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.

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.

Lost in science fiction: Ghostbusters

This time on Lost in Science Fiction, aka science in the movies, let’s turn to a classic.

Appropriately for our pre-Halloween theme, it’s Ghostbusters (1984), directed by Ivan Reitman (see, I told you he’d be back), and starring Bill Murray, Dan Aykroyd, Sigourney Weaver, Harold Ramis, Rick Moranis, Annie Potts, etc.

It might not sound like it, but I believe this is a great movie for science. To quote:

But is there any real science in Ghostbusters? Well, there are the proton packs, those portable particle accelerators they wore on their backs – remember “don’t cross the streams”?

Of course, they’re not terribly accurate – a real world proton accelerator (like, say, the Large Hadron Collider) would be far too big to carry on your back – but what’s really interesting is what they seem to do, which is to contain “negatively charged ectoplasmic entities”. Which, considering positive electrical charges attract negative charges, kind of makes sense.

That’s if you assume that these “ectoplasmic entities” are negatively charged; presumably they’re negative because they’re bad, in some way.

Ectoplasm, though, is a term first used in 1883 to describe a material supposedly excreted from the orifices of mediums in the Victorian era. You can see “ectoplasm” in a lot of spirit photographs from the time (needless to say, they were hoaxes, and mediums tend not to do that these days).

Another Victorian-era innovation referenced in the movie is the actual practice of ghostbusting, or “ghost hunting“. It’s still very popular today; perhaps too popular, as apparently police forces in the United Kingdom are being inundated with nuisance Freedom of Information requests from ghost hunters looking for any reports containing supernatural terms. Although there were far more complaints about ghost hunters than actual sightings of ghosts in at least one report, from Dyfed-Owes Police in Wales (PDF 30 KB).

Seemingly influenced by the movie is not only the concept of ghostbusting, but also the techniques they use. Many modern ghost hunters carry electromagnetic field detectors, very similar to the devices used by Dr Egon Spendler.

Why would people think ghosts generate electromagnetic fields? Considering ghosts could at best be described as “unknown to science”, it seems strange to use such a specific scientific technique. Although, ghost hunters do at least claim they find strong electric or magnetic fields in haunted locations…

Is it just the movie’s influence, or is something else going on? Could they be extrapolating from the fact that living organisms produce electromagnetic fields?

Intriguingly, there have been counter-suggestions that it could run the other way: that electromagnetic fields may cause people to see ghosts.

It’s been long known that strong electric or magnetic fields can cause people to see flashes of light, or phosphenes (PDF 8.4 MB). But even further than that, Canadian psychologist Michael Persinger claims that low-level magnetic fields applied to the temporal lobes of the brain can cause people to sense a mysterious presence, or even experience religious ecstasy. He’s even invented a device to generate this effect, called the God Helmet.

Persinger’s claims are very controversial, with critics saying that the power of suggestion, as well as prior susceptibility and beliefs, have far more to do with whether people experience any unexplained sensations under the influence of the God Helmet. At least, this seemed to be the case with the ‘Haunt’ Project, which attempted to use both electromagnetic fields and infrasonic (low frequency) sound to cause people to sense a ghostly presence (it didn’t work).

So perhaps a more likely explanation for the use of electromagnetic field detectors is that they’re essentially another data collecting device to take with you when hunting ghosts. And the more types of data you collect – preferably by using high-tech equipment that’s easy to misinterpret – the more likely it is that you’ll find something unusual purely by chance.

And when you’re casting a wide net for anomalies, anything you find takes on significance, no matter how random. Which is a mistake you often find in pseudoscience.

Still, it’s all very entertaining, and I’m possibly being too harsh on the ghost hunters. Maybe I should back off: they’re pseudoscientists.

Well they don’t call him Bob the Physicist

As a special pre-Halloween treat, this week on Lost in Science we’ll be talking about science in the movies.

Just as a taste, have a look at this clip from a popular children’s television program (sorry, but the embedding is disabled).

In case you didn’t watch it, here’s a transcript:

(Sound of thunder)

“Oh, dear me. One elephant, two elephant, three elephant, four elephant, five elephant, six elephant, seven elephant, eight elephant… Oh!”

“Um, why are you counting elephants, Bob?”

“No, I’m counting how many seconds there are between the thunder rolling and the lightning flashing. It takes one second to say ‘elephant’. There were eight seconds, which means the storm’s only eight miles away!”

I don’t think there’s much more to add. Although maybe I should cut Bob some slack: after all, he is trying to explain it to a bulldozer.

Nobel win for Aussie dark energy discovery

Every year, all eyes turn north to see who takes out the Nobel Prizes, the most prestigious scientific awards Sweden has to offer. And in 2011, Australian eyes got to see one of our own, Professor Brian P. Schmidt, receive the Nobel Prize in Physics.

Professor Brian P. Schmidt, seen here observing the expanding universe (Photo by Belinda Pratten, ANU)

Professor Schmidt shared the prize with Saul Perlmutter, from the Lawrence Berkeley National Laboratory and University of California USA, and Adam G. Riess, Johns Hopkins University and Space Telescope Science Institute USA (actually, in the complex way the Nobel Prize works, Saul Perlmutter got 50% and Brian Schmidt and Adam Reiss each got 25%).

