Custardy battle

Everybody likes custard, but who likes lumpy – or even chunky – custard? Today I’m presenting 4 simple techniques to keep the chunks out of your custard.

Before we start, I should clarify that by custard I mean the real deal: made with milk or cream, sweetened with sugar and thickened by either whole eggs or just the yolks.

This is not to disrespect the stuff you make using custard powder, which can be quite yummy and fascinating in its own right, being a non-Newtonian fluid that gets more viscous when you apply more pressure. But let’s be honest: real custard, or crème anglaise if you will, is a much better companion for plum pudding, and you can’t make ice cream out of custard powder.

Like I said, it’s thickened with eggs, but it’s very easy to go too far and end up with lots of lumpy, grainy curds. For this reason many people are intimidated and won’t go near it, despite the culinary temptations. But you can remove some of the mystery using science; and my main reference is as usual Harold McGee’s book On Food and Cooking, an invaluable resource for any kitchen.

Alright, we’ll get the primary but least interesting technique out of the way early: method 1 you will see in just about every recipe: strain out the lumps.

That may sound obvious, and if so, good on you. What you’ll find with most of the other methods is that you’ll reduce the lumps, but there’ll often still be a few. So it’s safest just to strain them.

Okay, method 2 is a bit more scientific: use a thermometer. Here’s where we need to understand how eggs work.

Eggs of course are wonderful devices, designed to protect and nourish baby chickens. If you take a 55g egg, most of it, about 41g, is water. There’s 6g of fat, only 0.6g of carbohydrate, 213mg of cholesterol, and what we’re interested in, 6.6g of protein.

There are many different kinds of protein in both the yolk and the white, all designed to do different jobs: obviously, they provide food for the chick, but they also protect against infection, prevent them from being nutritious for predators – laboratory animals fed on raw egg actually lose weight – and physically protect the embryo.

The proteins – and remember proteins are long chains of amino acids all joined together – are folded up tightly into little knots floating in the liquid. This is why the white starts off transparent.

Coagulation of egg proteins: on the left an uncooked egg, with tightly bound knots of proteins; on the right, the cooked egg with unravelled proteins forming new networks (picture adapted from Harold McGee's book, and is obviously not to scale - nor are eggs oblong)

But when you heat the egg, the proteins jostle around and break the bonds that fold them so tightly together – we say they denature, or lose their natural form.

They unfold into long chains which then bump into and bond to each other, forming a network or messy web of protein throughout the material. This is when the egg sets or coagulates, becoming solid and opaque.These proteins are actually joined together using disulfide bonds, that is, involving bonds between sulphur atoms. So you can add certain chemicals like sodium borohydride (NaBH₄) which break those bonds and can “uncook” the egg.

Anyway, the different proteins coagulate at different temperatures, but overall a whole egg, that’s yolk and white combined, sets at about 73°C. But in custard it’s diluted – a basic recipe is one whole egg to 1 cup of milk and 1 tbsp sugar – so it thickens at a higher temperature, about 78-80°C.

You get the lumps when you overheat it: then the proteins bind too tightly together, forcing out the water from their little networks and becoming hard little lumps. That happens at only another 5 or so degrees, so you need to watch the temperature very closely. Try to keep it below 85°C.

Method 3 is actually the most reliable: add flour or cornflour, or even cocoa. The starch granules absorb water and also dissolve a bit, all getting in the way of the proteins bonding.

You can even bring the temperature up to the boil, in fact, as McGee says, if you want a thick, stiff custard like crème pâtissière you must boil it, because the egg yolks contain an enzyme called amylase that digests starch and will make it all runny again unless you kill the amylase first by boiling it.

So there you go. Making custard is just like doing a chemistry experiment: understand the basic science, use the right ingredients, and watch the conditions.

Wait, what about method 4? That’s easy: practice, practice and practice, and get really good at it. Sounds facetious, but after enough times heating it slowly – like by using a double boiler – and using a kitchen thermometer to keep it just a bit over  80°C, you’ll get used to how it thickens, becomes glossy, and coats the back of a wooden spoon, as the professionals say.

Really, the art of cooking is just science with practice.

Now is more than an instant

What do we mean when we say “now”? How long is a “now”? It feels pretty quick, so quick that by the time you’ve read the word “now” it’s already past.

