What would happen to the human body moving at near lightspeed?

In science fiction, spaceships moving at or beyond lightspeed enable all manner of universal exploration. But in Earth-bound reality, traveling at the speed of light (299,792,458 meters per second, or 670,616,629 miles per hour, in a vacuum) in a clunky rocket is a physical impossibility. “It’s the speed at which massless things travel,” says Gerd Kortemeyer, an associate professor emeritus of physics at Michigan State University. So, anything with mass cannot reach that speed. And even massless particles are limited by lightspeed. “It’s often called the cosmic speed limit because nothing will go faster than that,” Kortemeyer says.
Doubly unfortunate for those among us eager to visit galaxies far, far away–even moving at near lightspeed isn’t in the cards. “It’s neither possible nor survivable to travel near the speed of light relative to our good old Earth,” the physicist says.
Wonky theoretical explanations aside, it would simply take far too much fuel and energy to propel any human-bearing spacecraft up to that speed. By Kortemeyer’s calculations, reaching 99% of lightspeed in a vessel weighing 10 metric tons (significantly lighter and smaller than most spacecraft), while accelerating at a tolerable g-force would use more than 200 times the amount of energy consumed on Earth in a year. And that’s assuming a perfectly efficient fuel, where mass converts to propulsive energy without any heat loss–another physical impossibility according to the second law of thermodynamics.