The trio won it for showing that the universe is not only expanding – the first evidence for which was found by Edwin Hubble in 1929 – but its expansion is actually accelerating.

They showed this by observing distant Type Ia supernovae (or supernovas, if you prefer). These incredible explosions occur when white dwarf stars get too big, specifically 1.38 times the mass of our Sun (they usually do this by pulling matter off a nearby star). When they reach this threshold, they collapse under their own weight, and their core compresses further until the pressure sets off a tremendous nuclear fusion reaction that blows the whole thing to smithereens.

What makes these Type Ia supernovae so special is that we know exactly how they work and they’re all exactly the same. So they make great markers that we can look at across the universe. From their brightness we can calculate how far away they are, and from their Doppler redshift we can figure out how fast they’re moving away from us.

The findings that Schmidt, Reiss and Perlmutter announced in 1998 demonstrated that the more distant the supernovas are, the faster they’re moving away from us – and this expansion is getting faster all the time.

The best known explanation for this acceleration is the existence of dark energy, a mysterious, invisible force that permeates all of space. We discussed dark energy – as well as dark matter – and some recent discoveries concerning them a few months ago on Lost in Science (see ‘Help our mass is missing‘). But it’s great to see the original discovery getting such recognition!

Incidentally, according to Wikipedia, Brian P. Schmidt is the 11th Australian to win the Nobel Prize, and our 2nd laureate in physics. Physiology and Medicine is clearly our strong point, with no less than 7 winners, but here’s hoping the physicists are on their way to catching up!

For more on his prize-winning work, see Brian P. Schmidt’s homepage.

Music vs physics

How come physicists think they know everything about everything? Is it just pure arrogance that makes them think they can explain how brains work, as well as things like black holes?

Well, brain cells, or neurons, do send signals to each other by building up an electrical voltage to a high enough level to discharge, which is all to do with physics. And the networks they form are just the kind of thing that physicists are good at modelling with mathematics.

As an example, in a recently published paper, Russian and Italian physicists have modelled how neurons in our auditory system might work to help us distinguish between harmonious and non-harmonious combinations of musical notes (Ushakov YV, Dubkov AA & Spagnolo B 2011, “Regularity of spike trains and harmony perception in a model of the auditory system”, Physical Review Letters, vol. 107, 108103).

The auditory system is good for this kind of study, because it’s a relatively simple system from a wiring point of view – plus, of course, we have a good understanding of how to turn sound into electrical signals.

Anatomy of the human ear. Sound waves are converted into electrical signals by hair cells in the cochlea (the purple bit). The rest of it mostly serves to focus and amplify the sound. (Image by Chittka L, Brockmann, via Wikimedia Commons)

For humans, this is done by hair cells in our inner ears, which, as the name suggests, have tiny hairs or cilia that pick up amplified sound waves and then turn them into electrical signals.

Of course, physicists like to simplify things to understand them, so in this case they used a model with only two receptor neurons, both connected to a third interneuron that passes the signal to the rest of the brain.

Nerve signals typically appear as a sequence of electrical impulses called spike trains, created when they build up a voltage, fire, relax and build up again. In the model the physicists used, the interneuron mixes different frequency spike trains from the two receptor neurons into a combined signal.

What they found was that, when the two tones were harmonious, the interneuron produced a nice, regular, coherent spike train. But when they were discordant, the spikes were all messy and blurred into each other.

Now, we’ve known since the days of Pythagoras in about 500 BC that pairs of notes with simple frequency relationships go well together – like octaves (2:1) or perfect fifths (3:2). But this research shows that those mathematical relationships create strong, regular electrical signals in the brain.

It also fits in with what people hear when two notes are combined, which is often a low frequency that wasn’t really there in the original sound waves. This third note could well be the combined, regular spike train that the interneurons create.

Of course, this theoretical 3 neuron system is a vastly simplified model. But it does show how networks of neurons can give rise to phenomena that we see as subjective experience.

Those tachy neutrinos

Like pretty much every other physicist in the world, I’ve been inundated with people sending me links to articles about neutrinos timed to be moving faster than the speed of light (thank you BBC, The Guardian, The Age, Slashdot, Bad Astronomy, Starts With a Bang, etc.).

And just like every physicist, I’m obliged to say that we’ll have to wait to see whether this is confirmed as a truly universe-shattering discovery, or some sort of statistical fluke or experimental error.

The implications of it being true are so great that we have to be careful about jumping to conclusions; plus, there are good results from previous experiments that showed neutrinos don’t move faster than the speed of light.

First though, the gist of today’s result: this was part of the OPERA experiment to measure neutrino oscillations (that’s where they change from one flavour to another).

Neutrinos are subatomic particles related to electrons. They have nearly zero mass, no electric charge, and they only interact via the weak nuclear force. This makes them very hard to detect, but it also means there are a lot of unanswered questions about them, and hence potential for revealing more about the fundamental nature of the universe.

