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starship-design: Re: Stellar Drive

>Interesting idea. Is the physics really OK on this one? It sounds dodgy. I
>don't follow the author's attempt to keep Newton's third. What if there
>were no charged matter in the direction of the escaping fields? If there
>were charged matter, would it not make a difference if it was mainly
>negative or positive? The magnetic forces would be in opposite directions
>for opposite charges. If there were no charged matter moving relative to
>the electromagnets, there would again be no possibility of future magnetic
>forces making up lost momentum. Does the concept depend on the statistical
>distribution of charge and velocity throughout the universe surrounding the
>magic drive? Surely the principle of reaction is not statistical in the
>sense thermodynamics is. I have a few other points. Timothy wrote:
>>Magnetic fields is just EM-radiation, usually magnetic fields are very low
>>frequency radiation, in some cases almost non-alternating. In any case
>>photons are exchanged.

While stating this, I should have mentioned that non-alternating/changing
magnetic fields are not EM-radiation. In this reactionless drive one could
make the switching times neglectable to the on-time, which I didn't do.
I believe the author of the article did point to this difference.

>How would you make a laser pull? Lasers, indeed all real photons, only
>push. The force on the target would be cancelled by the reaction to the
>laser emission in the emitter, and the system would experince no net force.
>With pulling virtual photons, the idea seems to imply an odd free lunch -
>the action and reaction are in the same direction. But we still pay for

Don't count on free lunches, even virtual photons have to get their momentum
from somewhere too.

>I think Kevin made a good point about the second electromagnet's field
>interacting with the first electromagnet by inducung a current in it. I
>don't think that shielding it with a superconductor would help - a current
>would be induced in the superconductor, and you would still have your
>unwanted reaction force. It would be like a bar magnet hovering over the
>eddy currents it produces in a superconductor.

There indeed will be a reaction force, but you could make it neglectable by
momentarily increasing the resistance of magnet 1. That would prevent the
formation of an induced magnetic field.

>My final point is that if the system doesn't violate Newton's Third Law,
>what's the point of it? Its not really a 'stellar drive' (how did
>reactionless drives get that name?) at all. I assume the point of a
>reactionless drive is to save a starship from using up its energy on
>reaction mass - one would prefer that it convert all its stored energy into
>KE for itself. But if the magnetic fields created eventually do accelerate
>a distant 'reaction mass', no energy has been saved. So is the concept
>really helpful for us?

No, like I said, you could just as well use a laser beam to propell yourself.
However the concept on itself is interesting. It would probably be possible
to do the same with electric fields:

- + ======= + - 

Where -+ are separated charges and ===== is the ship (or plastic rod).

Since the positive charges are closer to each other than the negative
charges, the net result is both sides reject each other a little.
By separating and rejoining the charges you would have a similar effect as
switching on and off the magnets.

>Just out of interest, is there any way of focussing virtual photons to
>concentrate a magnetic field in one direction? I doubt it's possible
>without making the photons real first.

I suppose that if you have two charges (or charged plates), there will be an
increased amount of virtual photons between them.
I don't know however, what the difference is between an "attracting" or
"pushing" virtual photon. (It may have to do with the fact that a virtual
photon has 3 degrees of freedom in contrast to the real photon which has
only 2 polarisations.)