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Hunting the Mysterious Monopole
by Eugenie Samuel Reich
New Scientist / May 6, 2009
They seem magical. Magnets -- every child's favorite science toy. Two otherwise ordinary lumps of
metal draw inexorably closer, finally locking together with a satisfying snap.
Yet turn one of them round and they show an entirely different, repulsive face. Try as you might to
make them, never the twain shall meet.
If magnets seem rather bipolar, that's because they are. Every magnet has 2 poles -- a North and a
South. Like poles repel, unlike poles attract. No magnet breaks the 2-pole rule. Not the humblest bar
magnet; not the huge dynamo at the heart of our planet. Split a magnet in two and each half sprouts the
pole it lost. It seems that poles without their twins -- i.e., magnetic "monopoles" -- simply do not exist.
That hasn't stopped physicists hunting. For decades, they have ransacked everything from moon
rock and cosmic rays to ocean-floor sludge to find them. There is a simple reason for this quixotic
quest. Our best explanations of how the Universe hangs together demand that magnetic monopoles
exist. If they are not plain to see, they must be hiding.
Theory predicts that monopole particles should exist. And
now we may have found them (Image: Steve
Bronstein/Riser/Getty)
Now at last, we have might have spied them out. The first convincing evidence for their existence
has popped up in an unexpected quarter. They are not exactly the monopoles of Physics lore. But they
could provide us with essential clues as to how those legendary beasts behave.
So what attracts physicists to monopoles? Several things.
First, there's symmetry. A purely aesthetic consideration, true. But one that for many physicists
reveals a theory's true worth. For over a century, we have known that Magnetism and Electricity are 2
faces of one force: ElectroMagnetism. Electric fields beget magnetic fields and vice versa.
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Accordingly, the Classical picture of ElectroMagnetism formulated in the late 19th century is pretty
much symmetrical in its treatment of Electricity and Magnetism. But although positive and negative
electric charges can separate and move freely in electric fields, magnetic "charge" remains bound up in
pairs of North and South poles that cancel each other out. "No monopoles" is another way of saying that
there is no such thing as a freely moving magnetic charge.
In 1931, this puzzling asymmetry caught the attention of the pioneering quantum physicist Paul
Dirac. He pointed out that Quantum Theory did not deny the possibility of monopoles. On the contrary,
they could be quite useful. His calculations showed that monopoles existing anywhere in the Universe
would explain why electric charge always comes in the same bite-size chunks (or quanta).
Even so, monopoles were little more than a curiosity. And the lack of any obvious examples nearby
dampened the enthusiasm for the chase. That all changed in the 1960s with the wide acceptance of the
'Big Bang' theory -- i.e., the idea that the Universe began in a fireball governed by a single force that has
since splintered into the fundamental forces we see today. The great ambition of Physics became to
construct a theory that would reunite these forces.
There are many different approaches to this goal and almost all have an odd feature in common.
They say that chunks of magnetic charge must have been created in the very first fraction of a
nanosecond of the Universe's existence. Some theories (like Dirac's original idea) suggest these
monopoles are very massive with a mass around 1016 times that of a proton. Other approaches suggest
more modest beasts with a mass only a few thousand times the mass of the proton. But all predict they
should be there.
Shady characters
Suddenly monopoles assumed a new significance. Not only would the detection of magnetic
monopoles be a major boost for "grand unified" theories of how the Universe began, but finding the
mass of a monopole would also help distinguish which of those theories were on the right track.
"The search has a low chance of paying off but a very high importance if it did," says Steven
Weinberg of the University of Texas at Austin who won the Nobel prize for physics in 1979 for his
work on force unification.
Sheldon Glashow of Harvard University (who also took a share of the 1979 prize) took the
monopole idea a stage further. That same year, he suggested that beefy, Dirac-type monopoles might
also be the answer to one of Cosmology's most important unsolved problems. They might be the
identity of the unseen dark matter that is thought to make up most of the Universe and to have formed
the structures that led to galaxies.
Physicists thus had a wealth of reasons to believe that these "cosmic" monopoles must exist
somewhere. But where Besides the odd tantalizing glimpse, no experiment has yet produced
convincing evidence of their existence (see "Race for the Pole").
