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!"

Comments

I "detect" a grant money detector at work...

[erick99]

That the sensor has never detected something doesn't tell you that it's working or not working - or am I am missing something here?

....Researchers from the Cryogenic Dark Matter Search II (CDMSII) say they are pleased with their first results, which show that their detector is working.

However since it started running in November last year, the detector has not seen a single WIMP.

Then they decide to make a more sensitive detector so that they can "not" detect at an even higher level?

Physicists with the CDMSII experiment say they will now add another 24 crystals to the detector, increasing its sensitivity tenfold.

Okay, maybe I am being a bit silly, but, I still don't see how they can know the detector is working. I don't even know how the WIMP can make the thing "ring" once it, itself, is subject to the 1/10 degree above absolute Zero conditions. And then, somehow, with no data, they can extrapolate more accurately how much dark matter is in the universe. Well, they would say the lack of WIMPS is data but I'm not buying it. Enough /. folks have worked in research to know better than to buy into those kinds of statistical games (you can prove almost anything with non-parametric statistics).

Happy Trails!

Erick

Re:I "detect" a grant money detector at work...

[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.

Re:I "detect" a grant money detector at work...

[reidbold]

A possible explanation:

The detector can measure interactions between protons and strong interactions (collisions with photons or what have you). But is not sensitive enough to detect interaction with WIMPs.

Re:I "detect" a grant money detector at work...

[confused one]

when we built a detector, we put in a mechanism for testing and calibration. So, we could apply artificial pulses to verify everything was working properly; and, because we knew the amplitude of the pulses we applied, we could calibrate the instrumentation.

I'm sure this is part of thier validation that the detector is working.

Re:I "detect" a grant money detector at work...

[Anonymous Coward]

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

Re:I "detect" a grant money detector at work...

[TMB]

That the sensor has never detected something doesn't tell you that it's working or not working - or am I am missing something here?

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]

Re:I "detect" a grant money detector at work...

[Ya Bolshoi!]

I'll bet it's easy to tell if equipment is running or not. A simple diagnostic--artificially creating what they're looking for--would tell them if the thing is working or not.

Then they decide to make a more sensitive detector so that they can "not" detect at an even higher level?

Basically, yeah, that's exactly what they're doing. If they can establish some kind of upper limit on the number of WIMPs, then that is a very important result that allows physicists to see what theories worked and which were just crap.

And then, somehow, with no data, they can extrapolate more accurately how much dark matter is in the universe.

They do have data. They know that there cannot be more that a certain number of WIMPs per unit volume, because otherwise they would have detected them. To use a vaguely similar analogy, if I'm trying to find out how many people are in a building, and I look in the first floor and see no one in it, that observation is data about the number of people in the building and it tells me things about how many people I can possibly expect to find.

At any rate, failed experiments are very important to physics. For instance, the Michelson-Morley experiments failed to detect anything, thus giving strength to the wave-particle theory of light. It would really have sucked for physics if after the first time, Michelson had said, "Oh, well, our equipment must be bogus, this is a waste of money."

Re:I "detect" a grant money detector at work...

[Goldsmith]

You're missing some physics, allow me to fill you in:

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 /., 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.

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.

Check the actual webpage...

[Xandu]

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.

But it's obvious...

[R2.0]

Anyone in high school knows that if a wimp hits anything, no one notices. If someone did notice, he wouldn't be a wimp.

Wimp?!

[Kjuib]

If a Wimp is about a thousand times more massive than a proton - what does that make a proton? a Wuss? or a Nerd?

The Real Dark Matter

[Nom du Keyboard]

The real dark matter in the universe is the massive SCO intellectual property rights that no one else has yet seen.

Gravity is wrong

[Ckwop]

I think the answer to the dark matter problem and the quantum theory of gravity is one in the same. Our description of gravity is wrong. It has recently been discovered that dark matter is 'missing' from three elliptic galaxies. One would think that on the scale of something as big as a galaxy and with WIMPs being so massive that you ought to detect some quite major effect..

Add that to the fact that the universe's acceleration is getting quicker rather than slowing down and I think we have a strong case for our description of gravity being incorrect.

Simon.

Re:Gravity is wrong

[Ckwop]

GR is perhaps the most well-tested physical theory yet developed and, as such, you can't say that it's "wrong". It plainly isn't once you remain within its field of reference.

You miss the point.. I'm citing the effects as evidence the theory is incorrect.

Simon.

Re:Gravity is wrong

[citdude]

The fact that dark matter is "missing" in eliptical galaxies means nothing. The amount of gravitational mass in young (or close) spiral galaxies is much higher than the visible mass that makes up the stars and the dust clouds around the galaxy. At some point, whenever my project gets approved, I will conduct similar research on older, farther away spiral galaxies that may give us some insight as to how spiral galaxies and the dark matter in them evolved together.
In other words, it a bit early to say that everything we know is wrong and that the "quantum theory of gravity" is right. No physicist is so arrogant to claim that they know what IS correct. Everything that we have is but a model. Hawking, Thorne, Preskill, Kamionkowski, Phinney, all realize this but the truth is that our current model of gravity works well most of the time and many people are working on finding a better one but notice that in the meantime, no on is abandoning our current model. It is still the best thing that we have.
You are also forgetting that while WIMPs may be big, they are neutral (like neutrinos) and interact very little with matter, making them difficult to detect. Give physists more time (or money) and they will get there. By the way, you are also forgetting gravitational lensing with things a big as galatic clusters. That is a major effect that cannot be explained by the mass that we can see too. And those galaxies incude all types of galaxies.

