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 difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about 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 there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
MOND does amazingly well at galactic rotation curves, less well at anything else. If you think it started with Vera Rubin in 1966 MOND seems natural, but if you know that it started with Fritz Zwicky in 1933 than dark matter is easier to believe.
MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.
But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things
you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?
It's funny how for MOND we cant accept that it has some unknowns yet but we are more than willing to accept the FULL UNKNOWN Dark Matter. it's easy. put "Dark" in front of something and you don't have to explain it at all, no matter that something else explains at least 60-70% instead of 0.
Dark matter is invisible, but it isn't magic. It is not significantly different from neutrinos. No one seriously denies the existence of neutrinos nowadays, even though they are invisible (i.e. they don't interact electromagnetically).
Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).
In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.
One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.
We make a similar guess for the stuff that is in the center of Earth. We measure local gravity and speed of sound velocity, and we guess here is liquid, here is solid, here is this rock, here is this another rock [1]. See for example https://www.livescience.com/64943-nobody-understands-the-gia... nobody has seen them, we guess they are there.
Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.
Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition
Dark matter is not another Guess. is no guess at all. its the same as saying the center of the earth is made of Dark Core. and when someone proposed liquid core, there would be papers how a liquid core doesnt explain 100 %, and neither does rock or solid so Dark Core it is because it's sIMpLer to have just one Dark Something than 3 different somethings to explain anything.
The fact that there are tentatives to identify what it might be does not ammegliorate the fact that at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves. I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury). just a tiny percentage of obeservable behaviour at that time but it was a different time. a time where you could bring down the existing science of the day for a tiny percentage and now we accept 90% observation disaccordance (dark energy+dark matter) with what the equation require.
> at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves
Another way to interpret dark matter is that we can observe something using several different ways, but all those ways use gravity. When trying to observe this something using electromagnetism, we see nothing. It doesn't seem so crazy then to hypothesize that this something only interacts with gravity, and not electromagnetism.
MOND is kinda like a dead horse now, that people like to keep flogging
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
MOND is dead is a true statement if we say MOND is dead as a general theory of gravity. It does not mean is does not have its success with explaining galactic rotation curves but failing at mostly everything else.
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.
Dark Unknown Matter would be a better name for lay people to understand what's going on. I'm no cosmologist but isn't it just a placeholder for something that gravity interacts with (and not much else) and we don't know what it currently is. When we discover what it is the name will change.
This comment and GP are two of the most concise and punchy descriptions I've ever heard of some of the deepest aspects of modern physics. On the one hand we have principles of locality and finite propagation speed, which limit the computational work to a small neighborhood, and on the other hand we have principles of non-locality and superposition, which cause the computation to explode as it swallows up potentially everything and every possible thing.
I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?
General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?
General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).
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 difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about 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 there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
MOND does amazingly well at galactic rotation curves, less well at anything else. If you think it started with Vera Rubin in 1966 MOND seems natural, but if you know that it started with Fritz Zwicky in 1933 than dark matter is easier to believe.
MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.
We are likely going to find out that both are unfixably faulty.
It'll take either the next Einstein or some groundbreaking experimental observation to get there in my opinion.
If it was possible to incrementally fix these theories, the army of postdocs working on these would have already done so in the last decade or so.
But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things
Interestingly, more often than not it happens the other way.
Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant.
It gets proven experimentally years or decades later.
Relativity was exactly like this.
you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?
It's funny how for MOND we cant accept that it has some unknowns yet but we are more than willing to accept the FULL UNKNOWN Dark Matter. it's easy. put "Dark" in front of something and you don't have to explain it at all, no matter that something else explains at least 60-70% instead of 0.
Dark matter is invisible, but it isn't magic. It is not significantly different from neutrinos. No one seriously denies the existence of neutrinos nowadays, even though they are invisible (i.e. they don't interact electromagnetically).
Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).
In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.
One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.
We make a similar guess for the stuff that is in the center of Earth. We measure local gravity and speed of sound velocity, and we guess here is liquid, here is solid, here is this rock, here is this another rock [1]. See for example https://www.livescience.com/64943-nobody-understands-the-gia... nobody has seen them, we guess they are there.
Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.
Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition
[1] I don't know enough about rocks https://xkcd.com/2501/
Dark matter is not another Guess. is no guess at all. its the same as saying the center of the earth is made of Dark Core. and when someone proposed liquid core, there would be papers how a liquid core doesnt explain 100 %, and neither does rock or solid so Dark Core it is because it's sIMpLer to have just one Dark Something than 3 different somethings to explain anything.
The fact that there are tentatives to identify what it might be does not ammegliorate the fact that at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves. I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury). just a tiny percentage of obeservable behaviour at that time but it was a different time. a time where you could bring down the existing science of the day for a tiny percentage and now we accept 90% observation disaccordance (dark energy+dark matter) with what the equation require.
> at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves
Another way to interpret dark matter is that we can observe something using several different ways, but all those ways use gravity. When trying to observe this something using electromagnetism, we see nothing. It doesn't seem so crazy then to hypothesize that this something only interacts with gravity, and not electromagnetism.
MOND is kinda like a dead horse now, that people like to keep flogging
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
MOND is dead is a true statement if we say MOND is dead as a general theory of gravity. It does not mean is does not have its success with explaining galactic rotation curves but failing at mostly everything else.
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.
Dark Unknown Matter would be a better name for lay people to understand what's going on. I'm no cosmologist but isn't it just a placeholder for something that gravity interacts with (and not much else) and we don't know what it currently is. When we discover what it is the name will change.
Or, you know, as aether.
It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior.
Does it seems to be kept together by duct tape? Maybe.
Is it yet useful? Yep.
Will it be discarded if anything more fitting will came up? You can be sure of it.
my understanding is that there are a few MOND champions who are still holding on to the idea while everyone else has moved on.
so MOND is the new String Theory...
String theory is still the leading contender for quantum gravity.
It never had the institutional imprimatur of string theory.
Once I joked that a lot of things in the universe make sense if you view it as a "simulation with optimizations like lazy loading".
Yeah until you get to quantum computing and then it seems as if the universe is doing enormously more work than you would think necessary.
This comment and GP are two of the most concise and punchy descriptions I've ever heard of some of the deepest aspects of modern physics. On the one hand we have principles of locality and finite propagation speed, which limit the computational work to a small neighborhood, and on the other hand we have principles of non-locality and superposition, which cause the computation to explode as it swallows up potentially everything and every possible thing.
It might just be a reflection of the architecture the universe simulation is running on...
Not necessarily.
You'd be correct given hidden variables.
But we know pretty convincingly that quantum anything does not have hidden variables.
https://en.wikipedia.org/wiki/Bell%27s_theorem
But only if someone observes it. The act of observation forces reality into existence.
Everything we don't understand we conceptualize using the most similar tools which we do have command over.
I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?
General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
[1]: https://pirsa.org/26030070
Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?
[delayed]
Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?
General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).
The test here is for the inverse square law of gravity. The rival theory in this case isn't GR, but MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
just another nag screen ...