Brahe & Oskar Klein Memorial Lecture - Sheldon Lee Glashow
My Random Walk Through the Wonderful World
of Physics, from 1957 to 2017... and Counting!
Sheldon Lee Glashow, Boston University, Harvard University Emeritus
Stockholm-Uppsala, 11-12 December 2017
(after the lecture Glashow was awarded the Oscar Klein Medal)
A personal account of my joyous years as a practicing particle theorist. The Standard Theory of Elementary Particles was developed, tested and confirmed during my tenure. I shall revisit those thrilling times of long ago and speculate briefly on developments yet to come.
What physics was like in the 1950's
There Were Four Forces of Nature:
Gravity: Einstein's Gen'l Theory of Relativity was twice tested by '19. Much later: Finkelstein Black Holes ('58), Pound & Repka's slowing of clocks by gravity ('59) Shapiro's time delay test ('64). Meanwhile Joe Weber fails to detect gravitational waves.
Electromagnetism: Quantum Electrodynamics arose in the 40's. Schwinger computes one-loop QED correction to electron magnetic moment ('48), Sommerfield finds exact two-loop result ('57), which CERN confirms ('62). Positronium ('51) offers arena for other tests of QED, which some see as paradigm for weak and strong forces.
Strong Force: Originally to bind nucleons. Yukawa proposed meson to mediate, but "Attempts to predict nuclear forces [from] meson fields... have failed." Weisskopf '52. First pion-nucleon resonance ('54), Strangeness & Gell-Mann('56)-Nishijima('53) Formula, Antiprotons ('55). Much ado about dispersion relations and analyticity; growing disaffection with QFT.
Weak Force: Fermi's current-current model of beta decay ('33) reactor neutrinos detected and parity violation discovered ('55) V-A theory proposed ('57). Yukawa, Fermi, Bludman, Zeldovich et al. ponder weak intermediaries, but only Schwinger imagines electroweak synthesis using Yang-Mills gauge theory... a project he assigned to me for my thesis research!
A Digression: Other Unifications and Dreams thereof:
Newton Unified Terrestrial and Heavenly Mechanics.
Faraday Unified Vitreous & Resinous Electricity.
Maxwell Unified Theories of Electricity and Magnetism.
Einstein Tried to Unify Gravity and Electromagnetism.
...So Did Kaluza and Klein. They failed as well.
Yukawa Tried & Failed to Unify Weak and Strong Forces.
Electroweak 'Unification' Works!
Grand Unification is just too Elegant to Fail!
String Theory does NOT Reproduce the Standard Model!
It Tolerates the SM as it would any other Gauge Theory.
Electroweak hints:
Both forces are Vectorial and Universal.
Yang-Mills theories display both properties!
Some divergences cancel in Y-M theories.
Y-M theories make sense in the massless limit
Electroweak Obstacles:
How to accommodate weak Parity Violation? [1960]
SU(2)XU(1) does it!
How do W and Z acquire their masses? [1967]
Spontaneous Symmetry Breaking! (Weinberg & Salam)
Can the Theory Include Hadrons? [1964]
Quarks w/ Color provide the Arena! (Gell-Mann & Greenberg)
And avoid Strangeness Violating Neutral Currents? [1970]
The GIM Mechanism! (Iliopoulos, Maiani & me)
Is the Electroweak Theory Renormalizable? [1972]
Proven by 'tHooft & Veltman!
Can the Theory describe CP violation? [1973]
A third quark family does it! (Kobayashi & Maskawa)
My thesis Overcame NONE of these Obstacles:
The poetic imagination comes in two varieties: Some are good at inventing fables, others are inclined to believe them.
"It is of little value to have a potentially renormalizable theory of beta processes without the possibility of a renormalizable electrodynamics. We should care to suggest that a fully acceptable theory of these interactions may be achieved only if they are treated together, in accordance with our identification of the neutral
Z-boson as the photon." ...SLG, Thesis, July 1958
Moscow, Nyet! Copenhagen, Ja! Geneva, Oui!
