Here's a great animated explainer video about String Theory


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Kurzgesagt is wonderful. I really enjoyed their video on optimistic nihilism. I particularly liked how well this video addressed the Heisenberg uncertainty principle. Thanks to popular media, many people seem to think of it as almost magic, like the things we’re trying to measure possess some kind of consciousness when in reality the phenomenon has a pretty basic explanation.


wow, awesome


I can follow just fine right up to the point where the ten dimensions show up. Length, width, depth, time … and what? Why ten?

I have questions.


I agree, except wtf was the part about the cruise ship?


Dup, Owen, Fleb and Blep.

That’s what I call the next four dimensions, anyway.


String theory, and the related M-theory (which assumes 11 dimensions), hypothesize that point particles are actually waves resonating in extra dimensions which are closed in on themselves (compactified). In other words, if you could head off along one of these dimensions, you would wind up back where you started. The distance to do so is unimaginably small, so small that a wave with a wavelength so short it appears to be a single point in the three expanded space dimensions can span it. Some wavelengths are the right distance to establish standing waves in some of these dimensions. The idea is that the wavelengths and frequencies of these waves correspond to the quantum numbers (the values one must plug into the Schrödinger equation to describe a quantum system over time). Different values correspond to different elementary particles (such as the various kinds of electrons and quarks).

The goal of string theory has been to figure out how many of these compactified dimensions you need to account for all the particles in the Standard Model of particle physics. The first consistent strong theory required 26 dimensions, but it was later realized that you could get the Standard Model with 10 dimensions.

The problem is that no one knows if it’s true. It’s widely regarded as an elegant idea, but that doesn’t make it correct. Moreover it’s extremely difficult to test because most of the results that would confirm it require studying particle interactions at energy densities well above what current high-energy particle accelerators can achieve.

Consequently some string theorists have turned to cosmology since the universe went through those energy densities during its very early expansion and may have left evidence in the structure of the observable universe.

One major criticism of string theory is that it posits a whole raft of supersymmetric “twins” for many known particles, for which there’s no experimental evidence. Searching for these supersymmetric particles is a major part of what string theorists do at particle accelerators such as the LHC, but so far to no avail.

Hope that helps!


Okay, my understanding trudges onward. Thanks.

So instead of a particle/wave, or a location dot, we have a string vibrating in ten dimensions.
Quantum theory got down to three fundamental particles - boson, photon, and gluon. (Good names for pet ferrets.) But what is gravity? The math for it as a particle just doesn’t work.

But the math for the three particles and gravity ‘particles’ does work if they each are the same kind of string just vibrating differently in those ten dimensions. I have to take it on faith that the math works out.

(I didn’t understand Special Relativity till I realized that the math had to work, and at those theorized speeds the only thing that could change would have to be the speed of light. A spiritual awakening of sorts.)

Sooo, yes I am going to go there. What is a quark, and what was all that fuss about it a little while ago? Is it smaller or bigger than our three fundamental ferrets?

I said I have questions.


I think the cruise ship is the ten dimensions, and the little boat is us in our dinky three dimensions trying to understand cruise ships.


And that is more problematic than it sounds. If the (previously) popular visions of Supersymmery were correct, they really should have found something.


I’ve definitely found some Kurzgesagt videos to be worthwhile. I didn’t expect to learn anything about string theory (that I’ve studied a bit) from this one but hoped it might be informative to my children. Pretty disappointed.

The explanation of the uncertainty principle, as being about the need to disturb a particle to measure it, doesn’t get at the heart of the matter: under the wave-function point of view on particles, they don’t have a simultaneously localized position and momentum. (The video merely explains that even if they did, you’d have trouble using light to find out what they were.)

The best I can do on the 10-dim issue is this. Mathematically, it turns out that dimensions that are multiples of 8 are better and easier than others. For example consider the kissing number problem: if you pack pennies into the plane, then each penny touches six others (“the kissing number in dimension 2 is 6”). In 3 dimensions one can use solid angles to figure out that it’s at most 13 (and 12 is easy to achieve); Newton showed it’s actually 12. In 4 dimensions it’s 24 or 25 – nobody knows! But in 8 and 24 dimensions we do know – it’s 240 and 196,560 respectively.

