atoms

Celebrating the Standard Model: Atoms, Quarks and the Strong Nuclear Force

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For the general reader:

Last week I showed you, without any technicalities, how to recognize the elementary forces of nature in the pattern of particle masses and lifetimes. This week we’ll start seeing what we can extract just from the particles’ masses alone… and what we cannot. Today we’ll focus on quarks and the strong nuclear force.

A key factor in nature, which plays an enormous role in everyday life, is the mass of a typical atom. [Note: on this website, “mass” always means “rest mass”, which does not increase with a particle’s speed.] This in turn arises mainly from the masses of protons and neutrons, which are about equal, and tiny: about 0.00000000000000000000000000167 kg (or 0.00000000000000000000000000368 pounds). Since those numbers are crazy-small, physicists use a different measure; we say the mass is about 1 GeV/c2, and more precisely, 0.938 GeV/c2. In any case, it’s tiny on human scales, but it’s some definite quantity, the same for every proton in nature. Where does this mass come from; what natural processes determine it?

You may have heard the simplistic remark that “a proton is made of three quarks” (two up quarks and a down quark), which would suggest these quarks have mass of about 1/3 of a proton, or about 0.313 GeV/c2. But something’s clearly amiss. If you look at websites and other sources about particle physics, they all agree that up and down quark masses are less than 0.01 GeV/c2; these days they usually say the up quark has mass of 0.002 GeV/c2 and the down quark has 0.005 GeV/c2. So if the proton were simply made of three quarks, it would naively have a mass of less than 1% of its actual mass.

What’s going on? A first little clue is that different sources sometimes quote different numbers for the quark masses. There are six types of quarks; from smallest mass to largest, they are up, down, strange (u,d,s, the three light quarks), charm, bottom (c,b, the two somewhat heavy quarks) and top (t, the super-heavy quark.) [Their names, by the way, are historical accidents and don’t mean anything.] But some websites say the up quark mass is 0.003 instead of 0.002 GeV/c2, a 50% discrepancy; the bottom quark’s mass is variously listed as 4.1 GeV/c2, 4.5 GeV/c2, and so forth. This is in contrast to, say, the electron’s mass; you’ll never see websites that disagree about that.

The origin of all these discrepancies is that quarks (and anti-quarks and gluons) are affected by the strong nuclear force, unlike electrons, Higgs bosons, and all the other known elementary particles. The strong forces that quarks undergo make everything about them less clear and certain. Among numerous manifestations, the most dramatic is that quarks (and anti-quarks and gluons) are never observed in isolation. Instead they’re always found in special combinations, called “hadrons“. A proton is an example, but there are many more. And the strong nuclear force can have a big effect on their masses.

The Modern Proton and the Masses of Quarks

A proton, in fact, is not simply made from three quarks, the way a hydrogen atom is simply made from a proton and an electron. As I described in this article, it’s vastly more complex; it’s made from three quarks plus lots of gluons plus lots of pairs of other quarks and anti-quarks. So the simple intuition we get from atoms does not apply to hadrons like the proton.

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The Size of an Atom: How Scientists First Guessed It’s About Quantum Physics

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Atoms are all about a tenth of a billionth of a meter wide (give or take a factor of 2). What determines an atom’s size? This was on the minds of scientists at the turn of the 20th century. The particle called the “electron” had been discovered, but the rest of an atom was a mystery. Today we’ll look at how scientists realized that quantum physics, an idea which was still very new, plays a central role. (They did this using one of their favorite strategies: “dimensional analysis”, which I described in a recent post.)

Since atoms are electrically neutral, the small and negatively charged electrons in an atom had to be accompanied by something with the same amount of positive charge — what we now call “the nucleus”. Among many imagined visions for what atoms might be like was the 1904 model of J.J. Thompson, in which he imagined the electrons are embedded within a positively-charged sphere the size of the whole atom.

But Thompson’s former student Ernest Rutherford gradually disproved this model in 1909-1911, through experiments that showed the nucleus is tens of thousands of times smaller (in radius) than an atom, despite having most of the atom’s mass.

Once you know that electrons and atomic nuclei are both tiny, there’s an obvious question: why is an atom so much larger than either one? Here’s the logical problem”

  • Negatively charged particles attract positively charged ones. If the nucleus is smaller than the atom, why don’t the electrons find themselves pulled inward, thus shrinking the atom down to the size of that nucleus?
  • Well, the Sun and planets are tiny compared to the solar system as a whole, and gravity is an attractive force. Why aren’t the planets pulled into the Sun? It’s because they’re moving, in orbit. So perhaps the electrons are in orbit around the nucleus, much as planets orbit a star?
  • This analogy doesn’t work. Unlike planets, electrons orbiting a nucleus would be expected to emit ample electromagnetic waves (i.e. light, both visible and invisible), and thereby lose so much energy that they’d spiral into the nucleus in a fraction of a second.

(These statements about the radiated waves from planets and electrons can be understood with very little work, using — you guessed it — dimensional analysis! Maybe I’ll show you that in the comments if I have time.)

So there’s a fundamental problem here.

  • The tiny nucleus, with most of the atom’s mass, must be sitting in the middle of the atom.
  • If the tiny electrons aren’t moving around, they’ll just fall straight into the nucleus.
  • If they are moving around, they’ll radiate light and quickly spiral into the nucleus.

Either way, this would lead us to expect

  • Rnucleus = # Ratom

where # is not too, too far from 1. (This is the most naive of all dimensional analysis arguments: two radii in the same physical system shouldn’t be that different.) This is in contradiction to experiment, which tells us that # is about 1/100,000! So it seems dimensional analysis has failed.

Or is it we who have failed? Are we missing something, which, once included, will restore our confidence in dimensional analysis?

