InwardBound

Inward Bound (Abraham Pais)

Along this incompletely traveled road inward man has established markers that later generations will rank among the principal monuments of the 20C.
These advances have by no means come smoothly. Between times of progress and stagnation, or order and chaos, of clarity and confusion, of belief and incredulity, of the conventional and the bizarre; also of revolutionaries and conservatives, of science done by individuals and by consortia, of little gadgets and big machines, and of modest funds and big moneys.
The main part is the history between the period of 1895-1945.
There are are many (overlapping) histories as there are historians. "He who is deficient in the art of selection may, by showing nothing but the truth, produce all the effects of the grossest falsehoods. It perpetually happens that one writer tells less truth than another, merely because he tells more truths.
The emergence of theoretical physics as a semi-autonomous discipline is a 20C phenomena.
The revolution was kicked off by two accidental discoveries made in mid-career by well-established scientists. Roentgen (X-rays) and Becquerel (Radiation)
The most paper from Marie Curie is when she showed that radioactivity is a property of individual atoms of certain species, once of which she announced was thorium. This atomic interpretation led her to use radioactivity as a diagnostic tool in the discovery of new elements.
The discoveries in 1900 by Rutherford of a half life and, and later that same year, by Planck of the quantum theory signal the end of the era of classical physics. Neither of them was at once aware of how profoundly his work was to change the course of science.
Rutherford quote discussing an offer to go to Yale: "Why should I go there? They act as though the University was made for students" (
Three new themes emerge as we move further into the 20C: beta-spectra, atomic structure, then early ideas on nuclear structure. (beta activity is one of the main subjects of the book and will stay with us until the end)
Spectra signified that atoms has to have substructure, as Maxwell had been among the first to recognize. It is therefore not surprising that the search for atomic models began quite early, long before the discovery of the electron.
Rutherford was the only physicist to do his greatest work after receiving the Nobel Prize.
Quantum theory before 1925, the old quantum theory, was part craft part art. Old first principles had been found wanting, new ones had not yet been discovered.
It was Fermi's great contribution to apply field theory to beta-decay and assert that electrons and neutrinos are created in the very act of beta-disintegration.
Fermi's theory is one of three developments in the 30s that set the stage for the postwar era. The other two are: the first appearance of "isospin" and the first attempts (by Yukawa) to associate nuclear forces with fields.
The discovery of the muon in 47 marks the beginning of a new and as yet unfinished chapter in the story of the structure of matter.
The neutral pion was the 2nd particle to be found at an accelerator. (The electron was the first)
The bizarre predicament of the 1930s how can a theory be essentially correct when treated approximately yet fail when treated more rigorously?

Shelter Island conference: a new generation of refined spectroscopic experiments in hydrogen and other materials gave the first evidence that corrections to the leading order predictions of QED had been observed.