There are reasons to believe they never shall. According to the Inflationary Theory of the Universe's
origin (which has gained wide currency since the 1980s), the cosmos expanded enormously fast just
after the 'Big Bang'. This expansion should have carried most -- if not all -- of the monopoles created in
the first instants of the Universe to a patch of the cosmos so distant that they (and information about
them) will probably never reach us. Game over?
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Perhaps not if the latest research is anything to go by. Monopoles might have been under our noses
for a while in a strange type of solid known as spin ice. When this material was reported in 1997 by
physicists Mark Harris of the University of Oxford, Steve Bramwell of University College London, and
their colleagues (Physical Review Letters, vol 79, p 2554), monopole searches were not high on the
agenda. The researchers were looking at something else entirely -- an odd property of certain solids
known as magnetic frustration.
Magnetism arises from spin -- a fundamental property of atoms and ions. Spin can be understood by
thinking of atoms and ions as tiny rotating magnets. The axes about which the atomic magnets rotate
generally point in random directions. But in magnetic materials, they all point in the same direction
(i.e., the North-South axis of the magnet), creating a magnetic field aligned with them.
Frustrated progress
In certain non-magnetic materials, spins would dearly love to line up but can't. That's the situation in
Holmium Titanate -- the compound that Harris and Bramwell were studying. Holmium ions align their
spins more than twice as readily as even Iron does. But in Holmium Titanate, the Titanium and Oxygen
atoms form a tight tetrahedral lattice with Holmium ions at the corners (see diagram).
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Thus corralled, the ions cannot align their spins all in one direction, so plump for the next best thing:
Two spins pointing inwards to the centre of the tetrahedron and two pointing out. "It's an unhappy
arrangement. The spins don't know where to go," says Oleg Tchernyshyov of Johns Hopkins University
in Baltimore, Maryland who studies similar instances of magnetic frustration.
The spin arrangement in Holmium Titanate mirrors the way that hydrogen ions are arranged in water
ice, so Harris and Bramwell coined the term "spin ice" to describe their compound. It and similar
frustrated materials soon revealed intriguing properties.
For example, when a magnetic field is applied to a normal magnetic material like Iron, the material's
magnetization increases smoothly as the atomic spins come into alignment with the field. But in 2004,
researchers in Japan found that in spin ice this relationship was more complex. Instead of increasing
smoothly, magnetization leaps up suddenly, suggesting that the external field was causing dramatic
changes in the spin configuration inside. The effect was reminiscent of the shifts in the arrangement of
atoms when a liquid turns into a gas. But no one could explain how something similar could be
happening with spins. And in a solid.
In 2007, Claudio Castelnovo of the University of Oxford and his colleagues Roderich Moessner of
the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany and Shivaji Sondhi
of Princeton University came to the rescue (Nature, vol 451, p 42). They looked at what happens when
as the temperature of spin ice rises, the additional heat energy causes an occasional ion to flip its spin.
It turns out that individual tetrahedra then develop a slight imbalance in their spins and act as a tiny,
localized North or South pole. The ion with the flipped spin is shared by the neighboring tetrahedron in
the lattice, which then has the opposite polarity. As the 2 poles are still attached, their charges cancel
out.
The clever part of the idea, though, was the realization that for not much more additional heat, the
spins in a chain of adjacent tetrahedra could be progressively flipped. Each link in the chain then
respects the two-in, two-out rule with only the ends of the chain violating it.
As the spin-flipping propagates through the spin ice, the North and South poles move apart from
each other. "If you could live inside a spin-ice sample, you could leave a South pole at home and walk
around with a North pole in your pocket," says Castelnovo. What the team had dreamed up was a
magnetic charge freely roaming about the spin-ice lattice -- i.e., a magnetic monopole.
Depending on the temperature, the monopoles behaved differently. Sometimes randomly distributed
and freely moving like the atoms in a gas. And at other times interacting like the atoms in a liquid. The
transition between the 2 states neatly explained why frustrated materials magnetize in fits and starts as
the Japanese research had found.
"Suddenly there was a community of physicists who became monopole hunters," says Peter
Holdsworth of the École Normale Supérieure in Lyon, France -- one of the people bitten by the bug.
