Thanks, Owen. Yes, it is indeed the case that in the modern
perspective of quantum field theory, forces are replaced by the
interchange of ("virtual") particles. I didn't want to make my
comments unnecessarily complicated by talking about this aspect of
field theory, but you're right.
I'd like to mention an amusing aspect of the contemporary physicist
concept "interaction". There are two ways to blast through a cement
wall. The crude, inelegant way is to throw lots of material at high
speed at the wall, and depend on the rather strong electric
interactions between the projectile and the wall to break through.
Clumsy. Inelegant. Crude.
The elegant way to get through the wall is to throw neutrinos at the
wall, and they go right through. Neutrinos have no charge, so they do
not undergo electric interactions. They are in a family ("leptons",
which includes electrons) that does not participate in the
strong/nuclear interaction that protons and neutrons engage in.
Gravity is intrinsically an extremely weak interaction: note that you
can hold a book in your hand, exerting upward electric interatomic
interactions and easily counteract the downward effects of the entire
massive Earth! In fact, the electric repulsion between two protons is
10 to the 40th bigger than their gravitational attraction!!!
That leaves only the "weak" interactions that neutrinos engage in, and
like gravity, these interactions are very weak, so weak that almost
all neutrinos coming from the Sun (produced in fusion reactions in the
Sun) pass right through the Earth without interacting at all. Result:
while cannonballs get through the wall by interacting very strongly
with the wall, neutrinos sneak elegantly through the wall by NOT
interacting.
I highly recommend John Updike's splendid little poem on the subject:
http://www.phys.psu.edu/~cowen/poetry/cosmic-gall.html
Bruce
On Sat, May 19, 2012 at 11:05 AM, Owen Densmore <[email protected]> wrote:
> Bruce: Wow, very nice! A+
>
> I was about to mention that although the impact mass has on spacetime gives
> a means for understanding gravity, it was outside of the interaction model
> of the Standard Model of elementary particle physics. In the SM, force
> arises from an interchange particle exchange:
>
> http://en.wikipedia.org/wiki/Fundamental_interactions
> In the conceptual model of fundamental interactions, matter consists
> of fermions, which
> carry properties called charges andspin ±1⁄2 (intrinsic angular
> momentum ±ħ⁄2, where ħ is the reduced Planck constant). They attract or
> repel each other by exchanging bosons.
>
>
> Unfortunately, the unification of gravity with the rest requires a graviton
> that has not yet been observed.
>
> Merging general relativity and quantum mechanics (or quantum field theory)
> into a more general theory of quantum gravity is an area of active research.
> It is hypothesized that gravitation is mediated by a massless spin-2
> particle called the graviton.
>
>
> BUT the question is: do we have any bounds on the requirements of observing
> such a critter? In a way, this would render General Relativity to a
> position to Newtonian physics, one in which has a still more fundamental
> underpinning.
>
> -- Owen
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