General Relativity For a general audience

Precession of Mercury Mercury’s entire orbit undergoes precession at a known rate. Most of it can be explained using Newton’s laws and gravitation (pull from other planets, etc.), BUT NOT ALL OF IT! Creative Commons

Theories to Explain the Extra Precession Asteroid belt inside the orbit of Mercury? An undiscovered planet (called Vulcan) closer to the sun than Mercury? Neither were found. Therefore, something about our understanding of the problem was missing…

Einstein’s General Relativity In 1905, Einstein published his special theory of relativity. In 1915, he published his general theory of relativity. General relativity exactly predicted the observed precession of Mercury’s orbit.

What is General Relativity? GR explains how space and time work around massive objects. Massive objects cause space to “warp” in towards the mass. Everything traveling through the “warped” space will appear to curve in towards masses. Creative Commons

Newton VS Einstein Newton Force tells mass how to accelerate – ƩF = ma Mass tells gravity how to exert force – F g = GMm/d 2 Einstein Curved space-time tells mass how to move Mass tells space-time how to curve

Time Dilation in GR Time runs slower closer to a massive object. NASA

Light Traveling Past the Sun Light always travels at the speed of light (c = 3 x 10 8 m/s) But near a massive object, time goes more slowly. As the photon travels past the sun (a very massive object), time slows down for it. We detect a longer travel time than we would otherwise expect for the photon to travel from the earth, to Venus, and back again. NASA

Testing the theory at Haystack Observatory Radio waves were sent from the 37 meter radio dish (radar) at Haystack Observatory in Westford, MA Those radio waves traveled past the sun, to Jupiter, then reflected off of Jupiter, traveling past the sun again, and back to the Haystack radio dish. The travel time was exactly what was predicted by general relativity. Creative Commons Haystack

Practical Application - GPS There are 24 GPS satellites in 12-hour orbits around the earth. Each contains a very accurate clock. A GPS receiver on the ground receives the time according to whichever satellites are within view in the sky. The difference between the received time and the current time is enough to calculate distance from each satellite. Triangulation then gives the receiver's position on the ground.

Why does GPS need GR? Wikipedia Creative Commons Due to GR, clocks on earth run slower than the clocks in the GPS satellites in orbit. This must be corrected for in order to accurately calculate positions on earth. If GR corrections were not done, positions would be off within minutes.

General Relativity Activity