It has been a while since I have read anything about String Theory in science journals (3-4 years ago), so I don't know its current status of String Theory in science.
According to what I remember about String Theory it is where various scientists giving different competing hypotheses on String Theory, in the hope to unified 2 current and accepted theories of today - General Relativity and Quantum Mechanics.
One of the hypotheses is that it is possible to have at least 11 dimensions (M-Theory), which allow for multiverse or alternate dimension (or reality).
This depends on what's meant by "multiverse" and "alternate dimension." Multiverses are a separate idea from string theory; as are ideas about different physics throughout the megaverse. (Megaverse is an often misunderstood term that just means the rest of the universe outside of the
visible universe. For instance, Leonard Susskind is a proponent of the Higgs field fluctuating throughout the megaverse even if the entire visible universe has roughly the same Higgs field values -- such that if you were to magically teleport 200 billion light years away you might end up in a place with different values and thus different physics).
In Susskind's "cosmic landscape" where physics changes throughout the megaverse it's not a different dimension; it's just what might happen if you went far enough out of the visible universe. Sort of like if you had a magnetic field with wildly different values on a plane but you were stuck only looking at a square centimeter of the plane, you might assume the magnetic field has only the values in that tiny square that you can see. In any case that's off topic and I'm just pointing out that variable physics megaverse ideas aren't necessarily inherent in string theory and don't necessarily incorporate multiverses.
Likewise, having 11 dimensions doesn't necessarily mean multiverses; but only by semantics. If a flatlander on a piece of paper on Earth were to glance somehow at the third spatial dimension we use, is that a multiverse? Usually we don't think of it that way; but it's not so easy to tell them apart: for instance, what if the flatlander discovers that there's another plane of flatland bisecting their plane? (Such that Flatland 2 is orthogonal or some other relation using the 3rd dimension to Flatland 1)?
That fulfills the definition of a "parallel dimension," since the word "parallel" is used very loosely in such cases -- but we 3rd dimension folk wouldn't call it a multiverse, we'd just say that the flatlanders have discovered the third dimension.
Well, that's exactly what a "multiverse" would be for us, though: another universe existing but separated from the one we know by some distance or relation over another spatial dimension that we're not familiar with. Is that a "multiverse" or is it just us discovering another dimension?
The terms are vague and sort of interchangeable. I just know that when I hear the word "multiverse" or "parallel dimension" that I imagine something more magical than scientific, but that's my own problem. I just like to think of it in terms of dimensions, plain and simple.
gnostic said:
The last time I had looked at this String Theory, the M-Theory is still untested hypothesis. There are competing mathematical proofs, but not testable and no evidences beyond the mathematics.
Has the String Theory made any headway since then?
Do scientists still believe it is possible to have dimensions greater than current dimensions (alternate dimension)?
String theory is in trouble and has been for a while. First it could only work for a negative or zero lambda universe and we discovered that ours is positive (i.e., universe is expanding and accelerating as it does so). This was entirely incompatible with string theory as it was formulated. Normally this is what we would call a falsification, but sort of like how Ptolemaic astronomers kept adding concentric orbits
ad hoc, that's what physicists have done with string theory today.
The only way to get string theory to work with a positive lambda universe was to use these crazy high dimensional spaces called calibai-yau manifolds -- the only reason they're given any thought at all outside of pure mathematics is because they so happen to save string theory (and I'd call that a bit
ad hoc if there isn't any other reason to bring them in, is my bias showing through?)
The problem with that besides its dubious
ad hoc nature is that there are something like 10^500 such possible manifolds, meaning that there are now around 10^500 possible string theories and very little* means to tell which one, if any, describes the actual physical universe that we observe.
(* -- by "very little" I mean there are
no known means but I'm padding myself a little in case some means is discovered)
On top of that, we derive answers from string theory perturbatively (with perturbation theory, which is a recursive approximation schema to put it gently)... which means the "answers" come out as infinite strings of terms.
I'm sure we've all heard or read that string theory is supposed to get rid of the infinities we encounter when we probe certain things with current physics... it's claimed to be "proven finite." Well, what they mean by that is that of the infinite perturbative terms we get from very manufactured questions (which idealize everything to an extreme -- sort of like if you were trying to figure out the mathematics behind a cow by assuming it's a perfect sphere) we have proven the first three terms to be finite.
Yep -- 3 terms out of an infinity of terms proven finite. That doesn't sound "proven finite" to me. Oh, and by 3 terms proven finite I mean that the first term is proven finite and the 2nd and 3rd terms are proven finite under "most normal conditions." (They can in fact blow up to infinity under a certain group of circumstances which are supposed to be possible within string theory, so why they say it's proven finite, I don't know...)
I could go on, but I'll finish by saying lastly that string theory as it stands is background dependent. That takes some time to explain and I'm tired, so let's just say that it ignores Einstein's relativity revolution by assuming a fixed spacetime. M-theory is supposed to be background independent (which is good) but I honestly haven't seen any attempts at putting it in such terms that aren't grotesquely contrived, but I admittedly don't grasp all the mathematics since I haven't even finished grad school yet. I know enough to know it looks unnecessarily ugly and inelegant though.
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Edit: Oh, and to answer your last question, even with the shortcomings of string theory -- that is entirely separate from the question of whether hyperspace (i.e. other spatial dimensions, or even temporal dimensions) exists. It is still entirely possible that there are extra spatial and/or temporal dimensions even if string theory fails. There are some indications that it may even be likely, but it's ultimately hypothetical at this point. There are precious few indirect evidences for higher spatial dimensions -- but they are weak evidences and in many cases have alternative explanations. There will come a time, perhaps with the LHC, where we will have access to the raw amounts of energy to test models with dimensions other than the three spatial and one temporal that we're familiar with.
