Remember when I asked for suggested topics for an upcoming Bloggingheads discussion with David Albert about quantum mechanics? The finished dialogue is up and available here:
I would estimate that we covered about, say, three percent of the suggested topics. Sorry about that. But perhaps it’s better to speak carefully about a small number of subject than to rush through a larger number.
And I think the dialogue came out pretty well, if I do say so myself. (And if not me, who?) We started out by laying out our respective definitions of what quantum mechanics “is,” in terms that should be accessible to non-experts. (One user-friendly answer to that question is here.) Happily, that didn’t take up the whole dialogue, and we had the chance to home in on the real sticky issue in the field: what really happens when we observe something? This is known as the “measurement problem” — it is unique to quantum mechanics, and there is no consensus as to what the right answer is.
In classical mechanics, there is no problem at all; you can observe anything you like, and if you are careful you can observe to any precision you wish. But in quantum mechanics there is no option of “being careful”; a physical system can exist in a state that you can never observe it to be in. The famous example is Schrodinger’s cat, trapped in a box with some quantum-mechanical killing device. (Someone must write a thesis on the ease with which scientists turn to bloodthirsty examples to illustrate their theories.) After a certain time has passed, the cat exists in a superposition of states: half alive, half dead. It’s not that we don’t know; it is really in a superposition of both possibilities at once. But when you open the box and take a look, you never see that superposition; you see the cat alive or dead. The wave function, we say, has collapsed.
This raises all sorts of questions, the most basic of which are: “What counts as `looking’ vs. `not looking’?” and “Do we really need a separate law of physics to describe the evolution of systems that are being looked at?”
In our dialogue, David does a good job at laying out the three major schools of thought. One, following Niels Bohr, says “Yes, you really do need a new law, the wave function really does collapse.” Another, following David Bohm, says “Actually, the wave function doesn’t tell the whole story; you need extra (`hidden’) variables.” And the final one, following Hugh Everett, says “You don’t need a new law, and in fact the wave function never really collapses; it just appears that way to you.” This last one is the “Many Worlds Interpretation.”
I want to actually talk about the pros and cons of the MWI, but reality intervenes, so hopefully some time soon. Enjoy the dialogue.
Comments
133 responses to “Quantum Diavlog”
It is perfectly fine for the amoeba to bet which side it will end up on.
Say it guesses that it will end up on the left
If you follow up with the amoeba on the left after the split, you will find that it was 100% correct.
I admit not understanding the many worlds theory, or the larger implications of wave collapses and would like to hear more…
But i would also like to have the issue of sperm addressed… as in, if every sperm reaches the destination, then the many worlds are of every potential variation of every potential life that ever could have transpired?
It isn
It doesn’t necessarily mean this. There is no reason to suspect that macroscopic probabilities due to ignorance of all of the relevant variables are the same thing as superpositions of quantum states. That is to say, sperm are quite large enough that they’re largely classical objects. So there is no reason to suspect that every one makes it in a different one of the many worlds.
The proliferation of worlds within quantum mechanics means that every possible outcome occurs, but this doesn’t mean that every imaginable outcome occurs. And it further doesn’t mean that every outcome which, given our limited information, we think might occur will do so. Many outcomes that we think might occur may, given fuller information, prove to be impossible.
As for the amount of information, well, the Hilbert space that describes quantum mechanics is an infinite-dimensional space. There is no limit to the amount of information.
-Thanks Jason…
The many worlds theory only comes into play when an observation is made of a quantum state? Ok, that is a relief of sorts…
That would mean only quantum physicists could be affected by this.
Does the multi-verse re-enter a symmetry with the rest of us after the observation, or are we all experiencing a butterfly effect from the various different observations made by these quantum observers?
Haha, well, it’s not quite that simple. Basically it’s just difficult to understand quite what the quantum mechanical effects on macroscopic, highly complex systems like our own bodies are. I mean, we understand quite well what the classical limit is, but we don’t really understand how this all translates back to quantum mechanics, so it’s not really worth worrying about (unless, of course, you’re interested in discovering those details).
What’s basically happening with the many observers is that they’re continually dividing, each one only seeing one of the possible outcomes. Recovering some sort of symmetry would be effectively the same as a reducing entropy. We see the effect as, in essence, a loss of information to the environment. These other worlds are inaccessible to us. Basically, portions of the wavefunction of the universe which we can now interact with will become inaccessible to us later through decoherence: the information content of those portions of the wavefunction are effectively lost to us.
information loss? hmmm…
ok, still, so i can envision the dividing observers… but what comes after?
do they un-divide at some point or splinter off infinitum?
If there are multiple observers there must be multiple universes to go along with them, right?
i think the information loss aspect allows for the infinite splitting to go on without having to consider it… if it happens, it happens in a way that we can’t see and therefore doesn’t matter… ok…
but if it’s supposed to only be a limited experience then there are issues… like, what to do with all these “you’s” that just saw different things but are supposed to react the same…
They splinter off ad infinitum until maximum entropy is achieved.
And it’s not so much that they’re multiple universes, but that there are multiple, approximately classical, approximately non-interacting “worlds” of the infinite-dimensional Hilbert space.
I personally think that “multiple universes” is just bad use of language, as “universe” means all that exists. So you can’t, by definition, have multiple universes. It’s just that different components of the wavefunction of the universe can’t interact with one another to any significant degree. These different components are viewed by observers within them as worlds unto themselves, because they can’t see what else is out there.
ok, so here’s where i try to play with this…
each observer takes some note of the trailing digits of collected data…
doesn’t matter what those digits are, but we know they are different to each observer…
we then use these trailing digits to formulate a date and time and show up to our favorite restaurant at that date and time…
now we have split the universe for more than just a few moments, for all our other observers are eating at different times, at different tables, and having different meals… further creating a butterfly effect on the macro scale on all the other people the observer encounters…
quantum physics can become contagious it seems…