6 ms·

> 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 b

> 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...
  - mikekchar 8y ago
    But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind boggingly massive structures.
    - saagarjha 8y ago
      I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?
      - pas 8y ago
        DM usually forms a halo as it orbits the center of mass of whatever it orbits (galaxy, globular cluster), and because it doesn't interact otherwise, it cannot slow down, cannot shed momentum, so it is likely mostly not a disk, but a big sphere, and probably a shell around galaxies.
        ianai 8y ago
        That sounds a little like an additional dimension or alternative something truly “out there”. Ie maybe gravity’s force relies on some process that reverses at the macro scale.
    - MauranKilom 8y ago
      I mean, If there were appreciable amounts of it (i.e. not on the scale of kilograms in or around the earth), so that it could have a measurable impact on planet orbits, we could gain information on its distribution within (or interaction with) the solar system. That's why this density estimate is interesting - it sort of determines what the smallest scale is at which we'd have a chance of observing interactions/structure with dark matter.
- ianai 8y ago
  Could it be antimatter? Ie are we sure through direct observation that antimatter reacts with light the same way matter does? I know antimatter has been made in laboratories, but plenty of phenomena act differently outside of the lab. (I’m pretty sure we’re sure dark matter isn’t antimatter, but just the same, worth the thought when 5/6th of the universe is unexplainable.)
  - daeken 8y ago
    I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter. (Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use it for practical things every day. For instance, PET scanners in medicine produce positrons and watch the annihilations inside your body. Astoundingly cool to me.)
    - ianai 8y ago
      Good point! I forgot about PET scans.
  - saagarjha 8y ago
    The problem with antimatter is that it very much interacts with normal matter, quite violently in fact. So if dark matter was antimatter we’d be bathed in a sea of radiation as it continually annihilated with ordinary matter, which would make it quite easy to detect ;)
- mirimir 8y ago
  It's also observable on the scale of galaxies. Evidence for dark matter includes observed inconsistency between galactic rotation curver and total mass estimates. But yes, some of the earliest evidence (decades of it) was unexpected behavior of galactic clusters. Anyway, what we expect to see here is what's typical at our distance from Sagittarius A[star], and distance from the galactic plane. Also, I gotta say that the galactic dark matter distribution reminds me a lot of Vinge's "Slow Zone" ;) A Fire Upon the Deep came out in 1992. I wonder whether he had dark matter in mind. He never used the term, as I recall.
dTal 8y ago
http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Short answer: 10^-18 as much as the mass of the sun, 1 proton-mass per 3 cubic centimeters. Not enough to be detected gravitationally - it just gets swamped.
- ISL 8y ago
  Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. Nevertheless, our gravitational experiments can say things about the properties of dark matter (it generally obeys the Equivalence Principle): https://arxiv.org/abs/1207.2442 https://arxiv.org/abs/1207.2442 Furthermore, for certain classes of ultra-light dark matter, gravitational and spin-coupled searches can have something to say, e.g.: https://arxiv.org/abs/1512.06165 https://arxiv.org/abs/1512.06165 We're trying.
  - thaumasiotes 8y ago
    > locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?
    - dTal 8y ago
      Dark matter doesn't physically interact with itself or regular matter, so it doesn't "clump" the way regular matter does. A particle of dark matter will fall towards the Sun or Earth, but it doesn't stop when it gets there - it just carries right on through, with just as much energy as it had before. We expect there to be a dark matter "wind" passing through the solar system at galactic speeds, so it doesn't stick around. But! There may be seasonal variations in the amount of dark matter reaching Earth due to a solar "lensing" effect. Attempts have been made to find this signal, and an annual modulation has been found, but the debate as to its cause is ongoing. https://physics.stackexchange.com/questions/194107/is-dark-matter-expected-to-be-equally-distributed-in-our-solar-system/194137#194137 https://physics.stackexchange.com/questions/194107/is-dark-m... https://en.wikipedia.org/wiki/DAMA/NaI https://en.wikipedia.org/wiki/DAMA/NaI https://www.quantamagazine.org/trouble-detected-in-infamous-dark-matter-signal-20180412/ https://www.quantamagazine.org/trouble-detected-in-infamous-...
ajross 8y ago
Not within the solar system, but within the stellar neighborhood. If you construct a voronoi volume around the sun, then very roughly you'd expect 5 solar masses of dark matter in that volume. But that is a HUGE volume, on the order of tens of thousands of AU on a side. In the area we can directly observe (via orbits of bodies we can see), the fraction of dark matter comes out to like one part in a trillion or thereabouts.
raattgift 8y ago
Great questions ! > How much [dark matter] [is] in the solar system ? A lot, because the solar system is a huge volume and dark-matter fills it fairly uniformly (there is a small overdensity inside the sun, and the planets will also cause small departures from essential uniformity). However, it's extremely sparse, so there isn't much in any small fraction of the solar system. Compare that with the planets: they are extremely dense, but do not fill more than the tiniest fraction of the whole volume of the solar system. However, even so, even Phobos and Deimos have much more mass than all the dark matter inside Mars's orbit (see a couple paragraphs below). ~ one third of a million proton-masses for every cubic metre ~ 6 * 10^-22 kg for every cubic metre (Earth's density is 524 kg/m^3) ~ 0.65 kg / earth volume http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Of course the total mass scales with volume. Inside Neptune's orbit there is about 10^17 kg of dark matter; that's about ten Phobos-masses, or about the mass of 253 Mathilde (a carbonaceous intermediate-belt asteroid). The volume of the galaxy is enormous, and with dark mater filling all of it roughly uniformly, the mass of all the visible matter is dwarfed -- there is an awful lot of space between star systems. > somehow observable It's not moving anywhere close to relativistically compared to the Earth's surface, and it doesn't feel electromagnetism. If we compare two other neutral particles, we have no practical ability to detect non-relativistic neutrinos (we can only spot a microscopic fraction of relativistic neutrinos from known sources) and have trouble spotting thermal neutrons (again, we generally need a known source that is "loud" with them, and additionally the collision momenta will still be larger than most collisions with solar system dark matter -- neutrons spit out of nuclear reactions are much faster than Earth's orbital motion through the extremely sparse dark matter the inner solar system sweeps through, and neutron beams used experimentally are generally a lot denser than ~ 3 neutron-masses per cubic centimetre).

