Re: Dark matter

On Sep 16, 4:27 pm, "Mike Dworetsky"
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"VoiceOfReason" <papa_...@xxxxxxxxxxxxx> wrote in message

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Mike Dworetsky wrote:
"VoiceOfReason" <papa_...@xxxxxxxxxxxxx> wrote in message
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Mike Dworetsky wrote:
"Féachadóir" <Féach@d.óir> wrote in message
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Any physicists out there to help with a mind experiment?

The question I have is, what if dark matter was the new phlogiston?
Or
to put it another way, if there is no dark matter, how different
would
our physics be? How wrong are we if it isn't there? How would the
universe behave differently?

Yes, its Saturday, and I'm bored.

--
'Donegal: Up Here It's Different'
© Féachadóir

Dark matter is postulated precisely because we do believe that our
laws
of
physics (as far as gravity is concerned) are correct. If you want to
play
with non-Newtonian and non-Einsteinian gravitation then you nead
something
like MOND ("modified newtonian dynamics").

There is plenty of evidence that DM exists, but so far no real
explanation
of what it consists of. It definitely fails to interact with ordinary
matter in any obvious way except through gravity, and its particles do
not
move particularly fast, hence "cold dark matter".

What really puzzles cosmologists is Dark Energy, the
force/stuff/whatever
that seems to make the Universe space structure "flat".

Give a rip what puzzles them. What puzzles ME is the observation that
the expansion of all the stuff in the universe is _accelerating_.
Talk about non-intuitive...

Um, that is exactly one of the effects of Dark Energy. Lots of science
is
non-intuitive, including General Relativity, but the theory has been
worked
out in detail and (for example in last week's Nature article) the
verifying
evidence is there in observations.

The fact that this may not agree with your private "intuition" is not
evidence of anything except your limitations.

You do realize I was joking around, right? And yes, I learned in
grade school that science is sometimes not intuitive.

Oh. (Flips on sense of humour) OK, but it remains true that lots of
physics is deeply counterintuitive. This sort of problem goes back to the
Greek philosophers doing physics by "thought experiments" instead of actual
experiments.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

This reminds me of my junior year of high school, where we learned
about the old theories of "impetus" that said a thrown object flies
straight as long as it has "impetus" and once it runs out it falls
straight down. The teacher showed a nice graph of what that looked
like. Of course we all scoffed at this as everyone knows thrown
objects such as cannonballs have parabolic trajectories dictated by
the laws of gravity.
Then in college I had to caculate the trajectory of an object taking
air resistance into account... and miracles of miracles guess what the
resulting graph looked like ! It appears these old natural
philosophers weren't so stupid after all...

I have all the more respect for the Renaissance scientists who managed
to abstract out what the world actually looked like and realized there
were simple laws underneath it all, that just happened to be
complicated by stuff like air resistance.


.

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