Discussion:
OT Alternatives to dark matter
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a***@gmail.com
2020-02-09 16:08:24 UTC
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What are the alternatives to dark matter? There is MOND, which is a modification of Newton. IIRC, in simulations it is more accurate than dark matter in predicting galaxy creation after the big bang. There are also models of ordinary matter like planets and stars that are not luminous, but they don't account for enough matter. There are also models of modified GR.

If GR can fail at the large scale, and it already fails in the small scale, then hopefully SR may fail also at some point.

Abhinav Lal

"This is madness"
Paul S Person
2020-02-09 17:47:17 UTC
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Post by a***@gmail.com
What are the alternatives to dark matter? There is MOND, which is a modification of Newton. IIRC, in simulations it is more accurate than dark matter in predicting galaxy creation after the big bang. There are also models of ordinary matter like planets and stars that are not luminous, but they don't account for enough matter. There are also models of modified GR.
One reaction to a firmly-accepted theory ("planets move in circles",
"GR") is to "save the appearances" by hypothesizing unseen actors
(epicycles, Dark Matter).

The other is to /replace the theory/ (Kepler's elleptical orbits, and
now MOND).

Of course, MOND is, no doubt, very speculative. It will take a great
deal of effort to replace GR!
Post by a***@gmail.com
If GR can fail at the large scale, and it already fails in the small scale, then hopefully SR may fail also at some point.
Only if the speed of light does not have the same value in all
inertial frames of reference. Keep in mind that experimental data open
this particular Pandora's Box by showing that it /was/. Well, provided
I understand what Michelson-Morley did properly.
Post by a***@gmail.com
Abhinav Lal
"This is madness"
Indeed.
--
"I begin to envy Petronius."
"I have envied him long since."
J. Clarke
2020-02-09 19:04:12 UTC
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On Sun, 09 Feb 2020 09:47:17 -0800, Paul S Person
Post by Paul S Person
Post by a***@gmail.com
What are the alternatives to dark matter? There is MOND, which is a modification of Newton. IIRC, in simulations it is more accurate than dark matter in predicting galaxy creation after the big bang. There are also models of ordinary matter like planets and stars that are not luminous, but they don't account for enough matter. There are also models of modified GR.
One reaction to a firmly-accepted theory ("planets move in circles",
"GR") is to "save the appearances" by hypothesizing unseen actors
(epicycles, Dark Matter).
The other is to /replace the theory/ (Kepler's elleptical orbits, and
now MOND).
Of course, MOND is, no doubt, very speculative. It will take a great
deal of effort to replace GR!
What leads you to believe that MOdified Newtonian Dynamics will
"replace" General Relativity? The basic premise of the hypothesis is
that there is a weak component to gravitation that we do not encounter
under ordinary circumstances that becomes effective at very tiny
accelerations. This does not as far as I can tell do any violence to
General Relativity--yes, it will have to be accounted for in
adjustments to the field equations but that's about it.
Post by Paul S Person
Post by a***@gmail.com
If GR can fail at the large scale, and it already fails in the small scale, then hopefully SR may fail also at some point.
The fact that you think that Special Relativity is somehow "different"
from General Relativity is rather touching but it suggests that you
need to do some more homework. Special Relativity isn't "different",
it is just a simplification in which it is assumed that the frame of
reference is experiencing no accelerations.

As for "failing", General Relativity does not "fail in the small
scale", it simply does not say anything in that realm unless the
density of matter is very, very high as occurs in, say, a black hole.
Post by Paul S Person
Only if the speed of light does not have the same value in all
inertial frames of reference. Keep in mind that experimental data open
this particular Pandora's Box by showing that it /was/. Well, provided
I understand what Michelson-Morley did properly.
Post by a***@gmail.com
Abhinav Lal
"This is madness"
Indeed.
a***@gmail.com
2020-02-10 03:25:56 UTC
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Post by J. Clarke
On Sun, 09 Feb 2020 09:47:17 -0800, Paul S Person
Post by Paul S Person
Post by a***@gmail.com
What are the alternatives to dark matter? There is MOND, which is a modification of Newton. IIRC, in simulations it is more accurate than dark matter in predicting galaxy creation after the big bang. There are also models of ordinary matter like planets and stars that are not luminous, but they don't account for enough matter. There are also models of modified GR.
One reaction to a firmly-accepted theory ("planets move in circles",
"GR") is to "save the appearances" by hypothesizing unseen actors
(epicycles, Dark Matter).
The other is to /replace the theory/ (Kepler's elleptical orbits, and
now MOND).
Of course, MOND is, no doubt, very speculative. It will take a great
deal of effort to replace GR!
What leads you to believe that MOdified Newtonian Dynamics will
"replace" General Relativity? The basic premise of the hypothesis is
that there is a weak component to gravitation that we do not encounter
under ordinary circumstances that becomes effective at very tiny
accelerations. This does not as far as I can tell do any violence to
General Relativity--yes, it will have to be accounted for in
adjustments to the field equations but that's about it.
Post by Paul S Person
Post by a***@gmail.com
If GR can fail at the large scale, and it already fails in the small scale, then hopefully SR may fail also at some point.
The fact that you think that Special Relativity is somehow "different"
from General Relativity is rather touching but it suggests that you
need to do some more homework. Special Relativity isn't "different",
it is just a simplification in which it is assumed that the frame of
reference is experiencing no accelerations.
As for "failing", General Relativity does not "fail in the small
scale", it simply does not say anything in that realm unless the
density of matter is very, very high as occurs in, say, a black hole.
According to my understanding, GR and quantum mechanics are incompatible. I believe that space is discrete, while GR assumes it is continuous.

Abhinav Lal

"Same difference"
Post by J. Clarke
Post by Paul S Person
Only if the speed of light does not have the same value in all
inertial frames of reference. Keep in mind that experimental data open
this particular Pandora's Box by showing that it /was/. Well, provided
I understand what Michelson-Morley did properly.
Post by a***@gmail.com
Abhinav Lal
"This is madness"
Indeed.
p***@gmail.com
2020-02-11 18:30:37 UTC
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Post by a***@gmail.com
What are the alternatives to dark matter? There is MOND, which is a modification of Newton. IIRC, in simulations it is more accurate than dark matter in predicting galaxy creation after the big bang. There are also models of ordinary matter like planets and stars that are not luminous, but they don't account for enough matter. There are also models of modified GR.
If GR can fail at the large scale, and it already fails in the small scale, then hopefully SR may fail also at some point.
MOND has real difficulty with certain observed systems. And it does
not even remove the need for dark matter.

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Responses_and_criticism

GR does not "fail" in the small scale. We fail at building a quantum
theory out of it. So far, no experiment has disagreed with GR.

So far, we don't really know what to do about that. We have string
theory, but it's currently extremely far from any kind of experiment
testing it. And we have loop quantum gravity, which shows some hopes,
but also has no experimental tests.

Quantum gravity is, to date, a place for good theoreticians to go
when they don't ever want to do anything else.

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