A: Does this sound familiar to you? Even though you were in pretty good shape last summer, over the winter you seem to have put on a little holiday poundage. And now, you’re probably wondering how you’re going to squeeze into those same swim trunks next summer. Well, here’s one word of caution: If you’re thinking about running, don’t run too fast or you’ll end up putting on weight. It’s true. And that’s not all, either – you’ll become shorter, too!
Of course, this will only happen if you run near the speed of light, but it’s still interesting to take note of. You see, as a consequence of special relativity, any object in motion will become shorter by a factor of gamma (it’s called the Lorentz contraction). We won’t bore (or confuse) you with the equation, but the phenomenon has been observed in laboratory experiments. But again, only near the speed of light is such a thing noticeable.
Similarly, when something is moving this fast, it will also increase in mass. But, just so you know, we were seriously kidding about the running concerns. Even if you were motoring along at 67 miles per hour (like a cheetah), your mass would only increase by one part in 100 trillion.