On hadron colliders, dark matter and black holes

[jfrazierjr]

Yep, there was a big article on the BBC as well talking about how the LHC was set to become the coldest place in the universe which isn't an unreasonable claim.

2 weeks to start up if nothing goes wrong.

Were the programs any good?

Hey, my birthday's in 3 weeks! Any chance for a short delay so I can get my birthday presents before the world implodes?

On a side note, everything I see this thread pop up, I misread the word collider and replace it with the word colander and wonder why I am all of a sudden very hunger for spaghetti.

Joe

[Robbie]

I just realized that torstan never commented on my rendering of the CERN site in 2012...I'm terribly offended (or maybe torstan is)

edit: I just lol'd at the tags

[torstan]

I was wondering how long it would take for someone to notice. Unfortunately only the 'dark matter' one makes the tag clud search

Good alteration to the CERN site actually. Geneva is one city that could only be improved by the addition of a black hole. Just my opinion of course.

[jfrazierjr]

I just realized that torstan never commented on my rendering of the CERN site in 2012...I'm terribly offended (or maybe torstan is)

edit: I just lol'd at the tags

I was wondering how long it would take for someone to notice. Unfortunately only the 'dark matter' one makes the tag clud search

Good alteration to the CERN site actually. Geneva is one city that could only be improved by the addition of a black hole. Just my opinion of course.

Heh.. you still should mock one up with some of the worlds monuments being sucked in....

[yu gnomi]

I hadn't expected to find a thread about high energy physics at this sight, but it is a very pleasant surprise.

Regarding Dark Matter: From what I can gather (mostly from watching TV programs like "Nova") theorists speculate that dark matter is exotic, and not the same as matter we are familiar with.

What I don't understand is why dark matter couldn't be ordinary bits of matter like space dust, space rocks, etc?

Or even less tangible, but ever-present particles like neutrons and neutrinos?

[Turgenev]

I'm not really a rap fan but the Large Hadron Rap has to be heard to be believed.

http://www.youtube.com/watch?v=j50ZssEojtM

[torstan]

We know that there is a certain amount of space dust in the universe - in fact there is a lot of it. Dust and gas make up the dominant source of 'normal' mass. However both dust and gas do interact with light. Hydrogen has a very specific spectrum at which it absorbs and re-emits light. That means we can 'see' it by looking for dips in a spectrum at the relevant frequency. The same actually goes for dust and rocks. Because dust absorbs light, we can see how much of it there is by noting how obscured light is from distant objects. Basically there's an important distinction here. 'Dark' in the context of dark matter means that it doesn't interact with light, not that it absorbs it. So gas and dust and rocks can't be the dark matter because they do interact with light.

Neutrons and neutrinos are another matter. Neutrons can't be dark matter for two reasons. Free neutrons are unstable, decaying after about 15 minutes into a proton, an electron and a neutrino. The proton will capture an electron and form hydrogen. So there's no chance of having neutron dark matter. Also, neutrons are strongly interacting. If there were 1 billion neutrons streaming through us, we'd notice. To an atom, an incoming neutron looks like a ten ton truck. Note that high energy neutrons are what split uranium nuclei to cause nuclear fission. They're pretty destructive.

Neutrinos are a different matter. They have mass (just), they are weakly interacting, they are electrically neutral and they are stable - so they cover all of the relevant criteria for dark matter, and indeed they were proposed as a solution as soon as it was discovered that they had mass. However those theories have been disproved. There are two reasons for this. Firstly, there simply aren't enough of them. They are weakly interacting, but not so weakly that we can't detect them. There are huge detectors in deep mines that have been studying neutrinos for years - one up in Sudbury, another in Japan and smaller ones around the world. They have pinned down a decent measurement of the number of neutrinos in the universe and it isn't even close to the number that would be required to make up the missing mass. Note that neutrinos are incredibly light - at least 10 billion times lighter than a neutron - so you would need a lot of them to make up all the missing mass. The second reason they can't be the dark matter is more subtle. Basically, neutrinos travel very fast - almost at the speed of light. s such they are known as 'hot' matter. This means that they don't really notice the effects of gravity.

To explain this I'm going to have to use a bit of an analogy. Gravity works a bit like a large shallow bowl. If you send a ballbearing around a shallow bowl it will roll in a spiral, dropping towards the center as it loses energy. If there were no friction, the ball-bearing would roll in a circle forever - like an object in orbit. The space around a massive object is curved, just as the bowl is and this causes objects to orbit the heavy object. Space is frictionless, so planets move in circles rather than spirals. Now consider the situation where you roll a small light ball bearing into the bowl at high speed - it just goes in one side of the bowl and out the other. It may well come out at a slightly different angle, but it won't stay in the bowl. This is the case with neutrinos. Because they are moving so fast, they don't get trapped in so called 'gravitational wells', even when the well is created by something as massive as a galaxy, or cluster of galaxies. They go in one side of the dip and come straight out the other side. This is a problem. We know from the way that galaxies rotate that the dark matter must be arranged in a cloud around each galaxy with the highest density at the center. This can only happen if the particle that makes up the dark matter can be trapped in a gravitational well. For this to happen, it must be a slow particle, or 'cold'.

So we now know that dark matter must be cold, as well as all the other properties that have been assigned to it. That rules out neutrinos.

[torstan]

@turgenev: That's great! Thanks for the link.

[RobA]

I'm not really a rap fan but the Large Hadron Rap has to be heard to be believed.

http://www.youtube.com/watch?v=j50ZssEojtM

You beat me to it! (another boingboing reader?)

-Rob A>

[yu gnomi]

Wow that was quick. Thank you Torsten for the thoroughness of your answer and the speed with which it was posted.

I figured that there had to be some reasoning as to why the normal everyday cases of matter were ruled out as being dark matter, but I've simply never seen it explained. Your responses all made sense to me.

I know that there are now theories that galaxies, including our own, circle around black holes. Could it be that dark matter is simply black holes (and maybe neutron stars, which I believe are similar) ?

I do realize that this would mean a lot of black holes and neutron stars.