[glymor]

You can't prove there aren't more dimensions. After all you could be hiding them in your garage with your invisible dragon or you could of rolled them up in a Calabi-Yau manifold.

Time is a unidirectional so it's considered to be a different kind of dimension than the spacial (bidirectional) dimensions.

[gchpaco]

.. Although depending on the particle theory you subscribe to, it may only be unidirectional to us (positrons being electrons moving backwards in time, etc.)