Shortly after Howard earned his Ph.D., his BU colleague John Stachel became the first editor of the Einstein Papers Project and invited Howard to be an assistant editor on the critically important volume 2 writings from 1900 to 1909.

Howard has written several papers on the early development of quantum mechanics up through the 1935 paper on the so-called Einstein-Podolsky-Rosen Paradox.

In more than one of his papers, Howard explored the deep question of "nonseparability," distinguishing it from nonlocality. Most historians of physics trace this problem to the EPR paper itself, or at the earliest to the 1927 Solvay Conference where Einstein did not present a paper but made some very insightful remarks and drew a puzzling diagram on the blackboard.

Einstein saw nonlocality in his earliest work on quantum mechanics, his photoelectric paper of 1905. But it was the nonlocal relationship between a single particle and its wave. Einstein first saw nonlocal behavior between two particles that had separated in 1933. He then made it, and nonseparability, famous with his 1935 EPR paper.

In 1984, Howard presented a paper, "Einstein on Locality and Separability," at the Boston Colloquium for the Philosophy of Science. Shimony chaired the session.

Howard noted the disregard for Einstein's work in the late 1920's and 30's, and he summarizes what Einstein was actually arguing, almost never appreciated by Einstein's colleagues...

too many physicists and philosophers have been inclined to dismiss Einstein's misgivings as being a result of the naive but forgivable failures of understanding of an old man still clinging to an outmoded, deterministic metaphysics...

Einstein called it his "Trennungsprinzip"

In brief, what Einstein argues is that the incompleteness of quantum mechanics follows from the conjunction of two assumptions. The first, which I call the 'separability principle', asserts that any two spatially separated systems possess their own separate real states. The second, the 'locality principle' asserts that all physical effects are propagated with finite, subluminal velocities, so that no effects can be communicated between systems separated by a space-like interval.

("Einstein on Locality and Separability," p.172)

the Bell experiments demonstrate not the existence of a peculiar kind of 'quantum non-locality', but instead the existence of quantum non-separability. And Einstein's analysis implies that therein lie the seeds of a conflict not with special-relativistic locality constraints, but with assumptions fundamental to general relativity and any other field theory. Given their importance in what follows, the separability and locality principles should be clearly distinguished. To repeat: separability says that spatially separated systems possess separate real states; locality adds that the state of a system can be changed only by local effects, effects propagated with finite, subluminal velocities. There is no necessary connection between the two principles, though they are frequently stated as if they were one.

(ibid, p.173)

In 1989, Howard wrote an article for the International School for History of Science meeting at Erice, Italy. It is what may be his most important contribution to Einstein Studies in general and the problem of nonseparability in particular.

It was boldly titled "NICHT SEIN KANN WAS NICHT SEIN DARF," OR THE PREHISTORY OF EPR, 1909-1935: EINSTEIN'S EARLY WORRIES ABOUT THE QUANTUM MECHANICS OF COMPOSITE SYSTEMS."

Howard describes Einstein's famous debates with Bohr and Einstein's well-known misgivings with quantum mechanics and writes

Why tell the story yet again? The answer is that there is more to be said. I will argue that the standard histories have overlooked what was from early on the principal reason for Einstein's reservations about quantum mechanics, namely, the non-separability of the quantum mechanical account of interactions, something ultimately unacceptable to Einstein because it could not be reconciled with the field-theoretic manner of describing interactions. Showing the significance of this issue for Einstein is important not only for the sake of setting right the historical record, but also because it makes Einstein's critique of quantum mechanics far more interesting—from the point of view of the physics involved—than if we see it resting merely on a stubborn old man's nostalgic attachment to classical determinism.

(Nicht Sein Kann Was Nicht Sein Darf," p.61)

Howard added a footnote,"To my knowledge, [Arthur] Fine (1986) is the only author who has so far hinted at the importance of this worry in Einstein's thinking about quantum mechanics prior to 1935.

Howard summarizes the conflict with field theory...

The argument is simple. In a field theory, the fundamental ontology, the reality assumed by the theory, consists of the points of the space-time manifold and fundamental field structures, such as the metric and stressenergy tensors, assumed to be well defined at each point of the manifold. Implicitly, therefore, any field theory assumes (i) that each point of the manifold, and by extension any region of the manifold, possesses its own real state, say that represented by the metric tensor, and (ii) that all interactions are to be described in terms of changes in these separate real states, which is to say that joint states are exhaustively determined by combinations of the relevant separate states, just as the separability principle demands. If this is correct (and I think it is), and if the quantum mechanical account of interactions denies separability, then there can be no reconciliation of the two. Moreover, Einstein had not inconsiderable (if not ultimately compelling) arguments—methodological, epistemological, and metaphysical—for retaining both locality and separability, which helps to explain his dogged commitment to the field theory program as an alternative to quantum mechanics.

(Nicht Sein Kann Was Nicht Sein Darf," p.65)

When Howard worked on the CPAE volume 2, he undoubtedly studied Einstein's 1905 paper on the light-quantum hypothesis carefully. He clearly sees Einstein worrying about these issues from his earliest paper on quantum mechanics. In his article for the 1997 Shimony Festschrift, Howard wrote the following.

the nonseparable manner in which quantum mechanics describes interacting systems..[is an]... aspect of the quantum had bothered Einstein since his own first paper on the quantum hypothesis in 1905, and from the time in 1925 when his work on Bose-Einstein statistics convinced him that it would be an unavoidable feature of any eventual quantum formalism.

