MadSci Network: Physics

Re: why is the propagation of light not an example of a perpetual motionmachine

Date: Tue May 12 17:55:57 1998
Posted By: Everett Rubel, Degree in Physics
Area of science: Physics
ID: 892670121.Ph


Thanks for an interesting question.  The field of perpetual motion machines 
is one filled with much controversy.  I think we need to think about what 
the term means.

Some people use the term to mean a device that gives out more energy/mass 
than it takes in.  Such a device would be a counter example to the first 
law of thermodynamics.  My opinion is that the first law is correct.

Some people use the term to mean a device that continues to operate 
indefinitely without receiving energy from outside.  Here I use the word 
"operate" to mean some action that is repeatedly detectable outside the 
device.  This sort of device does not defy the first law, so it is okay 
that far.  I believe such a device can only be practical however if it is 
designed to stop, since it cannot really do anything that affects the 
outside world without running down.  In my view, if the device affects the 
outside world, then some energy must leave the system, and then the device 
stops eventually due to a lack of energy.

I think a photon is an example of the second kind of device.  If it is not 
bothered, there is no reason that a photon could not go on forever.  
However, if we want to do anything useful with the photon, such as detect 
it or have it heat our tea, then the photon does not last forever, and is 
not a perpetual motion machine.

On a somewhat different tack,  since the photon is moving at the speed of 
light,  time has essentially stopped for the photon.  So from the point of 
view of the photon,  it does not move and is instantly destroyed after 
being created.

Regards and best wishes,


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