Slashdot Mirror
Missing Matter... Still Missing
squidfrog writes "Nature.com, PhysicsWeb, and the BBC all report on the latest results from the Cryogenic Dark Matter Search. 'The most powerful search yet for the Universe's missing matter has come up empty handed, contradicting an earlier study that claimed to have seen new particles.' 'A favoured theory is that the dark matter consists of Wimps (weakly interacting massive particles) about a thousand times more massive than a proton, one of the particles found in an atom's nucleus... on the rare occasions a Wimp strikes an ordinary atom, the effect should be noticeable.' 'Writing in the Physical Review Letters, the team says that while a detection has yet to occur, there is now a better idea of how much dark matter must exist.' They 'hope to improve the sensitivity of the experiment by another factor of 20 over the next few years.' What's 20 times 0? And don't tell me zero!"
For much more info, head to the CDMS homepage, which includes links to preprints of the mentioned Phys. Rev. Letters article (note, the paper hasn't been published yet), as well as other (published and unpublished) papers, as well as general info.
--Xandu
No, they don't extrapolate how much dark matter is in the universe. They say, if dark matter is of the 'WIMP' variety, we know that the mass and cross section (aka how easily they interact with other particles, namely the germainium nuclei in their detectors) of of these WIMPS is not in a certain range.
--Xandu
I'm sure this is part of thier validation that the detector is working.
The lack of WIMPs is data. If they reported "zero" WIMP events, the actual report would have been within some error, zero +/- some amount. Using that, you can set an upper limit to the actual amount of WIMP activity; you can say that the WIMP level must be below a certain amount (if the data is to be trusted).
You can tell a piece of equipment is working if it sees things you expect and if it behaves the way you expect it to from the theory. Getting "nothing" is not no data. You're being way too cynical.
Love,
A Published (albiet undergraduate) Physicist
Cooling is done in tiers (over a distance of many meters). I would assume that the outermost is cooled to 76K with LN2 since that is dirt cheap. And then inside that LHe cools it down to a couple Kelvin or so, maybe less if they use superfluidic Helium. This much is pretty standard by now. As far as the last degree or so, I would guess they mess with the pressure a bit to get the temperature as low as possible.
I hate to say it, but CDMS II (this experiment) was SUPPOSED to not find WIMPs in this range. There was an experiment called DAMA which had found a modulation in their noise consistent with their being WIMP dark matter, and they claimed detection. The whole purpose of this press release is to say that DAMA's claimed detection is now *ruled out*.
As for the description of gravity being incorrect, I hate to tell you this, but general relativity solves *so* many problems that cannot be solved otherwise that it's preposterous at this point to consider anything else. Gravitational lensing, bending of light by masses, binary pulsar decay, Mercury's perihelion precession... etc. etc... NO other theory of gravity explains any of this, unless it starts with General Relativity and expands on it.
As for your proof that there is no dark matter because it's there in small quantities in three (out of ~250,000) galaxies, give me a break. Normal matter clumps and interacts with itself, so it's quite reasonable to expect we will get some cases where we have more normal matter than dark matter.
On average, though, Dark Matter is well known (see my comment history for examples) to exist in about 6-7 times the abundance of normal matter.
Sorry if this is a rant, but talk about throwing the baby out with the bath water...
> > consists of Wimps (weakly interacting massive
> > particles) about a thousand times more massive
> > than a proton
> My training in physics is quite elementary, but
> I was led to believe the proton is relatively
> massive on the atomic level, especially when
> compared to an electron. How could a wimp be so
> large and yet unnoticed?
The key is the "weakly interacting" in the name. At the microscopic level, these particles (if they exist) can only interact via the weak force, which is both weak and short-range.
In particle physics the size of a particle has no relation to a physical size or a particle's mass. It is defined in terms of how strongly a particle interacts with other matter. (See the definition of cross section at PhysicsWorld.) So since the WIMP particle interacts only weakly, it is by definition "small," even if it is massive.
If the WIMP hypothesis is correct, then the WIMPs have hardly been "unnoticed." One of the chief motivations for looking for them is to explain the rotation of various galaxies which appear to be much more massive than can be calculated by adding up the mass of all the stars and dust in them. So if this missing mass does consist of WIMPs, then they have already been noticed!
From dictionary.com:
universe (yn-vûrs)
n.
- All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole.
-
- The earth together with all its inhabitants and created things.
- The human race.
- The sphere or realm in which something exists or takes place.
If WIMPS were outside the universe, I'd think the physisists would have a much, much larger problem. Either that or we'd have to redefine "universe"Uhh, did you read that article? That article only showed that three elliptical galaxies didn't have as much dark matter in them as expected. It did this by showing that these galaxies rotated in the exact fashion they should *in the absence of dark matter*. So, this study does not deny the existence of dark matter. It only poses the question "why do some galaxies have dark matter and others don't?"
