This is part of The Blogging Gauntlet of May 2016, where I try to write 500 words every day. See the May 1st post for full details.

As far as I know, time travel doesn’t exist. However, that doesn’t mean there’s nothing interesting to say about it. In fact, you can do a lot of theorizing about models of time travel.

Today, I made a time travel password. What I’m going to do is detail the argument for why everyone else should make one too.


Suppose we’re living in a world where time travel is possible, and that furthermore time travel will be invented within our lifetime. I know this is a huge assumption, but bear with me for a moment.

A world where people can transmit information to past versions of themselves is very hard to predict. The nature of that world depends on the model of time travel used, but the potential gains of time travel are huge.

However, that means the potential losses of forged future messages are also huge. If you’re in a world where time travel is possible, the strangest requests in the world can be explained away by saying the future depends on it. What makes decision making tricky is that it’s almost always done under uncertainty. Time travel removes that uncertainty, giving a perfect oracle towards achieving the best outcome. Or, if a message is forged, a perfect oracle towards achieving the worst outcome.

To prevent this, you need an authentication scheme that verifies future messages come from you. Hence, time travel passwords! In the present, you pre-commit to adding your time travel password to any message sent to a past version of you. That way, if you ever receive such a message, you can verify that message is legitimate.

Importantly, you never, under any circumstances, write down your time travel password or speak it out loud. After the password leaves your head, it’s no longer secure. As a corollary, every time travel password is one time use, and you must create a new one whenever you receive a future message.


There’s a very natural objection to this: isn’t time travel exceedingly unlikely? Why should I waste my time making a password for non-existent technologies?

Well, consider the following.

  • Creating a time travel password takes less than a minute.
  • If time travel is never invented, you lose one minute of your life.
  • If time travel is invented, you prevent a large class of attacks that could be made against you.

The expected utility is vastly, vastly in favor of making a time travel password. I’ll fully admit this argument is a variant of Pascal’s mugging, which you might reject for one reason or another.

Here’s my counterargument. Yes, maybe I’m mugging you out of one minute of your time. However, the value of one minute of your time is very, very low. And even if time travel is never invented, you get to tell people you have a time travel password! How awesome is that? You can explain this very argument, or link to help increase my view counts (nod nod, wink wink).

Having a time travel password makes you a more interesting person, and that’s surely worth the one minute of time it costs you.