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Suppose we made an observation that mercury's orbit precesses by a fraction of a degree every orbit. Heretofore every observation of perturbances has correctly

Suppose we made an observation that mercury's orbit precesses by a fraction of a degree every orbit. Heretofore every observation of perturbances has correctly presaged the presence of a large mass (a planet) that was quickly found, but this time we have looked and looked but not found anything. Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? > You can postulate an additional separate modification of gravity that comes into play on cluster scales. Or it might be something else altogether. Anyways, MOND has made predictions that people set out to disprove MOND went looking for negative evidence and had to turn around (e.g. external field effect), as well as straightforward predictions JWT may have confirmed (early galaxies) and explains many things that dark matter cannot, like how satellite galaxies don't get ripped apart by the dark matter halo.

tshaddox 4y ago
> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Are these substantially different? Of course using “must” isn’t appropriate, but spouting off the best theory you have at the time seems okay to me. Hopefully no one is saying that we cannot possibly ever find another better theory the same way Einstein’s was better than Newton’s or that we shouldn’t ever look for one.
- 2muchcoffeeman 4y ago
  I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.
- throwawaymaths 4y ago
  > Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.
  - pdonis 4y ago
    > draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.
    - deleted 4y ago
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    - throwawaymaths 4y ago
      > the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. Against three parameters, each of which is a single number. But it is three parameters! That's 3x more parameters! Moreover, we had a problem and we had to increase the number of parameters by 50%. So yeah One parameter is much, much better, especially since we have never changed that it's one parameter.
      - pdonis 4y ago
        > It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.
lmm 4y ago
> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? "we have no other way of detecting except through the same effect we use to infer its existence" is true of everything in the universe. We believe stars exist because we can see the light coming from them, but light is our only way of detecting them. There's no single answer to this kind of question, we have to actually do science and make observations. We should ask whether it behaves like a ball of matter (e.g. does it have a point location in space? Does it orbit the way a ball of batter does?) or like a modified law of gravity (can we reproduce the same conditions)? The likes of the Bullet Cluster makes a very compelling case that cold dark matter is real.
- throwawaymaths 4y ago
  > "We have no other way of detecting except through the same effect we use to infer its existence" is true of everything in the universe No. For example, staying within astronomy, we detect gravitational waves and if we find an optical event that correlates with the gravitational waves, it is multimodal (we are at n=1), and if you don't solicit events (or if you statistically blind it by adding fake events in your solicitation) it's multimodal independent (currently n=0). Going outside of astro, you'll find very rich multimodal studies "I know what this molecule is because I have an IR, UV, NMR, Mass spec, etc etc etc"
  - kotlin2 4y ago
    Light, in the GP’s example is also traceable back to a source. Observing an effect in a 3rd party isn’t directly attributable to anything without a plausible mechanism of action. Using light to detect stars as defense of dark matter isn’t a very strong argument.
  - wumpus 4y ago
    I'm not sure what you're trying to say -- there are many astronomy multimodal studies.
nlarion 4y ago
The main argument of this article seems to fall into the counterfactual fallacy category. Even if MOND has problems and ends up being wrong, that doesn't make dark matter exist.
pdonis 4y ago
> should we tweak Newton's laws of physics? But you can't tweak Newton's laws, because we know Newton's laws aren't the right laws of gravity. The laws of General Relativity make better predictions. So what you would have to tweak would be the laws of GR. And the basic problem with doing that is that you can't just tweak a single parameter in GR to get the kind of modified behavior that MOND does with Newtonian dynamics. In GR, there simply isn't any such free parameter to tweak. You have to adjust the whole theory. And any such adjustment ends up looking just like dark matter: you have to invent new fields or new forms of stress-energy that we have no other evidence for. And that destroys the whole point of MOND, which was to just modify the laws of gravity without changing our assumptions about what matter and energy is present in the universe. If you're going to have to add new forms of matter and energy anyway, why not just stick to that and keep the laws of gravity the same? That's what the dark matter hypothesis amounts to, and why it's actually the simpler hypothesis once GR is taken into account.
- deleted 4y ago
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- ejolto 4y ago
  > But you can't tweak Newton's laws, because we know Newton's laws aren't the right laws of gravity. The laws of General Relativity make better predictions. So what you would have to tweak would be the laws of GR The invisible planet OP is talking about is Vulkan[0]. We modified Newtonian gravity to explain the orbit of Mercury (instead of discovering Vulkan), the modification is GR. OP is implying that maybe we have to modify GR like we did Newtonian physics. [0]: https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
  - gpderetta 4y ago
    Sometime you have to modify the theory, but sometime the missing mass is really there. For example neutrinos were postulated to exist to explain missing quantities in some interactions. Then they were actually discovered experimentally . Incidentally neutrinos are dark matter, although not of the right kind.
shusaku 4y ago
> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? I mean this is literally how they discovered Neptune? Dark matter isn’t necessarily some magically invisible particle, we’re just trying to count up all the different types of objects in the universe.
- russdill 4y ago
  I feel like the author is assuming here that the reader knows this is how Neptune was discovered and that the procession of mercury caused by relativity was long thought to be caused by a hidden planet. A very literal example of something thought to be missing matter that instead was a modified Newtonian theory.
russdill 4y ago
Except the evidence of dark matter isn't just about the orbit of a single planet. It goes all the way to predicting the spectrum of the CMB.
- throwawaymaths 4y ago
  The explanatory power of something that has as many free (statistical) parameters as lcdm is so high it's stunning how many things it doesn't explain. You'll surely get some things in there by chance.
  - russdill 4y ago
    The number of free variables has gone down dramatically due to extensive theoretical and experimental work. And as already mentioned, there are many very different lines of evidence and it's not like different variables are being to fit each one in isolation.
    - throwawaymaths 4y ago
      It's still way more than 3 > different variables are being to fit each one in isolation. For the big picture, phenomena that get "explained" by dark matter are absolutely being fit by different individual variables. For example, the ratio of dark matter to baryonic matter is different between clusters and individual galaxies by an order of magnitude. I recall the DMD for CMB is also different but I can't substantiate that memory quickly In the small, too, there are many galaxies where we have no idea what dynamical situation led them to have the dark matter content they exhibit.
    - naasking 4y ago
      > The number of free variables has gone down dramatically due to extensive theoretical and experimental work. The number of free variables in MOND is one, visible mass, and it's surprising the number of observations it matches despite that. There is almost certainly something there. The superfluid dark matter idea that incorporated both was a promising unifying idea, but last I checked the proposed models didn't match some observations.
yongjik 4y ago
> Heretofore every observation of perturbances has correctly presaged the presence of a large mass (a planet) that was quickly found, but this time we have looked and looked but not found anything. Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Note the leap of faith from "a large planet" to "an invisible ball of matter." As one could imagine, given what we knew in the 19th century, "a small, hard-to-observe planet (or a group of small bodies) orbiting very close to the sun" was a totally reasonable hypothesis. "An invisible ball of matter" wasn't (and still isn't). It was natural that physicists never seriously considered such a possibility. On the other hand, given what we know now, "a diffuse cloud made of particles that do not interact with light" is a totally reasonable hypothesis. In fact we even know one such "invisible" particle: neutrinos. It's not crazy to assume that there are some other kinds of neutrino-like particles that fit the bill.
- atoav 4y ago
  I mean a primordial black hole with the size of a tennis ball would have 5 to 15 earth masses. It would not be invisible, but at that size it would be invisible to us even if it was in our solar vicinity. If primordial black holes are things that exist, one could reason that they might be an explaination for masses we cannot see maybe?
  - simonh 4y ago
    It's an interesting idea. I suspect such a large population of these objects would generate enough x-rays from interactions with dust and gas that we'd be able to detect their presence. We'd also probably see micro-lensing of various observable phenomena if they were that populous. Just guesses though.
  - rini17 4y ago
    Astronomers searched for microlensing events from black holes or similar dark objects, but did not see so many.
  - gpderetta 4y ago
gizmo686 4y ago
Notably, Mercury was not a motivation for the development of relativity. Even with the benefit of hindsight, I don't recall ever seeing anyone critisizing contemporary astronomers for looking for the missing planet. GR was developed based on theoretical concerns. The fact that it happended to solve an open issue of Mercury's orbit was a nice confirmation that it is on the right track. The good news is that we have a lot of physicists looking for an alternative theory of gravity called quantum gravity. It would be great if they come up with something that solves the theoretical problems with GR and happens to resolve dark matter.
wumpus 4y ago
It's a shame that a crackpot comment is at the top of this discussion. Vulcan was looked for and didn't exist. Einstein won that one. I'm unaware of any MOND theory which changes the orbit of Mercury.
- aeneasmackenzie 4y ago
  Thats the exact point the comment is making. The moNd theory you’re looking for here is GR. We don’t think of it as a modified theory anymore though.
  - wumpus 4y ago
    Sure, I was expecting that when I started reading the comment, but then it pretty much fell apart. Even if the comment was clear, it still isn't a very good comparison: GR has relatively easy to verify things like the motion of Mercury, and the factor of 2 when light is bent by the Sun. MOND, not so much. BTW I took a graduate class from one of the people who named WIMPs. It took considerable effort to overcome the objection of ApJ's editor to the cute name. Fun times.
- naasking 4y ago
  Yes, Vulcan was looked for just like DM has been looked for. The evidence for DM is largely indirect, just like the evidence for Vulcan. The OP's comment is a good summary of where we stand now. I suggest comparing all of the evidence for DM and MOND: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm There is definitely something to MOND. It would be a remarkable coincidence if MOND was able to make accurate predictions with only a single parameter, visible mass, what takes DM multiple parameters and a tuned mass distribution specific to each observable galaxy. DM definitely "explains more" in some sense because of how much more work it's received, but that's not saying much.
sylware 4y ago
Aren't there several MOND flavors? Heard there is one which is very good at comisc scale. At "our" scale, GR is accurate "enough" as far as my limited knowledge is.