(""Spacetime and Separability: Problems of Identity and Individuation in Fundamental Physics."," p.114)

Howard draws our attention to Einstein's 1905 description of his light quanta as "mutually independent" particles.

Already in this first paper on the light quantum hypothesis in 1905, Einstein realized that the full story of the quantum would involve some compromise with classical notions of particle independence. He remarked explicitly that the assumption that light quanta behave like independent particle-like carriers of electromagnetic energy would yield only the Wien end of the black-body spectrum. As to what kind of independence assumption we were making here, he was quite explicit. We are assuming, he said, that Boltzmann’s principle is valid for a collection of light quanta, meaning, as he again said explicitly, that the joint probability for two light quanta to occupy two specific cells of phase space factorizes as a product of the separate occupation probabilities

(""Spacetime and Separability: Problems of Identity and Individuation in Fundamental Physics."," p.116)

Einstein's 1905 paper is largely remembered as his explanation of the photoelectric effect. His Nobel Prize was specifically awarded for this work, not for his work on relativity. But the paper contains many more words on light quanta, and still more on the entropy of the radiation. In the three years before Einstein's "miracle year,' he derived the laws of statistical mechanics, mostly independent of Ludwig Boltzmann (whose work he did not fully know) and J. Willard Gibbs (whose work he did not know at all). Einstein found that the relation between entropy and probability depends on the statistical independence of non-interacting systems.

Einstein wrote...

If it is reasonable to speak of the probability of the state of a system, and furthermore if every entropy increase can be understood as a transition to a state of higher probability, then the entropy S₁ of a system is a function of W₁, the probability of its instantaneous state. If we have two noninteracting systems S₁ and S₂, we can write

S₁ = φ₁ (W₁)
S₂ = φ₂ (W₂)

If one considers these two systems as a single system of entropy S and probability W, it follows that

S = S₁ + S₁ = φ (W)

and

W = W₁ · W₂.

("A Heuristic Viewpoint on the Production and Transformation of Light," AJP translation, p.371)

This gives us entropy as the additive (extensive) quantity that Rudof Clausius wanted, that J. Clerk Maxwell found by assuming the x, y, z components of molecular velocities are statistically independent (which of course they are not exactly, for high energies, since energy is finite), and that Boltzmann confirmed with his "principle," as Einstein called it, that the entropy S = k logW, where W is the number of possible distributions of particles in available phase space cells.

Howard is very insightful to focus on the "mutual independence" that grounds Einstein's deep belief in "Boltzmann's principle." Howard shows us that Einstein's greatest concern all his life was not the question of indeterminism and the uncertainty principle, but interactions between separated quantum particles.

We argue that Einstein also saw as early as 1905 the so-called "collapse of the wave function," twenty years before there was a Schrödinger wave function. If the light wave carries a substance, Einstein worried, how could it collect all that substance instantly into a light quantum to eject a particle in the photoelectric effect? Would it have to move faster than the light, violating his new principle of relativity?

Recently, Howard posted thirty-six issues of the extremely valuable "Epistemological Letters", a privately published newsletter that were circulated to about 180 of the most prominent physicists in the world between 1973 and 1984. It contained contributions to the foundations of physics inspired by the work of Bell, which in turn had been inspired by David Bohm's revival of Louis de Broglie's "pilot wave theory." It documents the beginning of the "Age of Entanglement."

"Einstein on Locality and Separability.” Studies in History and Philosophy of Science 16, 171-201 (1985)

Nicht Sein Kann Was Nicht Sein Darf, or the Prehistory of EPR, 1909-1935: Einstein’s Early Worries about the Quantum Mechanics of Composite Systems.” In Sixty-Two Years of Uncertainty, ed. Arthur Miller. (1990)

Review of Arthur Fine's The Shaky Game Synthese Vol. 86, No. 1, pp. 123-141 (1991)

"Spacetime and Separability: Problems of Identity and Individuation in Fundamental Physics." in Potentiality, Entanglement and Passion-at-a-Distance: Quantum Mechanical Studies for Abner Shimony, Volume Two, pp.113-141. (1997)

Einstein: The Formative Years, 1879-1909. Springer Science & Business (2000)

"Who Invented the “Copenhagen Interpretation?” A Study in Mythology," in Philosophy of Science , Vol. 71, No. 5, pp. 669-682 (2004)

"Albert Einstein as a Philosopher of Science," Talk at the AAAS meeting, Washington, D.C. February 20, (2005)

"Albert Einstein as a Philosopher of Science,", Physics Today, 58(12), 34-40. (2005)

“Revisiting the Einstein-Bohr Dialogue.” Iyyun: The Jerusalem Philosophical Quarterly 56, 57-90. Special issue dedicated to the memory of Mara Beller. (2007)

Reduction and emergence in the physical sciences: some lessons from the particle physics and condensed matter debate (pp. 141-157). Oxford: Oxford University Press. (2007).

“Einstein and The Development of Twentieth-Century Philosophy of Science,” in The Cambridge Companion to Einstein, Janssen & Lehner. 2014, 354-376. (2014)

"Virtue in Cyberconflict," in The Ethics of Information Warfare, Springer (2014)