This old problems was revisited with far greater care.
The neutral K-complex is a veritable laboratory for the study not just of properties of specific particles but also of far more general aspects such as the superposition principle of QM. "There is scarcely a physical system that contains so many of the elements of modern physics."
The advent of new particles had made physicists become increasingly accustomed to selections rules, conservation laws, and symmetries that were, meant to apply to some but not to all interactions. In 1956, something different came to pass. Symmetries earlier believed to hold for all interactions, turned out not to be obeyed by weak interactions. It was the most startling development in all the physics of the 50s. Lee and Yang noticed that invariance under P,C and T had never been tested in weak interactions and proposed ways of doing so.
Then CP was found to be violated. These new violations were very small and very puzzling.
Instant reply is not the noblest form of history nor even of memoir.
The first round of experiments at SLAC led to a discovery (1968) of seminal importance, comparable to the observation of the hard alpha-nucleus scattering sixty years earlier: high energy inelastic e-proton (or neutron) scattering is hard; very many more electrons are deflected at large angle than had been anticipated.
A consistent description of the neutral current demands the existence of a new species of quarks "charmed quarks". Their discovery reaffirmed that the theory was moving in the right direction.
And so it goes, on and on, amidst a great deal of excitement. And also of confusion. In brief, business is continuing as usual.
Roentgen wanted to have his own readings controlled by an unprejudiced observer. (Blinded analysis)
A man whose soul is in his work always makes more progress than one whose aim is something not immediately connected by his occupation.
The terrestrial presence of He was only ascertained in 1895, not long before the discovery of alpha-rays; they were found to be present in a uranium bearing mineral.
names are one thing and science is another.
In "Medical Lessons from History" Lewis Thomas considers the early history of medicine to be an unbelievably deplorable story and goes on to remark: "Virtually anything that could be thought up for the treatment of disease was tried out at one time or another, and, once tried, lasted decades or even centuries before being given up. It was, in retrospect, the most frivolous and irresponsible kind of human experimentation, based on nothing but trial and error, and usually resulting in precisely that sequence."
Radioactivity was discovered in 1986, the atomic nucleus in 1911. Thus some 25 years would pass before one could understand why some, and only some, elements are radioactive.
Between 1998 and the early 30s it happened three times that the discoveries of new natural phenomena were so unsettling as to make prominent physicists waver in their faith in the universal validity of the law of conservation of Energy. Radioactivity (source of energy) / beta-decay spectra / Quantum effects (BKS proposal)
1 g of radium can heat 1.3 g of water from the melting point to the boiling point in 1 hour. 10 W
The first determination of the half-life for a radioactive decay was made by Rutherford.
Radioactivity represents one instance among many of a situation in which physicists of earlier days were unwittingly dealing with quantum effects.
It seemed (in fact, it was true) that radioactive decays were contrary to the classical concepts of cause and effect. During the 1st two decades of the 20C physicists had no reason to suspect that these paradoxes were not by any means typical for radioactivity only.
Those who wisely left aside the question: "Why does a radioactive atom change?" and focused on the more modest problem: "How does it change?" were able to make some further progress on a more descriptive level.
The lifetime problem simply did not lend itself to the statement of new hypotheses subject to test. The problem was so difficult that it was hard even to get a wrong idea about it.
George Gamow showed that alpha-decay was a consequence of quantum mechanical tunneling thought a potential barrier. The first explanation of the Geiger-Nuttal relation, known phenomenologically since 1912 which established a connection between the lifetime of an alpha-emitter and the range of the particles.
The Mossbauer effect, where a nuclear decay, even though it originates in a single nucleus, is properly described only by treating the decay as a collective quantum property of the entire crystal in which that nucleus sits.
Planck's 1900 paper the most outstanding example in the history of all science of the precept that to make a discovery is not necessarily the same as to understand a discovery.
Rutherford kept silent about the implications of the lifetime concept. Another example of a discoverer who did not at once grasp the enormity of what he had wrought.
Asked to designate just one single discovery in 20C physics as revolutionary I would unhesitatingly nominate Planck's. I count it among Einsteins great achievements that he may well have been the first to realize that the advent of quantum theory represented a crisis in science.
The history of the prediction and discovery of the photon ranks among the most fascinating chapters of 20C science. Never to this day has the suggestion of a new particle caused more and longer-lasting confusion, the principal reason being that one was not confronted with never yet seen phenomena but,instead with old phenomena in a new guise.
From the very first paper on the subject of relativity the theory was put on a firm, logically complete, axiomatic basis – this is quite unlike the quantum theory which remained a logically muddled subject for the first 25 years. If in the early years of the 20C some physicists could not accept the quantum theory, that was because to them it seemed (as indeed it was) to be too complicated a hodgepodge of ad hoc rules. IF at the same time some physicists could not accept SR that was because for them the theory was too simple to grasp.
Revolutions in science are rarely brought about by revolutionary souls. Like the rest of humanity, physicists tend to cling tenaciously to what they know or think they know, and give up traditional thinking only under extreme duress.
Pitfalls of simplicity are neither avoidable nor a mark of stupidity. They should rather be considered a necessary evil. It is the scientists task to seek for simplicity at the fuzzy frontiers of knowledge. He is privileged in that his latitude for choice of simplicity, necessarily ill-defined, is held in tight check by experiment.
Until 1947 (when the muon was discovered) beta-decay was the only manifestation of a specific type of force (weak) rather than one among many.
Why did it take so long to observe that the beta-spectrum was continuous? Electrons were quite novel objects, there was no qualitative guidance about what to expect. Secondary effects strongly obscured the nature of the primary spectrum.
The cloud chamber was a tool for counting the number of electrons per beta-disintegration was first used in 1926 by a group in Tokyo.
Hahn's new element (thorium) was probably "a mixture of Radium and stupidity"
With Rutherford's robust personality he was much in demand for lecturing to the engineers who were notoriously difficult to control.
Bohr said he had not been aware of Balmer until shortly before his work on the Hydrogen atom.
AS late as 1835 the French philosopher Auguste Compte had written "On the subject of the stars… We can conceive of the possibility of determining their shapes, sizes, and motions we shall never be able by any means to study their chemical composition or mineralogical structure. … We shall not at all be able to determine their chemical composition or density … I regard any notion concerning the true mean temperature of the various stars as forever denied to us". Spectral analysis is but one branch of science which has taught philosophers caution.
Kirchhoff and Bunsen founded spectral analysis.
"Spectrum analysis might be no less important for the discovery of elements that have not yet been found" Splendid example of how great advances in physics are possible in absence of first principles. In all those years, there was not a clue to the structure of the atom or the origin of the spectra, Yet the insight that an atomic spectrum is a unique label for an element was sufficient for making major progress.
Balmer was 60 years old and had never published any research paper.
Maxwell thought that the atomic model building would be a very difficult task "The spectroscope tells us that some molecules can execute a great many different kinds of vibration/ They must therefore be systems of a very considerable degree of complexity, having far more than six variables … every additional variable increases the specific heat … every additional degree of complexity which we attribute to the molecule can only increase the difficulty of reconciling the observed with the calculated specific heat." Thermodynamically there was not enough room for any additional variables. The resolution of this paradox belongs to the 20C.
The years 1858-64 where a vintage period in the discovery in science. Clausius wrote of the motion we call heat, Maxwell discovered his velocity distribution in gases, Le Verrier found and anomaly in the precession of Mercury, Kirchhoff found sodium in the sun, Darwin published his Origin of Species, Kirchhoff and Bunsen founded spectral analysis, Huggins realized that nebulae are gaseous clouds, and Maxwell wrote his equations.
Progress in science depends on a backlog of experimental data in need of interpretation. In all of the 20C the experimental backlog in physics was never great than during its opening years. For a century chemists had amassed data which called for physical interpretation, especially the period table of the elements. 50 years of spectroscopy had yielded a wealth of results about which, nothing was known other than that something in the atom was moving. It was known that atoms could break up by ionization, emitting electrons. It was known that some species of particles could break up in more violent ways.
Early in 1909 when he realized that he did not know enough probability theory for handling small samples of data, the Nobel laureate (Rutherford) became a regular student in a course on the subject by Horace Lamb.
The early successes of Rutherford's models were a mark of his ingenuity and of very good luck. All data at that time were obtained from scattering of low energy 5 MeV alpha on targets with high nuclear charge. Therefore the penetration of the nuclear Coulomb barrier was negligible so that deviations from the Rutherford formula due to the strong interactions were unimportant. Moreover for a Coulomb potential the classical and quantum scattering cross sections are identical.
First there was Plank who in 1900 started it all with his blackbody radiation law. Five years later, Einstein introduced light-quanta, an idea so alien to Planck that he resisted it for a long time. Then Einstein ushered in the quantum theory of the solid state in order to explain long-standing specific heat anomalies. Common to these three roads is their origin in statistical mechanics. The first strides on the fourth, dynamical road, were made by Bohr.
Bohr's first postulate: an atom has a state of lowest energy which, by assumption, does not radiate, was one of the most audacious hypothesis every introduced in physics. It contradicted Maxwell's equations. The derivation of the Balmer formula is the 1st triumph of quantum dynamics.

