Missing Matter... Still Missing 370
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!"
I "detect" a grant money detector at work... (Score:5, Insightful)
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... (Score:5, Informative)
Why do dark matter found (Score:2, Insightful)
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 (Score:5, Informative)
Re:Why do dark matter found (Score:4, Informative)
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 (Score:3, Insightful)
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. Everyon
Re:Why do dark matter found (Score:5, Interesting)
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 (Score:3, Interesting)
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
Basic assumptions may be wrong (Score:4, Insightful)
Re:Basic assumptions may be wrong (Score:3, Insightful)
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
Re:I "detect" a grant money detector at work... (Score:3, Insightful)
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... (Score:5, Informative)
I'm sure this is part of thier validation that the detector is working.
I "detect" someone jumping to conclusions (Score:5, Insightful)
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 (Score:5, Funny)
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 (Score:4, Funny)
>
>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 (Score:3, Funny)
Re:I "detect" a grant money detector at work... (Score:2, Informative)
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
Working detector? (Score:5, Informative)
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
Re:I "detect" a grant money detector at work... (Score:3, Informative)
Yes, you're missing something. :-)
The statement "the sensor has never detected something" is patently false. Figure 1 of the paper [lanl.gov] 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 shie
Re:I "detect" a grant money detector at work... (Score:3, Informative)
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 inter
Check the actual webpage... (Score:5, Informative)
But it's obvious... (Score:5, Funny)
Wimp?! (Score:5, Funny)
hehe (Score:2)
The Real Dark Matter (Score:4, Funny)
Thanks (Score:2, Funny)
Gravity is wrong (Score:4, Interesting)
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 [physicsweb.org] 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 (Score:2)
I agree.
A friend and I read The Elegant Universe and both came to the same conclusion - "dark matter" doesn't exist. The gravitational effects are due to gravitons entering our universe either from another brane, or from our own brane folded over in a higher dimension.
For those who haven't read it, according to string theory all particles except the graviton are bound to their "home" brane. Gravitons may m
Re: (Score:3, Interesting)
Re:Gravity is wrong (Score:2, Insightful)
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: (Score:2)
Re:Gravity is wrong (Score:3, Insightful)
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
Comment removed (Score:5, Informative)
Comment removed (Score:4, Informative)
Well, if it is... (Score:5, Informative)
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... (Score:3, Insightful)
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
Re:Well, if it is... (Score:3, Insightful)
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
Re:Gravity is wrong (Score:2)
Re:Gravity is wrong (Score:2, Informative)
As for cosmic acceleration, there's no particular reason to believe that that phenomenon has anything at a
Re:Gravity is wrong (Score:2, Interesting)
In other words, it a bit early to say that
Maybe - (Score:3, Funny)
What if software bugs emit gravitons? Wouldn't that explain the apparent extra mass in the universe?
Re:Maybe - (Score:2)
The Answer (Score:4, Funny)
Zero.
Opps. I meant, seven.
Re:The Answer (Score:3, Funny)
Opps. I meant, seven.
How about 42?
Re:The Answer (Score:5, Interesting)
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.
Enron? Worldcom? Tyco? (Score:2)
We have an excellent "culture" for a person possessing your skillsets.
Forgive my ignorance (Score:2, Interesting)
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 (Score:5, Funny)
You just described my entire high school career.
Re: (Score:2)
Re:Forgive my ignorance (Score:4, Interesting)
Re:Forgive my ignorance (Score:3, Informative)
> > 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 pa
Re:Forgive my ignorance (Score:2)
Re:Forgive my ignorance (Score:5, Informative)
--Tom
Great Location (Score:2, Funny)
The Cryogenic Dark Matter Search uses equipment at the bottom of a Minnesota mine to filter out all interference.
The underground observatory is some 1,000m beneath the surface. It is only from such an isolated place that scientists believe they have a chance of catching their quarry.
