Does Dark Matter Really Exist?

[NoXion]

I'm involved in a discussion on another forum concerning dark matter, and one of the posters there is very skeptical of it's existance, comparing it to the celestial spheres of geocentric cosmology or the epicycles of Ptolemaic astronomy. She does not seem as critical of alternatives like MOND, even posting a number of links to pages on the subject.

Now, previously my understanding was that dark matter was there, but that it's composition was unknown due to the indirect methods of detecting it. But she seems to be under the impression that dark matter is as substantial as phlogiston. Which one of us is closer to the truth, or are we both barking up the wrong tree?

[Samuel]

We can detect its gravitational tug. It is there all right.

[Twoyboy]

We can detect its gravitational tug. It is there all right.

Alright, I'm only a wanna be physicist, but wouldn't it be more correct to say that we can observe irregularities in the movement of distant objects which could be explained by extra gravitational force? We can't actually directly detect the gravity of dark matter, can we? If not, she could be correct, dark matter may now exist at all, other theories are around.

Having said that, iirc dark matter is considered the best explanation because it best explains the observations with the least disruption to current physics. I haven't read of a dissenting physicist for some time, so I assume belief in dark matter is almost unanimous.

[HRogge]

I'm involved in a discussion on another forum concerning dark matter, and one of the posters there is very skeptical of it's existance, comparing it to the celestial spheres of geocentric cosmology or the epicycles of Ptolemaic astronomy. She does not seem as critical of alternatives like MOND, even posting a number of links to pages on the subject.

MOND cannot explain stuff like the "bullet nebula", which is made from two colliding clusters of galaxies.

Most of the stars and gas is in one place, most of the mass is in another one, so it's an asymmetric distribution. You cannot explain something like this with wrong scaling.
http://www.damninteresting.com/?p=652

[starslayer]

Alright, I'm only a wanna be physicist, but wouldn't it be more correct to say that we can observe irregularities in the movement of distant objects which could be explained by extra gravitational force? We can't actually directly detect the gravity of dark matter, can we? If not, she could be correct, dark matter may now exist at all, other theories are around.

Having said that, iirc dark matter is considered the best explanation because it best explains the observations with the least disruption to current physics. I haven't read of a dissenting physicist for some time, so I assume belief in dark matter is almost unanimous.

Correct, although I would say "...which could only be reasonably explained by extra gravitation." This is also only true because DM operates on such large scales (I mean, no one's going to claim that you can't directly detect the Earth's gravity). IIRC, MOND does explain the rotation curves of galaxies, the original evidence for dark matter, but as HRogge has said, it most certainly cannot explain more recent observations.

NoXion, the other poster there is utterly off base in her comparisons. We have direct observations of DM's influence which MOND cannot explain. Can you link to the discussion so we can see exactly what she's saying? I'd like to provide more in-depth counters to her statements if I could.

[NoXion]

NoXion, the other poster there is utterly off base in her comparisons. We have direct observations of DM's influence which MOND cannot explain. Can you link to the discussion so we can see exactly what she's saying? I'd like to provide more in-depth counters to her statements if I could.

Sure thing: Darkmatter what is it?

[Formless]

Amazing that that person can so horribly misunderstand the Principal of Parsimony that they completely ignore the evidence cited (the bullet nebulas) in favor of a "simpler" solution. Tell them that Occams Razor cannot advocate a simpler theory if that theory doesn't take all the facts/evidence into account, because otherwise its not really simple at all! It still has to explain away those facts. If it cannot, it is invalid. That is the scientific method. A simple mathematical explanation: if you are saying that 1+1+2=2, you are an idiot because you forgot to add in the two! That is why parsimony only works after you take into account all the evidence, in this case the bullet nebulas that say Mond is fucked.

Have him answer that piece of evidence, don't let him try to railroad the discussion with cheap rhetoric.

[starslayer]

She doesn't seem to get, as most people don't, the reason Ptolemy's views prevailed even in the face of Copernicus, even though she flat says that the Copernican system was complicated: well, it was more complicated! They themselves said that not all of the epicycles that had been invented likely existed, and were calculational tools; this sort of approach has been used successfully many times in science. The one that comes off the top of my head is the Bohr model of the atom. Simply put, Copernicus' ideas did not explain the observations any better than the Ptolemaic system did; in fact, it was worse! It was not until Kepler, working with Brahe's much more accurate data, that it could be proved that the Sun was at the center of the Solar System, with the planets all having elliptical orbits. Copernicus assumed they were circles.

