and ultimately measured by the same effect: acceleration.

·         The Positivist viewpoint: Acceleration and gravity must be equivalent.

·         Einstein’s thought experiment

·         Look for a case where acceleration and gravity should produce different effects according to classical (i.e., Newtonian) physics.

 

Einstein’s Elevator, another site

·         In a closed elevator, acceleration feels like gravity.

·         Therefore, gravity should do the same to light as acceleration does, namely pull it toward the attracting mass.

 

A Real Experiment – The Angular distance between two stars

·         Einstein’s thought experiment points out the anomaly in the theory but is not testable as conceived because the distances are too short and the speed of light too fast.

·         The only practical way to test this hypothesis is with an astronomical event.

·         Consider two stars that can be observed (at night) and the angular distance between them measured.

·         Were it possible to see the stars in the daytime, the light travelling from them should be bent by the gravitational pull of the sun. The result would be that the stars would seem to be farther apart in the daytime than at night.

·         During a complete solar eclipse, the sky is dark enough to see the stars, but the sun lies in the path between the stars and the earth.

·         Einstein predicted that the starlight would be bent by 1.7 seconds of arc. In 1919, Sir Arthur Eddington confirmed that result. Details of Eddington’s analysis.

·         This is when Einstein became know to the public as Mr. Genius.

Another confirmation of General Relativity:

·         General Relativity “postdicts” the otherwise unexplained advance of the perihelion of Mercury by 43 seconds per century.

 

The Curvature of Space