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# one question

```Kevin C. Houston writes:
> Steve, I have a question for you. (BTW, Your sarcasm on the
> reactionless drive was not lost on me.  I am both overjoyed, and dismayed
> to have invented it  overjoyed for obvious reasons.  dismayed, because it
> cannot be --- although E E "Doc" Smith didn't think so ;)

I hope it did not seem harsh.  My intent was to demonstrate a
really nifty application for your, um, incredible invention.  :-)

> Out in intergalactic space (far far away from all outside gravitational
> influences)  you have the following setup:

[ Figure turned sideways for brevity -- top -> ]

|--------------------------------------|
|######~~~~~~~~~~~~~~~~~~~~~~~~        |
p = -E/c |######~~~~~L~A~S~E~R~~~~~~~~~~        | p = E/c
|######~~~~~~~~~~~~~~~~~~~~~~~~        |
|--------------------------------------|
black plate
absorbs laser beam
# = laser
~ = laser beam
- = support wires

> The top plate (the absorber) is in thermal equilibrium with a large
> amount of solid Unobtainium  m.p. 2.7 K, atomic mass = sqrt(-1) g/cm^3
> so that there is no radiation from heat effects.  power is provided by a
> small on-board power source

You must shop at the same physics experiment supply store that I
do.  That unobtainium sure is useful :-).

> What happens when you turn on the laser?
>
> I'd say that it would just sit there, although you can clearly see that
> without the wires to hold them together, the laser source and the top
> plate would to move away from each other.

You are correct.

Although an actual device like this would actually move, but not
with any permanent velocity.  As the laser heats the absorption
plate, mass is transferred from the laser to the plate.  As the
center of gravity shifts to the right (in my diagram) the
assembly would slide to the left.  When the laser is turned off,
it would stop, then begin to slide slowly rightward as the plate
reheats the laser by radiation.

> The wires are clearly dissipating a momentum equal to 2 E/C.
> that is what i was trying to say with my bb analogy.

I'm not sure "dissipating momentum" is quite the term for it.
After all, the laser with momentum -E/c is tugging through the
wires against the plate with momentum E/c.  The momenta cancel.

Like I said before, momentum means motion.  A non-moving object
has no momentum.  If the laser and the plate are held together
with wires and don't move relative to each other or your
observer, then you can't say that they have momentum.

The wires are indeed under tension, because there is a force
between the laser and the plate.  This tension was created in the
first instant the laser was turned on, and a small amount of its
energy went into stretching the wires before it was all spent on
heating the plate.

```

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