General Relativity For a general audience. Precession of Mercury Mercury’s entire orbit undergoes...
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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
Precession of Mercury Mercurys entire orbit undergoes
precession at a known rate. Most of it can be explained using
Newtons laws and gravitation (pull from other planets, etc.), BUT
NOT ALL OF IT! Creative Commons
Slide 3
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
Slide 4
Einsteins 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 Mercurys orbit.
Slide 5
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
Slide 6
Newton VS Einstein Newton Force tells mass how to accelerate F
= 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
Slide 7
Time Dilation in GR Time runs slower closer to a massive
object. NASA
Slide 8
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
Slide 9
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
Slide 10
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.
Slide 11
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.