Dear Yousuf Khan:

On Wednesday, October 19, 2016 at 11:46:24 PM UTC-7, Yousuf Khan wrote:
On 10/16/2016 1:05 PM, dlzc wrote:
Dear Yousuf Khan:

On Saturday, October 15, 2016 at 10:45:21 PM UTC-7, Yousuf Khan
wrote: ...
Maybe the issue here is not to find a new modification of
Newtonian gravity, but perhaps our reliance on still using
Newtonian gravity even 100 years after we found a better
theory of gravity might be the problem here?

I find it more likely that a nearly 100 year old
assumption that luminosity is directly proportional
to the amount of mass present, when it has long been
known that luminosity drops off rapidly with surface
temperature. If you have cooler objects, they simply
don't put out as much light... especially in the
visible light bands.

But they do still glow in the cooler invisible light
bands like IR and microwave and radio.

At a *much* lower luminosity. Remember, they use luminosity, essentially watts, and calibrate to normal-mass-present.

We're still using Newtonian gravity after all of these
years, because it's frankly much easier to calculate
with than General Relativity.

Paper on this subject for a "simple" galaxy, and
evaluating the possible error between Newtonian
gravity-as-a-force and GR, and in that galaxy, it
is a 1% (or so) error, not the necessary 300%
error.

That's the point I'm trying to make, they are using
"simple" galaxy models, rather than full galaxy models.

Even dwarf spiral galaxies need Dark Matter, however. And they have a few billion stars. This should be doable soon.

GR really kicks in:
- to handle light,
- to handle advancement of perihelion (for close objects),
- to handle gravitational radiation.

But in a many-body system such as stars in a galaxy or
galaxies in a universe, those simple inverse-distance
squared relationships simply don't work out anymore?

They do work out "well enough", for simple gravitation.

But we are "blind as bats" at these scales, and have
a full complement of "flatlander fallacies" that we
have to divest ourselves of.

So then we're basically agreeing on this. Newtonian
gravity might be one of those flatlander fallacies.

Remove the *serious* errors of (normal-mass / luminosity) calibration, and then see if you think a further 5 or 10% (max) correction is necessary.

We're still using Newtonian gravity in this day and
age because we still don't have computers strong
enough to do a GR calculation for an entire galaxy.

False. The amount of computer time might still be
abysmally long for an interesting galaxy, but it
would still be doable. After all, Nature does
this math in real time...

Nature has its own entire universe-sized quantum
computer to work with. We can barely put two qubits
together yet.

But GR (like Newton), is a classical solution. GR simplifies to Newton, under the right circumstances, circumstances suitable to galaxies "in the large".

I don't think GR explains "Dark Matter", better than Newton does. They both have to accept that there is more matter that our myopic vision cannot detect (except via gravity).

Using even our strongest supercomputers we can do
perhaps a simulation of only a few million stars in
a galaxy using GR, but our galaxy contains perhaps
as much as 400 billion stars, so we keep
approximating with Newton.

Yet, even small spirals show a need for Dark Matter.
Globular clusters, essentially don't.

Then we need to investigate where the globular clusters
differ from dwarf galaxies.

There is no significant rotation in a globular cluster, so the normal mass present, is explained by microlensing, and other methods that apply equally well to a spiral's nucleus, or a globular cluster (expected to be ancient cores of spiral galaxies).

If one day we can do a full simulation of the Milky Way
with all of its entire 400 billion stars, then likely
we'll see surprising results coming out of GR that are
inconsistent with Newton, and then we'll be finally
shaken of our illusion that Newton is "still good enough".

Maybe. But the speeds and curvature on something the
size of a galaxy, even the Milky Way, should present
minimal error in using Newton.

Well, that's been our assumption all along hasn't it?
Maybe our assumption is wrong?

We *know* it is still a classical theory, however.

Now what I wonder is, if the "perfectly mirrored,
massless box, containing photons", which has rest
mass, exists between a star and the gases / dust /
planets that give that star a background temperature
higher than the CMBR. So some Dark Matter (probably
less than 1%) might still be photons in transit
between intersystem objects...?

Or even neutrinos.

Amen. Absolutely "dark" too, just not very massive, and in order to stay in the halo (as we observe), would have to be moving damned slowly... so would have to be too numerous to be all of Dark Matter.

David A. Smith