But in fact “now” actually encompasses everything that happened in the last 80 milliseconds. This timespan is important for connecting the cause and effect of our own actions, and to some extent our understanding of time, our sense of self and our inability to tickle ourselves.

80 milliseconds is approximately the time it takes to integrate the sensory input from all the different parts of your body. If you touch your nose and your toe simultaneously, you feel them happening at the same time, even though the nerve signals take longer to come from your foot than your face – up to 80 milliseconds longer.

As another example, our brains can actually process sound quicker than they can process images, yet when a hand claps we see it move and hear the sound simultaneously. That is of course, until the person clapping gets so far away that the difference between the speed of sound and light causes them to be more than 80 milliseconds out of synch, at which point they suddenly become disconnected.

You can compare this with film and television. Video is typically screened at 25 frames per second, which means that 80 millisecond roughly corresponds to 2 frames. This is actually quite helpful for video editors and broadcasters, as it gives some leeway for synchronisation of sound and vision before it looks weird.

But this 80 millisecond span is not totally fixed: it can also be sped up and slowed down. I don’t mean some sort of slow motion, Keanu Reeves in The Matrix, bullet time sort of thing. Although subjectively you may think time slows down in stressful situations, that’s not really the case. Think about it: in, say, a car crash, do you hear people’s voices in slow motion?

This was actually demonstrated experimentally by David Eagleman and colleagues, who tossed volunteers backwards off a 45 metre tower. The subjects had devices strapped to their wrists that showed numbers alternating at varying rates. The hypothesis was that if people’s brains worked faster under stress then they would be able to read numbers oscillating at a quicker rate. (See Stetson C, Fiesta MP, Eagleman DM 2007, “Does time really slow down during a frightening event?”, PLoS ONE 2(12): e1295, doi:10.1371/journal.pone.0001295)

Volunteers in the time-slowing experiment had devices, called "perceptual chronometers", strapped to their wrists. They displayed alternating digits on their LED displays. When the digits alternated slowly they were easy to identify (a); but as they sped up, the numbers blurred into a uniform field (b). The people were then dropped backwards off a tower, into a safety net 31 m below (d).

What they found was that the experimental subjects – when they were able to actually concentrate on the watch – weren’t thinking any faster when falling then they were standing still. But afterwards, when they were asked to estimate the time of their fall, they recalled it as being at least a third longer than the time they guessed for other people falling.

The theory is that time seemed to move comparatively slower in their memory of the event because of the rapid rate of stimuli that their brains had to process in such a short time. This could go some way to explain how the years seem to go by faster and faster, because as you get older there are fewer new experiences.

Despite this, there are in fact ways you can train your brain to speed up and slow down beyond the 80 milliseconds. In another study, David Eagleman got people to push a button that made a light go on, but with a short delay (Stetson C, Cui X & Eagleman DM 2006, “Motor-sensory recalibration leads to an illusory reversal of action and sensation”, Neuron 51, pp. 651–659, DOI 10.1016/j.neuron.2006.08.006 [PDF 509 KB]).

As you’d expect, when the delay was less than 80 milliseconds, people thought the button-clicking and the light-lighting happened at the same time. But when the delay was consistently increased, the subjects’ internal chronometers could be recalibrated; they interpreted flashes up to 135 milliseconds later as being simultaneous with the click.

Then the researchers did something tricky: they suddenly decreased the delay to 44 milliseconds. When this happened, the people whose brains were recalibrated saw the flashes as coming before they pressed the button.

This breakdown in causality has led David Eagleman to the idea that schizophrenia may be a problem with perception of time. If, say, you were to “hear” yourself thinking something before your intention to think it, then it would seem like voices in your head coming from somewhere else. Or if you were to think about what you’re seeing on TV before your eyes register it, then it would seem like they’re broadcasting your thoughts. And indeed, in exercises with video games, Eagleman has found that schizophrenics have more difficulty recalibrating their brain clocks.

Tickling is a slightly more commonplace example. In 1998, scientists from University College in London showed that it was possible to tickle yourself by introducing a time delay (‎Blakemore S-J, Wolpert DM & Frith CD 1998, “Central cancellation of self-produced tickle sensation”, Nature Neuroscience vol. 1, no. 7, pp. 635-640 [PDF 271 KB]‎).

Machine used to tickle a person's palm with a small piece of soft foam. It can be operated either directly by the subject or, with a short delay, by an experimenter.

