Category Archives: Physics

Solid smoke and stardust

Recently I visited the Canberra Deep Space Communication Complex at Tidbinbilla in the Australian Capital Territory, where they have a sample of aerogel, aka “solid smoke”.

The author in front of the Big Dish at the Canberra Deep Space Communication Complex
Here’s me posing in front of the Big Dish at Tidbinbilla. Disappointingly, their cafe doesn’t have a meal called a ‘Big Dish’. Get onto that, CDSCC!

This ethereal substance was used on NASA’s Stardust probe to capture dust from comet Wild 2 and return it to Earth—along with a handful of precious samples of interstellar dust, giving a rare glimpse of material from outside our Solar System.

A 2 gram piece of aerogel holding up a 2.5 kg brick (image from NASA/JPL-Caltech)
A 2 gram piece of aerogel holding up a 2.5 kg brick (image from NASA/JPL-Caltech)

Listen to our show from 27 August 2015 to hear all about it.

You can also hear our interview with British scientist Maggie Aderin-Pocock, host of the long-running astronomy TV show The Sky at Night, and who’s also known for answering important questions like, do we really need the Moon?

Yes. Yes we do.

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6 truths about chemtrails

Recently a listener wrote to us in response to a story we aired dismissing chemtrail conspiracy theories—our listener asked us to consider both sides of the argument.

‘Chemtrails’ are a type of aircraft contrails—which is short for ‘condensation trails’, long clouds formed by water vapour in aircraft exhaust or due to pressure changes from wings.

The difference is that chemtrails are supposedly deliberately created, either to change the climate as a form of geoengineering, or some other form of weather control, or maybe mind control, or some other mass-poisoning depending on what’s being sprayed.

This is, of course, generally considered to be nonsense.

In saying that, I have considered both sides of the argument, albeit correctly weighted. As always, Lost in Science gives greater weight to peer-reviewed scientific evidence. And there is in fact a surprising amount of published research relevant to this topic.

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Contrails from USAAF B-17F Flying Fortress bombers over Europe, circa 1943. That is, propellor aircraft, 72 years ago. Scientists have attempted to analyse their effect on English climate at the time. (Photo: United States Air Force)

Now, I did try to respond to our listener, but unfortunately the email bounced, so I’m writing this blog post instead in the hope they see it. Hi! Please keep listening!

And since this a largely internet-based controversy, I’ve decided to go all out for clickbait and write this in the form of a listicle. Here we go…

1. Geoengineering is a thing

Geoengineering, a popular term for deliberately manipulating the climate, is becoming a hot topic as our inadvertent climate manipulation becomes more dire. But it comes with many moral, ethical and practical risks, so it’s currently much further down the list of preferred responses than stop burning fossil fuels.

We have in fact discussed it before on Lost in Science: in March 2013 we aired a discussion on the ethics of geoengineering between Peter Singer and Clive Hamilton (author of the book Earthmasters: Playing God with the climate).

And in 2011 we looked at a range of proposed mechanisms (Geoengineering as climate change plan B), largely collated from an excellent 2009 review from the UK’s Royal Society.

2. It includes solar radiation management

This is a fancy name for reducing the amount of sunlight reaching the Earth. This is not a new idea: apart from Mr Burns, volcanoes have been doing this forever by spewing ash into the atmosphere. The eruption of Mt Pinatubo in 1991 reduced global temperatures by about 0.1 °C for a couple of years.

In fact, one of the first people to push for injecting particles—or aerosols—into the atmosphere to counteract climate change was the father of the hydrogen bomb, Edward Teller. You can listen to me talking about him at the Laborastory.

3. Aerosols emitted by humans do affect climate

For instance, sulphur pollution from China’s coal-fired power stations are believed to have contributed to the slowing of global surface temperature rise.