Alternatives are often rejected out of hand by traditonalists in science; why they do this is not too clear since the history of science shows that these alternatives often win out:

Pure selection bias. She really has no idea how many times ideas come up and are rejected, and turn out to be wrong, does she? I'm quite serious here: if you're a scientist, and a mere 20% of your ideas are good ones, congratulations, here's your Nobel Prize. Scientists are wrong all the fucking time.

It's also how science stalls. There are plenty of examples of this over and above the 'extra epicycles' fiasco. For example, the discovery of Neptune, and then the non-discovery of the planet Vulcan (no, not the on from Startrek):

She also doesn't realize how crazy smart Einstein was; GR was almost entirely due to him, and it almost killed him. Thus, it is not in fact science stalling because of it's own structure; it is science stalling because most humans, even most physicists et al., just aren't smart enough to come up with some of this stuff. Also, often scientific progress and mathematical progress have gone hand in hand, as the example of Newton and calculus demonstrate. If you don't the mathematical tools to describe something, you'll get nowhere.

Mike, you must know that there are alternative explanations for such phenomena that do not require belief in this mysterious form of 'matter', conveniently invented to fill a gap in currently 'orthodox' theory (just like the extra epicylces dreamt up in the later stages of Ptolemaic astronony).

It's an odd sort of materialism that accepts a theory that tells us that 90% of the universe is missing!

It seems she has a fundamental opposition to the idea that we can't see most of our universe. She seems to think that the universe is obligated to make sense to humans; it most certainly isn't. It is not obligated to keep us around, it is not obligated to cater to us, and it sure as hell wasn't obligated to even let us or any other environment-changing form of self-replicating chemical reactions to come into existence. In fact, the most amazing thing about the universe is that as much of it makes as much sense to us as it does! To me, though, fundamentally altering the concepts of mechanics is far more repulsive and more complicated than just saying, "hey, there's something there that we can't see for whatever reason." Oh, and just because we can't see this stuff doesn't mean it's "missing." It's there, all right.

1) and 2) are exactly how a Ptolemaist would have defended his/her ideas 700 years ago. 3) I have already dealt with.

Recall, I am not saying that Dark Matter does not exist, just reminding you all not to jump on yet another 'scientific' band wagon.

DM isn't a scientific bandwagon; it's a thoroughly tested idea that has withstood all challenges thus far. Nor is it new; it was first proposed in the 1970's. She also again reverts to criticizing the scientific process with her dismissals of Whitemage's points. As I said, the Ptolemaists were completely correct in dismissing Copernicus' ideas at the time, as they didn't have accurate enough data to get the clear picture, and theirs was even simpler than what Copernicus proposed. They went with the data they had, just like all scientists do, just like everyone does.

This has nothing to do with anything I have said, so I do not know why you asked this.

Yes it does. See "It's an odd sort of materialism that accepts a theory that tells us that 90% of the universe is missing!"

It is appropriate that it is named after the mythical fifth element of ancient Greek Philosophy, and no doubt it will go the same way: into the trash can of history.

Don't be so sure when you don't understand the underlying observations and science, idiot. I'm sure you can fish the Arxiv for something substantive on dark energy.

But they always want to throw away the least amount of earlier principles which they consider already tested and reliable. We will see the greatest resistance to reopening the case for momentum and Kepler's laws.

I've no idea what mikelepore's saying there. Is he seriously contending that the idea of momentum and the ideas of Kepler's laws are wrong? If so he's a grade A fucking moron.

Anyway, who says nature should run according to our notions of 'simplicity'?

It doesn't have to simple; it has to be as simple as possible, and no simpler, and no more complex, either. This means that we add as few layers of complexity as possible that will still explain everything we observe. MOND adds more complexity than does DM, and thus it is set aside. Call me when you find observations that DM does not explain that MOND does. However, given the current status of things, it would probably require a new hypothesis.

We also say the Sun is the center of the Solar System, and that everything in it orbits it, because a) to an outside observer, the barycenter of the system is inside the Sun's surface, and b) it is by far the most massive body in the system, containing over 99% of said mass, even though we can do some convoluted coordiante transforms to put Earth at the center, or Mars, or Mercury, or Jupiter, or some little podunk speck of dust way the hell out in the Kuiper Belt. Oh, and the fact that the system is cylindrically symmetric about the Sun, indicating that it dominates matters here. Naw, couldn't be that.