Suppose that the world consisted entirely of point masses, moving in perfect accord with the Newtonian law of motion, under the influence of some particular collection of interparticle forces. And imagine that that particular law, in combination with those particular forces, allowed for the existence of relatively stable, extended, rigid, macroscopicarrangementsof those point masses—chairs (say) and tables and rocks and trees and all of the rest of the furniture of our everyday macroscopic experience.¹ And consider a rock, traveling at constant velocity, through an otherwise empty infinite space, in a world like that. And note that nothing...

Huckleberry:Why is it that the future can apparently be affected by what we do now, but the past apparently can not?

Jedediah:I’m not sure I understand the question. The past can not be affected by what we do now—I take it—precisely because it is thepast:because it’ssettled,because it’sgone,because it’sdone,because it’sclosed,because it’sover.The past can not be affected by what we do now because it is of the veryessenceof the past—whatever, exactly, that might turn out to mean—that it can not be affected...

Consider a classical Boltzmannian universe—a universe (that is) whose laws consist of a set of deterministic time-reversal-symmetric Newtonian or Hamiltonian equations of motion, and of a hypothesis about the initial macrostate of the universe, and of a probability distribution over the exact microstates compatible with that macrostate.

The distinctive contributions of the various individual components of a theory like this to the overall form of the world—and particularly the contribution of the hypothesis about theinitial macrostateof the universe—are worth attending to.

Imagine (to that end) that weremovethe hypothesis about the initial macrostate of...

A proposed complete scientific theory of the world counts asempirically adequateif it makes the right predictions about everything observable.

Putting things that way, however, suggests that in order to settle the question of whether or not some particular proposed complete scientific theory of the world is empirically adequate, we must first (among other things) settle the question of what the observable features of the worldare.And that isn’t right. It’s a sufficient condition of the empirical adequacy of any complete scientific account of the world (as a matter of fact) that it make the right predictions, under...

Consider a system of four distinguishable quantum-mechanical spin-½ particles. Call itS. And suppose that the complete history of the motions of those particles in position space—as viewed from the perspective of some particular Lorentz frameK—is as follows: Particle 1 is permanently located in the vicinity of some particular spatial point, and particle 2 is permanently located in the vicinity of someotherspatial point, and particles 3 and 4 both move with uniform velocity along parallel trajectories in space-time.¹ The trajectory of particle 3 intersects the trajectory of particle 1 at space-time pointP(as in...

The picture that almost everybody seems to have in their heads on first being introduced to the Bohmian mechanics of multiple-particle systems—call it the two-space picture—is of a world that unfolds simultaneously in two real, physical, concrete, fundamental spaces. One of these is a three-dimensional space inhabited byNmaterial corpuscles, and the other is a 3N-dimensional space inhabited by a real, concrete, physical wave function—a complex-valued field. The wave function undulates in the high-dimensional space in accord with the Schrödinger equation. And that wave function (in turn) tells the material corpuscles how to move around, back...

Here’s a way of looking at one particular train of thought about the quantum-mechanical mea sure ment problem:

The problem (to begin with) was put in its clearest and most urgent and most ineluctable form, in the first half of the twentieth century, by figures like Schrödinger and von Neumann and (especially and particularly) Wigner. They thought of quantum mechanics—at least in its first-quantized, nonrelativistic version—as a theory of fundamental material particles, moving around in a fundamental three-dimensional space. And they supposed that those particles were the sorts of things to which one could coherently attribute dynamical properties...

Let me start off by rehearsing what I take to be the simplest and most beautiful and most seductive way of understanding what it was that Everett first decisively put his finger on fifty years ago.

There is supposed to be a problem with the linear, deterministic, unitary, quantum-mechanical equations of motion. And that problem—in its clearest and most vivid and most radical form—runs as follows: The equations of motion (if they apply to everything) entail that in the event that somebody measures (say) thex-spin of an electron whosey-spin is initially up, then the state of...