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In 1990, world-famous physicist Stephen Hawking read papers of his colleagues proposing their version of a time machine, but he was immediately, skeptical. His intuition told him that time travel was not possible because there are no tourists from the future. Hawking also raised a challenge to the world of physics. There ought to be a law, he proclaimed, making time travel impossible.

The embarrassing thing, however, was that no matter how hard physicists tried, they could not find a law to prevent time travel. Apparently, time travel seems to be consistent with the known laws of physics. Unable to find any physical law that makes time travel impossible, Hawking has recently changed his mind. He made headlines in the British papers when he said, “Time travel may be possible, but it is not practical.”

Once(ア:consider)to be science fiction or fringe science, time travel has suddenly become a playground for theoretical physicists. The reason for all this confusion and excitement is that Einstein's equations allow many kinds of time machines. (Whether they will survive the challenges from the quantum theory, however, is still in doubt) In Einstein's theory in fact, we often encounter something called“closed time-like curves”, which is the technical term for paths that allow for time travel into the past. If we followed the path of a closed time- like curve, we would set out on a journey and return before we left.

A possible time machine involves a wormhole. There are many solutions to Einstein's equations that connect two distant points in space. But since space and time are intimately intertwined in Einstein's theory, this same wormhole can also connect two points in time. Theoretically, by falling down the wormhole, you could journey into the past. But passing ( イ ) the wormhole at the center of a black hole is a one-way trip. As physicist Richard Gott has said, “I don't think there's any question that a person could travel back in time while in a black hole. The question is whether he could ever emerge to brag about lt.”

Time travel poses, however, all sorts of problems, logical, social, and technical. The most difficult one is the so-called“grandfather paradox”:what happens if we travel back in time and kill our own grandparents or parents before we are born? This is a logical impossibility because it implies that we would not have been born and thus could not have gone back in time to commit the murder. Difficult as it may sound, there are, however, ways to resolve this paradox.

First, perhaps, you simply repeat past history when you go back in time, therefore fulfilling the past. In this case, you have absolutely no free will and are forced to complete the past as you know it.

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 actually doing this.

Third, the universe splits into two universes. On one time line, 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, which is also explored in the famous Hollywood movie Back to the Future, seems to be consistent with the quantum theory.

The key to understanding time travel and the possibility to put it into practice is a theory of everything. Such a theory would unite the four forces of the universe and enable to calculate what would happen when we entered a time machine. Only a theory of everything could successfully calculate all the radiation effects created by a wormhole and definitively settle the question of how stable wormholes would be when we entered the time machine. But even then, we might have to wait for centuries or even longer to actually build a Machine to test these theories.

(Adapted from Michio Kaku, Physics of the Impossible, New York: Anchor Books, 2009, pp.221-228)

• Playground: (ここでは)活動の場
theory:　量子論
• theory of everything:万物の理論

設問

1. 次の英文が、本文の内容と一致している場合にはTを、一致していない場合にはFを記しなさい。
   • (1) Stephen Hawking believed from the beginning that time travel is possible.
   • (2) The known laws of physics clearly rule out the possibility of time travel.
   • (3) Many theoretical physicists nowadays consider time travel a serious topic of research.
   • (4) Physicists have already built wormholes which make time travel possible.
   • (5) Richard Gott thinks that one could use a black hole for safely traveling back in time.
   • (6) The“grandfather paradox”is a clear proof that time travel is impossible.
   • (7) According to the third solution to the“grandfather paradox”, a time traveler could create a parallel universe.
   • (8) The definitive answer to the possibility of time travel could only be provided by a theory of everything.
2. ( ア )の語を適切な形に直しなさい。直す必要のない場合はそのまま記しなさい。
3. ( イ )に入る最も適切な語を記しなさい。