Chapter 5 - Dimensional Portals and Time Travel

In this chapter, Kaku discussed two theories that intrigued me. The first described “a universe in your bedroom” and the other “time paradoxes.” Both deal with time travel, and the second more specifically about the problems with time travel.

The first theory, is a simplified universe created by Charles Misner. In this universe, for example, your room would become an entire universe. The right and left wall of your bedroom would be identical, so as you tried to walk through the wall on your left, you would end up reappearing into the room from the wall on the right. This concept is also applied to the ceiling, floor, front and back of the room. In this way, you cannot escape from your room making it an entire universe.

In this case, all four walls would be transparent and you would notice that there would be exact carbon copies of yourself beside you, in front of you and behind you. There would be a infinite series of you, but you could never see your own face because if you were to look right, all the copies of you would be looking right and therefore you could only see the back of your head. It would also be advised not to point a gun at the clone in front of you because you will soon notice that the clone behind you will also be pointing one at your head.

In a Misner universe, we’ll say that the right wall is moving in at a speed of 2 m/s. So, if you leave through the left wall, you will reappear through the right but have your speed boosted by 2 m/s. If you continue to go through the loop, you can reach speeds close to the speed of light causing you to go back in time, or visit any point in space-time.

Hawking analyzed this scenario and compared it to two mouths of a wormhole, where both mouths of the wormhole would be identical. So, if we were to find a wormhole that moving in, we would be able to time-travel.

This universe is not stable so it has not gotten us much closer to time-travel, but created lively discussions in the physics world.

These discussions lead to time paradoxes. There are four main paradoxes which render time travel very complicated:

-The grandfather paradox where you alter your past, for example, killing an ancestor so you would not exist, and therefore not be able to go back in time to kill your ancestor.

- The information paradox where information comes from the future and so it has no origin in the present. So if someone from the future were to tell you how to make a time machine, your idea would not have an origin since you did not create the machine.

- Bilker’s paradox where a person knows the future, but does something in the present that makes the future impossible. For example, if you knew went into the future to find out who you’re going to marry, and then go back to the present to kill that person, your future cannot exist.

- The sexual paradox where you go back in time and father yourself, which is biologically impossible.

Having so many paradoxes means there are many theories explaining how to get around the paradoxes. Like the “many worlds theory” where all possible quantum worlds would exist, and so in one world you may have killed your parents and you would not exist, but in the other, nothing would have changed and life would continue normally.

Finally, time travel also stumbles upon free will. If we don’t have free will, and somehow, something is constraining us to “act properly” so we can’t kill our parents, we could eliminate certain paradoxes. So, I’m wondering, do you think time travel is possible? And, do you think we actually have free will?