Electromagnetism

• 1. Introduction

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  Around 600 B.C.E., the Greek philosopher Thales of Miletus rubbed a piece of amber with cloth and watched it pick up bits of straw. He also played with magnetic rocks that pulled on one another. Then he did something remarkable for his time. He hypothesized that these two odd behaviors, the cling of charged amber and the pull of magnetized stone, might be connected. He was right, though the proof would not arrive for more than 2,000 years. Electromagnetism is the interaction between particles that carry electric charge. It is one of the four fundamental forces of nature, and it governs almost everything you notice in daily life. Why does it take so long to turn a hunch about amber and lodestone into mathematics? How did two forces that look entirely different turn out to be the same thing? And how does an interaction between tiny charged particles end up holding your own body together?

• 2. The Force That Holds Matter

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  The electrostatic attraction between atomic nuclei and their electrons is what holds atoms together at all. Electric forces let separate atoms bond into molecules, including the large molecules such as proteins that form the basis of life. Magnetism enters here too. Magnetic interactions between the spin and angular momentum magnetic moments of electrons influence chemical reactivity, a relationship studied in spin chemistry. So this single force runs through both the structure of an atom and the chemistry of living things.

  The push you feel when you press on a solid object is electromagnetic. The forces that resist your hand come from intermolecular forces between the molecules in your body and the molecules in the object. Nothing actually touches in the way it seems to. Electromagnetism explains how materials carry momentum despite being made of individual particles and empty space.

  Electrons in motion add another layer to this. As electrons move between interacting atoms, they carry momentum with them. When a collection of electrons becomes more confined, its minimum momentum must rise, a consequence of the Pauli exclusion principle. The behavior of matter at the molecular scale, including its density, comes from a balance. On one side sits the electromagnetic force. On the other sits the force generated by the exchange of momentum the electrons carry. That balance is why a block of metal has the density it does.

• 3. Two Forces, One Phenomenon

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  Electric forces cause opposite charges to attract and like charges to repel, and that pull or push weakens with the square of the distance between them. Magnetism behaves in a parallel way. Magnetic poles attract or repel much as positive and negative charges do, but they always come in pairs. Every north pole is yoked to a south pole, with no single pole found on its own.

  An electric current running through a wire creates a magnetic field that circles the wire, and the direction of that circulation depends on the direction of the current. The reverse also holds. Move a loop of wire toward or away from a magnetic field, or move a magnet toward or away from the loop, and a current is induced in the wire. The direction of that induced current depends on the direction of the motion. These four effects have all been clearly demonstrated by experiment.

  Relativity later showed just how tightly bound these two faces are. In a moving frame of reference, a magnetic field transforms into a field with a nonzero electric component, and a moving electric field transforms into one with a magnetic component. They are two sides of the same coin, which is exactly why we speak of electromagnetism as a single word.

• 4. From Lodestone To Lightning Rod

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  Investigation into electromagnetic phenomena began about 5,000 years ago. There is evidence that ancient Chinese, Mayan, and potentially even Egyptian civilizations knew the naturally magnetic mineral magnetite had attractive properties, and many worked it into their art and architecture. They saw lightning and static electricity too, but had no idea of the mechanisms behind them. Lightning, in many cultures, was treated as a creation of the gods rather than a subject for science.

  In 1600, William Gilbert argued in his De Magnete that electricity and magnetism were distinct effects, even though both could attract and repel objects. Mariners had long noticed that lightning strikes could disturb a compass needle, a hint that lightning carried something electrical. That link was not confirmed until Benjamin Franklin's proposed experiments. In 1752, on the 10th of May, Thomas-Francois Dalibard of France carried them out, using a 40 ft iron rod instead of a kite, and drew electrical sparks from a cloud.

  Gian Romagnosi reached a different frontier in 1802. He noticed that connecting a wire across a voltaic pile deflected a nearby compass needle, one of the first published links between human-made electric current and magnetism. His finding stayed obscure for years. The same effect would not become widely known until another experimenter repeated it nearly two decades later.

