Slashdot Mirror
Good bye Dark Matter, Hello General Relativity
dr. loser writes "The CERN newsletter reports that a new paper by scientists at the University of Victoria has demonstrated that one of the prime observational justifications for the existence of dark matter can be explained without any dark matter at all, by a proper use of general relativity! What does this imply for cosmology and particle physics, both of which have been worrying about other aspects of dark matter?"
.. that Dark Doesn't Matter??
To Terminate, or not to Terminate, that's the question - SCSIROB
Science just isn't definite these days, is it?
Maybe.
Switch back to Slashdot's D1 system.
That's really interesting. It makes sense to model a galaxy as a fluid on a very large scale. After all, gravity is a relatively weak force. I haven't gone through the paper, but if their math is right, since the assumption is relatively benign, this seems like it would be experimentally verified.
Since the model assumes that a galaxy is a fluid (on a large scale), the model would predict fluid-like phenomena. What I wonder is if there is a galactic analogue to solitary waves. How would these manifest? (A friend wrote his thesis on solitons)
After all, I am strangely colored.
...was physicists around the world collectively slapping their foreheads.
Dark Reflection
Science never has been definite. The defining characteristic of science is that it accepts that all solutions to problems are tenative, and that some piece of information might turn up in the future that will cause us to doubt what we now believe. Intellectual process can't happen without replacing wrong old ideas with better new ones.
There's no point in questioning authority if you aren't going to listen to the answers.
The paper only concerns itself with the observed rotation speeds of galaxies, for which "maybe there's something we don't understad about gravity" has always been just as convincing an explanation as dark matter. However, the recent cosmic microwave background radiation data *also* implies dark matter, and doesn't have such an easy alternative explanation. The data tells us that (at least, at the moment the univers first became transparant) baryons only account for 20% or so of mass.
Socialism: a lie told by totalitarians and believed by fools.
TFA is just plain silly.
Every teccie knows that the universe is held together by gaffer tape, and the only problem has been to find the link between gaffer tape and dark matter.
If relativity does away with dark matter, well fine, but the cosmologists are missing the key issue here. All this means is that now we have to find the link between relativity and gaffer tape.
"The question of whether machines can think is no more interesting than [] whether submarines can swim" - Dijkstra
It means my physics paper is proper fucked, for one.
cmd-q.co.uk - some sort of stupid fucking internet bullshit
Comment removed based on user account deletion
Even if the article was correct, and dark matter was not needed to explain rotation curves in galaxies, dark matter is still needed to explain the acceleration of the universe, its large scale structure and the primordial anisotropies in the cosmic microwave background.
I've dealt with this same issue.
There seems to be, as you put it, a "dogmatic" belief, often from undergrads (I'm guessing), that their now current understanding of physics is "right", and that any questioning of dark matter is an excuse to call the qestioner ignorant.
I've asked numerous times why I should think dark matter is anything other than a mildly promising theory.
The responses questioning my intelligence, calling me names, and generally being assholes outnumber the cogent replies 3 to 1.
Since when did scientists start behaving like fundies?
When science teachers started teaching `facts' instead of the simplest hypotheses which were not disproved by any observational evidence.
I am TheRaven on Soylent News
The problem, as I see it, is often that those who question the theories don't have anything better to back them up -- they're just presented with skepticism or an alternative answer that has nothing to do with physics.
Of course, I think there's more room for that in astrophysics, given the focus on math and proofs rather than testing (due to rather obvious logistics). A new mathematical proof can come out that completely changes how people view space (or, heck, an appropriate use of an old mathematical proof, as the article shows).
But I can understand why some people would be a trifle edgy nowadays. I'm not saying that you provoked the argument, as I've dealt with scientists (heck, I live with one and hang out with her friends), but I have to ask -- when you said it was a 'mildly promising theory,' did you present an alternative opinion? One thing I learned is that scientists really dislike people saying "I don't believe that" or "I think that theory is wrong" but then don't offer what they DO believe in that's based on science. After all, that doesn't accomplish anything -- it just states a claim of belief, which isn't science.