They did this by creating a mechanical tickle device that people could use to touch themselves. The greater the delay, the more tickly they found the touch. This is consistent with the notion that a disconnect between an impulse and an action makes them seem unrelated, or coming from someone else.

But as well as helping us to keep track of our own actions, the ability to connect cause and effect is the basis for our understanding of how the universe behaves in time. So you could argue that this 80 milliseconds of assembling data is essential for making sure we experience most things in the right order.

It’s curious that we puzzle over the unexpected physics of time – one of the most curious results of Einstein’s Special Relativity being that simultaneity isn’t the same for all observers – when our subjective definition of “now” is inherently fuzzy.

Can we really hope to understand the whole universe when we don’t truly understand how we experience it?

Graphene: a Nobel Prize experiment in your own home

Carbon has been known to humankind since before recorded history, so it’s not surprising that discovering a new form of it – especially a form as remarkable as graphene, the wonder material of the 21st century – nets one a Nobel Prize. What is surprising is that it was discovered using everyday office supplies.

(Before you get alarmed about this unashamed and unfashionable pro-carbon stance, let’s be clear: too much carbon dioxide in the atmosphere leading to climate change = bad, carbon the versatile element that’s the basis for all life on Earth = good.)

Apart from the fact that there are more known compounds containing carbon than all the other elements combined (except for hydrogen of course, but we won’t mention that) pure carbon itself comes in many forms.

Diagram showing eight different forms or allotropes of pure carbon: a. diamond, b. graphite (with its layers of graphene), c. lonsdaleite, d. buckminsterfullerene or buckyball, e. C540 fullerene, f. C70 fullerene, g. amorphous carbon and h. a single-walled carbon nanotube (image by Michael Ströck, via Wikimedia Commons)

There’s non-crystalline amorphous carbon, found in coal and charcoal and soot and such. And of course if you subject it to high temperatures and pressures, like in Superman’s fist, it forms the crystals we call diamonds.

Then there are the more complicated structures. The year 1985 saw the arrival of buckyballs, or to give them their proper name, buckminsterfullerene (named after Richard Buckminster Fuller, inventor of the geodesic dome), which are spherical molecules of 60 carbon atoms arranged in the shape of a soccer ball.

But the most recent and perhaps the most amazing form of carbon comes from one of the most common.

Graphite is best known for being the raw material of pencil leads, but it has many other uses including being a dry lubricant. This is because it’s made out of millions of tiny flakes of 2-dimensional crystals with carbon atoms arranged in a hexagonal pattern (see allotrope b in the diagram above). These flat crystals can slip and slide against each other, or flake off to make marks on paper.

Individually, these 2-dimensional crystals are called graphene. And of course, they’re not really new – we’ve known they exist for a long time - but it wasn’t until 2004 that anyone figured out how to extract graphene from graphite and do experiments on it.

The secret is amazingly simple. Starting with graphite extracted from, say, a HB pencil, it’s possible to peel off individual layers using common sticky tape. And because the tape is transparent, you can put it under a microscope and find the pieces of graphene crystal.

Sticky tape can be used to peel off powdered graphite, leaving a single layer of graphene

It sounds nothing special, but because no one had thought of it before it won Andre Geim and Konstantin Novoselov the 2010 Nobel Prize in Physics.

But why the big deal? Well, graphene has a lot of interesting properties:

• Clearly, it’s very thin: just one atom thin. Which means it’s nearly transparent, letting through 97.7% of visible light.
• And yet it’s practically impermeable. The carbon atoms in graphene are only 0.142 nanometres apart, so not even the smallest gas atoms (i.e., helium) can get through.
• It’s also incredibly strong, more than 100 times stronger than steel of the same thickness.
• It’s a great heat conductor, about 10 times better than copper.
• It’s easily turned into a better electrical conductor than copper. In fact, electrons move through graphene as if they had zero mass, a fact that was the subject of that episode of The Big Bang Theory where Sheldon Cooper had to get a job at the Cheesecake Factory so that he could figure out why.
• Being both transparent and such a good conductor, graphene has great potential for use in the next generation of touch screens – that would be strong, flexible touch screens – as well as in lighting panels and solar cells.

Newer, more sophisticated techniques are needed to make sheets large enough for industrial purposes – now up to 70 cm wide – but it’s amazing that it all started with two physicists mucking around with sticky tape.

