A Physicist Who Denies That Dark Matter Exists • Hacker News Board

Re: A Physicist Who Denies That Dark Matter Exists

#31

Post by thechao » Mon, Feb 27, 2017, 2:43 PM UTC

corey_moncure wrote: post #25

mikeash wrote: post #23

Earlier quoted context omitted.

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…

>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?

You can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravi... .

Re: A Physicist Who Denies That Dark Matter Exists

#32

Post by contravariant » Mon, Feb 27, 2017, 2:44 PM UTC

Cyph0n wrote: post #17

contravariant wrote: post #9

Earlier quoted context omitted.

It's also worth pointing out that several similar 'holes' in theories have turned out to be correct. Things like missing elements in the periodic table, and fundamental particles like the Ω− baryon or the Higgs boson, where all things that seemed like holes in a theory, but were later discovered to exist.

And how many "holes" proved to be incorrect? I think that using the history of past true positives is a weak argument for dark matter.

Obviously it's not sufficient evidence for dark matter. But I'm pretty sure that more often than not predictions of unobserved phenomena based on theories with great predictive power and mathematical elegance, turned out to be correct. Although sometimes, like with the neutrino, things turn out to be slightly different than expected.

In my opinion Verlinde's theory is a better candidate as a replacement of dark matter, since it doesn't require changing the theory of gravity as drastically as MOND does. That said, his theory does require a combination of general relativity with quantum physics, which is tricky. Compared to both just adding a bit of dark (not necessarily invisible) matter is a lot simpler.

Re: A Physicist Who Denies That Dark Matter Exists

#33

Post by lngnmn » Mon, Feb 27, 2017, 2:45 PM UTC

Nothing exist unless experimentally proven and replicated.

The way they proved the fact that DNA is the genetic molecule.

Re: A Physicist Who Denies That Dark Matter Exists

#34

Post by simonh » Mon, Feb 27, 2017, 2:47 PM UTC

SagelyGuru wrote: post #15

There is another possible explanation instead of dark matter and that is electric charge and currents (plasma flows). Electric force is many orders of magnitude stronger than gravity. So, if the objects in the universe are charged, the repelling force between them could account for the expansion acceleration in a very simple way.

Except that electrical/magnetic forces are vastly more powerful than gravitational forces. Any such force that's noticeable at the cosmic scale would be powerful enough at the object scale (star, planet, even galaxy) to tear them apart or cause them to collapse.

Lets suppose the Earth and the Sun were both positively charged sufficiently that their repulsive force was enough to noticeably affect the orbit of the Earth. For that to be true, the repulsive forces due to the positive charge on earth itself would have to be so great that it would blast the planet to fragments.

Re: A Physicist Who Denies That Dark Matter Exists

#35

Post by yk » Mon, Feb 27, 2017, 2:50 PM UTC

corey_moncure wrote: post #25

mikeash wrote: post #23

Earlier quoted context omitted.

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…

>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?

The dark matter hypothesis may not actually be falsifiable, however that is not what physicists are working on. Physicists work on dark matter models, precisely defined theories which incorporate dark matter and these are falsifiable. For example roughly ten years ago models of weakly interacting particles (that is, particles that interact via the weak force) were very popular, but they have a influence on the abundance on nucleons in the early universe. Turning the argument around, one gets a interaction probability of the particles, and that part of the parameter space was excluded a few years ago. (You can of course try to mask the signal, but then you are generating different models, the original very simple models were falsified.)

Re: A Physicist Who Denies That Dark Matter Exists

#36

Post by pka » Mon, Feb 27, 2017, 2:52 PM UTC

pja wrote: post #21

fuhrysteve wrote: post #4

Earlier quoted context omitted.

> 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. I have always had the same inclination. It seems like a classic candidate for using Occam's razor. Would love to hear someone who knows more about physics weigh in with a more nuanced viewpoint.

(I have a physics degree, but it’s very rusty at this point) Particle physics has a long history of noticing gaps in the experimental data that don’t match up with current theories and positing that maybe a new particle would solve the problem. This goes right back to things like the neutrino (which was posited to preserve momentum in certain particle interactions IIRC) and even the neutron. Sometimes the suggestion…

My impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.

Re: A Physicist Who Denies That Dark Matter Exists

#37

Post by lngnmn » Mon, Feb 27, 2017, 2:54 PM UTC

thechao wrote: post #31

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?

You can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravi... .

You can observe nothing but the fact that mass distorts light. Everything else is imagination.

Re: A Physicist Who Denies That Dark Matter Exists

#38

Post by twoodfin » Mon, Feb 27, 2017, 2:56 PM UTC

amelius wrote: post #11

> when you study textbook material, you’re studying done deals. You still don’t see the effort that goes into making breakthrough science, when things are unclear and advances are made intuitively and often go wrong. They don’t teach you that at school. Why don't they ever fix that?

I don't think that's a fair characterization of how the history of science is taught. There are other examples in these threads, but even in a high school & EECS-biased undergraduate science education I was exposed to "ether" theories of light propagation and Lamarckian evolution.

Re: A Physicist Who Denies That Dark Matter Exists

#39

Post by zamalek » Mon, Feb 27, 2017, 2:57 PM UTC

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…

I am not a physicist.

> Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right.

I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). It is an effect or the sum of effects that we are not aware of. It's almost like the predictions that Mendeleev made with the periodic table.

Personally, I don't like dark matter because my "hack" alarm bells start ringing. That being said, I also don't know what I don't know.

> nobody has been able to come up with anything like a coherent modification of gravitation that explains the observations.

There have been promising "non-matter" dark matter hypotheses[1], though these hypotheses seem to be quickly relegated to the disinterested bucket. In that way, it wouldn't hurt if slightly fewer eyes were looking at particle dark matter and slightly more at "non-matter" dark matter.

[1]: https://phys.org/news/2016-11-theory-gravity-dark.html

Re: A Physicist Who Denies That Dark Matter Exists

#40

Post by sandgiant » Mon, Feb 27, 2017, 2:57 PM UTC

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 "dark matter" is just a placeholder name for a number of observational anomalies. All of which can be accurately explained by introducing a single extra particle that interacts weakly with photons. So what you call "invisible stuff" is not necessarily that outrageous. A lot of new particles have already been proposed for other reasons, and some of them fit the requirements quite well (SUSY, sterile neutrinos etc.).

That said, I believe the main reason that many physicists prefer some kind of dark matter "particle" is that it's extremely difficult to modify general relativity (GR) without breaking the theory. GR provides such accurate predictions that modifying it just the slightest is almost guaranteed to break something somewhere else (e.g. at some other scale), which forces you to introduce nasty (if you ask me) scale dependent effects.

This does not mean that we shouldn't modify GR, it just means that it's very difficult to make it work. And physicists tend to like simple theories that work (for reasons such as https://en.wikipedia.org/wiki/Occam's_razor).