About The Missing Six Dimensions of String Theory:

a reply to: Peeple

Everybody knows our 3 for space + time.
And how String Theory just would work out so beautifully.
So where are the missing 6 dimensions?

Compactification is hard to visualize. I could give you the goto "ant walking on a mobius strip", but that really doesn't help people...

It's basically like this, for string theory to work there needs to be WAY WAY WAY MORE STRINGS than 4 dimensions will allow.

To make the math line up (in the original 5 10 dimension variations) you need a bunch of extra parameters. Volume and time is nowhere near enough for the math to work. The dimensions are "superpositional". They exist in many different states simultaneously. Our universe just collapses that down a specific way. We can never perceive the entirety of the higher dimensions any more than you can look at a composite object (like a grain of salt) in two places at once.

Descirptions for higher dimensions taken from Phys.Org:

1-4: Length, width, depth, and time.
5: A world slightly different from our own that would give us a means of measuring the similarity
6: a plane of possible worlds, where we could compare and position all the possible universes that start with the same initial conditions as this one (i.e. the Big Bang).
7: you have access to the possible worlds that start with different initial conditions.
8: a plane of such possible universe histories, each of which begins with different initial conditions and branches out infinitely (hence why they are called infinities).
9: all the possible universe histories, starting with all the different possible laws of physics and initial conditions.
10: the point in which everything possible and imaginable is covered. Beyond this, nothing can be imagined by us lowly mortals, which makes it the natural limitation of what we can conceive in terms of dimensions.

The Spinal Tap Variation: In order to make all the 10 dimension versions of string theory compatible you add another dimension. A California surfer dude figured that out.

The strings became attached to cosmically undulating branes in a "bulk", which is like "subspace".

It did this by asserting that strings are really one-dimensional slices of a two-dimensional membrane vibrating in 11-dimensional spacetime.

When you get right down to it, it is super-contrived, but you wouldn't have a multiverse without it.

If that just raises more questions, there's tons more to this. Especially when dealing with superposition and general relativity.

Pulled off Stack Exchange:

How does string theory relate superposition and general relativity?

Unlike what you may hear in many places, we have a perfectly good theory of quantum gravity: you just treat general relativity as a quantum field theory! Now, it's true that the result is non-renormalizable. But that just means that it only gives useful predictions below a certain energy scale Λ, or for distance scales longer than some length Λ−1∼ℏGN−−−−√. We call this an "effective theory".

That theory deals perfectly well with superposition. The result is much the same as any other quantum theory: the dynamical variables can be in a superposition of different states. In gravity, the dynamics describes the geometry of spacetime. So spacetime can be in a superposition of different geometries!

String theory is more complete (it gives predictions even at the very short distances where the above description fails), but for the purposes of this question all you need to know is that at long distances or low energies it gives the quantum theory of GR described above. So in string theory too, spacetime can be in a state with a superposition of different geometries.

originally posted by: Degradation33
When you get right down to it, it is super-contrived, but you wouldn't have a multiverse without it.

Who has a multiverse?

Are you talking about the pseudoscientific multiverse or the religious multiverse, because Sabine Hossenfelder argues there's no such thing as a scientific multiverse. They've lied to us about string theory for decades, and they've also lied to us about multiverses, when they pretend they have scientific multiverse hypotheses.

The Multiverse: Science, Religion, or Pseudoscience?

Some highlights from the video:

6:00 talks about string theory having multiverses known as "landscapes" Elsewhere in the video talks about other types of multiverses but certainly the string theory variety is on topic here. She also notes that string theory has been somewhat abandoned because it never really led anywhere.

8:14 Multiverse theories postulate the existence of unobservable entities
-Pseudoscience: Pretending that unobservable universes exist and write papers about them.
-Religion: If you believe in multiverse while accepting that science says nothing scientific about them.
-Science? Nope, but see 10:00

10:00 Some scientists believe other universes exist because they show up in their mathematics. You see, they have mathematics and some of that describes what we observe. Then they claim therefore that everything else that their mathematics describes must also exist. They are confusing mathematics with reality.

I can give you another example, run your washing machine and it will use real power and reactive power. We use imaginary numbers to calculate the reactive power. They are just a handy mathematical tool, and they give accurate calculations, but that doesn't mean the imaginary numbers are real. I mean what number can you multiply by itself and arrive at the answer -1? We call it i (for imaginary). It's not a real number, it's math.

a reply to: blackcrowe
You too are confusing mathematics with reality. Lorentz transformations convert one observer's 4D space-time into another observer's 4D space time and it takes some fancy math to do the conversion. Don't confuse mathematics with reality.

a reply to: Arbitrageur

No worries.

a reply to: Arbitrageur

Are you talking about the pseudoscientific multiverse or the religious multiverse, because Sabine Hossenfelder argues there's no such thing as a scientific multiverse.

I mean the idea (or abstraction) of the multiverse came mostly from string theory's higher dimensions. No more, no less.

As far as string theory goes, it tries hard to seem elegant, but I am more partial to field theories (like SVT). I don't like particles as one dimensional selectively vibrating strings + tons on convolution to prove it. I don't think it's the occam's razor "Theory of Everything" people hope it is. It rectifies the weirdness of gravity with untold mounds of equations to make it all work and scatter itself sufficiently. It has to being rooted in S matrix theory, but it bugs me for that.

The crux of it being an attempt to explain gravity (and unify it with other forces) by reducing all elementary particles to vibrating multidimensional strings. With higher dimensions in place to accommodate all the infinities. Technically a "scattering theory".

But if all force carriers are strings vibrating in higher dimensions they still haven't really explained why the graviton string is orders of magnitude weaker than any other force carrying particle/strings. Makes me wonder how strong a photon is without the higher dimensions. I may be missing something there.

And what do I know, my degree didn't need anything beyond statistics. Please pardon my armchair physics.