Burger myth is a load of rot

If you hang out on the internet much, you’ve probably come across video/photos/email similar to the following, in which a McDonald’s Happy Meal doesn’t appear to rot:

What does this mean? Are McDonald’s burgers made out of undigestible material, meaning you can eat as many as you want and not get fat? Or are they a miraculous, non-spoiling food source that could feed hungry millions in places without access to fresh produce or refrigeration?

Well, J. Kenji López-Alt of the Burger Lab has found a more mundane yet scientific answer, by comparing spoilage of a McDonald’s Quarter Pounder with a homemade control burger using good, honest, all-natural ingredients. Both burgers ended up with pretty much the same lack of mould or visible rot.

The main reason seems to be that the burgers in question are small enough to quickly dehydrate when left out on the kitchen bench. Together with the fact that the burger patties are essentially sterilised in the cooking process, they lack both starting mould spores and the environment in which to grow them.

This was further verified by sealing a McDonald’s burger in a plastic bag, which caused it to grow mould just like that loaf of supermarket bread you’ve left for too long.

The moral of the story? Don’t believe everything you see on the internet, at least not without thinking about what it actully means. And one demonstration on YouTube is not the same as a controlled experiment…

For the proper study, with control burgers and everything, head on over to the Burger Lab at Serious Eats.

Oh the brain freeze will blind the weary runner

Like a cold chisel to the head, ice cream headaches are an unfortunate side-effect of what is otherwise a pleasant relief and an effective way to improve athletic performance in hot weather, as suggested by recent research.

Let me explain. (And let me also apologise for my attempted pun, which I blame on summer brain deactivation.)

An ice cream headache, also known as brain freeze or cold stimulus headache, or, if you want to get really fancy, sphenopalatine ganglioneuralgia, is the pain you get when something very cold touches the roof of your mouth.

It is supposedly the most common cause of head pain; at least, according to the seminal 1988 textbook Headache, by Dr Neil H. Raskin. Although, Dr Raskin also found it was more common in migraine sufferers, with 93% of them experiencing it compared to 31% of a control group (see the reassuringly titled article by Joseph Hulihan in the British Medical Journal, “Ice cream headache: no need for abstinence” [PDF 159 KB]).

The pain appears to start in blood vessels in the mouth or sinuses, which rapidly contract when cooled and then dilate again when they rewarm. It’s then transmitted to the brain by either the trigeminal, glossopharyngeal or vagus nerve (opinions are divided on the culprit).

The trigeminal nerve, shown in yellow, carries signals between the brain and most parts of the face. The glossopharyngeal and vagus nerves, not shown, largely serve the throat and chest, respectively. (Gray's Anatomy of the Human Body, 20th edition)

The problem is that each of these nerves serves many other parts of the head – like the forehead in the case of the trigeminal, or the membrane around the brain for the glossopharyngeal – and the brain misinterprets the signals as coming from one of these other areas. This is a phenomenon known as referred pain, which can also be experienced in heart attacks when people feel pain in the neck, shoulders or back instead of the chest.

There are a few treatments recommended for ice cream headaches, like tilting your head back, pressing your tongue against the roof or your mouth or drinking a warm drink. But for most people they’ll go away themselves some time between 20 seconds and 5 minutes – although for an unfortunate few they’ve been known to trigger migraines.

But as I mentioned before, it’s also the main drawback for a newly “discovered” method athletes can use to stave off overheating. Drinking “ice slurry” – basically slurpies – allows people to run for longer in hot weather and endure a higher body temperature, compared to those who only have a cool drink (Siegel R, Maté J, Brearley MB, Watson G, Nosaka K & Laursen PB 2010, “Ice slurry ingestion increases core temperature capacity and running time in the heat”, Medicine & Science in Sports & Exercise, vol. 42, no. 4, pp. 717-725, doi:10.1249/MSS.0b013e3181bf257a).

More recent follow-up research by the same group compared the ice slurry with full body immersion in cold water. Both methods were just as effective in increasing running time, but the slurpies were rather more convenient (Siegel R, Maté J, Watson G, Nosaka K & Laursen PB 2012, “Pre-cooling with ice slurry ingestion leads to similar run times to exhaustion in the heat as cold water immersion”, Journal of Sports Sciences, vol. 30, no. 2, pp. 155-165, doi:10.1080/02640414.2011.625968).

The one problem? 6 out of the 8 participants in the trial suffered from brain freeze.

