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

A Physicist Who Denies Dark Matter

nautil.us

Post reply on HN
Print view View original

101–110 of 182 posts

Re: A Physicist Who Denies Dark Matter

#101

Post by Sir_Cmpwn »

_ph_ wrote: post #49
vivekd wrote: post #41

Earlier quoted context omitted.

>How do you explaining gravitational lensing with no visible mass, without dark matter? Black holes? The physicist says: >What struck me was some regularity in the anomaly. The rotational velocities were not just larger than expected, they became constant with radius. Why? Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of…

>>How do you explaining gravitational lensing with no visible mass, without dark matter? >Black holes? Well, black holes are not really black, you can see their accretion disk. Also, a centralized huge mass would create a different velocity profile from what we are seeing. So black holes are pretty much out.

Only if they have an acretion disk. If they've cleared their neighborhood of most infalling matter then they would be invisible except by gravitational effects.

Re: A Physicist Who Denies Dark Matter

#102

Post by _0w8t »

_0w8t wrote: post #4

What is puzzling about MOND and to lesser extent about Dark Matter is that both uses violation of Newton mechanics to justify themselves when we know that Newtonian mechanics is wrong. The problem is that we cannot yet model behavior of galaxies using just General Relativity. Typically models assumes Newtonian physics with minimal accounting for relativity. However there is no sound proof that it is a valid approxima…

Here is a paper that discusses a possibility that the General Relativity alone can be enough to explain the observed dynamics of galaxies:

https://arxiv.org/abs/0712.0019

From the paper:

we are witnessing here the power of the nonlinearities inherent in general relativity in the context of weak gravity to effect very significant changes relative to the results expected on the basis of Newtonian theory. It suggests that hitherto unexplained astrophysical phenomena be re-considered on the basis of the application of general relativity to weak-field gravity.

Re: A Physicist Who Denies Dark Matter

#103

Post by Sir_Cmpwn »

hwillis wrote: post #98
signa11 wrote: post #46

Earlier quoted context omitted.

> How do you explaining gravitational lensing with no visible mass, without dark matter? black hole would produce the same result right ?

No. Dark matter is spread relatively homogeneously so you would need very small back holes, and those would be visible or wouldn't have existed this long. Baryon acoustic oscillations also limit how much normal matter we can have, and they say there has to be a great deal of non-baryonic matter.

I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.

Re: A Physicist Who Denies Dark Matter

#104

Post by TheOtherHobbes »

douche wrote: post #89
hwillis wrote: post #87

Earlier quoted context omitted.

That's total nonsense.

A.) God waved his finger, and the universe sprang into existence. B.) something happened, and the universe sprang into existence.

The difference is that god is a bizarre, nonsensical, and clearly wrong story that shuts down questioning.

The Big Bang is a debatable story - could be right, could be wrong, our best effort so far - that encourages questioning.

Re: A Physicist Who Denies Dark Matter

#105

Post by kradem »

boomboomsubban wrote: post #95
kradem wrote: post #84

Earlier quoted context omitted.

This subthread deals with the completely different issue, it's about bad logic. No matter how well Milgrom and/or his theory could be discredited, that doesn't change the fact we can dismiss something prior to fully understand it. It doesn't matter if that - what we dismissed - shapeshifts into something completely different (or similar...) afterward, on the contrary, regarding resources spent it would be sad if not.

>This subthread deals with the completely different issue, it's about bad logic. No matter how well Milgrom and/or his theory could be discredited, that doesn't change the fact we can dismiss something prior to fully understand it It's not bad logic, it only seems like it is because you're approaching it from an Aristotlean view of logic. These things can't be proven, so dismissing them is never an option in science.

We're actually dealing with one complex sentence here, the second of these two:

> But, dark matter isn't something to be denied. Because it isn't understood yet, to the point where we can doubt its existence

I'm lousy in English and my knowledge of dark matter is irrelevant. With those two lemmas in mind, please answer the only question in this subthread:

could something be dismissed if we don't fully understand it?

Re: A Physicist Who Denies Dark Matter

#106

Post by gus_massa »

jessriedel wrote: post #86
BoiledCabbage wrote: post #38

Earlier quoted context omitted.

And the second word of the only sentence in your comment is wrong. Please provide a constructive, substantive critique or don't comment at all.

The fact that someone with more expertise is warning that a discussion is going off the rails is a useful, constructive fact. Demanding that that person explain in detail why in order to contribute is a good way to make sure they don't bother.