We are missing quantum physics, and in particular Planck’s constant h. When we include h into our dimensional analysis, a new possible size appears in our equations, and this sets the size of an atom. Details below.

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Auroras — Quantum Physics in the Sky — Tonight?

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Maybe. If we collectively, and you personally, are lucky, then maybe you might see auroras — quantum physics in the sky — tonight.

Before I tell you about the science, I’m going to tell you where to get accurate information, and where not to get it; and then I’m going to give you a rough idea of what auroras are. It will be rough because it’s complicated and it would take more time than I have today, and it also will be rough because auroras are still only partly understood.

Bad Information

First though — as usual, do NOT get your information from the mainstream media, or even the media that ought to be scientifically literate but isn’t. I’ve seen a ton of misinformation already about timing, location, and where to look. For instance, here’s a map from AccuWeather, telling you who is likely to be able to see the auroras.

Don't believe this map by AccuWeather. Oh, sure, they know something about clouds. But auroras, not much.
Don’t believe this map by AccuWeather. Oh, sure, they know something about clouds. But auroras, not much.

See that line below which it says “not visible”? This implies that there’s a nice sharp geographical line between those who can’t possibly see it and those who will definitely see it if the sky is clear. Nothing could be further than the truth. No one knows where that line will lie tonight, and besides, it won’t be a nice smooth curve. There could be auroras visible in New Mexico, and none in Maine… not because it’s cloudy, but because the start time of the aurora can’t be predicted, and because its strength and location will change over time. If you’re north of that line, you may see nothing, and if you’re south of it you still might see something.  (Accuweather also says that you’ll see it first in the northeast and then in the midwest.  Not necessarily.  It may become visible across the U.S. all at the same time.  Or it may be seen out west but not in the east, or vice versa.)

Auroras aren’t like solar or lunar eclipses, absolutely predictable as to when they’ll happen and who can see them. They aren’t even like comets, which behave unpredictably but at least have predictable orbits. (Remember Comet ISON? It arrived exactly when expected, but evaporated and disintegrated under the Sun’s intense stare.) Auroras are more like weather — and predictions of auroras are more like predictions of rain, only in some ways worse. An aurora is a dynamic, ever-changing phenomenon, and to predict where and when it can be seen is not much more than educated guesswork. No prediction of an aurora sighting is EVER a guarantee. Nor is the absence of an aurora prediction a guarantee one can’t be seen; occasionally they appear unexpectedly.  That said, the best chance of seeing one further away from the poles than usual is a couple of days after a major solar flare — and we had one a couple of days ago.

Good Information and How to Use it

If you want accurate information about auroras, you want to get it from the Space Weather Prediction Center, click here for their main webpage. Look at the colorful graph on the lower left of that webpage, the “Satellite Environment Plot”. Here’s an example of that plot taken from earlier today:

The "Satellite Environment Plot" from earlier today; focus your attention on the two lower charts, the one with the red and blue wiggly lines (GOES Hp) and on the one with the bars (Kp Index). How to use them is explained in the text.
The “Satellite Environment Plot” from earlier today; focus your attention on the two lower charts, the one with the red and blue wiggly lines (GOES Hp) and on the one with the bars (Kp Index). How to use them is explained in the text.

There’s a LOT of data on that plot, but for lack of time let me cut to the chase. The most important information is on the bottom two charts.

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Breaking News: Two Great New Measurements

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Two new ground-breaking measurements reported results in the last 24 hours!  Here are very quick summaries. A group of atomic physicists, called the ACME collaboration, has performed the best search so far for the electric dipole moment (EDM) of the electron.  Unfortunately they didn’t find the EDM, but the limit |de| < 8.7  10-29 e … Read more

A Short Break

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Personal and professional activities require me to take a short break from posting.  But I hope, whether you’re a novice with no knowledge of physics, or you’re a current, former, or soon-to-be scientist or engineer, or you’re somewhere between, that you can find plenty of articles of interest to you on this site.  A couple … Read more

What is the “Strength” of a Force?

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Particle physicists, cataloging the fundamental forces of nature, have named two of them the strong nuclear force and the weak nuclear force. [A force is simply any phenomenon that pushes or pulls on objects.] More generally they talk about strong and weak forces, speaking of electromagnetism as rather weak and gravity as extremely weak.  What do the words “strong” and “weak” mean here?  Don’t electric forces become strong at short distances? Isn’t gravity a pretty strong force, given that it makes it hard to lift a bar of gold?

Well, these words don’t mean what you think.  Yes, the electric force between two electrons becomes stronger (in absolute terms) as you bring them closer together; the force grows as one over the square of the distance between them.  Yet physicists, when speaking their own language to each other, will view this behavior as what is expected of a typical force, and so will say that “electromagnetism’s strength is unchanging with distance — and it is rather weak at all distances.

And the strength of gravity between the Earth and a bar of gold isn’t relevant either; physicists are interested in the strength of forces between individual elementary (or at least small) particles, not between large objects containing enormous numbers of particles.

Clearly there is a language difference here… as is often the case with words in English and words in Physics-ese.  It requires translation.  So I have now written an article explaining the language of “strong” and “weak” forces used by particle physicists, describing how it works, why it is useful, and what it teaches us about the known forces: gravity, electromagnetism, the strong nuclear force, the weak nuclear force, and the (still unobserved but surely present) Higgs force.

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Why Do Protons and Neutrons Form Nuclei, and Why Are The Nuclei So Small?

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The Structure of Matter series continues: last week’s article on the basics of atomic nuclei is now supplemented with an article discussing the “residual” strong nuclear force which binds protons and neutrons inside of nuclei.  It further explains why nuclei are so small compared to atoms.  Or rather, it explains it in part, because I … Read more

Article on Atomic Nuclei

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Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week. Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to … Read more

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