Re: Conservation of momentum (and a weird space drive)

From: greywolf42 (mingstb_at_marssim-ss.com)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 17:28:43 GMT

"Tom Roberts" <tjroberts@lucent.com> wrote in message
news:Kbnad.12582$HA4.196@newssvr15.news.prodigy.com...
> Joe wrote:
> > But what I still
> > don't get is how the light "accelerates" the planet upwards to meet
> > the photon?
>
> English is inadequate to describe this accurately. As is my knowledge of
> the details (and, I suspect, so is anybody's knowledge of the details).
> But a general discussion is possible:
>
> Please remember that in GR all energy responds to gravitation, and all
> energy contributes to gravitation.

Except the Zero Point Energy required by quantum mechanics. (Or any other
energy source that would invalidate the theory.) :)

> In that previous paragraph "gravitation" means "the geometry of
> spacetime".

This is called argument-by-definition. And is useless.

> So read it as: any negligibly-small parcel of energy will
> follow a geodesic in the geometry determined by the non-negligible mass
> and energy in the manifold, and any non-negligible energy will affect
> the space-time geometry.

The translation of this statement is that GR exists only where experiments
do not disprove it. If you measure something that disproves GR, then -- by
Relativist definition -- your region is not 'local.'

> So the light "accelerates" the planet upwards in precisely the same
> manner that the mass m did so (in the previous gedanken). Granted, this
> too is opaque -- that's directly related to the modern dictum to discuss
> what is measured, and don't try to delve too deeply in unmeasurables
> such as "why?" or "how?"....

That is not a 'modern dictum.' That is Kantian philosophy. It is the
refuge of the priest.

> > How does the extra kick from the photon get ballanced?
> > Never mind carrying the flashlight up and down, I can see (probably
> > naively) that this doesn't impart a net motion in any direction.
>
> Not true, because the flashlight carried up is more massive than the
> flashlight carried back down (because it emitted the light while on top
> of the tower).

According to GR theory, this is correct.

> > But
> > what I'm confused about, is the light striking the surface. You can
> > quote that momentum is conserved in a closed system, but at least in
> > Newtonian Mechanics there's a reason for that, in that there are equal
> > and opposite forces.
>
> A better way of thinking about momentum conservation is via Noether's
> theorem -- as the Lagrangian is invariant over spatial displacements of
> the entire (closed) system, momentum is conserved, and the (closed)
> system cannot move no matter what happens inside it.

You'll note that Tom did not answer your question. He simply avoided it.

What Tom is hiding is the simple fact that GR simply assumes that momentum
and energy are conserved. No mechanism or reason for this is given. While
this is perfectly fine, it simply means that GR cannot answer your question.

> In Newtonian mechanics that is fine as it stands. But in GR the
> Lagrangian for this system is NOT invariant over spatial displacements
> of the "entire system", because gravitational radiation is emitted and
> the "entire system" includes the entire universe (manifold). Baically in
> GR no system can ever be completely closed (except for certain
> highly-symmetric cases).
>
> In GR the notion "spatial displacement" is ambiguous; instead
> one really looks for Killing vectors of the manifold, as I
> mentioned earlier.... Here there are none.

The above is simply more verbiage designed to confuse you while sounding
authoritative.

> > Does that mean that as the momentum falls, and
> > increases momentum, there is a corresponding increse in the momentum
> > of the planet in the opposite direction? Doesn't this violate no
> > "action" faster than light?
>
> You are attempting to apply Newtonian concepts to a GR situation and
> discussion.

Note Tom's attempt to put you on the defensive, before answering your
question.

> Yes, the falling light imparts an impulse onto the surface
> of the planet. This is offset by the difference in carrying a charged
> flashlight up and a discharged flashlight back down, plus a host of
> still smaller effects (e.g. differences in the stress internal to the
> tower and planet, gravitational radiation, ...). As I said befor, I
> don't know the details of summing all this up, but it is clear that
> nothing here travels faster than light.

What Tom means is that he simply assumes that nothing travels faster than
light -- because it would violate his Faith. But --- even though he doesn't
know the details, he "knows" that he (and GR/SR) is Right. 

--
greywolf42
ubi dubium ibi libertas
{remove planet for e-mail}

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