NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/astro/13367

Text File | 1992-12-22 | 6.5 KB | 135 lines

Newsgroups: sci.astro
Path: sparky!uunet!think.com!ames!pacbell.com!well!metares
From: [e-mail] (Tom Van Flandern)
Subject: Re: The Hole Story
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Whole Earth 'Lectronic Link
References: <[e-mail]> <[e-mail]>
Date: Wed, 23 Dec 1992 02:35:56 GMT
Lines: 123


[e-mail] (Chris Flatters) writes:

> If you are doing calculations in the Sun's rest frame then you need to
> calculate the gravitational forces in that frame: the motion of the Earth
> in this reference frame makes no difference to the Sun's gravitational
> field.

     To better understand the meaning of aberration, consider an analogy. Let
an arrow be shot out from the Sun, and replace the Earth with a train moving
in a similar direction.  Assume the train has open windows.  When the arrow
passes through those open windows, a fixed observer in the Sun's frame will
see the arrow always moving radially away from the Sun, even though it goes
into the train cabin through one window and emerges through another window
not opposite the first because of the train's motion during the arrow's
passage through the cabin.

     But for a passenger on the train, the arrow flys diagonally across the
cabin, and appears to have originated from a direction well out ahead of the
Sun in the direction of the train's motion.  If the arrow strikes the cabin,
one component of its force pushes back on the train and tries to retard the
train's motion slightly.

     So your statement that the Earth's motion makes no difference to the
Sun's gravitational field in the Sun's frame is correct, just as is the case
for the arrow.  But it does make a difference to the direction of the force
applied on the Earth by that field.  The effective direction of an applied
force must be the resultant of the speed and direction in which the force
acts and the speed and direction of the body it acts upon.

     When an effective force is independent of the speed and direction of the
body it acts on, as in the case of gravity, we are forced to conclude that
either a) the force acts faster than we can measure, or b) the force is not
propagating at all.  In the case of GR, choice b) has been adopted, since we
can easily measure that the force of gravity does not propagate as slowly as
light.

>> [TVF]: But MTW in Chapter 7 show at great lengths all the efforts that
>> have been made to make retarded potentials work, without success.

> No they don't.  Chapter 7 of Misner, Thorne and Wheeler shows that a
> consistent theory of gravitation can not be developed without introducing
> curved space.

     I think that's consistent with what I said.  Chapter 7 gives three
examples of attempts to solve this problem with propagating gravity, showing
that they all fail.  Only the GR approach in which space-time is curved and
nothing propagates, or ftl propagation, can explain what we observe.

>> [TVF quotes Eddington's argument]

> I can't check the context of this quotation since Eddington's book is
> checked out of the library here but I believe that this is also an argument
> against flat space-time.

     Eddington goes on to quote an analogy with electromagnetic forces to
argue that there must be a way to avoid ftl propagation speeds in gravity. He
does not show how that might be accomplished.  Later authors argued that
curved space-time must be the answer.  Eddington's argument still shows that
nothing about the gravitational force can propagate at anywhere near
lightspeed.

> Tom is basically repeating a demonstration that a consistent theory of
> gravitation can not be developed in flat space-time.  GR holds that
> space-time is not flat.

     I have not argued that there is something wrong with the GR argument.
I'm simply pointing out what it is.  Surely you agree with me that it is
important for all of us to understand exactly what constraints are set by
observations and experiment, and how our standard models explain those facts.
If you find the GR explanation a bit strained, and then seek an alternative
explanation, as I do, that's the way science is supposed to be done.  I think
I've even found one that works in flat space-time.  But in these postings,
I'm not arguing for an alternative, but just clarifying the GR explanation.


and [e-mail] (Steinn Sigurdsson) writes:

> calculating motions using retarded potentials does work, it's simply very
> cumbersome because the first few post-Newtownian terms vanish.

     I know MTW imply that, but I have never seen it shown.  Have you?  My
math says there is no way to make the first order accelerations proportional
to v/c vanish without ad hoc, unphysical assumptions, such as some sort of
average between advanced and retarded potentials.  Even then, the second-
order effects are too large to agree with observations.  If you know of a
plausible way to deal with this problem using retarded potentials, without
hand-waving, I'd be most interested.

> Let's build LIGO and find out!

     The trouble with this proposal is that, if some of these alternative
ideas have any merit, then the LIGO design is doomed to failure.  That's a
pretty expensive project just to learn how to do it right the next time,
given the state of the U.S. and world economies just now.  Expensive failures
are also sure to sap public support for science in general.

     I think we should be sure our models are right first, then design
gravity wave detectors.  In my opinion, the rush to do it now is ill-advised.


and [e-mail] (Thomas Clarke) writes:

> TVF. of course, would like that something to be wrong with GR, opening the
> door to alternate theories.

     Am I laboring under a false impression, or aren't we all suppposed to be
working to falsify our existing models?  Besides, I don't think there is
anything "wrong" with GR any more than there is something wrong with
Newtonian gravity.  Both are simply incomplete compared to the ultimate
models we seek to develop.  There are far too many aspects of gravitation not
explained by existing models for these to be close to ultimate models.  Why
gravity appears to act without detectable delay is only one of them.


     Travel will keep me from carrying on my end of this interesting
discussion until after the new year begins.  My new year's greetings to all.
May the old year go out with a big bang.  Or vice versa.  :-)  -|Tom|-

--
Tom Van Flandern / Washington, DC / [e-mail]
Meta Research was founded to foster research into ideas not otherwise
supported because they conflict with mainstream theories in Astronomy.