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comment by Fantômas
Fantômas  ·  1071 days ago  ·  link  ·    ·  parent  ·  post: What are your favorite thought experiments, hubski?

This thread is misnamed. If it's supposed to be about psychology experiments, which it seems to be so far, it should say so. A "thought experiment" is by definition something you don't actually do, either because it's wildly impractical - for example, it would involve somehow harnessing all the energy in the Universe - or because it's completely pointless. Shrodinger's Cat is an example of this, because although the experimental setup is quite simple, the bizarre result you're trying to observe - a cat which is simultaneously alive and dead - vanishes whenever you try to look directly at it, so you can never actually see it (there are also at least two other reasons why this experiment can never succeed).

Thought experiments are useful because they allow you to mentally explore aspects of science which can't be directly tested without worrying about your inability to do so, which may lead you to discover things which can be tested after all, thus indirectly confirming what you thought in the first place, or may reveal huge logical problems suggesting that your theory is broken. Some of general relativity's predictions about gravity have for the past century been a borderline thought experiment, since gravity is very hard to study because it's such a weak force, but have been assumed to be true because all the testable predictions work out just fine. However, in the next few years, we'll be launching satellites which can do the experiment for real. If they get the expected result, Einstein posthumously becomes officially even smarter. If they don't, Einstein is still very smart indeed, but there's a peculiar gap in his theory which the next generation of theoretical physicists will need to fill somehow, but that's OK because they like a challenge.

Superstring theory, in which the Universe has many extra dimensions but only over ridiculously short distances, is taken very seriously because if you take it to be true it solves a lot of problems, but it's definitely in the realm of thought experiments for the foreseeable future and probably forever, because unless science is completely wrong about a great many things, there's absolutely no way to test these ideas directly without building a machine similar to the Large Hadron Collider, only with a circumference the size of the the orbit of Neptune. But by assuming the Universe is 11-dimensional in a really subtle way that doesn't show, problems can be solved that otherwise couldn't be.

The interesting thing about this theory which we'll almost certainly never actually be able to test is that it doesn't need to be literally true to work. Quantum physics is so weird that "quantum weirdness" is a perfectly serious scientific term, and we're talking about the next level down, so the truth may be completely unimaginable by humans. But a mathematical model which approximates that unimaginable truth does the job pretty well. And since all those extra dimensions, assuming they exist in the first place, only matter over distances smaller in relation to the diameter of a proton than a proton is in relation to the solar system, whether or not we're 11-dimensional doesn't have any practical consequences whatsoever, so don't expect to turn into Doctor Manhattan any time soon.

So, if the thread title isn't a misprint, superstring theory is my favorite thought experiment, mainly because I'm impressed by the sheer audacity of those versions of it which suggest that everything is 10-dimensional apart from gravity, which radiates across the 11th dimension, losing almost all of its strength in the process, therefore there's a parallel universe where everything is insanely heavy that you could get to (a bad idea, since you'd instantly be squashed flatter than all previous definitions of flat) by moving less than a tenth of a millimeter, if you could do that in the 11th dimension. Which is 6 more dimensions than Mister Mxyzptlk knows about, so it's a good trick if you can do it!




unique_username  ·  1071 days ago  ·  link  ·  

The thread was supposed to be about thought experiments, not actual ones, but since the other user submitted one which I thought was interesting we started discussing.

The superstring theory looks interesting, what I would like to know about it is whether it is based on indications or it is simply an unfalsiable possible theory, like the infinite more we could create , as well as the reason that the dimensions are limited to 11 and not infinite. I would also like some recommendations for further reading in thought experiments in general or quantum mechanics , if you have any

Fantômas  ·  1070 days ago  ·  link  ·  

Superstring theory is at present unfalsifiable for the reason I've already given: it would need a ridiculously huge machine to test it (the diameter of the machine has to be enormous for technical reasons you can't get around unless general relativity is completely wrong, which is very unlikely indeed). The basic idea is that fundamental particles which appear pointlike are in fact scrunched-up tangles of "string" with multiple extra dimensions, but this only applies over incredibly tiny distances. The number of extra dimensions has changed over the years, but it's never been infinite - superstring theory used to have a lot more dimensions, but the most viable versions of it have now settled down to either 10 or 11 (that's the usual 3 space and 1 time, plus another 6 or 7 spacial dimensions). In some versions of the theory, the 7th extra dimension is a lot bigger than the others, and only applies to gravity. However, since the thought-experiment based on this idea suggests that if the 11th dimension was bigger than a tiny fraction of a millimeter we could detect it fairly easily, the fact that we haven't implies that either this is wrong, or the 11th dimension, despite being bigger than the others, is still very small indeed.

Superstring theory is a thought experiment because, in addition to the extreme practical difficulty of testing it, most of the people working on it admit that it's an abstract mathematical model that tries to make sense of things we can neither observe nor imagine. But if the theory works, it doesn't really matter whether or not these extra dimensions literally exist, so long as the mathematical model which assumes that they do describes reality better than the previous version.

I can't offhand think of any entry-level books on this particular topic, since it's one of the most complex ideas ever conceived by the human brain, it's still under development, and it quite possibly deals with things nobody has anything like the brainpower to genuinely understand. But generally, you can find quite a lot of texts that aren't overly technical that get into the less insanely complicated aspects of this kind of thing, including the thought experiments you're particularly interested in.

The classic "Two Slit Experiment" is both a thought experiment, in that you can do it in your head and logically figure out what has to happen, and a real experiment in that it's very easy to actually do, but unfortunately logic and reality give completely different results. Since it's mechanically simple but at the same time incredibly baffling, studying it until you more or less sort of understand it (if you really, truly do understand it, congratulations - you're the smartest person on the planet and then some!) is probably a necessary first step before moving on to anything more complicated. Most popular books on quantum physics will give you a detailed account of this experiment in easy-to-understand terms (if they don't, dump that book 'cos it's rubbish). The trick is not to grasp the mechanics of it, which shouldn't tax anyone's brain, but to see what's going on there and why it's so weird. I'm describing a real experiment here, but you don't have to actually do it (though I personally have, and I assure you that it gives results just as odd as the books say it does). You just need to think about the thought-experiment version until something clicks in your head - you'll definitely know when it has. Then you can move on to stuff like quantum entanglement.

If we're talking about specific books, offhand I would say that anything not too technical by Richard Feynman is a good bet. By the way, he won the Nobel prize for physics, so he probably knew what he was talking about. Avoid anything which in any way mentions eastern philosophy or religion, new age beliefs, or telepathy. I promise you that none of this has anything whatsoever to do with real physics.