David Z. Albert

Elementary Quantum Metaphysics

Once upon a time, the twentieth-century investigations of the behaviors of sub-atomic particles were thought to have established that there can be no such thing as an objective, observer-independent, scientifically realist, empirically adequate picture of the physical world.

Tails of Schrödinger’s Cat

IN HIS FAMOUS THOUGHT EXPERIMENT, Schrodinger (1935) imagined a cat that measures the value of a quantum mechanical observable with its life. Adapted to the measurement of x-spin of an electron, the experiment is this. If at the start of the measurement interaction the state of the cat is |R〉 (the ready to measure state), and the state of the electron is |↑〉 (the electron’s x-spin is up), then at the conclusion of the measurement the state of the cat and electron is |↑〉|Alive〉. And if at the start of the measurement interaction the state of the cat is |R〉 and the state of the electron is |↓〉 (the x-spin of the electron is down) then at the conclusion of the measurement the state of the cat and electron is |↓〉|Dead〉 The cat records the x-spin of the electron in its state of aliveness.

On a proposed postulate of state-reduction

Abstract A recent theory of the collapse of the wave-function due to Ghirardi, Rimini and Weber is described, and is applied to the case of a Stern-Gerlach-type spin-measurement, and is shown to run into some interesting difficulties there.

Special Relativity as an Open Question

There seems to me to be a way of reading some of the trouble we have lately been having with the quantum-mechanical measurement problem (not the standard way, mind you, and certainly not the only way; but a way that nonethe-less be worth exploring) that suggests that there are fairly prosaic physical circumstances under which it might not be entirely beside the point to look around for observable violations of the special theory of relativity. The suggestion I have in mind is connected with attempts over the past several years to write down a relativistic field-theoretic version of the dynamical reduction theory of Ghirardi, Rimini, and Weber [Physical Review D34, 470-491 (1986)], or rather it is connected with the persistent failure of those attempts, it is connected with the most obvious strategy for giving those attempts up. And that (in the end) is what this paper is going to be about.

Physics and Chance

I discuss the role of chance in the fundamental physical picture of the world, and in the connections between that fundamental picture and the various other pictures of the world that we have from the special sciences, and I make a few remarks about the sort of thing that chance would need to be in order to be able to play that role.

On a Theory of the Collapse of the Wave Function

The quantum-mechanical measurement problem is reviewed, and a recent attempt (due to Ghirardi, Rimini, and Weber) to solve that problem by means of a theory of the collapse of the wave function is described. The theory is applied to the case of a Stern-Gerlach type spin-measurement, and is shown to run into some interesting difficulties there.

Physics and Narrative

I present a very simple thought experiment—which has somehow been overlooked in the literature—that has surprising consequences about the Lorentz-transformation properties of the quantum states of multiple-particle systems. [Editors note: for a video of the talk given by Prof. Albert at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-29.]

On the Collapse of the Wave-Function

There is a conventional wisdom about what a workable theory of the collapse of the wave-function ought to be able to do, which runs like this: (i) It ought to guarantee that measurements always have outcomes (that is: it ought to guarantee that there can never by any such thing in the world as a superposition of ‘measuring that A is true’ and ‘measuring that B is true’). (ii) It ought to preserve the familiar statistical connections between the outcomes of those measurements and the wave-functions of the measured systems just prior to those measurements (that is: it ought to guarantee that a measurement of non-degenerate observable 0 on a system in the state | ψ> yields the result o’ with probability | |, where 0|φ>=o’|φ>). (iii) It ought to be consistent with everything which Is experimentally known to be true of the dynamics of physical systems (for example: it ought to be consistent with the fact that isolated microscopic physical system’s have never yet been observed not to behave in accordance with linear quantum-mechanical equations of motion; that such systems, in other words, have never yet been observed to undergo collapses).

On some speculations about the state reductions of photons

Abstract The spontaneous collapses of the wave functions of photons, within the framework of possible relativistic extensions of the theory of Ghirardi, Rimini, and Weber are discussed. It is shown that a recently published argument which claims that such collapses can account for the emergence of determinate outcomes of certain experiments involving fluorescent screens is inadequate, since it is based on an unreasonable speculation about the treatment of photons in the GRW theory.

Two types of prediction in Newtonian and quantum mechanics

Abstract Laplacian prediction in Newtonian mechanics, which requires the simultaneous measurement of 6N quantities for N particles, fails in quantum mechanics. We analyze single measurement prediction, which uses measurement of one present quantity to predict any one measurement at a prescribed future time, and which works in Newtonian and quantum mechanics.