New Scientist’s fledgling Explanimator series on YouTube has already explored the burning questions of how many friends you can have, and when you would be considered dead. The latest episode in the series tackles that favorite sci-fi staple, time travel.

As the narrator says, the laws of physics don’t explicitly forbid time travel, but that doesn’t mean we’ll be building a working time machine anytime soon that can let us whip back and forth through time and space at will. Among other challenges, we’d need to build a wormhole big enough for a large object to pass through it, and prop it open with some negative (dark) energy — except we don’t actually know what that is, although we have certainly measured its effects.


Enjoy this whirlwind tour of general relativity, time dilation, wormholes, the grandfather paradox, the Many Worlds interpretation of quantum mechanics, and other big speculative ideas in physics. Just remember: “Possible does not mean easy.”