Which just goes to show that even the most benign, natural-sounding treatments – or in this case, performance-enhancing drugs – can have unfortunate side effects. But still, remember that title from the British Medical Journal: “no need for abstinence”.

Because I don’t pick on The Age enough

While it’s true that in the past I’ve found the Herald Sun to be an easy target, let’s not forget the other Melbourne newspaper.

Browsing The Age website today, I came across the following link to an article in their Executive Style section:

The article itself gives a reasonable precis of the report which it’s quoting (Kim JH, Malhotra R, Chiampas G, d’Hemecourt P, Troyanos C, Cianca J, Smith RN, Wang TJ, Roberts WO, Thompson PD, & Baggish AL 2012, “Cardiac arrest during long-distance running races”, New England Journal of Medicine, no. 366, pp, 130-140, doi:10.1056/NEJMoa1106468).

Specifically, it repeats the article’s main conclusions, which are:

Marathons and half-marathons are associated with a low overall risk of cardiac arrest and sudden death. Cardiac arrest, most commonly attributable to hypertrophic cardiomyopathy or atherosclerotic coronary disease, occurs primarily among male marathon participants; the incidence rate in this group increased during the past decade.

That increase there being due to the greater number of people participating in marathons and half-marathons.

My concern is of course the sensationalist headline, which tries to emphasise the fear that running is in fact dangerous. A message somewhat at odds with the media releases from the institutions where the research was based, being the Massachusetts General Hospital and Harvard University. Their stories were, respectively, Participating in marathons, half-marathons not found to increase risk of cardiac arrest and Good news for marathoners.

What have we learned here? I can think of at least two things:

1. Never treat a newspaper headline as an accurate summary of scientific research; always read the actual story carefully, or preferably track down the original article.
2. Whatever you may think of the paper itself, don’t assume newspaper websites are a reputable news source.

Hot weather disproves climate change… Wait, what?

Happy 2012 to all you Lost in Scientists!

Here in Melbourne it’s been a sweltering start to the new year, hitting 40 degrees on 2 January. Of course, you can’t blame isolated spells of hot weather on climate change alone, as temperatures will always fluctuate around long-term changes in the mean (although extremes, particularly on the hot end, are expected to become more frequent).

But still, even though we’re all familiar with the tabloid notion that winter disproves global warming, the readers of the Herald Sun seem convinced that summer makes a good counter-argument too:

At last, summer. It used to go for months, now we get weeks. “Climategate” sure exposed the frauds.
(Note: it’s January)

It’s summer, not climate change. We’ve had 40 degree days every year for years.

One day of 40 degrees … it’s nothing out of the ordinary. Deal with it like people have for years.

The most at-risk are the pensioners who can’t afford their electricity bills. The carbon tax will only make things worse. Global warming is a scam … 40 degrees is not unusual.

And my favourite:

We’re told this is the hottest start to summer in over 100 years. Does that mean it has been hotter in the past – before airconditioning and maybe even before global warming?

Yes, I know I should find something better to read – but give me a break, I’m on holiday!

Regular coverage will resume shortly…

Magnetic cows test their mettle

Birds do it, bees do it, even sharks in the seas do it… But can cows detect magnetic fields?

That question is, surprisingly, hotly debated. It all started in 2008 when Sabine Begall and colleagues from Germany and the Czech Republic found, using Google Earth, that cows tend to align themselves north-south along the Earth’s magnetic field (Begall S, Červený J, Neef J, Vojtěch O & Burda H 2008, “Magnetic alignment in grazing and resting cattle and deer”, Proceedings of the National Academy of Sciences, vol. 105, no. 36, pp. 13451-13455, doi:10.1073/pnas.0803650105).

It's actually rather difficult to find suitable herds of cattle on Google Maps, given that they're mostly found in rural areas where the photo resolution is poorer. The cows pictured here are near the 12 Apostles in Victoria, and don't have any obvious consistency in their alignment- the pattern was actually found in the statistics of 8,510 cows (click to embiggen).

The obvious question to ask is whether the alignment is due to environmental conditions. However, the researchers claimed they could rule this out: the behaviour of cattle under heat stress or when basking in the sun is well known, and not seen in the cows studied; also, the varying local wind patterns didn’t match the orientations (interestingly, although the wisdom accumulated by farmers over thousands of years was sufficient to rule out these environmental causes, no one had ever noticed the north-south alignment before).

