Electricity and magnetism have a lot in common. They are linked by the unified theory of electromagnetism and, in many ways, two sides of the same coin. Both can exert forces on charges and magnetic fields. A changing electric field creates a magnetic field and vice versa. Elementary particles can have electrical and magnetic properties. There is one fundamental difference, however.
Electric fields are generated by electric charges. There are two types of electrical charge. We call positive and negative, and they exist independently of each other. For example, electrons only have a negative charge, while protons have a positive charge. The everyday objects around us are usually electrically neutral, but it is easy to easily charge objects positively or negatively. Static electricity is a good example of this.
Monopolies only have a north or south pole. Photo credit: Daniel Dominguez, CERN
This is not the case with magnetic charge. As with electric charge, there are two types of magnetic charge. For historical reasons we call them the North and South Poles. However, these magnetic poles always occur in pairs. Each magnet has a north and south pole. If you break a magnet in half, don’t separate the poles. Instead, you get two smaller magnets, each with a north and a south pole. Even magnetic elementary particles have this dipole property. As far as we can tell, there are no magnetic monopoles.
Monopoles would add symmetry to electromagnetism. Photo credit: Wikipedia user Maschen
Although we have never observed magnetic monopoles, we know how their properties would correspond to electromagnetism. One of the more surprising consequences is that magnetic monopoles would quantize the charge. If you had multiple electric and magnetic charges, the electromagnetic field they created would have angular momentum (rotation) that depends on the value of the electric and magnetic charges. In quantum mechanics, the angular momentum is quantized, which means that the charges would also be quantized. The existence of a single magnetic monopole in the universe would explain why fundamental particles always have an integral value of electrical charge. Magnetic monopoles would also add symmetry to electromagnetism. This is one of the reasons why models like string theory predict magnetic monopoles.
The discovery of magnetic monopoles would revolutionize our understanding of the universe. So it’s worth looking for them wherever we can. Recently, a team looked for evidence of her in the Earth’s magnetic field. In principle, their idea is simple. If magnetic monopoles exist, they could be captured by the Earth’s strong magnetic field. This would give the earth a magnetic charge that we should measure.
The swarm constellation examines the magnetic field of the Earth’s core. Photo credit: ESA / ATG Medialab
In practice this is a big challenge. For their study, the team used public data from the ESA swarm constellation for six years. This is a collection of orbiting satellites specially designed to create a high resolution map of the Earth’s magnetic fields. From this data, the team created a map of our planet’s magnetic field lines and applied a mathematical technique known as Gaussian law. Think of the earth surrounded by an imaginary sphere. If there are no magnetic monopoles, any magnetic field line that crosses out of the sphere must also cross back into the sphere. If there are magnetic monopoles, some magnetic field lines must cross the sphere without crossing each other. An excess of field line crossings would prove the existence of magnetic monopoles.
The team found that, up to the limits of their data, there was no evidence of excessive field line crossings. This does not mean that there are no magnetic monopoles, just that there is no evidence that they are trapped by the Earth’s magnetic field. So out of luck this time, but using public data is a great result.
Reference: Bai, Yang, Sida Lu and Nicholas Orlofsky. “Search for magnetic monopoles with the earth’s magnetic field.” arXiv preprint arXiv: 2103.06286 (2021).
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