Everything seemed to be falling into place . . . except that nature would not let physicists off so easily. Experiments performed over the next decade demonstrated the somewhat strange behavior of this new particle, at the time called the “mesatron.” (The term Yukon, after Yukawa, was briefly considered but quickly abandoned as too frivolous.) Yukawa’s strong nuclear force carriers should interact strongly with nuclei, and it was therefore predicted that the negatively charged mesatron should be captured by the positively charged nuclei in matter well before it could itself decay into lighter particles such as electrons and neutrinos. By 1947 it was clear that this particle interacted millions of times less strongly than these predictions suggested it should.
Instead, it turned out that this new particle, now renamed a “muon,” behaved exactly like an electron, except it was two hundred times heavier. This completely unexpected development caused the famous experimental physicist and Nobel laureate I. I. Rabi to make his now often repeated remark, “Who ordered that?” We are still wondering that today!
While 1947 brought the demise of the mesatron, it also heralded the discovery of the long sought particles proposed by Yukawa. Using a new technique involving photographic emulsions to record particle tracks—a technique that was claimed to be “so simple even a theoretician might be able to do it”—the British physicist Cecil Powell and Blackett’s erstwhile collaborator Occhialini were able to go to high altitudes to search for new cosmic ray signatures.
Occhialini, who had been a mountain guide, ascended to the Pic du Midi at 2,867 meters in the French Pyrenees and exposed his film to cosmic rays high in the atmosphere. Later that year, when he and Powell examined the developed emulsions in London and Bristol, Powell remembered feeling as if they had entered a whole new world. As he later wrote; “It was as if, suddenly, we had broken into a walled orchard, where protected trees flourished and all kinds of exotic fruits had ripened in great profusion.”
I have rarely read a more poignant description of the joy of scientific discovery, of seeing something absolutely new, something that no human has ever witnessed before. It is what drives individuals to scale mountains, metaphorical or literal: the hidden universe, previously unknown and unobserved, but actually present, that we all seem hardwired to crave so deeply.
Powell and his collaborator’s discovery of the particles that became known as pions was not the end of the road, merely a new beginning. An even stranger discovery occurred in the same year, although it took until 1950 before it was independently confirmed. In 1947, working at Manchester, George Rochester and Cecil Butler observed two unusual events involving forked tracks in cloud chambers that appeared to be due to the decays of new particles, about five times heavier than the newly discovered pions, and half as heavy as protons. In 1950, again at Pic du Midi, using a cloud chamber carted up to this high altitude just for this purpose, Blackett’s group observed similar events. The situation still remained somewhat confused until 1952, when a new refined type of cloud chamber resolved that there were actually two different types of these new sorts of particles. What made the decay events associated with these objects so strange, literally, is that while the particles involved were indeed strongly interacting, they lived about ten million million times longer than one would estimate for unstable, strongly interacting particles. Whatever property caused them to live so long was dubbed by physicists, in an act of linguistic creativity worthy of a primary school student, “strangeness,” and the mysterious entities themselves became known as “strange” particles. Powell’s cosmic ray data produced yet one more shock for the physics community, much higher on the Richter scale than even the discovery of strangeness itself. In 1949, in one of the observations associated with the discovery of strangeness, Powell noticed a strange particle, which he dubbed a tau meson, that decayed into three pions. (We now call it a kaon.) Shortly thereafter came the discovery of the theta particle, which decayed into two pions. This in itself was not especially surprising, but when careful measurements were later made, it was found that the two particles had identical masses and identical lifetimes. Why should two such different particles be otherwise so identical?
One suggestion was that they were, in fact, the same particle. However, that was impossible because the final states of the two decays behaved very differently in one crucial respect—indeed, a respect that is of great significance in the context of this book. If Lewis Carroll’s Alice were to observe the three-pion outgoing particle state in her looking glass, it turns out that it would be distinguishable from the three particle state as seen in her own room, just as a left hand in her world becomes a right hand when viewed in the mirror. The three different particles arrange themselves to have a certain “handedness,” just as pointing three fingers in the x, y, and z directions with your right hand produces a “right-handed coordinate system,” while pointing three fingers from your left hand in these three different directions produces a coordinate system that is left handed. Try it. There is no way you can rotate one configuration into the other.
By contrast, it turns out that the two-pion state would look identical in the mirror to the state as observed in the real world. There is no “handedness” to this distribution. Now, there is no way that a single particle could on the one hand produce a final state that was distinguishable from its mirror image, and on the other hand decay into a state that was identical to its mirror image, at least as long as the fundamental laws of physics governing the decays themselves don’t distinguish left from right. The tau-theta puzzle, as it became known, persisted for over five years until two young theoretical physicists, Tsung-Dao Lee and Chen Ning Yang, working for the summer at the new Brookhaven National Accelerator Laboratory in 1956, asked a remarkable question: What evidence was there that the new force responsible for the decay of these particles, the socalled weak force, which was also responsible for the decay of the neutron, actually didn’t distinguish left from right?
