Thanksgiving

This year we give thanks for something we’ve all heard of, but maybe don’t appreciate as much as we should: electromagnetism. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, the Jarzynski equality, the moons of Jupiter, space, and black hole entropy.)

Physicists like to say there are four forces of nature: gravitation, electromagnetism, the strong nuclear force, and the weak nuclear force. That’s a somewhat sloppy and old-fashioned way of talking. In the old days it made sense to distinguish between “matter,” in the form of particles or fluids or something like that, and “forces,” which pushed around the matter. These days we know it’s all just quantum fields, and both matter and forces arise from the behavior of quantum fields interacting with each other. There is an important distinction between fermions and bosons, which almost maps onto the old-fashioned matter/force distinction, but not quite. If it did, we’d have to include the Higgs force among the fundamental forces, but nobody is really inclined to do that.

The real reason we stick with the traditional four forces is that (unlike the Higgs) they are all mediated by a particular kind of bosonic quantum field, called gauge fields. There’s a lot of technical stuff that goes into explaining what that means, but the basic idea is that the gauge fields help us compare other fields at different points in space, when those fields are invariant under a certain kind of symmetry. For more details, check out this video from the Biggest Ideas in the Universe series (but you might need to go back to pick up some of the prerequisites).

The Biggest Ideas in the Universe | 15. Gauge Theory

All of which is just throat-clearing to say: there are four forces, but they’re all different in important ways, and electromagnetism is special. All the forces play some kind of role in accounting for the world around us, but electromagnetism is responsible for almost all of the “interestingness” of the world of our experience. Let’s see why.

When you have a force carried by a gauge field, one of the first questions to ask is what phase the field is in (in whatever physical situation you care about). This is “phase” in the same sense as “phase of matter,” e.g. solid, liquid, gas, etc. In the case of gauge theories, we can think about the different phases in terms of what happens to lines of force — the imaginary paths through space that we would draw to be parallel to the direction of the force exerted at each point.

The simplest thing that lines of force can do is just to extend away from a source, traveling forever through space until they hit some other source. (For electromagnetism, a “source” is just a charged particle.) That corresponds to field being in the Coulomb phase. Infinitely-stretching lines of force dilute in density as the area through which they are passing increases. In three dimensions of space, that corresponds to spheres we draw around the source, whose area goes up as the distance squared. The magnitude of the force therefore goes as the inverse of the square — the famous inverse square law. In the real world, both gravity and electromagnetism are in the Coulomb phase, and exhibit inverse-square laws.

But there are other phases. There is the confined phase, where lines of force get all tangled up with each other. There is also the Higgs phase, where the lines of force are gradually absorbed into some surrounding field (the Higgs field!). In the real world, the strong nuclear force is in the confined phase, and the weak nuclear force is in the Higgs phase. As a result, neither force extends farther than subatomic distances.

Phases of gauge fields.

So there are four gauge forces that push around particles, but only two of them are “long-range” forces in the Coulomb phase. The short-range strong and weak forces are important for explaining the structure of protons and neutrons and nuclei, but once you understand what stable nuclei there are, there work is essentially done, as far as accounting for the everyday world is concerned. (You still need them to explain fusion inside stars, so here we’re just thinking of life here on Earth.) The way that those nuclei come together with electrons to make atoms and molecules and larger structures is all explained by the long-range forces, electromagnetism and gravity.

But electromagnetism and gravity aren’t quite equal here. Gravity is important, obviously, but it’s also pretty simple: everything attracts everything else. (We’re ignoring cosmology etc, focusing in on life here on Earth.) That’s nice — it’s good that we stay attached to the ground, rather than floating away — but it’s not a recipe for intricate complexity.

To get complexity, you need to be able to manipulate matter in delicate ways with your force. Gravity isn’t up to the task — it just attracts. Electromagentism, on the other hand, is exactly what the doctor ordered. Unlike gravity, where the “charge” is just mass and all masses are positive, electromagnetism has both positive and negative charges. Like charges repel, and opposite charges attract. So by deftly arranging collections of positively and negatively charged particles, you can manipulate matter in whatever way you like.

That pinpoint control over pushing and pulling is crucial for the existence of complex structures in the universe, including you and me. Nuclei join with electrons to make atoms because of electromagnetism. Atoms come together to make molecules because of electromagnetism. Molecules interact with each other in different ways because of electromagnetism. All of the chemical processes in your body, not to mention in the world immediately around you, can ultimately be traced to electromagnetism at work.

