Electromagnetic fields as they would be generated by positive and negative electric charges, both at ... [+] rest and in motion (top), as well as those that would theoretically be created by magnetic monopoles (bottom), were they to exist.
Out of all of the known particles both fundamental and composite there are a whole slew of properties that emerge. Each individual quantum in the Universe can have a mass, or they can be massless. They can have a color charge, meaning they couple to the strong force, or they can be chargeless. They can have a weak hypercharge and/or weak isospin, or they can be completely decoupled from the weak interactions. They can have an electric charge, or they can be electrically neutral. They can have a spin, or an intrinsic angular momentum, or they can be spinless. And if you have both an electric charge and some form of angular momentum, youll also have a magnetic moment: a magnetic property that behaves as a dipole, with a north end and a south end.
But there are no fundamental entities that have a unique magnetic charge, like a north pole or south pole by itself. This idea, of a magnetic monopole, has been around for a long time as a purely theoretical construct, but there are reasons to take it seriously as a physical presence in our Universe. Patreon supporter Jim Nance writes in because he wants to know why:
You've talked in the past about how we know the universe didn't get arbitrarily hot because we don't see relics like magnetic monopoles.You say that with a lot of confidence which makes me wonder, given that no one has ever seen a magnetic monopole or any of the other relics, why are we confident that they exist?
Its a deep question that demands an in-depth answer. Lets start at the beginning: going all the way back to the 19th century.
When you move a magnet into (or out of) a loop or coil of wire, it causes the field to change around ... [+] the conductor, which causes a force on charged particles and induces their motion, creating a current. The phenomena are very different if the magnet is stationary and the coil is moved, but the currents generated are the same. This was the jumping-off point for the principle of relativity.
A little bit was known about electricity and magnetism at the start of the 1800s. It was generally recognized that there was such a thing as electric charge, that it came in two types, where like charges repelled and opposite charges attracted, and that electric charges in motion created currents: what we know as electricity today. We also knew about permanent magnets, where one side acted like a north pole and the other side like a south pole. However, if you broke a permanent magnet in two, no matter how small you chopped it up, youd never wind up with a north pole or a south pole by itself; magnetic charges only came paired up in a dipole configuration.
Throughout the 1800s, a number of discoveries took place that helped us make sense of the electromagnetic Universe. We learned about induction: how moving electric charges actually generate magnetic fields, and how changing magnetic fields, in turn, induce electric currents. We learned about electromagnetic radiation, and how accelerating electric charges can emit light of various wavelengths. And when we put all of our knowledge together, we learned that the Universe wasnt symmetric between electric and magnetic fields and charges: Maxwells equations only possess electric charges and currents. There are no fundamental magnetic charges or currents, and the only magnetic properties we observe come about as being induced by electric charges and currents.
It's possible to write down a variety of equations, like Maxwell's equations, that describe the ... [+] Universe. We can write them down in a variety of ways, but only by comparing their predictions with physical observations can we draw any conclusion about their validity. It's why the version of Maxwell's equations with magnetic monopoles (right) don't correspond to reality, while the ones without (left) do.
Mathematically or if you prefer, from a theoretical physics perspective its very easy to modify Maxwells equations to include magnetic charges and currents: where you simply add in the ability for objects to also possess a fundamental magnetic charge: an individual north or south pole inherent to an object itself. When you introduce those extra terms, Maxwells equations get a modification, and become completely symmetric. All of a sudden, induction now works the other way as well: moving magnetic charges would generate electric fields, and a changing electric field can induce a magnetic current, causing magnetic charges to move and accelerate within a material that can carry a magnetic current.
All of this was simply fanciful consideration for a long time, until we started to recognize the roles that symmetries play in physics, and the quantum nature of the Universe. Its eminently possible that electromagnetism, at some higher energy state, was symmetric between electric and magnetic components, and that we live in a low-energy, broken symmetry version of that world. Although Pierre Curie, in 1894, was one of the first to point out that magnetic charges could exist, it was Paul Dirac, in 1931, who showed something remarkable: that if you had even one magnetic charge, anywhere in the Universe, then it quantum mechanically implied that electric charges should be quantized everywhere.
The difference between a Lie algebra based on the E(8) group (left) and the Standard Model (right). ... [+] The Lie algebra that defines the Standard Model is mathematically a 12-dimensional entity; the E(8) group is fundamentally a 248-dimensional entity. There is a lot that has to go away to get back the Standard Model from String Theories as we know them.
This is fascinating, because not only are electric charges observed to be quantized, but theyre quantized in fractional amounts when it comes to quarks. In physics, one of the most powerful hints we have that new discoveries might be around the corner are by discovering a mechanism that could explain why the Universe has the properties we observe it to have.
