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Dark matter in particular strikes me as… “yes, obviously”. There’s about a dozen quantum fields corresponding to particles. These form a graph, which is by no

Dark matter in particular strikes me as… “yes, obviously”. There’s about a dozen quantum fields corresponding to particles. These form a graph, which is by no means fully connected; the fields each interact with a subset of each other, and neutrinos in particular only interact with gravity and the weak force. If the connections are in some sense random, then it should come as no surprise whatsoever that the graph has disconnected subsets. In fact dark matter theory is effectively stating that the subset we’re a part of is one of many, which also agrees with the copernican principle.

antonvs 2y ago
That's all essentially true, but it doesn't necessarily mean that there's a quantum field that matches the properties needed to explain the phenomena dark matter is supposed to explain. It just means that it's plausible, and wouldn't be surprising, if there were such a field.
- MichaelZuo 2y ago
  How are they so beyond direct detection if they similarly permeate the entire universe?
  - antonvs 2y ago
    Every quantum field only interacts ("couples with") a subset of other quantum fields. Undetectable quantum fields would be ones that don't couple with any fields we interact with. We already see something like this with the neutrino: they only interact via the weak force and gravity, so we need massive detectors measured in kilometres and buried underground to detect them.[1,2] It's estimated that about 100 trillion neutrinos pass through your body every second, traveling close to the speed of light. Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. (Even particles that interact only via the weak force and gravity are candidates, but it's generally believed they'd have to be more massive than neutrinos.) There's also the possibility that, if it turns out that gravity is an emergent property arising from quantum interactions or something along those lines, that there could be fields that don't participate in the gravitational interaction. But that's highly speculative territory. [1] Ice Cube neutrino detector: https://icecube.wisc.edu/science/icecube/ https://icecube.wisc.edu/science/icecube/ [2] Super Kamiokande neutrino detector: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/ https://www-sk.icrr.u-tokyo.ac.jp/en/sk/
    - ninkendo 2y ago
      > Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. I think even calling it a particle is presupposing things we don't observe. All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe. This could be due to a particle of a field that only interacts via gravity… or it could be that there are things other than particles that bend space-time. (Like maybe there’s an entirely separate dimension of space we can’t see, but its gravity affects ours…) Or it could be that spacetime warpage propagates in different ways than we thought (“gravity is different at large distances” etc), or it could be something else entirely we haven’t even thought of. Quantum mechanics gives us a mental framework (fields and their corresponding particles) which has been extraordinarily useful for understanding the visible universe so far, but I think it’s important to remember that it’s just a model. It isn’t reality itself.
      - antonvs 2y ago
        > I think even calling it a particle is presupposing things we don't observe. Right, that's why I said it's a possible candidate for dark matter. Your philosophical point about quantum field theory isn't really relevant here. The point is that, as the original commenter pointed out, we observe evidence of the presence of a set of quantum fields that act and interact in very well-defined ways. Given the nature of those interactions in the fields we can observe, it shouldn't be surprising if there are also other fields we can't observe - all it takes is a different set of coupling parameters. And we already see examples of this - we're virtually blind to interactions besides electromagnetism and gravity, and have to build enormous multi-billion dollar machines to explore other such interactions. No-one is saying that this must be the explanation for dark matter, or that there must be other invisible fields. It's simply a natural prediction that arises from a successful theory.
        ninkendo 2y ago
        I agree no one seems to be saying that dark matter must be part of a quantum field, but my point is more to lament the overgeneralizing of “quantum fields” as a concept to explain unobserved things. I’m absolutely a layman here, so apologies if any of this is misguided or flat out wrong… but its something that’s always gnawed at me about the constant tendency to “quantize” everything including gravity and dark matter… I’ll try my best to articulate: Fields and particles are a mathematical construct we use to describe our observations. Quantum field theory seems to be a consistent mathematical framework you can use for particle interaction once you’ve observed the properties of the particles and how strongly force is carried, etc. But each field has to have its parameters set via observation; we don’t have a good reason why particle strengths are what they are (the fine structure constant doesn’t have an understood “cause”, for example.) But my problem with this is that it can be so general that you can postulate a new field every time you don’t understand why something works a certain way. Like, we have the em field with photons, the weak field with neutrinos, the strong field with gluons… so dark matter? Must be another field with some new particle. Gravity? Must be another field with its own graviton or something… It’s like string theory’s 10 dimensions… I certainly don’t have the expertise to understand string theory or why the extra dimensions are required, but something rubs me the wrong way if you can just postulate another dimension every time the theory is having trouble describing reality. I’m not saying there aren’t multiple fields or that QFT is wrong, but I will say that QFT is a model we have that usefully describes parts of reality, and we shouldn’t confuse the model with reality itself. Some subset of the phenomena we observe can be described and predicted by it, but there are many phenomena that are not predicted by it (gravity is the big one.) I’m not convinced that the fix is to just narrow down the parameters of a graviton, but instead to accept that we simply don’t know if QFT and the standard model can ever be a truly unified theory of everything or if it’s just a useful tool for a few categories of phenomena. It’s possible that we simply don’t have (and might never have) a better theory. So when we have a problem like dark matter, to say “maybe it’s a particle in its own field” can feel a bit like physicists going back to the same old well again.
