Astrophysics was entangled in the DNA of the Aspen Center for Physics from the beginning, even if it was not immediately apparent. Although George Stranahan, the visionary founder of the Physics Center, practiced condensed matter physics during his research career, he read astronomy books on his own time during his stint at the Hotchkiss School and did astrophysical research as an undergraduate at Caltech, long before the Center was even a gleam in his eye. As a senior at Caltech, George worked with Leverett Davis, Jr. on measuring interstellar magnetic fields by detecting the polarization of dust–scattered starlight. Real astrophysics! In addition, one of the first attendees at the brand new, Navy–supported, Physics Division of the Aspen Institute for Humanistic Studies in 1962 was the renowned Hans Bethe. Bethe donated part of his winnings from his 1967 Nobel Prize for elucidating nucleosynthesis in stars to build the Bethe Building.

There was no obvious work on astrophysics in that first year (Who knows what Bethe was thinking?), but there were smatterings of astrophysics in the first few years of the Center. In 1963, Al Schild from the new Center for Relativity at the University of Texas at Austin gave a seminar on the “Principle of Equivalence” on July 23 and Curtis Callan gave a seminar on “Cosmological Solutions with Inhomogeneous Matter Distributions” on August 27.

One of the clear contributions of the Center to astrophysics was to catalyze the transformation of hardcore physicists into astrophysicists when the lure became too enticing to ignore. Among the astrophysicists–to–be was David G. Ravenhall in 1962. By 1963 the list included Sid Bludman, Steve Frautschi (a bit of a reach, but his work on the Hagedorn equation of state had implications for cosmology and neutron stars), Daniel Z. Freedman, Al Petschek, David Pines, and Ray Sawyer, all of whom would contribute significantly to astrophysics when the time came.

By 1964, Bethe was on the Center’s Advisory Committee. Joe Weber, a pioneer in the search for waves in the fabric of space predicted by Einstein, began the first of many visits, so the Center can rightfully claim to have a founding role in the effort to measure gravitational waves. Weber gave a seminar on July 15 entitled ”Gravitational Waves.” In addition to Bludman, on the list of attendees for 1964 was a young graduate student, perhaps one of the first non–PhDs to attend, Mr. Leonard Susskind. Perhaps Susskind should not be classified as an astrophysicist–to–be, but his career led to his deep studies of black holes as they play a role in the information crisis and to his role in expounding the hypothesis of a holographic universe, an area right on the dramatic boundary of physics and astrophysics.

The astrophysics report for 1965 is not in the ACP archives, but Pines recalls that Mal Ruderman gave a talk on neutron stars. The conclusion, at least according to Pines, was that the topic was interesting, but basically hopeless, because they would never be observed.

The report for 1966 shows more evidence for astrophysics and astrophysics–to–be. In addition to Weber and Pines, Ruderman, a Columbia University astrophysicist still studying neutron stars who was then at NYU, made his first recorded appearance. Ruderman gave a seminar on July 1 entitled “Some Astrophysical Many–Body Problems” and reported that he worked on cooling white dwarfs and talked with Pines about physics at high astrophysical densities 104 – 109 g cm–3. Pines himself was working on dilute solutions of He3 in He4 and gave a seminar on that topic on July 15.

In 1967, Weber gave a seminar on August 4 entitled “Gravitational Theory and Recent Experiments” and worked that summer on scalar–tensor theories of General Relativity, a hot topic of the time. Gordon Baym and Chris Pethick made their first recorded appearances. Baym gave a seminar on June 23 on his work with Pines, ”Dilute Solutions of He3 in superfluid He4.” Pethick worked on periodic ionic lattices in simple metals and ferromagnetic Fermi systems.

Neither Baym’s nor Pethick’s report for that year makes reference to any astrophysical topic. Pines apparently did not manage to turn in a report. Within a year, all three would be active, impassioned astrophysicists and the Aspen Center for Physics would make its first broad impact on the world of astrophysics. The Center was about to be transformed.

