At "galaxy hopping" velocities, i.e. relativistic, even light can be a problem. Even the emptiest of empty space still has photons moving around from the cosmi

At "galaxy hopping" velocities, i.e. relativistic, even light can be a problem. Even the emptiest of empty space still has photons moving around from the cosmic microwave background, and go fast enough, and these photons will turn into deadly ionizing radiation because of blueshift. And even before that, the term "object" starts to lose meaning when it comes to collisions, it becomes a bunch of elementary particles with enough energy to cause fusion reactions. It could be one of the xkcd "What If?" scenarios, Randall loves to imagine stuff moving at relativistic speeds, and usually, it doesn't end well...