🚀 go-pugleaf

sehari@iastate.edu (Babak Sehari) writes:

>Well after learning years of physics, I have no idea why two objects
>at a distance feel each others gravitational force, or for that mater
>why two charges at a distance can feel each other? ( I know How they
>do it ie. newton and coulomb law, but why they do it? )

Nobody knows!  We can describe such things in an elaborate fashion
(I could say that Coulomb's law arises from the structure of the
virtual photon propagator in QED, that Newton's law of gravity is an
approximation to certain properties of timelike geodesics in general
relativity, etcetera, etcetera) but none of that really tells us
why any of this happens.  If you keep asking "why?", after about
three or four iterations you invariably come to the limit of human
knowledge.  While it's a useful exercise to find the limit in this
fashion, I doubt you'll ever encounter a situation where you don't
run into a brick wall at some point.

I heard a story a few years ago, probably an academic legend, about
a student who majored in physics for years and then asked a
professor the reason for gravity.  Upon receiving no satisfactory
answer, he dropped physics the next day.  If you specialize in
physics expecting to learn the ultimate explanation for all things,
you will undoubtedly be disappointed.  The best we can hope for is
a simple and elegant model that describes all things in a way that
possesses logical consistency and aesthetic unity.
 
>I know the charge disturbance propagate by photons with the speed of light,
>as has been formulated by maxwell? But what about two masses, how does the
>gravitational field changes propagates in the space? How fast does that
>travel? 

This is a thorny question that is debated to death on sci.physics
at irregular intervals.  It appears that, at least under non-exotic
conditions, GR is causal; if you shake a mass at point A by nongravitational
means, the gravitational effects propagate at the speed of light according
to general relativity.  But systems in which objects are in free fall
and have been in free fall for a long time have effects which possess
no delay; objects gravitate toward each others' true positions rather
than some sort of retarded position, or even a retarded position 
extrapolated along straight-line motion in space (the motion must
instead be extrapolated along a geodesic in spacetime).  This
doesn't transmit information faster than light, since the motion
under these conditions is necessarily completely determined
gravitationally given initial data.  There isn't anything really
propagating from the current position of the gravitating object. 

The short answer, then, is that the disturbance propagates at the
speed of light, but the issue is subtle.
 
>What is charge, why is it that we have Positive & negative charges, and
>why the smallest unit of positive charge is equal to smallest unit of
>negative charge? ( do not give me electron and proton story, my question
>here is why electron has negative charge and proton has positive charge
>of exact opposite magnitude) Why an electron can not have twice the charge?
>For that mater why it can not be found with 1.5 times the charge?  Why negative
>charge attaches to smaller mass than positive charge?

Nobody knows why electric charges are what they are.  It appears that
the existence of a magnetic monopole would require charge quantization
in order to construct a self-consistent quantum mechanics, but nobody
has ever definitively seen a magnetic monopole.  Certain constraints
can be put on the charges of particles by requiring anomaly cancellation
in the standard model, but that doesn't tell you much.

As for why it's the electron that has the negative charge and the proton
that has the positive charge and not the other way around, that's just
a convention.  If someone working 200 years ago (Benjamin Franklin?)
had decided differently, we'd be calling the electron positive and the
proton negative.

> Does Anti-mater exists?

Yes.  You might want to read the article in the December (I think)
Scientific American by my old quantum prof Gerry Gabrielse, in which
he describes means by which antiprotons can be manipulated with
some finesse.  Particle physicists encounter antimatter all the time,
in the form of individual antiparticles.  But nobody's built a whole
atom of the stuff yet.
  
>that is can we attach negative charge to nucleus of matter?  What is causing
>negative charge property, why should be attracted to creator of positive charge
>property and repeals from negative charge?

The fact that like charges repel and opposite charges attract can be
shown to result from the fact that the photon has a spin of 1.  But that's
not much of an explanation, since you can then ask why forces should
be described by quantum field theory!

>What is causing the magnetic force, that is why spin of a charge in one
>direction influences other charge spins?

Nobody knows why Maxwellian electromagnetism works.  It emerges from
the electroweak theory, but that's not an explanation, just a description
that sort-of-unifies (accent on the "sort of") electromagnetism and
some other phenomena.

>Why does not natural mass spin
>generate similar effects?

According to general relativity, it *does*, but they're very small!
There has been in the works for many years an project to launch a
satellite containing gyroscopes that should precess very, very
slightly as a result of the earth's "gravitational magnetism."  It
hasn't been launched yet, as a result of the generally mangled nature
of the U.S. space program.
  
> ( What causes the Amper's law, or the Lenses law)

See the comment on electromagnetism above.
-- 
Matt McIrvin