Watch the most famous paradox of time travel explained (without a single equation)

Of all the thought experiments of modern science – thought, because they cannot actually be carried out yet – that have stormed pop culture, the Grandfather Paradox is probably the best-known one.

Often mentioned in science fiction, used as a premise for the first Back to the Future movie, it questions the possibility of time-travelling into your past to change it. Simply put, what if you went back in time and killed your own grandfather when he was a child? Then you would never be born to go back in time to kill your grandfather. Aka, a paradox.

A subset of this is the Hitler Paradox, since quite a few people who want to time travel fantasise about killing Hitler. The paradox being if you went back in time and killed Hitler, there would be no reason to go back in time in the first place.

Now there is a new video (above) by MinutePhysics, claiming to have solved the Grandfather Paradox.

Before Henry Reich, the creator of the channel, solves the Paradox, he gives the "boring solution" that sidesteps it – your grandfather's death occurs in a different reality and any changes made will only affect this alternate reality, not the reality you came from.

But that's not satisfying. So, Reich invites us to "follow the paradoxical timeline to beyond its paradoxical conclusion. You go back in time, kill your own grandfather, thus you aren't born, thus you cannot go back into time, thus your grandfather isn't killed, so you are born, so you go back in time to kill your own grandfather".

The explanation? "It's two entangling histories happening in parallel." Meaning, both the eventualities are happening, at the same time. Your grandfather is both alive and killed. You are, by extension, both born and unborn.

Is that even possible? According to Reich, sub atomic particles regularly do multiple things in parallel. It's called Quantum Superposition. Your grandfather is in both states at once, so there is no paradox.

There is another paradox-free solution which is a steady-state solution, which Reich doesn't go into in any understandable form.

However, "nothing about these solutions suggests that closed time-loops are actually possible. In fact, some of the implications these kinds of time loops have in the study of complexity theory suggest that time loops and thus time travel into the past must be impossible".

"But the main point is sometimes we think a situation creates a paradox when it doesn't. And really the only paradox is how our thinking can be twisted enough to dream up time-travelling murdering grandsons but not twisted enough to think about twisting time."

For your sanity – or confusion, depending on how you look at it – differing views are available here.