SAC.LONG@sri-unix.UUCP (05/23/84)
From: Steve <SAC.LONG>@USC-ISIE
As I have been reading recent discussions on this net, a question
has come to mind which I do not recall ever hearing an answer to
in my past studies. Perhaps one of our more knowledgeable contributors
on the net can address this whithout extremely technical terms.
I have heard the idea that space is warped due to the presence of
matter, especially in the case of supposed black holes. One
experiment done to confirm this was locating stars on the opposite
side of the sun during eclipses. It was found that the apparent
position of certain stars was changed according to that which would
be expected when relativistic effects are taken into consideration.
Question 1* Is the above paragraph correct?
Question 2* Is it known that matter warps space or is this a theory?
Question 3* Is it not also possible that gravity bends light thus
giving the appearance that space is warped due to our
means of perception (including all detectable EMR)?
I would appreciate any reasonable responses to these questions.
Perhaps this is elementary to many, but its been a while for me
since I looked into my Physics (before I changed to Comp Sci).
-- Stevegwyn@Brl-Vld.ARPA (05/24/84)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> The idea of general relativity (1914 version), which is still considered our best theory of gravitation after 70 years, is that gravitation is a manifestation of the curvature of space-time (not just space). The curvature can be considered to be caused by matter (mass-energy-stress-momentum), or one can take the equivalent view that what we call matter is just curved space-time. The technical expression of this (in Einstein's original terms) is: Rik = Tik - 1/2 T where T = gikTik (summed on i,k) or Tik = Rik - 1/2 R where R = gikRik Here, T is the matter tensor density and R is the curvature tensor. The above are field equations; i.e., they apply at each point of space-time. There will be different curvatures and different matter densities at different points of space-time. (I am skipping over several details but these are the main ideas.) Additional standard general relativity ideas include the rule that a gravitational test particle will follow a geodesic (think of this as a "shortest" path in curved space-time, although it is not really) and that light will follow a null geodesic. A point mass such as the sun (well, it's a fat point) generates a curved space-time described by the "Schwarzschild solution" of the field equations; such a mass is parameterized by a number M which represents the mass (e.g. in grams) of the body generating the curvature. Planets are modeled as test particles following geodesics in the curved space-time generated by the sun, and so forth. If one computes what happens to light from a distant star passing near the sun, he finds that it is deflected from the path expected if the sun were not there. This effect has been measured many times (I think 1917 was the first time but am not sure about the exact date), and it agrees with the general-relativistic prediction. Although a similar effect can be derived from Newtonian physics, the observed deflection of starlight by the sun differs from what such an approach would predict. I hope this explanation has been sufficiently clear. The calculation of the predicted planetary orbits, light deflection, and so forth is straightforward but tedious. For normal laboratory-sized objects it agrees pretty well with what the Newtonian theory predicts. For very fast (near the speed of light), very dense (neutron star), or very large (cosmological) objects, the Newtonian theory is hopeless and general relativity is normally used instead. So, yes, it is "just a theory", but it is well corroborated.
jlg@lanl-a.UUCP (05/25/84)
i Partial answer to question 3: In general relativity the statement that gravity bends light and the statement that matter distorts spacetime are equivalent. That is the whole point of general relativity; that the mechanism of gravity is the curvature of spacetime. I am not familiar with the non-relativity theories of gravity, so I can't comment on what explanation they give for light bending around the limb of the sun (or any other massive body).
elt@astrovax.UUCP (Ed Turner) (05/29/84)
Aside from the specific theory of General Relativity, it can be shown by simple thought experiments that any theory which obeys the Equivalence Principle (the identity of gravitational and inertial mass) and Special Relativity, both of which have been verified to exquisite precission experimentally, must also attribute non-Euclidean geometric properties (i.e., curvature) to space. Part of the elegance of GR is that it identifies this necessary curvature with gravity. Ed Turner astrovax!elt