The superfluid was about to hit the vortex.

There is no archival report for 1968, but history records that Jocelyn Bell made her staggering discovery of “little green men.” These rhythmic radio pulses were soon established to be of astrophysical, not intelligent, origin. There was a flurry of activity in the attempt to explain these remarkable signals, as normal stars, white dwarfs (my favorite being laser emission from the perimeter of a white dwarf), were eliminated. The gang from Illinois had a hammer in the theory of superconducting fluids and Tommy Gold’s (Gold was the first to propose that, or at least loudly broadcast, the idea that pulsars were neutron stars) suggestion that these new signals were from neutron stars looked like a nail. They were right.

The discovery of pulsars as rotating magnetic neutron stars and the rapid developments that arose from the discovery of glitches, leading to theories of superfluid cores, put Pines, Baym, Pethick, their collaborators, and the Aspen Center for Physics at the core of a huge new field of astrophysics. By 1969, Aspen was the intellectual center of this activity, drawing researchers from around the globe.

Pines notes that things were simpler back then. He began to organize a conference on pulsars in July of 1969 and it convened only a month later, in the week of August 19 – 23.-23. The attendees were the new who’s who of the pulsar game: Gold, Bethe, Pines, Baym, Pethick, and Ruderman, among others.

Bethe had received a 1967 Nobel prize for his contributions in the theory of nuclear reactions, especially his discoveries concerning the energy production in stars. He gave a 1969 talk o n the properties of matter in neutron stars. He also noted in his report of that summer that “In private discussions I gave D. Ruderman (sic) some advice on calculating the superfluidity of neutron matter.” Earlier (August 7) he also gave a public talk on the anti–ballistic missile system. Bludman, newborn astrophysicist along with Pines, Baym, and Pethick, gave a seminar on ultradense matter. Frautschi reported that he enjoyed learning about pulsars and had some ideas about how “bootstrap” physics might apply to central regions of neutrons stars with excited hadrons. In a footnote, I, a new postdoc at Caltech, wrote a paper with Frautschi that year on a “bootstrap” equation of state, not knowing he was already indirectly under the sway of Aspen. I had not yet been to Aspen, actually had not even heard of it at that time, but Frautschi had participated and he was one of the people who implicitly spread the spirit of Aspen. The Aspen participants were out in the world influencing young people whether they advertised it explicitly or not.

Jim Hartle was also present in 1969, working on neutrino cosmology, neutron star matter, and symmetry violations. Weber continued his work on detection of gravity waves. Ruderman worked with Bludman on causality and instability in superdense matter, and, of course, Pines, Baym, Pethick, and Ruderman worked on superfluidity in neutron stars and the sudden change that had been observed in the period of the Vela pulsar.

Stirling Colgate had purchased a house on Galena Street, right off the ski slope in 1968. In that summer of 1969, he got wind of the pulsar meeting. He felt strongly that the proposed studies of neutron stars needed to be done in the context of supernova research and that astrophysics should be formally recognized as a component of the Center. He had lunch at the Weinerstube with Pines and Pethick and pressed his case. Some funding for the Center was now coming from the NSF and Colgate thought that the Center needed a presence in both astrophysics and biophysics to keep the NSF happy. Pines eventually bought the idea and presented it to a meeting of the ACP Board of Trustees and the proposal to include astrophysics as a formal subject of study at the Physics Center was accepted. Colgate, Al Cameron and George Field dedicated time over the next eight years obtaining funding from NASA for what would become the June astrophysics workshops.

That summer of 1969 also brought the first overt mention of afternoon volleyball.