Together with his colleague Ludovic Jaubert, he has produced independent confirmation of the
monopole idea. In a paper published last month (Nature Physics, vol 5, p 258), the pair revisit an
experiment reported in 2004 by a group led by Peter Schiffer at Pennsylvania State University in
University Park.
Schiffer's team had shown that when a magnetic field was applied to spin ice at low temperatures
and then removed, the spins were surprisingly slow to revert to their original state (Physical Review B,
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vol 64, p 064414). Jaubert and Holdsworth calculated that monopoles explain this perfectly. At low
temperatures, monopoles do not have enough energy to move freely and so make the magnetic response
of the entire system sluggish by just the amount the experiments had found.
It seems the elusive monopoles have been pinned down at last. But Blas Cabrera -- who looked for
monopoles in cosmic rays passing through his laboratory at Stanford University in the 1980s -- sounds a
note of caution. The monopoles discovered in spin ice are rather different beasts from those that he and
others were looking for.
For a start, they are some 8000 times less magnetic and are free to move only within the spin ice -not to roam the wider Universe. So they are not really analogous to electric charges and it doesn't look
as if they are going to solve the dark matter problem.
Do they count at all? Quite possibly. When Dirac dreamed up his cosmic monopoles, he imagined a
vacuum as the lowest possible energy state that free space could assume. Monopoles then represented a
higher-energy "excitation" of a vacuum in much the same way that the low-energy two-in, two-out spinice state is excited to create monopoles.
The new research even borrows elements of Dirac's description of free-space monopoles such as the
invisible "strings" that he envisaged between pairs of poles that have separated. The similarities mean
that the interactions of spin-ice monopoles could provide a way to learn about cosmic monopoles by
proxy (for example, how they might have interacted in the early Universe).
Quite apart from that, the more down-to-earth monopoles might turn out to be practically useful,
says Tchernyshyov. Most computer memories store information magnetically. The ability to use
magnetic rather than electric charges to read-and -write bits to-and-from those stores could have great
advantages in speed and flexibility. What's more, the 3-dimensional configuration of spin ice might
allow for memories of much higher density than is currently possible.
That's for the future. For Holdsworth, the mere fact that we have found monopoles somewhere -anywhere! -- is reason enough to make a song&dance about them. "These might not be exactly the
monopoles that Dirac dreamed of. But that doesn't mean they're not remarkable."
Race for the Pole
The history of monopole searches is one of high hopes and dashed expectations.
● In 1969, Paul Buford Price at the General Electric labs in New York combed through samples of deepocean sludge in the hope that monopoles had lodged there earlier in geological history. Nothing
doing.
● The following year, Luis Alvarez of the University of California, Berkeley got his hands on samples
of moon rock from the Apollo missions and passed them close to a loop of superconducting wire.
He hoped an embedded monopole might trigger a detectable current. Again, nothing.
● In the 1970, Price (now also at Berkeley) shifted focus from the deep ocean to the high atmosphere,
performing a series of balloon experiments to spy passing monopoles in cosmic rays. In 1975,
he thought he had one. Alvarez disputed the claim. Lacking further confirmatory evidence,
Price's group backed down.
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● Perhaps the most tantalizing signal yet was the "Valentine's Day monopole" seen by Blas Cabrera of
Stanford University, California in 1982. He had set up a highly sensitive superconducting
induction coil in a basement lab, figuring that the magnetic forces from a passing cosmic-ray
monopole would induce an easily detectable jump in current.
For 150 days, nothing happened. But on the night of 14 February, just such a sudden and
obvious blip occurred. Cabrera reported the observation to great excitement in the journal
Physical Review Letters (vol 48, p 1378). But longer, more sensitive follow-up experiments
failed to replicate it. Cabrera now thinks it was caused by a rare release of mechanical stress in
the electronics attached to the coil rather than by a monopole falling from the sky.
● If the Inflation Theory of how the Universe began is true and most monopoles were carried off to
distant parts of the cosmos, we're probably best off recreating one rather than waiting for one to
pass our way. A detector called MoEDAL being designed at the Large Hadron Collider at the
CERN particle physics laboratory near Geneva, Switzerland will be able to spot monopoles
produced in the LHC's collisions.
