Re: Dark matter/energy - is it real?

: "Tue Sorensen" <sorensonian@xxxxxxxxx>
: Why should light have gravity? Just because gravity affects light and
: light's momentum, doesn't mean light has gravity.

Because of newtons' third law. If newton's third law if right, then
that's exactly what it *does* mean.

: Matter, yes. Radiation, no.

What's the difference? The only way you know that the dust mote floating
in the air is affecting the earth, instead of just the earth affecting it,
is because of newton's third law. Maybe only masses above a certain
threshold affect other things by gravity? Taking that position wrt light
is just setting the threshold at zero. But there's no particlar reason
to do that. So nobody does.

And IF light is deflected, and it does NOT affect the thing that deflected it,
then momentum isn't conserved; it's just that simple. Physicists have strong
opinions about basic conservation laws tied to strong symmetries, such
as conservation of momentum; it is enough "proof" to be very very very
(and did I say "very"?) convincing to them. If it's not convincing to
you... well... shrug. It should be. It's got the track record that
justifies how seriously physicists take it.

If you say "but it's so *tiny*; maybe it doesn't matter if it's so tiny",
well then, do it a billion times; or a billion billion. A quadrillion here,
a quadrillion there, and pretty soon you're talking real momentum deficits.

: It seems to me there should be other possibilities for solving that
: equation, like perhaps some quantum physical effects.

Shrug. If you ever come up with one, and can state it formally and
rigorously rather than just wishing somebody else would do the work,
you be sure and let everybody know. In the meantime, I'm pretty sure
physicists will muddle through without it.

Yeah, OK, so I sound snarky. But still, seriously, it's not dogma that
makes conservation issues so persuastive. It has both the practical track
record of being right, and has VERY deep formal roots, via Noether's
Theorem. The fact that quantum mechanics is steeped in group theory
and symmetries means you won't find refuge from conservation laws there.

http://en.wikipedia.org/wiki/Noether's_theorem


Wayne Throop throopw@xxxxxxxxx http://sheol.org/throopw
.

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