4. There’s no evidence anyone’s doing it on purpose

Yes, it’s hard to prove a negative. However I can direct you to the physical science working group of the 5th Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), who said, in their Summary for Policymakers (PDF 2.3 MB):

Limited evidence precludes a comprehensive quantitative assessment of both Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) and their impact on the climate system… Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing. CDR and SRM methods carry side effects and long-term consequences on a global scale.

It doesn’t sound to me like they’re actually doing it, or that they think it’s a good idea. And surely if it was a global conspiracy, they’d be in on it?

5. The effect of contrails has been measured, and it’s small

This is the kicker: scientists have actually studied the impact of contrails. That same IPCC report has a whole section on it (section 7.2.7.1, Contrails and Contrail-Induced Cirrus).

They find that, unlike in solar radiation management proposals, contrails actually increase the temperature, because they block the outgoing infrared (heat) radiation from the ground more than they block the incoming sunlight.

You can read the whole thing yourself (Chapter 7, PDF 19.2 MB), but I’ll save you the the trouble by quoting their conclusion:

Persistent contrails from aviation contribute a RF of +0.01 (+0.005 to +0.03) W m–2 for year 2011, and the combined contrail and contrail-cirrus ERF from aviation is assessed to be +0.05 (+0.02 to +0.15) W m–2. This forcing can be much larger regionally but there is now medium confidence that it does not produce observable regional effects on either the mean or diurnal range of surface temperature.

RF here is radiative forcing, the net thermal energy reaching the Earth. For comparison, the IPCC puts the total anthropogenic, or human-caused contribution at about 1.5 W m–2.

This is based on numerous published scientific papers. I’m not going to bore you with them here—instead you can click my Read more link and see the list. You’re welcome.

6. But aircraft emissions do have an impact overall

One of those references, Lee et al. (2009) added up the combined effect from aircraft carbon emissions and cirrus clouds caused by contrails. They found:

Total aviation RF (excluding induced cirrus) in 2005 was ~55 mW m–2, which was 3.5% of total anthropogenic forcing. Including estimates for aviation-induced cirrus RF increases the total aviation RF in 2005–78 mW m–2, which represents 4.9% of total anthropogenic forcing.

(Emphasis mine. And for readability I’ve taken out the uncertainties.)

According to the International Civil Aviation Organisation (ICAO), this 4.9% means that:

If global aviation was a country its emissions would be ranked 7th between Germany and South Korea on CO2 alone.

That’s quite a bit. So although the contribution of contrails alone is small, when you add in their carbon emissions then yes, flying planes does affect global temperature.

But even so, all this science suggests that, whatever you call them, contrails or ‘chemtrails’ are not being used as a deliberate global conspiracy.

However, we should still be concerned about the impact of air travel on the climate.

Continue reading 6 truths about chemtrails

Pouring wine… with a twist

It’s coming into the summer imbibing season, so expect a lot of wine to be poured. And as the wine was poured at a recent pre-summer BBQ, I was asked the question: what’s up with that little twist thing that waiters do when they finish pouring your glass? Is it just for show, or does it really stop the drips?

Now don’t get me wrong, it is showy; but the showiest I’ve seen also involved a snazzy napkin around the bottle, which does raise doubts about its anti-drip efficacy… Maybe science can answer the question!

Well, as far as I’ve found there isn’t any actual research on this topic—but there is some related work on teapots, which I shall attempt to adapt.

In 2009, some French physicists published a paper on what they called the “teapot effect”; that’s when your teapot dribbles when you finish pouring, leaving a puddle on the saucer or on the tabletop (Duez C, Ybert C, Clanet C & Bocquet L 2009, “Beating the teapot effect”, Physical Review Letters, vol. 104, 084503, arXiv:0910.3306).

They found it was due to the relationship between inertial flow from the spout and surface wetting, i.e., liquid sticking to the surface. As the flow decreases, the water forms drops that cling to the lip of the spout.

