In 1887 Michelson and Morley built an interferometer which is shown below.
In this interferometer, you shoot
a beam of light at a beam splitter. The beamsplitter splits the light into two parts,
which each bounce off of their own mirror. The light is then recombined by the beam
splitter and goes to a detector. Light is a wave, so when the path lengths are equal,
the waves add to produce a bright spot at the detector. When the path length changes
by only half a wavelength (which for visible light comes out to be about 250nm) the
waves cancel each other to produce a dark spot. For more information about this, see
Interference. Even though this instrument
can be a few meters big, it can detect changes in distance of hundreds of nanometers.
PICTURE of INTERFEROMETER
Why didn't they just think the Eather was being dragged along with the earth like
air inside of a plane?
Where was their reasoning flawed? It took 18
years before Einstein came up with a
satisfactory explanation. He said that there
was no eather. The electromagnetic
waves predicted by Maxwell's Equations travel
at the speed of light regardless
of the reference frame you're observing them in.
This is the major break from Galeleo's relativity where
if you were traeling with velocity
in the direction of light, you were
supposed to measure the light's velocity as
How does Einstein's new postulate solve the mystery of the MM interferometer? First let's
see how it's inconsistent with our analysis of the moving interferometer. In our analysis,
the speed of light was a constant relative to the eather (the grid underneath.) According
to Einstein, if we travel with the interferomter, the speed of light should also be c.
TRAVELING WITH INTERFEROMETER 2
Whereas before the light would always cover equal distances in equal times, now it's
speeding up or slowing down when it changes directions. Constant speed of light would
look like the first picture where both wavefronts get to the detector after traveling
the same distance. No matter how you rotate the interferometer, as you travel with it,
you would see that both wavefronts will cover the same distance.
So what assumptions did we make in our analysis that was false?
It turns out that even though the two arms of the interferometer
were at equal distances at rest, when moving, space itself is
contracted along the direction of motion. This shortens the journey
for the light along that arm exactly enough to allow it to catch up
with the light in the top arm. This is called .
LORENTZ CONTRACTED INTERFEROMETER 2