As the official self appointed Em-Drive denier here, my recommendation is to provide more information in the comments. Sometimes it's difficult. Most commenters here are not experts in the particular field, but are curious. Sometimes it's better to center in the worst part of the comment if it has too many errors.

Sadly, I don't know enough cosmology and general relativity, so I use this approach to this problem https://xkcd.com/793/

I think that general relativity give minimal corrections in the scale of a galaxy, only some local corrections that are important near black holes and some spots. So it's enough [citation needed] to use Newtonian physics, perhaps with some correction near the galaxy center and perhaps a small effective field with the average distribution of matter.

To track an individual star during many galaxy revolutions it may be necessary to be more careful, specially if the stars pass near another star. But to have a good estimation of the rotation speed it is possible to almost ignore general relativity.

Question time: Do the current simulations in the area use special/general relativity corrections? Do you have a link to a recent paper? A link to a simulation program? [bonus point if the link points to the option page to enable/disable relativity or select the approximation method] Is there a Wikipedia page about this? [Most of the times the Wikipedia page is easier to understand than the paper.]

Re: A Physicist Who Denies Dark Matter

#107

Post by nilson »

jcranmer wrote: post #94
douche wrote: post #59

Earlier quoted context omitted.

Whoo, boy, the astrophysics fanboys that come out of the woodwork to suppress the notion that the emperor has no clothes

Even as someone who would rather MOND be true than dark matter, I have to admit that dark matter is internally consistent in ways that MOND is not. By comparison to the theories of planetary movement (since you seem to love comparing dark matter to them so much), MOND is rather like the Copernican model and dark matter rather like Kepler's model. It's well-known that the traditional equations are off. MOND provides a…

Why do physicists believe that formulas should be elegant, beautiful etc.? Maybe that is not how universe works.

Re: A Physicist Who Denies Dark Matter

#108

Post by hwillis »

Sir_Cmpwn wrote: post #103
hwillis wrote: post #98

Earlier quoted context omitted.

No. Dark matter is spread relatively homogeneously so you would need very small back holes, and those would be visible or wouldn't have existed this long. Baryon acoustic oscillations also limit how much normal matter we can have, and they say there has to be a great deal of non-baryonic matter.

I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.

There aren't isolated examples of strange gravitational lensing. The lensing being discussed occurs at the scales of galaxies and clusters of galaxies, so you need an incredible number of tiny black holes, brown stars, or other MACHOs.

Re: A Physicist Who Denies Dark Matter

#109

Post by shakna »

hwillis wrote: post #73
gaius wrote: post #66

Earlier quoted context omitted.

Right it's like when people say, I don't believe in Creation, that stuff is just nonsense. And you say, so what's your alternative theory then? And they reply, well there was a "big bang" and everything just sort of appeared, for no reason. Really, that's the alternative explanation?

Well yes, if you ignore all the evidence of something then you can confidently say there is no evidence. Christian genesis: There was a garden of Eden, and a god. Evidence: No garden of Eden, no evidence of angels or god, no reason those things would suddenly stop being apparent. Direct contradictory evidence that YHWH[1] was a minor god in the original jewish pantheon rather than an actual creator god. Big Bang: The…

> Christian genesis: There was a garden of Eden, and a god.

That's a really bad example to go to.

The tale of Eden follows a poem, and if you take both literally, then you end up with contradictions.

For why? Genre.

Neither were intended to be literal. They were designed to convey meaning.

> YHWH[1] was a minor god in the original jewish pantheon rather than an actual creator god.

Origins are highly disputed, but El seems to be the most likely title, who is the head of the Caananite pantheon, and influential enough to be remembered in Babylon. Not a minor deity.

I understand you were responding to a statement which sort of ignores the basic premises of science, but that doesn't mean your narrative should ignore a scholarly approach to documents whose origins predate writing.

Re: A Physicist Who Denies Dark Matter

#110

Post by Sir_Cmpwn »

hwillis wrote: post #108
Sir_Cmpwn wrote: post #103

Earlier quoted context omitted.

I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.

There aren't isolated examples of strange gravitational lensing. The lensing being discussed occurs at the scales of galaxies and clusters of galaxies, so you need an incredible number of tiny black holes, brown stars, or other MACHOs.

You're right, gravitational lensing anomolies explained by dark matter are more commonplace than I had thought.
Post reply on HN