3 ms·

Has dark matter finally been detected?

46 points by indy 17y ago

anigbrowl 17y ago
Surprisingly accurate for a newspaper. Project: http://cdms.berkeley.edu/ http://cdms.berkeley.edu/ Nice to see that upgrades are planned to the detection equipment. Besides confirming theoretical predictions and presumably improving our understanding of cosmology, are there any potential applications for this knowledge at an earthly scale?
- amichail 17y ago
  Besides confirming theoretical predictions and presumably improving our understanding of cosmology, are there any potential applications for this knowledge at an earthly scale? One idea is to use dark matter as fuel for interstellar travel: http://www.newscientist.com/article/mg20427361.000-dark-power-grand-designs-for-interstellar-travel.html http://www.newscientist.com/article/mg20427361.000-dark-powe...
- andrewcooke 17y ago
  > are there any potential applications for this knowledge at an earthly scale? in broad terms, it's unlikely that any new physics has applications at an earthly scale. the argument is quite simple - if there are significant differences between whatever is "new" and what we currently know at the kinds of scales that are appropriate for applications then it would be easy to do the experiments. the very fact that physics is reduced to using astronomy or the lhc to explore extreme conditions indicates that anything new applies only at scales that are largely irrelevant to us here and now.
simon_ 17y ago
IANAP, but wouldn't confirmation of the existence of supersymmetric WIMPs not necessarily prove that they account for a large portion of dark matter? EDIT: Not to imply that it wouldn't be a huge deal anyway.
- hugh_ 17y ago
  IAAP,BNTRTOP but I assume you could count how many you detect to get a reasonable idea of how common they are in this part of the universe, at least. Then you could adjust it for the fact that WIMPs tend to prefer to hang out in gravity wells, like around here, and get a pretty good idea of the overall distribution. I haven't done the maths, but if you're detecting _any_ of 'em then they've got to be reasonably common, since they should be relatively uniformly distributed throughout the galaxy. What I didn't understand until just now was this: if WIMPs interact by gravity why don't they fall into stars and planets, or even form stars and planets of their own? The answer is that they do fall into stars and planets, but just keep going through the middle (since they are not subject to any ordinary interactions) and out the other side. And they can't form dark-matter planets of their own since they don't interact strongly with each other either, so they won't stick together. Neato!
  - rms 17y ago
    BNTRTOP?
    - JacobAldridge 17y ago
      But Not The Really Top Order Physicist? Boy, Newton Totally Responded To Our Planet? Basically, Nobody Trusts RMS To Out-think PG?
      - hugh_ 17y ago
        The others who said "but not the right type of physicist" correctly ascertained my meaning, though yours are all quite accurate as well.
        carbocation 17y ago
        I was imagining, "But not the relevant type of physicist." Excellent new acronym.
    - samdk 17y ago
rms 17y ago
http://resonaances.blogspot.com/2009/12/cdms-live.html http://resonaances.blogspot.com/2009/12/cdms-live.html "18:20 Summarizing, no discovery. Just a hint of a signal but with a very low statistical significance. Was fun anyway." As the newspaper article said, a 75% chance of finding new particles.
- hugh_ 17y ago
  Also from Cosmic Variance: http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/dark-matter-detected-or-not-live-blogging-the-seminar/ http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/... 2:50: Bottom line: “The results cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject the possibility that either event is signal.” –Risa To which I reply "Hang on, so you made a big deal out of this for what? Wake me when you have something which _can_ be interpreted as significant evidence for something"
thras 17y ago
No it hasn't. This is 2 events in an expected background of 0.8. According to reports it's a 1.5 sigma event. Noise. This can be dismissed until they get to 3-5 sigma.
- gjm11 17y ago
  It shouldn't be dismissed. Suppose two candidate hypotheses are (a) no dark matter and (b) enough dark matter to produce an expected signal at +1sigma in this experiment. Then a 1.5sigma event is about [EDIT: following ratio was wrong, sorry] 3:1 Bayesian evidence for (b) over (a). Of course that's nowhere near enough to justify saying (b) is right and (a) is wrong, and of course the simple normal-distribution model I've assumed is likely too simple; but I think a 1.5sigma event should certainly lead you to adjust your probability estimates a little.
roundsquare 17y ago
One of the great mysteries is why time only goes in one direction, and one candidate to explain that is a dark matter particle. How?
richardw 17y ago
IANAP. "To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate" Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. It would simply have to be anywhere near us (or a nearby body we could observe carefully) to observe weird gravitational effects. Nearby ones, not 'out there in other galaxies' ones. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. How far out must we search before we could reasonably assume it doesn't exist and there might be another explanation? [edit] And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Does it exist inside all galaxies? Is it in the middle, or does it rotate like normal 'matter'? If it exists in the perimeter, is much heavier, and spins like visible matter, do we have to invent a super-ultra-dark matter to explain why common-or-garden dark matter doesn't fly off into space? If it's in the middle and clumpy, could it collapse into a dark-matter black hole, especially since there's no radiation to force particles apart? Is it only one particle type, or a family like visible matter? Since it has mass, is there another form it can convert to as visible mass does? Since it doesn't radiate (being dark) what happens to the energy when (if) it converts? If it's clumpy, stop worrying about visible asteroids. It's the invisible ones that can really hurt you :)
- Locke1689 17y ago
  Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. Well, if the dark matter is ~90% of the mass of everything, that doesn't necessarily mean that 90% of the mass of every thing is dark matter. As far as I understand it, we see the effects of the large amount of extra matter but we don't really know where it is or how it's distributed. That could mean that there isn't any dark matter clustered in large enough quantities close by for us to detect. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. Well, a really spread out distribution across the universe would explain that. And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Well we have a prediction for what the particle would look like (/act like). That's what the detector is there for. Well, not just dark matter. Think neutrinos and Cherenkov radiation. As far as the other questions go, I'm not sure enough to answer and I'm pretty sure that some of those questions are open.

