The Big Questions: Physics (16 page)
Read The Big Questions: Physics Online
Authors: Michael Brooks
Modern measurements of the Earth’s magnetic field only began two centuries ago, but we do have older evidence of shifting fields. Examine the orientation of over a hundred Danish churches built during the 12th century, for example, and you’ll see that they sit around 10 degrees off from today’s magnetic east–west line. As with the Olmec buildings, it is highly likely that when these churches were built, compasses pointed in a different direction than today.
A more reliable account of Earth’s magnetic field began at the start of the 19th century, when Alexander von Humboldt took field measurements while travelling in the South Atlantic. He found that the intensity of the field decreased in this region. In 1804, von Humboldt reported his finding to the Paris Institute, but counter-claims soon emerged, throwing the issue into confusion. Von Humboldt eventually took the matter to the German mathematician Carl Friedrich Gauss, and asked for help in constructing an atlas of magnetic observations. Gauss, a
polymath who had made important discoveries in disparate scientific fields, was already investigating terrestrial magnetism, and was only too keen to help. By 1840 he had written three significant papers on magnetism – including a way to define the Earth’s field – and constructed a mobile magnetic observatory that would exclude all fields but the Earth’s.
Gauss’s geomagnetic atlas was published in 1836. Measurements of Earth’s field have continued since Gauss’s first efforts, and we now have a 150-year record. One of the key findings is that the magnetic North Pole moves. It was first pinpointed by explorers in 1831, then again in 1904. During that interval it had moved 50 kilometres (31 miles). During the 20th century, the pole has moved north at around 10 kilometres (6.2 miles) per year, though that movement appears to be speeding up. It is currently moving at around 40 kilometres (25 miles) per year.
That’s not the only change: records show that at mid-latitudes, compass needles are drifting about 1 degree per decade. There really is a blip in the South Atlantic too: satellite measurements tell us that under the Atlantic Ocean, west of South Africa, field lines appear to converge, forming a magnetic pole. This ‘South Atlantic anomaly’ exhibits its own reversed magnetic field lines, which now cover much of South America, and confuse our overall view of the Earth’s field. Then there’s the issue of the general weakening of the field. Taken as a whole, Earth’s magnetic field has lost 10 per cent of its strength since Gauss’s measurements began. To understand what that means for the future, scientists have tried to uncover the roots of the field.
The fact that the Earth has a North and a South Pole might tempt us to think that its field arises from something like a bar magnet buried deep in the Earth. Unfortunately, things are nowhere near as simple as that. The Earth’s magnetic field arises from a churning sphere of molten iron and nickel deep in the heart of the planet. This, the Earth’s inner core, is a solid ball of iron 1,250 kilometres (775 miles) across. It is fiercely hot, at thousands of degrees, and only the pressure bearing down on it from the weight of the rest of the planet keeps it from melting.
ANIMAL MAGNETISM
There is no doubt that some animals can sense magnetic fields. Many of the animal kingdom’s most ambitious migrations involve navigation by the Earth’s magnetic field. The 8,000-mile trek of the loggerhead turtle, the great American journey of the monarch butterfly, and the continent-crossing osprey all involve sensing the magnetic field. It’s still not clear exactly how they do it, but we are gathering clues. The tissues of many animal species – frogs, bees, yellowfin tuna and bacteria, for example – contain the mineral magnetite, which aligns to an external magnetic field.
Migrating birds such as the bobolink have magnetite in cells in their brains. But birds have also been shown to have ‘magnetic sight’. The visual neurons of migratory garden warblers contain proteins called cryptochromes, which seem to be sensitive to weak magnetic fields. When exposed to fields of different orientation, the proteins produce different combinations of chemicals. The ‘blue’ light of evening seems to be particularly good at stimulating the proteins to produce these chemicals, which corresponds to the time of day when birds are orienting themselves.
It’s not only migratory animals that sense magnetic fields; cows are thought to be magnetically sensitive too. Satellite images of grazing dairy and beef herds, taken over six different continents, seem to show that they stand oriented to within 5 degrees of the north–south line. There are question marks over the data; it could be to do with prevailing winds, for example. Nevertheless, it is an intriguing observation, and the data seems to tie in with the various shifts between geographic and magnetic north. In Oregon, where there is a strong field, cows face 17.5 degrees off geographic north, towards magnetic north. Deer herds have been observed to do the same. So, if so many animals have this sense, what about humans?
There is no evidence we have a conscious sense of magnetic fields, but there are studies that link human health issues with magnetic fields. Research in Russia, Australia and South Africa has found links between periods of geomagnetic activity and increased suicide and depression rates. The root cause remains a mystery, but researchers have suggested that geomagnetic variations might affect melatonin production and circadian rhythms – both of which have been linked to mood disorders.
Surrounding the inner core is the molten metal that creates the magnetic field. Heat from the inner core courses through this liquid, creating convection currents that move hot liquid metal up towards the mantle, the layer beneath the crust. This hot liquid cools as it rises, and so falls back down. The motion of this metallic conductor creates electricity, which is always accompanied by a magnetic field. The combination creates a self-sustaining ‘geodynamo’ that maintains Earth’s field.
This geodynamo creates an extremely complex magnetic field. As the Earth spins on its axis, the field lines become twisted, creating new currents within the liquid outer core. This creates new magnetic field lines, and a new magnetic field can sometimes grow within the core. Typically, this adds to the current field, but if its orientation is shifted relative to the dominant field, it can sometimes detract from the overall magnetization of the Earth.