deleted 2y ago
[deleted]
adrian_b 2y ago
However the connections are not at all random, but they observe certain strict symmetries. For instance, in each of the 3 "generations" of quarks and leptons, separately for particles and for antiparticles, all the different kinds of charges and spin sum to zero, e.g. for the 8 particles that are the 3 kinds of u quarks + the 3 kinds of d quarks + the electron + the electronic neutrino. Moreover, in a 3-dimensional space of the "color" charges and electric charge, the 8 particles and 8 antiparticles of a "generation" are located in all the corners of 2 cubes, not in arbitrary positions. So the set of elementary particles that we know is complete, there are no random locations where there could be extra particles. Any so-called "dark matter", if it would exist, would have to be something completely different and not related in any way with the known elementary particles.
- mikhailfranco 2y ago
  Not true, right-handed neutrinos are a dark matter candidate. They don't experience the weak force, so would only interact through gravity. See recent work by Neil Turok. This podcast is one of the best popular accounts of his work: The (Simple) Theory That Explains Everything https://www.youtube.com/watch?v=ZUp9x44N3uE https://www.youtube.com/watch?v=ZUp9x44N3uE
throwawaymaths 2y ago
> Dark matter in particular strikes me as… “yes, obviously”. Why? Historically there have been two "dark matter" theories prior to this one (we speculated non-visible mass in a place to explain motion of celestial bodies). One turned out to be neptune. The other was vulcan, which turned out to be general relativity. So historically, the inclination to invoke some form of "dark" matter is batting 50/50. im not sure i would put 50/50 into "obvious" territory
- Keysh 2y ago
  The "batting average" is a bit higher than that. For example, measurements of the proper motion (motion across the sky) of Sirius led to the prediction in 1844 that it was in an orbit with an (observed) faint or dark companion; the latter (the white dwarf Sirius B) was not directly observed until 1862, when better telescopes were available. One could also argue that detections of planets from spectroscopic observations of stars is another example. The first observations of transiting exoplanets -- where the planet blocks some of the light of the star -- were actually cases where the existence of the planet had been previously inferred from Doppler shifting of the parent star (e.g., https://en.wikipedia.org/wiki/HD_209458_b https://en.wikipedia.org/wiki/HD_209458_b). As another example, the first evidence for dark matter came from observations in the 1930s of the Doppler shifts of galaxies in galaxy clusters, which suggested much more mass in the clusters than could be explained by the masses of the individual galaxies. Some of this "missing mass" was actually observed in the 1960s and 1970s, when orbiting X-ray telescopes showed X-ray emission from very hot, dilute gas within the clusters (unobservable from the ground because the Earth's atmosphere blocks X-rays). It turns out that the hot, X-ray-emitting gas has about five times the mass of the (stars in) the individual galaxies. So some of the missing mass has been found -- though you still need significant, as-yet-undetected extra mass in clusters to explain why they haven't flown apart long ago.
  - throwawaymaths 2y ago
    Good examples.
euroderf 2y ago
Waaaiiit a minute... Neutrinos interact with gravity ? How did anyone manage to prove THAT ?
- itishappy 2y ago
  They have energy and exist in our universe. GR does all the heavy lifting.
- scotty79 2y ago
  I think one way to prove that would be to observe electromagnetic activity of the sun and neutrinos originating from it. If you manage to find correlation that looks like neutrinos and photons arrive at roughly the same time it would indicate that neutrinos are affected by gravity. I think it was done but I'm not sure.
Ekaros 2y ago
Could there then be some particle that would interact electromagnetically, but not via gravity? And should we not be able to observe this?
- fartsucker69 2y ago
  the current thinking is no because quantum fields still act within spacetime, which is curved by gravity. there could be no particle that exists outside of this, so every particle has to interact gravitationally. but obviously, this is a concept from general relativity which is currently not compatible with quantum field theory at all. for the purposes of QFT every particle exists within some magical separate space where all those considerations are ignored because nobody really knows how to incorporate them without breaking predictions.
- gpderetta 2y ago
  I don't think it could be possible. IANAP, but as far as I understand, anything that contributes to the stress-energy tensor "gravitates". So electromagnetic interaction per-se already "gravitates".

antonvs 2y ago

That's all essentially true, but it doesn't necessarily mean that there's a quantum field that matches the properties needed to explain the phenomena dark matter is supposed to explain. It just means that it's plausible, and wouldn't be surprising, if there were such a field.