1958-1960: No Soviet visa, so I spent two years as NSF Fellow at the Bohr Institute and CERN...
...Networking with postdocs from all over. I collaborated with Swedes, Norwegians, Poles and Americans & spoke in Lund, Paris, London, Warsaw, Israel &c. Just before departing in spring '60 I wrote my Nobel-winning paper:
Partial Symmetries of Weak and Electromagnetic Interactions
I am indebted to NSF for my funding, to CERN and the Nobel Institute for hospitality... but most of all to the Soviet Government for denying my visa.
"The simplest partially-symmetric system exhibiting all known interactions of leptons, both weak and electromagnetic has been determined... It requires the existence of at least four vector boson fields including the photon."
...submitted to Nuclear Physics 9 September 1960.
[Salam and Ward proposed electroweak schemes based on SU(2) in 1959 and again in 1964. Their theories lacked neutral currents, in addition to their more immediate conflicts with experiment.]
1960-61: A Fruitful Year at Caltech
Unitary Symmetry(the 8-Fold Way) was proposed by Gell-Mann and Ne'eman in '61. Sidney Coleman and I, using 3X3 matrices rather than clumsy 8X8's, deduced the Coleman-Glashow mass formula and several approximate relations among baryon magnetic moments. We became world-traveling advocates of the theory... until its dramatic confirmation by Samios in '64.
MGM and I collaborated only once: in Ann.Phys. 15(1961)437, a rarely cited paper touching upon...
The Cabibbo Angle in SU(3) Symmetry
Separate Arenas for Strong and Weak Forces
Possible Unification of all Elementary Particle Forces
1964: A Most Remarkable Year for Basic Physics
Quarks Invented!
Quarks Acquire Color!
The Charmed Quark Debuts
The Bell Inequality!
Cosmic Background Radiation!
CP Violation Discovered!
Nick Samios' Omega-Minus Confirms the 8-Fold Way!
Brout-Englert-Higgs-Guralnik-Hagen-Kibble Mechanism!
Quasars Discovered!
Spring 1964: A Return to Copenhagen
ONE QUARK MORE MAKES SU(4)
"A new quantum number, CHARM, is violated only by the weak interactions. We predict the existence of many CHARMED particles" ...B.J. Bjørken and S.L. Glashow (Copenhagen) 19 Jun 1964.
Yet we did not recognize charm as a device worn to avert evil! Neither would anyone else for another six years!
1970: The GIM Mechanism
The conventional theory of weak interactions cannot be extended to a gauge theory without contradicting experiment because:
The neutral gauge boson leads to strangeness-changing decays involving neutral lepton pairs, and to ∆S = 2 decays at order G .
But with the addition of the charmed quark these difficulties do not appear!
Thereby can the electroweak theory be extended to describe all known particles...
And later on would describe the third family of quarks and leptons that was soon to be discovered!
S.L. Glashow, J. Iliopoulos, L. Maiani (Harvard) 5 Mar 1970
The Stupendous Seventies:
1972 Electroweak Renormalization: 'tHooft & Veltman
1973 Neutral Currents Observed at CERN!
1973 Asymptotic Freedom: Gross, Politzer & Wilczek
1973 CP Violation (Theory): Kobayashi & Maskawa
1974 SU(5) Unification: Georgi & Glashow
1974 J/Ψ Particle: Ting & Richter et al.
1974 Black Hole Radiation: Stephen Hawking
1975 Tau Lepton: Martin Perl
1975 First Charmed Baryon: Nick Samios
1976 First Charmed Meson: G. Goldhaber et al.
1977 Upsilon a.k.a. Bottomonium: L. Lederman et al.
1979 Abdus Salam, Steve Weinberg and I Visit Stockholm
Entr'acte: The Unification of All Elemental Particle Forces
Also known, regrettably, as Grand Unified Theory
"A series of hypotheses... leads inescapably to the conclusion that SU(5) is the gauge group of the world... that all elementary particle forces (strong, weak and electromagnetic) are different manifestations of the same fundamental interaction... The uniqueness and simplicity of our scheme are reasons enough that it be taken seriously."
