Download Why did we spend so much time discussing swimming in a river?

Why did we spend so much time discussing
swimming in a river?
Answer: because this example explains the underlying concept
Of the Michelson-Moreley experiment – one of the “milestone
experiments” in the history of physics.
Some facts:
● In 1805, Thomas Young makes his historic observation of interference
of light rays emerging from two narrow slits. It proves that light has a
wave-like nature. The Newtons theory according to which light is a beam
of micro-particles, or “corpuscles”, is dead.
● In the 1860s, a Scottish physicist and mathematician, James Clark
Maxwell, discovers a set of four differential equations that “unify
electricity and magnetism” – i.e., they provide a complete description
of electric and magnetic fields. Based on these equations, he predicts
the existence of electromagnetic waves.
Continued from the preceding slide:
● Soon, in 1888, the existence of electromagnetic waves of radio
frequency is confirmed experimentally by Heinrich Hertz. Also,
physicist realize that LIGHT is also an electromagnetic wave, only
of much higher frequency than the waves generated by Hertz.
● For XIX-century physicists it is clear that all types of waves need
a medium for propagating.
For instance, as you know, sound waves can propagate in media
fluids
and solids. But not in vacuum – it’s not a “medium”!
such as gases,
…. (please
answer).
● However, light does propagate in vacuum – it propagates extremely
well! Light rays travel large distances in vacuum (e.g., in interstellar
space) without losing any intensity, whereas in the most transparent
materials we know (such as, e.g., perfect diamond crystals) there is
always some intensity loss.
In view of that, XIX-century physicists postulated the existence of
a LUMINOFEROUS ETHER, a hypothetical substance (weightless,
colorless, odorless….) filling the entire Universe (including all material
bodies), and acting as a medium for light propagation (luminoferous
means “light carrying” in Latin).
LUMINOFEROUS ETHER, continued:
● Maxwell believed that the equations he discovered actually represented
the elastic properties of ether (similarly, the equations describing sound
waves in solids are derived from equations describing elastic distortions
of solids). Maxwell argued that ether actually IS a solid! Why?
Well, we know two types of waves – longitudinal waves and transverse
waves (recall Ph212 course). Light is certainly a transverse wave. How do
we know that? The answer is simple: there is an effect that only
transverse waves exhibit, but not longitudinal waves!
Question: What is the name of this effect?
Yes, you are right – the name of this effect is POLARIZATION!
Sound waves in air and other gases, and in fluids can only be of
longitudinal nature. Therefore, there is no such thing as “voice
polarization”. However, transverse sound waves do exist – but
exclusively in solids. Then, they may be polarized. For instance,
polarized sound waves are used in certain studies of solids by
a technique known as “sonography”.
IMPLICATIONS OF THE ETHER HYPOTHESIS:
The “ether hypothesis” seemed to be so well grounded that physicists
overwhelmingly accepted it. In the closing decades of XIX-th Century
a large number of papers discussing the properties of the hypothetical
ether was published in scientific journals. One conclusion that emerged
from those speculations was that light propagation in ether should
obey the principles of the classical (Galilean) relativity.
Accordingly, if an observer were sitting in the middle of “ether wind”
– we can also think of it as of a “river of ether” – light would behave like
the swimmer in the example we have discussed.
Ether wind
If the speed of light in stationary ether were c , then the observer would see
light propagating “downstream” with c+u speed, and light propagating
“upstream” with c-u speed (where is the “current speed” of the “ether river”).
Well, Michelson’s idea was to test the above predictions experimentally.
But where to find a suitable “ether river”?
“No problem!” -- Michelson said. “Earth is orbiting Sun with a velocity of 30 km/s,
Which is one part in a thousand in the speed of light”.
So, the observers on Earth see this situation as if they were sitting
In the middle of an “ether river” (or “ether wind”).
Michelson’s idea was
very simple: let’s measure the difference between the speed of light
propagating “downstream” and propagating
“upstream”.
Such an effect, he reasoned, should yield a difference of 60 km/s.
In Michelson times, direct measurements of c with the necessary precision
(error margin much less than 0.1 %) were not yet feasible. Various
measurements performed by the
best expert of that period, Fizeau
and Fucault from France, and
others, yielded values that
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differed by as much as
1000 m/s, or more (see
the plot to the right).
http://www.setterfield.org/report/report.html
This is an interesting link. The authors of this document present
a highly controversial hypothesis, according to which the speed of light
changes with time. I definitely do not want to “advertise” this theory.
Actually, in the opinion of most experts it is “trash science”.
However, there is one valuable element in the paper. In order to support
their claims, the authors had made meticulous literature searches, and
In the paper they presented an extensive historical review of speed of
light measurements performed over a period of more that 300 years
– including many details that one can hardly to find in books.
If you skip the sections in which the authors try to “indoctrinate” the
reader, and you focus only on the “historical” parts of the document,
it may really be a very interesting reading!
Michelson’s idea was to use an ingenious interferometry technique
Observer
looks
To explain the idea of the historic 1887 Michelson-Morley experiment,
let’s assume that there is such thing as an “ether wind”. Let’s now
run an animation:
http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/mmexpt6.htm
After the show:
For the
“cross-stream”
beam (blue) it
takes less time
to travel forth
and back along
the path L than
for the
“upstreamdownstream”
beam (red).
Suppose that the time difference corresponds
to one-half wavelength. Then, when the beams
are recombined, the interference is destructive
– the observer sees a dark spot at the center of
the interference pattern.
Now, it is the red beam which is the “cross-stream”
beam. Now this beam arrives first, so again there is
a phase shift of one-half wavelength, and the interference again is destructive.
Again there is a dark spot in the center of the interference image.
Michelson-Morley experiments – conclusions:
When the apparatus is rotated by 90°, the “ring pattern” goes through
a full cycle:
dark central spot → bright central spot → dark central spot again
Michelson-Morley experiment was repeated many times, with better and
better precision. The experiments always yielded the same result: the
apparatus rotation never caused an change in the interference image,
clearly indicating: “There is no ether wind!”
Michelson very reluctantly accepted the negative results of his
experiments. He seemed never to fully accept that there is no such
thing as “cosmic ether”.