R(He+)/R(H) = 4.0016 instead of the naive guess 4. Bohr at once rose to the occasion; he had neglected the mass of the electron relative to the mass of the nucleus. In addition to Z one had to use the reduced mass. Doing so he found exact agreement!
Stern and Von Laue: said that if the Bohr model turned out to be correct they would leave physics. It did and they didn't.
A horseshoe hung over the entrance of Bohr's house. His guess asked him if he really believed in it. He said "Of course not, but I am told that it works even if you dont believe in it." The old quantum theory, was much like the horseshoe.
The first to raise the issue of causality in regard to Bohr theory was Rutherford. How does an electron decide with what frequency it is going to vibrate when it passes from one stationary state to another ? It seems that you would have to assume that the electron knows beforehand where it is gong to stop. (Interesting that Rutherford made this comment about Bohr's model, but failed to notice the same issues in his lifetime model of the nucleus.
Bohr was among the last to accept the photon as a real particle.
Mosley was ready to call the roll of the elements. Ending with Uranium there had to be 92 of them. Seven were missing. All these have since been discovered, some by X-ray analysis some by later techniques. Rutherford put Moseley's work on a par with the discovery of the periodic table.
The H-particle-electron picture of the nucleus is another example of simplicity as a necessary evil. It was a model as inevitable as it is wrong.
No consequence of Maxwell's theory was more profound than the unification of light with electromagnetism. No part of the new theory appeared to be better understood than the wave nature of light as it travels through empty space. Consequently no part of the old quantum theory appeared more dubious than Einsteins idea that under certain circumstances light behaves as if it has particle structure.
The introduction of probability in the sense of QM may well be the most drastic scientific change yet effected in the 20C.
The era of the old quantum theory, from 1900 to 1925 constitutes the most protracted revolutionary period in modern science. Six theoretical papers appeared during that time which are revolutionary: Planck on the discovery of Quantum theory Einstein on light quantum Bohr on the hydrogen atom Bose on quantum statistics Heisenberg on matrix mechanics Schrodinger on wave mechanics These papers each contain at least one step which could not be justified
The end of this revolutionary period was brought about by four people: Heisenberg, Schrodinger, Born and Bohr. Schrodinger never found peace with what he had wrought
Born's great novelty, the correct transition probability concept, entered physics by way of a footnote.
The motion of particles follows probability laws but the probability itself propagates according to the law of causality.
"Unknowablity relations" is a better term than uncertainty relation
"It seems to me that the deliberate utilization of elementary symmetry properties is bound to correspond more closely to physical intuition than the more computational treatment." - Wigner
The QM linearity, the real novelty, is directly related to the superposition principle: in QM one can add two states. There is nothing like this in classical physics. Linearity is also the basic reason why symmetries in QM determine, all by themselves, many properties of systems.
Bose's derivation of Planks lay is a confused masterpiece. His reasoning is correct, but as he himself once said, he had no idea that it was novel.
In QFT the Hydrogen atom can no longer be considered to consist of just one proton and one electron. Rather in contains infinitely many particles.
It may well be that other had seen positrons before Anderson. But then many people had seen apples fall from trees before Newton.
25 years after the light-quantum did the second prediction of a new particle, the anti-electron, enter physics. (Half a year earlier Pauli had suggested the neutrino, but was not yet prepared to publish.)
The discovery of the positron was wholly accidental.
1920-1930 was the final decade in which the physics at the frontiers was quintessentially European. Moving into the 30s we encounter fundamental contributions to QFT from the US and Japan.
Oppenheimer played the lead role in bringing QED to the US and making it flourish there.
Oppenheimer: vast insecurities lay forever barely hidden behind his charismatic exterior, whence an arrogance and occasional cruelty befitting neither his age nor his stature. "In Oppenheimer the element of earthiness was feeble"
Oppenheimer entered practical QED on the ground floor. He was Americas leading teacher in QFT. He created the greatest school of theoretical physics that the US has ever known. "His physics was good but his arithmetic was awful"
Dilemma in QED: Lowest-order effects are finite and work well. Higher-order contributions are small but infinite, small because of powers of alpha, infinite because of the integrations.
Thus the classical answer returns if one makes the absurd assumption of an electron radius large compared with the Compton wavelength. Whatever the future may hold, the tiny marble with energy e²/a is gone forever.
Light-by-light scattering implies that Maxwell's equations are no longer linear.
In the 1930s the tools to push the theory much further were in hand, but the experimental incentive was lacking and there were so many other interesting things to do in theoretical physics: nuclear physics, beta decay, cosmic rays, mesons, …
The scattering and the absorption of neutrons became a main source of information about nuclear structure. Neutron bombardment led to the discovery of nuclear fission. Nuclear fission changed the world in which we try to live.
Two main themes dominate the physics of the 30s: struggle with QED, first rational theory of nuclear phenomena
Its hard to overrate Fermi's contribution (theory of beta-decay) He showed how beta-radioactivity is due to a new force. He liberated the neutron from the vestiges of the proton-electron model. And he silenced Bohr's conjectures about non-conservation of energy.
In nuclear physics the deuteron matches in importance, but not in simplicity, the hydrogen atom in atomic physics.
After the discovery of the neutron with an alpha source (in Feb 1932) all basic particles are discovered either in cosmic rays, or with accelerators or reactors.
Heisenberg observed that the higher-order processes became more important than the lower-order ones at a sufficiently high energy, which he estimated to be ~1000 MeV. perturbation theory breaks down at such energies. "In this way Fermi theory gives a quantitative explanation for the creation of cosmic ray showers.
Yukawa's paper is a primitive improvisation, yet it contains three major lasting points: the dissociation of the strong nuclear force from the weak beta-decay force; a sensible first estimate of the meson mass; and the remark "the massive quanta may also have some bearing on the showers produced in cosmic rays."

