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

On Tue, 17 Oct 2006 18:48:22 -0700, TrekJunky wrote:

Hello Sue,

*ahem* I haven't been reading Sue's posts in this thread but if she's up
to her usual form you may wait a long time before you get a simple,
straightforward answer from her. She posts a lot of interesting links but
tends to answer questions with questions.


I would like to answer you question about how I would measure the mass
of light.

OK, first point: Light has zero _rest_ mass. In other words,
hypothetically, if you could stop a photon and weigh it, it would weigh
nothing. (You can't stop one, of course, they always travel at C.)

However, light has energy, and energy is equivalent to mass, so a photon
in flight does carry something which could reasonably be called "mass".

A closed box covered on the inside with perfect mirrors which has lots of
photons bouncing around inside it would weigh _more_ than the same box
without the photons.

I would like to approach it by stating a few facts to see if
you agree with them:
1) Solar Sail space ships are propelled by the pressure of light on the
"sails" not solar wind(subatomic particles) as some might think.

Exactly. (The solar wind, OTOH, most likely just punches little holes in
the sails and makes periodic maintenance necessary...)


2) Light can exert pressure.

Yes. It's often called "radiation pressure". In fact, inertial
containment fusion devices are designed to use the radiation pressure of a
large number of very powerful lasers to smash a tiny bead of frozen heavy
water with such force that the hydrogen fuses.

(NB -- there are other things going on in such a device and the pressure
exerted on the pellet by ablating D2O no doubt plays a role too but the
radiation pressure is certainly a big part of it.)


3) F(pressure)=ma [force=mass times acceleration] 4) Therefore light
has mass that can be measured indirectly by measuring the force acting
on the Solar Sail because the acceleration is the speed of light (C).

You may run into trouble and get the wrong answer if you try to
apply Newtonian mechanics to the problem since photons -- particles of
light -- are distinctly non-Newtonian. However the basic idea is
certainly right: if you measure the pressure exerted by light you will
get some idea of the "mass" it must be carrying with it. But you need to
be careful of exactly what you mean by "mass" and how you define it and
how you try to calculate the "mass" of the light.



Please tell me where I am going wrong. Thanks!!!

You're not going wrong, as far as I can see, but in order to get any real
numbers out (at least, with any assurance that they were correct) you
would need to go quite a bit farther into the theory of relativity.



Cane Kostovski AKA TrekJunky

Sue... wrote:
TrekJunky wrote:
Hello Sue,

For an object to have momentum (inertia), it would have to have mass.

Light has angular momemtum but no mass. An attraction to all the
universe is all that is necessary for inierial.
http://chaos.fullerton.edu/~jimw/general/inertia/index.htm

I
have an amature knowledge of light being a wave of energy and at the
same time have a measurable amount of mass.

How will you measure it ?

I don't understand the
prevoius phrase, but accept it as fact. And if it is true that light
has mass and intertia, it must exert a tiny bit of gravity. It's
gravity affects other mass so I agree with your statement below even
though the effect is minimal in the situation below. Thanks!!!!


Ya weclomed.

Sue...

TrekJunky


Sue... wrote:
TrekJunky wrote:
Inverse sound waves cancel each other out, so why can't there be
a gravity equivalent?

TrekJunky

<< Principle of Operation: Fig 1b shows a more detailed look at how
an optical trap works. The basic principle behind optical tweezers
is the momentum transfer associated with bending light. Light
carries momentum that is proportional to its energy and in the
direction of propagation. Any change in the direction of light, by
reflection or refraction, will result in a change of the momentum
of the light. If an object bends the light, changing its momentum,
conservation of momentum requires that the object must undergo an
equal and opposite momentum change. This gives rise to a force
acting on the object. >>
http://www.stanford.edu/group/blocklab/Optical%20Tweezers%20Introduction.htm

http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:physics/0107015
http://www.chem.purdue.edu/gchelp/liquids/inddip.html
http://www.mypage.bluewin.ch/Bizarre/GRAV.htm
http://www.esa.int/SPECIALS/GSP/SEM0L6OVGJE_0.html
http://www.research.ibm.com/grape/grape_ewald.htm

Sue...

--
Nospam becomes physicsinsights to fix the email
I can be also contacted through http://www.physicsinsights.org

.

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