Magnetic Musings

I love astronomers.

Here’s one reason why.  Currently the solar system is passing through an interstellar cloud.  It is called the Local Interstellar Cloud, which is amusing enough considering its scale — but do you know what astronomers have nicknamed it?  The Local Fluff.

All right, so I’m easily amused.  But here’s the interesting thing about it.  According to this report from NASA, this cloud shouldn’t exist. 

The reason being that where it is right now there used to be supernovas.  Those supernovas did what stars of their type eventually do: ten million years ago they exploded, leaving behind a giant bubble of million-degree gas that should have ripped through the Local Fluff and destroyed/dispersed it. 

And yet the Local Fluff lives long enough to have been named a cutesy name by cheeky astronomers.

According to the NASA report the reason, discovered by the still-voyaging Voyager spacecraft, is that the Local Fluff is magnetized.  This magnetic field of about 4 to 5 microgauss provides sufficient pressure against the supernova exhaust that the Local Fluff can maintain its integrity.  The NASA article shows a picture of the sun’s magnetic field pushing against the cloud as well.  Very dramatic. 

On a much smaller scale the Earth’s magnetic field protects us from the constant rain of cosmic charged particles that would otherwise have prevented or at least drastically changed life as we know it.  (Cosmic radiation is not fun for living tissue).  I can imagine it: the constant rain of the solar wind, all those little electrons and protons rushing toward us, swept magnificently away to the poles by our giant, invisible shield.  Who says science is not magical?

I’m remembering a certain seminal moment in scientific history, when the Dutch scientist Oersted was doing an experiment to demonstrate the heating of wires by an electric current.  He had his circuit set up; a magnet — a compass — happened to be nearby.  He noticed that every time the current was switched on and off, the compass needle moved. 

What this odd observation led to, eventually, was the realization that electricity and magnetism are aspects of one force: electromagnetism.  So moving charged particles give rise to magnetic fields, and changing magnetic fields give rise to electric currents (a fact that power companies employ to generate electricity).  This was the first great unification in physics.

We now have four fundamental forces of nature: electromagnetism, gravity, the weak nuclear and the strong nuclear force.  There is something quite mysterious about these forces — the fact that they act invisibly, over a distance.  19th century physicists had a lot of problems with “action-at-a-distance” forces. Hold a ball in your hand.  Let go, and it falls.  But how does the ball “know” the earth is there?  How does the earth sense the presence of the ball?  Unlike contact forces like pushes and pulls, these “action-at-a-distance” forces are inherently mysterious.

Michael Faraday, that dreamy son of a blacksmith who is sometimes described as Sir Humphrey Davy’s greatest discovery, came up with the idea of a force field, an invisible aura or region of influence that surrounded an object (electric field if it was a charged object, magnetic field if it was magnetic, gravitational field by virtue of its mass, etc.).  Thus we are immersed in Earth’s gravitational field, which is how the ball and the Earth sense each other’s presence. 

But on to the subject of unification.  The Standard Model of particle physics, while flawed (some call it the sub-Standard Model), does manage to unify electromagnetism and the weak force.  The strong force has problems that make it difficult to calculate with, but it can be described in a language not dissimilar to that of electromagnetism and the weak force.  It is gravity that makes attempts at super-grand unification so difficult. 

When I was an undergraduate at Delhi University, my four friends and I would walk across the green lawns to the chai shop and talk about how (maybe) one day we would unify gravity and the other forces.  Once Abdus Salaam, the Pakistani physicist who was one of the architects of Electroweak unification, came to speak on campus, and we actually got to exchange a few words with him.  In those days it seemed only a matter of time before gravity would succumb, reveal itself as just one aspect of a superunified force.  Drunk with possibility on cheap chai and Tibetan noodle soup, we dreamed of doing the great synthesis ourselves.  Of course, we grew up.  I ended up working in the realm of strong forces, and then left academia for a decade, and particle physics research forever, for reasons I could not have imagined in those heady days.  But now — especially after the broken promises of string theory and the like, I wonder if things are that simple —- that perhaps you can’t simply add the forces up like a child making a structure out of lego blocks.  The physicist George Sudarshan, who came up with the theory that correctly describes the weak force, told me in a conversation many years ago that perhaps there was no one superforce, that maybe grand unification was a mirage.  Perhaps the laws of Nature are more like a tapestry than a simple hierarchy. 

