3 ms·

> 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 w

> 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.
  - antonvs 2y ago
    You may be assigning too much meaning to the term "field". All that a field is in physics is a physical quantity that has a value at every point in spacetime. So if we observe an electron, and we believe that electrons can appear anywhere in spacetime, then we define a field with the relevant properties, as an abstraction that allows us to model electron activity in spacetime. Aside from the mathematical details of the representation, that's all there is to it. (Clarification: "anywhere in spacetime" can actually be a subset of spacetime, e.g. many particles could not have appeared in the early universe before the electroweak phase transition. We say the universe then had a different vacuum structure, which is equivalent to saying it had a different set of fields.) This seems to me to negate your objection about "so general that...", at least for particles we observe. We define the fields to match the observed properties, and that's our model of that type of particle. The second issue I see with what you're saying seems to be a kind of conflation of established theories supported by evidence, with theoretical developments in progress. Many scientists would agree that gravitons may not be the right approach - particularly those working on alternate theories, such as emergent gravity, AdS/CFT correspondence, loop quantum gravity, string theory, etc. (The full list is much longer!) Those people working on the theories of quantum gravity are of course going to talk about gravitons, but they can't claim we know gravitons exist because we can't observe them and there isn't even a complete and consistent theory that describes them. That's still being worked on. But it's certainly an obvious avenue of research. The same thing goes for what you said about the "constant tendency to quantize everything". For quantum objects we observe, we observe that they're quantum so there shouldn't be any controversy there. For possible objects we haven't yet observed, like dark matter or gravitons, exploring the possibility that they're quantum just makes sense, if the behavior of what we're looking for is consistent with that. It doesn't prevent research on other possibilities. > 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. If we're looking for missing mass, there's already a whole theory of how mass works, which is QFT. The theory predicts that anything with mass must be a quantum particle. Of course the theory could be incomplete, but it wouldn't make sense not to explore the possibility that other massive particles could explain our observations. Besides, everything in the physical universe we've ever observed fits under either QFT or GR. As far as we know, that's how the universe works. It's natural to explore new phenomena from that perspective. > (the fine structure constant doesn’t have an understood “cause”, for example.) Funnily enough what was being discussed in this subthread can completely explain this. Imagine a very large, if not infinite number of quantum fields, each coupled to others in all sorts of possible ways. In that case, the fine structure constant we observe can be explained by the weak anthropic principle: somewhere in that large possibility space, there are likely to be fields with properties that can support the existence of observers like us. The idea of "hidden" quantum fields is known as hidden sectors: https://en.wikipedia.org/wiki/Hidden_sector https://en.wikipedia.org/wiki/Hidden_sector . According to the Copernican principle, we should take it seriously. There's no real reason to think that the particular set of quantum fields we're able to interact with are the only ones, just as it turned out we didn't live on the only planet, or in the only solar system, or in the only galaxy. (Although, the nature of gravity's interaction with quantum objects could constrain the possibilities here.)
drdeca 2y ago
> All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe I think we can say more than that. For observations to be compatible with the hypothesis that dark matter is particles or fields that interact gravitationally but not electromagnetically or strongly enough with itself to cause too much friction with itself, It doesn’t just need to be the case that “if there was such matter at these locations at this time, then the gravitational effects on what we do see would be like what we see”. There is an additional major constraint: the dynamics of the dark matter particles/field(s) have to work with the distribution we see. Like, if we have two regions which each have some normal matter and some dark matter, and they ran into one another, the dark matter shouldn’t be slowed by the friction in the way the visible matter is, Etc. etc. Like, in order for the observations to be compatible with the hypothesis that dark matter particles are the thing, the distribution of “where the dark matter would have to be in order to explain the gravitational effects” would have to be a distribution that would make sense for it to be in, under the assumption that it is dark matter particles.

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.
- antonvs 2y ago
  You may be assigning too much meaning to the term "field". All that a field is in physics is a physical quantity that has a value at every point in spacetime. So if we observe an electron, and we believe that electrons can appear anywhere in spacetime, then we define a field with the relevant properties, as an abstraction that allows us to model electron activity in spacetime. Aside from the mathematical details of the representation, that's all there is to it. (Clarification: "anywhere in spacetime" can actually be a subset of spacetime, e.g. many particles could not have appeared in the early universe before the electroweak phase transition. We say the universe then had a different vacuum structure, which is equivalent to saying it had a different set of fields.) This seems to me to negate your objection about "so general that...", at least for particles we observe. We define the fields to match the observed properties, and that's our model of that type of particle. The second issue I see with what you're saying seems to be a kind of conflation of established theories supported by evidence, with theoretical developments in progress. Many scientists would agree that gravitons may not be the right approach - particularly those working on alternate theories, such as emergent gravity, AdS/CFT correspondence, loop quantum gravity, string theory, etc. (The full list is much longer!) Those people working on the theories of quantum gravity are of course going to talk about gravitons, but they can't claim we know gravitons exist because we can't observe them and there isn't even a complete and consistent theory that describes them. That's still being worked on. But it's certainly an obvious avenue of research. The same thing goes for what you said about the "constant tendency to quantize everything". For quantum objects we observe, we observe that they're quantum so there shouldn't be any controversy there. For possible objects we haven't yet observed, like dark matter or gravitons, exploring the possibility that they're quantum just makes sense, if the behavior of what we're looking for is consistent with that. It doesn't prevent research on other possibilities. > 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. If we're looking for missing mass, there's already a whole theory of how mass works, which is QFT. The theory predicts that anything with mass must be a quantum particle. Of course the theory could be incomplete, but it wouldn't make sense not to explore the possibility that other massive particles could explain our observations. Besides, everything in the physical universe we've ever observed fits under either QFT or GR. As far as we know, that's how the universe works. It's natural to explore new phenomena from that perspective. > (the fine structure constant doesn’t have an understood “cause”, for example.) Funnily enough what was being discussed in this subthread can completely explain this. Imagine a very large, if not infinite number of quantum fields, each coupled to others in all sorts of possible ways. In that case, the fine structure constant we observe can be explained by the weak anthropic principle: somewhere in that large possibility space, there are likely to be fields with properties that can support the existence of observers like us. The idea of "hidden" quantum fields is known as hidden sectors: https://en.wikipedia.org/wiki/Hidden_sector https://en.wikipedia.org/wiki/Hidden_sector . According to the Copernican principle, we should take it seriously. There's no real reason to think that the particular set of quantum fields we're able to interact with are the only ones, just as it turned out we didn't live on the only planet, or in the only solar system, or in the only galaxy. (Although, the nature of gravity's interaction with quantum objects could constrain the possibilities here.)

drdeca 2y ago

> All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe I think we can say more than that. For observations to be compatible with the hypothesis that dark matter is particles or fields that interact gravitationally but not electromagnetically or strongly enough with itself to cause too much friction with itself, It doesn’t just need to be the case that “if there was such matter at these locations at this time, then the gravitational effects on what we do see would be like what we see”. There is an additional major constraint: the dynamics of the dark matter particles/field(s) have to work with the distribution we see. Like, if we have two regions which each have some normal matter and some dark matter, and they ran into one another, the dark matter shouldn’t be slowed by the friction in the way the visible matter is, Etc. etc. Like, in order for the observations to be compatible with the hypothesis that dark matter particles are the thing, the distribution of “where the dark matter would have to be in order to explain the gravitational effects” would have to be a distribution that would make sense for it to be in, under the assumption that it is dark matter particles.