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Comment by Filligree | Hacker News Reader

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.

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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.

How are they so beyond direct detection if they similarly permeate the entire universe?

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/

[2] Super Kamiokande neutrino detector: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/

> 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.

> 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.

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.

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 . 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.)

> 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.

Would such a particle not accumulate after a slowdown at a la grange point? its acummulation altering then other properties of that space we could measure?

Such particles would be frictionless and collisionless - they don't interact via the electromagnetic force. As such, they would just sail through a Lagrange point on whatever geodesic they're on.

There's no particular reason to expect them to collect there, except possibly a small subset of particles that has a low enough velocity to actually be gravitationally captured. But that's unlikely if they're coming from some distance from the Lagrange point. The expected quantity of particles that could be captured like this is too low to be measurable.

Keep in mind that much of the nature of what we observe in the solar system is a consequence of friction or collisions having occurred in the past: the flattened disk shape, with all the planets orbiting on a plane, the accumulation of matter into objects like stars, planets, asteroids, and comets, and the parameters of their orbits were all heavily influenced by friction and collisions. Particulate dark matter would experience none of that, so doesn't end up being as organized as ordinary matter.

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.

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

> 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

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).

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.

Good examples.

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

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.

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".

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

They have energy and exist in our universe. GR does all the heavy lifting.

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.

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