Showing posts from December, 2011

Ah Bottomium!

When I was an undergraduate student, I had thought about becoming a particle physicist, but a summer school first at the Rutherford Labs (working on proton-anti-proton scattering) and then CERN (search for charged Higgs particles) beat that out of me :)

There is a lot of chatter about the Higgs out there at the moment, but unfortunately I think that a lot of it illustrated the poor understanding of statistics by journalists (and even scientists). p-values make me weep.

But I thought I would talk about something else, the the discovery of bottomium at the LHC. Funnily enough, it's not actually a new particle, and that's something I thought I would try and explain.

Let's start with a picture - what's this?

Of course, it's an atom. Well, except we know this is just a schematic picture of an atom, a nucleus with some electrons whizzing around. A real atom is more complex than this, being described by the laws of quantum mechanics. Electrons are not little particles on …

What shape is the ocean on a cubical planet?

Clearly, they are circular!
The question is, however, how do you calculate this?

The first step is to realise that the surface of a liquid is an equipotential surface. This means nothing more than the value of the potential energy over the surface is the same. This is easy to think about if you remember your classical mechanics as if there were differences in the potential energies in the water, so one bit of water was higher than the other, then that potential can be converted to kinetic as the water flows downwards. When could all this be static? When the surface of the water is at the same potential, so no one bit is "higher" (in terms of potential energy) than another.

So, to work out the shape of the ocean, we just need to calculate surfaces of the same potential energy. But how do we do that for our cubical world?

In the following, I've made a few simplifying assumptions. I assume my planet is not rotating (that adds a further set of energy terms) and that I can ig…

The Sydney-AAO Multi-object Integral field spectrograph (SAMI)

When I say modern astronomy, what do you think of? Large telescopes peering into the sky, looking to unlock the secrets of the Universe? Of course, telescopes drive astronomical research, but what is often forgotten is the business end of the telescope, the instruments that collect the light, are the things that really define the science we can do. And such instruments can be extremely complex, and extremely expensive.

Here at Sydney we have a group working on astronomical instrumentation and a new instrument, The Sydney-AAO Multi-object Integral field spectrograph (SAMI), has recently be commissioned and the first paper has been accepted.

So, what does this new instrument do? Well, spectroscopy is an essential part of astronomy. Basically, it just means collecting light and dispersing it into a rainbow. Over to Pink Floyd
We don't use prisms any more (we use volume phase holographic gratings, which sounds much more science fiction). Looking at the light tells us lots and lots ab…

Gravitational Waves and the Wild Wild West

I started this post at Perth airport, but the wireless was too slow to complete it. So here goes (again).

I'm back in Sydney after a few days in Perth. I gave a talk at ICRAR and the The Australian International Gravitational Research Centre at the University of Western Australia. Perth clearly is a boom town, with plenty of money flowing from the mining (a bottle of house white cost $58!), and it still has a slight feel of the wild-wild western - the police were touring the lounge where I was sitting at the airport as, as you can imagine, several hundred miners were heading out on a Friday afternoon and, apparently, there has been trouble in the past.

I had several really interesting meetings in Perth, especially with regards to writing grants in the next ARC round (which has come round really quickly again). I'll write about those at a later date. On Thursday I was heading up to Gingin, north of Perth, for a BBQ at the Gravitational Wave Centre, and I was invited to take a …

Scary monsters (and supermassive black holes)

A quick post this morning as I have spent a few hours plumbing in a new dishwasher (the previous one decided it would like to be like the Nile, and so flooded every so often), and have a children's party this afternoon. But, I've had a new article published on The Conversation titled Scary monsters (and supermassive black holes).

It's a review of the discovery of the most massive supermassive black holes yet. As I note in the article, the discovery itself is not such a surprise as we know that there is a well known relation, the M-sigma relation which shows that larger galaxies have larger black holes (the astronomers in the article weren't just blindly looking for black holes - they knew where to look).

Also, in the article, I touch on another article in The Conversation called Black holes might exist, but let’s stay sceptical, which was in response to a previous article I published on black holes. This article seemed to suggest that astronomers believe black holes a…

More dark matter shenanigans

The internet is alive again with another cry that dark matter is dead (again) and the slashdot-eratti are getting themselves into the usual lather and "DM is BS" claims.

I've written my views on slashdot commenting previously, and will not reiterate them here, but will comment on the paper and the "meaning" of dark matter to astronomers.

OK. The paper can be read here and here's the press image that goes with it.
The picture is correct. If we just considered the gravitational attraction of stars, then their rotation speed should follow the red curve, but when we measure it (and we can measure it far outside the stellar disk by looking at the rotational velocities of HI gas) it actually follows the white curve.

So, the big question is why? The prevailing hypothesis is that there is more mass there than we can see, i.e. dark matter. But others suggest that dark matter is not there, and there is some other influence, usually by modifying the laws of physics (i…

How many tanks?

The German tank problem is a fav of mine. The wikipedia page on it is a little long winded, but I think it can be looked at a lot faster with a little numerical mucking about.

The problem is quite simple. The enemy are producing tanks, and each has a sequential serial number (for simplicity, let's assume that the numbers are reset every month). You encounter this scene on the battle field;
and we see that this is tank number, say, 15 of a particular months production. How many tanks were produced in that month? Can we even answer the question?

This is the problem that faced the Allies in WWII; you really wanted to know how many panzers are out there. Intelligence officers were reporting productions of more than a 1000 tanks per month, but based on statistics, the predicted number was significantly fewer than that, in the hundreds. After the war, the numbers were checked against records and the statistical answer was amazingly correct (read the wikipedia page for more details).


The Beast with Four Tails

The Sagittarius Dwarf galaxy is a pretty cool object. It was discovered by a very close collaborator of mine, Rodrigo Ibata of the Strasbourg Observatory, way back in the ancient past (well, 1993), and it is clearly a little galaxy in trouble. It's orbit brings it dangerously close to the Milky Way, where its tidal gravitational pull is ripping it apart.

During its destruction, stars have been continuously pulled from the dwarf and are now wrapped around the Milky Way. These tidal streams are really interesting, and can be used to work out how the dwarf has been pulled apart. More importantly, if we can work out the orbit of the streams, then we can measure the amount of dark matter surrounding the Milky Way, a very important thing to do.

But look at this
This is a map of the sky in coordinates where the tidal stream of Sagittarius wraps around the equator.  You can clearly see the Milky Way galaxy. The colour are fields from the Sloan Digital Sky Survey where stars in the halo of…

Probing planetary mass dark matter in galaxies: gravitational nanolensing of multiply imaged quasars

One of the big questions of modern astrophysics is "what is dark matter?" Of course, we all know that there is a possibility that it is not a material substance at all, and the extra gravitational influence we need to explain observations may be due to a modification in the laws of physics, but it being matter is the simplest of hypotheses and it seems to work very well (but, of course, to the media and rabid slashdotters/internet trollers, we are nothing but religious fanatics pushing our wheelbarrow of dark matter, blinkered to the geniuses out there!).

If it is a material substance, then we have ruled out a number of candidates (in terms of stellar mass black holes etc), and the weight of evidence is pointing towards a subatomic particle. As the Universe evolves, dark matter clumps together to make dark matter halos within which galaxies like our own Milky Way form.

How small do dark matter halos get? Well, some think it continues down to planetary mass scale. If it does,…