This may be what is happening with the South Atlantic anomaly, and it may be what is causing the apparent weakening of the Earth’s field. However, researchers can’t be sure, because the dynamics of the field created by such an enormous geodynamo are too complex to yield up their secrets to mathematical models. Frustrated geodynamo researchers are supplementing their mathematical models by creating their own real-world geodynamos. Typically, this involves some highly perilous equipment. If you want molten spinning metal in your lab, you can’t use a metal that melts at thousands of degrees. The best candidate is something like sodium, which melts at temperatures of just under 100 celsius.
That said, sodium has its own dangers. It can burn in a fierce explosion on contact with water or air, for example. Nevertheless researchers have succeeded in spinning balls of molten sodium to simulate what is going on beneath our feet. The results have been impressive: self-sustaining magnetic fields do
indeed form, and they do exhibit the kind of complex behaviour seen in the Earth’s geodynamo. They even exhibit occasional ‘reversals’, when the North and South Poles swap places. During that process, the magnetic field fades and becomes much more complex, then grows again, but with a reversed polarity.
For a period of time, during a reversal there is no clearly defined field. So, could this happen with the Earth’s magnetic field, with potentially disastrous results? Unfortunately, even these simulations have not yet proved accurate enough for us to make forecasts for the Earth’s field. The best we can do, it seems, is to look at the evidence frozen into the planet’s rocky crust, and try to extrapolate our findings.
In the molten rock that pours from volcanoes and the gaps between tectonic plates in mid-ocean ridges, magnetic crystals – tiny grains of magnetite, for example – are free to move, and will orient themselves to the direction of the Earth’s magnetic field. When that rock cools, that orientation is frozen in, creating a rock whose magnetic field points towards its era’s magnetic north. By dating rocks and noting their magnetic orientation, researchers can build up a picture of how the direction of ‘north’ has changed over millennia. This is how we gained the first evidence for a shield failure. In 1904, geomagnetic studies of the Massif Central mountains of southern France revealed that the orientation of the magnetic crystals in the rocks was significantly shifted from what it would be today. In the 1920s, similar observations were made across the world, and the field of palaeomagnetism was born.
We now have evidence that, during the past 20 million years, the Earth’s field has collapsed and reversed more than 60 times. These reversals have occurred every half-million years or so, and can take thousands of years to complete. However, it is by no means a clockwork phenomenon. Sometimes, as happened during the age of the dinosaurs, no flips happen for tens of millions of years. We haven’t seen a reversal for 780,000 years now. So, does that mean we are due one? Is that why the Earth’s field is currently fading at what seems like an alarmingly fast rate?
We know, thanks to the logbooks kept during Captain Cook’s journeys in the South Seas, that the current failure only started relatively recently. We have mariners’ logs that date back to 1590, which record, amongst many other things, the direction of the Earth’s field and the angle at which the field lines enter the Earth. It was a useful navigational trick – in many ways the sailors’ lives depended on it. We have recorded a decline in field strength since Gauss began measuring it in 1840, but the ship’s logs show no change between the 1590 value and Gauss’s field strength.
Of course, it may be that we haven’t enough data to draw any firm conclusions; the ‘South Atlantic anomaly’ may be leading us astray, for example. So should these strange measurements and discoveries give us cause for concern? Given the crucial role Earth’s magnetic field has played – and continues to play – in the development of life on Earth, the answer has to be yes.
The blue-green planet we call home sits roughly 93 million miles from the sun. We are in the ‘Goldilocks Zone’, where life can thrive in certain regions because the climate is not too hot and not too cold. But the sun produces more than heat. The surface of the sun is a turbulent mass of plasma, a gas composed of charged, high-energy particles. The sun is constantly losing these particles, which travel through space as the ‘solar wind’. Our magnetic field directs most of these around Earth. Crucially, only a small proportion of the particles reach the Earth’s surface.
WHEN THE SUN ATTACKS
Our magnetic field really comes into its own when the sun creates what is known as a ‘solar storm’. This often coincides with the appearance of sunspots, which are indicators of hugely intense magnetic fields beneath the sun’s surface. The chaotic motion of particles means these magnetic fields writhe around, twisting and turning, and occasionally creating a whiplash that throws out a massive ball of plasma. If flung towards the Earth, its intense magnetism interacts with our magnetic field, the magnetosphere.
Depending on the relative orientation of the two fields, two things can happen. If the fields have the same alignment, they slip round one another. In the worst-case scenario, when the field of a particularly energetic plasma ball opposes the Earth’s field, things get much more dramatic: the magnetic field of the plasma ball opens up a hole in the Earth’s field, and the particles surge through. The result can be devastating, damaging satellites, and causing havoc to the Earth’s power grids. In March 1989, for example, such a solar storm blacked out a huge swathe of the Canadian province of Quebec, leaving 6 million people without electricity for nine hours.
When particles from the solar wind hit the atmosphere at higher latitudes, some create a cascade of energized particles. That energy is released as the fluorescent, shimmering glow of the
aurora borealis
, the Northern Lights. Some of the particles, though, reach the surface of Earth as radiation. This radiation from the solar wind has been, in some ways, a positive force. It may, for instance, be responsible for directing some of life’s evolution on Earth. This radiation can damage DNA, which imposes mutations in the genetics of terrestrial life, facilitating the process of evolution.