[nickdrozd]

Dark matter skeptics believe, in effect, that everything that interacts with gravity also interacts with electromagnetism. They tend to believe this really strongly, but I've never heard anyone give a reason for it. It's a strangely abstract thing to be so dogmatic about.

[dogma1138]

Is that true? This would be the first time I’ve heard that, neutrinos don’t interact really with anything other than gravity and via the weak force… in fact I’m not sure if “interacting” with gravity is even the right way to look at things photons interact with gravity but they don’t have any mass.

So at best their axiom might be that if it has mass it interacts with electromagnetism which we already know not to be be true in all cases.

[ianburrell]

Neutrinos interact with barionic batter through weak nuclear force. Dark matter can't be normal neutrinos because they move at close to light speed and we know that dark matter is cold, slow moving. The best dark matter candidate are WIMPs which only interact with gravity and weak force. Heavier, unknown neutrinos are possibility but would mess up Standard Model.

There are detectors that are trying to find weak force interactions. They haven't found anything yet which is constraining paramters of dark matter particles.

[dogma1138]

I didn’t think neutrinos are a good candidate for dark matter despite being wimps all I said is that this is the first time I heard of that people who don’t accept dark matter think that interaction with ‘gravity’ requires also interaction with electromagnetism despite us having pretty solid examples of things that we know for sure that they exist that do not follow that requirement.

[consilient]

> Is that true?

They're not literally thinking those thoughts in those terms, it's just what pops out when you try to translate complaints about "tangibility" and "seeing things directly" from folk-physical intuition into something physically meaningful.

[dogma1138]

I’m not sure I’m following, are you saying that the likes of Milgrom have difficulties articulating their thoughts?

[consilient]

No, I'm just talking about the local commentariat. Milgrom's reasons for holding out (all the other MOND people are MOND + DM people now; the Bullet Cluster was a death blow for pure MOND) are presumably more idiosyncratic.

[dogma1138]

Still doesn’t align with the original assertion that dark matter “deniers” assert that interaction with gravity requires interaction with EM.

Currently I would group the “dark matter” into 3 camps, dark matter as a new form particles, dark matter as a combination of any sort of existing particles and matter e.g. wimps, primordial blackholes, brown dwarfs, cold baryonic matter etc. and dark matter as modified gravity this is MOND and TEVES et al.

None of which as far as I know make a statement around having to have interaction with EM if you are interacting with gravity.

[zinclozenge]

General relativity links geometry to the stress energy tensor, so even if photons are massless I don’t think calling the effect “interaction” is necessarily wrong.

[dogma1138]

Well gravity isn’t a force or at least not on the same level of other forces which the interaction with is mediated by a force carrying particle.

How gravity works in GR is more like a topological property of space time rather than say like the electromagnetic force which is why I was asking about what exactly they mean by interacting with gravity.

[ninkendo]

Well, I would turn this back and say Dark Matter proponents believe that everything that causes gravity must also be matter, which is a strange thing to be dogmatic about.

The only thing we know is that spacetime is curving without any visible/detectable thing causing it (at least not in a way our equations predict.) Calling it “matter” is already making a huge set of assumptions.

[ncallaway]

> Dark Matter proponents believe that everything that causes gravity must also be matter

I don’t think that’s correct. Most Dark Matter proponents are ardent adherents to general relativity. General Relativity is pretty clear that massless non-matter particles like the photon contribute to the bending of spacetime. So it seems like most Dark Matter proponents would stridently object to the claim that “everything that causes gravity must be matter”.

[ninkendo]

> most Dark Matter proponents would stridently object to the claim that “everything that causes gravity must be matter”

Given that the only thing we’ve observed is a change in the gravitational field (curvature of spacetime), then why call it matter? Why assume that it’s matter in the first place? This is where my criticism comes from. It seems like they’re eager to assume something that affects gravity must be matter, hence why it’s called dark matter.

The fact that the leading candidate for dark matter is a “weakly interacting massive particle” further cements this criticism IMO… it’s something we’ve never observed, but it’s just assumed to exist because it must be some sort of matter, right? Because we can’t think of anything else that bend spacetime…

[ncallaway]

> It seems like they’re eager to…

My understanding of the position of the various physicists that I follow is that they are *not* eager to assume this. It’s just, after quite a bit of investigation, the best explanation that fits the sum total of information that we have the best. I wouldn’t describe any of the cosmologists that I’ve listened to describe their position as “eager” for it to be matter. The ones I’ve listened to, in fact, would be absolutely thrilled if we had evidence that ruled out the “matter” explanation.

> Why assume that it’s matter in the first place?

I… don’t think physics assume it must be matter. It’s the leading theory largely because we’ve spent a decade ruling out various other possibilities and ideas. Dark Matter is what we have left that best fits the sum of available evidence. But, again, every physicist I’ve heard talk about this subject has acknowledged that it’s basically an area with a lot of question marks, and we don’t really know much about it, so we’ll have to keep studying the question to try and learn more.

> Because we can’t think of anything else that bend spacetime…

Again, we can think of lots of things that can bend spacetime that aren’t matter. That’s trivial. But (again, my understanding from physicists I listen to) is that we’ve thought of lots of those things and ruled many of them out (because the observation we have are more than just “bends spacetime”). Dark Matter is what’s left after ruling lots of our ideas out.

Does that mean DM is definitely the cause of the behaviors we see? No! Of course not. It could absolutely be some matter-less cause that we haven’t ruled out yet.

I would throw out their that I don’t feel like you are able to provide a summary of the Dark Matter proponents position that most Dark Matter proponents would agree with (or, at least the ones I’ve listened to; I’m sure there are some proponents who hold the positions you describe, I’m just not familiar with them).

[consilient]

> Calling it “matter” is already making a huge set of assumptions.

Not really. "Matter" is not really a formal term of art, but in practice it usually means "not an elementary boson", or at the strictest "fermionic".

[AnimalMuppet]

Are there any massive elementary bosons that we currently know of?

[magicalhippo]

The W and Z bosons[1] have mass, a fair bit of it actually.

[1]: https://en.wikipedia.org/wiki/W_and_Z_bosons

[andreareina]

I'm not aware of anyone reputable claiming that e.g. photons are matter, or that they don't gravitate.

[Retric]

Dark matter was initially just a fudge factor because planets etc didn’t emit light. That kind of dark matter obviously exists, reusing the term may not actually imply the stuff is matter.

[Ma8ee]

> Dark matter skeptics believe, in effect, that everything that interacts with gravity also interacts with electromagnetism.

I’ve been following this debate at least casually for a long time, and I’ve never heard that argument. Can you present even one example of any respectable scientists arguing that?