There is a sense in which the things that are familiar to us are at the same time full of magic.  We all feel the tug of gravity.  Most of us have played with magnets.  But gravity remains aloofly distant from other forces, and as for magnets…

I’m thinking of magnetotactic bacteria.  These critters contain magnetic crystals that enable them to sense the Earth’s magnetic field.  Migratory birds have this ability too.  Magnetotactic bacteria were discovered by chance twice, once by Italian scientist Salvatore Bellini in the 1960’s, and then again by Richard Blakemore at Woods Hole Oceanographic Institute in 1975. 

Imagine being a bacterium.  You are very light, so much so that gravity is not strongly felt.  You are generally in some kind of watery medium, and for you the drag force due to the water is so significant that you never experience inertia.  That is, if you suddenly stop moving your cilia, you stop dead in the water — you never coast.  If there is an Isaac Newton among you, he would never have come up with the First Law of Motion.

For magnetotactic bacteria, too much oxygen is poison.  Immersed in their watery environment, they have to be able to distinguish up and down.  Down is safe — less sunlight, less oxygen.  But how to figure out where down is?

Earth’s magnetic field lines, of course.  In the Northern and Southern hemispheres the field lines are inclined in such a way that by swimming south-to-north or north-to-south you can actually go up or down.  We speculate that this is why these bacteria have magnetic crystals inside them.  (See, for instance, this great popular article in Strange Horizons).   

But we also find magnetotactic bacteria in the equatorial zone, where the Earth’s magnetic field lines are parallel to the surface.  What purpose can the magnetic crystals embedded in their cells serve?  As far as I know, nobody knows for certain. 

I once met a physicist, Karl Canter, who had gotten fascinated by this problem.  He had observed these bacteria responding in a peculiar way to strong magnetic fields.  He was a gentle, quiet man, who died before he could complete his research.  When we met he told me about a curious magnetotactic organism called an MMP, which is a collection of some 20-odd independent, identical cells that drift about in close proximity.  Each has a magnetic crystal.  If you push apart the cells of the MMP (they are not connected physiologically) beyond a certain distance apart, they die.

He wondered whether the MMP used their mutual magnetic fields to communicate in some way. He had a charmingly romantic view of these organisms, that, he admitted, his more down-to-earth biologist colleagues found naive. 

But it did make me wonder if one can communicate using magnetic fields alone.  We do it all the time with electromagnetic signals, but magnetic fields?  Hmm…

It is not surprising that humans (and other animals possessing nervous systems that run on electricity) generate electromagnetic fields of their own.  And also that strong magnetic fields affect the human brain, from possibly triggering “genius”-like abilities   to curing depression and waking people up from comas.  There is also some speculation that magnetic anomalies could be behind sensations that a place is haunted.   Wild, isn’t it?

For some reason my train of thought (which is, of course, a time-travel device) is going to Frankfurt in 1922, where Otto Stern and Walther Gerlach conducted an experiment that helped usher in the quantum age.  Using a beam of silver atoms shot through an inhomogeneous magnetic field, they revealed that the classical prediction of what would happen was wrong.  The way this was done shows to what extent sheer luck and the concatenation of circumstances leads to results in science. 

The beam of silver atoms emerging from the magnet hit a glass plate.  The thin veil of silver on the glass was difficult to see.  According to an account I read in physicist-philosopher Karen Barad’s amazing book Meeting the Universe Halfway, this is what happens next.

 Stern, who is smoking a cheap cigar, holds the plate closer to him.  His salary is so low that by his own account he cannot afford a good cigar.  Cheap cigars have a lot of sulphur.  Sulphur and silver form silver sulphide, which is black.  Voila!  Black marks appear on the glass plate and Stern and Gerlach have a result. A result that came to be not only because the universe is quantum-mechanical but also because Stern just happened to be poor and addicted to cigars.  And because of the way silver atoms couple with sulphur, and the optical properties of the resulting compound. 

Whew!

It took several years to interpret Stern and Gerlach’s results correctly, but what they revealed was that the electron has spin.  What that means is a whole other musing. 

Magnets.  Magnets inside bacteria.  Magnets as probes, revealing the inner secrets of the electron.  Magnets in electrical generators that give us electricity and carbon dioxide pollution.  Children playing with magnets, magnetic field of the Earth deflecting away the solar wind, the sun’s own magnetic field pushing up against the Local Fluff, and the magnetic pressure of the Local Fluff keeping it intact against the violent, hot remains of supernova explosions. 

Enough for a hundred science fiction stories, or a thousand random imaginings on a snowy Friday  morning.

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