MIB and Jacob are the Opposite ends of a Magnet by numbersarebad

Now that we know MIB and the smoke monster are one and the same, I theorize that MIB and Jacob represent the opposite poles of a magnet.

If the ash is magnetized dust, that might explain why the MIB was trapped or repelled by the ash circles. The magnetism of the ash pushes MIB away the way the north end of a magnet pushes away the then north end of another magnet.

Smokey scans and records people's thoughts. Magnetic Resonance Imaging (MRI's) scan people to find problems in their bodies, Smokey uses magnetism to scan people. Cassette's, VHS tapes, and hard drives use magnetic coating to record data, this is how MIB and Jacob seem to know about people's pasts. Jacobs touch may have recorded or downloaded memories as well.

Older TV screens used electromagnets to guide electrons to the screen. This may be how the smoke monster takes on different forms, it projects images similar to the way a TV works.

If MIB is magnetic, maybe the energy at the Swan Station kept MIB from leaving or getting very far. Perhaps the Swan Station being blown up by Desmond (or Juliet) was the loophole Jacob referred to.