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> Also, dark matter doesn't interact much with regular matter, which makes the search even harder. This is (one of the many places) where I get lost. It has ma

> Also, dark matter doesn't interact much with regular matter, which makes the search even harder. This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass? Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?

analog31 8y ago
As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.
- beefield 8y ago
  But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth due to dark matter is calculated in e.g. kilograms. That's why I would like to know the expected density of the dark matter.
  - acqq 8y ago
    Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! No "clumping" even around e.g. Sun can be observed. The "hanging around" is on the level of the whole galaxies, and sometimes the dark matter even remains outside of the whole galaxies, being too slow to follow their gravitational interaction(!) That's the famous example of the "bullet cluster": https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/ https://www.forbes.com/sites/startswithabang/2017/11/09/the-... That's how weakly the dark matter interacts with anything else. And it obviously doesn't even interact strong enough to "fall" to the center of the galaxy. Otherwise it would be there, but it remains to the outside of even where the "normal" matter is (mostly the stars and black holes, the "supermassive black hole" in the center is only at most 1e-5 of the estimated total mass of our Galaxy).
    - acqq 8y ago
      Or more precise: https://medium.com/starts-with-a-bang/ask-ethan-if-dark-matter-is-everywhere-why-havent-we-detected-it-in-our-solar-system-67ca11f94b1f https://medium.com/starts-with-a-bang/ask-ethan-if-dark-matt... There’s “just a little more” of the dark matter in our Solar system but way below the levels it could affect the measurements we can perform.
    - sago 8y ago
      > Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! Would I be right in thinking that also puts severe limits on how much it interacts with itself? Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly?
      - acqq 8y ago
        > Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly? Now I have a little of Newton for you: look at our Solar system: you see the planets, and even more interesting, all the small asteroids circling around the Sun? Can you answer why don't they all fall to the Sun, but move in the orbits? The way the gravitation works was not "intuitive" before Newton, 300 years ago, and now it's obviously still so for many non-professional readers. What's actually happening, according to the dark matter model, and the dark matter actually more easily fits much more of our cosmological observations than anything else, is that there is a lot of dark matter but it is simply much more "spread" around the volume of the galaxies. And just like all the visible stuff of the whole galaxy doesn't fall to the galaxy center (like the planets don't fall to the Sun!), the dark matter remains "around" the galaxies, where more of dark matter is "outside" (as in "in the outer regions of it") than in the "inside" of the galaxy (and in the case of the "Bullet Cluster", that I've mentioned in some other comment, dark matter is obviously lagging all the movement of non-dark matter! (1)). That dark matter that is in the inside of the galaxies actually initially "clumped" somewhat, but that "somewhat" is, according to our estimates, significantly below what we are able to measure, when we're interested in the gravitational effect on the Solar system. 1) https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bullet_cluster.jpg https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul... and https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0657_scale.jpg https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...
  - siweiz 8y ago
    They are not distributed evenly throughout space. So if you measure more gravity in a region of space that you cannot see (because dark matter doesn’t interact with em), is it dark matter, or an error in your theory of gravity?
  - analog31 8y ago
    Indeed the mass of dark matter is why we think there is dark matter -- discrepancies such as you mention, but evident from astronomical observations. The problem for characterizing the particles is: Mass and what else? The what else is the thing people are trying to detect.
  - BurningFrog 8y ago
    We don't have that good idea of what the earth consists of. I think much of those theories are based on counting backwards from knowing the mass, and if we have 5% dark mass orbiting Earth, it wouldn't make any material difference.
- acqq 8y ago