Electromagnetism doesn’t get all the credit for the structure of matter. A crucial role is played by the Pauli exclusion principle, which prohibits two electrons from inhabiting exactly the same state. That’s ultimately what gives matter its size — why objects are solid, etc. But without the electromagnetic interplay between atoms of different sizes and numbers of electrons, matter would be solid but inert, just sitting still without doing anything interesting. It’s electromagnetism that allows energy to move from place to place between atoms, both via electricity (electrons in motion, pushed by electromagnetic fields) and radiation (vibrations in the electromagnetic fields themselves).

So we should count ourselves lucky that we live in a world where at least one fundamental force is both in the Coulomb phase and has opposite charges, and give appropriate thanks. It’s what makes the world interesting.

19 Comments

19 thoughts on “Thanksgiving”

  1. When I was training to be a H.S. physics teacher, my “Master” teachers taught their students that in everyday life gravity was the most important force. When I started student teaching, I corrected this by pointing out that astronauts do just fine without gravity, but without electromagnetism there is no chemistry or biology. So I really like this post.

  2. ” but once you understand what stable nuclei there are, there work is essentially done, ”
    there->their

  3. >but once you understand what stable nuclei there are, there work is essentially done, as far as accounting for the everyday world is concerned

    Tall order! There’s still much to learn.

  4. Maria Fátima Pereira

    Tendo estudado fisica e quimica, apenas até ao 12 ano, e, seguido para outra área, tento aprender ( porque gosto) sempre que posso.
    Mais uma vez, graças por conseguir explicar de uma forma acessivel, interessante, de forma a que, outros, de áreas diferentes consigam compreender.
    Graças, eletromagnetismo!

  5. MARK WEITZMAN,
    You probably meant as a joke that “astronauts do fine without gravity”. But in case some H.S. student gets a wrong impression, you should say what happens if, for a space station in orbit, the earth’s gravity is suddenly turned off. You know, the space station with the astronauts will move tangentially away from the earth and the astronauts will be lost forever!!

  6. Really enjoyed this, I look forward to every thanksgiving article. Also, Thank You for “Biggest Ideas…” it helped get me through the pandemic lock down!

  7. I often wonder if photons could be thought of as being the primary agent of causality (QED). Apparently, even our cells may communicate via light (biophotons). And even our concept and perception of time / causality is defined via the electromagnetic field. Thank you for this lovely ode to electromagnetism for Thanksgiving, Sean! Essence… Infinitely sustained. I hope you and yours have wonderful holidays!

  8. Enjoyed it.
    Electromagnetism, gravity and other forces/phenomenon are the way we know them because LIFE exists here on earth and the physical constants have been tuned to support life as we know it.
    On planet B, those constants will most certainly have different values and the forces/fields on B will interact differently simply because they support a different life system (also intelligent)
    Since the life experience (and the evolution of the laws of physics) on B will be so different from ours, we can imagine that their Einstein and General Relativity came about before Newton.

  9. If forces can be in a phase, then can they exhibit phase changes? Can electromagnetism “melt”?

  10. Sean,

    Thank you for your thoughts on electromagnetism. Light is especially appreciated in these dark days of COVID.

    Your words “… both matter and forces arise from the behavior of quantum fields interacting with one another” are reassuring for two reasons. First, they reinforce the notion that everything comes from quantum fields. Second, that such “emergence” is due to the interactions between quantum fields.

    The idea that these interactions can be treated as quantum measurements was noted in a John Preskill blog (https://quantumfrontiers.com/2013/06/07/entanglement-wormholes/comment-page-2/#comment-82042).

    Almost five years to the date, it was also pointed out that what one observes in a quantum measurement can depend on the tools used, or the circumstances of observation (http://www.preposterousuniverse.com/blog/2016/11/23/gifford-lectures-on-natural-theology/#comment-7295910552604313840).

    As the universe is made up mostly of dark energy and dark matter, we look forward to light being shed on these subjects in future Thanksgivings.

    KC

  11. Hi, you may want to replace “there” by “their” in
    “…but once you understand what stable nuclei there are, there work is essentially done,”
    after the first figure.

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