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...
- mikekchar 8y ago
  But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind boggingly massive structures.
  - saagarjha 8y ago
    I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?
    - pas 8y ago
      DM usually forms a halo as it orbits the center of mass of whatever it orbits (galaxy, globular cluster), and because it doesn't interact otherwise, it cannot slow down, cannot shed momentum, so it is likely mostly not a disk, but a big sphere, and probably a shell around galaxies.
      - ianai 8y ago
        That sounds a little like an additional dimension or alternative something truly “out there”. Ie maybe gravity’s force relies on some process that reverses at the macro scale.
  - MauranKilom 8y ago
    I mean, If there were appreciable amounts of it (i.e. not on the scale of kilograms in or around the earth), so that it could have a measurable impact on planet orbits, we could gain information on its distribution within (or interaction with) the solar system. That's why this density estimate is interesting - it sort of determines what the smallest scale is at which we'd have a chance of observing interactions/structure with dark matter.
ianai 8y ago
Could it be antimatter? Ie are we sure through direct observation that antimatter reacts with light the same way matter does? I know antimatter has been made in laboratories, but plenty of phenomena act differently outside of the lab. (I’m pretty sure we’re sure dark matter isn’t antimatter, but just the same, worth the thought when 5/6th of the universe is unexplainable.)
- daeken 8y ago
  I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter. (Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use it for practical things every day. For instance, PET scanners in medicine produce positrons and watch the annihilations inside your body. Astoundingly cool to me.)
  - ianai 8y ago
    Good point! I forgot about PET scans.
- saagarjha 8y ago
  The problem with antimatter is that it very much interacts with normal matter, quite violently in fact. So if dark matter was antimatter we’d be bathed in a sea of radiation as it continually annihilated with ordinary matter, which would make it quite easy to detect ;)
mirimir 8y ago
It's also observable on the scale of galaxies. Evidence for dark matter includes observed inconsistency between galactic rotation curver and total mass estimates. But yes, some of the earliest evidence (decades of it) was unexpected behavior of galactic clusters. Anyway, what we expect to see here is what's typical at our distance from Sagittarius A[star], and distance from the galactic plane. Also, I gotta say that the galactic dark matter distribution reminds me a lot of Vinge's "Slow Zone" ;) A Fire Upon the Deep came out in 1992. I wonder whether he had dark matter in mind. He never used the term, as I recall.

dTal 8y ago

http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Short answer: 10^-18 as much as the mass of the sun, 1 proton-mass per 3 cubic centimeters. Not enough to be detected gravitationally - it just gets swamped.

