Foundations of Physics, Vol . 29, No. 3, 1999

For Daniel Greenberger on His 65th Birthday

In order to make contributions to the foundations of quantum mechanicsone must be an intellectual tightrope walker, balancing so as to avoid fall-ing into the abyss of trivial formalism on the one side and the abyss ofspeculative confusion on the other. Dan Greenberger is one of the mastersof this kind of high-wire performance ( even if he does not look like one),as this Festschrift intends to celebrate.

He has clarified theoretically the physics of neutron interferometry,particularly in a review article with Albert Overhauser. And as a VisitingScientist at Clifford Shull’s Neutron Diffraction Laboratory at MIT, heparticipated in the design of several imaginative experiments, such as thesearch for a neutron Aharonov± Bohm effect and a neutron version of thedelayed choice experiment.

Over many years he has investigated the complicated relation betweenquantum mechanics and general relativity theory. Notably, he has pointedout that these two fundamental physical theories have very different treat-ments of mass and proper time, and as a result, quantum mechanics raisesdifficulties for the equivalence principle.

He has thought deeply about the argument for hidden variablespresented by Einstein, Podolsky, and Rosen and the theorem of J. S. Bellproving that no hidden-variables theory of the local variety envisaged byEinstein could agree with all the predictions of quantum mechanics. Hewas the prime mover in a collaboration with M. Horne and A. Zeilingerthat improved Bell’s original theorem by showing that even some of thepredictions which quantum mechanics makes with certainty, rather thanstatistically, cannot be replicated by a local hidden-variables theory.

He has been one of the pioneers in the extension of multiphoton inter-ferometry from pairs to n-tuples of photons with n greater than 2. This isa field which is just beginning to be developed, and it promises a very richarray of phenomena. Dan has not only theoretically analyzed entangledstates in Fock space and examined the variety of multiparticle superposi-tions, but he has participated in the design of experiments to exhibit someof the multiparticle phenomena.

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Dan’s character and temperament are as remarkable as his intellectualachievements. He is tenacious and passionate in his curiosity about thephysical world, wonderfully generous and free from self-aggrandizement,deeply concerned about social problems, and delightfully humorous andhigh-spirited at the same time that he is serious. Those who have beenfortunate enough to collaborate with him in research have been thebeneficiaries of these qualities. Collaboration with him is both a serioussearch for the truth and a high-spirited game. His qualities also enabledhim to organize two of the best conferences on the foundations of quantummechanics that any of us has had the privilege to attend, one in January1986 entitled `̀New Techniques and Ideas in Quantum MeasurementTheory,’’ and one in June 1994 entitled `̀Fundamental Problems in QuantumTheory.’’ The first was dedicated to Eugene Wigner and the second to JohnA. Wheeler, each of whom attended and participated in the conferencededicated to him. For both conferences Dan obtained the sponsorship of theNew York Academy of Sciences, found the funding, did most of the organiza-tional work, and infused the sessions with his own inimitable enthusiasm.

Finally, Dan’s humor is unforgetable. One of his papers is entitled`̀Evidence for the Law of the Excluded Muddle,’’ and another lists as authors`̀Greenberg, Greenberger, and Greenbergest.’’ Some of his imprompturemarks deserve to be anthologized. For example, in a discussion of`̀Theories of Everything,’’ he commented, `̀ I would not want to live in auniverse that I could completely understand.’’

Dan has enriched both the discipline of physics and the lives of thecontributors to this Festschrift .

Michael A. HorneAbner ShimonyAnton Zeilinger

324 For Daniel Greenberger on His 65th Birthday