"If time travel is possible, then where are the tourists from the future?"
"[Time travel] is against reason," said Filby.
"What reason?" said the Time Traveler
-H. G. Wells
Kaku's second group of impossibilities-- Class II impossibilities-- are "technologies that sit at the very edge of our understanding of the physical world. If they are possible at all, they might be realized on a scale of millennia to millions of years in the future." An example that will be discussed here is the idea of time travel.
Like invisibility, time travel has been discussed by everyone-- from scientists and philosophers to innocent children, all throughout the ages. Yet most have so far assumed that such ideas of traveling to the past or to the future is an impossible task, as that would bring out numerous well-known paradoxes. (Tor example, if you go to the past and prevent your parents from meeting, will you exist?)
And for a while in scientific history, the impossibility of time travel was supported by physics: in Newton's universe, time was labeled as being absolute-- a second on earth was thought to be a second in the entire universe. Time would be an unstoppable arrow.
This all changed with Einstein's new, revolutionary developments in physics, in which he showed that time is like a river, speeding up and slowing down as it flows throughout the universe. In his special theory of relativity, he explained how in a rocket, for example, time slows down the faster it moves. As it approaches the speed of light, time slows down.
Thus, the idea of time travel being impossible was broken: now, we know that time travel is possible (albeit not as nearly as close to the level that we imagine). In fact, the world record for traveling into the future is held by Russian cosmonaut Sergei Avdeyev, who was hurled 0.02 seconds into the future.
Yet even this picture may not be complete! In Einstein's perspective, time travel to the past would still be impossible, as an object would have to travel faster than the speed of light to accomplish such a journey. For instance, in Superman I Superman tries to go back in time by circling around the Earth faster than the speed of light:
(The scene from Superman: The Movie)
However, in order for this to happen, an object would have to have an infinite mass! Yet new discoveries are already suggesting that the speed of light barrier has been broken: researchers at CERN have possibly found that neutrinos (which certainly do not have infinite mass) can travel faster than the speed of light!
This further reveals just how a standard approach to science only hinders scientific achievement. Had we fully taken Einstein's ideas of science to heart, such discoveries would have never been made, and the scientific community would have been covered in a veil of ignorance. A pluralistic and counterinductive approach, on the other hand, would ensure that (at least) attempts are made to directly challenge such "well-established" theories.
(Referring to the paradox mentioned above, Kaku offers three possible answers/hypotheses:
First, perhaps you simply repeat past history when you go back in time, therefore fulfilling the past. In this case, you have no free will. You are forced to complete the past as it was written. Thus, if you go back into the past to give the secret of time travel to your younger self, then it was meant to happen that way. The secret of time travel came from the future. It was destiny. (But this does not tell us where the original idea came from.)Second, you have free will, so you can change the past, but within limits. Your free will is not allowed to create a time paradox. Whenever you try to kill your parents before you are born, a mysterious force prevents you from pulling the trigger. This position has been advocated by the Russian physicist Igor Novikov. He argues that there is a law preventing us from walking on the ceiling, although we might want to. Hence, there might be a law preventing us from killing our parents before we are born.Third, the universe splits into two. On one timeline the people whom you killed look just like your parents, but they are different, because you are now in a parallel universe. This latter possibility seems to be the one consistent with the quantum theory.)