MichaelZuo 2y ago
How are they so beyond direct detection if they similarly permeate the entire universe?
- antonvs 2y ago
  Every quantum field only interacts ("couples with") a subset of other quantum fields. Undetectable quantum fields would be ones that don't couple with any fields we interact with. We already see something like this with the neutrino: they only interact via the weak force and gravity, so we need massive detectors measured in kilometres and buried underground to detect them.[1,2] It's estimated that about 100 trillion neutrinos pass through your body every second, traveling close to the speed of light. Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. (Even particles that interact only via the weak force and gravity are candidates, but it's generally believed they'd have to be more massive than neutrinos.) There's also the possibility that, if it turns out that gravity is an emergent property arising from quantum interactions or something along those lines, that there could be fields that don't participate in the gravitational interaction. But that's highly speculative territory. [1] Ice Cube neutrino detector: https://icecube.wisc.edu/science/icecube/ https://icecube.wisc.edu/science/icecube/ [2] Super Kamiokande neutrino detector: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/ https://www-sk.icrr.u-tokyo.ac.jp/en/sk/
  - ninkendo 2y ago
    > Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. I think even calling it a particle is presupposing things we don't observe. All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe. This could be due to a particle of a field that only interacts via gravity… or it could be that there are things other than particles that bend space-time. (Like maybe there’s an entirely separate dimension of space we can’t see, but its gravity affects ours…) Or it could be that spacetime warpage propagates in different ways than we thought (“gravity is different at large distances” etc), or it could be something else entirely we haven’t even thought of. Quantum mechanics gives us a mental framework (fields and their corresponding particles) which has been extraordinarily useful for understanding the visible universe so far, but I think it’s important to remember that it’s just a model. It isn’t reality itself.
    - antonvs 2y ago
      > I think even calling it a particle is presupposing things we don't observe. Right, that's why I said it's a possible candidate for dark matter. Your philosophical point about quantum field theory isn't really relevant here. The point is that, as the original commenter pointed out, we observe evidence of the presence of a set of quantum fields that act and interact in very well-defined ways. Given the nature of those interactions in the fields we can observe, it shouldn't be surprising if there are also other fields we can't observe - all it takes is a different set of coupling parameters. And we already see examples of this - we're virtually blind to interactions besides electromagnetism and gravity, and have to build enormous multi-billion dollar machines to explore other such interactions. No-one is saying that this must be the explanation for dark matter, or that there must be other invisible fields. It's simply a natural prediction that arises from a successful theory.
      - ninkendo 2y ago
        I agree no one seems to be saying that dark matter must be part of a quantum field, but my point is more to lament the overgeneralizing of “quantum fields” as a concept to explain unobserved things. I’m absolutely a layman here, so apologies if any of this is misguided or flat out wrong… but its something that’s always gnawed at me about the constant tendency to “quantize” everything including gravity and dark matter… I’ll try my best to articulate: Fields and particles are a mathematical construct we use to describe our observations. Quantum field theory seems to be a consistent mathematical framework you can use for particle interaction once you’ve observed the properties of the particles and how strongly force is carried, etc. But each field has to have its parameters set via observation; we don’t have a good reason why particle strengths are what they are (the fine structure constant doesn’t have an understood “cause”, for example.) But my problem with this is that it can be so general that you can postulate a new field every time you don’t understand why something works a certain way. Like, we have the em field with photons, the weak field with neutrinos, the strong field with gluons… so dark matter? Must be another field with some new particle. Gravity? Must be another field with its own graviton or something… It’s like string theory’s 10 dimensions… I certainly don’t have the expertise to understand string theory or why the extra dimensions are required, but something rubs me the wrong way if you can just postulate another dimension every time the theory is having trouble describing reality. I’m not saying there aren’t multiple fields or that QFT is wrong, but I will say that QFT is a model we have that usefully describes parts of reality, and we shouldn’t confuse the model with reality itself. Some subset of the phenomena we observe can be described and predicted by it, but there are many phenomena that are not predicted by it (gravity is the big one.) I’m not convinced that the fix is to just narrow down the parameters of a graviton, but instead to accept that we simply don’t know if QFT and the standard model can ever be a truly unified theory of everything or if it’s just a useful tool for a few categories of phenomena. It’s possible that we simply don’t have (and might never have) a better theory. So when we have a problem like dark matter, to say “maybe it’s a particle in its own field” can feel a bit like physicists going back to the same old well again.

