[Soul Surfer (OP)]

This week's New Scientist has got an article in it about a new electromagnetic drive for spacecraft that could be much more efficent than the currently being tested ion motors.  

It is so way out that I double checked the date wasn't  1st April

It has been dsigned and tested and found to work by   Roger Shawyer a respected aerospace scierntist at Portsmouth University.

Apparantly the idea has been around for some time but it is the way that it works is so far off beam I still find it difficult to believe and am looking for the catch.

You feed microwave radiation into a high Q resonant cavity that is shaped.  The shape is a tapered cylindrical waveguide with one end greater than the other and the idea is to get as much energy stored in the cavity that you possibly can.  He is feeding about a kilowatt into a cavity with a Q in the region of 6,000 - 50,000.

The enegy in the cavity produces a force on the cavity along its axis and in the direction of the largest end.  He explains it as the enegy in the cavity produces a force on the walls of the cavity and the force is greater on the wider end than the narrow end so there is a net external force on the cavity.  Note, no throwing energy out the back like a rocket! the energy is entirely contained in the cavity and the force depends on how much you can get in there.

OK the force is not very great, his first prototype only produced a force of 16 millinewtons but a later model with a higher q gave 300 millinewtons,  a great deal more thrust than that produced by the recent ESA SMART-1 Ion engine.

It is claimed that in theory using a superconducting cavity of the quality used in big accelerators forces of  30,000 newtons per Kilowatt might be achieved. this is enough to lift a car!

The downside.  You are not getting something for nothing and the moment you start accelerating the object the force drops and energy is drawn from the cavity and needs to be replaced  but the device could produce a static anti gravity force very efficiently.

This is the nearest thing to an antigravity machine I have ever seen and is coming from a very reputable scientific source but I am still finding it difficult to believe.


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[Nieuwenhove]

I can't believe it either. I guess that waves are somehow escaping from the device.

[Soul Surfer (OP)]

No they are not for the cavity to have a very high Q all significant losses must be reduced to an absolute mininmum and I guess the only way for EM radiation to get out of the cavity will be via the input waveguide which will probably have a special coupler to prevent this.

I think that I understand how it could work but I am still amazed that the opposing effects do not balance.  

Electromagnetic radiation posesses no mass but it does posess momentum and this is well known and can be measured.  Inside a waveguide cavity the radiation goes backwards and forwards between the two ends.  the Q factor gives a measure of how many times it can do this before the energy is lost.  The assymetrical design of the cavity means that the group velocity of the radiation in one direction is different from the group velocity in the other  this difference in group velocities means that there is a slight momentum imbalance between the waves travelling in one direction and the waves travelling in the other  this is seen as an attempt for the cavity as a whole to gain momentum,  which if this is prevented, appears as a force.  If the body is free to move it will gain momentum but as it does it will accelerate and energy will be taken out of the cavity and must be restored to keep the force acting  so the laws of conservation of energy and momenum are not being violated.

What amazes me is that it is apparently possible to make this assymetric cavity.  I would have expected things to have balanced themselves out.

THe existence of this process makes me wonder if there are any particle physics situations when this may happen and if so, this may have important consequences in our understanding of dark matter, dark energy and inflation.

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[thebrain13]

soul surfer, you said that once it accelerates it loses its energy, so doesnt that mean that it would lose its energy in the presene of a gravitational field, even if it is stationary relative to a ground observer?

[bostjan]

My E/M professor in grad school told me I was foolish when I asked him about the possibility of harnessing E/M momentum to make a sort of ion engine.  But then, he also told me that there was no future in ion engines.

If this thing really works, it is a true breakthrough for the possibilities in space travel.  Not that anyone seems to get nearly as excited about space travel as we do.

[science_guy]

that would make sense.

two reasons that people arent excited about space travel:
      1)many people dont believe in aliens
      2)many people dont want to pay more taxes on what may become useless missions.

_________________________________________________________________________________________

I would engage you in a battle of wits, but it is against my moral code to attack the unarmed.

[Soul Surfer (OP)]

No I believe that it would not lose energy if the object is not accelerating in its own frame of reference and that includes hovering in a gravitational field if it can produce enough thrust to support its own weight.  mind you looking at the numbers it is unlikely that a device could be built to achieve this in earth's gravity. however if you compare its performance wit that of ion engine thrusters for spacecraft it could easily be much more efficient and it uses no consumable fuels to generate the thrust.  All you need is electrical energy  and you can get that from solar cells or a thermal isotope source.  It might even be the route to a practical starship design.  

It is important to note that such an engine would not be detectable from any great distance because it profuces no visible or particle radiation.  If aliens were using more converntional rockets or ion motors for getting around using large ships they would be detectable at very long ranges.