I'm done rambling now.
Scott

Re:Gravity is wrong

[Anonymous Coward]

If you think it is, give me one example of an experimentally observed physical process which violates general relativity. Just one.

Cheap shot: The rotation curve of spiral galaxies. Given the presumed distribution of mass, GR does not predict the star orbits. Either we aren't perceiving mass that's there, or GR is wrong.

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).

-- dn

Re:Gravity is wrong

[cyril3]

That might have been the wrong phrase for him to use as it does leave him open to riposts such as yours. But is it true that that vigourous defence of a position is not acceptable in the scientific method. This is after all only a discussion forum. Other posts have already suggested reasons why the data referred to may not destroy the GR/Gravity links.

Others have given you examples of why GR may be incorrect as it relates to gravity

But that doesn't mean their interpretation of the data is correct. If the accumulated evidence against GR as it relates to gravity was so clear this would be a discussion between scientists and the GR equivalent of the flat earth fringe who keep pushing the old GR/gravity line. But it hasn't come to that yet so discussion is still valid.

Is this the first time ever that data has suggested that GR may be incorrect as it relates to gravity. If not, what was the outcome.

prove your point with evidence

I thought the scientific method was to disprove your point with evidence. If you fail to do that you strengthen your view that the point you have is less likely to be incorrect than any alternative. As far as I can tell he merely suggests that the state of current observations are insufficient to show GR AIRT gravity to be unsustainable.

.I'm not a scientist either but this isn't a scientific forum. So it's onl a laymans view of how it works.

Maybe -

[Black Parrot]

What if software bugs emit gravitons? Wouldn't that explain the apparent extra mass in the universe?

The Answer

[theraccoon]

What's 20 times 0? And don't tell me zero!

Zero.

Opps. I meant, seven.

Re:The Answer

[Nom du Keyboard]

Zero.
Opps. I meant, seven.

How about 42?

Re:The Answer

[infinite9]

Opps. I meant, seven.

IBM may agree with you! Try this code on AIX:

#include"stdio.h"

int x,y,z;

int main() {

x=1;

y=0;

z=x/y;
printf("%d", z);

}

On most unix implementations you get floating point exception since the divide operator takes floating point operands. On AIX, when the denominator is cast to a float, it's a zero approximation rather than the official floating point zero. The result is that instead of a core dump, you get... 15.

Forgive my ignorance

[jm92956n]

A favoured theory is that the dark matter 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?

Re:Forgive my ignorance

[Mr. Bad Example]

> How could a wimp be so large and yet unnoticed?

You just described my entire high school career.

Re:Forgive my ignorance

[stox]

The Top quark was only discovered in 1995, and it is around 200 times the size of a proton.

Re:Forgive my ignorance

[jmtpi]

> > A favoured theory is that the dark matter
> > 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!

Re:Forgive my ignorance

[radtea]

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

Dark Matter

[dustmote]

I don't know, even though science has a track record of proving (at the time) absurd claims, dark matter just seems.....silly. (I typed darl matter here as a typo, that would have led to yet another SCO thread I'm sure) What are the other theories about the missing mass? I'd like to shop around and see if I can find one a little more reasonable-sounding. :)

Missing Matter

[Detritus]

I thought the usual rule in science was, if your theory conflicts with your observations, there is something wrong with your theory. Maybe there is no "missing matter", just an incomplete or defective theory of gravity.

Re:Missing Matter

[k98sven]

Well, for one: It's the only theory of gravity we've got really. If you can come up with one from as few (or fewer) postulates, which fits as well into the what we already know, and make the same predictions, I'm certain people will listen.

It's not as if everyone here has 'decided' that dark matter simply exists. There are plenty of people at work with alternative explanations.

However: If the theory is correct, and dark matter does exist, how are you supposed to find it without looking?

Re:Missing Matter

[Anonymous Coward]

That's what dark matter/dark energy always sounds like at first glance. However, it's not quite so simple, and cosmologists are not quite so stupid.

Actually, the "dark matter" hypothesis agrees with practically every observation you can think of: x-rays from galaxy clusters, the rate of formation of large-scale structure, the formation of deuterium/helium/lithium in the Big Bag, the cosmic microwave background, gravitational lensing by large clusters, AND (of course) the familiar argument that 'the outer parts of galaxies rotate too fast)'. The only odd thing about the hypothesis, is the fact that the dark matter apparently interacts only very weakly: with photons, not at all, thus 'dark'; with gas, stars and planets hardly at all, thus the extended shapes of galactic halos; and with CDMS, too weakly to have been detected so far, apparently.
Weakly interacting particles shouldn't surprise us - think of neutrinos, no? - and indeed a very powerful theory called SUSY predicts the existence of dark matter, based on elementary particle physics constraints.