1970 brought a diversity of activity that would mark the future of astrophysics at Aspen. Work on neutron stars was still prominent. George Greenstein gave a seminar on August 6 entitled “Pulsar Burps and Wobbles.” He worked on superfluidity in neutron stars, talked with Morrel Cohen, Alfred E. Glassgold and Bludman on the topic and with Steve Berry drafted a paper on the effects of an airplane falling on a neutron star, thus foreshadowing some later models for cosmic gamma–ray bursts models. Bludman reported working on the equation of state of superdense matter in neutron stars. Weber gave a seminar on July 29 on “Gravitational Radiation Experiments” and discussed problems of gravimetry with Jean–Paul Richard. Peter Goldreich worked on molecular masers in magnetic fields as related to OH radio sources in the Galaxy. He talked with Greenstein and Colgate, who, Goldreich remarks, “visited the Center in an unofficial capacity.” This is the first formal mention of Colgate in the record. Setsuo Ichimaru of Illinois worked on strong turbulence in plasmas and a model of pulsar radiation based on strong plasma turbulence. He gave a seminar on “Plasma Turbulence as a Possible Mechanism of Pulsar Radiation.”

In 1971 there was another Pulsar Study Group. Bethe reported that he “enjoyed particularly the talk by Sterling (sic) Colgate on Supernovas, and that by A. Cameron on conditions of a star preceding the supernova outburst.” Bethe gave a seminar on August 19 on the equation of state at extremely high densities. Gold had discussions with Bethe, Ruderman, Pines, and Baym on the difficulty of explaining starquakes and advocating the speed of light circle as the origin of pulsar radiation. He shaped a program to find high–speed pulsars. Ruderman worked on his paper on the nature of matter in fields of 1012 Gauss showing that atoms would link up in one–dimensional metals. A Classic! Ruderman also worked with Baym and Pethick on rotating superfluids, vortex lines, and the effects of external torques. Baym worked with Bethe and Pethick on the equation of state at high densities. Baym, Pethick and Bethe began work on the equation of state of a star undergoing a supernova explosion, a result of discussions with Colgate and Cameron, just the result Colgate had encouraged. Their work on the equation of state at high temperatures was another classic of the era. Sawyer, another newly converted astrophysicst, reported being influenced by Bethe’s talk to work on multi–pionic interactions in nuclear matter at superhigh densities. Ed Groth worked on Fourier transform algorithms and application to pulsar searches. Ichimaru, who was in Tokyo by now, worked on radiation transfer in astrophysical plasmas. Bruno Coppi and Aldo Treves worked on plasma instabilities near speed of light cylinder. Bethe reported talking with Gerhard Borner on energy loss from pulsars and Parbahan Kabir on proton/nucleon collisions. Greenstein continued work on superfluidity in neutron stars. Jon Arons, then at the Institute for Advanced Studies, worked on absorption line spectra of QSOs and with Jerry Ostriker on a theoretical model for QSOs based on a central cluster of pulsars.

The diversity of astrophysical research at the Center also continued to expand that summer. Fred Lamb worked on spectral line formation in magnetic fields in the solar atmosphere and Ap stars. He worked on magnetic white dwarfs with Peter Sutherland and on stellar opacity sources in strong magnetic fields. He had discussions with Martin Rees on observational data on magnetic white dwarfs (Lord Martin as an observer!), with Gabriel Karl on molecular spectra in magnetic fields with Ichimaru on electron–ion correlations in magnetic plasmas. Remo Ruffini talked about research on black holes. Greenstein started work with Martin Olsson of Wisconsin on small black holes. Weber did a noise analysis of gravity wave experiments.

Geoffrey Bath of Oxford spent his time preparing the first–ever undergraduate lecture course on astrophysics at Oxford, and Greenstein worked on a new course on astrophysics for non–science majors. These efforts gave Aspen a role in pedagogical as well as research astrophysics.

Joel Primack, Freedman and Carl Akerlof all attended that summer. Though they would all make substantial contributions to astrophysics, they were still mired in particle physics at the time.

In the summer of 1971, Colgate, Cameron, and Field submitted the first proposal to NASA to fund an astrophysics workshop. The first NASA proposal netted $5,000 to support the workshop, to which NSF added another $3,000.