tshaddox 4y ago

> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Are these substantially different? Of course using “must” isn’t appropriate, but spouting off the best theory you have at the time seems okay to me. Hopefully no one is saying that we cannot possibly ever find another better theory the same way Einstein’s was better than Newton’s or that we shouldn’t ever look for one.

2muchcoffeeman 4y ago
I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.
throwawaymaths 4y ago
> Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.
- pdonis 4y ago
  > draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there. No, it isn't. The distribution of stress-energy is a single free parameter in GR; there is no law in the theory that says the density of any particular kind of stress-energy has to be the same in different galaxies. So adjusting the distribution of stress-energy to match the observed behavior in different galaxies does not require adding a new parameter for each galaxy; it only requires adjusting the single parameter that already exists, in different regions of spacetime.
  - deleted 4y ago
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  - throwawaymaths 4y ago
    > the single parameter that already exists, Ok I concede. It is a single parameter that contains a millions of subparameters, at least one for each galaxy. Against three parameters, each of which is a single number. But it is three parameters! That's 3x more parameters! Moreover, we had a problem and we had to increase the number of parameters by 50%. So yeah One parameter is much, much better, especially since we have never changed that it's one parameter.
    - pdonis 4y ago
      > It is a single parameter that contains a millions of subparameters, at least one for each galaxy. No, it's a single parameter with a distribution in space and time. It's the same one we already have to vary in space and time anyway based on our observations of light. Dark matter just means adjusting the same distribution further based on observed motions even if we can't see the source of gravity that is producing the motion. If MOND actually gave the choice of adjusting a single parameter in place of having to adjust the distribution of matter and energy, that would be one thing. But, as I noted in another comment upthread, once you take into account that the theory of gravity we are using is actually General Relativity, not Newtonian gravity, there is no such single parameter anyway. Relativistic versions of MOND actually end up looking just like dark matter: they have to invent new fields, new forms of energy, for which we have no other evidence. There is no way to get the desired effect in GR by just tweaking a parameter in the law of gravity.