The idea that you can fix the 'dark matter problem' by modifying gravity is interesting, and it has been thought about a lot. It's hard to make any sort of theory agree with all of the measurements I mentioned. Google for "MOND" (Modified Newtonian Gravity" to learn more.

Obvious, but ...

[nicodaemos]

Missing Matter ... still missing

Did anyone check under the cushions on the couch?

Why do dark matter found

[Anonymous Coward]

because there isn't any.

The astronomers have been seeing something they do not understand, and so they assume it is dark matter. The same result could be gotten by a decaying speed of light.

Unfortunately, that requires another rewrite of physics, from the ground up. However, looking for something understood gets more grant money.

Re:Why do dark matter found

[Profane MuthaFucka]

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.

Re:Why do dark matter found

[gewalker]

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.

Re:Why do dark matter found

[Profane MuthaFucka]

No, I like all those theories, including decaying lightspeed as explanations equally well. That is to say, I have no preference for any of them, except the one that will eventually have the most evidence. Decaying lightspeed is off to a spectacularly bad start...

But, I don't think that scientists are staying away from that theory because they don't want to rewrite the Physics textbooks.

Look at the last guy who rewrite the Physics textbooks. He's got one of the most recognizable faces in the world. Everyone considers him to be wonderful, despite the fact that he doesn't comb his hair. His name is considered to be the definition of intelligence. His ideas helped to invent the Atomic Bomb yet the Einstein Peace Prize is named after him.

Now tell me who in the world wouldn't want to be considered the Albert Einstein of the 21st century? The conspiracy theory proposed was just ludicrous.

Re:Why do dark matter found

[TMB]

There seems to be a common sense here that all of the evidence for dark matter could be equivalently explained by changing the force law.

However, that isn't true. One unique test of dark matter is that it is dynamical; it can move. And there are a bunch of tests that have started to be made that show evidence for dynamical dark matter:

- in order to explain rotation curves without dark matter, models like MOND require force laws that would make the derived "shape" of the dark matter halo spherical at large radius. You can test this by looking at the shapes of clusters using X-ray emitting gas (eg. Buote et al. 2002, ApJ, 577, 183; Lee & Suto 2003, ApJ, 585, 151; Lee & Suto 2004, ApJ, 601, 599) or the Sunyaev-Zeldovich effect (LS03,LS04). You can also look at the shapes of dark matter halos around galaxies using weak gravitational lensing (Hoekstra et al. 2004, ApJ, 606, 67). So far all of the tests indicate that dark matter halos are not spherical, but flattened exactly as predicted by cold dark matter.

- the bars in barred spiral galaxies should slow down and disperse quickly in a spherical static halo potential, like you'd get from modifying the force law, but they can be maintained for long periods of time if they can exchange angular momentum with the dark matter (Athanassoula 2002, ApJ, 569, L83; Valenzuela & Klypin 2003, MNRAS, 234, 459).

- there's a weak gravitational lensing observation of a group that is falling into a cluster, where the mass of the infalling group is offset from the light - the gas is moving slower because it's interacting with the cluster gas, while the dark matter has kept moving (Clowe et al. 2004, ApJ, 604, 596).

[TMB]

Re:Why do dark matter found

[Thing 1]

I like the idea that, once we develop nanotechnology a goal should be to begin to develop Dyson Spheres, so we can capture 100% of each star's output and save it in batteries to be rationed later. We can make the universe last longer that way (a year or two ago it was determined that we won't contract: we were sentenced to a heat death. So we might as well conserve as much as possible; think big.

So if that's a goal of ours, perhaps it's a goal of another race's. And perhaps they got a head start on us, and that large percentage of "dark matter" actually consists of Dyson Spheres which capture everything, so are "undetectable" by us. That's pretty scary, to think that we just lost that much playground, and will eventually have to deal with the bully--on his own terms perhaps.

I mentioned this a year ago or so, and someone pointed me in the direction of Matrioshka Brains, so I will include some links for that as well. And an excellent discussion.

I would add to the last part that the larger planets could be taken apart by space elevators as well. They'd just start with the upper atmosphere; then work their way down. All the time the mass is getting smaller, and the elevators are pulling mass out so they can make themselves bigger in order to reach deeper. I think it's workable, and appears to be the most efficient way to do it--get the mass all out into "orbit" first. Actually, when you're about halfway done you can then start shipping what you mine off to other locations, and taking that amount of mass out of the elevators as well since they won't need to counterbalance as the planet's now smaller. (I don't know what the mathematical "middle point" where you start dismantling the elevators actually is--it could be something other than 50%.)

We could have "planet splitter seeds" which we shoot off to other stars, and they start with a tiny, correctly-placed elevator and build more of them as fast as is physically possible; the seed would be smart enough to calculate all the masses and start with the most effective one that would lead to the earliest date at which the entire mass of the star system is being used for computation.

The only problem is if we encounter life. Will our machines just assimilate it? Are the ones out there programmed to preserve us? Have they already done so?

20 times 0...

[aaribaud]

... would be the efficiency of the experiment (assuming it would fail the same way as this one), not the sensitivity of the equipment used.

Thanks

[Anonymous Coward]

I was wondering how this story was going to be tied into SCO or Microsoft.

