From: pauld@exploratorium.edu
Date: Mon Nov 04 2002 - 14:59:09 PST
Message-Id: <200211042259.gA4Mx7a11397@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole gravity Date: Mon, 4 Nov 2002 14:59:07 US/Pacific
Hi Jennie
Scientists treat forces on different levels.
At the simplest level Gravity is a force of attraction which is proportional to
the product of the masses of two objects. (And falls off as the inverse square of
the distance between the centers two spherical objects.) You can use the force to
calculate the motion of an object.
Eistein created general relativity in which one more step is added, mass warps
spacetime and then objects move in this warped spacetime.
This more complicated explanation came with a prediction that when light crosses
space in the presence of a mass it falls twice as much under Einsteins model as
under the simple Newtonian model. This prediction was verified by observing stars
during total solar eclipses and noting how much their apparent position was
deflected by the gravity of the sun. (Newer tests are much better than this now
questioned original data.) There is no observable difference in the prediction of
general relativity from Newtonian mechanics on the motion of a basketball dropped
near the surface of the earth.
Another model has gravity originating by the exchange of virtual gravitons,
essentially, masses exchange these particles which are carriers of the gravity
force.
In the same way. Electromagnetic forces are carried between particles by the
exchange of virtual photons.
Which model you choose depends on the questiuons you are asking and the detail
you need.
Paul D
> Hi,
>
> Today i introduced the concept of gravity as a force of attraction between
> all objects and the reason why objects fall towards the center of the earth
> on our planet. my students were perplexed by the abstract concept that
> there is this invisible pulling on things (by the earth/planet) to make them
> fall. am i right in saying that we don't REALLY know exactly what gravity
> is- what we know is how planets, objects, etc. behave and so have tried to
> formulate theories to explain that behavior, one of which is that there is
> this force of attraction called gravity attracting things to each other? are
> there further theories about where this force comes from or what it is more
> precisely? any info on the "why?" i think einstein's ideas about gravity
> dont' call it a force at all, but a curvature in space-time(!)- but are
> there any explanations for the theory that it is a force? or do most
> physicist believe einstein's theory and don't believe its a force of
> attraction at all? any ideas on how to make this very abstract concept more
> concrete would be very helpful!
>
> thanks for you help,
> jennie
>
>
>
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