The faster a particle travels through vacuum, the more likely it is that the particle will interact with virtual particles contained in the vacuum. (At a quantum level, a vacuum is far from being empty.) Based on this quantum phenomena, scientists have predicted a maximum energy with which particles can travel long distances through vacuum (or the "empty space" in the universe). If the kinetic energy ("speed") of a particle exceeds this limit, the particle will eventually get involved in a collision with a virtual particle in the vacuum, creating new particles, each real and with lower kinetic energy.
Related article at http://www.europhysicsnews.org/full/12/article4/article4.html
Kurt Riesselmann, Fermilab
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