In Our Time: The Earth’s Core

Despite being relatively close to us the inside of the Earth, and particularly the core of the Earth, is difficult to investigate. Primarily because we can’t just look at it – and the deepest mines or boreholes are only 10km deep which is tiny compared to the 6,000km that is the Earth’s radius. So everything needs to be logically deduced from the readings that we can take. Discussing what we know about the Earth’s core and how we know it on In Our Time were Stephen Blundell (University of Oxford), Arwen Deuss (Utrecht University) and Simon Redfern (University of Cambridge).

Prior to the 19th Century the assumption was that the Earth was the same all the way through – rocks where we can see, so rocks everywhere. But in the 19th Century scientists realised that the theory of gravity required a denser Earth than is possible if it’s just rocks and so they postulated an iron core. This was also the time when scientists began to be interested in how the Earth was formed. The consensus at the time was that it formed by condensation out of a hot cloud, and it was still cooling. This explained (at a time when radioactivity wasn’t known) why it got warmer the further you went underground. So at the time the best explanation for the structure of the Earth was that it had a hot liquid iron rich core surrounded by a rocky shell.

However even in the 19th Century it was clear that there were problems with this explanation. If you spin an uncooked egg, it wobbles – so why doesn’t the earth? During the 20th Century it began to be postulated that the core was two phase – a solid core with a liquid coating. One of the experts on the programme, Arwen Deuss, used seismological readings to show that this was the case. When there is an earthquake seismographs on the other side of the Earth detect the shockwaves that have travelled through the planet. Before Deuss’s work it was thought that there was a shadowzone where no waves were detected because they had failed to pass directly through the centre of the Earth – so it was thought that the core was a different phase to the rest of the planet and the waves couldn’t travel through it. Deuss showed that there are very faint delayed waves detectable in that shadow zone, and that mathematically the best model to describe how these waves are delayed and how they are diminished is one where the core is solid but it is surrounded by liquid. The seismic waves cannot travel through liquid in the same state as they travel through solid, and each transition between states uses up some of the energy in the wave. A wave that travels directly through the core will transition from solid to liquid to solid to liquid and lastly to solid again. As well as each transition using up energy it takes time (hence the delay) and changes direction (so the waves aren’t in quite the same places you’d expect if they had no transitions).

The current theory is that the inner core is an iron crystal that is forming out of a less pure molten iron fluid around it. This iron crystal is about the size of the Moon, a fact which I find mind-boggling. The crystal is still growing and this is not a consistent process, sometimes it grows more quickly and sometimes more slowly. The experts said there is evidence of some sort of discontinuity that formed 500 million years ago, but no-one knows what caused it. The crystal is also split into two pieces. One of the experts made an analogy with the land/sea divide up here on the crust, but I didn’t really follow that. The crystal is also different in the north/south direction as compared to the east/west direction – seismic waves take longer to travel east & west than they do north & south. It’s not known why this is: perhaps to do with crystal alignment, or perhaps it tells us something about the shape of the core.

This solid iron crystal is rotating within the liquid it sits in, I think at a slightly different (quicker?) speed than the whole of the Earth rotates. It’s this rotation that is the cause of our magnetic field (which is another piece of evidence in favour of the two phase theory). And the magnetic field is what protects us from cosmic radiation so in some sense you can say that the two-phase spinning core of the Earth is why there is life on Earth. The current theory is that Mars and Venus have cores that are too solid or too small to generate enough of a magnetic field to protect against radiation. That’s an untested hypothesis, and so Deuss would like to put seismographs on one (or both) of the other planets to see what she can detect about their internal structure.

Bragg closed up the programme by attempting to encourage them to talk about practical uses that have come out of this blue-skies research – but it seems at the moment this is still in the blue-skies phase.

In Our Time: Absolute Zero

Absolute zero, or 0°K is the minimum possible temperature, and there has been a race of sorts over the last couple of centuries to reach that temperature in the laboratory. The experts discussing it on In Our Time were Simon Schaffer (University of Cambridge), Stephen Blundell (University of Oxford) and Nicola Wilkin (University of Birmingham).

The programme started with the Greeks (as a sort of in-joke I think) and mentioned their idea of cold as being radiated in the same way heat is. And then we fast-forwarded through a couple of millennia to the 17th Century when Boyle (amongst others) was speculating about the existence of a supremely cold body which was in effect the essence of cold. And in the 18th Century a French man (Guillaume Amontons) was measuring temperature by means of an air thermometer. He saw this as the effects of heat on the “springiness” of air which increased as the temperature went up. So he postulated that at some low temperature there would be no springiness left in the air and so this must be the lowest possible temperature. In the 19th Century this was taken further, by Kelvin, who used thermodynamics to calculate the lowest possible temperature and set this as the zero point on his temperature scale which is still used by physicists today. 0°K is -273°C, and Bragg unfortunately kept misspeaking through the programme and saying “-273°K” when he meant absolute zero.

