May the weak force be with you

I follow science news the way normal dudes follow sports. If you’re geekily inclined like me, you may have heard that the particle physics people are getting closer to producing the Higgs boson. You may have wondered what that is exactly, and why you should care. The science press has nicknamed the Higgs “the God particle,” which is poetic but doesn’t move me any closer to understanding. Here’s my best effort to wrap my head around the idea — maybe you’ll find it helpful, or at least entertaining. If you’re a real scientist and want to clarify or correct anything I’m saying here, please jump in on the comments.

Okay. So. The Higgs boson is involved in the weak nuclear force, which is the weirdest and least famous of the basic four forces in the standard model of particle physics. I have a pretty firm handle on the other three. There’s gravity, you know what that is. There’s electromagnetism, which is behind pretty much everything in our direct experience aside from gravity. And there’s the strong force, which sticks quarks together into protons and neutrons, and sticks those together into atomic nuclei.

But then the weak force. This one is harder to nail down in a single pithy sentence. When you get to the section on the weak force in most popularly-oriented physics texts, the language becomes vague and evasive. There’s usually a feeble bit about how the weak force “is involved in certain kinds of radioactive decay.” This is true, but it doesn’t begin to tell the story. What the weak force really does is transform one kind of particle into another.

Here’s the weak force at work, transforming a neutron into a proton.

This might be happening inside an unstable atom, like nitrogen-16 (nitrogen with too many neutrons.) Time goes from bottom to top, as indicated by the arrow. At the bottom is the neutron, with its three quarks: up, down and down. As the nucleus wobbles, it’s possible for its components to come within the weak force’s extremely short range. A weak force carrier particle, represented by the wavy line labeled W-, changes the flavor of one quark from down to up. That changes the neutron into a proton, and in the process spits out an electron and an antineutrino. With its new proton, the former nitrogen atom is now oxygen. Shazam!

Weak interactions are part of the fusion reactions powering the sun. They’re the reverse of the one pictured above, since the sun turns protons into neutrons, spitting out positrons and neutrinos. Weak interactions produce some of the heat coming from the Earth’s core, and there are more weak interactions in the upper atmosphere as high-energy particles slam into air molecules. Aside from nuclear reactors and particle accelerators, there isn’t too much weak force happening in our day to day lives. Your best chance to experience the weak force first hand is in a hospital PET scanner. Anna asks, looking over my shoulder, if it scans to see if you have any pets. Um, no.

Before you lie down in the scanner, you get injected with a radiotracer containing unstable isotopes of oxygen, nitrogen or some other biologically-oriented element. These isotopes come attached to glucose or water molecules. You lie in the scanner, and as your body metabolizes the radiotracers, weak interactions emit positrons (thus the term Positron Emission Tomography.) The positrons are antimatter, and they don’t get far before they smack into electrons, mutually annihilating into pairs of high-energy photons that zip away in exactly opposite directions. The scanner ring registers all the photons hitting it, and devotes mammoth amounts of computer power to ignoring all of them except the ones originating on opposite sides of the ring. From there, the computer can deduct where the photons are originating, and voila, you get a 3D animated picture of your metabolism in action.

There are a couple of weird and cool things about the weak force. One is that it has a preferred handedness. All particles spin, either clockwise or counterclockwise. Physicists call these spin directions right-handed and left-handed. If you think of your thumb as your spin axis, then your fingers curl in the direction of spin. The weak force defies common sense by only acting on left-handed particles (and right-handed antiparticles.) This is a startlingly odd asymmetry. Gravity and electromagnetism act the same on left and right-handed particles and there’s no obvious reason why the other forces shouldn’t behave the same way. Could this asymmetry could be related to the slight imbalance between matter and antimatter produced by the big bang? Is it necessary for a universe that’s hospitable to our existence? What do you say, scientists?

Another weird and cool thing about the weak force is its relationship to electromagnetism. They turn out to be different aspects of the same force. This is where the Higgs boson comes in. It’s the particle aspect of the Higgs field, which is thought to pervade all of space. The standard model of particle physics credits this invisible energy field with giving the various force-carrying particles their various masses. The idea is that the Higgs field gives the universe a weak force charge, the way electric charge pervades a cloud before a thunderstorm. The weak force has such a short range because the W and Z particles that convey it drag against the Higgs field and quickly lose their juice. Photons have an infinite range because they don’t interact weakly, so they pass right through the Higgs field.

So here’s the thing. At very high temperatures, the Higgs field evaporates. W and Z particles no longer drag, so they become massless and indistinguishable from photons. In other words, electromagnetism and the weak force reveal themselves to be different aspects of the same force, the electroweak force. The standard model predicts this, and experiments have confirmed it. This fact gives physicists hope that at higher energy levels still, the electroweak force will unify with the strong force, and at even more ridiculously high energy levels, they’ll all unify with gravity. Sadly, testing the unification of the electroweak and strong forces is very far out of our technological reach. Testing to see if the other forces unify with gravity would require a particle accelerator bigger than the solar system. That hasn’t stop physicists from dreaming of finding a single unified theory of the forces, a mathematical description of the entire universe that would fit on a t-shirt. Einstein spent decades of his life searching for such a theory, without success.

The Higgs boson is the particle form of the Higgs field, the way the photon is the particle form of the electromagnetic field, and the electron is the particle form of the lepton field. The Higgs boson has never been spotted. It’s assumed that it takes more energy to produce Higgs bosons than particle accelerators have been able to bring to bear. A major mission of the Large Hadron Collider is to produce Higgs bosons.

Here’s what Higgs boson production would look like on the LHC computers:

And so who is this Higgs guy? Here’s what he looks like.

He was one of the major movers behind the electroweak theory and is presumably twiddling his thumbs in a university office somewhere, waiting to see if they do indeed find his particle. I’m rooting for him.

6 thoughts on “May the weak force be with you

  1. Thanks for your write up on the weak force.

    Everything I’ve read on the weak force suggests that it is not technically a force in the sense that it doesn’t produce a change in momentum over time. If it really were a force (rather than a transformative interaction), I ought to be able to find an equation for it of the form: F_weak = dp/dp.

    I realize that the particles produced by beta decay experience a change in momentum after they are created, but it seems that the weak interaction creates the particles, and other forces accelerate them.

    If you can shed any light on this I would greatly appreciate it.

    BTW, gravity is not considered a force in general relativity.

  2. Found a thesis on beta decay, which seems to say that there is a “change in momentum for the [beta decay] transition”:

    “The total spin of the beta-neutrino system is coupled to the intrinsic spin operator ?, and ? represents the rank of the spherical tensor operator produced from this coupling indicating the total change in momentum for the transition.”

    page 15, right before equation 6:
    http://astro.uconn.edu/Wilds_Thesis.pdf

    This discussion also seems to say that weak interactions involve a change in momentum:

    “The Z0 boson is especially interesting for astronomers, because it is produced in the so-called neutral-current reactions, which involve the scattering of neutrinos on nucleons or electrons; for example,

    n + v = n’ + Z0 + v = n’ + v’ (2.3)

    where the primes are used to indicate a change in momentum and energy…”

    http://nedwww.ipac.caltech.edu/level5/Zeldovich/Zel2.html

  3. Hey Forrest, thanks for reading and engaging. I wish I could answer your questions, or even really understand them, but the sum total of my knowledge of the subject is represented in the post. If there are any scientists reading who want to help this man out, please do.

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