Why exacly 11 dimensions?

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  • #51
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ok so maybe "string hypothesis" would have been a better name but even cursory reading reveals string theory is based on perturbative (approximate) math, is incomplete even by string theorists standards themselves, and the basic underlying principles, which we assume are there, remain obscured. So far it has no proven to be the magic bullet once envisioned, er, twice, counting the frist and sec ond "revolutions"....
 
  • #52
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Post 38 mentions:
... All of them suggested 10 dimensions. Then some bright spark tried imposing 11 dimensions on the different theories, and suddenly they all appeared to be the same theory seen from different perspectives.
The "bright spark" was Ed Witten (Princeton) who was able to go beyond the approximations of the five basic string theory approximations and show that an additional, or tenth, space dimension had been missed in earlier approximations...so what appeared to be five different string theories could be "unified" with an additional space dimension.....and to add to the complexity it was discovered that p dimenisonal objects or p-branes were also embedded in the theory and that the tenth space dimension does not have to necessarily be much smaller than the others. Lisa Randall (Harvard) and collaborators have interesting theories on large size dimensions. (described in non mathematical tems in her book WARPED PASSAGES).
 
  • #53
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I think of string theory as something like medieval theologians trying to logically prove the existence of God. Yes it turns out that you can't logically prove the existence of God, but you don't know that until you spend a lot of time and effort trying and not getting anywhere.
 
  • #54
DaveC426913
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I think of string theory as something like medieval theologians trying to logically prove the existence of God.
Well, string theory had once been described by some as a piece of 21st century mathematics that fell into the hands of 20th century mathematicians....
 
  • #55
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Well, string theory had once been described by some as a piece of 21st century mathematics that fell into the hands of 20th century mathematicians....
My hunch is that physicists are going to continue spinning their wheels as long as they keep trying to find a theory of everything that attempts to explain a finite universe. It's not going to happen. The universe (or multiverse if you prefer) has to be infinite.
 
  • #56
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I can explain why 11 dimensions.

Let's go back to the standard model. You have three matrices that describe the forces in the universe. A one dimensional matrix that describes EM. A two dimensional matrix that describes the weak force. A three dimensional matrix that describes the strong force. Now you are a theorist, and you see that gravity is described by a four dimensional matrix. So the logical thing to do is to lump all of these matrices together and get a 10 dimensional matrix. And you argue that the extra six dimensions that describe all of the forces are just tiny little dimensions so 1+2+3+4 = 10. And you end up with a nice elegant theory that quite nicely seems to describe all of the forces in nature.

Except for one little detail.....

You have all of the forces in your 10 dimension matrix, but you don't have any matter..... Hmm.... it turns out that you can try to fix this problem by adding one more dimension which turns a force into matter and matter into forces. So this is where you get 11. Now the only problem is to then look at the big matrix and then take a sledge hammer to it until you get back the standard model.

Now in 1980, it wasn't obvious that if you think about it long enough you'll come up with neat math trick for getting the standard model out of this 11 dimensional theory. But people have been trying for a lot of years, and no one has getting it to work.........
 
  • #57
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The funny thing is that string theory posits billions of possible realities, and theorists tune it based on the standard model. And then all theorists seem to invent multiple universes to explain the strange fact that goldilocks porridge is perfectly warm.
 
  • #58
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The funny thing is that string theory posits billions of possible realities, and theorists tune it based on the standard model. And then all theorists seem to invent multiple universes to explain the strange fact that goldilocks porridge is perfectly warm.
I don't like the term multiple universes, the "multiverse" should be the Universe, and each little "universe" should be considered spatial regions. Let's face it, in an infinite universe an infinite amount of events will occur. Nobody is inventing universes.
 
  • #59
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I don't like the term multiple universes, the "multiverse" should be the Universe, and each little "universe" should be considered spatial regions. Let's face it, in an infinite universe an infinite amount of events will occur. Nobody is inventing universes.
give me one single piece of evidence that there is more than one universe. "spacial regions" is nonsense in terms of dimensionality.
 
  • #60
DaveC426913
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give me one single piece of evidence that there is more than one universe. "spacial regions" is nonsense in terms of dimensionality.
It is a hypothesis. No one is suggesting there is evidence that there is more than one universe. The MWI is another such hypothesis.
 
  • #61
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It is a hypothesis. No one is suggesting there is evidence that there is more than one universe. The MWI is another such hypothesis.
But is it reasonable to accept that every sub atomic interaction divides reality such that every possibility did occur? I'd suggest that all our evidence suggests that once a decision is made, it's final.

It's just that in the macro world, the decision is made Very quickly. Decoherence seems to me to be exponentialy related to the plank length, and yet there is an almost quantum nature to the border where the decisions get made - in effect - instantly.

And yet the uncertainty remains in the macro world because firstly there are too many variables for us to be capable of making sense of their interactions, and secondly because at some fundamental level our consciousness lives in the place where quantum uncertainty is normal.

I'm not suggesting
anything like indecision etc. I'm proposing that human will is non deterministic. And I'm certainly not suggesting that consciousness and quantum uncertainty are directly connected. But it is a fact that we are a long way from starting to understand either.

I feel sorry for all the people that have no intellectual curiosity because they believe people far more intelligent than themselves are close to understanding everything. It's cruel that science nowadays is constructed to give that false impression, when the reality would stir and exite so many more to hunger to understand the great mysteries that stand before us all.

If science was more honest about the fact that most of it's current "facts" will be at least partially modified in a decade or two, the crazies that reject reason altogether would have the rug taken from under their feet. And maybe more would hunger for truth.
 

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