However, none of that provides any evidence that magnetic monopoles actually do exist, it simply suggests that they might. On the theoretical side, quantum mechanics was soon superseded by quantum field theory, where the fields are also quantized. To describe electromagnetism, a gauge group known as U(1) was introduced, and this is still used at the present. In gauge theory, the fundamental charges associated with electromagnetism will be quantized only if the gauge group, U(1), is compact; if the U(1) gauge group is compact, however, we get magnetic monopoles anyway.
Again, there might turn out to be a different reason why electric charges have to be quantized, but it seemed at least with Diracs reasoning and what we know about the Standard Model that theres no reason why magnetic monopoles shouldnt exist.
This diagram displays the structure of the standard model (in a way that displays the key ... [+] relationships and patterns more completely, and less misleadingly, than in the more familiar image based on a 4x4 square of particles). In particular, this diagram depicts all of the particles in the Standard Model (including their letter names, masses, spins, handedness, charges, and interactions with the gauge bosons: i.e., with the strong and electroweak forces). It also depicts the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry, and how the properties of the remaining particles change as a consequence.
For many decades, even after numerous mathematical advances, the idea of magnetic monopoles remained only a curiosity that hung around in the back of theorists minds, without any substantial progress being made. But in 1974, a few years after we recognized the full structure of the Standard Model which in group theory, is described by SU(3) SU(2) U(1) physicists started to entertain the idea of unification. While, at low energies, SU(2) describes the weak interaction and U(1) describes the electromagnetic interaction, they actually unify at energies of around ~100 GeV: the electroweak scale. At those energies, the combined group SU(2) U(1) describes the electroweak interactions, and those two forces unify.
Is it possible, then, that all of the fundamental forces unify into some larger structure at high energies? They might, and thus the idea of Grand Unified Theories began to come about. Larger gauge groups, like SU(5), SO(10), SU(6), and even exceptional groups began to be considered. Almost immediately, however, a number of unsettling but exciting consequences began to emerge. These Grand Unified Theories all predicted that the proton would be fundamentally stable and would decay; that new, super-heavy particles would exist; and that, as shown in 1974 by both Gerard tHooft and Alexander Polyakov, they would lead to the existence of magnetic monopoles.
The concept of a magnetic monopole, emitting magnetic field lines the same way an isolated electric ... [+] charge would emit electric field lines. Unlike magnetic dipoles, there's only a single, isolated source, and it would be an isolated north or south pole with no counterpart to balance it out.
Now, we have no proof that the ideas of grand unification are relevant for our Universe, but again, its possible that they do. Whenever we consider a theoretical idea, one of the things we look for are pathologies: reasons that whatever scenario were interested in would break the Universe in some way or another. Originally, when tHooft-Polyakov monopoles were proposed, one such pathology was discovered: the fact that magnetic monopoles would do something called overclose the Universe.
In the early Universe, things are hot and energetic enough that any particle-antiparticle pair you can create with enough energy via Einsteins E = mc2 will get created. When you have a broken symmetry, you can either give a non-zero rest mass to a previously massless particle, or you can spontaneously rip copious numbers of particles (or particle-antiparticle pairs) out of the vacuum when the symmetry breaks. An example of the first case is what happens when the Higgs symmetry breaks; the second case could occur, for example, when the Peccei-Quinn symmetry breaks, pulling axions out of the quantum vacuum.
In either case, this could lead to something devastating.
If the Universe had just a slightly higher matter density (red), it would be closed and have ... [+] recollapsed already; if it had just a slightly lower density (and negative curvature), it would have expanded much faster and become much larger. The Big Bang, on its own, offers no explanation as to why the initial expansion rate at the moment of the Universe's birth balances the total energy density so perfectly, leaving no room for spatial curvature at all and a perfectly flat Universe. Our Universe appears perfectly spatially flat, with the initial total energy density and the initial expansion rate balancing one another to at least some 20+ significant digits. We can be certain that the energy density didn't spontaneously increase by large amounts in the early Universe by the fact that it hasn't recollapsed.
Normally, the Universe expands and cools, with the overall energy density being closely related to the rate of expansion at any point in time. If you either take a large number of previously massless particles and give them a non-zero mass, or you suddenly and spontaneously add a large number of massive particles to the Universe, you rapidly increase the energy density. With more energy present, suddenly the expansion rate and the energy density are no longer in balance; theres too much stuff in the Universe.
This causes the expansion rate to not only drop, but in the case of monopole production, to plummet all the way to zero, and then to begin contracting. In short order, this leads to a recollapse of the Universe, ending in a Big Crunch. This is called overclosing the Universe, and cannot be an accurate description of our reality; were still here and things havent recollapsed. This puzzle was known as the monopole problem, and was one of the three main motivations for cosmic inflation.
