4 ms·

To me dark matter has always seemed inelegant, like a hack, which is exactly why and how it was introduced. Now nature doesn't care what's elegant or not, so ma

by eloff 5y ago

To me dark matter has always seemed inelegant, like a hack, which is exactly why and how it was introduced. Now nature doesn't care what's elegant or not, so maybe that's how things actually are. I always thought it'd be nicer if we found out our equations for gravity were off instead. Something that's a rounding error at small scales but that matters at large scales and could explain the faster orbits at the edges of galaxies and in galaxy clsuters. However, as Sabine points out, it seems that really, at the mathematical level it's the same thing. Add a field, add a particle, both ways you're just adding some terms to the equations . Fields and particles are two sides of the same coin. I guess MOND and dark matter aren't as different as I thought.

nicklecompte 5y ago
I am sympathetic to the recently-advocated view that the effects of dark matter are really “gravitational magnetism” associated with rotating large masses[1]. That said: dark matter is no more a “hack” than the discovery of Neptune or Alpha Centauri C: the existence of both were inferred from deviations from theoretically expected gravitational motion, and only later confirmed by direct observation. If Neptune did not exist it might have implied Newton’s equations of gravity were wrong - but it’s existence and correctly-predicted mass were instead a major validation of those laws. So I think the dark matter hypothesis and the subsequent research activity are fully justified areas of scientific inquiry. [1] https://link.springer.com/article/10.1140/epjc/s10052-021-08967-3 https://link.springer.com/article/10.1140/epjc/s10052-021-08...
- posix_me_less 5y ago
  Neptune hypothesis was successful, Vulcan wasn't [1] [1] https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) In both cases, the assumption about another planet was a reasonable thing to investigate. Dark matter is much worse, because we are not introducing single planet to explain another planet behaviour, but we are introducing new "stuff" that can be distributed in space largely arbitrarily and we have great amount of freedom to fit the observations. Add a little dark matter here, remove little dark matter there, now we can fit a bear.
  - Keysh 5y ago
    In fact, assuming extra undetected mass to explain the anomalous motions of galaxies in clusters (as originally noted by Fritz Zwicky in the 1930s) was at least partly correct, because we now know that galaxy clusters contain rarified, extremely hot intergalactic gas (detectable only by its X-ray emission) in amounts equal to five or ten times the stellar mass of the galaxies. You still need even more mass in some as-yet-unidentified form (i.e., "dark matter") to fully explain cluster dynamics, but the basic hypothesis was at least partly successful. (The "undetected mass" hypothesis also turned out to be successful in the cases of binary stars, including things like Sirius B, where the companion turned out to be a bizarre type of star never before seen or theorized.) And, no, dark matter can't be distributed "arbitrarily": its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations (consistent with those seen in the cosmic microwave background).
    - posix_me_less 5y ago
      > its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations That may be a restriction that some may operate with, but it is non-obvious why only models obeying this restriction are to be allowed. Some people do not care about cosmology and want to fit just rotation curves. And even if someone does care about cosmology, why would initial uniform distribution would be the only acceptable one?
- Sniffnoy 5y ago
  The gravitomagntesim (note: this effect is more commonly known as "frame-dragging") paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north-pole-of-dust.html https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
posix_me_less 5y ago
That argument with field/particles is not right. MOND isn't adding arbitrary new position-dependent field like dark matter hypothesis does. MOND changes the universal law of gravity. That is a priori much better thing to try. Yes, there is arbitrary interpolating function there (so also a great degree of freedom to fit the observations), but the resulting modified law is supposed to be universal, valid for all points of space and all material particles. Dark matter hypothesis, on the other hand, just adds immense number of new obscure quantities(dark matter density at all points of space) that can be tuned to fit (almost) anything.
candiodari 5y ago
I've always thought it's weird that it isn't the other way around. Matter, electrons, even photons ... isn't the building block of the university. Fields are. Fields have gravity. I think it's really the reverse: why aren't there more fields that are simply self-sustaining? Such fields, if sufficiently reluctant to interact with normal matter and/or photons, would be exactly dark matter.

by eloff 5y ago

nicklecompte 5y ago

I am sympathetic to the recently-advocated view that the effects of dark matter are really “gravitational magnetism” associated with rotating large masses[1]. That said: dark matter is no more a “hack” than the discovery of Neptune or Alpha Centauri C: the existence of both were inferred from deviations from theoretically expected gravitational motion, and only later confirmed by direct observation. If Neptune did not exist it might have implied Newton’s equations of gravity were wrong - but it’s existence and correctly-predicted mass were instead a major validation of those laws. So I think the dark matter hypothesis and the subsequent research activity are fully justified areas of scientific inquiry. [1] https://link.springer.com/article/10.1140/epjc/s10052-021-08967-3 https://link.springer.com/article/10.1140/epjc/s10052-021-08...

posix_me_less 5y ago
Neptune hypothesis was successful, Vulcan wasn't [1] [1] https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) In both cases, the assumption about another planet was a reasonable thing to investigate. Dark matter is much worse, because we are not introducing single planet to explain another planet behaviour, but we are introducing new "stuff" that can be distributed in space largely arbitrarily and we have great amount of freedom to fit the observations. Add a little dark matter here, remove little dark matter there, now we can fit a bear.
- Keysh 5y ago
  In fact, assuming extra undetected mass to explain the anomalous motions of galaxies in clusters (as originally noted by Fritz Zwicky in the 1930s) was at least partly correct, because we now know that galaxy clusters contain rarified, extremely hot intergalactic gas (detectable only by its X-ray emission) in amounts equal to five or ten times the stellar mass of the galaxies. You still need even more mass in some as-yet-unidentified form (i.e., "dark matter") to fully explain cluster dynamics, but the basic hypothesis was at least partly successful. (The "undetected mass" hypothesis also turned out to be successful in the cases of binary stars, including things like Sirius B, where the companion turned out to be a bizarre type of star never before seen or theorized.) And, no, dark matter can't be distributed "arbitrarily": its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations (consistent with those seen in the cosmic microwave background).
  - posix_me_less 5y ago
    > its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations That may be a restriction that some may operate with, but it is non-obvious why only models obeying this restriction are to be allowed. Some people do not care about cosmology and want to fit just rotation curves. And even if someone does care about cosmology, why would initial uniform distribution would be the only acceptable one?
Sniffnoy 5y ago
The gravitomagntesim (note: this effect is more commonly known as "frame-dragging") paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north-pole-of-dust.html https://www.overcomingbias.com/2021/03/what-holds-up-a-north...

posix_me_less 5y ago

That argument with field/particles is not right. MOND isn't adding arbitrary new position-dependent field like dark matter hypothesis does. MOND changes the universal law of gravity. That is a priori much better thing to try. Yes, there is arbitrary interpolating function there (so also a great degree of freedom to fit the observations), but the resulting modified law is supposed to be universal, valid for all points of space and all material particles. Dark matter hypothesis, on the other hand, just adds immense number of new obscure quantities(dark matter density at all points of space) that can be tuned to fit (almost) anything.

candiodari 5y ago

I've always thought it's weird that it isn't the other way around. Matter, electrons, even photons ... isn't the building block of the university. Fields are. Fields have gravity. I think it's really the reverse: why aren't there more fields that are simply self-sustaining? Such fields, if sufficiently reluctant to interact with normal matter and/or photons, would be exactly dark matter.