As for cosmic acceleration, there's no particular reason to believe that that phenomenon has anything at all to do with the nature of gravity. It very well could be related to how our universe formed (see some of the alternative theories to inflation that involve pre-big-bang states).
I should probably RTFA before posting, but hey, this is /. So, my guess is they have an outer vessel with LN2 surrounding a bath of Liquid Helium 4 which in turn is surrounding the rest of the refrigeration equipment. This most likely has a He4 pre-cooling stage (works just like any other refrigeration cycle...reduce pressure over a given amount of liquid and the temp drops). Next stage is most likely a dilution refrigeration cycle, which uses a mixture of He4/He3. Not going into details here, but this will get you to tens of millikelvin or so. To get lower temps, use adiabatic demagnetization of a large copper block and you can get into the micro-kelvin range...assuming no heat leaks...tada!
Nice with the conspiracy theory, AC. Too bad that you're wrong. The first tip-off that there's dark matter is the rotational speed of galaxies. Your decaying speed of light won't explain that.
Fascism trolls keeping me up every night. When I starts a preachin', he HITS ME WITH HIS REICH!
How do they do it?
I assume you mean how do they cool it that low rather than how they found an abonadoned mine in Minnesota.
First, I imagene you have a series of refrigerators. If you've seen the movie Akira you have an idea what I'm talkign about. You put various types of refrigerators inside of eachother to limit the heat coming in from outside.
Take Helium (He) and put under pressure till it is in liquid form. If you let it boil, it will cool down to about 4K at atmospheric pressure. if you lower the atmospheric pressure by pumping out all the atmosphere, it will cool lower. This will take you to about 1K.
To get lower you can use a mixture of He3 and He4 (Helium atoms with different atomic weights) and cool it to make a dilution refrigerator. The lighter He3 will spearate from the He4. The He4 works to absorb the He3. You pump off He3 out of the He4 at the othe end of the tube and it cooles the remaining He3 as it is disolved into the He4. This should take you to the temperatures needed for this experiment. Simply put your experiment inside of the cold He3.
You can get even lower with various magnetic traps that allow fast atoms to "evoprorate" out of the traps but this tends to be for a small amount of atoms.
It took me a while but i found it, its halfway down.
2writer: You freak! you actualy looked up the episode # cq webpage?
[snip] Bart: But I have 52 million shares! What's 52 million times zero? AND DON'T TELL ME IT'S ZERO! [snap]
Hivemind harvest in progress..
Size, mass and interaction strength are unrelated. For example, imagine trying to detect clouds by throwing rocks at them. Clouds are big, but they only interact with rocks very weakly.
--Tom
Blasphemy is a human right. Blasphemophobia kills.
CDMS detectors detect heat (vibrational energy) which is deposited in their superconductors when any kind of particle flies in and hits them. The localized heat causes the hit region to go non-superconducting, and as a result they can measure a reduced current as would be expected from a normal conductor.
All sorts of particles are constantly flying in and creating signals in their detectors. This is how they know that it is working. The trick is to veto the known signals by surrounding their superconductors with other detectors which can detect ordinary matter, but not dark matter. Therefore if the other detectors tell you that some ordinary matter entered the superconductor, then you would reject that signal.
In the context of a dark matter flux (flow) measurement, greater sensitivity means a greater ability to detect low fluxes. So far they've measured 0 dark matter particles in a few years of running. This means that the flux is less than 1 particle per detector area per few years (also per detector efficiency).
Suppose the numerical value of their measurement is that the flux is less than 100/m^2/year (just to use round numbers). Then, if the true flux given to us by nature is 1/m^2/year, then they would have to run for another ~100 years in order to detect 1 dark matter event. On the other hand, if they make their detector 100 times larger, then they can detect the 1 dark matter event with only 1 more year of running. This is what they mean by increased sensitivity by building a larger detector. Meanwhile, in the time taken to see the 1 dark matter event, they probably reject several trillion false events which are caused by ordinary matter particles.
A. Physicist
Yes, you're missing something. :-)
The statement "the sensor has never detected something" is patently false. Figure 1 of the paper shows all of their detections - and there are lots of them! WIMPs aren't the only things that interact with Germanium. ;-) However, once you exclude all of the events which are consistent with being cosmic-ray produced interactions with the shielding, you get Figure 4... all of the detections in the red region (which is where the WIMPs would show up) are gone.
So the detector works great and detects lots of things! But no WIMPs yet.
