No. 100: Jul-Aug 1995
Over the past few years, more than one theorist has proposed that our universe coexists with at least one, perhaps many, other universes. Said universes are constituted of particles possessing properties so different from own own that we cannot normally discern the reality of these other "existences." In other words, astronomers cannot visually see the stars of these "shadow universes, nor do our detectors of electricity and magnetism acknowledge them. Normally, the subatomic "shadow" particles do not interact with our own particles either. Then, why even bother to contemplate shadow universes? Well, physicists say that none of their laws prohibits the existence of these other universes, and that's reason enough to search for a "looking-glass" entrance of some sort.
Just suppose that the particles of one of these shadow universes do possess mass (or whatever shadow physicists call it). Some speculate that this shadow mass could be the "missing mass" that cosmologists have been looking for and can't find. Cosmologists need something palpable out there to explain the puzzling dynamics of galaxies and other phenomena. Some physicists in our universe have conceived of a situation where our universe may "rub together" with a shadow universe. [Honestly!] During such less-than-cataclysmic encounters, some of the electric charge on our-world particles could be "scraped off" and transferred to shadow-world particles, enabling us to finally detect them!
This possibility obviously calls for experiments aimed at detecting other-world particles with electric charges of, say, one-thousandth of an electron's charge. The search is now underway at SLAC (Stanford Linear Accelerator Center).
(Travis, John; "How Quixotic Is SLAC's Quest to Detect 'Crazy Particle'?" Science, 267:1424, 1995.)
Shadow Comment. Obviously, weird phenomena might transpire when and where universes rub together. One area of frequent rubbing is probably the Bermuda Triangle! Had enough? We have.