What’s the matter?

Mysterious and invisible dark matter is still mysterious, but possibly a little less invisible after hints of its existence were announced at CERN yesterday, in a cautious but read-between-the-lines-nudge-nudge-wink-wink presentation.

The Alpha Magnetic Spectrometer (AMS) experiment, led by Professor Samuel Ting – who won the Nobel Prize in Physics in 1976 for his part in the discovery of the J/ψ meson and hence the charm quark – confirmed that there are more positrons in space than you’d expect, and this could be due to dark matter. Of course, it could also be due to something else, but Professor Ting sounds like he’s betting on dark matter.

Graph showing the fraction of positrons compared to electrons in cosmic rays, plotted against their energy. The AMS results (red dots) are compared to those of previous experiments, notably PAMELA and FERMI, which also saw an excess of positrons but had much higher uncertainty. The levelling off occurs at an energy of about 350 GeV – after that, who knows? (Image AMS-02 collaboration)

What AMS did was look at cosmic rays, which are charged particles that zip through outer space, and which would likely kill us all if we weren’t protected by Earth’s atmosphere and magnetic field.

Based on the International Space Station, AMS uses magnets and other clever devices to measure the electric charge, energy and momentum of the cosmic ray particles and so work out what they are.

In particular, it looked at the ratio of electrons to anti-electrons, known as positrons (because they have positive charge). The universe is full of electrons, but positrons are generally only produced when other particles interact. This means that there should be fewer positrons at higher energies, as there are fewer parent particles at higher energies to create them.

But AMS found that although there is a dip at an energy of around 10 GeV, after that the proportion of positrons increases. So something is creating more positrons at higher energies.

This could be dark matter, which many people believe to be WIMPs, short for weakly interacting massive particles. These particles would interact via the weak nuclear force, as do electrons and positrons. So if a WIMP and an anti-WIMP happen to collide (or possibly if WIMPs are there own anti-particles), they would produce an electron-positron pair.

The numbers of positrons produced would then be expected to rise until you reach the energy corresponding to the mass of the WIMPs (E=mc²), after which they’d suddenly drop off. This would be a good sign that what we’re looking at is indeed due to dark matter.

If not, it could be something else in the universe, like a pulsar. However, the signal that AMS found appears to be coming from all directions, so that seems  unlikely. But then it’s still possible that it’s being produced by something else that astrophysicists don’t know about. Which would also be cool.

The previous ‘smoking gun’ of dark matter, the so-called Bullet Cluster. Two galaxies collided, separating the ordinary matter (pink), distorted into bullet shapes by the crash, from the dark matter (blue), which passed right through. The caption is my little joke about dark matter occupying the universe… (Image from NASA)

So what are we seeing? Well, at the moment we’re just seeing the number of positrons rise with higher energy, but levelling off at around 350 GeV. Beyond that, the AMS team haven’t detected enough positrons to say. There were only 72 found at 350 GeV, and the fewer there are the greater the statistical uncertainty. Professor Ting’s demeanour hinted that he has an idea about what might be found at the higher energies, but he refused to be drawn.

This is admirable restraint, which actually shouldn’t be that surprising coming from a guy who won a Nobel Prize for experimental physics. But these days there’s often a tendency to call a press conference as soon as there’s even a hint of an exciting discovery, which then evaporates as more data comes in. Remember those faster-than-light neutrinos?

So it makes sense that Professor Ting is being cautious by refusing to release “preliminary results”, and saying he’ll only make an announcement when they’re statistically confident they have something. Plus as he pointed out, doing experiments in space is very difficult, so it takes time to get it right. And since he’s been working on this for 18 years, he’s prepared to wait a little longer.

As for what any WIMPs actually are, that’s up to the theoretical physicists to work out. And it has to be compatible with what’s being found – or rather, not being found – at the Large Hadron Collider at CERN.

But whatever is the origin of the signal seen by AMS, it’s likely to be something new to physics, which is exciting. And considering the last big announcement from CERN was that the Higgs boson is probably really the Higgs boson, we could do with some new physics.

We just have to wait until Professor Ting is ready to tell us.

Climate change and droughts and flooding rains

The crazy weather of the 2012-13 Australian summer – which apparently isn’t over yet – prompts the question of whether climate change means this is what we should expect from now on. The answer is yes, that’s the trend, but the details are a bit more complicated, and definitely worth knowing.

