MadSci Network: Physics

Re: Is it possible to create a continual motion device in space?

Date: Thu Feb 25 10:05:36 1999
Posted By: Jason Goodman, Graduate Student, Massachusetts Institute of Technology
Area of science: Physics
ID: 919622235.Ph

There are several questions here. The most basic can be stated as:

Temperature is a measurement of the jiggling of molecules. How do we measure temperature in the vacuum of space?
You are correct that it's difficult to define a temperature when there is no matter. We generally say that the temperature in space is the temperature that a small light-absorbing glob of matter would become if we put it into the vacuum. (Imagine a black-painted thermometer drifting through the stars). This temperature is not zero: while the thermometer isn't being heated by atoms bumping into it, it is being heated by light shining on it from the sun and stars. Near the sun this can be quite large; if you go far, far away from every star and galaxy in the universe, it will still not be zero. This is because the universe is awash with the glow of light leftover from the Big Bang which began the universe! It's a very weak glow, red-shifted down into the microwave region of the electromagnetic spectrum, and the temperature associated with it is 3 Kelvin. Which is very cold, but not absolute zero.

Is perpetual motion possible in space?
Pretty much, yes, but it's not very useful. "Objects at rest tend to remain at rest; objects in motion tend to continue that motion." The Earth has been moving around the sun with essentially zero change in orbit for almost its entire 5 billion year history... which is close enough to "forever" for human purposes. The galaxies have been moving with respect to one another for even longer. Your question seems to be asking about superconductivity, in which many substances lose all resistance near absolute zero. Yes, you could make a wire out of a superconducting substance and have it carry electrons around in a circle "forever", if it was in deep space at a temperature of 3 Kelvin.

However, most people's idea of a perpetual motion machine is one which runs forever, doing useful work. All processes require energy to do work: if you use the electricity in the circulating wire to light a light bulb or drive a motor, the electrons will slow and stop as their energy is used up. This is a consequence of the "first law of thermodynamics", also known as "conservation of energy", or "There ain't no such thing as a free lunch".

To summarize: things move forever in space if you leave them alone, but using them to do work will slow them down.

If I've misunderstood your question, please send me e-mail.

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