  "Think of the amount of mass required to generate a gravitational pressure needed to overcome the electromagnetic binding force between molecules inside the mass--the equilibrium occurs, basically, when an object in space becomes spherical. This happens at about 10^20 - 10^21kg. Divided by the mass of a proton implies you need about 10^47 atoms to generate the amount of gravitational pressure to break the electromagnetic strength between atoms." (1) Richard Feynman would tell you that this immense difference is "what's keeping you from not falling through the floor down to the Earth center." Or why the apple hanging on the "few atoms" of its stalk is not falling from the tree, when the summary gravitation of all atoms of the whole Earth are pulling it down. Or, again to compare such big numbers, there are "only" 10^86 atoms in the whole Universe observable to us! (2) 1) https://www.physicsforums.com/threads/how-are-the-gravitational-and-electric-force-comparable.660195/ https://www.physicsforums.com/threads/how-are-the-gravitatio... 2) https://www.universetoday.com/36302/atoms-in-the-universe/ https://www.universetoday.com/36302/atoms-in-the-universe/
seppel 8y ago
> It has mass, so it by definition interacts with anything with mass? Yeah, but the only interaction is gravity. And nothing that clumps these particles together (such as the electro-magnetic force for ordinary matter). > Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? The additional mass can be measured, this is why we suspect the particles are there in the first place :)
- beefield 8y ago
  > The additional mass can be measured, this is why we suspect the particles are there in the first place :) Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?
  - mikekchar 8y ago
    The thing you are missing is that the "missing mass" isn't observable on the scale of the solar system. It's missing on the scale of large scale galactic structures -- i.e. lots and lots of galaxies. When we look at those structures, we can't figure out how they got that way unless there is a whole bunch more mass than there looks to be. Apart from that, we know nothing. People speculate that maybe there is some weird particle that we can't see in any other way than through looking at the mass of these large (really, really, really ridiculously large) structures. But is there any of it in the solar system? In our galaxy? Nobody knows. Does it hang out between galaxies? Between galaxy clusters? Does it even really exist? Nobody knows. You're just jumping way, way, way too far down the way ;-) Literally, dark matter could be anything -- even a misunderstanding about how the universe works. That's what's interesting about it.
    - beefield 8y ago
      Okay, let's try to reformulate. What is the average density of dark matter in places we know there is dark matter? Then, assuming here were dark matter with that density in earth, how much that would be? I am just trying trying to get my head around how sparse the dark matter actually is, wherever it actually exists. Would we have one kilogram of dark matter to observe? Damn. Should have googled in the first place. Here is an estimate of dark matter density in solar system: http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... 6x10^-28 kg/cm3 Volume of earth is around 10^27 cm3 so that makes the dark matter mass within earth around 600 grams. Admittably that is a bit difficult to measure...
ozmbie 8y ago
Dark matter’s interaction with “regular” matter via gravity is covered in the article. It also interacts with light via gravity. Measuring the interactions between galaxies, and things like gravitational lensing are providing evidence of dark matter’s existence.
InclinedPlane 8y ago
We've already detected on type of dark matter: neutrinos. A single neutrino can travel through a light-year of solid lead (if such a thing existed) and only have a 50% chance of interacting. Every day something like 10^20 neutrinos pass through your body (most from the Sun) without doing a thing. So weakly interacting particles are not completely unknown, we already have examples of them. Dark matter is just some particle we haven't discovered yet (again unsurprising because we know our theory of particle physics is incomplete) which is even less interacting and possibly more massive than neutrinos.
EamonnMR 8y ago
You need to think about what types of interactions we normally perceive-they're all dependant on the electromagnetic force. DM does not appear to absorb or emit light, so it stands to reason that it also won't repel electromagnetically (ie touch things.)
InclinedPlane 8y ago
We can detect dark matter via its mass, that's why we know it exists. We can see it speeding up galactic rotations and acting to gravitationally lens more distant galaxies and so forth. But we have yet to detect it on the small scale, especially at the individual particle level. Keep in mind that one of the key differences between dark matter and atomic matter within a galaxy, for example, is that dark matter has a nearly uniform density over very large volumes (many light years across) whereas atomic matter has enormous density variations, with huge expanses of near vacuum punctuated by ultra dense stars, planets, neutron stars, etc. Within our own Solar System the amount of dark matter inside a spherical volume that would extend out to Neptune's orbit is only as much as a comparatively small asteroid. As you scale out to larger and larger scales the fact that the density of dark matter is relentless causes the mass enclosed inside a volume to start to match (and ultimately exceed at the largest scales) the mass of stars, nebulae, planets, etc.

