Re: Confused about General Relativity, Help needed

From: Tom Roberts (tjroberts_at_lucent.com)
Date: 10/09/04 

Date: Sat, 09 Oct 2004 00:58:02 GMT

Joe wrote:
> Another question that I have is about the parallel between a rocket
> ship and a uniform gravitational field. Let's assume the rocket ship
> is thrusting in outer space, at uniform acceleration. This
> corresponds to uniform thrust. Now, I take a mass up to the top of
> the space sip, and "drop" it. Suddenly, the "mass" of the space ship
> is less, because it is not accelerating what you "dropped".

Yes.

> But this
> means there is a sudden lurch of the craft, because it is moving at
> constant thrust.

Yes.

> This is different with gravity (isn't it?), if the
> space ship is on the ground, and I drop something from the top, there
> is no sudden lurch!

Yes. The difference is in the ratio of (mass dropped) to rocket or
earth, not any difference of principle.

Note also that the equivalence principle applies only for "small"
regions and "small" masses -- so the mass you drop must be negligible
compared to the mass of either rocket or earth, in which case the lurch
is also negligible.

> Notice depending on the size of this object, and
> the ratio that it takes up with to the rest of the craft, this lurch
> could be enormous! But there is no accompanying lurch on ground.
> This means that for the equivalence principle to be true, the space
> ship must adjust its thrust to compensate for the dropped object.

No. The EP only holds for small masses.

Tom Roberts tjroberts@lucent.com

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