For comparison, they also looked at “beds”, or body prints left in snow by red deer and roe deer. These were even more highly correlated along north-south alignment, and being created at night were clearly unrelated to the position of the sun. (Additionally, although the satellite photos of cows weren’t high enough resolution to see the actual direction they’re pointing, i.e. if it’s north or south, the deer beds showed that they face north when resting.)

Perhaps the clincher though is that at high latitudes, where there’s a big difference between magnetic and geographic north, the cows and deer were much more aligned to the magnetic.

So how do they do it? Is it because beef is so high in iron?

Actually, the mechanism that animals use to detect magnetic fields – a skill known as magnetoreception - is still largely unknown. But it has been studied more in some animals than others.

Birds, for instance, are known for their navigational abilities, and they have a few features sensitive to magnetic fields. They have a region in their upper beak that is known to contain magnetite (Fe₃O₄); also, their eyes use the light-sensitive protein cryptochrome, which is affected by magnetism, so they may be able to “see” magnetic fields.

We might even share this ability: human sinuses have been found to contain magnetite, and the cryptochrome in our eyes is also potentially magneto-sensitive (see Baker RR, Mather JG & Kennaugh JH 1983, “Magnetic bones in human sinuses”, Nature 301, pp. 78-80, doi:10.1038/301078a0 and Foley LE, Gegear RJ & Reppert SM 2011, “Human cryptochrome exhibits light-dependent magnetosensitivity”, Nature Communications 2, doi:10.1038/ncomms1364).

It’s actually with humans that the whole cow compass affair started: Sabine Begall had been studying naked mole rats, which always sleep on the south side of their burrows, and she got to wondering whether sleeping humans also had a preference.

Begall tried using Google Earth to examine campsites to see if there was a pattern, but it turned out to be very hard to see which way people were sleeping in tents. But cows were much more visible, and they indeed showed a pattern.

However, not everyone agrees. In January 2011 another Czech team did their own analysis of satellite photos of cows across Europe, and found no alignment (Hert J, Jelinek L, Pekarek L & Pavlicek A 2011, “No alignment of cattle along geomagnetic field lines found”, Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, vol. 197, no. 6, pp. 677-682, doi:10.1007/s00359-011-0628-7).

But it doesn’t end there. In November, Begall’s team published a response, in which they re-analysed the same photos. They claim that the second study had used a different statistical technique and included a lot of “noise” in their data, including poor quality photos, pastures on slopes and herds near power lines, which supposedly disrupt the magnetic effect  (see Begall S, Burda H, Červený J, Gerter O, Neef-Weisse J & Němec P 2011, “Further support for the alignment of cattle along magnetic field lines: reply to Hert et al.” Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, vol. 197, no. 12, pp. 1127-1133, doi:10.1007/s00359-011-0674-1).

So the great magnetic cow controversy rages on. But the original question is still unanswered: what about humans?

This is a topic waiting to be solved, perhaps with better quality satellite photos, or by choosing suitable locations: people lying on beach towels are much easier to see, but the beach is a possible confounding factor.

Or maybe like the deer, we need to examine campgrounds up close, or even just go down to the local park at lunchtime and see which way people are lying. It could be a fun summer research project!

WIMPs may occupy the universe

As I type this post, the physics community is holding its collective breath waiting for news from the Large Hadron Collider, in the hope that it may give us a clue for the next step in understanding how the universe works. In the meantime, any unusual result like possible faster than light neutrinos is seized in the hope of finding something new.

Despite this, I can’t help wondering if we’re missing an obvious piece of the puzzle: dark matter and dark energy together account for about 96% of the total mass of the universe.

So if everything we can see and experiment on is a mere 4% of all that there is, is there any wonder it doesn’t make sense?

Okay then, do we know anything at all about what dark matter may be? Well, for a long time the leading candidates have been either MACHOs or WIMPs. Yes, those are the actual names.

MACHO stands for Massive Astrophysical Compact Halo Object, and it basically refers to clumps of normal matter that sit in the “haloes” around galaxies, but for some reason can’t be seen. They could be objects like planets, brown dwarfs (unignited stars) or even black holes.

Although MACHOs don’t emit enough light for us to see, they can be detected by gravitational microlensing, which is when their gravitational fields bend the light rays from distant stars when they pass in front of them.

