I take Isaac Asimov's book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science when he wrote the book (1959 - 60). In these posts I am reviewing what he reported then noting what's changed since. For this post I am starting with the section he titled "Strange Particles". I will then move on to a section he titled "Inside the Nucleus". Both are from the chapter he titled "The Particles".
Nuclear Physics was in a state of chaos at the time he wrote the book. Scientists knew something was wrong but they didn't know what it was. Asimov includes a table in this section that lists and gives a few of the properties of 27 "subatomic particles". In fact, at this time or shortly thereafter, there were more than a hundred "subatomic particles" that had been identified and characterized. Given that there were only 92 "naturally occurring" elements, this was ridiculous and everybody knew it. Before going into the "fix" let me review what Asimov had to say.
Antiparticles like the Positron, which figures in Asimov's extremely popular "Robot" Science Fiction series under the guise of the "positronic brain", and the anti-Proton had been discovered. This was annoying but did not disturb particle physicists much. The idea of a particle - anti-particle duality seemed interesting but did not "threaten the very foundations of particle physics". So, okay, it doubled (roughly) the number of subatomic particles. That wasn't seen as any kind of big problem.
And the Neutron, the "neutral Proton", was another discovery that was disconcerting but only mildly so. What did concern them were the "mass mysteries". It seemed reasonable that if a nucleus emitted or absorbed a particle then the combined mass/energy of everything at the beginning should match the combined mass/energy at the end. But it didn't. There was the whole "E equals M C squared" thing. It could be used in circumstances where mass got turned into energy or vice versa.
"X" quantity of mass was equivalent to "Y" quantity of energy. Physicists got into the habit of converting everything into its energy equivalent. If the "energy" going in was the same as the "energy" coming out, problem solved. If, for instance, a photon of the required energy was either emitted or absorbed as part of the nuclear reaction, then that contribution to total mass/energy could be figured in. But in a lot of cases, even these kinds of adjustments did not cause the "mass/energy equation" to balance.
One pleasant side effect was that nuclear physicists could go hunting around for a heretofore unknown particle to put things back in balance. That turned out to be an extremely fruitful strategy. This is how the neutrino was first predicted, then named (by Enrico Fermi), then discovered. The neutrino solved the mass balance problem for a common nuclear reaction. The "predict" and "name" step often got combined. So it soon became common to see a "predict/name" event be followed by a "discover" event.
And that led to a game. The "mass balance" for a particular nuclear interaction would fail to balance. That gave you the mass/energy of a new particle. And subatomic particles have other properties like charge and spin. So you looked for imequalities in those other particle attributes. It soon became a game of nuclear "Clue". The new particle, rather than being identified as Colonel Mustard in the Library with the Candlestick became a particle with a mass of "X" and a spin of "Y" and an electric charge of "Z". Whoever got there first got to slap a name on that particular configuration.
But it soon transpired that there were three "flavors" of neutrino, the "tau", the "mu", and the "electron". And there was an anti-neutrino. So, all told, six subatomic particles were eventually added to the list by the time the dust settled completely. (And the "mu" neutrino's name was changed to "muon" neutrino for obscure reasons.)
A fruitful environment in which to play the "find the new subatomic particle" game grew out of an attempt to figure out what was gluing the nucleus of atoms together. Heisenberg, started this investigation in 1932, Asimov notes. He hypothesized that "exchange forces" would come into play as particles continuously switched identities with other particles in the nucleus. It quickly became apparent that these exchange forces could be modeled as particles shuttling back and forth. And if it's a particle, we can play nuclear Clue and come up with a list of attributes. And at that point we can start looking for proof of existence.
Evidence was shortly found for a "Meson" that seemed to fit the bill. But things did not exactly work out. That led to the original meson being renamed to the "mu meson" (and eventually the "muon" meson). A new particle, named the "pi meson" got added to the list of subatomic particles. It fit the original requirements better than the mu meson. But there were still missing pieces of the puzzle. I'll get back to that in a minute.
An interesting side effect of this investigation that Asimov notes, is that it hinted that Protons and Neutrons were not simple indivisible particles. It looked like they had internal structure. This hint was still hanging out there tantalizingly when Asimov completed his book. Nuclear physicists didn't know what to do with it.
Anyhow, it turned out to be fairly easy to come up with a nuclear physics experiment that would turn up something puzzling. And the solutions to all of these puzzles were more and more subatomic particles. This led shortly to the "K-mason". And now there were three, and we are only talking about mesons. Something called "hyperons" was quickly added. And then "lambda particles" and "sigma particles" and "xi particles". And some of these were actually closely related families of particles in the mode of neutrinos. And, of course, you had particles and anti-particles. It seemed to go on and on.