The neutrinos in this experiment were emitted from the CERN facility in Switzerland and detected in Gran Sasso, 732 km away in Italy. Using atomic clocks to time their emission and detection, and accurate GPS measurements of the distance, the particles were found to arrive about 60.7 nanoseconds earlier than they should have if they were travelling at the speed of light.

Diagram of the neutrinos' subterranean path, known as CNGS, short for "CERN neutrinos to Gran Sasso" (Image from OPERA)

This means that they must have been moving at a speed of 1.0000248 ± 0.0000028 (stat.) ± 0.0000030 (sys.) times the speed of light (c).

Those numbers with all the zeroes after the plus-minus sign ± are the statistical and systematic uncertainties respectively. The statistical uncertainty is calculated from the standard deviation of the number of measurements made, whereas the systematic uncertainty is an estimate of inaccuracies inherent in the experimental equipment or methodology.

It’s pretty clear that the speed measured is well out of the range of the uncertainties, which is why the result is so worth commenting on. But that still doesn’t mean it’s right…

According to Albert Einstein’s Special Theory of Relativity, normal matter cannot travel faster than the speed of light. Anything that does would hypothetically be what we call a tachyon, unable to travel slower than the speed of light and capable of travelling backwards in time. Which raises serious questions about causality.

Alternatively, Einstein could be wrong. But that would also be very troubling, because in every other case relativity works very well. It’s really not clear what you could replace it with that would agree with relativity in every known instance except this one.

But apart from these theoretical reasons to doubt the result, the speed of neutrinos was previously measured to great accuracy in the explosion of Supernova 1987A. When a star explodes as a supernova, it first emits a burst of neutrinos from its collapsing core; the core then rebounds off itself, triggering an incredible explosion. And about 3 hours later, it emits a burst of light that can be seen across the universe.

And that’s exactly what was observed in 1987. A burst of neutrinos was detected and then, about 3 hours later, the light of the explosion became visible.

The timing was so close to theoretical predictions that it puts a limit on any faster-than-light – or superluminal - neutrinos of less than 1.000000002 times the speed of light. Or to put it another way, if the OPERA experiment is correct, the neutrinos should have arrived more than 4 years before the light did, not 3 hours.

So what happens now? Do we just dismiss the OPERA result?

No, we do the same thing we always do: see if other scientists can reproduce the result (which of course plenty of them are clamouring to do). And we find out what explains this discrepancy, whether it’s an error in one of the experiments, or if there are unique conditions, like different energy neutrinos behaving in different ways, or something weird happening in the rocks between Switzerland and Italy.

After all, even though there are some really wacky theoretical ideas being put forward – like neutrinos taking a shortcut through higher dimensions – it’s unexpected results like this that have the chance of giving us new physics. Especially now that we’re staring down the barrel of a possible deadlock with the Higgs not being found at LHC, we have to hope for mysteries like these to show us where to look next.

To find out more, read the full OPERA paper at the physics e-print repository, arXiv.org (PDF 4.7 MB).

Higgs update

One of the most publicised goals for the Large Hadron Collider (LHC) in Switzerland was to search for the Higgs boson, the hypothetical particle that could explain how other subatomic particles get their mass.

Well, the LHC has been running for nearly 18 months now, so what has it found? Any sign of the Higgs?

The LHC’s two general purpose experiments, ATLAS and CMS, announced their preliminary findings at August’s Lepton-Photon conference, held in Mumbai, India. And so far it’s not looking good for the Higgs boson.

(There are actually six experiments running at the LHC. As the name suggests, the Large Hadron Collider is a very large particle accelerator that collides hadrons, i.e. protons, at very high energy. Examining the outcome of these collisions are a range of detectors, run by six different groups; these make up the LHC’s six experiments.)

Plot of ATLAS experiment's Higgs boson search results to date. The wobbly black dashed line shows the prediction from simulations, and the green and yellow bands are the uncertainty in the predictions (at one and two standard deviations respectively). The solid black line is the actual result from the collected data. The Higgs boson is ruled out with 95% confidence wherever the solid black line dips below the horizontal line at 1. (Image ATLAS)

Things were looking promising a couple of months ago, when there were a couple of anomalous signals that could have been the Higgs, but twice as much data has been collected since and the signals have faded.

At the moment, there’s still a slight chance the Higgs exists with a mass between 115 and 145 GeV, or maybe 232-256 GeV, or possibly 282-296 GeV, or potentially even above 466 GeV. All other mass ranges have been ruled out with 95% confidence (GeV, or giga-electron volt, is the unit used to measure the mass of subatomic particles; protons have a mass of 0.938 Gev).

There’s still more data to collect though, so it’s possible there’ll be a definite result either way by the end of the year.

But it’s not necessarily a bad thing if the Higgs isn’t discovered: it simply means that some other, unexpected mechanism must explain the origin of mass. Which is kind of good, because it means new physics, and the potential for new discoveries.

What those might be is unknown – and in fact so far the LHC has ruled out a few popular theoretical possibilities. But that just means the real truth is likely to be something unexpected, and that’s where the really interesting discoveries come from.

All of which makes it an exciting time to be a particle physicist – not that it ever isn’t.

Read more about the results from the ATLAS experiment