It is Yukawas great and lasting contribution that he has changed our thinking about forces. To this day his extension to the strong interaction of the relation between forces and virtual particle exchange, his analogy with QED, has remained our guide to thinking about all fundamental forces.
The first prediction of a particle on symmetry grounds: Neutral pion. There was no evidence for a neutral meson, but it could easily have escaped detection since it can decay into photons with a lifetime of 1e-16s.
"The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine" - Rutherford When Lawrence was asked for his reaction to this statement he said "Personally I have no opinion as to whether it can ever be done, but we're going to keep on trying to do it"
Abe Pais gave an invited paper while he was living in hiding from the Nazis. He often discussed his ideas with Kramers, the only physicist who knew where he was and who would visit him from time to time.
What else was the muon good for other than being the pions favorite decay product ? The divine laughter continues unabated.
The need for renormalizing mass and charge implies that the the theory cannot predict their values but borrows them from experiment. These are phenomenological parameters. QED is nevertheless a theory with enormous predictive power. A finite number of parameters treated phenomenologically make possible an infinite number of prediction. Such a theory is called renomalizable.
QED cannot predict the electrons mass because it is the only scale. m is therefore as uncalculable in this theory as Planks constant is in quantum theory, or c is in relativity. It is not unreasonable to hope that these may be explained by theories with larger particle content.
Does the expansion of the renormalized amplitude converge? We do not know, but especially as a result of lessons learned from simpler but non-trivial model theories, we believe the answer is probably no.

