From: Gary Horne (gary@horne.cc)
Date: Wed Nov 06 2002 - 21:56:00 PST
Date: Wed, 6 Nov 2002 21:56:00 -0800 Subject: Re: pinhole gravity From: Gary Horne <gary@horne.cc> Message-Id: <9775EA14-F215-11D6-95B1-0030654C6092@horne.cc>
I heard a good explanation of gravity which helped me grasp the more
abstract nature of it. Start with two people in an elevator at the top
of a building. Suppose the cable breaks and the elevator falls.
Everyone floats around inside the elevator because everyone is moving
the same....so far so good (all things considered!)
Now imagine that the elevator shaft runs from the top of the building
all the way to the center of the earth. Both people in the elevator
are going to fall towards the center of the earth..a point...so, as
they fall, they will move towards each other. A Mysterious and
Invisible force has pulled them towards each other.
Likewise, things with mass run along these gravitational space/time
geodesics. Masses distort the lines and cause them to come together.
At least, that's what I remember. Perhaps someone can clean that up a
bit?
g
On Monday, November 4, 2002, at 02:59 PM, pauld@exploratorium.edu wrote:
> 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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