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The Michelson-Morley Experiment (Analogy)
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 "pre-Einstein" 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 tpar is given by  
 
and for his/her twin, swimming perpendicular to the direction of motion of the boat, the time tperp is given by  
 
where vd 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 vd = v and if (vd2 - v2) = 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 Michelson-Morley 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 Michelson-Morley experiment  
   
*I confess that this is not my idea but can't remember where I read it. As this is a not-for-profit site I hope the original author will accept my using his/her idea for this animation.  
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