deleted 2y ago

[deleted]

adrian_b 2y ago

However the connections are not at all random, but they observe certain strict symmetries. For instance, in each of the 3 "generations" of quarks and leptons, separately for particles and for antiparticles, all the different kinds of charges and spin sum to zero, e.g. for the 8 particles that are the 3 kinds of u quarks + the 3 kinds of d quarks + the electron + the electronic neutrino. Moreover, in a 3-dimensional space of the "color" charges and electric charge, the 8 particles and 8 antiparticles of a "generation" are located in all the corners of 2 cubes, not in arbitrary positions. So the set of elementary particles that we know is complete, there are no random locations where there could be extra particles. Any so-called "dark matter", if it would exist, would have to be something completely different and not related in any way with the known elementary particles.

mikhailfranco 2y ago
Not true, right-handed neutrinos are a dark matter candidate. They don't experience the weak force, so would only interact through gravity. See recent work by Neil Turok. This podcast is one of the best popular accounts of his work: The (Simple) Theory That Explains Everything https://www.youtube.com/watch?v=ZUp9x44N3uE https://www.youtube.com/watch?v=ZUp9x44N3uE

throwawaymaths 2y ago

> Dark matter in particular strikes me as… “yes, obviously”. Why? Historically there have been two "dark matter" theories prior to this one (we speculated non-visible mass in a place to explain motion of celestial bodies). One turned out to be neptune. The other was vulcan, which turned out to be general relativity. So historically, the inclination to invoke some form of "dark" matter is batting 50/50. im not sure i would put 50/50 into "obvious" territory

Keysh 2y ago
The "batting average" is a bit higher than that. For example, measurements of the proper motion (motion across the sky) of Sirius led to the prediction in 1844 that it was in an orbit with an (observed) faint or dark companion; the latter (the white dwarf Sirius B) was not directly observed until 1862, when better telescopes were available. One could also argue that detections of planets from spectroscopic observations of stars is another example. The first observations of transiting exoplanets -- where the planet blocks some of the light of the star -- were actually cases where the existence of the planet had been previously inferred from Doppler shifting of the parent star (e.g., https://en.wikipedia.org/wiki/HD_209458_b https://en.wikipedia.org/wiki/HD_209458_b). As another example, the first evidence for dark matter came from observations in the 1930s of the Doppler shifts of galaxies in galaxy clusters, which suggested much more mass in the clusters than could be explained by the masses of the individual galaxies. Some of this "missing mass" was actually observed in the 1960s and 1970s, when orbiting X-ray telescopes showed X-ray emission from very hot, dilute gas within the clusters (unobservable from the ground because the Earth's atmosphere blocks X-rays). It turns out that the hot, X-ray-emitting gas has about five times the mass of the (stars in) the individual galaxies. So some of the missing mass has been found -- though you still need significant, as-yet-undetected extra mass in clusters to explain why they haven't flown apart long ago.
- throwawaymaths 2y ago
  Good examples.

euroderf 2y ago

Waaaiiit a minute... Neutrinos interact with gravity ? How did anyone manage to prove THAT ?

itishappy 2y ago
They have energy and exist in our universe. GR does all the heavy lifting.
scotty79 2y ago
I think one way to prove that would be to observe electromagnetic activity of the sun and neutrinos originating from it. If you manage to find correlation that looks like neutrinos and photons arrive at roughly the same time it would indicate that neutrinos are affected by gravity. I think it was done but I'm not sure.

Ekaros 2y ago

Could there then be some particle that would interact electromagnetically, but not via gravity? And should we not be able to observe this?

fartsucker69 2y ago
the current thinking is no because quantum fields still act within spacetime, which is curved by gravity. there could be no particle that exists outside of this, so every particle has to interact gravitationally. but obviously, this is a concept from general relativity which is currently not compatible with quantum field theory at all. for the purposes of QFT every particle exists within some magical separate space where all those considerations are ignored because nobody really knows how to incorporate them without breaking predictions.
gpderetta 2y ago
I don't think it could be possible. IANAP, but as far as I understand, anything that contributes to the stress-energy tensor "gravitates". So electromagnetic interaction per-se already "gravitates".