On 4/16/2019 6:25 PM, agrayson2...@gmail.com wrote:
On Tuesday, April 16, 2019 at 5:41:35 PM UTC-6, Brent wrote:
On 4/16/2019 7:56 AM, agrays...@gmail.com <javascript:> wrote:
On Monday, April 15, 2019 at 9:26:59 PM UTC-6, Brent wrote:
On 4/15/2019 7:14 PM, agrays...@gmail.com wrote:
On Friday, April 12, 2019 at 5:48:23 AM UTC-6,
agrays...@gmail.com wrote:
...
If gravity is a fictitious force produced by the choice
of coordinate system, in its absence (due to a change in
coordinate system)*how does GR explain motion?* Test
particles move on geodesics in the absence of
non-gravitational forces, but *why do they move at all?* AG
Maybe GR assumes motion but doesn't explain it. AG
The sciences do not try to explain, they hardly even try to
interpret, they mainly make models. By a model is meant a
mathematical construct which, with the addition of certain
verbal interpretations, describes observed phenomena. The
justification of such a mathematical construct is solely and
precisely that it is expected to work.
--—John von Neumann
*This is straight out of the "shut up and calculate" school, and
I don't completely buy it. E.g., the Principle of Relativity and
Least Action Principle give strong indications of not only how
the universe works, but why. That is, they're somewhat
explanatory in nature. AG*
Fine, then take them as explanations. But to ask that they be
explained is to misunderstand their status. It's possible that
they could be explained; but only by finding a more fundamental
theory that includes them as consequences or special cases.
Whatever theory is fundamental cannot have an explanation in the
sense you want because then it would not be fundamental.
Brent
*I don't think I asked them to be explained, and I don't think** I
misunderstand their status. In the examples I gave, the principles are
pretty fundamental and nonetheless seem to explain something
substantive about the universe even though they're not part of a
deeper theory. AG
*
You wrote, "...how does GR explain motion? Test particles move on
geodesics in the absence of non-gravitational forces, but why do they
move at all?"
GR hypothesizes that force-free motion of test particles is along
geodesics. In 4-space they "move" because there is a time coordinate
and a particle is by definition something that persists in time (in
contrast to an "event").
Brent
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