111–120 of 162 posts
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so MOND is the new String Theory...
String theory is still the leading contender for quantum gravity.
It has fallen far from its glory days, and is more an old reliable tool than a leading physical theory.
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String theory is still the leading contender for quantum gravity.
I don't think "leading" is correct, more like "ill regarded but still standing". It has fallen far from its glory days, and is more an old reliable tool than a leading physical theory.
This is different from MOND, which is a distant second contender compared to dark matter.
If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side. As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter. I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but t…
When I see dark matter in the news I'm always reminded of the story of Vulcan. In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors. Le Verrier, the astronomer who made these observations, wondered how to explain the di…
There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
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There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
MOND explains 1. 2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). 3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g. MOND explains severa…
3. I would wager that what we call the CMB is more of a local effect than a big bang remnant.
I lack the karma to downvote, don't blame me. :)
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There are galaxies that appear to be free of dark matter and rotate accordingly. How does MOND account for that? My understanding is that these observations are a fatal blow to any serious MOND models.
MOND obviously don't have to account for the lack of dark matter, as all galaxies lack it under MOND. You have to actually do the calculations and compare what MOND output to the observed behaviour of the galaxies in question.
When I see dark matter in the news I'm always reminded of the story of Vulcan. In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors. Le Verrier, the astronomer who made these observations, wondered how to explain the di…
There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
This reminds me of arguments on the alt.athiesm newsgroup back in the day.
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> Only catch is that it hasnt been directly observed. Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause. For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong. People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about. The problem…
This is the key thing. As we still don't have a theory of everything, which explains the fundamental reasons why every particle is the way it is, dark matter is a very straightforward and simple explanation.
The name makes it sound stranger than it is. It's just particles that have mass and are affected normally by gravity, but which are unaffected by the electromagnetic force.
It's not a particularly exotic or implausible or wild idea. In fact it's so simple it's almost boring. It's just obviously inherently difficult to test directly, since gravity is such a weak force.
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> Only catch is that it hasnt been directly observed. Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
Because stuff like the Bullet Cluster exists. Dark matter is in fact the simplest answer. https://en.wikipedia.org/wiki/Bullet_Cluster
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Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter. It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.
The "pass through matter" is a consequence of not interacting electromagnetically. That's not that uncommon. Think neutrinos. (Also, "not" might just mean "very much reduced")