Ask MetaFilter questions tagged with quantumphysics

How are particles measured?

Citizen Premier — Thu, 11 Sep 2008 19:35:41 -0800

How are the attributes of atomic and subatomic particles measured? I guess you can't measure a particle without slamming something into it, right? How is this actually done?

Bonus: Is it this that forces us to use uncertainty in our current model? Does measuring a particle's location, say, change its momentum?

Oh, boy, parallel universe #57339! That's where I'm a Viking.

Rhaomi — Sat, 24 May 2008 17:39:43 -0800

How would the many-worlds interpretation work on the human level, if at all? (I'm no quantum physicist, so please forgive me if the following is woefully simplistic, ridiculously naive, and/or hopelessly wrong)

The way I've heard it explained, the many-worlds interpretation of quantum mechanics holds that for every situation in which multiple outcomes are possible, each one of those outcomes does happen -- albeit in its own universe. That the universe we perceive is just part of an inconceivably large multiverse of infinitely branching possibilities, and that every interaction between every atom everywhere in the universe creates another one, or multiple ones, all the time.

I've also heard that because every possible outcome occurs, even the most bizarrely improbable event has happened in at least one universe. This makes sense if the ideas in the above paragraph are true.

For instance, it is incredibly unlikely that a fair coin could come up heads twenty times in a row -- the odds are about 1,048,576 to 1. But if each coin toss branches into a universe where it lands heads and a universe where it lands tails, then at the end of the line one of the million+ universes would see it land heads all twenty times. Of course, most of the rest of the million branches would see a mixed outcome, so from the point of view of a single universe the odds are still very unlikely. But the many-worlds theory says it does happen somewhere.

But when you think about it, it wouldn't be that simple. For starters, each coin toss would make more than two universes. A lot more. For example, a single coin toss could have two universes where the coin lands tails, but one universe sees it land one centimeter further to the right than the other. And there would be a small set of worlds where the coin landed perfectly on its side. And an even smaller minority where all the molecules of the coin spontaneously evaporated at the same time. A colossally improbable event, but possible.

And hey, if this holds true for molecules and small objects like coins, could you not also extend it to the rest of the world (which is just a collection of 10^huge molecules)? Must there be a universe out there where, say, every person who bought a Florida State lottery ticket happened to pick the same number, which was the winning one? Or where every building on Earth suffered simultaneous structural failure? Or where everyone spontaneously decided to break into Broadway-style song and dance? And a trillion variations on these and other scenarios, each slightly different from the other? And that the only reason we (most of our selves?) don't experience these things is because the infinity of "normal" universes where probable things happen outnumbers the infinity of universes where "impossible" things happen?

It feels absurd, like I'm talking about the Infinite Improbability Drive instead of a theory of physics, but I'm not seeing why it shouldn't be true. I've tried finding answers, but most of the literature out there (with the occasional exception) deals with quantum physics in a dry, academic context that limits the discussion to the atomic level. And of the material that imagines crazy outlier universes like the ones I described, I don't have a good way of telling if the physics involved is real or just taking artistic liberties for the sake of interesting fiction.

Must I mourn for the Earth somewhere out there that suffered Total Existence Failure? (And the one that suffered it five minutes later, and the other one that suffered it sixteen years later, and the one that suffered it partially, losing the western hemisphere, and the one that lost the eastern hemisphere, and the one that suddenly split into two planets, and the one where Australia turned into gelatin, and etc.)

Oh, and I know that we still aren't sure about which interpretation of quantum mechanics is correct, and that even if we knew the many-worlds theory were right, we'd have no way to observe other universes. I just want to know if the things I described are allowable in the context of the theory as it's understood today.

Why can't I move shit with my mind?

Nattie — Sat, 29 Mar 2008 06:18:51 -0800

On what grounds does one draw the line between quantum physics/mechanics and pseudoscientific claims about the powers of consciousness? I'm naturally skeptical of claims like the following that purport to be supported by quantum physics/mechanics:

- Telekinesis.
- Telepathy.
- Healing yourself with your mind, etc.

I don't know enough about quantum physics to understand where the reasoning goes south, though. When I read skeptical articles, they usually dismiss the claims by saying "they misunderstand the science," but they don't go into detail. That's what I'm looking for.

If it's helpful, these sort of things tend to take the idea that your consciousness acts on everything it observes and extrapolates from there.

Does it have something to do with the idea of a "conscious observer" not necessarily meaning "a human being with a conscious," depending on your interpretation of quantum mechanics? Is it conflating superposition with observer effect? Something else?

Are these claims actually possible in some "legitimate" interpretations but we're doubtful those interpretations are true? Or are all the claims based on misunderstanding?

I feel like I have a very vague grasp of these concepts, and whenever I try to pinpoint the flaw in reasoning my mind can't process it.

What do quantum physicists mean when they say "observer"?

L. Fitzgerald Sjoberg — Sat, 29 Dec 2007 18:01:23 -0800

In quantum physics, what's an observer? Okay, so the term "observer" comes up a lot in explanations of quantum physics. When I hear about Schrödinger's cat or the double-slit experiment, I hear "observer" and I think of some guy in a lab coat and bow tie leaning in and POOF! There's a cat! And of course that gets me thinking things like "What if you videotaped the live/dead cat and watched it later?" and "Hey, why can't the cat observe itself?"

When online discussions head in that direction, generally someone steps in and says "No, it doesn't work that way, when we say observer we don't mean necessarily a person playing peek-a-boo." But when they try to explain what they DO mean by observer, well, I'm sad to say I haven't quite grasped it.

So can anyone out there explain it the way Isaac Asimov would have? (Or did, I suppose.) Or can you point me to someone who can?

What the bleep do we know about weaponizing Quantum Mechanics?

basicchannel — Thu, 16 Nov 2006 00:10:28 -0800

Have humans' evil brains yet conceived of ways to weaponize the ideas and implications relating to Quantum Mechanics? It took roughly 30 years for humanity to weaponize General Relativity. It's been, what, roughly half a century since QM/QP has gained acceptance? This shocking lapse in evil human ingenuity can mean only one thing: I am ignorant to the various sundry DARPA projects that have been undertaken to solve this vexing problem! Please hope me MeFi!

State of Quantum Physics

evinrude — Thu, 18 Dec 2003 22:13:25 -0800

I'd like to read a readable, yet not dumbed-down account of the current state of quantum physics, addressing the famous paradoxes and directions modern research is taking. Any recommendations? [more inside] I'm a mathematician by training but not a physicist, and I'm inspired to ask this question because I just finished reading Paradigms Lost (and its sequel) by John L. Casti and found his explanations less than clear. I'm not sure if this was my fault or his. I guess I'm probably looking for something less technical than a journal article, but more rigorous than the science section of the New York Times. Please help me!