First though, the simple answer is that, as the Australian Government’s Climate Commission pointed out in their report, The Angry Summer:

Extreme heatwaves and catastrophic bushfire conditions during the Angry Summer were made worse by climate change… All extreme weather events are now occurring in a climate system that is warmer and moister than it was 50 years ago. This influences the nature, impact and intensity of extreme weather events.

Having said that, it doesn’t mean that it’s always-always going to be like this. People tend to think that any weird weather is completely unprecedented and at the same time a permanent change, when in fact variation is a normal part of the system.

To take an example: Melbourne weather is famously variable, so you’d think Melburnians would be used that. But every year when the first hot days arrive in September or October everyone says “summer’s come early! Time to put away the warm clothes.” Then it gets cold again, and everyone says “where did that come from? Oh well, guess there’s not going to be much of a summer this year.” And so on. Every year. Even this past summer.

However, under climate change, the odds are skewed towards hotter weather, even as this variation continues. If there was no global warming, you’d still expect occasional records broken for extreme hot and cold weather. But with the 0.8°C warming we’ve seen under the past 100 years, that same variation around the average will produce slightly more hot records than cold ones.

Image from the Climate Commission’s Angry Summer report, showing the connection between a shifting average and the proportion of extreme events (adapted from IPCC report, 2007)

It’s important to remember this, so you don’t get fooled into thinking that global warming has stopped when it gets cold again in a few months time. After all, 0.8°C isn’t a very noticeable change when considered on a daily basis.

So that’s the big picture, but to find out exactly what it means for weather in your area you have to turn to the climate scientists and their computer models. You can look them up yourself: for an overview, see www.climatechange.gov.au. For more technical and detailed report from the CSIRO and the Bureau of Meteorology, see www.climatechangeinaustralia.com.au.

When you go to these sites, you’ll see patterns that are largely a continuation of what we’ve experienced so far. Temperatures will rise on average, with the greatest warming in the middle of Australia and the north-west.

But there will also be more hot days and warm nights, including an increase in the number of days per year over 35°C. For instance, Melbourne currently has 9 days above 35°C per year (that’s average: in 2013 we’ve already had 14), but by 2070 it could be as high as 26. Brisbane will go from an average of 1 to up to 21. Darwin could be up to 308 days above 35°C every year.

Rainfall is a little more complicated. On average, warmer weather puts more moisture into the atmosphere, but changing temperatures also change wind patterns. The top end is not going to have much of a drop in average rainfall, but the rest of the continent will – especially the southwest. Perth is already experiencing much drier conditions.

Rainfall trend map showing the changes in annual totals across Australia from 1910 to 2012 (Image from the Bureau of Meteorology)

But not only will rainfall reduce, it will also vary much more. We’re likely to have more very wet days and more very dry days. So yes, more droughts: again, the south-west will be worst hit, with an 80% increase in drought by 2070.

What’s that you say, Mr Andrew Bolt? Doesn’t the recent flooding in Queensland and NSW prove these predictions wrong? No, because I said there’d be more variation.

When you can see the sea in the dark

You may have seen bioluminescence, also known as phosphorescence, in the movie Life of Pi. Or possibly in the Gippsland Lakes (“Plankton put on a show at Lake Victoria”, The Age, 29 January 2013).

Bioluminescent plankton lighting up splashes of water in the Gippsland Lakes in January 2009 (Photo by Phil Hart)

The short explanation for what causes it is that it’s oxidisation of the chemical luciferin, in the company of the enzyme luciferase, releasing energy in the form of light. This reaction is found in all kinds of animals and fungi, from fireflies and glow worms, through to Anglerfish and Colossal Squid, and of course marine plankton.

The long answer is best found by listening to Beth in our podcast from 24 January 2013. Go do that, along with our other previous shows.

Why toast lands buttered side down

In one of the very first posts on this blog, we tackled the question of how cats always land on their feet. Now, two years later, it’s time to tackle another species with a similar skill: namely toast.

A demonstration of the typical behaviour of falling toast. It’s physics, not bad luck, that causes it to land butter-side down.

As with the cats, the answer turns out to be physics – and not, as you may have expected, Murphy’s Law, which is more of an engineering principle.

You see, Murphy’s Law is frequently taken as a pessimistic prediction that things will always go wrong. This is clearly not true: the principle says “anything that can go wrong will go wrong”, but usually there are many, many things that can go wrong, and they can’t all happen at once.

A more charitable interpretation – which was supposedly one of the original meanings when the law was named for USAF Captain Ed Murphy, who built rocket sleds in the 1950s – is that if there’s a known flaw you should try and rectify it, rather than just leave it and hope that it won’t happen.

