At relativistic speeds, you'd be vaporized, or at least ionized by interstellar gas, e.g. there are about 1 atom of hydrogen in one cubic cm. There was a paper

At relativistic speeds, you'd be vaporized, or at least ionized by interstellar gas, e.g. there are about 1 atom of hydrogen in one cubic cm. There was a paper considering making a shield against it, either lead, or electric field, or magnetic field, and resources required for it.