Beating the teapot effect with a superhydrophobic coating. Top: water flow under the spout of an (hydrophilic)
(a) A normal teapot, pouring tea—note the stream bending back towards the spout; (a’) as the flow decreases, the drips stick to the spout. (b) A spout with a superhydrophobic coating (they used black soot) has a smooth stream, (b’) even as the flow slows. (Image: Lydéric Bocquet lab, Ecole Normale Supérieure, Paris)

Their solution was to use a superhydrophobic surface. OK, so in the science of wetting, you classify surfaces in different ways. There are surfaces that like to get wet; they’re called hydrophilic. These are usually smooth surfaces of a material that has high energy, i.e., strong chemical bonds between the molecules, which override the surface tension of the liquid. So things like metal, glass and glazed ceramic. If you think how drops of water spread out on the window—glass wets easily.

Then there are surfaces that are hydrophobic, and they usually have low energy. Things like hydrocarbons, fats and oils. Think about when you wax your car and the water forms beads: that’s hydrophobic.

Then there are superhydrophobic surfaces, which are extremely hard to wet. You find these in nature, like on plant leaves, insect wings or yes, even duck feathers. These often have surfaces that are rough on the nanoscale. The roughness stops the water drops from spreading out, forcing them to form little globules.

As the figure above shows, if you use a superhydrophobic surface on the teapot spot, the liquid doesn’t cling to it and instead it pours neatly, no matter how slow the flow.

Now, there have been other attempts to make dripless teapots by controlling the flow, like using a “speed bump” to stop it getting slow enough to dribble, or by having a really sharp edge that’s harder for drops to cling to. But I like the French research because it identifies the mechanism, which we should be able to generalise to wine. 

Red wine being poured into a glass
Note how the wine clings to the mouth of the bottle, just like the tea in the previous image (Photo: theloushe via Compfight cc)

Essentially the same thing happens: the liquid bends towards the glass when pouring, and as you stop the last few drops hang on, running down the bottle and leaving a trail over the tablecloth.

So that’s the cause, but what’s the solution? Can we insist on wine makers switching to superhydrophobic bottles?

Maybe, or maybe the twist is easier. This appears to simply embrace the fact that a drop is going to cling to the glass, but instead of letting it fall down, the twist turns the drip so it runs around the mouth of the bottle instead of down.

Wine dripping as it's poured into a glass
Or maybe it’s up to the pouree to catch the drips (Photo: sintixerr via Compfight cc)

One piece of advice I did find is to try it with a slow-flowing liquid, like honey. It’s pretty easy to do, and a great demonstration of how the twist diverts the drip; just be prepared to get sticky if you get it wrong on the first go.

But it does work, and is maybe a good way to practise before moving on to a grown-up liquid. Try it yourself!

And if you don’t like that, well one other old trick for stopping teapot dribbles is apparently to rub butter on the spout. Butter, being hydrophobic, should also reduce wetting and prevent drops.

So the same trick should also work for wine—as long as you’re not afraid of having greasy vino. Something else to try at your next BBQ.

Time travel model tests quantum theory

Even though the laws of physics seem to permit time travel, many physicists and non-physicists still worry about the paradoxes that arise when you try to change the past (for an example see… well, pretty much any movie involving time travel). But quantum mechanics can give a way out of the mess, and an experiment performed at the University of Queensland has tested what happens when you try this.

Although not a dinkum time machine, the experiment simulated the effect of sending a photon (a particle of light) back in time by using two photons, one acting as the past version and one as the future version, and having them interact (Martin Ringbauer, Matthew A. Broome, Casey R. Myers, Andrew G. White & Timothy C. Ralph 2014, “Experimental simulation of closed timelike curves”, Nature Communications 5, Article number: 4145, doi:10.1038/ncomms5145).

Diagram of the time travel simulation experiment
A diagram of the experimental set-up, in which two single photons, generated in a nonlinear β-barium-borate crystal, are sent by optical fibres into the top and bottom paths. The bottom photon represents the time traveller, which is polarised according to a theoretical quantum consistency condition. The two photons interact in the middle in a polarising beam splitter and then are detected by photo-diodes at the outputs (Image Ringbauer et al., via Nature)

To make it act like a time machine, they imposed theoretical conditions established in 1991 by David Deutsch (Deutsch 1991, “Quantum mechanics near closed timelike lines”, Physical Review D 44, 3197, DOI:10.1103/PhysRevD.44.3197 [PDF 4.7 MB]).