• 5. The Danish Compass Needle

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  In April 1820, Hans Christian Orsted watched an electrical current in a wire move a nearby compass needle. At the moment of discovery he offered no satisfactory explanation and made no attempt to cast the phenomenon in mathematical form. Three months later he returned to it with more intensive investigation, then published his findings. He had proven that an electric current produces a magnetic field as it flows. The CGS unit of magnetic induction, the oersted, carries his name.

  Orsted's results influenced the French physicist Andre-Marie Ampere. Ampere worked out a single mathematical form to describe the magnetic forces between current-carrying conductors. By determining a force law for the interaction between elements of electric current, he set the new subject on a solid mathematical foundation. The field he opened gained a name of its own: electrodynamics.

  This unification did not stop with Ampere. It was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside and Heinrich Hertz. Together their work stands as one of the key accomplishments of 19th-century mathematical physics, and it pointed straight toward the true nature of light.

• 6. Maxwell And The Light Within

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  James Clerk Maxwell's 1873 work, A Treatise on Electricity and Magnetism, changed how physicists saw the world. Electricity and magnetism had been treated as two separate forces. Maxwell showed that the interactions of positive and negative charges are mediated by one force. His treatise pulled the earlier developments together into a single theory.

  The heart of that theory is a set of equations now known as Maxwell's equations, four partial differential equations that completely describe classical electromagnetic fields. The discovery culminated in the 1860s. These equations gave a sound mathematical basis for the relationships between electricity and magnetism that scientists had explored for centuries. They also predicted self-sustaining electromagnetic waves.

  Maxwell postulated that such waves make up visible light, and that was later shown to be true. The same family of waves spans a vast range of frequencies. Radio waves sit at the lowest frequencies, visible light at intermediate frequencies, and gamma rays at the highest. Gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, and radio waves are all electromagnetic radiation, differing only in their range of frequencies. In his theory, Maxwell proposed that light propagated through a medium called the luminiferous ether.

• 7. A Crack That Made Relativity

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  Maxwell's equations carry a strange consequence. They say the speed of light in vacuum is a universal constant, set only by the electrical permittivity and magnetic permeability of free space. That claim violates Galilean invariance, a long-standing cornerstone of classical mechanics. Classical electromagnetism turned out to be hard to reconcile with classical mechanics, even as it sat comfortably with the new physics to come.

  One escape was to assume a luminiferous aether through which light propagated, but later experimental efforts failed to detect it. After important contributions from Hendrik Lorentz and Henri Poincare, Albert Einstein solved the problem in 1905 with special relativity. He replaced classical kinematics with a new kinematics compatible with electromagnetism. The requirement that observations stay consistent across moving frames of reference, together with a speed of light fixed by permeability and permittivity, helped inspire that theory.

  Quantum mechanics reshaped the picture again. Quantum electrodynamics, or QED, modifies Maxwell's equations to fit the quantized nature of matter. In QED, changes in the electromagnetic field are expressed as discrete excitations called photons, the quanta of light. Light is no longer a smooth wave in an ether but quantized, self-propagating oscillatory field disturbances.

• 8. Strength, Reach, And Open Questions

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  The electromagnetic force is the second strongest of the four known fundamental forces, and its range is unlimited. Every non-fundamental force we meet, including friction and contact forces, is derived from the four fundamental forces. At high energy, the weak force and the electromagnetic force merge into a single interaction called the electroweak interaction.

  This force drives much of modern technology. It underlies electrical energy production, transformation, and distribution. It governs the production and detection of light, heat, and sound, along with fiber optic and wireless communication, sensors, computation, electrolysis, electroplating, and mechanical motors and actuators. The theory itself is used to understand and design electric circuits, magnetic circuits, and semiconductor devices.

  For all that reach, a few problems remain unsolved. Among them are the lack of magnetic monopoles, the Abraham-Minkowski controversy, the location in space of the electromagnetic field energy, and the mechanism by which some organisms can sense electric and magnetic fields. That last puzzle returns the story to where it began, with living creatures responding to a force that Thales of Miletus first glimpsed in a rubbed piece of amber.