Quoting Asimov:
All in all, nuclear physics is currently a wonderland -- or a jungle, if you prefer -- awaiting further exploration. At the latest count there are some 29 or 30 particles, detected or predicted, and no one could say that this was the end.And bear in mind, that this "29 or 30" figure actually collapses many particle/anti-particle pairs or families of similar particles into a single entry. At the time Asimov wrote the book particle physicists were pretty sure they were missing a fundamental insight. They just didn't know what it was.
The missing insight was the "Quark". Things continued to go from bad to worse. Then some people started doing the same thing Mendeleev had done when he organized the first periodic table of the elements. By arranging subatomic particles appropriately then noting regularities in the progression of properties the idea of sub-sub-atomic particles emerged. A short list of Quarks could be combined in various ways to produce combinations that matched many of the particles in the list.
Particles like the Proton and Neutron were composed of three Quarks. The Mesons were composed of two Quarks. And so on. Going hand in hand with this idea was the idea of "Gluons", exchange particles that moved from particle to particle to "exchange" some attribute like mass or charge or spin or whatever. Listing all the combinations of how three (or two) Quarks could be combined reproduced many of the particles in the list. Organizing Gluons along similar lines resulted in a nice neat list that was based on a list of sub-components now of reasonable and manageable length.
And in a manner similar to the periodic table of elements, holes in the pattern were detected. That led to a hunt for particles with the appropriate attributes. They were all eventually found. The last to be found was something called the "Higgs Boson". The LHC at CERN was built specifically to find the Higgs Boson and it was a big deal when it was found.
And with its discovery, one of the classic doors to new discovery was closed. The "Standard Model", the modern model of how nuclear physics works, has no holes needing to be filled. This is driving the current generation of particle physicists nuts. New physics is what makes working in the field interesting and exciting. With no holes in the list, physicists don't know where to go to look for new physics.
When Asimov was writing, the modern conception of four fundamental forces was just coming into focus. They are Gravity, Electromagnetism (what many people think of as two forces, electricity and magnetism, are actually one force that manifests itself in two apparently dissimilar ways), and what we now call the "Strong Nuclear Force" and the "Weak Nuclear Force".
The idea that there were two "Nuclear" forces was a new one at the time of the book's writing. Of the two, the Weak force was by far the least weird. The Weak force is what causes nuclear decay. The Strong force is what binds the nucleus of atoms together.
What makes the Strong force weird is that it gets stronger as you move particles away from each other. If you move two electrons further apart their electrical charges repel each other more weakly. If you move two masses farther apart they gravitationally attract each other more weakly. This "it gets weaker as distance increases" behavior is what we commonly associate with forces. But the Strong force gets stronger the further apart you move two particles subject to its influence.
It is now easy to see how this keeps atomic nucleuses together. If you want to inflate the size of the nucleus you have to add energy. And the farther you inflate it the more energy it takes to inflate it just a little more. Beyond a certain distance the amount of energy to inflate it a little more goes up literally astronomically.
If you have everything else you need then you can make a Quark if you have enough energy laying around. So, if you pull two particles that are subject to the Strong force apart far enough, there is enough "Strong Force" energy available to create an additional Quark. And if you do that (create a third Quark between the first two) then the now shorter distances between the various particles reduces the "Strong Force" energy of the system as a whole by enough to balance off the energy you used to create the Quark.
So if you inject enough energy to almost smash things apart what you end up doing is creating more Quarks instead. So you end up with a nucleus that is again stable. It just has an extra Quark or two in it. This goes a long way to explaining why many nucleuses are stable in spite of the fact that each Proton is mightily resisting being in close proximity to other Protons a short distance away.
What's going on with this "inverse distance" thing with the Strong force? I don't know. Physicists don't either. But the whole Quantum Mechanics thing that happened several decades earlier caused a sea change in the way nuclear physicists think. They now focus on figuring out what the rules are. They have stopped trying to figure out why the rules are the way they are. At this point, you can't blame them. This stuff is weird beyond imagination.
Asimov then talks about "strange particles". He is talking about interactions (and the Gluons that "mediate" those interactions) that are associated with the Weak force. That usage has gone out of fashion. There is now a "Strange" Quark and to continue to use the "strange particles" construction would invite confusion.
Instead, all of the mediating particles are now classed as Mesons. There are something like 150 of them. This large number stems from the fact that the number of different environments in which an interaction can take place is quite large. But, in a lot of cases it is more a matter of degree than it is a matter of something truly different going on. So, physicists consider many of these "particles" as variations on a theme. They don't worry all that much if the list is complete and correct.
