What do black holes, magnetars and supernovae have in common? They all emit X-rays. But it's difficult, if not impossible to study certain aspects of the X-ray emissions from these powerful objects. And there's much we don't understand about how black holes distort space-time around them, or how magnetars affect their surroundings, or how cosmic rays are accelerated by shocks in supernova remnants. A proposed new NASA mission called Gravity and Extreme Magnetism (GEMS), will use a new...