But I don't think I need to explain why an "accepted theory" will have people assume that it's accurate and "true" and be reluctant to drop it just at some new information or test or mathematical proof. That older theory generally has plenty of evidence to back it up -- the new theory has none. So people will look at the new theory, run through the math or tests on their own, and confirm, therefore changing the general understanding. That's how science works. The reluctance to accept just any new information without seeing a lot more proof is one of the reasons science tends to add to a base of knowledge, rather than jumping down any old path.
Comment removed based on user account deletion
Wrong. Copernicus still had the planets moving in circles. The big difference (and the reason of the rejection by the church) was that in his model he put the sun instead of the earth in the center.
It was Kepler who found that the planets don't really move in circles, but in ellipses.
The Tao of math: The numbers you can count are not the real numbers.
I'm not in this field anymore, but I spent 4-5 years in quantum gravity, black hole astrophysics, and inflationary cosmology. Summarizing my reactions to other comments in this thread:
First, I will say that I have not gone through this new paper in detail. I'm skeptical at a gut level that their results seem to depend on general relativity, because GR should not be relevant on the scale of galactic rotation curves: there is good reason why all the calculations ignore GR. It makes me think that there is a flaw in their calculation, and indeed another poster referred to a potential rebuttal of their GR analysis.
Second, as yet another poster mentioned, galactic rotation curves are just ONE evidence for dark matter. We have evidence from the aforementioned stellar orbits in galaxies, plus the motions of satellite dwarf galaxies, gravitational lensing, measurements of galactic gas temperatures (depends on the local gravitational neighborhood), anisotropies in the CMBR, the rate and structure of large-scale cosmological structure formation, etc.
(There are also a bunch of theoretical reasons to believe that dark matter particles could exist purely on the basis of particle physics, even if you ignore the astrophysical evidence; see axions, supersymmetry, etc.)
It's not surprising to come up with an alternative that can explain ONE of these phenomena. In fact, there is already another alternative that can also explain galactic rotation curves: MOND (MOdified Newtonian Dynamics), an alterating of Newton's laws of gravity. (There is a relativistic extension by Bekenstein, although it's currently even more ad hoc than dark matter appears to be.)
The problem is coming up with explanations for ALL of these phenomena. Dark matter is the only theory that has been able to do so, and it's not for lack of trying. Contrary to popular Slashdot groupthink, scientists are not in love with coming up with the most absurd and exotic possibilities they can. Most astronomers hated dark matter. For decades. I even know one who only came around to it a few years ago. It's simply that dark matter works, and everything else people tried to propose in its place didn't. As Carl Sagan said, "No physicist started out impatient with commensense notions, eager to replace them with some mathematical abstraction... Instead, they began, as we all do, with comfortable, standard, commonplace notions. The trouble is that Nature does not comply."
Now, this is not to say that dark matter is the end-all, unassailable dogma. It's possible there are alternatives, including modifications to gravity. I like to compare it to the discovery of Neptune and the perihelion precession of Mercury. People say that it's ad hoc to postulate unseen matter to explain gravitational anomalies. But that's precisely what led to the discovery of Neptune: its gravitational effects on Uranus. On the other hand, you can't always get away with postulating unseen matter: when Mecury's orbit wasn't behaving right, people tried inventing an unseen planet ("Vulcan"), but it turned out that general relativity was the answer, modifying the laws of gravity. Either can be right a priori.
In the dark matter case, it was once true that the evidence in its favor was strong and there were a number of competing theories, but now there is a lot more evidence, and higher standards for theories, and dark matter is pretty much all that's left. People should and do continue trying to come up with alternatives, but as of now, dark matter is still the best game in town. Far from claims of ad-hockery and epicycles, dark matter is actually a robust physical theory: most theories of dark matter have already been falsified because they make such specific predictions about what we should see. It's only a very specific type, quantity, and distribution of dark matter that can work. That's the hallmark of a good theory, not unfalsifiable wish-fulfillment.