...H. Georgi, S.L. Glashow (Harvard) Feb 1974
[In that same year, Pati and Salam sort of unified the three forces with the semi-simple group SU(4)XSU(2)XSU(2).]
BUT...
Charm Discovered!
"Is Bound Charm Found?" we asked upon J/Psi 's discovery.
Yes! "We argue that the newly-discovered narrow resonance at 3.1 GeV is a bound state of charmed quarks - Charmonium. The crucial test of this notion is the existence of charmed hadrons near 2 GeV."
A. De Ru`jula (Harvard) and S.L. Glashow (MIT) 27 Nov 1974
"Is Charm Found?" we asked, after Gerson did his homework. Yes! "A new neutral meson was recently discovered... What has been seen coincides with what was expected by advocates of charm."
A. De Ru`jula, H. Georgi & S.L. Glashow (Harvard) 7 Jun 1976
...And we all lived happily ever after. I have many other tales to tell, but I suspect that my time may be running out.
Nobel Prizes en route to our Standard Theory reflect work of the lucky few of thousands of scientists who made it all possible. Starting from 1957 --
1957 Parity Violation
1959 Antiproton
1960 Bubble Chambers
1965 QED Formulated
1968 Alvarez's Discoveries
1969 Quarks (Theory)
1976 Charmonium
1979 W & Z Bosons (Theory)
1980 CP Viola'n (Exp't)
1984 W & Z Bosons (Exp't)
1988 Two Neutrinos
1990 Quarks (Exp't)
1992 Particle Detectors
1995 The Tau Lepton
1995 Neutrino Discovery
1999 Renormalizability
2004 QCD Understood
2008 CP Violation (Theory)
2008 Symmetry Breaking
2013 Higgs (Theory)
2015 Neutrinos Oscillate
2017 LIGO's Triumph
The 21st Century
Transient Mention of Superluminal Neutrinos.
The Higgs Boson is Found!
Gravitational Waves from Merging Black Holes
And Striking Gold with Merging Neutron Stars!
The New Science of GW Astronomy is Born!
All's Quiet on the LHC Front,
With Higher Energies a Distant Dream,
Dark Matter & Energy Remain Mysterious.
String Theorists Continue to Do... whatever it is that String Theorists do,
But surely we may expect further surprises!
The Future
Things are quiet on the high-energy frontier, with little found beyond the Standard Theory. Neutrinos are well described, but their CP violation remains to be measured. No evidence has appeared for more than three neutrino states, sterile or otherwise. Dark Energy is probably attributable to Einstein's cosmological constant, but its tiny value poses a vexing problem. Dark Matter remains puzzling. Ever more sensitive searches fail to find it as theorists propose ever more baroque explanations. No evidence has been found for supersymmetric particles or extra-dimensional phenomena or 'fourth forces' or forbidden decay modes or additional gauge bosons or proton decay.
Fortunately both CERN and China plan to continue the push toward higher energies. China plans two new and more powerful facilities: a 250 GeV circular electron-positron collider CEPC and, in the same tunnel, a stupendous proton-proton collider CPPC with a design energy of 70 TeV, upgradable to 100 TeV!
Long Ago, Advocates of Super String Theory
... promised to give us a theory of everything from which the Standard Theory follows: Such a truly unified theory would explain its group structure, spontaneous symmetry breaking and the appearance of three fermion families with their most curious masses and mixings.
Although the Super-String yields a seemingly consistent quantum theory of gravity, its current realization as the Multiverse reveals it to be a theory of nothing, in the sense:
It neither implies the Standard Theory nor determines its dozens of free parameters. Rather, the Super-String merely tolerates the Standard Theory as one of a virtually infinite number of plausible (and implausible) causally disconnected universes. Fermion masses, for example, are simply accidents of the birth of our particular Universe. This is not particle physics; it is a philosophy of despair.