        He looked back at the field.  In one of Asimov’s short stories, an engineer had created an antigravity field which was surrounded by a glowing magenta field, caused by the air molecules accelerating to lightspeed in the field and then slowing once they encountered normal atmosphere again.  Was that what the bubble was?  That story had ended when the engineer had been killed by a billiard ball traveling at light speed which went through his heart.  What would happen to him if she flung the marbles at the bubble?  How did she get in and out of it without injury?  The first time, the big one had put something on her.  Maybe that was the key.