Finally: this is a
My question about dark matter has always been "Why is it more acceptable to make up a new type of matter, rather than deal with the idea that the fundamental forces may work differently than is believed?"
Well, because there was no theoretical framework to explain the data without the use of darkmatter. Let's face it, the whole darkmatter hypothesis is extremely ad-hoc, a fudge factor added into galactic rotation calulations to make them fit to what was expected. The outcome was a predicition that darkmatter must exist.Now, there is nothing particularly unscientific about this. Go take a look at particle physics where all kinds of particles were predicted to exist, and as a result many particle physicists went out looking for these particles. When they were found, this confirmed the theory, when the particles were not found, they continued to look, or they revised the theory.
The same kind of thing happened here. People have been looking for darkmatter for quite some time, however, it appears that a revision to the models used to predict galactic rotation curves *and* galactic clustering is what's needed.
Why was the existance of darkmatter more "acceptable?" 1) Basically, because it was a prediction that fit the models. That's something that scientists like a lot, it gives the experimentalists something to really sink their teeth into. And 2) there was no way to predict that a change in the theory was needed without having already developed a theoretical framework that could explain galactic rotation curves without the need for darkmatter.
As an astronomer, I would say that you're not wrong to ask your question, however, without having any idea of how our theory might need to be changed, it's kind of a pointless question. And in this case, it sounds like we really don't need to change our theory at all, it turns out that the range of validity of Newtonian gravity is a lot smaller than we thought.
I think the bigger question in my mind is why hadn't someone tried to do this before now? In some sense, it's one of those things that just kind of surprises you, because all of a sudden you realize that *everyone* has been operating under the same false assumption about Newtonian gravity, and then you wonder why nobody thought to check that out.
Of course, this all assumes that this new model using relativity is correct... It probably is, but I think it does need to under go the usual scrutiny just to be sure.
Shop Smart, Shop S-mart!
A valid criticism of science teachers, but not of scientists. A difference sometimes overlooked.
Socialism: a lie told by totalitarians and believed by fools.
I think that overall this is a good point, but I've seen many people who make their living doing hard science who, once they open their mouths, start putting their personal credibility where it does not yet belong.
Although I don't do it for a living, I'm dedicated to science and it's progress and I have a real love for both the process and the results. But I'm afraid that one of the biggest factors that has made science vulnerable to inroads by fundamentalists is that scientists have, of late, embrace three (admittedly hastily constructed) levels of credibility on scientific subjects:
1) We very strongly believe this is true because it has been repeatedly verified through controlled experimentation.
2) We very strongly believe this is true because it can be strongly inferred from existing verified data.
3) We understand that we don't have all the facts, but we are critical thinkers for a living and our theories are worthier than your theories.
Number 1 is where scientists should be, but in debates, articles, and various other discussions on the battle between religion a science I have seen prominent and credible scientists arguing 2 and sadly, much more often 3. I understand it, but many scientist should reign themselves in.
Understand, I'm not saying don't fight, just that we should fight from our position of greatest strength, which is being "fundamentalist" about the scientific method. If we can teach kids, or anyone, how it works and why we're devoted to it, all the while showing by example how to be scientific in thought, then we win. There's plenty of room for religion in the world even with hard science, and there's plenty of amazement and wonder to be had in science too. I just don't want to see scientists try to expand their own role in human exploration way beyond the data.
The only acceptable defense of scientific results is to say that they were the product of the Scientific Method.
I'm so happy this main reason for Dark Matter finally got explained with standard physics.
I didn't quite buy the whole idea of Dark Matter, it wasn't scientific enough. We took a stab and said that there were phantom particles that we couldn't see and they were causing our observations to be different from what they should be. It just seems like we assembled a mythos. DarkMatter, the God of the Slow Galactic Turn, floats unseen at the edge of all galaxies. 90% of all matter is dark matter, and no you cannot see any of it (short of one lensing effect from an unknown object). So verifying this theory is next to impossible. And after a while we took the leap to say that we were correct. Even though we just invented stuff to 'fix' the flawed equations. Not that we can't guess right the first time, but just inventing a solution with no basis shouldn't hit the nail on the head.
