Re: Can inverse gravity waves cancel out Earth's gravity in selected areas?

[And see my previous post also -- this one is sort of an afterthought]

On Fri, 20 Oct 2006 12:48:52 -0700, TrekJunky wrote:

Has anyone ever heard that
light sometimes acts as a wave and sometimes acts as a particle?

Yes of course. In general you can think of it in whichever way is the
most convenient in a particular situation. If you're brilliant you can
think of it both ways at once but most of us need to be content with just
using one of the models at a time and not worrying too much about the fact
that they're apparently contradictory -- welcome to the weird world of
quantum mechanics.

For understanding radiation pressure you're probably best off thinking of
it as a particle and forgetting about the wave nature; you can deal with
radiation pressure either way but believe me the wave approach to this is
harder (check out antenna theory some time).


I have a
hard time understanding how energy can apply a force if it has no mass. I
apologize for my ignorance, but I would like to learn. In my simple mind,
the reaction in matter to light is heat.

When matter absorbs radiation, something inside it jiggles as a result
(the radiation gets turned into linear motion of some charged particle,
someplace -- typically an electron). That jiggling of particles within a
solid shows up macroscopically as heat.

When matter _reflects_ radiation it doesn't get hotter (it didn't absorb
any energy, after all).


Is that heat from the matter or
from the light? I am not good in math either and Sue sent me to a link
that used variations on E=mc(squared).
http://farside.ph.utexas.edu/teaching/em/lectures/node90.html In my mind m
is mass which can be converted to E (energy) and back again. How does that
relate to radiation pressure? sal wrote:
On Wed, 18 Oct 2006 08:59:19 -0700, Mike wrote:


Igor wrote:
Mike wrote:
TrekJunky wrote:
Hello Sue,

I would like to answer you question about how I would measure the
mass of light. I would like to approach it by stating a few facts
to see if you agree with them:

Light has no mass, or if it has some it is beyond any measurement
accuracy.

1) Solar Sail space ships are propelled by the pressure of light
on the "sails" not solar wind(subatomic particles) as some might
think.

That is not your usual notion of pressure.

Why not? Light has momentum. Momentum changing direction exerts
force.
And force per unit area is pressure. It's that simple.


Idiot. Are you the sam eposter asking these stupid questions? How do
you make light change direction other than making it pass through a
gravity field?

Well, as one example, a mirror works pretty well.

And by the way, that's what a solar sail is. In the simplest case of
the sail perpendicular to the incoming light, the photons reverse
direction when they hit the sail, their momentum flips sign as a result,
and the sail gains twice the momentum of each photon in the process.

The sail feels a force as a result of reflecting the light, and if
someone on the ship measures the force on the sail as a whole and
divides by the area of the sail, they find the radiation pressure which
is being exerted on the sail.


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