Posted: Aug 03, 2011 2:31 pm
by Evolving
M319 wrote:
Evolving wrote:Even if many-worlds is correct, the thought experiment is only valid if everything that happens in the macro-world, including somebody's death, is the result of (or attended by) the collapse of a wave function at a quantum level.

A sweeping assumption, or not?

Perhaps this has not occurred to you but quantum phenomena underly EVERYTHING - considering that everything is ultimately constructed from subatomic particles :roll:

I see that I should explain my point in more detail (guilty of assuming that we are physicists amongst ourselves on this forum and can therefore afford to hand-wave at an argument, rather than derive it step by step!). Apologies if this time around I am being too detailed.

A very small physical entity, such as an electron confined in an atom, or a photon being emitted from a source of radiation like a light bulb, is described by a wave function, which is associated with a probability function. In respect of a measurable value which that physical entity can have, the wave function determines how two or more possible values (eigenvalues) are superposed upon each other. In certain circumstances, which may be created by a measurement carried out by a human being, or may simply arise, without human intervention, because of what is going on around the physical entity that we are considering, the physical entity is compelled to assume one of those eigenvalues, and we have no way of knowing beforehand which of those eigenvalues will be measured: all we can say is the probability that it will be this eigenvalue rather than another. When this occurs, we talk about the wave function of the physical entity collapsing, and say that it is behaving like a particle: like a electron going through a single slit in a diffraction grating, rather than propagating wave-like through more than one slit and interfering behind the grating.

The many-worlds interpretation conjectures that, on such occasions, it is not the case that one eigenvalue is selected from among those on offer, and all others are discarded, but in fact several parallel universes emerge, in each of which one of the possible eigenvalues is measured.

An electron is pretty much the largest particle of which this is true: anything larger would have a de Broglie wavelength far too small to have any physical significance, so that we can say it always behaves like a particle, and never like a wave.

Now, all this may or may not be correct, and is all well and good in (possibly) describing how a single quantum-sized physical entity behaves when forced to make up its mind which of its eigenvalues to adopt. The question arises, however, as to how much of macroscopically observed events can be associated with the collapse of quantum-level wave functions, as just described. Clearly there are macroscopic events which are so associated: the field of solid state physics with its multitudinous applications provides many examples. And, to reverse the direction of causality, macroscopic events may well (one assumes) cause innumerable wave functions to collapse among the microscopic entities that are involved in, or affected by, those events.

But outside of areas like solid state physics (and Schrödinger's famous thought experiment involving a cat), which are deliberately engineered so that a quantum event will have a macroscopic effect, how likely is it that a macroscopic event - the death of a human being, say - will turn out differently, depending on whether a particular electron, for instance, is measured to have its first, second or third of its three eigenvalues? Highly unlikely, I should have thought; so if somebody willingly embraces his own death in the confidence that there will be at least one parallel universe in which he survives, I think he is probably making a mistake. There may well be some parallel universes emerging as a result of whatever steps he takes to cause his own death; but in all of them he is likely to be dead.