The LHC isn't energetic enough to make the high-mass monopoles originally conjectured by
Paul Dirac. But the smaller monopoles predicted by some theories of grand unification and some
versions of string theory could well pop up there.
Readers' Comments
1. An Arrogant Proxy
by Anonymous / Wed May 06,2009 19:10:45 BST
Yes, let us suppose "monopoles" based on mathematical niceties. Further, let us suppose that
"spin ice", a complete accident of molecular activity, represents the abstract monopoles. Further,
let us suppose dark matter = monopoles = spin ice.
Suppositions leading to suppositions. Mathematics is the enemy.
I have no respect for this nonsense. The article is expertly written, however.
2. An Arrogant Proxy
by Anne O. Nymous / Wed May 06, 2009 19:22:31 BST
A good sized chunk of modern technology works on this sort of mathematics and supposition.
Perhaps you should become Amish?
Scientists are forced to make guesses when direct evidence is tough to come by. The beauty of
the scientific method is that theories change as new information becomes available through more
sensitive instruments, more powerful devices, more clever experiments, or just dumb luck.
3. An Arrogant Proxy
by PeterY / Thu May 07, 2009 08:45:28 BST
You obviously have no idea about what real Physics is like.
This is beautiful Physics! One theory creates a mathematical model that is an analog for
something else. You see this all the time in the history of Physics and Mathematics. The next step
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is to use the new mathematical theory to make predictions for the new area of application. Perhaps
it will be useful, perhaps not. Only time will tell.
4. Why Not Reject Theories That Require Monopoles?
by Crackpot / Wed May 06, 2009 19:42:21 BST
If our best explanations of how the Universe hangs together demand that magnetic monopoles
exist, then maybe we should reject those explanations. At least physicists could be a little less
hostile to alternative ideas...
5. If It's Discovered That Such Monopoles Exist, In Such Case What Would Be The Behavior Of
These Poles In The Earth's Magnetic Field?
by Anand Goadha / Wed May 06, 2009 19:59:21 BST
If we start plotting the magnetic lines in the Earth's magnetic field with these monopoles, it
will be interesting to note the 'starting' and 'ending' of the field lines and where we would end up in
locating the 'null points'.
This will be wonderful and lead to new inventions in the world of Physcis. Thus old theories
will get modified.
6. If It's Discovered That Such Monopoles Exist, In Such Case What Would Be The Behavior Of
These Poles In The Earth's Magnetic Field ?
by Vendicar Decarian / Wed May 06, 200920:14:36 BST
> Anand: "If we start plotting the magnetic lines in the Earth's magnetic field with these
monopoles, it will be interesting to note the 'starting' and 'ending' of the field lines and
where we would end up in locating the 'null points'."
There is no such thing as a magnetic "line of force". The magnetic field is continuous and
smooth just like an electric field.
It should also be remembered that magnetic fields are also entirely fictitious and arise from
charge imbalances resulting from the relativistic motion of charge carriers.
Spin magnetism is presumed to be innate to fundamental particles like the electron (although
there is no proof or reason why this is the case).
Again, Magnetism is the result of Einsteinian Relativity as applied to accelerating charged
particles.
In this context, monopoles cannot be generated without violating General Relativity. And this
is exactly how they are generated in inflation models of the 'Big Bang' (a period in which space
and the forces it carries were expanded at rates exceeding c).
7. Kinds if ElectroMagnetism
by Andre / Wed May 06, 2009 20:23:09 BST
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I'm a complete layman. But the idea of "Ying" without "Yang" sounds wrong. I can believe
that the other pole might not be obvious or maybe not even nearby. But it seems to me the other
pole would still be there somewhere.
So my question is are there different types of ElectroMagnetism? Like one kind could
magnetize right through a different type of magnetism without affecting it? To reach its partner?
Couldn't the 'Big Bang' have something beyond it balancing it all out?
8. Magnetic Poles
by Mike / Wed May 06, 2009 20:23:35 BST
How is this for you. The magnets in computer hard drives have 4 poles. North and South on
one side. But on the other side, the poles are reversed-4 poles and in the center is a null point.
