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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Has anybody noticed last week's New Scientist front cover article, "An end to wings and wheels"? An engineer named Shawyer is claiming that a truncated cone filled with microwave photons is givin him a reactionless force . Not a lot at the moment, milliNewtons for a kWatt input, but any such force appears to violate conservation of momentum. There's a short write-up on Wikipedia under EmDrive, and their website goes by that name.
His claim is that a proper relativistic treatment of the photons means that there is an unequal force on the ends, that is not fully compensated by the force on the sloping sides, giving a net force.
Neglecting the measurements and the detail of the theory for the moment and going for first principles, my thoughts are as follows. Relativity is a self-consistent theory, and is also consistent with the conservation of both momentum and energy. Therefore any calculation done using it should produce predictions which are also consistent, it is not logically possible to produce a contradictory answer (like doing addition with integers, it can only produce only integers and not fractions). If they are not consistent with the conservation laws, it indicates that there is an error in the maths. Or he is using something other than relativity to do the sums. Do I reason correctly or not?
Is it the case that in classical (Newtonian) mechanics, there is no momentum associated with a wave? If so, then a quantum treatment is required to give the photons momentum. He has been claimed to have been using group velocity in his solution to the field density in the waveguide, which is not the speed of anything physical so should not have momentum associated with it.
Relativity also requires the strict equivalence of all inertial frames of reference. If a force is being produced without an equal and opposite reaction on something, surely this means that the total work done in different frames is different, by moving a net force through different distances in different frames, an objection that an equal and opposite force neatly makes constant. Do I reason correctly that lack of a reaction force would imply violation of conservation of energy as well as momnentum by the above argument?
It will be interesting to see where his work goes. His claimed forces are still small enough to be experimental error, given the cables and copious cooling air that will be disturbing his force balance. He has attracted funds from various sources, and has NASA and the Chinese space agencies interested, so I guess he is happy for the moment regardless of the physics.
Registered Member #69
Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116
He has some way to account for conservation of energy, that the Q of the chamber and thus thrust drops as its velocity is increased so that lost em energy balances gained kinetic energy. This would seem to imply that there is some absolute frame of reference to measure the chamber velocity from. In other words, if you are flying along with your 'engine', by measuring Q you can deduce your velocity. This would seem to pose big problems for the theory of special relativity.
Looks like it shouldn't take much to reproduce his experiment, a scale, a magnetron and some sheet metal work... and a six pack to give you hope.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The main reaon I posted was to clarify my feeble understanding of physics. It's quite a (long) while since I did the first part of my degree in physics before switching to engineering, so I could use a bit of help from people who've done physics more deeply or more recently.
Does classical (Newtonian) physics have any concept of wave pressure, or does it need QM? Is work done in different inertial frames equal, is this mandated by energy conservation? Please confirm that is should be impossible to produce momentum conservation breaking predictions from relativity without having a mathematical error somewhere. Does conservation of momentum strictly imply all forces must have an equal and opposite reaction? At the simplest level, every interaction between a photon in the cavity and electrons in the walls of the cavity obeys the conservation laws, so there should be no way that an ensemble of them can break them.
Though switching between QM and classical approaches like group velocity is a good way to confuse oneself, or ones finacnial backers
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Newton itself probably didn't study radiation pressure itsef, but conservation of momentum is unique for everything and no force from 'inside' can move a body in a direction because of it's own reaction. No matter how our resonant cavity is shaped radiation pressures will always cancel out. (waverider can explain this better)
If you were able to ignore conservation of momentum you could make the motor go in one direction, then reverse it and accelerate in another direction virtually destroying the energy, I guess conservation of energy is good enough reason why such a thing would't work.
If you overcome this by assuming Q is dependent on velocyty (Eric^^) you could have 'velocity' relative to nothing, and special relativity is stuffed.
Thing seems as impossible as making perpetuum mobile...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Look, the damned thing's impossible, if nobody is going to try to answer my very specific questions to try to clarify exactly why it's impossible, then this thread gets closed too.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I tried to give he answer: it violates conservation of energy. By simply driving our motor back and forth you can destroy energy.
Or imagine our reactionless motor is moving towards some point in space (exp. earth) having some kinetic energy, adn starts braking.
Once it stops, both it's kinetic and used braking energy are gone
With reactive drives, either mass (chemical etc.) or electromagnetic, both kinetic and 'braking' energy are transferred to exhaust, either ending as kinetic energy of exhausted particles or increased frequency of EM radiation due to doppler effect.
Momentum is always conserved no matter of drive type used...
And I don't know how could you help there in reality ;/
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
wrote ...
Does classical (Newtonian) physics have any concept of wave pressure, or does it need QM?