By the end of the 19th Century (i.e. before quantum mechanics was thought of) there was a theoretical consensus that temperature could be measured by the energy of atoms of the substance. As the temperature increases the atoms move around more, as it decreases the atoms move around less. Absolute zero is the point where the atoms and their electrons etc. have stopped moving, everything is fixed in place.

And so practical physicists started to try and reach this temperature. The first experiments were done by Faraday, who used pressure to liquify chlorine. The principle behind this is the same was why tea made up a mountain doesn’t really work – as the pressure lowers (because you’re up high) then the water for the tea boils at a lower temperature and so boiling water is no longer hot enough to make tea properly. So in these experiments Faraday increased the pressure that the chlorine was under until the boiling point of it was above room temperature, so the chlorine liquefied. They didn’t spell out the next bit, so I’m guessing here – but I think it’s that once you return the pressure to “normal” then you end up with very cold chlorine liquid (-30°C). He liquefied several gases, but regarded the noble gases as being “impossible” to liquefy, this became the next goal for physicists interested in absolute zero.

At this point in England (which was at the forefront of such research) there were two main players, James Dewar and William Ramsay. Both Scots working in London, and they loathed each other. Which was a shame, as that meant they didn’t work together instead trying put the other one down or prevent him from getting hold of reagents for experiments. Both were interested in liquefying the gases thought to be impossible – as a side-effect of building his research equipment Dewar invented the thermos flask, and Ramsay discovered (and liquefied some of) the noble gases. Ramsay had control of the country’s supply of Helium, which was one of the newly discovered gases (first seen in the sun before being discovered on Earth, hence the name), and prevented Dewar from getting enough to be able to try liquefying it. So instead this was left to a German scientist called Heike Kamerlingh Onnes to achieve. Helium liquefies at about 4°K so we’re down to pretty close to absolute zero here.

Onnes also started to investigate the properties of materials at these low temperatures. In particular he looked at electrical resistance in mercury as you lowered the temperature – one major theory had been that as you reduced the temperature then the electrons would slow down, so resistance would increase. However Onnes found the exact opposite – mercury at the temperature of liquid helium had no measurable resistance at all. He set up an experiment with a loop of mercury at this low temperature and introduced a current to it, after a year the current was still flowing just the same as it had been to start with.

This superconductivity was the first quantum mechanical property to be seen at a macro scale – normally you don’t see quantum mechanical effects at this scale because the jittering around of the atoms disguises and disrupts it. Superfluids are the other property seen at these low temperatures – this is where a liquid has no viscosity.

One other effect of quantum mechanics on the story of absolute zero is that it has changed the understanding of what it actually is – the 19th Century understanding was that everything had stopped moving, there was no energy. However this cannot be the case because the Heisenberg Uncertainty Principle states that you can either know where a particle is or how it’s moving but not both. And if everything has stopped moving then you’d know both, so this can’t be the case. So now there is a concept of “zero point energy” for the energy that remains at absolute zero.

Modern physicists are still trying to reach as close to absolute zero as possible, but it is now thought to be a limit in the mathematical sense – they can tend towards it but not reach it. Part of the reason for this is Zeno’s Paradox – any cooling method cools a body by a fraction, say half. So you can halve the temperature, and halve it again, and halve it again and so on, but if you do that then you never reach your goal. But they’ve got within a billionth of a degree.

This has all been very blue skies – science for the sake of curiosity alone. But along the way there have been inventions and engineering solutions that have had significant practical applications. I mentioned the thermos flask above, but the much more significant invention is the fridge which relies on principles and apparatus designed in this quest for absolute zero and now underpins modern civilisation (think of a world where we couldn’t freeze or refrigerate our food).

And right at the end Bragg flung in a “rabbit out of the hat” question, as he called it. A group of German scientists have managed to get a substance to below absolute zero. Wilkins answered this with one of those physics explanations that makes it all seem like black magic to me – whilst it has a negative temperature in one sense it will be hotter than absolute zero in another sense. And even tho that temperature was reached it won’t’ve been reached by going via absolute zero. She didn’t have a chance to expand before they were out of time, but I rather suspect it would require both high level mathematics & a strong grasp of quantum mechanics to understand!