To give an example, consider the Death Star. It was built with the engineering fault of an exhaust vent that, if you hit it just right, would cause the whole thing to explode. But Grand Moff Tarkin apparently just assumed that the Rebels would never be good enough shots to hit it (perhaps he was used to the lousy accuracy of Imperial Stormtroopers, not to mention targeting computers). If he had have known of Murphy’s Law, he might have decided to put a cover over it instead.

But enough sci-fi nonsense: back to toast. As I mentioned, this is actually straightforward physics, first documented by Robert Matthews of Aston University, an achievement for which he won an IgNobel Prize (Matthews RAJ 1995, “Tumbling toast, Murphy’s Law and the fundamental constants”, European Journal of Physics, vol. 16, no. 4, p. 172, doi:10.1088/0143-0807/16/4/005).

Toast usually falls by tipping over the edge of something, like a plate or the edge of a table. When it does, it teeters first, rotating about the edge of the plate/table, before dropping down. It continues to rotate while in the air, and under typical conditions does a half-turn and lands upside-down.

Diagram of the dynamics of falling toast, taken from “The Anthropomurphic Principle”, by Roland Krenn

I say ‘typical conditions’ because the amount it rotates depends on a number of factors, like the height of the table, the amount the toast is overhanging the edge when it’s released, how fast it’s moving horizontally, the friction between the toast and the table, and the size of the toast.

It’s fairly straightforward to model a simplified scenario where the toast doesn’t slip against the edge of the table (see Roland Krenn 2005, “The Anthropomurphic Principle”, Karl Franzens Universität Graz). In the no-slipping case, the toast doesn’t fall until it’s aligned vertically (θ = 90° in the diagram above).

Using those calculations, you can show that you’d need to drop the toast from a height of about 3 metres for it to do a full rotation and land right-side up again. Needless to say, this rarely happens.

When you add in the slipping it gets more complicated, and you need sophisticated computer modelling to do the calculations. Fortunately, people have done just that (Bacon ME, Heald G & James M 2001, “A closer look at tumbling toast”, American Journal of Physics, vol. 69, no. 1, pp. 38-43, DOI: 10.1119/1.1289213, PDF 475 KB).

They found that slipping causes the toast to rotate faster, but for small amounts of overhang – which is realistic for natural toast dropping – it will still land upside-down.

So, going back to Murphy’s Law, how can we save our toast from dirty butter? Well, the experts have a number of suggestions:

1. carry the plate above your head at about 3 metres high
2. equivalently, only use toast about 2.5 cm wide (maybe that French mini-toast)
3. when the toast starts to fall, pull the plate away quickly so it doesn’t rotate.

Considering that last point, I have an alternative idea: if you are able to pull the plate away, you should also be able to push it back under the plate. Why let it fall at all?

Murphy would be proud.

Smelling asparagus wee is a mutant superpower

Some people report that their urine smells funny after eating asparagus, and some don’t. But is it because their wee is different or their sense of smell is different? Cue: science!

Actually, you can test this at home fairly easily. If you’re able to smell asparagus wee but you know someone who can’t, simply go into the bathroom after they’ve visited it following an asparagus meal. I’ve tried it – in the name of science – and I can say that I could definitely smell their urine, even though they couldn’t (a condition called specific anosmia).

However, that’s not a terribly rigorous experiment, and the plural of anecdote is not data. But answering it properly turns out to be a rather tricky puzzle, and one that has mildly interested scientists for centuries.

If you’re an asparagus-wee-smeller, you know exactly what the result of eating these will be. If you’re not, then you’re probably wondering what all the fuss is about (Photo by Jeremy Keith from Brighton & Hove, United Kingdom, via Wikimedia Commons)

The phenomenon of asparagus wee was first documented in the 18th century, by French botanist Louis Lémery in 1702 and English physician John Arbuthnot in 1735. Arbuthnot particularly observed that it’s more common after eating tastier, young asparagus.

Not a scientist, but renowned for his appreciation of smell and taste, was Marcel Proust, who said that asparagus “as in a Shakespeare fairy story transforms my chamber-pot into a flask of perfume.”

But the cause of it has taken a while to pin down, partly because it doesn’t appear to have any medical significance, so there isn’t really a pressing need to solve it. This is unlike, say, the tendency of some people to have red urine after eating beetroot, which has been linked to things like absorbing too much iron (see Mitchell SC 2001, “Food idiosyncrasies: beetroot and asparagus”, Drug Metabolism & Disposition, vol. 29, no. 4, pp. 539-543).