[TMB]
Expensive shot: Gravity in the presence of delocalization. One can, in principle, convert huge amounts of mass-energy (gigagrams) into photons, and diffract those photons over large distances (hundreds of meters). GR predicts that those photons will create a substantial gravity field, one that is "trivially" measurable with a spatial resolution of meters. GR does not predict the values of those measurements. Ergo, GR does not describe the universe. It is easy to devise other quantized systems for which GR predicts a particular average result, but makes no predictions about the distribution of results.
I'm not saying that GR isn't a useful approximation when you have vast numbers of localized particles, but it ain't the Truth. This has implications for neutron stars in particular, as they may contain on the order of a solar mass of Bose-Einstein condensate (superfluid neutrons and such).
Is this Dark Matter the same thing as or related to anti-matter or something whole different?
Something compeltly different. Matter is dark because we cannot see it normally with telescopes. However, some dark matter might be anti-matter.
The basis for dark matter is that the galaxies are spinning and by how fast they are spinning and realative movements, we can figure out how much mass they contain. Yet, with telescopes we can only detect aobut 10% of the mass needed for make galaxies as massive as they are. Through various methods we can figure out how much normal matter (including anti-matter) might be around that we can't see because it's behind other matter, really dark, or otherwise undetecatble. This dark normal matter is only about another 10% of the mass needed. Thus we are needed much more mass to make up the difference.
Various explanations consist of superblack holes, exotic matter such as WIMPS, etc. one such solutions is MOND which says that our equations governing gavity need to be revised to match what we are seeing experimentally with the telescopes and other data. This theory has just as many problems as they others. Various phsysists have their pet theories but most seem to beleive there is mass or energy out there we cannot directly detect yet.
The numbers come from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) which measured fluctuations in the Cosmic Microwave background (afterglow of the Big Bang). There's a good review of their results in hep-ph/0308251 accessible from the LANL preprint server though it might be a bit technical for most.
OK, so you don't like decaying lightspeed as an explanation (and I agree, though it could explain some other issues and is given serious consideration by real scientists).
There is a thery that there is little or no dark matter, and the difference is accounted for by the assumption that the inverse square law for gravity fails at large distances -- based on a theoretical model of graviton particle exchanges that would not follow inverse square -- This just happens to match the observed data pretty well without need for dark matter.
A second alternative is combines light speed decay along with big change in assumed age of universe, so that spiral galaxies look the way they do because they are quite young compared to the standard model.
I'll bet there are other non-darm matter models that are explain observed data as well as the dark matter model too.
They use a "dilution refrigerator" to get that cold. Dilution refrigeration uses a mixture of He3/He4 (mash) and cycles between two phases of the mixture (a He3 rich phase and a He3 dilute phase). The He3 and He4 are both liquids at this point.
Here's a basic overview of cryogenics. Liquid Nitrogen (LN2) liquifies at 77 K in 1 atmosphere. N2 is abundant, and LN2 is priced cheaper than milk. LHe4 liquifies at 4.2 K, and costs (here in the USA) about $4 per liter. I think it's much more expensive elsewhere in the world, but helium is mined w/ natural gas companies, so is more plentiful here than elsewhere. LHe3 is a rare isotope of Helium and vastly more expensive. It liquifies (I think) around 3K, and costs several hundred dollars for a few gaseous liters (here in USA).
So one can easily get to 4.2 K by dipping something in LHe4. One can employ evaporative cooling, and 'pump' on the LHe4 dewar, and get down to temperatures of about 1.5K. Perhaps slightly lower for bigger pumps. This cooling is quite easy and cheap to do, but often doesn't get low enough in temperature. If one has LHe3, that can be pumped on to get down to about 200 mK. But this is difficult because LHe3 is so expensive, and closed-cycle pumps are needed so as not to waste the cryogen.
Dilution fridges can get to lower temperatures. We just got one of these fridges in our lab, and using that I've cooled some samples down to about 20 mK. Dilution fridges have fundamental limits around 6 mK or so, but physical limits usually kick in earlier than that due to equilibrium between cooling 'power' and heating (mostly due to radiation and vibration). The basic thermodynamics are actually quite similar to your standard fridge, and you can think of it as He3 'evaporating' out of the mash, absorbing energy as they do so. And later the He3 is condensed back into the mash.
