Well, let's say you wanted to travel in time.

Let's set a few rules. You can only time travel in the time you have been alive.
Also, you need a way to send a message to yourself.

So if you were in the future, what message would you send so that you knew it was you? You can only send a message back in time to the point where you have had this thought.

It is the constant I was talking about.

Maybe your limit on time travel only rely on this constant. So if you pick one, you need to be able to find it.

My constant is Pi.

Then again, there are different ways to look at a quantum mirror :-P