Archive for: April, 2013

My reaction (as student & teacher) to "let's have class outside!"


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The Higgs Boson: a talk in Second Life tomorrow morning (April 6)

It's been a year since I've given a public outreach physics and astronomy talk in Second Life. I used to do these things fairly regularly as a part of MICA (the Meta-Institute of Computational Astronomy). However, the MICA project has completed, its island in Second Life has gone online, its Second Life groups have been disbanded, and MICA no longer really exists. (Its website is still up, and should stay up for at least a little while. If I were smart, I'd probably make sure to download and archive elsewhere all of the audio recordings of my own talks....) A write-up of what MICA did and was all about is available at, and was published in the conference proceedings of a SLActions conference on virtual worlds

I've always meant to find other venues for continuing to do popular talks in virtual worlds. Someday, I'd like to escape from Second Life's walled garden and start doing these talks in an OpenSim grid, and even did the first steps for trying to get set up to do them in my own region on OSGrid. However, of course, the audience in Second Life for now is still far bigger.

Fortunately, the Exploratorium, the excellent science museum in San Francisco, has a presence in Second Life. This Saturday (tomorrow, 2013 April 6) at 10AM pacific time (17:00 UT) I'll be giving a talk about the Higgs boson in the Exploratorium region in Second Life. Remember, basic Second Life accounts are free. Drop by if you're interested.

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Dark Matter found? Don't break out the champagne just yet.

You may have seen announcements that Dark Matter has been "found". I don't believe there's a publicly available scientific paper on this yet, so the original source for this is two press releases from CERN: One from four days ago and one from today.

First, I want to say what is meant by dark matter being "found" here. It's not evidence that previously-uncertain Dark Matter exists. We already know that Dark Matter exists; the Bullet Cluster observations several years ago was unambiguous confirmation that non-baryonic dark matter exists. We don't know what it is from that, but we know it exists. (Here is a podcast I did three years ago about the evidence for the existence of Dark Matter, including the Bullet Cluster.) So what does it mean to say that these new CERN results may have "found" Dark Matter?

Although we know Dark Matter exists, there remain a huge number of mysteries about it. Many of these can be summarized under: what is it? All we really know is that it's not made out of baryons, that is, protons and neutrons. So, it can't be an excess of dim stars or rogue planets (a model that was once considered a real possibility for our Galaxy's dark matter). Thus far, we've observed it because of the effects of its gravity. We've seen it in comparisons of the structure in the Universe to models of structure growth from early-Universe conditions; in the dynamics of galaxy clusters and galaxies; and through gravitational lensing. It would be nice to observe it in other ways.

To "find" Dark Matter, we'd like to do one (or more) of two or three things. Either, we'd like to see the results of decay products in our atmosphere or in space because of interactions of Dark Matter particles out in space. Or, we'd like to have an actual Dark Matter particle interact with a particle detector we have on Earth (analogous to how we see neutrinos from the Sun). Or, we'd like to actually make some of the stuff in a collider like the LHC at CERN in Switzerland, and see its decay products or signature there.

The current announcement from CERN is potentially of the first type. There is a detector, the "Alpha Magnetic Spectrometer" or AMS, on the International Space Station. This spectrometer is measuring electrons and positrons (the antiparticles of electrons) coming from space— that is, cosmic rays that are electrons and positrons. They see too many positrons for what we'd expect. One possible reason for the excess of positrons is that they are the result of very rare Dark Matter annihilations in our Galactic halo. (Although such annihilations, if they are happening, would be rare, there is so much bloody Dark Matter out there that if it's doing this, it would produce enough excess positrons for us to observe.)

What's really been detected is a positron excess, which is interesting all by itself. Whatever it turns out that this positron excess is coming from, it's going to be at least new astronomy, and potentially also new physics. It may not be as sexy and headline-worthy as "WE FOUND TEH DARK MATTER!!!1!!one!", but it will still be interesting, and will tell us something about nature. What's been seen is consistent with it coming from the Dark Matter halo of our galaxy, but other sources can't be ruled out yet. As more data is collected, the investigators running this experiment will be able to test whether the details of what is seen remain consistent with what would be expected from Dark Matter, versus other possible sources.

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