Asimov concludes this section with a discussion of "Parity". What's Parity? It's complicated so I am not even going to bother. What is important is that, just like mass and spin and charge, it is another attribute that is supposed to be "conserved" in nuclear interactions.
The net Parity of the result was supposed to be the same as the net Parity of the starting components. But some early experiments suggested that this was not true. Shortly before the book was written, the definitive experiment was done that proved that Parity conservation does not occur in some circumstances. That's where things sat when Asimov finished his book.
Since then, a ton of research, both theoretical and experimental, has been done around what is now referred to as the "Charge-Parity violation" problem. It is often short handed as "CP violation". I am not going to dive into the subject because frankly I don't understand it all that well. Let's just say it is a very active area of current investigation and leave it at that. And with that, we can move on to "Inside the Nucleus".
The simplest model for the shape of the nucleus is that of a sphere. The best experiments done then, and experiments that have been done since, indicated that's about right. There is a slight deviation from purely spherical but it is small. Another idea that had been developed by the time Asimov was writing was the "liquid-drop" model of the nucleus. This is important if you trying to figure out what will happen if you hit the nucleus with something.
If you hit a drop of water with a tiny piece of sand, what happens? Well, if it is going very slowly it is likely absorbed by the drop. If it is going very fast, then it tends to drill straight through and leave without having changed the drop much. But if it is going at an intermediate speed it may smash the drop into smithereens. If you think about it, this kind of understanding is important if you are trying to make a nuclear explosion.
You need to figure out what will happen if you throw a neutron at the nucleus of a Uranium atom. Fast neutrons tend to bore through and leave an intact nucleus behind. Not good. Slow neutrons tend to be absorbed and also not change things much. But neutrons hitting at the right speed will smash the Uranium nucleus to pieces.
A "smashed to pieces" Uranium nucleus releases a lot of energy. It also, if you are lucky, flings out some more neutrons that can, if they end up moving the right speed, smash more Uranium nucleuses apart. A lot of atomic bomb design issues center around ways to get lots of neutrons of the right speed flying around in such a way that they would smash other Uranium nucleuses to pieces.
If you know how to do this, call Los Alamos. They have a job waiting for you. Of course, even if Asimov knew about the tie between the liquid-drop model and atomic bomb research, he wouldn't have been allowed to say anything. But over time this sort of thing eventually dribbles out. Designing an atomic bomb is complex. But a lot of the "how to" is now out there if you are smart and know where to look.
We mostly now depend on the fact that highly enriched Uranium is hard to come by. With "natural" Uranium, the kind found in the ground, the ratio between the U-235 and the U-238 isotopes is about 0.7% U-235 and about 99% U-238. With "bomb grade" Uranium the U-235 ratio has been "enriched" to the point where it constitutes about 90% of the total. Fortunately, the physical process of Uranium enrichment is very hard to do. That's true even though learning "how to" is again easy to do. Hint: You need specially designed "bleeding edge" centrifuges and you need a lot of them.
Asimov dishes out more information that would be helpful to a bomb builder without quite saying that's what he is up to. In general, his observation that large nucleuses, like that of Uranium and the trans-uranic elements (those with atomic numbers higher than Uranium's 92) tend to be "unstable", is correct. They can be broken into smaller pieces fairly easily so they make good fuel for nuclear weapons.
Asimov also discusses the "shell" model of the nucleus. This is where "nucleons" (Protons and Neutrons) are arranged in concentric shells. More powerful colliders have allowed physicists to get a clearer picture of what the situation is than what was known in Asimov's day. The shell model seems to be generally accurate.
Of course, things are complicated by the fact that nucleons are constructed from Quarks. So there are about three times as many particles rattling around in the nucleus than was thought in Asimov's time. Worse yet, the weird way the Strong Force works makes things devilishly complex.
It takes really smart people to figure out what's going on even with modern supercomputers and much more powerful colliders. I'm not going to go into it further except to note something Asimov observes. There are certain "magic numbers".
In the same way that electrons are organized in shells in the periodic table of elements, nucleons (or Quarks or whatever) seem to be organized into shells. That results in configurations containing certain numbers of nucleons tending to result in nucleuses that are more stable (less radioactive). That means they tend to stick around longer before they decay. They have a longer half life.
This idea is now called "islands of stability". The highest atomic number of an element that has been created so far is 118. If you subscribe to this theory then it suggests that specific elements with higher atomic numbers might have relatively long half lives if you can arrange for them to have the right number of nucleons. And that means that it might be possible to manufacture them and then have them stick around along enough to be detected. The theory says this can be done but no one has actually done it yet. And that's where I am going to leave this installment.
No comments:
Post a Comment