With this new NASA support, the first astrophysics workshop was held that was not devoted to neutron stars. This was also the first of the series of June astrophysics workshops, in this case two weeks from June 12–23. The topic was “The Physics of the Early Universe.” Thirty–five people attended, including Cameron, Colgate, Joe Silk, Chris McKee, and Dick McCray. The format of morning discussions on the patio was adopted. The indelible memory remains of Cameron ruling the patio in shorts and an Hawaiian shirt, a beer in one hand and a cigar in the other. The discussion leaders that year were Gary Steigman on the Hadron Era, Bob Wagoner on Cosmological Nucleosysthesis, Charlie Misner on Chaotic Universes, Ted Harrison on Galaxy Formation, Bill Saslaw on Dynamics of Dense Stellar Systems, Field on Intergalactic Matter, Jim Bardeen on Rotating Massive Objects, and Rees on Quasi–Stellar Objects.

McCray worked on a paper with Joseph Swartz and Bob Stein on filaments in fossil HII regions and with Jim Buff on a time–dependent model for the ionization structure of interstellar hydrogen and the exposure of the interstellar medium to a soft X–ray burst. Gordon MacAlpine gave a seminar on July 27 entitled “Photoionization Models of Quasars and Related Objects.”

Hartle gave a seminar on July 13 on the topic of “Tidal Friction in Black Holes.” He had conversations with Susan Keyser of Chicago State about general relativity and relativistic astrophysics.

Neutron stars remained an important theme. Ruderman worked with Jacob Shaham and Pines on a review of glitches and timing noise, on superfluids and vortices, on the stability of a vortex lattice in a superfluid, and on a model for the 35.7 day on/off period of Her X–1. Sachiko Tsuruta (at least two women that year!) worked on cooling of dense stars and the composition of matter at high densities. She reported conversations with Ruderman, Shaham, and Kayser and with “Dr. C. Wheeler; properties of ultradense stars and dense star models in these high densities.” There was no report from this C. Wheeler fellow.

In the summer of 1972, Colgate, Cameron, and Field wrote a renewal proposal to NASA that funded the 1973 June Workshop on Interstellar Physics. The span was increased from two to three weeks and the format of nine topics, three per week, was instituted to try to prevent the morning discussion sessions from expanding to fill the day and preclude informal research. In attendance were Field, McCray, Charlie Townes, Lyman Spitzer, Ed Purcell, Silk, Frank Shu, Alex Dalgarno, Goldreich, Dave De Young, Russell Kulsrud, Don Cox, J.Randy Jokipii, Phil Solomon, Paul Woodward, Peter Biermann, Teije de Jong, Catherine and Diego Cesarsky, Peter Martin, Peter Meszaros, John Kwan, Dan Watson, Bill Watson, Minas Kafatos, J. Mayo Greenberg, Telemachos Mouschovias, Reuben Ramaty, Marvin Litvak, Chun M. Leung, Donald W. Goldsmith, Steigman, Moshe Elitzur, Mike Oppenheimer, Jon Weisheit, John Black (as a grad student of Dalgarno), Saslaw, William Klemperer, Berry, Goldsmith, John Lyon, Humberto Gerola, Trinh Thuan, John Krizan, Jim Truran, Theodore Stecher, and Jim Felten among others.

Cross fertilization was in full swing. De Young worked on the interaction of galactic and intergalactic gas, and had discussions with Cox, Silk, and Field on those topics, with Jokipii on winds from galaxies, and on the motion of supernova remnants with Colgate. Shu had discussions with Cameron, Meszaros with Colgate. Jokipii talked with Ramaty about Sco X–1. Felten talked about cosmic pion decay with Goldsmith. Kwan talked about astrophysical masers with Litvak, DeJong, Townes and Goldreich. Spitzer allowed that he “expects other astronomers have benefitted from his discussions of Copernicus satellite.”