The obvious solution

[fiannaFailMan]

All they have to do is reverse the polarity of the anti-proton injectors in the warp core, re-route the resulting subspace pulse through the plasma conduits, synchronise the comm-system to transmit the frequency of the subspace distortion field to the deflector dish and emit a sub-tachyon particle scan over a wide area. That'd surely reveal what they're looking for!

Re:The obvious solution

[JaxGator75]

Pshaw... Like they didn't already try THAT ...

I "detect" someone jumping to conclusions

[Vellmont]

That the sensor has never detected something doesn't tell you that it's working or not working - or am I am missing something here?


Yah, you're missing the scientific paper. This is a one page write-up written by a journalist. The one page write up doesn't describe how they know the detector works, but I'm sure they have _some_ means of testing that it does. Blame the article, but at this point you can't really accuse anyone of doing shoddy science for grant money.

Re:I "detect" someone jumping to conclusions

[Uber Banker]

The one page write up doesn't describe how they know the detector works, but I'm sure they have _some_ means of testing that it does.

Sure they do... the system has a green light on. If the red light were on it would be on standby and no light may mean there is no power, or the light is broken. But as long s the green light is on they know it's working.

Surely everyone knows that. Now please increase my grant.

Re:I "detect" someone jumping to conclusions

[Tackhead]

> Sure they do... the system has a green light on. If the red light were on it would be on standby and no light may mean there is no power, or the light is broken. But as long s the green light is on they know it's working.
>
>Surely everyone knows that. Now please increase my grant

You forgot the third possibility. Suppose the power indicator LED is orange: it's hard to tell if we're in a superposition of states or merely oscillating very rapidly.

Or I just want a high-speed digital camera for Christmas.

Re:I "detect" someone jumping to conclusions

[Dun Malg]

You forgot the third possibility. Suppose the power indicator LED is orange: it's hard to tell if we're in a superposition of states or merely oscillating very rapidly.

...or that some bonehead wired the unit up to 24VAC instead of DC....

What's 20 times 0?

[Animus Howard]

Dunno. But 20 divided by zero is &)%*$%*_))[LOST CARRIER]

Re:What's 20 times 0?

[Barryke]

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]

ObBart

[sharkey]

1:00 - Still just a potato.
2:00 - "
3:00 - "

Comment removed

[account_deleted]

Comment removed based on user account deletion

Not completely zero

[kyoko21]

I am not a physics/math expert, but assuming that dark matter does exist, it only proves that the equipment currently used has a sensitivity that is approaching zero, but not zero. But anyone who has seen a graph of an asymptope, it is not very promising especially if you push x approaching infinity. Even if you were to multiply x by 20, while you are out to infinity, by not knowning where exactly they are relative to the origin on the graph, a factor of 20 may not be all that significant... :-/

But at least they are still trying... and trying... and trying some more.

Re:Chilled out

[citdude]

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.

Well, if it is...

[Pi_0's don't shower]

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...

Re:Well, if it is...

[websensei]

general relativity solves *so* many problems that cannot be solved otherwise that it's preposterous at this point to consider anything else

while I agree that the case for GR is pretty compelling, this same line of thought is why it took so long for ptolemy's ridiculous (in hindsight) orbits to be debunked. "but they solve so many problems that cannot be solved otherwise that it's preposterous to consider anything else"... ditto for many other "givens" (sun circles the earth, etc) in history. my point is that a scientific mind is always prepared to be repeatedly shown completely wrong - and in fact delights in this process, as it moves understanding closer to fullness. /$0.02

Re:Well, if it is...

[blincoln]

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.

Newton's physics accurately describe a lot of things - and are still very useful - but they are *not* a correct description of the way the universe really works.

General Relativity is the same way. It accurately describes many things, but eventually it will be superceded by a more complete theory.

Re:Well, if it is...

[Stalyn]

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*.

Maybe I'm confused but in the article they use words like incompatible and inconsistent with DAMA. The also say this is the new limit on WIMP-nucleon scalar cross section. So apparently unless I'm wrong.. which I probably am CDMS II went further then DAMA and didn't find anything. Anyway check out fig. 5, and if I am wrong an explanation of it would be nice.

Re:Well, if it is...

[Stalyn]

i'm an idiot i confused now with not... stay off the drugs man

No events != 0 sensitivity

[Shurhaian]

Wow, what a suprise, hyperbole in the Slashdot summary.

The fact that the detector hasn't found the thing it was designed to detect doesn't mean that it has a zero sensitivity or that the hypothesis is bogus(you can't readily prove a negative except by proving a contradictory positive), just that, in the finite time it's been running, it hasn't been sensitive ENOUGH to detect anything. 20 x 0.00000000000000000(you get the picture)001 is still an improvement, and may be enough to make progress.

Re:No events != 0 sensitivity

[Shurhaian]

This is certainly true. Obviously, common sense needs to be applied. The only point I was initially trying to make is that the "20 times zero is what?" comment in the post was at best an exaggeration.

There definitely comes a point where these people should be told to fund it themselves if they're so keen on it, but a lack of results does not in and of itself guarantee that the notion is unsound. A whole bunch of conflicting results is another matter, but a lack of evidence isn't statistically significant(sample size is zero).