Excellent location choice! Even if they don't find any dark matter, at that depth they at least have a chance at locating the remnants of ex-Gov. Jesse Ventura's political career.
Soudan Mine (Score:2)
If you're in the Ely area, it's definitely worth a quick side trip to see the mine.
Hidden outside the horn of the universe (Score:2)
Re:Hidden outside the horn of the universe (Score:2, Informative)
From dictionary.com:
universe (yn-vûrs)
n.
If WIMPS were outside the universe, I'd thin
Dark Matter (Score:3, Interesting)
Massive wimps? (Score:2, Funny)
Missing Matter (Score:4, Insightful)
Re:Missing Matter (Score:3, Insightful)
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 (Score:2, Interesting)
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
Obvious, but ... (Score:4, Funny)
Did anyone check under the cushions on the couch?
20 times 0... (Score:2, Insightful)
The obvious solution (Score:5, Funny)
Re:The obvious solution (Score:4, Funny)
What's 20 times 0? (Score:4, Funny)
Re:What's 20 times 0? (Score:2, Informative)
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 (Score:2, Funny)
2:00 - "
3:00 - "
Not completely zero (Score:5, Insightful)
But at least they are still trying... and trying... and trying some more.
No events != 0 sensitivity (Score:5, Insightful)
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 (Score:3, Interesting)
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(s
Unusual science (Score:5, Interesting)
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 (Score:4, Interesting)
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 (Score:3, Interesting)
We know the missing matter is there because we can verify it. One example of how we know is that we can measu
Re:Unusual science (Score:3, Funny)
Of course, since Shiva will then destroy the world, you won't be feeling silly very long, which is a blessing.
Too much herb... (Score:2)
The answer lies on the other side of the aether (Score:3, Interesting)
Re:The answer lies on the other side of the aether (Score:4, Informative)
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.
Crawford math (Score:5, Funny)
I'd just like to be the first to say that it's an honor, Mister President, to count you amongst the Slashdot readership.
Wow (Score:2, Interesting)
[Dark matter] is thought to come from a variety of heavy particles that rarely interact with regular matter and can pass through conventional objects unseen.
That sounds like another phenomena of a less scientific nature... ghosts! In some belief systems spirits or souls are more massive or dense then normal matter as well.
If this were true, I would suggest the reason that this experiment didn't find any "dark matter" is because there wasn't any in the vicinity, becaus
But wait a minute (Score:3, Interesting)
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.
"Dark matter" sounds like (Score:2)
Lemme know when someone figures out what is really going on.
Maybe the current theory is wrong. (Score:5, Insightful)
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.
BBC Got it Wrong (Score:5, Informative)
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 [lanl.gov] though it might be a bit technical for most.
Thier detector may not have found anything... (Score:3, Interesting)
The case for dark matter (abridged) (Score:5, Insightful)
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...
Well (Score:3, Funny)
No kidding? I guess that's why it's referred to as MISSING matter!
Dark matter may not exist (Score:3, Interesting)
There are versions [sciamdigital.com] of M-theory which do not require one or both of these. There is also a theory [arxiv.org], 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 (Score:3, Funny)
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
Re:Chilled out (Score:5, Informative)
Re:Chilled out (Score:2, Informative)
Re:Chilled out (Score:5, Funny)
How do they do it?
Ever been to Minnesota? In the winter? You wouldn't have to ask.
Re:Chilled out (Score:4, Informative)
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:Chilled out (Score:4, Informative)
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.
Re:That explains it (Score:2)
(sorry, i couldn't resist)
Fortunately, we have large quantities of ... (Score:2)
Despite it being described as an unstable substance which every ethical scientist in the galaxy has denounced as dangerously unpredictable, I've found that it is the only way to solve certain problems.
Re:Multi Dimensions? (Score:5, Interesting)
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.
Re:Dark Matter? (Score:3, Informative)
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
Re:Another Hare-Brained Idea (Score:3, Interesting)
I said based on almost no facts, you haven't said anything here which challenges that. T