lmm 4y ago

> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? "we have no other way of detecting except through the same effect we use to infer its existence" is true of everything in the universe. We believe stars exist because we can see the light coming from them, but light is our only way of detecting them. There's no single answer to this kind of question, we have to actually do science and make observations. We should ask whether it behaves like a ball of matter (e.g. does it have a point location in space? Does it orbit the way a ball of batter does?) or like a modified law of gravity (can we reproduce the same conditions)? The likes of the Bullet Cluster makes a very compelling case that cold dark matter is real.

throwawaymaths 4y ago
> "We have no other way of detecting except through the same effect we use to infer its existence" is true of everything in the universe No. For example, staying within astronomy, we detect gravitational waves and if we find an optical event that correlates with the gravitational waves, it is multimodal (we are at n=1), and if you don't solicit events (or if you statistically blind it by adding fake events in your solicitation) it's multimodal independent (currently n=0). Going outside of astro, you'll find very rich multimodal studies "I know what this molecule is because I have an IR, UV, NMR, Mass spec, etc etc etc"
- kotlin2 4y ago
  Light, in the GP’s example is also traceable back to a source. Observing an effect in a 3rd party isn’t directly attributable to anything without a plausible mechanism of action. Using light to detect stars as defense of dark matter isn’t a very strong argument.
- wumpus 4y ago
  I'm not sure what you're trying to say -- there are many astronomy multimodal studies.

nlarion 4y ago

The main argument of this article seems to fall into the counterfactual fallacy category. Even if MOND has problems and ends up being wrong, that doesn't make dark matter exist.

pdonis 4y ago

> should we tweak Newton's laws of physics? But you can't tweak Newton's laws, because we know Newton's laws aren't the right laws of gravity. The laws of General Relativity make better predictions. So what you would have to tweak would be the laws of GR. And the basic problem with doing that is that you can't just tweak a single parameter in GR to get the kind of modified behavior that MOND does with Newtonian dynamics. In GR, there simply isn't any such free parameter to tweak. You have to adjust the whole theory. And any such adjustment ends up looking just like dark matter: you have to invent new fields or new forms of stress-energy that we have no other evidence for. And that destroys the whole point of MOND, which was to just modify the laws of gravity without changing our assumptions about what matter and energy is present in the universe. If you're going to have to add new forms of matter and energy anyway, why not just stick to that and keep the laws of gravity the same? That's what the dark matter hypothesis amounts to, and why it's actually the simpler hypothesis once GR is taken into account.

deleted 4y ago
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ejolto 4y ago
> But you can't tweak Newton's laws, because we know Newton's laws aren't the right laws of gravity. The laws of General Relativity make better predictions. So what you would have to tweak would be the laws of GR The invisible planet OP is talking about is Vulkan[0]. We modified Newtonian gravity to explain the orbit of Mercury (instead of discovering Vulkan), the modification is GR. OP is implying that maybe we have to modify GR like we did Newtonian physics. [0]: https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.m.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
- gpderetta 4y ago
  Sometime you have to modify the theory, but sometime the missing mass is really there. For example neutrinos were postulated to exist to explain missing quantities in some interactions. Then they were actually discovered experimentally . Incidentally neutrinos are dark matter, although not of the right kind.

shusaku 4y ago

> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? I mean this is literally how they discovered Neptune? Dark matter isn’t necessarily some magically invisible particle, we’re just trying to count up all the different types of objects in the universe.

russdill 4y ago
I feel like the author is assuming here that the reader knows this is how Neptune was discovered and that the procession of mercury caused by relativity was long thought to be caused by a hidden planet. A very literal example of something thought to be missing matter that instead was a modified Newtonian theory.

russdill 4y ago

Except the evidence of dark matter isn't just about the orbit of a single planet. It goes all the way to predicting the spectrum of the CMB.