Newtonian physics specifically doesn't say much about light beyond its optical effects. (Newton himself thought light was made up from particles but that's slightly off track.) The momentum of a photon as calculated by QM, however, is ordinary momentum and you can use Newtonian physics to handle it (with the usual caveats).
So, really, it's more that in Newton's time no one understood exactly what light was than that Newtonian physics couldn't handle this effect.
wrote ...
Is work done in different inertial frames equal, is this mandated by energy conservation?
Your concept of work and energy depends on your reference frame. This is easy to show.
Imagine you're on the back of a truck going at 20m/s and you roll a 1kg ball forwards at an extra 1 m/s. From your point of view, the ball has .5*mvv=.5J of kinetic energy and when it hits the other end of the truck tray and stops it loses just that.
From the point of view of someone outside standing on the road, the ball is going at 21m/s and has 220.5J of kinetic energy. When it "stops" in the tray and is going at 20m/s, it has 200J of kinetic energy.
So from the point of view of the tray, the ball loses .5J of kinetic energy. From the point of view of the bystander it loses 20.5J.
Edit: I guess I'd better clarify. This is not a way of generating 20J of teh free energy, it is simply showing that when calculating energy you need to keep a consistent point of view. This 20J of "free energy" is like the 20m/s of "free velocity" you get when you change perspective from the truck to the roadside.
wrote ...
Neglecting the measurements and the detail of the theory for the moment and going for first principles, my thoughts are as follows. Relativity is a self-consistent theory, and is also consistent with the conservation of both momentum and energy. Therefore any calculation done using it should produce predictions which are also consistent, it is not logically possible to produce a contradictory answer (like doing addition with integers, it can only produce only integers and not fractions). If they are not consistent with the conservation laws, it indicates that there is an error in the maths. Or he is using something other than relativity to do the sums. Do I reason correctly or not?
I think this is a reasonable enough argument. Relativity does incorporate conservation laws so if you can use relativity to break the conservation to break the conservation laws... well, you probably can't. At least, forget the drive, relativity would be broken.
wrote ...
He has some way to account for conservation of energy, that the Q of the chamber and thus thrust drops as its velocity is increased so that lost em energy balances gained kinetic energy. This would seem to imply that there is some absolute frame of reference to measure the chamber velocity from. In other words, if you are flying along with your 'engine', by measuring Q you can deduce your velocity. This would seem to pose big problems for the theory of special relativity.
When I read the New Scientist article I had similar thoughts. This guy seemed to be complaining that the drive didn't work so well at high velocity, which is an extremely odd thing to say for a relativistic effect. Unless perhaps the article really meant acceleration rather than actual velocity but frankly I don't care enough about this drive.
I have better things to study. It reeks of pseudoscience (albeit not as badly as some, but that's not saying much). It's not just the fact the drive is outlandish but the way that it's supposedly based on theory that no one's writing papers about.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
The basic conservation laws in Relativity are the same in Newtonian mechanics (with minor modification).
1. Conservation of (rest) mass (in the absence of particle decay or creation). 2. Conservation of momentum 3. conservation of angular momentum 4. Conservation of charge 5. Conservation of energy 6. ......etc.
Quantum mechanics is not necessary to show this invention does not work. The properties of radiation pressure were well known from Maxwell's electrodynamics in the 19th century..
In this thread. it was generally established that it was BS. It is fairly straighforward to show that Shawyer (the inventor) made a critical mistake in his calculations regarding the radiation pressure on the cavity walls. In short, I am suprised that New Scientist published the article without running Shawyer's claims by someone with experience in electrodynamics. His mistake would have flunked him out of my EM theory class!
The fact that is has attracted funding is not a complete suprise, since Shawyer seems more of a insistent salesman than a competent engineer.
I know this thread should be locked, but it could be interesting to see if others on the forum understand why this invention cannot work. Who here is studying EM field theory this semester? If you are, let's here from you!!!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't think this thread ought to be locked, since it seems to be a well mannered scientific debate. I did think of one thought experiment that seems to make the whole thing look stupid:
If his claim that the Q of an asymmetrical cavity depends on velocity is true, then someone would have noticed that microwave gear on board aircraft and spacecraft in motion had lower Q than it did on the ground. (Or higher, depending if the pointy end of the cavity was facing in the direction of travel...) As far as I know, nobody ever has noticed that, and there are some space probes with microwave equipment travelling at very high speeds.
If his claim is false, and the Q stays the same, then his engine is a perpetual motion machine, which makes it seem even less probable.
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spaceship drive snake oil
. Not a lot at the moment, milliNewtons for a kWatt input, but any such force appears to violate conservation of momentum. There's a short write-up on Wikipedia under EmDrive, and their website goes by that name.