46 points by indy 17y ago

Surprisingly accurate for a newspaper. Project: http://cdms.berkeley.edu/ http://cdms.berkeley.edu/ Nice to see that upgrades are planned to the detection equipment. Besides confirming theoretical predictions and presumably improving our understanding of cosmology, are there any potential applications for this knowledge at an earthly scale?

amichail 17y ago
Besides confirming theoretical predictions and presumably improving our understanding of cosmology, are there any potential applications for this knowledge at an earthly scale? One idea is to use dark matter as fuel for interstellar travel: http://www.newscientist.com/article/mg20427361.000-dark-power-grand-designs-for-interstellar-travel.html http://www.newscientist.com/article/mg20427361.000-dark-powe...
andrewcooke 17y ago
> are there any potential applications for this knowledge at an earthly scale? in broad terms, it's unlikely that any new physics has applications at an earthly scale. the argument is quite simple - if there are significant differences between whatever is "new" and what we currently know at the kinds of scales that are appropriate for applications then it would be easy to do the experiments. the very fact that physics is reduced to using astronomy or the lhc to explore extreme conditions indicates that anything new applies only at scales that are largely irrelevant to us here and now.

simon_ 17y ago

IANAP, but wouldn't confirmation of the existence of supersymmetric WIMPs not necessarily prove that they account for a large portion of dark matter? EDIT: Not to imply that it wouldn't be a huge deal anyway.