Just as inflation stretches the Universe, whatever its geometry was previously, to a state indistinguishable from flat (solving the flatness problem), and imparts the same properties everywhere to all locations within our observable Universe (solving the horizon problem), so long as the Universe never heats back up to above the grand unification scale after inflation ends, it can solve the monopole problem, too.
If the Universe inflated, then what we perceive as our visible Universe today arose from a past ... [+] state that was all causally connected to the same small initial region. Inflation stretched that region to give our Universe the same properties everywhere (top), made its geometry appear indistinguishable from flat (middle), and removed any pre-existing relics by inflating them away (bottom). So long as the Universe never heats back up to high enough temperatures to produce magnetic monopoles anew, we will be safe from overclosure.
This was understood way back in 1980, and the combined interest in tHooft-Polyakov monopoles, grand unified theories, and the earliest models of cosmic inflation led some people to embark on a remarkable undertaking: to try and experimentally detect magnetic monopoles. In 1981, experimental physicist Blas Cabrera built a cryogenic experiment involving a coil of wire, explicitly designed to search for magnetic monopoles.
By building a coil with eight loops in it, he reasoned that if a magnetic monopole ever passed through the coil, hed see a specific signal due to the electric induction that would occur. Just like passing one end of a permanent magnet into (or out of) a coil of wire will induce a current, passing a magnetic monopole through that coil of wire should induce not only an electric current, but an electric current that corresponds to exactly 8 times the theoretical value of the magnetic monopoles charge, owing to the 8 loops in his experimental setup. (If a dipole were to pass through, instead, there would be a signal of +8 followed shortly after by a signal of -8, allowing the two scenarios to be differentiated.)
On February 14, 1982, no one was in the office monitoring the experiment. The next day, Cabrera came back, and was shocked at what he observed. The experiment had recorded a single signal: one corresponding almost exactly to the signal a magnetic monopole ought to produce.
In 1982, an experiment running under the leadership of Blas Cabrera, one with eight turns of wire, ... [+] detected a flux change of eight magnetons: indications of a magnetic monopole. Unfortunately, no one was present at the time of detection, and no one has ever reproduced this result or found a second monopole. Still, if string theory and this new result are correct, magnetic monopoles, being not forbidden by any law, must exist at some level.
This set off a tremendous interest in the endeavor. Did it mean inflation was wrong, and we really did have a Universe with magnetic monopoles? Did it mean that inflation was correct, and the one (at most) monopole that should remain in our Universe happened to pass through Cabreras detector? Or did it means that this was the ultimate in experimental errors: a glitch, a prank, or something else that we couldnt explain, but was spurious?
A number of copycat experiments ensued, many of which were larger, ran for longer times, and had greater numbers of loops in their coils, but no one else ever saw anything that resembled a magnetic monopole. On February 14, 1983, Stephen Weinberg wrote a Valentines Day poem to Cabrera, which read:
Roses are red,
Violets are blue,
Its time for monopole
Number TWO!
But despite all the experiments weve ever run, including some that have continued to the present day, there have been no other signs of magnetic monopoles ever seen. Cabrera himself went on to lead numerous other experiments, but we may never know what truly happened on that day in 1982. All we know is that, without the ability to confirm and reproduce that result, we cannot claim that we have direct evidence for the existence of magnetic monopoles.
These are the modern constraints available, from a variety of experiments largely driven from ... [+] neutrino astrophysics, that place the tightest bounds on the existence and abundance of magnetic monopoles in the Universe. The current bound is many orders of magnitude below the expected abundance if Cabrera's 1982 detection was normal, rather than an outlier.
Theres so much that we dont know about the Universe, including what happens at energies far in excess of what we can observe in the collisions that take place at the Large Hadron Collider. We dont know whether, at some high energy scale, the Universe can actually produce magnetic monopoles; we simply know that at the energies we can probe, we havent seen them. We dont know whether grand unification is a property of our Universe in the earliest stages, but we do know this much: whatever occurred early on, it didnt overclose the Universe, and it didnt fill our Universe with these leftover, high-energy relics from a hot, dense state.
Does our Universe, at some level, admit the existence of magnetic monopoles? Thats not a question we can presently answer. What we can state with confidence, however is the following:
Its nearly 40 years since the one experimental clue hinting at the possible existence of magnetic monopoles simply dropped into our lap. Until a second clue comes along, however, all well be able to do is tighten our constraints on where these hypothetical monopoles arent allowed to be hiding.
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Ask Ethan: What Impact Could Magnetic Monopoles Have On The Universe? - Forbes
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