Actually, I take that back. The sample size of this technique is insufficiently large to reject the hypothesis on a result of zero. Given that it's made to detect inherently rare events, a size of 0 would be within its data scatter. There's probably a point at which they can state that if they're not detecting WIMP interactions, then WIMP - at least alone - can't account for all of dark matter(or the proportion they thought it did according to the hypothesis). They might be some of it, but other things will then need to be looked at.

In short: It's possible for a limit value(in this case, zero events) to not be statistically significant. Consider it as a truncated normal curve, with 0 within a few standard deviations. You have two basic ways to avoid the insignificant limit problem: increase the mean value, or decrease the standard deviation. The former is possible by making a more sensitive detector(which could simply be bigger), the latter by making it more precise(which would probably mean, among other things, covering a MUCH larger area and likely multiple sites, and is in general harder to accomplish if you aren't even quite sure what you're looking for outside of unproven math).

So, while WIMPs wouldn't be disproven by a lack of results, it could eventually be stated that, since a count of zero is no longer within the variance of the hypothesis and the instruments used to test it, a persistant zero count would mean that the WIMP count is significantly lower than the prediction of the hypothesis, so the hypothesis would need to be revised.

Re:Chilled out

[r_j_prahad]

The detector is also chilled to within a tenth of a degree of absolute zero [...]

How do they do it?

Ever been to Minnesota? In the winter? You wouldn't have to ask.

Unusual science

[Control Group]

I freely admit to not being a physicist, cosmologist, or astronomer. However: when Einstein formulated general relativity, he discovered that his model demanded either an expanding or a contracting universe. Since he "knew" the universe to be static, he introduced the cosmological constant to "fix" the model. Later, of course, when Hubble (I think?) demonstrated the universe to be expanding, the cosmological constant was dropped, and Einstein referred to it as his greatest mistake.

This research, though, seems to be taking the same route: rather than questioning the model, they continue a so-far fruitless search for the "missing matter." If the model demands something the existence of which we are completely unable to verify, shouldn't we be questioning the model? Doesn't the very fact that there's all this "missing" matter indicate that perhaps our understanding is flawed?

Or am I just displaying rampant ignorance of the current state of physics and cosmology by asking this?

Re:Unusual science

[rblum]

You might want to read up on VSL theories. They do make sense of the cosmological constant, and they solve several other problems. Homogeneity amongst them, which, AFAIK, is a rather big deal to cosmologists :)

It's not proven or anything, and it competes with inflation theory. But it looks like it might be experimentally verifiable, as opposed to inflation.

Re:Unusual science

[stand]

This research, though, seems to be taking the same route: rather than questioning the model, they continue a so-far fruitless search for the "missing matter." If the model demands something the existence of which we are completely unable to verify, shouldn't we be questioning the model? Doesn't the very fact that there's all this "missing" matter indicate that perhaps our understanding is flawed?

We know the missing matter is there because we can verify it. One example of how we know is that we can measure the rotational velocity distribution of a spiral galaxy as a function of radius from the center. We can also measure the luminosity distribution. Given the latter, and the fact that we have a pretty good understanding about how stellar luminosity relates to stellar mass, we expect the velocity distribution to vary in a predictable way according to gravitational laws. This comparison indicates that there is a lot of mass in galaxies that doesn't produce any luminosity.

This is a case where independent measurements don't produce consistent results, not of a theoretical model failing to match up with measurements.

Re:Unusual science

[SEE]

You know, when our probes hit the sphere surrounding the solar system and reveal the entire cosmos beyond 250 AU is just an animated image on the shell of the Universe, you're going to feel awfully silly for believing in dark matter.

Of course, since Shiva will then destroy the world, you won't be feeling silly very long, which is a blessing.

The answer lies on the other side of the aether

[eljasbo]

I am not a physicist, but I think the dark matter is a totally false idea fabricated to explain things we cannot explain with our current perception and knowledge of physics. It is similar to the aether idea that was fabricated to explain Maxwell's equations on a cosmological scale so they did not collide with Newton's theories. The more we figured out about the properties of the aether, the more magnificent it needed to be. Einstein realized that Newton's common sense laws were actually different than we perceived and rewrote physics by determining that the existance of the aether was incorrect, and what we observed was caused by relativity. I think the same holds true with dark matter. What we are observing is the effect of gravity traversing dimensions other than the four we normally encounter. The other eletromagnetic forces do not cross into these dimensions, but gravity does. This would also explain why gravity seems so much less powerful than the electromagnetic forces, it is spread out through multiple dimensions. We know there is a force somewhere and lots of it, but can see no evidence of it because it is beyond our perception. We only see the effect of gravity particles (gravitons) that are traversing into our dimension from the others. Perhaps there really is the aether all around us, and it is more spectacular than ever imagined. This aether would be multidimensional and be everywhere. We cannot see or cross the dimensions we are in into another one. But they are there on the other side of the aether. The gravitons pass right through it and that is what we observe.

Re:The answer lies on the other side of the aether

[pavon]

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.

Re:Hidden outside the horn of the universe

[benchbri]

IAKAP, but I do have access to a dictionary, and the universe, as defined, is EVERYTHING. Nothing exists outside the universe, by definition.

From dictionary.com:

universe (yn-vûrs)
n.

