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Comment by pchiusano | Hacker News Reader

You might be interested in quantized inertia http://physicsfromtheedge.blogspot.com/ which is a model that does away with the need for dark matter and that has been gaining some interest. For instance, author claims it replicates the success of MOND but also predicts where MOND and GR both fail (see wide binaries posts on the blog).

I 100% agree that when the model doesn’t fit the data, making up some hitherto undetected kind of matter, distributed in just the right way to explain the discrepancies, is kinda odd... the QI author is constantly complaining about this.

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"making up some hitherto undetected kind of matter, distributed in just the right way to explain the discrepancies, is kinda odd"

That seems totally backwards to me. Matter, by definition, is what is distributed spatially in just the right way to explain our observations. Physical laws, by definition, are not; they are universal. So why keep trying to explain what appears to be matter with adjustments to physical laws?

It's like you kept seeing elephant footprints appear out of nowhere, but insisted that rather than believe in invisible elephants, the most parsimonious explanation is that they are a natural consequence of a corrected law of gravity. That is, to me, turning Occam's Razor inside out.

Now, imagine no one has ever seen, felt, smelled, or heard the invisible elephant yet it makes up 85% of matter.

I'd start to wonder if the tracks could be explained by another phenomenon.

I think, philosophically, dark matter has been "seen" as much as anything. Like, radio waves are invisible, but you accept that we've "seen" them, right? So-called visible light isn't what activates your visual cortex, it's a cascade of rube goldberg machinery triggered by photons; the inside of one's head is dark. Nothing is seen directly, but all the things which we think we've seen have various intermediate steps before we can "see" them. We've detected gravity waves - is that "seeing", and do people who doubt dark matter exists doubt the gravity wave detectors work?

I think your response makes sense (neutrinos are another example where the theory predicted an as yet undetected particle and it was eventually found).

But the other reply to your comment is good too - at what point, after failing to detect any OTHER signs of these invisible elephants, do you wonder if there’s other explanations? What it would take to falsify dark matter, given that you can invent it in whatever distribution you like to fit the data?

There’s nothing that logically compels us to assume the existing model of gravity is correct, and its discrepancy with observation is explained by assuming the inputs aren’t what we can observe. It is also logical to consider that the existing model of gravity isn’t quite right. Of course any tweaks to gravity have to be compatible with a lot of other observations but it would be wrong to rule out the possibility of an alternate model.

"What it would take to falsify dark matter, given that you can invent it in whatever distribution you like to fit the data?"

What does it take to falsify any observation of any kind of matter? Isn't the chair you are sitting on just as unfalsifiable? In an effort to be consistent, you might concede that the belief your chair (or anything!) exists is not scientific, so then I ask if that's not a problem, why is dark matter a problem?

Ok, so epistemologically we can’t know what reality “really” is, only what we observe, and we can come up with models that fit our observations. Your (I’d say vacuous) point is that my model of reality that includes a chair is epistemologically on the same level as a model of the cosmos that includes dark matter... though it’s also on the same level as a model that gravity is caused by invisible purple dinosaurs. :) So this perspective doesn’t really get us anywhere in judging models of reality.

I’d say models are useful when they are simple and predictive. Dark matter isn’t really predictive, it’s what is hypothesized to compensate for the prediction error of our current model of gravity. (Doesn’t mean it’s wrong but am just pointing this out)

So I don’t think dark matter is inherently problematic, just that it’s not predictive and it’s worth considering whether the model of gravity we have needs tweaking instead (much like GR refined Newtonian gravity).

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