End of the YearTime permitting, I like to end the academic year with Special Relativity with an emphasis on how it allows time travel. We then watch the Nova episode Time Travel with Kip Thorne. One segment uses the Many Worlds (MW) interpretation of Quantum Mechanics to suggest that the single-photon double slit experiment shows that time travel is possible between parallel universes.
DAVID DEUTSCH: The photon that we can't see is a photon in a parallel universe which - a nearby parallel universe which is interacting with the photon in our universe and causing it to change its direction. The result of these single photon inference experiment is the strangest thing I know. It is conclusive evidence that reality does not consist of just a single universe because that result could not come about unless there were another nearby universe interacting with ours.
NARRATOR: If true, this idea has profound implications for time travel.
DAVID DEUTSCH: When one travels back in time one does not in general reach the same universe that one starts from. One reaches the past of a different universe.
To illustrate MW (that when an action has different possible outcomes, the universe splits so that each possible outcome happens in a different universe), I grab a ball from my desk, wind up, and "throw" it at a student (I actually drop it behind my back). There is a collective gasp and then relief that no ball was thrown. I then tell them that according to MW, there is a universe where I did throw the ball. We all giggle a little and move on.
This year, in my last class, the ball didn't drop behind me. Fortunately, I narrowly missed a student. After a moment of shock and then profuse apologies, I got back on track and said "In another universe, I didn't miss. (pause) And I am now being lead away in handcuffs. (pause) And because I can't grade your finals, you all get 100's."
Next year, I'll use a foam ball.