I think the comparison between Luminiferous Aether and Dark Matter is one of the most prudent ones I've heard in a long while. Making something up to force your data to fit is a pretty bad idea. We can't be wrong. There's something that we cannot see that exists (does some calculations)... here; that makes the data roughly fit. It might as well have been the law of invisible elves of slow rotation.
And yes, if by some odd happening this gets peer reviewed dead... I still believe everything I said.
It is no longer uncommon to be uncommon.
I'm in Dr. Cooperstock's General Relativity (GR) class this semester. I must agree, he's a great teacher, and definitely a little quirky. Also he's quite old and some of that quirkiness may come from age.
As an aside, what's different about his lectures is that he uses a transparency roll and an overhead projector instead of the blackboard, and writes/derives everything with us in class, unlike many other professors who merely present slide shows or just talk a lot and write very little (very common among astronomy professors). I really dislike slideshows, and prefer Dr. Cooperstock's method because as he does so, we learn about how he thinks, why he makes the decisions he makes in the derivations, and the usual pitfalls in dealing with all the notation used in GR. That for me is far more valuable than just seeing an amalgamation of details presented on slide shows with a short verbal summary from a professor. Any textbook could provide me with that. The other advantage with the transparency roll is that if we ever need to go back to a previous lecture to revisit something that was discussed there, he just has to put up the roll corresponding to that lecture, and we have it right there in front of us. If we missed any lecture notes, we can just borrow the transparency rolls from him and copy the notes from them.
Back to the topic, I believe that what's important is that we must realize that dark matter is still just a hypothesis. There may be overwhelming signs pointing to something that we call dark matter, but this paper reminds us that dark matter is still only just a hypothesis. It is one of the easiest hypotheses to make, because simply adding a spherical distribution of dark matter to a galactic halo produces the observed rotation curve, but is not the simplest, because it postulates the existence of particles that we cannot yet prove to exist, at least not in such large quantities. If simplicity is a valid reason to accept or refute theories, then Dr. Cooperstock's model appears to me to be simpler because it requires fewer postulates to make things work.
However there are other observations such as satellite galaxies and gravitational lensing and galaxy clusters, all of which appear to require a huge amount of dark matter that we cannot observe. While Dr. Cooperstock's model may not explain all of these yet, this is work that has yet to be done, and so his model cannot be ruled out. One must realize that dark matter is really just a fudge factor to make the theory work out the same way as observations. Until there is good evidence from astronomers and from particle physicists, the arena should be open and impartial to other candidate hypotheses. It is good to see that despite most of the world jumping on to the dark matter band wagon, there are people who stand back and persist with their own ideas. We've seen this happen so many times in history.
Besides, it is still possible that despite GR explaining the galactic rotational curves, dark matter may still exist, but then its role and distribution would change. Oh, the fun of physics...
Within the jargon of science, "true" means "useful and predictive". Scientists sometimes forget that they have a nonstandard meaning for "true", especially if speaking casually. It's worth noting that this jaron definition of "true" is more useful and predictive than the standard meaning. :)
Socialism: a lie told by totalitarians and believed by fools.
I think this demonstrates a fundamental misunderstanding of scientific research. Any real scientist will tell you they're wrong more than they're right.
You start with an observation, come up with a reasonable hypothesis to explain it, then test it.
Eventually your hypothesis fails at some level. So based on your observation, you create another reasonable hypothesis....
That's scientific progress. Each step along the way we learn more. And often, we get led down the wrong path, for any one of many reasons -most are not evil.
As a scientist, I can tell how I think many things work. Of course that leads to the question, "Don't you know for sure?" from a non-scientific public who wants to know that the levy will hold or the vaccine will protect them from disease and not cause it.
No, I don't know for sure. But that's not what anybody wants to hear. And that's not what anybody will report in the press. That's not what politicians base decisions on. The overwhelming majority of times you see science misused as you stated above its by companies/politicians/people taking scientific data and theory and restating it as scientific fact. Its rarely the scientist doing the study who says such things.