1. All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole.
2. 1. The earth together with all its inhabitants and created things.
   2. The human race.
3. 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"

Re:Chilled out

[Anonymous Coward]

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!

Crawford math

[MoxCamel]

What's 20 times 0? And don't tell me zero!

I'd just like to be the first to say that it's an honor, Mister President, to count you amongst the Slashdot readership.

But wait a minute

[Billly Gates]

Only according to some sources 1/4th of the matter in the universe is dark matter. How can it not be undectable then if its this massive particle 1000x more dense then a proton?

I personally think its a lack of understand of space/time that creates the illusion of dark matter. The string theory could also prove that bends in the time/space contium alot like threades of lint in carpet exist. When light passes through them they amplify when they reach the bend.

There is alot of stuff in the 4th, 5th, and other dimensions that we do not know about.

Re:Chilled out

[painandgreed]

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.

Re:Hidden outside the horn of the universe

[DaZedAdAm]

Alright, maybe I'm just behind the times or a bit confused, but for the life of me I can't figure out IAKAP. Perhaps it was meant to be IANAP (I am not a physicist)?

The only other thing I can come up with is "I ain't knowledgeable about physics," and I'm just hoping that a statement like that wouldn't be abbreviated.

Working detector?

[Anonymous Coward]

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

Maybe the current theory is wrong.

[Jagasian]

Maybe there is no dark matter. Science only describe predicted observations. Reality doesn't necessarily obey the laws of science. Belief in such is similar to belief in a deity. Maybe the universe is governed by the laws of science, but then again, maybe it is governed by such-n-such a deity.

So if a theory isn't cutting it, then create a new model of whatever observation that you are trying to describe. It seems silly to try to fit nature to the theory, and not the theory to nature.

Re:Multi Dimensions?

[Chris Ashton 84]

One of the possible outcomes of string theory is multiple universes, each separated by a fairly small distance (of course this distance is in a higher dimension so we can't notice them). If these alternate universes do exist, it is thought that the gravity from particles in our universe affects the other nearby universes. Imagine our universe as a flat sheet and another universe is a parallel flat sheet close to ours. In this model, gravity would still be three dimensional - ie, it would be able to bridge the gap between universes and affect the other universe. Perhaps this is what we're noticing - the gravity of massive particles in another universe?

BTW, I am not a physist but I have read up on this stuff. The theory of gravity carrying over to other universes actually does make sense - it explains why gravity is so much weaker than the other forces, because much of gravity's effect isn't on this universe. There's experiments going on now to test and see whether this is actually the case but I don't know the outcome. Anyway, this is just my thought on perhaps why we can't detect the dark matter - because it's not physically in our universe.

Basic assumptions may be wrong

[arminw]

The theory of dark matter is based on the assumption that the basic properties of the Universe have never changed over time. If the intrinsic properties of space itself HAVE changed significantly, then there is no need to postulate such a thing as dark matter. Scientists are very reluctant to accept new data that shakes their preconceived pet ideas to their foundations. It took over 200 years after Roemer first measured a finite light speed, for the majority of scientists to accept the fact that light did not get instantaneously from point A to point B, as was the belief for centuries. In the same way, the majority of scientists today refuse to even consider the idea that some very fundamental "constants" may have changed dramatically since the beginning of time. For example, the cause for the "Red Shift" of distant star light is traditionally attributed to the Doppler effect, and in light of that INTERPRETATION of the cause for an observed fact, (the shifted light) all sorts of cosmological observations are very difficult to explain. Humans (including scientists) like to assume that certain things stay the same for all time, but that is a fervently desired wish based on faith, not observed fact. It seems that in the physical universe, there is nothing as constant as change! AAW

Re:Basic assumptions may be wrong

[Mant]

Scientist assume things stay the same, unless they have some evidence to the contrary. This is just Occam's Razor (Entities should not be multiplied beyond necessity).

If you don't have evidence the constants change, and they are called constants becuase they have always been observed to be the same, don't start assuming they have been without evidence. It isn't about faith, you assume it has not changed based on the observed facts that nobody has seen it change. If there is evidence of change, you rethink your assumptions.

Sometimes new ideas do take a while to displace old ones, as there is indeed resitance to paradigm shifts in science. Sometimes for the all too human reason of disliking change and what you thought you knew being swept away. It's better than jumping on any new theory or assuming things never observed to change have done becuase it is convenient to current thinking.

If the basic assumptions are wrong, they will eventually be disproved under the weight of evidence, but they should only be thrown out at that point.

Re:Basic assumptions may be wrong

[lyphorm]

Nice essay. Too bad it's wrong. The theoretical model is continuously being revised to try and match it with what is observed. The term Dark Matter (and Dark Energy too) is just there to describe the discrepancies between the model and the observations. Some scientists are sure Dark Matter is real matter, but currently only detectable by its effects on other observable matter. Other scientists believe it is the physical model that is flawed and Dark Matter isn't matter at all. Both camps (and others) are trying to find evidence to support their case.

Of course the not-so-constant constant idea has come up, but so far there is no convincing evidence to support it.

The humorous part is how you made a bunch of assumptions about how science works, while chastising the scientists for making assumptions...

Re:Dark Matter?

