I have often heard that there are more than 4 (3 space and 1 time) dimensions of spacetime. What are the theories that say so, and how many does each predict? Has any experimental evidence been conclusive? And how are these extra-dimensional theories to agree with our daily observations?

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    $\begingroup$ Related to the point of being a near duplicate: physics.stackexchange.com/q/29078/520 . You may also be interested in physics.stackexchange.com/q/10527/520, physics.stackexchange.com/q/4079/520, or physics.stackexchange.com/q/22542/520 . $\endgroup$ – dmckee --- ex-moderator kitten Jun 1 '12 at 12:55
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    $\begingroup$ Even after I edited, my vote is still a -1. This is a question that can never be good enough. $\endgroup$ – Abhimanyu Pallavi Sudhir Jun 23 '13 at 14:15
  • $\begingroup$ I think asking about what theories predict extra dimensions, the number of them (called critical dimension), and about the experimental evidence for them (I would bet quite a large amount of money that the OP does not know the difference between direct evidence which needs access to the Planck scale and indirect phenomenological hints that can possibly be seen at our "low" LHC energy scales) is way to much for one question. In addition, all of these subquestions have been asked and answered already on the site as far as I know. If needed I could find the corresponding questions ... $\endgroup$ – Dilaton Jun 24 '13 at 22:38
  • $\begingroup$ My downvote disappeared ? ! $\endgroup$ – Abhimanyu Pallavi Sudhir Aug 11 '13 at 15:49

We are not really sure.

Classical theories are based on 4 total: 3 for the observed spatial directions, and 1 for time. However, string theory introduces a whole collection of other possible numbers. Some commonly used ones include 10, 11, or 26. Those extra dimensions are supposed to be "compactified" to explain our inability to observe them.

  • $\begingroup$ ... and F-theory which gives quite a nice particle physics phenomenology has 12 (two of them are only infinitesimal). $\endgroup$ – Dilaton Jun 24 '13 at 22:42

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