Some dead stars heavier than our sun move across outer space at 450 kilometers per second. Now physicists from Penn and CERN (a European high-energy physics lab) have suggested that the dead stars, or pulsars, were kicked to that speed by the puniest of all particles known to physicists: the neutrinos.

What an unlikely candidate. Neutrinos are so puny they cannot push even our most sensitive detectors on Earth. Neutrinos simply fly through matter like ghosts fly through walls. How could they kick something they pass through like ghosts?

As ghosts, they cannot kick anything. But, the scientists suggest, if the ghostly neutrinos can materialize, even for a moment, they will be able to kick.

Communication is where the World Wide Web home page comes in. "When a star runs out of nuclear fuel, in a couple of seconds its gravitational force crushes all its matter into a pulsar the size of an asteroid," says Penn Physics Professor Gino Segré, who, in collaboration with Alexander Kusenko from CERN, published a paper on the theory in Physical Review Letters. "Everything is trapped inside except neutrinos, which explode freely out of the pulsar in all directions like a growing balloon.

"Neutrinos might not be strong, but they are many. And they are traveling under the presence of the pulsar's huge magnetic field." The field, because it has positive and negative poles, can control the direction that matter moves.

"Our idea is that such a huge magnetic field facilitates about 1 percent more neutrinos escaping in the direction of the field than in any other direction," says Segré. "This is like a balloon with an open airway hole: the balloon will recoil. If for a brief moment the neutrinos materialize before going back to their ghostly nature, then our model predicts that those 1 percent additional puny neutrinos have enough force to kick the pulsar to 450 kilometers per second."

Because models of Kusenko and Segré's explanation correctly predict the speed of pulsars, the theory may be right. Although other explanations for the speed of pulsars have been given, Kusenko and Segré's explanation is one of only two that predict the right speed.