In Deutsch’s theory, any attempt to meddle with the past gives mixed results, i.e. a quantum mixture of meddled and non-meddled. These separate possibilities exist simultaneously, and are often interpreted as alternative timelines created by the act of travelling back in time.

There are other theories of time travel,  such as those that forbid any changes to the past that haven’t already happened, but Deutsch’s model is a particularly popular one.

The study of even a simulated time machine gives clues to how our understanding of physics may have to change to accommodate such bizarre circumstances.

To find out more about it, we spoke to one of the experimenters, PhD student Martin Ringbauer. Continue reading Time travel model tests quantum theory

Doppler affects you and me, quite frequently

It’s making the news in oceans both Indian and Saturnian, tracking the movements of space probes and missing Malaysian airliners. And yet you encounter it every day, when you hear a car passing you on the road change from high to low pitch. So what exactly is the Doppler effect, and how does it work?

(Q: What sound does a cat make when it goes past at high speed? A: Meeeeeeeeeeeeeeeowwwww.)

As you might expect, the Doppler effect was named after the Christian Doppler, an Austrian physicist—although he only became a physicist because he was too frail to enter his father’s stonemason business—who proposed it in Prague in 1842.

It happens whenever there is movement relative to a source that’s emitting waves, whether they’re light, sound, water or something else. In the case of the moving car, think of its soundwaves as a series of peaks and troughs. The car emits one wave, i.e. one peak, and then another about 1 millisecond later.

But in that millisecond the car has moved closer to you, so the second peak has less distance to travel. It therefore reaches you less than 1 millisecond after the first peak does. This means that for you each peak is separated by less than a millisecond, so you hear the sound at a higher frequency.

OK, that maybe a little hard to picture, so try it visually instead. Imagine the waves as concentric rings being emitted by the source, they bunch up at the front and stretch out behind it. Or don’t imagine it: look at the picture below.

Doppler effect showing circular wave fronts emitted from a source moving to the right
Doppler effect from a source moving at 0.7 the speed of wave propagation (Image by Lookang with many thanks to Fu-Kwun Hwang and author of Easy Java Simulation Francisco Esquembre, via Wikimedia Commons)

However you imagine it, the frequency change due to the Doppler effect makes a very convenient way to measure velocity, so it has many applications. Talking about moving cars, well it’s the Doppler effect that the police radar uses to tell whether you’re speeding (see the NSW Police Radar Manual [PDF 4.3 MB]).

It’s also famously what we use to measure the expansion of the universe. When a light source like a star or a galaxy is moving away from us, the electromagnetic waves it emits go to the low frequency or red end of the spectrum, so we say it’s red-shifted. If it’s coming towards us, it’s blue-shifted. By measuring the redshift of galaxies depending on how far away they are from us, we can calculate how fast the universe is expanding (due to the expansion of the universe, the further something is, the faster it is moving away).

But if understanding the history of the universe isn’t enough, the Doppler effect still makes the news; specifically, in the hunt for missing Malaysian Airlines flight MH370.

Using what the BBC called “cutting-edge methods”, the British satellite firm Inmarsat received radio pings from the missing plane, and by comparing how the frequency of the signal differed from what it’s supposed to be when it’s transmitted, they could work out how the plane was moving. That’s how they determined it flew to  the Southern Indian Ocean, where the search is currently focussed.

