Download Relativity

Einstein’s Universe
20th Century Physics and Relativity
Late 1800s
• “Many wise people believed that there was
nothing much left for science to do.”
• Newtonian Mechanics was mostly right, but some
things turned out to be wrong:
– Space and time are constant
– Light travels as a wave through the “ether”, a
mysterious imaginary medium
– Speed of light should seem different depending on how
fast you are moving
Speed of light is constant
• Michelson-Morley Experiment
– American physicists, late 19th Century
– Perpendicular beams of light bounced off
mirrors
Ether
Time difference between
paths is same, whether
pointed North, South, East,
or West, whether noon or
midnight, spring or fall
• Speed of light (c) is constant
–It’s the same in ANY frame of reference
Albert Einstein (1905)
• Photoelectric effect
– Evidence for photons
• Brownian Motion
– Evidence for atoms
• Special relativity
– Explains why speed of light is constant
• General relativity (1917)
– Gravity and the structure of space
Special Relativity:
The World of Fast Things
• Based upon observation that speed of light is
constant in all reference frames
• The laws of physics hold true for all frames of
reference (does not mean that we will get the
same experimental results in differing frames)
• Implications
–
–
–
–
Time is not constant (time dilation)
Space is not constant (length contraction)
Mass is not constant
E = mc2
Speed of light is constant Time is NOT constant
 Twin Paradox
• Twins, Al and Bert, both of whom are 10 years old.
• Al goes to summer camp in the Alpha-3 star system (which is 25 light-years
away), at 99.99 percent the speed of light.
• Bert stays home on Earth.
• The trip to the star and back takes 50 years.
• What happens when Al returns?
• His twin brother is now 60 years old, but Al is only 10 and a half.
• Has Al just discovered the fountain of youth?
• Not at all. Al's trip into space lasted only a half year for him, but on Earth 50
years passed.
• Does this mean that Al can live forever?
• Nope. He may have aged by only half a year in the time it took 50 years to pass
on Earth, but he also only lived half a year. And since time can slow down but
never goes backwards, there's no way he could grow younger.
 Time is physically different in the two different frames of reference
 Time actually slows with motion but it only becomes apparent at
speeds close to the speed of light.
Length is NOT constant
Lengths become contracted
• When an object (with mass) is in motion, its measured
length shrinks in the direction of its motion.
• Only a person that is in a different frame of reference
from the object would be able to detect the shrinking as far as the object is concerned, in its frame of
reference, its size remains the same.
Rulers have different lengths!!!
Implication for energy
• Conservation of Energy and Conservation
of Mass are not exactly true, because one
can be converted into the other.
• E = mc2
• Radioactivity!
• Implications for stars, age of earth (Chapter
10), nuclear energy
Stars
• The energy generated as hydrogen deep within our star
continuously fuses to form helium.
Earth
• Einstein's formula accounts for the heat in our planet's
interior, which is kept warm by a steady barrage of E =
mc2 conversions occurring within unstable radioactive
elements such as uranium and thorium.
Nuclear energy
• Fission reactors in nuclear power plants generate
electricity by unlocking the energy tied up in fissionable
materials. Fusion: when two hydrogen atoms fuse to
form a helium atom, the mass of the resulting helium is
less than the two hydrogens, with the missing mass
manifesting itself as fusion energy.
• Activity: How much energy is released by 1
gram of hydrogen? (1 J = 1 kg m2/s2 )
c = 3x108 m/s
• How does this number compare to the
energy released by 1 gallon of gasoline =
100 million J?
General relativity (1917)
• Gravity is caused by
the bending of
spacetime caused
by objects with
mass
• A uniform
gravitational field
(like that near the
Earth) is equivalent
to a uniform
acceleration.
Matter tells space-time how to curve
Curved space-time tells light and matter how to move.
But is it real?
• The Sun warps space:
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
– Mercury’s orbit inexplicable
without relativity
– During eclipse, apparent
positions of stars near Sun
different
• Galaxies warp space:
– Gravitational lensing (see
picture at left)
– bending of starlight by gravity
• GPS won’t work without
corrections for general
relativity
J. Rhoads (STScI) et al., WIYN, AURA, NOAO, NSF
Einstein’s “Biggest Blunder”
• His theory of General Relativity predicted that a
static universe would not be stable
– Universe should either expand or contract
• He did not believe it. He believed in a “steadystate” (like Hutton and the mountains!)
• So he put an extra piece into his equations, a
“cosmological constant” to keep universe from
expanding! An artificial fix!
• Slipher and Hubble measured doppler shift
• They found the UNIVERSE IS EXPANDING - in
agreement with Einstein’s equations before the
“fix”. So the fix was a major blunder.