The other problem is that it’s subjective. Most studies have involved simply asking people “does your urine smell weird after eating asparagus?” But how do you define “weird”?

And we still don’t even know what causes the smell. Chemical analysis of urine can alter some of the volatile compounds that might be responsible. And even if you do find something unusual, how do you know that that’s actually causing the smell?

What you really need to look at is the vapour above the urine. Fortunately, a few dedicated souls have done just that, revealing a number of possible candidates, all of them sulphur compounds (see for example Waring RH, Mitchell SC & Fenwick GR 1987, “The chemical nature of the urinary odour produced by man after asparagus ingestion”, Xenobiotica, vol. 17, no. 11 , pp. 1363-1371, doi:10.3109/00498258709047166).

The main one that people have identified – and for a long time was believed to be the primary source of the smell – is methanethiol (CH₃SH). It’s also found in faeces, bad breath, farts and other decaying organic matter, and it smells like rotten cabbage. It’s sometimes added to natural gas to give it a smell, for safety reasons.

So it’s famously smelly, which is one reason to doubt that it’s the culprit. After all, everyone can smell things like faeces and farts, but asparagus smell is a little more idiosyncratic. Which means it’s probably a combination of it and the other sulphur compounds; things like dimethyl sulphide, dimethyl disulphide, bis-(methylthio)methane, dimethyl sulphoxide and dimethyl sulphone.

Because all these chemicals contain sulphur, they have to originate from a sulphur compound that’s unique to asparagus. The only possible candidate is called, surprisingly, asparagusic acid (S₂(CH₂)₂CHCO₂H). It’s deadly to insects, and found more in young asparagus, presumably to protect them from pests. So Dr Arbuthnot back in 1735 was right.

However, it’s not the asparagusic acid itself that ends up in the urine, it’s what the body metabolises it into. Which is the methanethiol and all the dimethyl et ceteras.

So we have some possible culprits, but we still can’t isolate the recipe for the smell. Which means we can’t just hand people a flask of the odour, and instead we have to go back to smelling urine.

One of the most comprehensive urine-sniffing studies was published in 2010. It used a technique called ‘two factor forced choice’, where they presented people with asparagus and non-asparagus wee, both their own and from other subjects, and they had to say which was tainted. Pelchat ML, Bykowski C, Duke FF & Reed DR 2010, “Excretion and perception of a characteristic odor in urine after asparagus ingestion: a psychophysical and genetic study”, Chemical Senses, published online 27 September 2010, doi: 10.1093/chemse/bjq081

Unexpectedly, in this study they got a spread of results, rather than a simple yes-no, can-can’t smell it. Only 3 people out of 38 did not produce smelly urine, i.e. when other subjects were asked which of the samples came after eating asparagus, results were no better than chance. And only 2 were unable to smell it at all. But there was certainly a range of ability to smell, and a range of smelliness.

Alongside this they also did some genetic testing. This concentrated on a single nucleotide polymorphism – that’s a variation in just a single base-pair in the DNA molecule – near a gene call OR2M7 (the OR is for ‘olfactory receptor’). They showed that that variation was strongly associated with the ability to smell the asparagus in urine.

But there was no association with the ability to produce it. Although there was a range of smelliness, which would have to be related to how their bodies process asparagusic acid, the cause is as yet unknown.

So it seems that the main difference between people is due to this mutation in OR2M7. If you have it, you can smell asparagus wee, and if you don’t… Well, you can probably smell it at high concentrations, but not as well as the mutants.

Which all concurs with my not-so-scientific, DIY test. Which is reassuring, but I still wouldn’t expect my study to be published in a peer-reviewed journal.

(This story first aired on 24 January 2013 – you can listen to the podcast.)

Negative absolute temperature will burn your brain cells

The new year in science is so far shaping up to be a confusing one, with the first surprising physics result being the reaching of temperatures below absolute zero.

If that sounds bizarre to you, well it does to me too; but it’s thermodynamics and I’ve always found that difficult. But having calmed down and taken a bit of time to read and think about it, I think I can write something sensible.

What’s happened is that a team of German physicists have manipulated ultracold potassium atoms using lasers and magnetic fields to achieve a state that technically has negative temperature (Braun S, Ronzheimer JP, Schreiber M, Hodgman SS, Rom T, Bloch I & Schneider U 2013, “Negative absolute temperature for motional degrees of freedom”, Science, vol. 339 no. 6115 pp. 52-55, doi: 10.1126/science.1227831).

