A post for general readers:
This is the first of several posts celebrating the hugely successful Standard Model of particle physics, the concepts and equations that describe the basic bricks and mortar of the universe. In these posts, I’ll explain (without assuming readers have a science background) how we know some of its most striking features. We’ll look at simple facts that particle physicists have learned over the decades, and use them to infer basic features of the universe and to recognize deep questions that still trouble the experts.
The Elementary “Forces” of Physics: A Classification of Nature
Perhaps you’ve heard it said that “There are four fundamental forces in nature.” Whether you have or not, today I’ll show you how to verify this yourself. (Actually, there are five forces, though we’ll only see a hint of the fifth today.) The force everybody knows from daily life is gravity; ironically, this force has no measurable impact on particle physics, so it’s the only one we won’t be looking at in this post.
I’d better emphasize, though, that the word “force” is slippery. Normally, in everyday life, a force means something that will push or pull objects around. But when physicists say “force,” they often mean something much more general. Because of that they sometimes use the word “interaction” instead of “force”.
For example, static electricity that holds socks together when they come out of the dryer is an example of an honest electromagnetic force — the socks really are pulled together. So is the force that pulls a magnet to a refrigerator door. But when a light bulb glows, that doesn’t involve a force in the limited sense of a push or pull. Yet it still involves the “electromagnetic interaction”, i.e. the “electromagnetic force” in a generalized sense. That’s because, although it is far from obvious, the emission (or absorption) of light involves the same basic phenomena that govern the force between the socks.
[Physicists use “electromagnetic” rather than “electric” or “magnetic” separately because these two forces are so deeply intertwined that it is often impossible to distinguish them.]
So when physicists say there are “four forces” (or five), they are imposing a classification scheme on the world around us. They mean:
- All fundamental physical processes in nature can be divided up into five classes.
- Each class involves one of the following types of interactions:
- gravitational (holds the planet together and holds us to the ground),
- electromagnetic (creates light, controls chemistry and biology, and dominates daily life),
- weak-nuclear (essential in stars and in supernova explosions),
- strong-nuclear (forms protons, neutrons, and their agglomerations in atomic nuclei),
- Higgs-related (associated with the masses of all known elementary particles).
There are currently no verified exceptions to this classification scheme. And by examining basic facts about the various particles found in nature, we can see these classes (other than gravity) in operation.
Particle Lifetimes and Masses: A Tool for Understanding
The protons and electrons we’re made of last forever, as far as we know, as do neutrons inside of stable atoms. But most particles that particle physicists make in experiments have a short “lifetime”; they “decay” away, transformed into other particles, in less than a second. Even a neutron, left on its own, has a lifetime; it decays, on average, about 15 minutes after it was created.
Each particle’s decay involves one or more of the universe’s “forces”, and by examining their lifetimes, we can observe these “forces” at work. The figure below illustrates this; each dot indicates the mass (on the vertical scale) and average lifetime (on the horizontal scale) of a type of particle. Some of the particles shown are elementary, while others are “composite” objects made from multiple elementary particles. Most of the composites are “hadrons” (examples of which are protons and neutrons) which are made of quarks and anti-quarks.
I know the font is small, but as you’ll see, we don’t need the details, just the overall patterns. If you’re interested in the details, please note:
- The masses are given in terms of “GeV/c2“. For scale, a single hydrogen atom has mass of roughly 1 GeV/c2, an electron has a mass of roughly 1/2000 GeV/c2, and the Higgs boson, the particle whose discovery we’re celebrating this week, has a mass of 125 GeV/c2.
- The lifetimes are given in seconds; they are generally very short! [I am using scientific notation on this axis: “104” means a 1 with 4 zeroes after it (10000); 10-6, means a number smaller than 1 that has 6 zeroes, 0.000001; 10-16 similarly requires 16 zeroes, etc.]
I’ve color-coded the particles in the following way:
- At center, the elementary particles muon (μ) and tau (τ), two heavier cousins of the electron, are shown as blue dots.
- At the upper left, with the largest masses and very short lifetimes, are four elementary particles: the top quark t (green), the W and Z bosons (light blue), and the Higgs boson H (red).
- At the bottom, in purple, is positronium (ee), an exotic atom made from an electron and a positron (the electron’s antiparticle); it plays a central role in PET medical scans.
- All of the other particles, shown in gray, black, and brown, are hadrons (particles made from quarks, antiquarks, and gluons) somewhat like the proton. Their names and the color scheme can be ignored for today.
These particles decay on time scales which vary widely, from trillionths of trillionths of seconds to millionths of a second, excepting the neutron (n) which looks quite unusual on this plot. Nevertheless, you can see that the dots aren’t randomly distributed. They cluster in interesting ways, and the question is: why?!
Learning From Particle Lifetimes
In the following figure, I’ve put some ovals around these clusters of dots, in order not only to draw your attention to the clustering but also to direct our thinking about their origins. In fact, each of these clusters reflects one of the forces of nature.
The one that’s most obvious is the cluster that lies along the blue dashed line. All of these particles are decaying by the weak nuclear “force”, in what we may call its “low-energy” or “virtual” manifestation, which we observe for particles of masses much below 50 GeV/c2. (Today I won’t discuss the origin of the blue dashed line, or why the particles lie along it; in my next post I’ll explain that it arises from so-called “virtual” W bosons.)