ISL 8y ago
Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. Nevertheless, our gravitational experiments can say things about the properties of dark matter (it generally obeys the Equivalence Principle): https://arxiv.org/abs/1207.2442 https://arxiv.org/abs/1207.2442 Furthermore, for certain classes of ultra-light dark matter, gravitational and spin-coupled searches can have something to say, e.g.: https://arxiv.org/abs/1512.06165 https://arxiv.org/abs/1512.06165 We're trying.
- thaumasiotes 8y ago
  > locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?
  - dTal 8y ago
    Dark matter doesn't physically interact with itself or regular matter, so it doesn't "clump" the way regular matter does. A particle of dark matter will fall towards the Sun or Earth, but it doesn't stop when it gets there - it just carries right on through, with just as much energy as it had before. We expect there to be a dark matter "wind" passing through the solar system at galactic speeds, so it doesn't stick around. But! There may be seasonal variations in the amount of dark matter reaching Earth due to a solar "lensing" effect. Attempts have been made to find this signal, and an annual modulation has been found, but the debate as to its cause is ongoing. https://physics.stackexchange.com/questions/194107/is-dark-matter-expected-to-be-equally-distributed-in-our-solar-system/194137#194137 https://physics.stackexchange.com/questions/194107/is-dark-m... https://en.wikipedia.org/wiki/DAMA/NaI https://en.wikipedia.org/wiki/DAMA/NaI https://www.quantamagazine.org/trouble-detected-in-infamous-dark-matter-signal-20180412/ https://www.quantamagazine.org/trouble-detected-in-infamous-...

ajross 8y ago

Not within the solar system, but within the stellar neighborhood. If you construct a voronoi volume around the sun, then very roughly you'd expect 5 solar masses of dark matter in that volume. But that is a HUGE volume, on the order of tens of thousands of AU on a side. In the area we can directly observe (via orbits of bodies we can see), the fraction of dark matter comes out to like one part in a trillion or thereabouts.

raattgift 8y ago

Great questions ! > How much [dark matter] [is] in the solar system ? A lot, because the solar system is a huge volume and dark-matter fills it fairly uniformly (there is a small overdensity inside the sun, and the planets will also cause small departures from essential uniformity). However, it's extremely sparse, so there isn't much in any small fraction of the solar system. Compare that with the planets: they are extremely dense, but do not fill more than the tiniest fraction of the whole volume of the solar system. However, even so, even Phobos and Deimos have much more mass than all the dark matter inside Mars's orbit (see a couple paragraphs below). ~ one third of a million proton-masses for every cubic metre ~ 6 * 10^-22 kg for every cubic metre (Earth's density is 524 kg/m^3) ~ 0.65 kg / earth volume http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Of course the total mass scales with volume. Inside Neptune's orbit there is about 10^17 kg of dark matter; that's about ten Phobos-masses, or about the mass of 253 Mathilde (a carbonaceous intermediate-belt asteroid). The volume of the galaxy is enormous, and with dark mater filling all of it roughly uniformly, the mass of all the visible matter is dwarfed -- there is an awful lot of space between star systems. > somehow observable It's not moving anywhere close to relativistically compared to the Earth's surface, and it doesn't feel electromagnetism. If we compare two other neutral particles, we have no practical ability to detect non-relativistic neutrinos (we can only spot a microscopic fraction of relativistic neutrinos from known sources) and have trouble spotting thermal neutrons (again, we generally need a known source that is "loud" with them, and additionally the collision momenta will still be larger than most collisions with solar system dark matter -- neutrons spit out of nuclear reactions are much faster than Earth's orbital motion through the extremely sparse dark matter the inner solar system sweeps through, and neutron beams used experimentally are generally a lot denser than ~ 3 neutron-masses per cubic centimetre).