hugh_ 17y ago
IAAP,BNTRTOP but I assume you could count how many you detect to get a reasonable idea of how common they are in this part of the universe, at least. Then you could adjust it for the fact that WIMPs tend to prefer to hang out in gravity wells, like around here, and get a pretty good idea of the overall distribution. I haven't done the maths, but if you're detecting _any_ of 'em then they've got to be reasonably common, since they should be relatively uniformly distributed throughout the galaxy. What I didn't understand until just now was this: if WIMPs interact by gravity why don't they fall into stars and planets, or even form stars and planets of their own? The answer is that they do fall into stars and planets, but just keep going through the middle (since they are not subject to any ordinary interactions) and out the other side. And they can't form dark-matter planets of their own since they don't interact strongly with each other either, so they won't stick together. Neato!
- rms 17y ago
  BNTRTOP?
  - JacobAldridge 17y ago
    But Not The Really Top Order Physicist? Boy, Newton Totally Responded To Our Planet? Basically, Nobody Trusts RMS To Out-think PG?
    - hugh_ 17y ago
      The others who said "but not the right type of physicist" correctly ascertained my meaning, though yours are all quite accurate as well.
      - carbocation 17y ago
        I was imagining, "But not the relevant type of physicist." Excellent new acronym.
  - samdk 17y ago

rms 17y ago

http://resonaances.blogspot.com/2009/12/cdms-live.html http://resonaances.blogspot.com/2009/12/cdms-live.html "18:20 Summarizing, no discovery. Just a hint of a signal but with a very low statistical significance. Was fun anyway." As the newspaper article said, a 75% chance of finding new particles.

hugh_ 17y ago
Also from Cosmic Variance: http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/dark-matter-detected-or-not-live-blogging-the-seminar/ http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/... 2:50: Bottom line: “The results cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject the possibility that either event is signal.” –Risa To which I reply "Hang on, so you made a big deal out of this for what? Wake me when you have something which _can_ be interpreted as significant evidence for something"

thras 17y ago

No it hasn't. This is 2 events in an expected background of 0.8. According to reports it's a 1.5 sigma event. Noise. This can be dismissed until they get to 3-5 sigma.

gjm11 17y ago
It shouldn't be dismissed. Suppose two candidate hypotheses are (a) no dark matter and (b) enough dark matter to produce an expected signal at +1sigma in this experiment. Then a 1.5sigma event is about [EDIT: following ratio was wrong, sorry] 3:1 Bayesian evidence for (b) over (a). Of course that's nowhere near enough to justify saying (b) is right and (a) is wrong, and of course the simple normal-distribution model I've assumed is likely too simple; but I think a 1.5sigma event should certainly lead you to adjust your probability estimates a little.

roundsquare 17y ago

One of the great mysteries is why time only goes in one direction, and one candidate to explain that is a dark matter particle. How?

richardw 17y ago

IANAP. "To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate" Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. It would simply have to be anywhere near us (or a nearby body we could observe carefully) to observe weird gravitational effects. Nearby ones, not 'out there in other galaxies' ones. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. How far out must we search before we could reasonably assume it doesn't exist and there might be another explanation? [edit] And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Does it exist inside all galaxies? Is it in the middle, or does it rotate like normal 'matter'? If it exists in the perimeter, is much heavier, and spins like visible matter, do we have to invent a super-ultra-dark matter to explain why common-or-garden dark matter doesn't fly off into space? If it's in the middle and clumpy, could it collapse into a dark-matter black hole, especially since there's no radiation to force particles apart? Is it only one particle type, or a family like visible matter? Since it has mass, is there another form it can convert to as visible mass does? Since it doesn't radiate (being dark) what happens to the energy when (if) it converts? If it's clumpy, stop worrying about visible asteroids. It's the invisible ones that can really hurt you :)

Locke1689 17y ago
Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. Well, if the dark matter is ~90% of the mass of everything, that doesn't necessarily mean that 90% of the mass of every thing is dark matter. As far as I understand it, we see the effects of the large amount of extra matter but we don't really know where it is or how it's distributed. That could mean that there isn't any dark matter clustered in large enough quantities close by for us to detect. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. Well, a really spread out distribution across the universe would explain that. And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Well we have a prediction for what the particle would look like (/act like). That's what the detector is there for. Well, not just dark matter. Think neutrinos and Cherenkov radiation. As far as the other questions go, I'm not sure enough to answer and I'm pretty sure that some of those questions are open.