Long before Henry Cavill’s Superman was destroying World Engines and Brandon Routh’s Superman was lifting Kryptonite islands, Christopher Reeves’ Superman accomplished an amazing feat: altering time. Many of us remember in 1978’s Superman, that after Lois Lane died in a car accident, the Man of Steel grew angry and flew around the Earth so fast that he turned back time and was able to save her. It’s worth noting that this act was merely Superman flying fast enough to travel back in time rather than literally spinning the Earth in the opposite direction. However, if he had indeed reversed the planet’s spin, a group of physics students have figured out how fast he would have needed to travel to accomplish this, though it would have come with enormous consequences.
Students at the University of Leicester have calculated that in order for Superman to spin the Earth backwards, he would have to travel at "an angular velocity 46.296 radians per second," or approximately 660,000,000 miles per hour, which is nearly the speed of light. In order to do this, he would have had to "increase his mass 13.7 million times," which is impossible, right? Well, not according to Einstein’s mass–energy equivalence equation E=mc². As Caltech applied mathematician Dr. Spyridon Michalakis explained to Nerdist, "relativistic mass" refers to the mass of an object moving very fast, so if Superman had hit close to the speed of light flying around Earth, he could have transferred this inertia to the planet and essentially braked it.
Even assuming you can travel that fast, there comes a lot of big problems aside from likely damaging the time stream. Namely, with all that energy comes gravity. The study noted that Superman’s actions would have resulted in objects near Earth, like asteroids, heading towards us. Then there would also be changes in "atmospheric pressure and wind speed," which would unsurprisingly result in extinction. So the characters in the movie should count themselves lucky that the Kryptonian didn’t actually try to pull this off in the first movie or any of the sequels.
Superhero story science is always inherently ridiculous, but even in this case, the denizens of the DC Universe should tell Superman not to pull a stunt like this. Reeves’ Clark Kent would have been better off leaving Lois dead and moving on with his life rather than than killing all humans in an effort to save her by messing with Earth. When in doubt, if you’re capable of reaching super speeds and don’t have a time machine, you’re better off simply trying to reach the speed of light. Of course, then you get into paradoxes, like stepping on "butterflies." And, if The Flash is any indication, maybe you even get temporal singularities. But, hey, one problem at a time.
