What Counts as a Fundamental Force?

I’m giving a presentation next Wednesday for Learning Unlimited, an organization that presents educational talks to seniors in Woodstock, Ontario. The talk introduces the fundamental forces and talks about Yang and Mills before moving on to introduce my work.

While practicing the talk today, someone from Perimeter’s outreach department pointed out a rather surprising missing element: I never mention gravity!

Most people know that there are four fundamental forces of nature. There’s Electromagnetism, there’s Gravity, there’s the Weak Nuclear Force, and there’s the Strong Nuclear Force.

Listed here by their most significant uses.

What ties these things together, though? What makes them all “fundamental forces”?

Mathematically, gravity is the odd one out here. Electromagnetism, the Weak Force, and the Strong Force all share a common description: they’re Yang-Mills forces. Gravity isn’t. While you can sort of think of it as a Yang-Mills force “squared”, it’s quite a bit more complicated than the Yang-Mills forces.

You might be objecting that the common trait of the fundamental forces is obvious: they’re forces! And indeed, you can write down a force law for gravity, and a force law for E&M, and umm…

[Mumble Mumble]

Ok, it’s not quite as bad as xkcd would have us believe. You can actually write down a force law for the weak force, if you really want to, and it’s at least sort of possible to talk about the force exerted by the strong interaction.

All that said, though, why are we thinking about this in terms of forces? Forces are a concept from classical mechanics. For a beginning physics student, they come up again and again, in free-body diagram after free-body diagram. But by the time a student learns quantum mechanics, and quantum field theory, they’ve already learned other ways of framing things where forces aren’t mentioned at all. So while forces are kind of familiar to people starting out, they don’t really match onto anything that most quantum field theorists work with, and it’s a bit weird to classify things that only really appear in quantum field theory (the Weak Nuclear Force, the Strong Nuclear Force) based on whether or not they’re forces.

Isn’t there some connection, though? After all, gravity, electromagnetism, the strong force, and the weak force may be different mathematically, but at least they all involve bosons.

Well, yes. And so does the Higgs.

The Higgs is usually left out of listings of the fundamental forces, because it’s not really a “force”. It doesn’t have a direction, instead it works equally at every point in space. But if you include spin 2 gravity and spin 1 Yang-Mills forces, why not also include the spin 0 Higgs?

Well, if you’re doing that, why not include fermions as well? People often think of fermions as “matter” and bosons as “energy”, but in fact both have energy, and neither is made of it. Electrons and quarks are just as fundamental as photons and gluons and gravitons, just as central a part of how the universe works.

I’m still trying to decide whether my presentation about Yang-Mills forces should also include gravity. On the one hand, it would make everything more familiar. On the other…pretty much this entire post.

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7 thoughts on “What Counts as a Fundamental Force?

  1. Zim the Fox

    I find the alt-text of the XKCD comic to be even more relevant than the comic itself:

    “Of these four forces, there’s one we don’t really understand.” “Is it the weak force or the strong–” “It’s gravity.”

    Neat article, by the way :3

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  2. ohwilleke

    Teach the controversy! Seriously, in the case of gravity, the choice between efforts to conceptualize gravity as a force mediated by gravitons, in contrast to efforts to conceptualize gravity as geometry, is a cutting edge theoretical issue that at least deserves mention, and it really isn’t inappropriate at all to say that the Higgs field really is a fifth force if one wants to be conceptually coherent. Contrasting the Higgs field which acts on the rest mass of fundamental particles with gravity which couples to all matter-energy, is also a useful point to elucidate.

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      1. clumma

        Howabout this:

        gravity: falling, tides, seasons
        electromagnetism: fire, electricity, chemistry
        strong nuclear force: the sun, nuclear power, nuclear weapons
        weak nuclear force: volcanoes, hot springs, carbon dating

        The weak force also prevents Earth from becoming like Mars.
        (OK, only ~ 80% of geothermal energy is radiogenic, and not all of that is due to the weak force.)

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        1. 4gravitonsandagradstudent Post author

          That’s workable. Most people aren’t familiar with the idea that radiation contributes to geothermal energy, though, so it would take some unpacking for most audiences. Still, if all you want is a summary that makes every force sound fundamental with macroscopic examples, that’s a good one.

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