analog31 8y ago

As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.

beefield 8y ago
But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth due to dark matter is calculated in e.g. kilograms. That's why I would like to know the expected density of the dark matter.
- acqq 8y ago
  Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! No "clumping" even around e.g. Sun can be observed. The "hanging around" is on the level of the whole galaxies, and sometimes the dark matter even remains outside of the whole galaxies, being too slow to follow their gravitational interaction(!) That's the famous example of the "bullet cluster": https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/ https://www.forbes.com/sites/startswithabang/2017/11/09/the-... That's how weakly the dark matter interacts with anything else. And it obviously doesn't even interact strong enough to "fall" to the center of the galaxy. Otherwise it would be there, but it remains to the outside of even where the "normal" matter is (mostly the stars and black holes, the "supermassive black hole" in the center is only at most 1e-5 of the estimated total mass of our Galaxy).
  - acqq 8y ago
    Or more precise: https://medium.com/starts-with-a-bang/ask-ethan-if-dark-matter-is-everywhere-why-havent-we-detected-it-in-our-solar-system-67ca11f94b1f https://medium.com/starts-with-a-bang/ask-ethan-if-dark-matt... There’s “just a little more” of the dark matter in our Solar system but way below the levels it could affect the measurements we can perform.
  - sago 8y ago
    > Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! Would I be right in thinking that also puts severe limits on how much it interacts with itself? Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly?
    - acqq 8y ago
      > Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly? Now I have a little of Newton for you: look at our Solar system: you see the planets, and even more interesting, all the small asteroids circling around the Sun? Can you answer why don't they all fall to the Sun, but move in the orbits? The way the gravitation works was not "intuitive" before Newton, 300 years ago, and now it's obviously still so for many non-professional readers. What's actually happening, according to the dark matter model, and the dark matter actually more easily fits much more of our cosmological observations than anything else, is that there is a lot of dark matter but it is simply much more "spread" around the volume of the galaxies. And just like all the visible stuff of the whole galaxy doesn't fall to the galaxy center (like the planets don't fall to the Sun!), the dark matter remains "around" the galaxies, where more of dark matter is "outside" (as in "in the outer regions of it") than in the "inside" of the galaxy (and in the case of the "Bullet Cluster", that I've mentioned in some other comment, dark matter is obviously lagging all the movement of non-dark matter! (1)). That dark matter that is in the inside of the galaxies actually initially "clumped" somewhat, but that "somewhat" is, according to our estimates, significantly below what we are able to measure, when we're interested in the gravitational effect on the Solar system. 1) https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bullet_cluster.jpg https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul... and https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0657_scale.jpg https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...
- siweiz 8y ago
  They are not distributed evenly throughout space. So if you measure more gravity in a region of space that you cannot see (because dark matter doesn’t interact with em), is it dark matter, or an error in your theory of gravity?
- analog31 8y ago
  Indeed the mass of dark matter is why we think there is dark matter -- discrepancies such as you mention, but evident from astronomical observations. The problem for characterizing the particles is: Mass and what else? The what else is the thing people are trying to detect.
- BurningFrog 8y ago
  We don't have that good idea of what the earth consists of. I think much of those theories are based on counting backwards from knowing the mass, and if we have 5% dark mass orbiting Earth, it wouldn't make any material difference.
acqq 8y ago
"Think of the amount of mass required to generate a gravitational pressure needed to overcome the electromagnetic binding force between molecules inside the mass--the equilibrium occurs, basically, when an object in space becomes spherical. This happens at about 10^20 - 10^21kg. Divided by the mass of a proton implies you need about 10^47 atoms to generate the amount of gravitational pressure to break the electromagnetic strength between atoms." (1) Richard Feynman would tell you that this immense difference is "what's keeping you from not falling through the floor down to the Earth center." Or why the apple hanging on the "few atoms" of its stalk is not falling from the tree, when the summary gravitation of all atoms of the whole Earth are pulling it down. Or, again to compare such big numbers, there are "only" 10^86 atoms in the whole Universe observable to us! (2) 1) https://www.physicsforums.com/threads/how-are-the-gravitational-and-electric-force-comparable.660195/ https://www.physicsforums.com/threads/how-are-the-gravitatio... 2) https://www.universetoday.com/36302/atoms-in-the-universe/ https://www.universetoday.com/36302/atoms-in-the-universe/