[painandgreed]

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.

BBC Got it Wrong

[Roger W Moore]

Just in case you got as far as the BBC article they got the dark matter percentage way off. It's actually about 23% that is dark matter. The 70% number is for dark energy which is a completely different beast which nobody yet really understands (at least to my knowledge) since it is actually gravitationally repulsive and is what is thought to be causing the Universe's expansion to accelerate.

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.

Re:BBC Got it Wrong

[fizbane]

The BBC Article says that: "This form of matter comprises more than 70% of the Universe's mass, far more than the stars and galaxies we can see." This is technically correct. The WMAP results are that 73% of the energy density of the universe is "dark energy." The remaining energy density is matter. From galactic rotation curves, gravitational lensing, etc. we suspect that dark matter accounts for around 80% of the matter (mass) in the universe.

Thier detector may not have found anything...

[ifwm]

but my bullshit detector has been pinged since this dark matter nonsense started. I have yet to see a single piece of credible evidence that shows the dark matter hypothesis is anything more than a hastily concocted attempt to explain why some physicists theories don't jive with experimental results. Just admit that you don't know why, rather than attempting to pass off a clearly inferior piece of intellectual flotsam as the next great thing. Show me some evidence, ANY evidence, and then you'll get my money.

Two atoms were sitting in a bar...

[Linker3000]

"Oh no", says the first
"What's up?", Says number two
"I've lost an electron", says #1
"Are you sure?", Asks 2
"Yep, I'm positive" Says 1

The case for dark matter (abridged)

[jpflip]

The thing that makes the dark matter explanation compelling is that it makes so many different observations work. We don't have to fine tune things so much - it all fits together. Here are some examples.

1. Galaxy rotation curves - you can watch the orbits of stars in a galaxy to determine the distribution of matter in the galaxy. This shows that there is a lot more matter than can be accounted for by the stars and that it is distributed differently.

2. Gravitational lensing - you can see how light is bent by distant galaxies to map out their matter distributions. Again, there's a lot more matter than the stars can account for, distributed differently.

3. The cosmic microwave background - this one is complicated, but the idea is that you look at the "afterglow" of the big bang, released when the universe was as dense and hot as the surface of a star. We understand the physics of matter at these temperatures very well, and by studying the signatures of vibrations in this hot plasma, we can measure the properties of the early universe. We can see from this that the universe contains a lot of matter, and that the large majority of this matter is not composed of ordinary atoms (hard to explain, but fairly rock solid).

4. Light elements - Most of the universe's helium, deuterium, lithium and beryllium were created in the early universe, not in stars (the conditions aren't right). Again, the physics is very well-understood, nothing fancy. By studying the relative ratios of these elements, we can figure out the properties of the plasma in which they were formed (a bit hotter and you get less deuterium, the temperature falls too quick and you get less helium, stuff like that). Again, the universe has a lot of matter, and most of it isn't made of atoms.

5. Structure formation - if you work things out on supercomputers, you find that (if the universe containst only ordinary matter) the universe hasn't been around long enough to form the galaxies and galaxy superclusters that we see. Adding dark matter to the mix makes galaxies form faster - just enough faster!

And the beautiful thing is that all of these different arguments give essentially the same answer for the amount of dark matter and its basic behavior. You can tweak your theories to explain some of these observations, but no one has been able to explain them all - except with dark matter, the SIMPLEST explanation!!

Before you say something is "clearly inferior intellectual flotsam", learn what you're talking about...

Re:The case for dark matter (abridged)

[Grayswan]

...no one has been able to explain them all dark matter, the SIMPLEST explanation!!

So, dark matter is just like God. It explains everything and has never been experimentally observed. Ya just gotta believe. Physics is the new religion and physicists the new priesthood. I think someone predicted this a while back.

Re:Chilled out

[wass]

Some other posts describe this a little, here's some more info.

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.

Well

[Lord Kano]

The most powerful search yet for the Universe's missing matter has come up empty handed,

No kidding? I guess that's why it's referred to as MISSING matter!

Maby Suntola is right with his DU theory

[Anonymous Coward]

* in DU there is no dark matter (if I understood right)
* DU is simpler than General Relativity
* DU predictions are close to GR, but differs slightly - those exceptions should be verified
* DU theory predicted also that the speed of light is getting slower - phenomenon found by Australian scientists lately

Check:
http://www.sci.fi/~suntola/DU,%20Main/DU %20Main.ht m

Re:Another Hare-Brained Idea

[shaitand]

"Actually the amazing this is that we CAN claim to be aware of the size of the universe! Since everything is accelerating away from us as the Universe apparently expands we can extrapolate this expansion back in time until all the matter in the observable Universe was at a point i.e. the Big Bang. Since we now have a time when the Universe began we now have a maximum size limit for the Universe ~ speed of light x age."

I said based on almost no facts, you haven't said anything here which challenges that. The big bang is a theory, that makes any facts determined based on when it happened a theory also. Your basing the starting point on pure theory. There are no facts in anything you've just said. And you certainly can't presume to impose proofs which rely on another contradictory theory on his theory.