Gary Steigman recalls that in the summer of 1972, Dave Schramm was on his way East after receiving his PhD from Caltech. Steigman invited him and his family to stop off at Aspen and stay with him for a long weekend. This was Dave’s first time in “real” mountains, and it was then and there that the bug bit him, leading to his love of mountaineering and his commitment to the Center for the remainder of his all too short life. In their hike that weekend, Steigman met, for the first time, Bob and Elaine Williams who were in Colorado visiting family and just happened to be on the same trail. This may also have been Bob’s first introduction to the Center.

Gary Steigman and Holly, a welcome companion on many hikes.

In addition to cross–fertilization, intense collaborations were started or furthered. Cameron worked with Truran. Sutherland worked with Lamb. Sawyer worked with Sasha Migdal on pion condensation. In 1974, the summer featured the newly traditional June Workshop on the Physics of Galaxies organized by De Young. A 1974 letter from Heinz Pagels to the Executive Committee mentions 213 applicants. One of the new stars among them was Beatrice Tinsley. Colgate, DeYoung, and Pines, astrophysicists all, were on the Executive Committee. The age of astrophysics at the Aspen Center for Physics was in full swing.

The latter half of the decade of the 70’s saw the flowering of the June astrophysics workshops sponsored by NASA as well as the NSF. The astrophysicists were banned to June, early in the summer, but they made the most of it. In this interval, it is somewhat difficult to point to specific developments that were uniquely tied to Aspen, but Aspen clearly nurtured breaking developments, drawing luminaries from throughout the world including the Soviet Scientist Program. This program was a proactive attempt by Aspen to get scientists from behind the iron curtain to Aspen. It was not limited to astronomers, but I think included some astronomers. To make this work, a coterie of dedicated astrophysicists devised and organized the workshops and wrote the proposals to fund them. Among them were Arons, Cameron, Colgate, De Young, Susan Lea, McCray, Steigman, Saslaw and Truran. At the same time, to avoid an “old boys network” aura, great effort was made to capture the most exciting astrophysics and to get new people to attend each year. Diversity would remain an issue (Susan Lea being the lone female in an organizational role in this era), but there was great success in the range of topics and the number of new, young, people introduced to the wonders of astrophysics on the patio of the Aspen Center for Physics.

The early 1970’s had seen the success of Uhuru to open the X–ray sky to the startling understanding that the Universe was not a quiet place, but ablaze with violent activity. In particular there was the dramatic discovery of candidate neutron stars and black holes in binary star orbits. Aspen capitalized on the excitement of this burgeoning era with a workshop on Physics of Compact X–ray Sources organized by Cameron in 1975. Many of the principals of this new field attended: Russell Hulse, who would win the Nobel Prize in 1993 for his co–discovery of the binary pulsar that showed orbital decay at the rate predicted by Einstein, and Riccardo Giacconi, who would later win for work with Uhuru and other missions. Among others in attendance were Kip Thorne who was doing his pioneering work with Igor Novikov on the structure of accretion disks, Lamb, Ethan Schrier, Mel Ulmer, John Bahcall, Ostriker, Kevin Prendergast, Jim Pringle, McCray, Craig Wheeler, Pines, Rees, Sutherland, Truran, Paul Joss, and Stu Shapiro. The topics discussed were neutron stars and black holes, accretion disks, radiative processes, and common envelope evolution. The strange “dips” in the light curve of Hercules X–1 were broadly discussed by many people with Paul Boynton, who had done pioneering work on them. Work continued on neutron star interiors with Phil W. Anderson, who would become a Nobel Laureate in 1977, with Pines, Ruderman, Baym and Shaham all actively involved. Cameron talked about parallels between stellar disks and the proto–solar disk. Cox absorbed discussions of pulsations of magnetic white dwarfs by Pringle and supernova light curves by Arnett to consider what might be computed with facilities at Los Alamos. Bill Rose, Sumner Starrfield, Carl Hansen, Hugh Van Horn and Truran talked about the physics of nova and dwarf nova explosions. Conversations on binary evolution occupied H.–C. Thomas, Wheeler, Hulse, Mike Lecar, and Ostriker. Steigman was involved with his important translation of the book by Yakov B. Zel’dovich and Novikov on astrophysics: Stars and Relativity. First time attendee, John Stewart remarked that there were too many morning talks, a price paid for many years by packing so many astrophysicists onto the patio, all of whom had much to contribute.

