so many worlds to tease my soul... quantum mechanically speaking

The link said “Time Travel: Fiction or Reality?” And I am forced to admit, I am a sucker for newsish articles on time travel. I know (rationally) that anything I find about it on is going to have less depth than a wombat’s contact lens. But I couldn’t help myself (my own knowledge is only as deep as the wombat’s reading glasses) I had to see what they said. The article was a puff piece reviewing Crichton’s new book “Timeline” and essentially said that the science he bases his story on is theoretically possible.

The article focused on Crichton’s maneuvering around the oft-used ‘grandfather conundrum’ by appealing to Everett’s many universe hypothesis. (The many universe hypothesis essentially says [great, now I am guilty of oversimplifying things too…sheesh!] that when there is an apparent collapse of a wave function, the universe actually splits [though ‘splits’ is not quite right] and there is a true (but separate) reality in which each possible outcome definitely occurs).

I have thought (or have been taught, rather) however, that Everett’s hypothesis didn’t form a coherent model of the universe for two reasons. First is a statistical reason. A good portion of our awareness of quantum mechanical conundrums stems from the statistical nature of the outcome of measurements. However, if in each measurement all results are one hundred percent guaranteed to occur, how do our observations is ‘this’ reality make any sense at all? Secondly, the number of universes that are created seems to depend on the terms of the equation, terms arbitrarily chosen by the experimenter.

So here (finally) are my questions. The article said

Why? Have there been advances to further Everett’s hypothesis, or have new logical arguments been put forth that would support the hypothesis? Or are no new logical arguments necessary, and the hypothesis can stand on its own.

Though there is a collection of theories put forth that are somewhat similar (quantum foam, etc.) I am hoping to see responses that deal mainly with Everett, because that is what I am most familiar with. Allow me to come to some understanding of one theory before I try and understand the next.

And lastly, I beg your forgiveness for my sloppy terminology, oversimplifications and for asking a question the answer to which I have little hope of truly understanding. Wish me luck.


Once in a while you can get shown the light
in the strangest of places
if you look at it right…

If anyone answers the OP in support of the many-worlds hypothesis, I’d like to add a question.

Everything I’ve ever seen about many worlds involves lookign at cases with two, or at most a finite number of discrete possibilities. How does many-worlds handle the case where there is a continuum of possible outcomes?

It is too clear, and so it is hard to see.

Well I’m a theorist, and I know nonrelativistic QM pretty thoroughly, and I don’t give a damn about many-worlds, nor do I know any theorists who do. I’ve heard of nothing recently that would change my opinion, notwithstanding. And who the fuck are they to issue an opinion anyway? Get your latest physics news from Physics Today or Science, if you want something worth the paper (or phosphor) it’s written on.

I don’t think either of your criticisms of many-worlds are valid. In the case of statistics, many-worlds would presumably assert that the number of worldlines exiting a junction with the universe in eigenstate J is proportional to the probability amplitude of measuring J. This preserves your traditional experience of probability as you trundle on down your worldline.

Second, the number of eigenvalues of the measurable operator A is independent of the experimenter’s choices. He can’t affect the number of outcomes at all. Of course, he can choose to measure one thing versus another, and this will presumably determine the nature of the many-words junction.

Which brings us to the real problem: the meaning of measurement. Why should the universe jump its rails every time I make a measurement? And what happens when there are no conscious beings measuring things? Does it still? This is worth debating, but I have little useful to say on the subject, because I know my limits.

Oh, I forgot: if your observable operator has a continuous spectrum (the second question), then you have over time an undenumerable (aleph-1) infinity of worlds, instead of a denumerable (aleph-null) infinity of worlds. Nothing interesting about this that I can see.

There are a number of interpretations still in the running as the ‘correct’ interpretation of results in quantum physics. Anyone who says otherwise should point to the experiment that rules out an interpretation, rather than appeal to the majority opinion of physicists. For an FAQ on many-worlds, see:

cgrayce, if space time is quantized, how do you get to aleph-1?