throwawaymaths 4y ago
The explanatory power of something that has as many free (statistical) parameters as lcdm is so high it's stunning how many things it doesn't explain. You'll surely get some things in there by chance.
- russdill 4y ago
  The number of free variables has gone down dramatically due to extensive theoretical and experimental work. And as already mentioned, there are many very different lines of evidence and it's not like different variables are being to fit each one in isolation.
  - throwawaymaths 4y ago
    It's still way more than 3 > different variables are being to fit each one in isolation. For the big picture, phenomena that get "explained" by dark matter are absolutely being fit by different individual variables. For example, the ratio of dark matter to baryonic matter is different between clusters and individual galaxies by an order of magnitude. I recall the DMD for CMB is also different but I can't substantiate that memory quickly In the small, too, there are many galaxies where we have no idea what dynamical situation led them to have the dark matter content they exhibit.
  - naasking 4y ago
    > The number of free variables has gone down dramatically due to extensive theoretical and experimental work. The number of free variables in MOND is one, visible mass, and it's surprising the number of observations it matches despite that. There is almost certainly something there. The superfluid dark matter idea that incorporated both was a promising unifying idea, but last I checked the proposed models didn't match some observations.

yongjik 4y ago

> Heretofore every observation of perturbances has correctly presaged the presence of a large mass (a planet) that was quickly found, but this time we have looked and looked but not found anything. Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Note the leap of faith from "a large planet" to "an invisible ball of matter." As one could imagine, given what we knew in the 19th century, "a small, hard-to-observe planet (or a group of small bodies) orbiting very close to the sun" was a totally reasonable hypothesis. "An invisible ball of matter" wasn't (and still isn't). It was natural that physicists never seriously considered such a possibility. On the other hand, given what we know now, "a diffuse cloud made of particles that do not interact with light" is a totally reasonable hypothesis. In fact we even know one such "invisible" particle: neutrinos. It's not crazy to assume that there are some other kinds of neutrino-like particles that fit the bill.

atoav 4y ago
I mean a primordial black hole with the size of a tennis ball would have 5 to 15 earth masses. It would not be invisible, but at that size it would be invisible to us even if it was in our solar vicinity. If primordial black holes are things that exist, one could reason that they might be an explaination for masses we cannot see maybe?
- simonh 4y ago
  It's an interesting idea. I suspect such a large population of these objects would generate enough x-rays from interactions with dust and gas that we'd be able to detect their presence. We'd also probably see micro-lensing of various observable phenomena if they were that populous. Just guesses though.
- rini17 4y ago
  Astronomers searched for microlensing events from black holes or similar dark objects, but did not see so many.
- gpderetta 4y ago

gizmo686 4y ago

Notably, Mercury was not a motivation for the development of relativity. Even with the benefit of hindsight, I don't recall ever seeing anyone critisizing contemporary astronomers for looking for the missing planet. GR was developed based on theoretical concerns. The fact that it happended to solve an open issue of Mercury's orbit was a nice confirmation that it is on the right track. The good news is that we have a lot of physicists looking for an alternative theory of gravity called quantum gravity. It would be great if they come up with something that solves the theoretical problems with GR and happens to resolve dark matter.

wumpus 4y ago

It's a shame that a crackpot comment is at the top of this discussion. Vulcan was looked for and didn't exist. Einstein won that one. I'm unaware of any MOND theory which changes the orbit of Mercury.

aeneasmackenzie 4y ago
Thats the exact point the comment is making. The moNd theory you’re looking for here is GR. We don’t think of it as a modified theory anymore though.
- wumpus 4y ago
  Sure, I was expecting that when I started reading the comment, but then it pretty much fell apart. Even if the comment was clear, it still isn't a very good comparison: GR has relatively easy to verify things like the motion of Mercury, and the factor of 2 when light is bent by the Sun. MOND, not so much. BTW I took a graduate class from one of the people who named WIMPs. It took considerable effort to overcome the objection of ApJ's editor to the cute name. Fun times.
naasking 4y ago
Yes, Vulcan was looked for just like DM has been looked for. The evidence for DM is largely indirect, just like the evidence for Vulcan. The OP's comment is a good summary of where we stand now. I suggest comparing all of the evidence for DM and MOND: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm There is definitely something to MOND. It would be a remarkable coincidence if MOND was able to make accurate predictions with only a single parameter, visible mass, what takes DM multiple parameters and a tuned mass distribution specific to each observable galaxy. DM definitely "explains more" in some sense because of how much more work it's received, but that's not saying much.

sylware 4y ago

Aren't there several MOND flavors? Heard there is one which is very good at comisc scale. At "our" scale, GR is accurate "enough" as far as my limited knowledge is.