seppel 8y ago

> It has mass, so it by definition interacts with anything with mass? Yeah, but the only interaction is gravity. And nothing that clumps these particles together (such as the electro-magnetic force for ordinary matter). > Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? The additional mass can be measured, this is why we suspect the particles are there in the first place :)

beefield 8y ago
> The additional mass can be measured, this is why we suspect the particles are there in the first place :) Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?
- mikekchar 8y ago
  The thing you are missing is that the "missing mass" isn't observable on the scale of the solar system. It's missing on the scale of large scale galactic structures -- i.e. lots and lots of galaxies. When we look at those structures, we can't figure out how they got that way unless there is a whole bunch more mass than there looks to be. Apart from that, we know nothing. People speculate that maybe there is some weird particle that we can't see in any other way than through looking at the mass of these large (really, really, really ridiculously large) structures. But is there any of it in the solar system? In our galaxy? Nobody knows. Does it hang out between galaxies? Between galaxy clusters? Does it even really exist? Nobody knows. You're just jumping way, way, way too far down the way ;-) Literally, dark matter could be anything -- even a misunderstanding about how the universe works. That's what's interesting about it.
  - beefield 8y ago
    Okay, let's try to reformulate. What is the average density of dark matter in places we know there is dark matter? Then, assuming here were dark matter with that density in earth, how much that would be? I am just trying trying to get my head around how sparse the dark matter actually is, wherever it actually exists. Would we have one kilogram of dark matter to observe? Damn. Should have googled in the first place. Here is an estimate of dark matter density in solar system: http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... 6x10^-28 kg/cm3 Volume of earth is around 10^27 cm3 so that makes the dark matter mass within earth around 600 grams. Admittably that is a bit difficult to measure...

ozmbie 8y ago

Dark matter’s interaction with “regular” matter via gravity is covered in the article. It also interacts with light via gravity. Measuring the interactions between galaxies, and things like gravitational lensing are providing evidence of dark matter’s existence.

InclinedPlane 8y ago

We've already detected on type of dark matter: neutrinos. A single neutrino can travel through a light-year of solid lead (if such a thing existed) and only have a 50% chance of interacting. Every day something like 10^20 neutrinos pass through your body (most from the Sun) without doing a thing. So weakly interacting particles are not completely unknown, we already have examples of them. Dark matter is just some particle we haven't discovered yet (again unsurprising because we know our theory of particle physics is incomplete) which is even less interacting and possibly more massive than neutrinos.

EamonnMR 8y ago

You need to think about what types of interactions we normally perceive-they're all dependant on the electromagnetic force. DM does not appear to absorb or emit light, so it stands to reason that it also won't repel electromagnetically (ie touch things.)

InclinedPlane 8y ago

We can detect dark matter via its mass, that's why we know it exists. We can see it speeding up galactic rotations and acting to gravitationally lens more distant galaxies and so forth. But we have yet to detect it on the small scale, especially at the individual particle level. Keep in mind that one of the key differences between dark matter and atomic matter within a galaxy, for example, is that dark matter has a nearly uniform density over very large volumes (many light years across) whereas atomic matter has enormous density variations, with huge expanses of near vacuum punctuated by ultra dense stars, planets, neutron stars, etc. Within our own Solar System the amount of dark matter inside a spherical volume that would extend out to Neptune's orbit is only as much as a comparatively small asteroid. As you scale out to larger and larger scales the fact that the density of dark matter is relentless causes the mass enclosed inside a volume to start to match (and ultimately exceed at the largest scales) the mass of stars, nebulae, planets, etc.