Though extra dimensions may sound like the stuff of science fiction, they are taken quite seriously by contemporary physicists and philosophers of physics. In addition to the three spatial dimensions we’re familiar with — up/down, left/right, forward/back — theories such as string theory postulate as many as 7 additional spatial dimensions. If such a theory were correct, the landscape of the three-dimensionalism / four-dimensionalism debate would need reformulation; perhaps objects are really perduring eleven-dimensional spacetime worms! But, why can’t we see these extra dimensions? The short answer is that they’re too small, certainly smaller than Bas van Fraassen’s threshold for observability. According to a recent paper by astronomer Oleg Gnedin and his colleagues at the University of Michigan, the extra dimensions must be less than half the size previously thought — less than 3 microns, or more than ten times smaller than the diameter of a human hair.
The evidence for this new restriction came from an unlikely place — one of the oldest, smallest black holes ever observed. According to a leading formulation of string theory, extra dimensions explain the relative weakness of gravity (1040 times weaker than nuclear forces) since gravity could ‘leak’ out through them, decreasing its magnitude over large distances. The ‘leaking’ would also cause black holes to emit more energy in the form of Hawking radiation and thus evaporate faster (the larger the extra dimension, the faster the black hole’s disappearance). If such a theory were correct, the particular black hole identified recently, only ten times the mass of our sun yet an incredible ten billion years old, should have evaporated long ago. The fact that it is still around, in a Globular cluster in the galaxy NGC 4472, is evidence that the extra dimensions of string theory must be smaller than physicists previously thought.
GRW: A Case Study in Quantum Ontology
By Peter J. Lewis, University of Miami
(Vol. 1, February 2006)
Probability in the Everett Interpretation
By Hilary Greaves , Rutgers University
(Vol. 1, December 2006)