Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

A Physicist Who Denies That Dark Matter Exists

cosmos.nautil.us

Post reply on HN
Print view View original

121–130 of 193 posts

Re: A Physicist Who Denies That Dark Matter Exists

#121

Post by mcguire »

cwmma wrote: post #18

So this doesn't actually get into the actual big issue with MOND which is that observational results [1] tend to imply some sort of dark matter so current MOND theories actually include some dark matter, just less of it. 1. https://en.wikipedia.org/wiki/Bullet_Cluster

"While the Bullet Cluster phenomenon may provide direct evidence for dark matter on large cluster scales, it offers no specific insight into the original galaxy rotation problem. In fact, the observed ratio of dark matter to visible matter in a typical rich galaxy cluster is much lower than predicted.[14]"

Re: A Physicist Who Denies That Dark Matter Exists

#122

Post by agentgt »

The way I see the MOND vs Dark Matter is that there are two philosophies. One is we need to add something new to explain the behavior and the other is we need to adjust the existing theories.

This is key because people (IMO particularly engineers like myself) think when stuff is added it is less likely to be correct which in some cases true but others not so much.

The thing is MOND actually adds a bunch of stuff and doesn't explain a whole bunch of other stuff (yet). What sell me on dark matter is the clustering of galaxies and the universe itself (like how did we get these massive super black holes in the galaxy with MOND?).

On a rather tangental note... do you remember when maverick scientist were shunned and if you go back far enough in some cases heretics? Now days it seems to pay nicely to be contrarian in terms of media coverage (at the cost of being revolted by your peers. There was a SA article on this a while back (famous scientist being hated).

Re: A Physicist Who Denies That Dark Matter Exists

#123

Post by Paul-ish »

mikeash wrote: post #28
corey_moncure wrote: post #25

Earlier quoted context omitted.

>particles invented before being directly observed as support of dark matter hypothesis Correct me if I'm wrong, which is quite likely, but isn't it one of the qualities ascribed to dark matter that it doesn't interact with 'ordinary' matter or photons in any way other than gravity, and therefore can't be directly observed? Or to turn the question around, is the dark matter hypothesis actually falsifiable?

It obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but…

> And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it.

Whats to stop us from "adding epicycles" when that happens?

Re: A Physicist Who Denies That Dark Matter Exists

#124

Post by danielhooper »

roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

I used to feel pretty similar, but then I realized that the people working on these problems are even more skeptical than I am. That's why they work on this stuff. If there is a name for this specific kind of bias, let me know!

Re: A Physicist Who Denies That Dark Matter Exists

#125

Post by jerf »

blueprint wrote: post #53
jerf wrote: post #13

Earlier quoted context omitted.

One of the mental problems that I think people get in this area is that you can't help but mentally imagine that there's just these few points of data that we need to hit, and so we have a lot of freedom in how to construct formulae that match the observation, so why are scientists being so obstinate about the possibility that MOND may be true? But that's not the case. We have petabytes of data from space (if not mor…

> but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass That's honestly not so "weird" at all. There are TONS of examples of objects and substances having that kind of behavior in nature. If I crash…

"That's honestly not so "weird" at all."

Recall the operant definition of "weird" here is something that fits into the sentence "I find the idea of 23% of the universe being invisible particles that we are completely incapable of detecting is weirder than the idea that we have gravity slightly wrong."

Mathematically, if we add non-linear terms to gravity, it isn't that weird that you might get a memory-like effect, among other things. But would someone who agrees with the previous sentence agree that this is still less "weird" than 23% of the universe being so invisible that we still can't pick it up, even in 2017?

Re: A Physicist Who Denies That Dark Matter Exists

#126

Post by platz »

danielweber wrote: post #73
dogma1138 wrote: post #71

Earlier quoted context omitted.

Actually dark matter and dark energy are "fudge factors" that are being inserted in order to explain observations. There are multiple "contenders" for dark matter, none of them actually explain observations and measurements. Dark Energy is also in that same category especially in it's purest form as a "universal constant" that was added then removed then added back again to GR when we were sure if the universe is sta…

Shouldn't there be a big lump of dark matter at the center of every planet, moon, and star? If we were to hollow out an asteroid, would there still be a blob of intangible dark matter inside of it?

I like to look through the lens of quantum field theory instead of 'dark matter particles'.