"until all the matter in the observable Universe was at a point"

For a second let's pretend to give you the big bang. Now, you say we determine this from knowing when all matter we can presently observe was at a single point based on the rate of theoretical expansion that may or may not be happening. No matter how far we've looked, and been able to look, we've found more universe. So wouldn't it be fairly reasonable to believe it might just be a tad arrogant to believe that how far we can presently see is as far as can be seen?

And if so, what about the time it takes for the rest of the matter we CANNOT see yet to get back to this single point of yours? Ok, so now we've discovered that both the method by which you propose to gauge the size of the Universe and the Age are full of holes. And as you propose them are Paradoxically reliant upon one another. How can you determine the size of the universe by determining how long it would take for all of it to be retracted back to a single point, when you don't know the size and thus how far it stretches to begin with? Further how do you know the Universe expands at a constant rate (if it expands)?

If it accelerates, what makes you believe it accelerates at a constant rate. Unfortunately physics in the present day is full of these kinds of holes at almost every turn and is pretty shakey at best.

"Well that is not quite true. We can observe a tremendous amount of the Universe and we see no evidence for mass being created since creating mass from nothing would violate probably the most fundamental law of physics, namely conservation of energy. This conservation law is caused by invariance of all the physical laws (that we know of) under spatial translations. So in order to create matter/energy you would need to introduce a new physical law that does not hold the same for all spatial coordinates."

Yes, we have observed a tremendous amount of the Universe, relative to our perception of it. A field mouse on a hill might believe he's observed the entire world, or most of it, but that hardly makes it true. We have no way of knowing in fact the size of the Universe, and therefore have no way of knowing if that portion we know about constitutes a billionth of a percent of it, 50%, or even no percent in an infinite universe.

Further, nobody has proposed creating mass from nothing here, someone has however proposed that mass from another location is moved to this location. That in itself violates none of the fundemental laws of physics that I'm aware of.

"So while we cannot categorically rule out mass being created somewhere in the Universe it requires lots of new physics for which there is absoulely no evidence. Thus it is far more likely, given our current understanding, that mass is NOT being created and added to the Universe."

Odd, I was under the impression that about the only thing in physics we could be absolutely sure of was that mass was at one point created. After all it had to come from somewhere right?

Comment removed

[account_deleted]

Comment removed based on user account deletion

Dark matter may not exist

[Phragmen-Lindelof]

The assumption that "dark matter" exists is a common one based on (some of) the observations of the universe. Dark matter does not explain the increased rate of expansion of the universe at great distances. This requires another assumption - "dark energy" (or a positive "cosmological constant").
There are versions of M-theory which do not require one or both of these. There is also a theory, as yet unpublished (since it upsets physics journal editors), which eliminates the "clock hypothesis" and accounts for inflation and accelerated expansion. (One has to be careful to take each new (and old) theory in physics with a big grain of salt.)

Just as the biological community "sold" the human genome project as THE ANSWER (one gene = one protein) and is trying to sell the protein folding problem as the NEW ANSWER (and it is an important problem), the (majority of) the (astro)physics community is trying to sell "dark" (matter or energy). "Dark" may well exist. I believe that it is important to allow a variety of views in the physics community to be heard (i.e. published) and let scientists design experiments to test various hypotheses. The "popular" theory may (or may not) correspond to observations.

Socks

[shadowbearer]

The should be concentrating on where that missing sock always goes when you do laundry at a laundromat. Find that, they'll probably find the missing matter. At least the research should be cheaper :)

SB

Comment removed

[account_deleted]

Comment removed based on user account deletion

There's NO MISSING MATTER it is aSimple as that!

[Chris Coles]

The problem with the debate about missing matter is that the underlying theory, Big Bang, is fatally flawed and thus all the derived associated theories are built upon incredible foundations. Like a house built on soft mud, no matter what you try, it keeps falling down. You may have noticed that Steven Hawking recently abandoned his Theory of Everything. UK Sunday Times Colour Magazine "Hawking's Big Bang". We believe he did that because he has read a new book, The Universe is a Cloud by me, Chris Coles. Further, we have produced a e-book of a second edition http://www.lrsp.com/ebooks.html that goes even further into where the present theories are wrong. One of the consequences of this book is that you should by now have noticed that the singularity has dissappeared. Take for example the big bit in Scientific American about time this month. In essence, they have made some really silly mistakes that, because they are so silly, really stupid mistakes, they are not even prepared to debate them. The first BIG mistake was that when a stars mass, (any star's mass), grew to the point that light was prevented from escaping because of the strength of gravity, (what is described as an Event Horizon - the point beyond which light does not transmit), they always proposed that from that moment all the additional mass that was sucked into the star was also, from that point onwards, always inside that event horizon. But think about that. The event horizon is simply a mathematical point; mass sufficient to prevent light escaping. Thus that mass is always the point where light cannot escape. Adding mass is like trying to pour more coffee into a cup once the cup is full. For once the cup is full, all the additional coffee must be outside of the cup. It is mathematically impossible for the coffee to be added to the cup; same with an event horizon. The event horizon is simply a notional point where gravity is so high, light cannot escape. Beyond that point, all mass must be OUTSIDE of the event horizon and that leads us into a completely new view of the universe. Read it and find out why Steven Hawking has abandoned his theories. http://www.lrsp.com/ebooks.html