Check it out..
9. Magnetic Monopoles
by Mtafu Norwin Maseleka / Wed May 06, 2009 20:56:53 BST
Reading of Physics literature tells me that electric charge is an inherent characteristic of an
"electrically charged" body.
Q = F/E
However, a magnetic charge is a result of the motion of an electric charge and I suspect the
velocity vector to be a strong factor in the polarity of Magnetism.
Qv = F/B
That is, to create a monopole, we might have to let the displacement vector to approach zero.
10. Good Writing
by Dan Conine / Wed May 06, 2009 21:07:24 BST
Regardless of the outcome, the article does a good job of explaining the aspects of monopoles,
I think. Comparing them to electric charges is tricky. A "chicken-and-egg" type of argument,
perhaps. If the existence of magnetic monopoles is dependent upon a state of the vacuum, then
perhaps so is the rest of matter and thus everything about the structure of the Universe as we know
it depends whether or not there is a source of 'aether' flow to maintain the quantum states.
Relativity is a nice mathematical convenience as well as Uncertainty. But Nature came into
existence and remains in existence without Einstein or Heisenberg. So whether-or -not the
mathematics works out is unimportant in the end.
11. Atomic Spin
by Vendiar Decarian / Thu May 07, 2009 00:34:31 BST
There is a symmetric form of Maxwell's equations that permit them to be written such that
charge results from the motion of magnetic fields. So the search persists for monopoles.
As to spin causing a magnetic field, it is an experimental fact that electrons and other particles
possess a magnetic field. This field is quantified as the particles spin.
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To the extent that they have been observed, electrons (unlike protons) have no internal
structure. Needless to say, this begs the question as to what is spinning.
Physicists have avoided that question for the last Century and have come to regard the
magnetic field of fundamental particles like electrons to be inherent to the particle itself rather than
arising from the acceleration of charge.
Whether this magnetic field is truly fundamental or results from a circulation bias of the
vacuum field about the electron itself, remains to be seen. My bets are on a circulation bias.
12. Atomic Spin
by joshua / Thu May 07, 2009 10:14:49 BST
> Vendiar: "This field is quantified as the particles spin. Needless to say, this begs the
question as to what is spinning."
Spin arises from the charge that occurs when you observe it. It orientates itself in an 'up' state
or 'down' state. So for this, a monopole nonsense in particle physics is null and void. But if you
can find a huge empty interstellar region of space where dark energy and anti-matter frequently
mingle, you might be onto something. Who knows? We've found regions of space with 13.7
trillion latent amps.
13. Individual Scientist's Imaginary Scientific Philosophies Might Have Been Reported By Default
by Vendicar Decarian / Thu May 07, 2009 00:40:57 BST
> Anand: "Monopole can never exist without the opposite."
Similarly charges never exist without the opposite. But that doesn't prevent charged particles
from being considered independent.
You might ask yourself what it means for an electron to be an independent entity when it was
produced in a reaction that produced a particle of opposite charge as well.
When are two particles that originated from the same fields at the same time and at the same
location be considered independent? What is it about the poles of a magnet that prevent this from
happening to them?
The article being commented on describes a situation in which North/South poles are free to
wander about a crystal as seemingly independent entities. In what way aren't they independent?
14. Means And Ends
by Amy / Wed May 06, 2009 23:30:19 BST
Can you have a piece of string with only one end?
15. Means And Ends
by John Dudding / Thu May 07, 2009 09:03:49 BST
Amy has hit this one on the head. This is NOT a monopole at all! It is a bipolar situation
where the two poles can wander around. They are still linked.
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Just as with a bar magnet, if you cut the chain of Tetrahedra, then you do not have 2 separate
(mono) poles. You have 4 poles in two separate pairs.
Good writing, interesting Physics, Monopoles? - not even close !
16. Instantaneous Communication By Monopole Creation
by Larry Pendarvis / Wed May 06, 2009 23:31:41 BST
Dirac proved that the existence of a magnetic monopole anywhere in the Universe implied that
electric charge is quantized in certain units depending upon the units of the magnetic charge of the
monopole. This effect affects the entire Universe everywhere. Even beyond any inflationary
horizons.