It is hard to overstate the striking boldness of this question. After all, everything we experience about nature suggests that the world in the mirror behaves identically to our own world. Being able to distinguish left from right is simply an accident of our location. If one was out in the open ocean on a cloudy night, for example, so that one couldn’t see the stars to navigate, there would be nothing on the horizon that would suggest one direction was different than any other. Or, to take a more modern example, if one was in empty space and performed any physics experiment, it would be ridiculous to expect that somehow its results should distinguish between right and left.
But there it was. In 1956 Lee and Yang realized this assumption was so ingrained in people’s psyches that no one had ever bothered to test it for the weak interaction. By contrast, for both electromagnetism and for the strong interaction, this property had been verified by a host of detailed measurements. Not only did Lee and Yang recognize that no tests of left–right symmetry (or, as it has become known, parity) had been performed for weak interactions, they also proposed several experiments that could be performed to verify it. Within a year of the publication of their paper, two studies, both performed by physicists at nearby Columbia University, had been carried out, and both revealed the same startling conclusion: The weak interactions indeed distinguished left from right!
The first such experiment, performed by the eminent physicist Madam Chien-Shiung Wu along with collaborator Ernest Ambler at the National Bureau of Standards and his colleagues, involved nothing other than a careful observation of the decay of neutrons in the radioactive nucleus cobalt 60. Neutrons behave as if they are spinning, and if one cools down neutrons in nuclei to a very low temperature and puts them in a magnetic field, one can arrange to have most of their spin axes pointing in the same direction. When this was done for neutrons in cobalt 60, Wu and collaborators observed an angular asymmetry in the distribution of the electrons that were emitted in the decay of the neutron: More electrons were produced heading in one direction than another. With resp
ect to the neutron spin axis, nature favored left over right. Within weeks, Leon Lederman and colleagues, also at Columbia, observed the weak decays of the recently discovered pions and muons and obtained a similar result. Both experiments reported that the left–right asymmetry associated with weak decays was not small. Not only did nature, through weak interactions, provide a way to distinguish right from left, but it produced the maximal possible distinction. No longer could knowledgeable scientists look into the mirror and wistfully imagine a world behind the looking glass identical to their own. Like Alice, they would find that the rules in this new world were in fact different than the rules in their own world.
The astounding significance of this totally unexpected prediction of Lee and Yang’s is perhaps best reflected in the fact that they were awarded the Nobel Prize in 1957, only a year, almost to the day, from the date their paper first appeared in print. Indeed, the surprise was so great that it was realized after the fact that the violation of parity had, in fact, been experimentally observed as early as 1928, even before the discovery of the neutron, in the experiments of R. T. Cox in England, who measured the scattering of electrons from the decay of radium and who detected a different scattering rate in one direction than another. His contemporaries, however, discounted his results. Sometimes, alas, it doesn’t pay to be too far ahead of one’s time.
The newfound complexity of the elementary particle world was both a mystery and a challenge. It also completely changed the framework for thinking about unification of forces in nature, especially along the lines of Kaluza and Klein’s extra-dimensional arguments. If electromagnetism and gravity were not the only forces in nature, and if a host of new objects and strange new forces played a fundamental role, then treating electromagnetism as a residue of a purely gravitational, and thus geometric, phenomenon in higher unobserved dimensions would no longer suffice. What is surprising is that the attempt to address the mysteries brought on by these new complexities provided a completely independent impetus to consider extra dimensions.
C H A P T E R 1 1
OUT OF CHAOS . . .
The day will perhaps come when physicists will no longer concern themselves with questions which are inaccessible to positive methods, and will leave them to the metaphysicians. That day has not yet come; man does not so easily resign himself to remaining forever ignorant of the causes of things.
—Henri Poincaré, Science and Hypothesis
The startling revelations about nature discovered through cosmic rays stepped up in pace once accelerators came online, as the number and complexity of the particles produced by colliding high-energy beams on targets continued to multiply. Physics had proceeded up to that point with the presumption, generally supported by experiment, that as one probed to smaller and smaller scales the apparent complexity of the world was reduced, with increasing simplicity and economy of ideas prevailing. But this new data suggested precisely the opposite. The subatomic world appeared to be proliferating endlessly.
Two questions then naturally arose in the particle physics community: (1) Was there anything fundamental at all about any, if not all, of these particles? and (2) Would they continue to proliferate indefinitely?
By the early 1960s these concerns had given rise to several drastic proposals. One that became particularly fashionable had a certain Zen quality about it, and was for a while the dominant fad in particle theory. It was originated by physicist Geoffrey Chew at Berkeley, then the center for much of elementary particle research.