Fridge operation basically has a mixing chamber, which is the 'cold' point of the system. One hopes to create the phase boundary between the two phases here. The mixture absorbs heat from the sample, and the dilute phase travels up to the still, where it's pumped on by some big-ass pumping lines. The liquid is effectively warmed up, gets circulated around and re-condensed by a cold block at about 1.5 K. [This block is called the 1-K pot and is only pumped LHe4]. There's a flow impedance put in (to calibrate the pumping power with the circulation to get the phase separation at the right place). Then it's back into the mixing chamber. Meanwhile there are many heat exchangers along the way, exchanging heat from the incoming rich phase and outgoing dilute phase. The cooling power of the fridge is greatly increased depending on these heat exchangers. The effective sample size in our fridge is a cylinder about 1 inch diameter and 10 inches long. The dewar itself is about 7 feet tall and 3 feet diameter, and there's a rack of electronics and four pumps to go with it. So it's a big unit for a relatively small cooling volume.
Dewers are designed using stainless steel and other components to minimize thermal conductance to room temperature as much as possible. Radiative heating, however, is a problem. The dewar is evacuated between the 'cold' part and the outside, to minimize conductance. Radiation goes as T^4, and this power law is greatly exploited in dewar design. If one surrounds the 'cold' part of the dewar with a LN2 shroud, the cold part sees radiation at 77K instead of 300K. This factor of ~1/4 translates to a drop in radiative heating power of about 1/250.
Beyond this dewars use superinsulation, whereby aluminized mylar is wrapped around many times (with spacers), so each successive layer sees a colder temperature. So the 20mK part of the dewar might only be surrounded by an effective layer of a few K. These methods cut radiative heating down by factors of millions or more.
make world, not war
Now, about this additional dimensions. You don't know what you are talking about.
He was making a reference to string theory. One of the ideas thrown around by string theorists is that while the particles for the electromagnetic, and strong and weak nuclear forces can only move in our 4 relativistic dimensions, gravitons can move in many more of the 11 dimensions. This would explain why it is so much weaker than the rest of the forces, since it expands in so many more dimensions.
But if gravity is weak because gravitons are "leaking" out into other dimensions, then it makes you wonder why it wouldn't also leak in as well, which is the point he is getting at.
The reason gravity is so weak is because the gravitational constant G is much smaller that the electric constant k. End of story.
Constants are simply numbers that we have determined experimentally, and thoughout the history of science we have often developed new theories that explain why the constants are what the specific values they are. What we know about subatom particles today is most assuredly not the end of the story, and there is no reason to think that we won't someday discover explainations for why the different particles have different constants.
Comment removed based on user account deletion
Comment removed based on user account deletion
You're missing some physics, allow me to fill you in:
/., it's very popular to put down statistics, but in real life, scientests use statistics for nearly everything. My theory is that most people hate statistical mechanics, so they don't realize the power and accuracy which it has, and don't trust it in general. Marketers and politicians, on the other hand, know little to nothing about statistical mechanics and abuse the math of statistics for personal gain. This further poisons people's minds against the elegance of statistics.
Particle detectors of these kinds detect all weakly interacting particles. Weakly interacting means that it will generally go strait through matter without "touching" it at all. Thermal equilibrium (temperature) is a non-issue in that weakly interacting particles don't contact enough with "normal" matter to ever come to thermal equilibrium. They cool it down so that stray vibrations don't give a false positive reading. A good example of a weakly interacting particle is a neutrino. You probably know that most neutrinos go strait through the earth, other stars, and a whole lot of the universe without even knowing that anything else exits. These guys probably pick up all sorts of neutrinos, and they use that measurement to calibrate their instrument. Neutrinos have a very low mass, and their energy spectrum on earth is pretty well mapped out now. If these guys get anything out of the ordinary, they'll jump on it and call that a WIMP.
So we get to the statistical part. Now, on
These guys aren't playing statistical games. They're calculating the probability that they get no measurement of WIMPS with the parameter being how easily they interact with the normal matter that makes up their instrument. They then use that to put an upper bound on how often they interact with matter. That bound is defined by the probabilities in their calculations (which you can probably trust since just about every theoretical particle physicist in the world has checked them). The exact level, be it 90%, 99%, 99.999999% or whatever is something the guys in the field have decided on, and it's in some paper somewhere.
The end result that these guys are looking for is how much dark matter interacts with the rest of the universe. Being able to say, "It interacts less than X with 99% accuracy" is pretty nice. At some point that "X" is going to get so low, the theorists will throw the idea out and move on. Unless, of course, the holy grail is found and someone detects dark matter interactions.
I hope that helps.
Where you are directly on target is to critisize these guys for doing all this research and gathering all this money for something that is so far unmeasurable and theoretically not much more than the "aether" of 150 years ago (I've met theorists who will admit to as much). The bottom line is that it hasn't been disproven yet either (people are trying to do that too). As long as that's the case, people probably should still be out there looking for this stuff.
I tried this on AIX 5.2, gcc 2.95, xlc 6. Code compiled with both compilers returns.. 0. (not segfault !)