Diagram showing how by triangulating the pings from the MH 370 with a calculation of its speed as determined by the Doppler effect, it was possible to calculate the aircraft's path

The other bit of recent Doppler effect news was the discovery of an ocean under the icy surface of Enceladus, a moon of Saturn. Again, the scientists used changes in the frequency of radio signals, this time from the spacecraft Cassini, which was flying past it (Iess L, Stevenson DJ, Parisi M, Hemingway D, Jacobson RA, Lunine JI, Nimmo F, Armstrong JW, Asmar SW, Ducci M & Tortora P 2014, “The gravity field and interior structure of Enceladus”, Science, vol. 344, no. 6179, pp. 78–80, DOI: 10.1126/science.1250551).

By looking at how Cassini’s speed changed as it flew past Enceladus, they could determine the forces of gravity acting on it, which in turn allowed them to calculate the distribution of mass inside the moon. These were changes in speed of mere millimetres per second, but allowed them to figure out there was liquid water—which is denser than ice—and a relatively light rocky core.

Cross-section image of Saturn's moon Enceladus, showing its rocky core and liquid ocean at the southern pole, emitting geysers through the icy crust
Diagram of the theorised interior of Saturn’s moon Enceladus, based on measurements by NASA’s Cassini spacecraft and NASA’s Deep Space Network. The gravity measurements suggest an ice outer shell and a low density, rocky core with a liquid water ocean sandwiched in between. This is also responsible for the plumes of water vapour shown at the moon’s South Pole (Image by NASA/JPL-Caltech)

So it may be commonplace, everyday science, but it’s good to see the Doppler effect is still making waves after all these years.

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, and comparing the AMS results with previous experiments like PAMELA and FERMI (click to embiggen)
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.

WIMPs may occupy the universe (click to read more)
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.

(This story aired on 18 April 2013 – you can listen to the podcast.)

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.

Animated clip of a piece of toast falling from a plate on the edge of a table, turning in the air and landing butter-side down
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, showing the angular rotation forces of gravity, friction, reaction of the table
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.

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 of entropy between minimum and maximum energies
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).

Electromagnetic shielding foiled again

The ineffectiveness of tinfoil hats against government mind control rays received a bit of media and internet attention this year, despite the fact the study in question is 7 years old and that mind control doesn’t appear to exist.

In 2005, four electrical engineers at MIT tested the shielding of three different helmets made of ‘tinfoil’ – actually aluminium foil – over radio frequencies from 10 kHz to 3 GHz (Rahimi A, Recht B, Taylor J & Vawter N 2005, “On the effectiveness of aluminium foil helmets: an empirical study”, published online 17 Feb 2005).

The principle is based on the Faraday cage, invented by English physicist Michael Faraday in 1836. This is a box made of conducting material: when an electric field is applied to the outside, the charges in the conductor realign and cancel out the electric field inside. For a more detailed explanation, see The Feynman lectures on physics vol. 2, 1964, section 5.10.

A homemade Faraday cage, made of a box covered in aluminium foil. In an external electric field, represented by arrows running from positive to negative, the charges in the conductor move accordingly. Negative charges are attracted towards the external positive charges, and positive to the external negative.
My homemade Faraday cage. When an external electric field is applied (traditionally represented by arrows running from positive to negative), the charges in the conductor move accordingly. Negative charges are attracted towards the external positive charges, and positive to the external negative. This redistribution of charges sets up its own electric field, which is equal and opposite to that from outside. The two fields cancel each other out, so inside the cage there is no field (Photo own work)

Faraday cages work really well at low frequencies, and they’re the reason you’re not affected if you’re in an airplane that’s struck by lightning: the metal fuselage shields you from the high voltages outside.

At higher frequencies, you need to make sure you’re using a good conductor (refer to this table of electrical conductivity, by TIBTECH).

You also need to make sure that any holes in the enclosure are small enough so that the electromagnetic waves don’t fit through. The basic rule is that the holes need to be less than about half a wavelength (with the wavelength equal to the speed of light divided by the frequency).