Why do we call this the “weak nuclear force”? It is weak in the following sense: the cluster on the blue dashed line lies far to the right of the red dotted line. The red dotted line shows the minimum lifetime that a particle of a certain mass can have and yet still be called a particle. A particle with a shorter lifetime would fall apart before it could even form, so it’s no surprise that we don’t see anything to the left of that line. But the particles on the blue dashed line have lifetimes far, far longer than this minimum. That’s because the “force” that is causing them to decay is relatively ineffective — i.e., weak!
(For an advanced article on the forces and their relative strengths, see here.)
With this in mind, our eyes are now drawn to the next obvious cluster, circled in purple, which in contrast to the previous cluster lies very close to the red dashed line. These particles have lifetimes that are nearly as short as they possibly can be, which indicates they decay through a strong force — the strong nuclear force!
Then there are a few particles scattered between these two clusters, which I’ve circled in orange. They don’t organize themselves as easily as the previous two clusters, but roughly speaking they all involve the electromagnetic “force”, which is intermediate in strength. A sign of this is that these particles, as they decay, emit one or more gamma rays, which are high-energy “photons”, i.e. particles of (an invisible form of) light.
Also there’s a cluster of green and blue dots up in the upper left: the top quark t and the W and Z bosons. These elementary particles also decay by the weak nuclear force, not by its virtual manifestation but directly. This “high-energy” version of the weak nuclear force is quite a bit stronger than the low-energy version, so the lifetimes are relatively shorter, and lie to the left of the blue dashed line, if you were to extend it that far. In fact, these particles’ decays lie relatively close to the red dotted line, which tells us that for particles with masses near and above 100 GeV/c2, the weak nuclear “force” isn’t weak after all!
There are still a couple of notable outliers. First, there’s the red dot up at the top, the Higgs boson, discovered in 2012; this week marks the 10th anniversary of that discovery. (However, we’ve been making these particles for 34 years; see yesterday’s post.) The Higgs boson’s decay doesn’t occur by any of the forces that I’ve mentioned so far; indeed it’s the only particles known that decays mainly through the Higgs “force”. One thing you can see is that this force is a little weaker than the direct weak nuclear force, because the Higgs boson lies well to the right of the top quark and W boson. But there’s more to that story, which I’ll tell you more about in a later post.
Let’s now look at the neutron, n; it’s way off on the right, with a lifetime more than a billion times longer than its nearest rival. But actually, it’s not as mysterious as it first appears. The neutron does have a large mass, but it always decays to a proton, along with an electron and an anti-nutrino. The proton’s mass is very close to the neutron’s mass; their masses differ by only a part in a thousand. The tiny gap between the neutron’s mass and the proton’s mass (plus the electron’s little mass) suppresses the neutron’s decay rate, and makes its lifetime a million billion times longer than you’d expect! (We’ll see why in a future post.)
We can account for this by plotting the neutron’s dot not at the neutron’s mass, 0.939 GeV/c2, but at the the mass equal to the gap between the neutron’s mass and the sum of the proton’s mass and the electron’s mass. That gap is only 0.0007 GeV/c2, and so we move the neutron’s dot down, following the green arrow, to the location of the star. The star actually lies quite close to the blue dashed line! Thus we learn that, despite initial appearances, the neutron decays via the same virtual weak nuclear “force” as all the particles in the blue cluster!
Unification of Forces?
So, as claimed, all known particle decays can be easily classified as due to one of the known “forces”. What else can we learn from this plot?
Isn’t it interesting that everything seems to be converging towards the upper left? The electromagnetic “force”, the weak nuclear “force”, and the strong nuclear “force” all seem to be approaching one another, near that red dotted line. Even the Higgs force isn’t that far away.
The fact that all of these lines are converging is the first evidence (quite thin to be sure, but provocative and interesting) for what is sometimes called “grand unification”, or perhaps, somewhat less grandly, “coupling-constant unification” [i.e. unification of the strengths of the forces.] The idea is that the forces shown on this plot may all be manifestations of just a single force. Their identity may only become obvious far above this plot, at much higher masses than those of a top quark or Higgs boson. This is a speculative idea that’s very popular, and you can see why a plot like this helps make it compelling. Unfortunately we don’t have any clear evidence for it yet.
However, if it were right, could we fit gravity in with it? Maybe; in some versions of string theory, for instance, that’s what happens. But that’s even more speculative, and we’re not going to go into the stratosphere today. We can discuss that another time.
Still, not everything about this convergence of lines is speculative. The coming together of the weak-nuclear and electromagnetic forces turns out to reflect how the Standard Model is put together! The key step in the Standard Model’s construction is that these two “forces” represent a reorganization of two even more elementary “forces,” of comparable strength, which are called SU(2)-weak and U(1)-hypercharge. The experimental evidence for this smaller-scale quasi-unification is overwhelming. Indeed this reorganization of the forces is the main role of the Higgs field, whose ripples are Higgs bosons. But that’s a long story, not for today. (Readers who are game for some math, albeit pre-university math, could tackle my recent series explaining some of the details, both in the Standard Model and in an extension of it that could potentially explain a possible small shift in the W boson mass.)
So there you have it. Just by staring at the properties of the known particles, you can see for yourself what physicists and science writers always claim: that as far as we know today, there are four [well…, five] elementary “forces” of nature. And you can also see why scientists speculate about the possibility that these forces may, someday, prove to be fewer, and perhaps just one.