The central idea of his “bootstrap” model was that perhaps all elementary particles, and at the same time none of them, are fundamental. Put another way, perhaps all elementary particles could be viewed as being made up of appropriate combinations of other particle states. It is like imagining, for example, that combinations of the three colors red, blue, and green could make up all other colors, including themselves . . . so that red combined with blue might make green, and green combined with blue might make red. In such a case (unlike in the real world), where these three colors can be considered fundamental, the choice of which colors one considered fundamental, and which ones are composite, would clearly be arbitrary. If you’re bothered by this kind of circular thinking—oddly reminiscent of the famous “Oroboros,” the snake from Indian philosophy whose head devours its own tail, ultimately disappearing completely—don’t be too dismayed. Remember that the quantum mechanical world is full of apparent classical paradoxes, most of which reflect the fact that our classical notions fail to capture what are truly the essential concepts. Ultimately what the bootstrap model suggested was that perhaps particles themselves, which seem so fundamental to us, are really not the important objects to focus on, but instead are merely different reflections of some other, more basic quantities.
Perhaps instead, it was suggested the quantities that one should concentrate on were simply the mathematical relations between the different configurations that could be obtained by scattering particles off one another. The laws of quantum mechanics and relativity provide many elegant constraints on these mathematical relations, independent of the specific particles involved. Since what one actually measures in a laboratory are the processes of interactions and scattering, maybe everything that could be experimentally measured could be derived from the mathematical relationships that described the scattering of particles, and not from the classification of the properties of the particles themselves. I am probably not doing justice to the bootstrap model, as it has since been confined to the dustbin of history. It is thus tempting to dismiss all of the work done during this period as merely a diversion, but that would not be fair. Concentration on the mathematical properties of so-called scattering amplitudes did reveal many illuminating and unexpected relations between states in the theory and the mechanisms for transformations between them. One of the realizations that arose out of this kind of analysis was a particularly disturbing one. As more and more strongly interacting particle states were discovered, an interesting relation was discovered between the masses of particles and their “spin.” Recall that many elementary particles behave as if they are spinning, and thus have an “angular momentum” similar to that of a gyroscope, which remains aligned in a certain direction and can precess about that direction and so on. The faster a top spins, the more energy it possesses, and the larger its angular momentum. Thus, it was perhaps not too surprising to find that strongly interacting elementary particles with higher-spin angular momentum tended to be heavier than their lower-spin counterparts. What was notable, however, was the roughly linear relation between the square of particle masses and their spins that was discovered.
In particular, it was tempting to predict that more and more new heavy states would be discovered as one attempted to produce states of higher and higher spin. Indeed, this prediction was verified as far out as it could be tested, so there was no reason to believe it would not carry on indefinitely. There is a problem with this suggestion, however. If one applies the rules of quantum mechanics and relativity to calculate the scattering rates when one causes fundamental particles of higher and higher spin to collide, these rates become very large as the energy of scattering increases—much larger, in fact, than the behavior observed in actual particlescattering experiments. Considerations of the mathematical relations associated with scattering rates, however, offered a possible way out of this dilemma. It turned out that while the calculated rates for individual scattering processes involving the exchange of a specified number of intermediate particles of a fixed spin might grow large, it was just possible that if there were instead an infinite number of possible intermediate states and if the total scattering rate was determined by summing up over this infinite number of possibilities, then it might just be that the infinite sum could be better behaved than any of the individual terms.
I know this must sound weird in the extreme. First, how could an infinite number of particles be involved in some specific scattering process?
This is made possible, howeve
r by the uncertainty principle. Remember that quantum mechanics allows for the existence of virtual particles that can spontaneously appear and disappear over short time intervals. If the interaction time is short enough, it turns out that an arbitrarily large number of virtual particles can be exchanged between the external particles undergoing a collision, with the heavier particles existing for progressively shorter times.
The second weirdness is worse, however. How could an infinite sum of terms end up being smaller than the magnitude of the individual terms in the sum? Let’s warm up with a simple example. Imagine the individual terms in a sum alternate in sign—something like 1 − 1⁄2 + 1⁄3 − 1⁄4 and so on. In this case the sum of this series seems to be clearly less than 1. Namely, the sum of the first two terms is 1⁄2, the sum of the first three is 5⁄6, the sum of the first four is 14⁄24, and so on. (Try adding more and more terms.) But it turns out that infinite sums behave even more strangely. Indeed, the mathematics of infinite sums is quite fascinating and unintuitive, based as it is on the properties of infinity itself.
Hiding in the Mirror: The Quest for Alternate Realities, From Plato to String Theory (By Way of Alicein Wonderland, Einstein, and the Twilight Zone) Page 14