Beginning with O(alpha²) one finds in the guts of the radiative corrections contributions from all species of charged particles in the physical world. This shows that the question of convergence will not say much about the real world.
In Rutherford's day scintillations were observed in solids of a kind which rapidly absorb the light flashes. When it was found that certain organic crystals (like naphthalene, anthracene) were transparent to their own fluorescent radiation, one was able to use a large amount of such substances and send the radiation into a photomultiplier. Thus was the scintillation counter created.
In 1950 a new particle (pi0) was discovered in an accelerator experiment for the first
Isospin invariance first introduced in phenomenological nuclear potentials, then built into Yukawa equations, became a free-floating invariance in search of a badly needed appropriate dynamics. This was to recur in subsequent years: start with dynamical equations, abstract some of their general features, forget the starting points.
G-parity in its simplest, though not most general, form says that an even number of neutral pions cannot go into an odd number and vice versa
The rho, omega and eta mesons where all discovered by the bubble chamber. It has enormous advantages over the cloud chamber in respect of sensitivity, rapid cycling, spatial resolution, and high stopping power.
Alvarez stressed in 1955 "data analysis is the bottleneck in cloud chamber and emulsion work all over the world…The rate at which interesting events will occur in the large bubble chamber is very much greater. One day of bubble chamber operation could keep a group of "cloud chamber analysts" busy for a year. He concluded that track and scanning needed to become semi-automatic; computers should be used for analysis and storage of data. And that is exactly what happened. Such were the techniques which led to the discovery of the rho, omega and eta meson. There were initial reservation of quite a few members of the profession in regard to Alvarez' managerial approach which they felt, would remove physicists yet another step from experimentation done in the old ways.
With limited statistics and without prodding by theorist, did the cosmic ray physicists usher in the era of new particles, and by their conjecture of one and only one charged K-particles pave the way toward the discovery of non-conservation of parity.
Cosmic ray studies did not come to a halt in the mid 50s, but everybody know that they had finished their task in elementary particle physics. (They will probably be back at the frontiers, sooner or later)
The physical content of parity is bound to interactions. There is no distinction between free spin field of even and odd parity. This fact is unnecessarily obscured by the term "intrinsic" parity.
P and C invariance provide excellent illustrations for the distinction between invariance of physical laws and non-invariance of initial conditions. P is not violated because our heart is on the left or because polarized light is twisted to the left when going through a sugar solution. Nor is C violated because in our universe protons and electrons dominate over their anti-particles. All these situations are created by locally asymmetric initial conditions.
Charmoniuum (the hydrogen atom of QCD) strongly reinforced the belief in QCD and in electro-weak theory. This earn the name the "Standard model"
A new charged lepton the tau add one unit to R (where it was first indirectly observed) The firm identification of the tau as a lepton took some doing, especially b/c it arrived before the sought-for D^+/-.
The angle observed between the jet axis and the beam direction was found to be 1+cos² θ just as what one would expect from spin 1/2 quarks.

InwardBound

Inward Bound (Abraham Pais)

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