A microwave oven is a good example. Microwaves have a frequency of about 2.45 GHz, i.e. a wavelength of 12 cm. So the holes in the metal mesh in the oven door are too small for the microwaves to fit through, but big enough for you to see in (light is also electromagnetic radiation, but with wavelength between 390 and 750 nm). For more technical details, see ‘Practical electromagnetic shielding’, by Learn EMC.

Incidentally, the microwave oven’s shielding is meant to block radiation from the inside, not the outside as in a basic Faraday cage. For this purpose the principle is pretty much the same, except that the enclosure needs to be grounded. Without the ability to bring in more charge from the ground, the conductor is basically transparent to any charges inside (thanks to Gauss’s law).

It is fairly easy to build your own Faraday cage using aluminium foil and test it by placing a mobile phone inside. We did this on air, risking violation of the rule of not using a mobile phone in the radio studio. Fortunately, the shielding worked and the phone didn’t ring when we called it.

Aluminium works well because it’s a fairly good conductor, not far behind gold (which itself ranks below silver and copper). But you really need to make sure you crimp all the seams, as these are the main weak points. Even though they’re not very wide, they can easily be long enough to let in the electromagnetic waves (in Australia, 3G mobile networks use 0.85-2.1 GHz, with 4G or LTE on 1.8 GHz, or a wavelength of 17 cm).

This is the big problem with the foil hats. Because they have to fit a head in, they’re never totally enclosed. So right away, basic electromagnetic theory tells us they won’t work as Faraday cages.

However, although they won’t totally block all radiation, we should at least see some attenuation of the signal. Which is what the MIT researchers looked for, and indeed what they found.

There was a 10 decibel (dB) reduction in signal strength at most frequencies, with the greatest attenuation being 20 dB at 1.5 GHz.

But the biggest surprise was amplification of the signal at 2.6 GHz and 1.2 GHz! Those frequencies saw increases of 30 dB and 20 dB respectively.

Now, I don’t have specific data on the dimensions of the hats involved, but considering those frequencies correspond to wavelengths of 11 cm and 25 cm, I suspect they’ve hit on resonant frequencies of the cavities they’ve created. Effectively, the radiation is bouncing around inside, echoing and building up in strength.

The engineers go on to point out that frequencies in the range of 1.2-1.4 GHz are reserved by the Federal Communications Commission, nominally for GPS and other satellite use. And in the US, 2.6 GHz is used by mobile phone companies, i.e. multi-national corporations.

This leads to the ‘conclusion’ that perhaps the very idea of foil hats was seeded by the government and their corporate cohorts as a bluff to get people to wear them and so amplify the mind control rays. Which is really doubling-down on the conspiracy theory.

So if you’re paranoid, this may be enough to amplify your paranoia.

For the rest of us, what it does show is that although electromagnetic shielding is quite possible, it requires a bit more care and crimping than you might have thought.

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

Massive, supermassive and superdupermassive black holes

There’s a monster lurking in the middle of our galaxy. You might not be able to see it, but we know it’s there. Its diameter is 10 times that of the Sun, but its mass is 4 million times. It’s what we call a supermassive black hole.

OK, it’s 27,000 light years away, so it’s probably not going to get you, but still: a supermassive black hole. Let that sink in, so to speak.

‘Normal’ black holes sound pretty massive themselves. If a star is bigger than about 3 times the mass of the Sun, then eventually it reaches a point where it can no longer hold up under its own weight, and it collapses into an object with gravity so strong that even light cannot escape. These are called stellar black holes.

The biggest stellar black hole so far confirmed is about 16 solar masses, but there are indications they can get up to around 33 solar masses.

However, the black holes believed to be at the centres of most galaxies are much, much bigger: more than 100,000 times the mass of the Sun. Hence the label supermassive black holes.