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[lightarrow]

I'm sorry, Soul Surfer, but I can't understand the physical principles of this effect. Nothing escapes from the cavity? Are you sure there can be a force on it? It seems impossible to me. Explain what you know about it.

[Nieuwenhove]

Hi Soul Surfer,

I couldn't find the article you mentioned in New Scientist. Could you please indicate the issue (date) and page number ?

[lightarrow]

Momentum must conservate: if the device gain momentum in a direction, something else must gain momentum in the opposite direction and this is impossible if there is no energy or particles flying back.

[lightarrow]

quote:Originally posted by bostjan

My E/M professor in grad school told me I was foolish when I asked him about the possibility of harnessing E/M momentum to make a sort of ion engine.

I don't understand the connection between E/M momentum and ion engine; however, of course is very simple to use E/M momentum to make an engine: a powerful laser beam directed back (photon engine). The force on the space-ship is W/c where W is the laser (or whatever light source) power in Watts and c the speed of light. For a 1 Megawatt laser, the thrust is about 3300 N (= 340 Kgf).

[syhprum]

I find this figure rather high when the designers of space vehicles speak of using sails in the order of square kilometers to propel their vehicles by sunlight which I understand has a power of about 1.3 KW per sqare meter.
Is there a different calculation for photons. bouncing of a surface as against locally generated ones. 10^6/3*10^8 = 0.00333 what units are employed ? or did you use a million megawatt laser.

syhprum

[lightarrow]

quote:Originally posted by syhprum

I find this figure rather high when the designers of space vehicles speak of using sails in the order of square kilometers to propel their vehicles by sunlight which I understand has a power of about 1.3 KW per sqare meter.
Is there a different calculation for photons. bouncing of a surface as against locally generated ones. 10^6/3*10^8 = 0.00333 what units are employed ? or did you use a million megawatt laser.

You are right, I'm sorry. I was thinking "Terawatt" and for some misterious reason I wrote "Megawatt"! (Can someone solve this mistery? Mah!)

[syhprum]

The NewScientist article quotes the case of a light sail of 600 square meters which would adsorb about 1 MW of sunlight and producing 0.0033N force

syhprum

[bostjan]

E/M waves carry linear momentum proportional to Poynting's vector, even in static fields.

Think about if you put an electron with finite mass in an e/m field.  It accelerates, and thus gains momentum, but where does the momentum come from?  It comes from the field.  Pretty cool, I think.

[lightarrow]

quote:Originally posted by bostjan

E/M waves carry linear momentum proportional to Poynting's vector, even in static fields.

Think about if you put an electron with finite mass in an e/m field.  It accelerates, and thus gains momentum, but where does the momentum come from?  It comes from the field.  Pretty cool, I think.

Yes, and what generate the field? Other electric charges that acquire a momentum in the opposite direction.

[lightarrow]

quote:Originally posted by syhprum

The NewScientist article quotes the case of a light sail of 600 square meters which would adsorb about 1 MW of sunlight and producing 0.0033N force

Exactly. With 1 Terawatt (= 10^12 Watt = 1000 Gigawatt = 1000,000 Megawatt) you have 3300N.

[Soul Surfer (OP)]

The new sciemtist article is on pages 30 - 34 of the 9 September issue.

Re conservation of momentum  I agree and find it difficult to believe that this will work but there seems to be strong evidence that it does.  Let me try to explain again.  Photons do not have mass but they do have momentum  In the tapered waveguide cavity the group velocity of the photons moving in one direction is different from the group velocity in the other direction this means that there is a net imbalance in the  momentum imparted on the two ends of the cavity.  Apparrantly the cavity with the largest end has the biggest momentum (This would tie in with waveguide theory where the group velocity in a larger waveguide is higher than that in a smaller waveguide)  so if the obgect is free to move this imbalanced momentum is transferred to the whole object and it can accelerate.  

This force is vaguely  related to the much talked of casimir effect,

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[syhprum]

Light arrow

I see the possibility for a school lab demonstration here a one watt laser could be fitted to the bottom of a one meter pendulum enclosed in a vacuum jar so that it has a very large 'Q' factor and the laser pulsed a the resonant frequency of the pendulum.
1 watt would give a force of 1/(3*10^8) Newton's but if you had enough patience you would eventually get it swinging.

Although a 1 Megawatt laser is just about at the bounds of present day technology I don't think one with a continuous output of 1 TW is possible

syhprum

[Soul Surfer (OP)]

I do not think that you could create the required assymetric group velocity very easily at optical frequencies. It is far better to use microwave frequencies and 1 kw magnetrons are dirt cheap and reliable (standard microwave oven kit)

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evolution rules in all things
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