|MadSci Network: Physics|
SSG, 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, Everett
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