```ReplyFrom : Timothy
Subject   : Humans and stairs

>> Does it TAKE energy to walk down the stairs or does it GIVE energy?
>
>Actually both take energy since your accelerating and decelerating masses of
>your body.  Though obviously going down you don't need to bost yourself up a
>gravity/potential energy well.

When walking downstairs most of the accelerating is done by gravity, so that
doesn't make you tired. Decelerating is done by the steps, so that does not
take energy either.

How do you respond to this?

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Subject   : Using the SIM

>I listed some assumptions on my Explorer Web page in LIT.  I think bottom
>assumption was a hydrogen atom every cubic centameter or 4.

Now the question what are we going to do with the hydrogen? First catch it
and decelerate a bit. Then use it both as fuel and reaction mass to
decelerate even more.
4 per cubic centimetre --> 4E6 per cubic metre

How much would we scoop? Total deceleration length: 2 ly = roughly 2E16 metres.
2E16*4E6*surface_of_the_scoop=8E22 hydrogen atoms per square metre of
scooping area during the total deceleration. Thats a little more than one
tenth MOL! and has a mass equivalent of: 0.0001 kilogram.
This value seems to be too small to be useful! Unless we are planning a
scoop of more than 1000 kilometres in radius. (Earth has a radius of 6500
kilometres)

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Subject   : Plasma mirror

>Sorry that won't work.  As the outer mirror moves away from the ship it has
>to continuosly reshape itself to refocus on the smaller catcher mirror/sail
>on the ship.

I don't think refocussing is necessary, the mirror itself can be just a flat
mirror so the reflected beam is nothing different than the beam from Earth.

>Also without the anchor on the ship it will tend to flutter and
>shift off course due to slight variations in beam, ISM, mirror reflectvity,
>seperation torque, etc..  This of course ignores the fact the sail isn't
>rigid, and will tend to crumple once its free of the ship.

Of course the mirror has it's own "gyro-system" it can compensate slight
movements by using a small side reflectors.
The same principle would be used when the Asimov is accelerated by a beam.

>Forward realized this, thats why he had an army of autonomus robots go with
>the outer sail to keep it working.

The mirror will be quite heavy so that it doesn't start moving too fast.
(what is fast...) Most of that mass can be used as shielding for the mirror.

Of course this whole thing would make the Asimov about twice as heavy. But
that seems to be the price of any solution for deceleration.

===============================================================================

Subject   : nanoAI

About AI and nanotech. If AI and nanotech would be sufficiently advanced in
about 40 years. One or more small vessels could be send to TC all with their
own fuel to decelerate. These small vessels would contain AI and nanotech or
even a combination of both. This nanoAI could build the same facilities on
TC that would "create" the energy for the Asimov on Earth.
The advantage is that the small vessels would use much less energy to make
the trip.
After nanoAI has build the facilities, accelerating and decelerating the
Asimov would be the same and make the whole design a lot easier.

===============================================================================

Subject   : Why use the Dragonfly

>In some ways I think the Dragonfly sail (if I was sure that
>Forward invented it, I would call it the Forward sail both in his
>honor and for the mnemonic appropriateness) is a much cleaner and
>possibly more effective design than any of the other
>externally-powered ship designs that have been considered so
>far.

Indeed it doesn't need massive engines or difficult energy transformations.
The only problem may be pointing the mirrors and beams acurately enough.

What I don't see is why the mirror can't be flat. Why does it need a
focusing action?

As far as I can see problems about redshifts always arise if one tries to
use the energy coming from Earth to decelerate the Asimov.

===============================================================================

Subject   : Lost letter?

Steve, I haven't heard it from you, yet: Do we agree that at least between
us the word relativistic mass is merely an other word for relativistic energy?

Also in my letter of november 29th, I replied to you:

>Yes, this is also true.  Hot objects are heavier than cold ones
>(although not by an amount we have equipment to measure).  A
>mirrored box full of photons is heavier than the empty box.

And it doesn't matter if they all move parallel and in the same direction
all the time, right?

Here is another one: If two particles feel the gravity of each other, then
they are heavier together than if they are separate because of the extra
gravitational energy.

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