```ReplyTo  : Kelly
Subject  : The bathtub is flowing over (EUREKA)

>>I HAVE SOLVED THE PROBLEM OF SLOWING DOWN!!!! <smug grin>
>
>>> First convince me... then celebrate...
>
>>First, there is _no_ way to reduce the momentum imparted by the photons.
>>However, we _can_ change the _direction_ of the thrust.
>
>>> NO and yes, you cannot change the initial size and direction of the photon
>>> thrust, but you can (of course) choose the direction of the thrust when
>transmitting a photon.
>
>>> In better words:
>>> You should see the reflection of a photon as two steps, independant of
>each
>>> other.  The first step is receiving the photon where
>>> in the same direction as the photon CAME FROM.
>>> The second step is transmitting(reflecting) the photon, hereby is the
>>> momentum of the photon added in the opposite direction the photon GOES TO.
>>> ----- <snip>
>
>thing the fundemental part of kevins argument is correct.  That the thrust
>vector on a photon sail is tangential to the surface of reflection, and the
>amount is related to the angle of incidence of reflection.  (thats one of the
>standard solar sailing tricks.)

Agreed, completely. But if you want to use all the photon's energy you have
to stop them. Stopping them makes the ship go forward. Kevin tried to change
the direction of the photons in a way that stopping them would not add to
forward but sideward momentum. Doing the same thing for the other sideward
momentum cancelled out the movement to either side.
But as soon as you change the direction of a photon a little you get a small
forward thrust. But if you want to change the direction so much to use
Kevin's trick, then you have the same forward thrust as you would have if
you captured the photons immediately.

I referred to Kevin's trick. I'm still not completely sure if I understand
Kevin's idea correctly since I don't know how and where he want to tap the
photons' energy.

>If of course you absorb the photons in a rectena array the result is a bit
>less clear, but presumably you could reflect the energy off the sail to
>converters, at an angle that would cut down the forward thrust.  The price of
>this would be a lot of extra thrust, and stress, on the guy wires.

Reflecting the photons to another direction adds some forward momentum to
the ship. And as soon as you stop the photons in the converters, you
transfer the rest of the forward momentum of the photons!

>>> You tried to trick the photons and thereby violated the preservation of
>>> momentum: If you receive an amount of photons, all their momentum is
>>> transferred to you.
>
>Yes, but if it is transfered at an angle; part the momentum will be disapated
>in the structure, rather than acting to accelerate the ship.

In that preservation also the direction is mentioned. So if before the
absorption you have a forward momentum, than that momentum should be there
after the absorption still in forward direction only now transferred to the
ship.

>>> Once more, whatever ingenious construction you can think of, to receive a
>>> certain amount of photons and use their energy, you ALWAYS get ALL their
>>> momentum in the same direction as they went to.
>
>Define went to?

Went to = The direction of the photons before they encountered the Asimov.

Subject  : Drawings

>Huh??  Well as a hint you generally need to have a copy before you can use
>it.

I use an IBM-compatible computer and have programs to convert many
image-formats. I assumed that for Mac computers there would be similar programs.

Subject  : Multi generation ship

>The ship would need a degree of selfsuficency that no country, much less
>industrial country, on Earth has had in generations.

Ah now I see what you mean. The industry on the ship would be completely
different than that on Earth. The different production lines should be very
generalized and not specialized. This means that one line can do all kinds
of synthetic material moldings. Probably this results in a less efficient
line. Not less efficient with energy but with time. My guess is that time is
not the most important factor here.
Another important thing would be automation of the production process. A lot
of equipment would be made many times again, so then automation would ease
the job.
About specialties, of course it would be easier if we had specialist for
everything but one person in a certain field can do a lot if he/she has the
right information available. Again that would mean less efficient with time.
But don't forget, on Earth lots of energy and manpower is used to innovate.
The thing that we are concerned most about, is to keep what we have.
Rebuilding what one has is a lot easier than creating new things. To keep up
with Earth's technology we could simply copy what they had created.
Furthermore almost all equipment should be recycled, a lot of energy can be
spared if the equipment is designed for easy recycling. So this will and
should reduce the need for raw materials a lot.
Maybe we can recycle some of the energy if we enclose the ship with a kind
of thermos flask and use heat exchangers to re-use some of that energy.

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