This means that we can communicate throughout the Universe by the simple expedient of
creating a magnetic monopole of a desired charge. The recipient of the message then measures his
local quantization of electric charge. Then we destroy the monopole (perhaps by tossing it into a
black hole) and create another with different charge. This process is repeated until the message is
complete.
There are 2 minor problems with this.
First, it is impossible to send a secret message because everyone in the entire Universe can
measure electric quantization.
Second, Dirac never figured out what would be the result if TWO magnetic monopoles existed
in the Universe with two different charges, so only one sender can operate properly at a time.
17. Not A Particle
by Gunboat / Wed May 06, 2009 23:42:14 BST
The monopole is supposed to be a particle (with mass). This article does not describe a
particle. What it describes is massless domain and is very similar to the "hole" in a P-type
semiconductor.
18. Very Heavy
by William Garrett / Wed May 06, 2009 23:45:42 BST
I will now declare my ignorance. Since the monopole has such a large mass, exactly where
does it reside so as to be not noticed since presumably it is present in a normal magnetic field?
Ditto Higg's boson.
19. Magnetic Monopoles and Mathematics
by Shannon Lieb / Thu May 07, 2009 00:30:57 BST
One interesting point left out of the article is the fact that if magnetic monopoles exist, then the
magnitude of the positive and negative charges can be shown to be equal. However, the fact that
positive and negative charges are equal does not prove that magnetic monopoles exist.
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Certainly, the notion that magnetic monopole implies equal magnitudes of + and - charges
along with the asymmetry of Maxwell's electromagnetic equations due to the lack of experimental
evidence for magnetic monopoles makes for a very attractive area to research.
Mathematics does not prove physical reality. It only gives us a more precise way of expressing
our discoveries as well as a broader hint at how to perceive reality.
20. Anti-magnetism
by Amy / Thu May 07, 2009 03:45:34 BST
It's amazing what you can do with Mathematics.
Anti-magnetic monopoles, anyone?
But wait! There's more ...
Monopolium: a monopole - antimonopole bound state.
http://arxiv.org/abs/hep-ph/0701133
Whats next? Anti-monopolium? Sounds quite repulsive.
21. Hunting The Mysterious Monopole
by Tom Besmer / Thu May 07, 2009 04:21:14 BST
A monopole denies Maxwell's equations. How do you justify the monopole and how do you
fit it in with Maxwell's equations. I would be very interested.
Presently, I am skeptical because denial of Maxwell's equations must be followed with new
data that expands Science.
22. Not An Example Of Monopoles
by Allan Gillard / Thu May 07, 2009 04:49:05 BST
Let's be clear. This is not an example of evidence for the discovery of "monopoles". It is a
temporary and localized aggregation of magnetic charges that results in a overall "North"- or
"South"-oriented field.
It is definitely not a magnetic monopole in the same context that an electron and a proton can
be classed as electrical "mono-charges". So I do not really understand the slant taken for the
article. Saying "no monopoles" is NOT another way of saying that there is no such thing as a
freely moving magnetic charges. There is no relationship! The statement that "These might not
be the monopoles of physics lore. But they could provide the first clues as to how those legendary
beasts behave" cannot be justified.
The other question to consider is the concept that since the 'Big Bang', this universe "expansion
should have carried most, if not all, of the monopoles ... to a patch of the cosmos so distant ..." The point is that the theory holds that most, if not all, matter existing was created in the 'Big
Bang' and has been subject to the same expansion forces and so should be present today. The
other consideration would be that like many of the other elementary particles we have discovered,
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monopoles are so short lived in isolation before they combine with other particles (such as their
opposite monopoles to produce the currently seen magnetic characteristics of particles).
23. Classical Ed
by Max / Thu May 07, 2009 06:58:31 BST
The article contains a gross error in saying that the Classical ElectroDynamics treats the
electric and the magnetic fields "pretty much the same way".
Actually, the Gauss laws for the electric and the magnetic fields are markedly different in the
classical ED. In fact, the Gauss law for magnetism explicitly rules out the existanse of magnetic
monopoles.
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