Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: operationally inertial frames



Please don't keep your mouth shut; you have exposed the truth about
general relativity just right, and we need more of that on this list. The
popular myths often seem to take over and go unchallenged.

On Wed, 13 Oct 1999, Jerome Epstein wrote:

I had intended to stay out of this after this point, but I'll be foolish
and stick in one more comment.

In a smallish region, say a space ship, you can treat it is an inertial
frame to a good approximation. So for ordinary physics lab experiments
over a short period of time it is OK to call it an inertial frame.

But from the standpoint of fundamental theory, the region is not
equivalent to a field free region. The Earth's field is radial. Thus the
field on one side of the space ship is in a slightly different direction
from the field at the other side. Also the field on the side farther
from the Earth is slightly weaker than the field on the other side. If
you have two separated masses in the space ship (just freely floating
for a while), if you wait long enough they will acquire a measurable
acceleration relative to each other with no apparent source of this
acceleration. Thus the frame is not inertial over the extended region.

Think even larger. COnsider the whole region around a rotating frame. To
try to treat this as an inertial frame, one must postulate a radially
*outward* gravitational force. One looks out there for masses
surrounding you to be the source of this "gravitational field" (this
"field" can be quite large for a rapidly rotating frame), and you don't
see them. Thus you have to postulate more and more mass farther and
farther away, until (as Eddington discusses) you need an infinite amount
of mass in all directions infinitely far away to account for this
"gravitational field".

There is so much mythology in popular press about relativity. it is NOT
true in GR that all frames are equivalent. Accelerated frames are NOT
equivalent to a gravitational field except locally (essentially at a
point). Rotation relative to "absolute space" is still a fact.

I think I have said more than enough, and I will attempt to keep my
mouth shut for a while.
Jerry Epstein


A. R. Marlow E-MAIL: marlow@loyno.edu
Department of Physics, Box 124 PHONE: (504) 865 3647 (Office)
Loyola University (504) 864 7315 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453