Wednesday, May 6, 2015

Space drive

People are asking me about  Shawyer/White's "EM drive."  Years ago I did some work on the idea of "pushing against empty space," (R. Jones, American Journal of Physics,  vol. 37, pg 1187,  1969 ) but it was research more along the lines of  Jack Wisdom's work ( Science, 21 March 2003, pg 1865 ). Shawyer and Yang Juan  and White have all offered various different explanations for their results. Most of the work is unpublished and none of it has been independently verified. Extraordinary claims require extraordinary evidence. And, again, the majority of scientific papers may well be wrong (New Scientist, 30 Aug. 2005)

Shawyer and White's device:

It may well be that we need to explore the details of the measurement systems being employed.  Some years ago I had a Geiger counter that would respond when I turned on an electric motor in the same room.  The counter was battery powered and I assume its circuitry was acting like a radio and picking up RF interference generated by the motor.  When I did radiation background measurements with my mechanical roughing vacuum pumps turned off I measured a low level background.  When I was doing (plasma) experiments the pump motors were turned on and the Geiger counter registered a higher level of radiation.  At first I thought this was all coming from my plasma. 

One of the suggested EM drive spaceships is to have a mass of 90,000kg, be powered by a 2,000,000 Watt nuclear reactor and have a thrust of 800 Newtons (.4N/kW).  This would give an acceleration of almost .009 m/s/s and in a week or so of powered flight the ship would have a speed of almost 5400 m/s and kinetic energy of over 1.3 teraJoules.  But the nuclear reactor will have only supplied an energy of  E=P t , perhaps 1.2 teraJoules.  The violation of conservation of energy gets worse for longer powered flight times.

 With a constant thrust to mass ratio the acceleration is constant so the velocity increases linearly with time.  So the energy output, the ship's kinetic energy, must increase as time squared. But the input power from the nuclear reactor is constant so the input energy only increases linearly with time. So conservation of energy will always be violated if the powered flight time is long enough. This is the same issue I used to discredit the Dean Drive back in the 1960s. The EM drive would be an even better energy source than it is a propulsion system.

So the EM drive not only pushes on the vacuum it gets energy for free (from the vacuum??) as well.  2 miracles, each extremely unlikely to be true.  Like with cold fusion, when the number of miracles required exceeds 1, at that point I give up hope. It's just simple probability theory.

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