So why do string theorists like 10 and 26? In a sense, the one time dimension eats one of the space dimensions, leaving over the 8 and 24 that are well-behaved.

Here’s another great property of 24, with which one can find out that 196,560 fact with enough work. Consider the sum 1^2 + 2^2 + … + n^2 of the first n squares. When is that a square? Well, n=0 and n=1 work, obviously. But in fact n=24 also works, and, it’s the only other one that does. Armed with that one can build a “lightlike” vector (0,1,2,…,23,24,70) in 26-dim spacetime, and a little more fiddling brings about the arrangement of 24-dim pennies alluded to above.

For a much more illuminating popularization of quantum field (but not string) theory, I strongly recommend Feynman’s book QED (quantum electrodynamics). Great bedtime reading.

  1. Length
  2. Depth
  3. Width
  4. Height
  5. Time
  6. Girth
  7. Zen
  8. Market penetration
  9. The Phantom Zone
  10. The Upside Down


You lost me right here. I understand your point about uncertainty being about how you can’t have both position and momentum (wave).

But what does it mean that some dimensions are easier than other dimensions? Who is using dimensions and what for? Easier to what? They have better personalities?


I did give an example right there: easier to compute sphere kissing number in. (I’m not claiming that kissing number is particularly relevant to string theory, nor explaining why 8n is easier than other dimensions, but that is an example of how.)

Incidentally, sphere packing is useful: if you need to round off a number in the range [0,1] to one of nine values then you’ll obviously pick 0.1, 0.2, …, 0.9 to minimize error, but if you need to round off a list of oh, say, eight numbers than instead of rounding them individually, which is like looking for the nearest point in a packing of cubes, you’ll have less total error by rounding the vector to the nearest point in a packing of spheres, which we know how to do more densely.


Bott perodicity?


It’s a metaphor for how we figure things like theoretical physics out. A science fiction metaphor might have worked better: What if we suddenly came into possession of part of an alien spacecraft? We have our “rowboat” in the form of things like fighter jets, rockets, space stations and the like, but there’s a considerable distance between that and a craft that can warp space-time to travel vast distances almost instantly. It’s unlikely that we’d be able to replicate a whole new alien spacecraft because the picture we have is incomplete, and there are lots of aspects of its construction that we won’t understand for a long time, but with the help of that partial picture we’d at least be able to make our “rowboats” a little bit more like the “cruise ship” and it would give us new perspectives and problems to solve that we might have taken centuries to get around to seeking or trying to solve otherwise.

It’s not the best metaphor, but I think they were shooting for brevity and accessibility. To explain it without a metaphor, it’s the difference between Newtonian physics and relativity. Fundamentally they both set out to explain the same thing, they just do it at different levels of detail. Relativity explains all the things that Newtonian physics tried to explain, only with much greater precision, but we still use Newtonian physics all the time because for most day-to-day applications, we don’t need the extra precision. Newtonian physics is “close enough” at the scales of mass and speed that we generally encounter, so it’s still useful and had we not had the “row boat” of Newtonian physics, we would not have figured out relativity.

That’s not because people in the old days were any less intelligent than we are, in the 50,000 years or so that human civilization has existed, we’ve not made significant genetic departures from our “primitive” forebears. They weren’t any less intelligent, they just had necessarily different priorities and lacked the wealth of collective human knowledge that we have now–and in spite of that, they still managed to do a lot of astonishingly clever things, but those clever things were built on top of the clever things we’d thought of before that.

So in a nutshell what they’re saying is that having a small working model plus an incomplete picture of a bigger, more accurate model, you can use the one to make predictions about the other which can then be tested experimentally and those observations can help make the incomplete model better.

I hope that helped, I’m trying to stop myself before I waste my morning writing a gigantic, unsolicited physics essay :-p


I see what you did there.


:wink: Shhhhh!


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