Question over dimensions, something that's been bothering me

In summary, the conversation discusses the concept of string theory and its different dimensions. It is mentioned that there are five stable string theories in ten dimensions, but the idea of "M Theory" allows for countless theories in eleven dimensions. It is also noted that there are other string theories in lower dimensions but they are not as popular due to instability. The conversation suggests reading more about the subject on Wikipedia, specifically on string theory and non-critical string theory.
Maybe someone could inform me a little on what I'm talking about, because, honestly, I have no idea. If 11 dimensions=1 string theory -> 10 dimensions=5 string theories, is it possible to derive(right word? maybe) more string theories using lesser dimensions? Thanks. Even a yes or no answer would help, with a link to the appropriate wikipedia article. Or if you had time I'd be more than happy to read over whatever you have to say.

A while back there were five string theories in ten dimensions that were all stable in that they didn't predict infinities, then Witten among others noticed that you could have one 11 dimensional theory in which the other five were like different ways of looking at the same thing (=M thory). Now however M Theory is not one theory but countless theories - leading to otherwise smart people believing that there are countless universes despite the fact we don't have a single bit of evidence for anything other than the one universe we observe.

Of course there are many other theories using less dimensions, in fact those are what string theories came from, but there are all kinds of issues with them and if you're interested you really need to get yourself one of the good books on the subject.

If you're looking for stuff on wikipedia I suggest

http://en.wikipedia.org/wiki/String_Theory#Number_of_dimensions
http://en.wikipedia.org/wiki/Non-critical_string_theory

Basically the idea as I understand it is this:

You can formulate string theory in any number of dimensions you like.

However, the background space where the strings live must be "critical", or else the theory is unstable. With supersymmetric strings, the only way anyone knows to do this is if the space is 10-dimensional.

The M-Theory people have a trick that let's them bump that up to 11 dimensions while keeping the strings critical. But nobody knows how to turn it down to lower than 10 dimensions. If anyone knew of such a trick then surely they would use it, because the more dimensions you have past 4 the more dimensions you have to explain away.

You can again just go ahead and create "non-critical" string models with fewer dimensions, and sometimes these models have mathematical uses. But because of the instability you cannot use these models for a force-unification "theory of everything", which is what people would really like to use string theory for, so people are not as interested in the non-critical string theories.

What are dimensions?

Dimensions refer to the measurements of length, width, and height of an object or space. In physics, dimensions also refer to the different planes or axes in which objects can exist, such as the three dimensions of space (length, width, and height) and time as the fourth dimension.

How many dimensions are there?

In traditional physics, there are four dimensions (three dimensions of space and one dimension of time). However, in more advanced theories such as string theory, there can be up to 11 dimensions.

Can humans perceive more than three dimensions?

While humans are limited to perceiving three dimensions in our physical reality, there are theories that suggest there may be more dimensions that we are unable to perceive directly. Some scientists believe that our perception of time could be a fourth dimension, while others explore the possibility of extra dimensions existing beyond our current understanding.

How do dimensions affect our understanding of the universe?

Dimensions play a crucial role in many scientific theories and models, such as the theory of relativity and the Big Bang theory. They also help us understand the behavior of particles and the structure of the universe. Additionally, the concept of dimensions allows scientists to explore the possibility of parallel universes and alternative realities.

How can we visualize dimensions?

While it is difficult for humans to visualize dimensions beyond the three we experience, there are ways to represent them mathematically or through analogies. For example, the fourth dimension of time can be represented as a line moving through space, or the 11th dimension in string theory can be visualized as a Calabi-Yau manifold.

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