Dark matter may be some field or interaction of existing fields that we don't understand yet.

Re: A Physicist Who Denies That Dark Matter Exists

#127

Post by openasocket »

benibela wrote: post #99

> In fact, MOND’s constant equals the speed of light squared, divided by the radius of universe. That does not sound like a coincidence. Did the constant change when the universe was smaller? Or does the light speed depend on the size of the universe?

Complete guess: it might not be a constant, but a function of the average curvature of the universe.

Re: A Physicist Who Denies That Dark Matter Exists

#128

Post by jerf »

waqf wrote: post #58
jerf wrote: post #13

Earlier quoted context omitted.

One of the mental problems that I think people get in this area is that you can't help but mentally imagine that there's just these few points of data that we need to hit, and so we have a lot of freedom in how to construct formulae that match the observation, so why are scientists being so obstinate about the possibility that MOND may be true? But that's not the case. We have petabytes of data from space (if not mor…

> What's true is true, regardless of how we feel about how beautiful it is. I think that's, in a certain sense, not really correct. Without some form of Occam's Razor, you can't do science — or at least, you can't come to any definite conclusions, because there are always infinitely many models that would explain any observation. Turning that around, it means that for any fixed amount N of data in support of your hyp…

"Without some form of Occam's Razor, you can't do science — or at least, you can't come to any definite conclusions, because there are always infinitely many models that would explain any observation."

I'm afraid that's a category error. I wasn't talking about science. I was talking about truth.

Many truths are literally incomprehensible. There are many systems out there in the world that may even consist entirely of atomic parts that we fully understand but we still can not understand the totality of all the parts, that simply consist of too many bits (in the information theoretic sense) for us to ever understand. The truths are true nevertheless.

I'm a bit old fashioned that way. The idea that there is no objective reality, or that if we can't understand it it can't exist, may be very exciting philosophies among the word-loving crowd, but I do not find them useful models of reality.

It actually wouldn't surprise me that much to discover that we can't quite nail down the laws of physics the universe operates on even in principle for some reason. (And I don't mean with any sort of copouts like "it might be a simulation" or anything; I mean the laws of physics that appear to govern the universe we appear to live in, regardless of whether that is itself sitting on top of something else.) Based on our progress in understanding the universe, I'd still guess that it's reasonably likely that we can find all the basic, atomic principles. But I wouldn't be stunned to be wrong.

Re: A Physicist Who Denies That Dark Matter Exists

#129

Post by Paul-ish »

snowwrestler wrote: post #92
arenaninja wrote: post #12

Earlier quoted context omitted.

Evidence for atoms and protons was already there before they were "invented". I think neutrinos and the Higgs boson were theorized before their discovery. Physics also has an unfruitful history of invisible stuff: ether, strings (string theory), etc. Personally I'm glad something other than dark matter is being considered. I was only told that the rotational velocities of galaxies did not match the predictions. IME s…

> Evidence for atoms and protons was already there before they were "invented". Right, and there is also evidence for dark matter.

I think arenaninja is saying that the theoretical models predicted those things, then empirical experiments later confirmed them. (I don't know if this is actually the case) This gives the theories credibility as being predictive.

In this case we have a theoretical mode, but we later notice our empirical data doesn't match this. Both MOND and dark matter are an attempt to patch up our existing theory to explain what we have seen. This is less compelling, because it is post-hoc and these theories might just work because we force it to work by over fitting. To use an analogy to machine learning, it is like training on your test set; no on is impressed when you then have good performance on your test set.

With that said, I think that point of view would apply to both theories equally.

Re: A Physicist Who Denies That Dark Matter Exists

#130

Post by Florin_Andrei »

mikeash wrote: post #23
roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

Remember the discovery of Neptune? I mean, not directly, since everyone from that time is dead, but.... That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct. Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relati…

> Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relativity. So it's not always the right answer.

If anything, the Vulcan hypothesis was not bold enough. It was a much greater leap of faith to assume that Newton's mechanics was just a low energy approximation, hence the twist in Mercury's orbit.

Anyway, we don't really know what this whole dark matter business is, but for now the prevailing hypothesis is the dark-matter-as-a-new-type-of-particle. Of course, you're welcome to pursue a different line of thought, and let the truest hypothesis win. This is how science works.

Post reply on HN