Unfortunately, although surveys of gravitational microlensing have found MACHOs, there’s not enough of them to account for the universe’s missing mass.

So we turn to the other option: WIMPs, or Weakly Interacting Massive Particles. These are a postulated type of subatomic particle that only interacts via gravity and maybe the weak nuclear force.

This means they would have properties similar to neutrinos (i.e. very hard to detect), only heavier (hence the “massive”). And that’s why we haven’t yet been able to create them or detect them on Earth.

However we do have some indications they exist, from observing collisions between galaxies like in the famous Bullet Cluster (pictured).

The so-called Bullet Cluster, formed from the collision of two galaxies, as captured by NASA's Hubble Space Telescope and Chandra X-ray Observatory. Ordinary matter, shown in pink, has been distorted into a bullet shape by the collision, whereas the clusters of dark matter, shown in blue, appear to have passed straight through. This supports the theory that dark matter particles only interact by the force of gravity. (Image from NASA)

In these collisions, we see matter like hot gas piling up in the middle, but the centres of mass of the colliding galaxies – presumably made out of dark matter – pass straight through. This suggests that the dark matter is something that doesn’t interact much at all, like WIMPs.

Lately though, there have been some intriguing signals suggesting we’re getting closer to actually detecting these dark matter particles. Experiments like CRESST and DAMA in Gran Sasso (the same underground lab that detected those pesky neutrinos) have seen hints of WIMPs  colliding with nuclei.

And most recently, the balloon-borne experiment ARCADE has seen a radio signal coming from space that could be the result of WIMPs colliding with each other to produce pairs of electrons and positrons (Fornengo N, Lineros R, Regis M & Taoso M 2011, “A dark matter interpretation for the ARCADE excess?”, arXiv:1108.0569v1).

These are all very small, very early results, so we have to be careful not to jump to conclusions. But when we’re contemplating the universe and trying to come up with a “theory of everything”, we really shouldn’t ignore the other 96% we can’t see.

Strigolactone, straight up

Plant growth is controlled by many different hormones (hormones being chemicals that influence the development and metabolism of both plants and animals). But recent research by the University of Queensland’s Dr Phil Brewer, among others, has shown that one hormone called strigolactone plays a number of important roles.

Striga asiatica or witchweed, a parasitic plant, the seeds of which germinate in the roots of their host when triggered by the hormone strigolactone - hence the name (image by Florida Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org, via Wikimedia Commons)

Firstly, in a paper published three years ago, Dr Brewer and colleagues showed that strigolactone inhibits the growth of side-branches, making plants instead grow straight up (Gomez-Roldan V, Fermas S, Brewer PB, Puech-Pagès V, Dun EA, Pillot J-P, Letisse F, Matusova R, Danoun S, Portais J-C, Bouwmeester H, Bécard G, Beveridge CA, Rameau C & Rochange SF 2008, “Strigolactone inhibition of shoot branching”, Nature, no. 455, pp. 189-194 doi:10.1038/nature07271).

Levels of strigolactone increase when light or nutrients are limited, which is a good time to grow taller than your neighbours, or to quickly reproduce before the food runs out (by inhibiting side-shoots, more energy is available for making flowers and seeds for reproduction).

Conversely, when nutrients are plentiful strigolactone levels fall, the plant grows bushy and is able to take advantage of its environment.

Now a follow up paper has revealed that strigolactone helps plants grow tall in another way: by making their stem thicker and stronger and so able to support the weight. It does this in response to signals issued by another hormone, auxin (Agusti J, Herolda S, Schwarz M, Pablo Sancheza, Ljung K, Dun EA, Brewer PB, Beveridge CA, Sieberer T, Sehr EM & Greb T, “Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants”, Proceedings of the National Academy of Sciences, published online 28 November 2011, doi:10.1073/pnas.1111902108).

Strigolactone also seems to encourage growth in other areas too, like tiny hairs on the roots needed to extract nutrients from the soil and encourage the growth of symbiotic fungi.

Of course, like anything it comes with negative effects too: presence of strigolactone in the root systems seems to stimulate the germination of seeds of some parasitic weeds, like those of the genus Striga (from which strigolactone gets its name).

It could also be used by foresters to make plantation trees grow faster with straighter trunks, which probably counts as a positive for both us and the plants; until the whole chopping down thing, of course. Despite their signalling hormones, the trees are a little quiet about whether it’s good or bad overall.