The location of our galaxy's supermassive black hole, hidden behind opaque dusk in the Milky Way, in the constellation Sagittarius. An inset shows a photo of it taken in the X-ray spectrum (click to embiggen)
Our galaxy’s supermassive black hole is hidden behind opaque dusk in the Milky Way, in the constellation Sagittarius. It can’t be seen in visible light, but it can be seen in the radio or X-ray spectrum, as seen here in the inset photo taken by NASA’s Chandra X-ray Observatory. Click to see a bigger image (Photo sources Moondigger and NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al., via Wikimedia Commons)

So if there’s something that big in our galaxy, then why can’t we see it? Well, between it and us there’s an awful lot of stuff.

You’ve probably seen the Milky Way in the sky, a cloudy band visible at night when you’re well away from the city. That’s the main plane of our galaxy. If you could stand outside and away from it, you’d see that it’s a spiral galaxy, i.e. a sort of disc shape made of four swirling arms, with a pronounced bulge in the centre.

From the inside, you just see a cloudy band stretching across the sky, with a lot of opaque dust and gas blocking out the good bits like the dense middle. But it’s there alright, in or near the constellation Sagittarius (see the picture above).

Even though we can’t see it directly – at least not with visible light – we can detect it with radio waves. And in the radio spectrum we see a very, very powerful radio source called Sagittarius A*. The radio waves are believed to be electromagnetic radiation given off from the accretion disk of the black hole: that’s where things spin around it really, really fast before they fall in. And when charged particles spin around fast like that they give off electromagnetic radiation (which actually means they lose energy and so fall in even faster. Not a good idea perhaps, but you can’t fight physics).

So we can see the radio waves, but how do we know Sagittarius A* is a black hole? Well, we can also detect 28 other stars orbiting it. One of them, called simply S2, orbits every 15.2 years and gets as close as 122 times the distance from the Earth to the Sun.

From the speed and distance of S2, we can calculate that the object in question has a mass of about 4.1 million times the mass of the Sun. That much mass in that small a volume has to be a black hole.

Its dimensions are given by something called the Schwarzschild Radius, which tells us that the black hole’s event horizon – the point at which light is no longer able to escape – is at about 13.3 million kilometres. That’s only about 10 times the diameter of the Sun, or 9% of the distance from the Sun to the Earth.

And yet it has a mass 4 million times that of the Sun. For comparison, the Sun is 333,000 times the mass of the Earth. The difference between the black hole and the Earth is the same as that between you and a grain of pollen.

Even so, there are bigger black holes out there. Much, much bigger (you can see where this is going).

Recently, one with a mass of 17 billion suns was discovered in a galaxy only 250 million light years away (van den Bosch RCE, Gebhardt K, Gültekin K, van de Ven G, van der Wel A, Walsh JL 2012, “An over-massive black hole in the compact lenticular galaxy NGC1277”, Nature, vol. 491, no. 7426, pp. 729-731, doi:10.1038/nature11592, arXiv:1211.6429v1 [astro-ph.CO]).

I call it a superdupermassive black hole, although the authors called it ‘over-massive’.

This term is actually appropriate, because it’s much larger compared to its host galaxy than previously discovered black holes. Although small in comparison, our galaxy is in more typical proportion, with the central black hole being 0.1% the mass of all other stars. But the black hole in NGC1277 is 14% of its galaxy’s stellar mass.

The animation embedded below shows how the black hole was identified, using measurements of stars in the galaxy to calculate their orbits and hence the mass at their centre. The photo in the background was taken by the Hubble Space Telescope (NASA/ESA/Fabian/Remco C. E. van den Bosch MPIA).

But even though it’s so big, this superdupermassive black hole isn’t a unique freak. The researchers have also found five other galaxies with similar extreme proportions. Instead, it suggests we may need to rethink our theories of how galaxies form. After all, we’ve been using our own galaxy as a typical example, but there seems to be a much bigger and more complex variety.

What we can say for certain is that it shows what huge objects are out there in the universe. Much too huge for our puny human adjectives.

I spoke to Professor Rachel Webster from the University of Melbourne about this discovery, on our show that aired on 13 December 2013. You can listen to the podcast.

A transcript follows after the break…

Continue reading Massive, supermassive and superdupermassive black holes