[extropy-chat] Relativity drive: the end of wings and wheels?
brian at posthuman.com
Sat Oct 14 21:43:58 UTC 2006
There's a few letters and a reply in a more recent NS:
Emdrive on trial
* 07 October 2006
* NewScientist.com news service
* Paul Friedlander
The article on "flying by light" describes a machine that uses microwaves to
generate thrust. As I read it, I, like the thousands of other physicists who
will have read it, immediately realised that this was impossible as described (9
September, p 30). Physicists are trained to use certain fundamental principles
to analyse a problem and this claim clearly flouted one of them.
To understand how, consider this. A "Shawyer drive" is installed in a spacecraft
floating in deep space far from any other object. Let us say that it got there
using nuclear power, since it cannot use sunlight. Switch on the Shawyer drive
and the craft begins to accelerate. The craft changes speed and in so doing it
changes its momentum without any other external change. Except it doesn't,
because this is impossible. Momentum, according to one of our basic principles,
is conserved and cannot be created or destroyed. The craft is breaking this rule.
In a conventional rocket, thrust is achieved without breaking the rule because
the combined momentum of the craft and the exhaust gas from the rocket cancel
each other out as they move in exactly opposite directions. The principle of
conservation of momentum is every bit as true in the world of relativity and
quantum mechanics as it was when set down by Newton. The Shawyer drive is as
impossible as perpetual motion.
From Greg Egan
Relativistic conservation of momentum has been understood for a century, and
dictates that if nothing emerges from Shawyer's device then its centre of mass
will not accelerate.
This statement holds true in all reference frames. It is likely that Shawyer has
used an approximation somewhere in his calculations that would have been
reasonable if he hadn't then multiplied the result by 50,000. The reason
physicists value principles such as conservation of momentum is that they act as
a reality check against errors of this kind.
Tuart Hill, Western Australia
From Dick Atkinson
Your cover story describes Roger Shawyer's plan to power a space vehicle by the
pressure exerted by microwaves in a vessel shaped like a truncated cone. Because
one end is smaller it is suggested that there will be a smaller force acting on
it, so the net forces will accelerate the device in the direction of its larger end.
Surely there is a facile error in this idea: every photon striking the big end
has to be reflected there. Photons which impact on the sloping sides exert
forces too. In effect, the narrowing walls of the vessel are part of the little
end, and a little vector analysis should show that their contribution neatly
balances the two ends.
I think this is as unreal as Jonathan Swift's account of the Big-endians and the
Little-endians in Gulliver's Travels, and any journey that Shawyer's drive
facilitates is rather less likely than Gulliver's voyage to the flying island of
Laputa. Having said that, I hope I'm wrong.
South Shields, Tyne and Wear, UK
From Paul Warren
I have seen some comments that question the academic integrity of your reports
on Shawyer and his emdrive concept. I feel New Scientist has an important role
to fulfil in exploring maverick or contentious science, and thus I am glad you
published the article. But I would like to request that you present both sides
of this kind of argument, and with more academic rigour. Can we hear more on the
emdrive and its sceptics?
Scarborough, North Yorkshire, UK
Roger Shawyer replies:
The momentum exchange is between the electromagnetic wave and the engine, which
is attached to the spacecraft. As the engine accelerates, momentum is lost by
the electromagnetic wave and gained by the spacecraft, thus satisfying the
conservation of momentum. In this process, energy is lost within the resonator,
thus satisfying the conservation of energy.
The emdrive concept is clearly difficult to comprehend without a rigorous study
of the theory paper, which is available via emdrive.com or the New Scientist
website (http://tinyurl.com/npxv8). This paper, which has been subjected to a
long and detailed review process by industry and government experts, derives two
equations: the static thrust equation and the dynamic thrust equation.
The law of the conservation of momentum is the basis of the static thrust
equation, the law of the conservation of energy is the basis of the dynamic
thrust equation. Provided these two fundamental laws of physics are satisfied,
there is no reason why the forces inside the resonator should sum to zero.
The equations used to calculate the guide wavelengths in the static thrust
equation are very non-linear. This is exploited in the design of the resonator
to maximise the ratio of end plate forces, while minimising the axial component
of the side wall force. This results in a net force that produces motion in
accordance with Newton's laws.
We are now in the process of negotiating a trial flight programme.
From issue 2572 of New Scientist magazine, 07 October 2006, page 24
Singularity Institute for Artificial Intelligence
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