Re: one question

[Kevin C Houston <hous0042@maroon.tc.umn.edu>]

On Tue, 28 Nov 1995, Steve VanDevender wrote:

> Kevin C. Houston writes:
> 
>  > Out in intergalactic space (far far away from all outside gravitational 
>  > influences)  you have the following setup:
> 
>  [ Figure turned sideways for brevity -- top -> ]

Fine with me.

> 
> 
>          |--------------------------------------|
>          |######~~~~~~~~~~~~~~~~~~~~~~~~        |
> 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

that's the reson for the unobtainium, the melting never shifts above 
cosmic backround radiation level

> 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.
> 

YES!  That's what I've been saying all along!  that a physical structure 
can transmit (through tugging) opposite momenta that can then 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 laser imparts momentum when it leaves, and when it strikes the black 
plate.

> 
> 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.
>

so you're saying that the plate heats more if it is held in place than if 
it was free to move?

Kevin