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Please forgive my simplistic questions, I do not have any higher education in physics and in my reading of pop science magazines I doubtless remain ignorant of important developments.

What is the proof that there are only three spatial dimensions?

You say "three spatial dimensions and time." Is time a dimension just like the spatial ones but we perceive it differently? Or is time something other than a dimension? And when was this proved or demonstrated?

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.

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

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