Anna Zytkow, a native of Poland, was at the time a postdoc with Thorne. They were doing their pioneering work on the structure of red giant stars with accreting neutron star cores. Kip loaned Anna his ageing Chevy Suburban to make the trip from Pasadena to Aspen. Kip said he thought it might break down, but that having a vehicle break down in remote circumstances and getting it repaired were a formative part of the American experience he hoped that Anna could share. She drove. It broke. She coped and arrived in Aspen to share the patio and the mountains.

The 1976 workshop was organized by Cameron and Truran on the topic of “Environments for Element Production.” This workshop brought together theorists and optical and infrared observers to discuss how measurement of the elemental and isotopic distributions in a variety of astrophysical objects could lead to deeper understanding of the Universe. Steigman talked about primordial nucleosynthesis. Ostriker talked about the possibility of zero–metallicity stars and the conundrum that there seemed to be a floor to the observed metallicity of stars. Dave Arnett talked about the advanced evolution of stars and the conundrum that stars of 4 to 8 solar masses (which Bohdan Paczynski has shown all converged to the same electron–degenerate structure) seemed to explode, not collapse. Later work showed that was a feature, not a bug, and the physics was closely involved with Type Ia supernovae. Dave Schramm talked about the origin of the r–process. There were also discussions of the convective Urca process, another development from Paczynski. There were many discussions of the role of neutrinos, especially neutrino scattering and neutrino degeneracy, in the context of stellar core collapse. The discovery that neutrinos could become degenerate, leading to trapping and adiabatic collapse was just breaking. This insight demanded a hot collapse and hence a hot nuclear matter equation of state, dramatically altering the understanding of the collapse ambience. Unfortunately, despite the qualitative change in physics, models of core collapse continued to fail to explode. Cameron talked about the newly discovered 26Mg anomaly in meteorites. The important implication was that if this isotope came form the relative short decay of 26Al, a nearby supernovae was probably required to inject this short–lived species into the proto–solar nebula.

Somewhere in this era, the Wheeler family invented the concept of marshmallow fights. This was done with uncooked marshmallows that did not hurt and left only a little powdery residue after the mayhem. This began as just a family affair, but one memorable evening, when they hosted many of the workshop attendees at a party, left an indelible image: dozens of world–famous astrophysicists thronging the living room and up the stairwell to the bedroom area, pelting one another with marshmallows in a happy burst of harmless violence. Stale marshmallows were found behind the sofa for weeks afterward.

1978 showed some breaking of the mold. Steigman organized a workshop on Particle Physics and Astrophysics under the auspices of the NSF program, rather than as a June Astrophysics workshop. A June workshop was organized by Colgate and De Young on Astrophysical Plasmas to discuss magnetospheres, particle accelerations, the origin of magnetic fields, nuclear fusion, and the application of plasma physics to planets, stars, and galaxies. A special astrophysics workshop on The Instruments of the 80’s was organized by McCray and Saslaw.

The 1979 workshop on Stellar Collapse and Neutrino Physics was organized by Lamb. In his exit report, Wagoner notes that he talked to Ed Kibblewhite about his new techniques for batch processing photometry. Wagoner recognized that this was a key development in his goal to discover distant supernovae and use them to measure cosmological parameters. It would take two more decades, but these ideas would flower, with Type Ia supernovae replacing Type II as the tool of choice (Type Ia have complex physics, but uniform behavior; Type Ia have a simple composition, hydrogen, but the hydrogen lines arise in strongly in non–thermal equilibrium conditions, making them difficult to analyze in practice), leading to the discovery of the accelerating Universe, the pre–eminent physics problem of the day as the Aspen Center for Physics celebrates its 50th anniversary.

This post was written in celebration of ACP’s 50th Anniversary