In order to illustrate some of the basics
behind this experiment, I ask you to imagine the following "thought
experiment"... 



Suppose we could find two friendly dolphins*
who would agree to swim at a constant speed (relative to the water)
near a simple square shaped boat. 



If the dolphins swim at
exactly the same speed (relative to the water) then they will
obviously take the same time and will arrive back at their starting
points simultaneously, as shown here. 



However, if we now get into our boat and
start the motor, so that the boat also moves relative to the water, the
situation will be a little more complicated, as shown
here. 

We must, of course, remind the dolphin swimming perpendicular to the
direction of motion of the boat to stay along side the boat all the
time... 



It is now far from certain that the dolphins
will arrive back at the same time. 



In fact, it is easy to show using our
"preEinstein" relative speed ideas (and a bit of Pythagoras and so
on) that the times taken by the two dolphins are as follows: 

for the dolphin moving parallel to the
direction of motion of the boat, the time t_{par} is given
by 



and for his/her twin, swimming perpendicular
to the direction of motion of the boat, the time t_{perp} is
given by 



where v_{d} is
the speed of the dolphins relative to the water and v the speed of
the boat relative to the water. 

These times are clearly not the same... ok,
except in a couple of special cases... if v_{d} = v and if
(v_{d}^{2}  v^{2}) = 1... but stop being
pedantic! 



By the way, if you're
not sure about why we're thinking about trained dolphins, well, you
see, one dolphin is like one of the beams of light in the
MichelsonMorley experiment, travelling parallel to the motion of
the earth through the aether and the other dolphin is like the other
beam of light. travelling perpendicular to that motion. 

One would expect that, if the direction of
motion of the boat were to be rotated through 90° then the dolphin
which "won the race" in the first case would now lose, so you would
expect a change in the dolphin interference pattern... ok so now
it's getting silly but you get the idea! 



return to MichelsonMorley experiment 



*I confess that this is not my idea but
can't remember where I read it. As this is a notforprofit site I
hope the original author will accept my using his/her idea for this
animation. 