This works because temperature isn’t exactly what you think it is. We normally think of it as representing the average kinetic energy of the atoms or molecules in a sample, i.e. how rapidly they’re all jiggling around. So absolute zero corresponds to when they’ve stopped moving altogether, which is why it’s an absolute.

Well that’s almost right. The technical definition relates temperature to the distribution of energy in the system, and the rate that the entropy changes when the energy changes.

Entropy is an expression of the disorder in a system. You may remember it from the Second Law of Thermodynamics. That’s the one that says that the entropy in a closed system can never decrease: everything tends to move towards a more disordered state.

This is the reason that a cup that falls off a table won’t spontaneously put itself together again and jump back up. For one thing, it would need to convert heat energy – the random jiggling of atoms – into uniform motion of the whole cup. And of course the random jiggling is more disordered, i.e. has more entropy.

At least, that’s how it works at positive temperatures. Adding more energy increases the jiggling and increases the disorder. And until they receive more energy, most of the atoms can be found in the low energy, low entropy states.

Graph showing how entropy varies when there are both minimum and maximum energies. The temperature is actually the inverse of the slope of the curve, so it’s -0 at minimum energy and +0 at maximum energy. In the middle, where the entropy curve flattens out, it switches from positive to negative infinity (Image by Braun et al, Science)

But in this latest experiment they turned that around. First of all, they cooled the atoms down to a few billionths of a Kelvin. The atoms would normally repel each other, but they used lasers to trap them in a lattice arrangement.

Then they flipped it over. They changed the force between the atoms to an attractive one, and used the lasers to try to push them apart.

The result is that all the atoms were suddenly in a maximum energy state, but a very ordered one because they were all in this lattice arrangement. Any lower energy states were more disordered.

This reversed the relationship between energy and entropy, and so corresponds to a negative temperature.

Not only that, but because the Second Law of Thermodynamics means systems want to increase their entropy, it also means that the atoms want to lose their energy and move to a lower energy state.

So if you put a negative temperature system in contact with one with positive temperature, energy will tend to flow from the negative to the positive temperature. This has led some people to claim that negative temperatures are hotter than any positive temperature. Even though they’re a few billionths of a Kelvin below zero.

The way to understand this is that the temperature scale also doesn’t work the way you think it does. Normal number systems go from negative infinity, through zero and up to positive infinity. Like this:

−∞ … 0 … +∞

But with temperature, it works more like this:

+0 K … +∞ K, −∞ K … -0 K

So absolute zero is still a limit, it’s just a limit at either end. And the infinities meet in the middle.

I’ll let you go away and think about that, but leave you with one very cool (sorry) consequence.

Pressure and temperature are proportional to each other, so a negative temperature should also have negative pressure. And negative pressure is the very strange property exhibited by dark energy, which causes the accelerating expansion of the universe. It’s possible that by studying these weird, idiosyncratic atomic systems, we may get a better idea of how the cosmos works.

So it’s strange stuff, but worth understanding. If you still don’t get it and would like to read more, with clever analogies, see What the Dalai Lama can teach us about temperatures below absolute zero (Empirical Zeal), or Leprechauns and laser beams (Coffeeshop Physics).

Every cigarette is doing ectoparasites damage

¡Feliz Año Nuevo! If your new year’s resolution is to quit smoking, consider donating your used butts to Mexican birds, who appear to be using them to get rid of parasites.

After noticing that local birds were incorporating cigarette butts into their nests, researchers in Mexico City decided to test whether they might be doing because of the parasite-repellent properties of nicotine (Suárez-Rodríguez M, López-Rull I & Garcia CM 2013, “Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe?”, Biology Letters, vol. 9, no. 1, 20120931, doi: 10.1098/rsbl.2012.0931).

Sure enough, nests with more butts were found to have fewer ectoparasites (creatures like mites that live on the outside of organisms) than those without. Although, they also found that smoked butts worked better, as they were more toxic to parasites.

Further research is needed to determine whether the birds are choosing the butts for their anti-parasite properties, or if it’s just because they make good insulation. Also, the researchers hope to find out whether using cigarettes does actually benefit the birds, or if the toxicity harms them as well.

But it’s good at least to see that birds can adapt and make use of urban environments, even if it is through poisonous litter. A kind-of good news story to start the year!

(This story first aired on 20 December 2012 – you can listen to the podcast.)