Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [64]
To theorists the possibility of time machines is very unsettling. If time travel is possible, all sorts of impossible situations, or “paradoxes,” raise their ugly heads. The most famous is the grandfather paradox in which a man goes back in time and shoots his grandfather before he conceives the man’s mother. The problem is, if he shoots his grandfather, how can he ever be born to go back in time and do the dirty deed?!
Embarrassing questions like this have prompted the English physicist Stephen Hawking to propose the Chronology protection conjecture. Basically, it’s just a fancy name for an outright ban on time travel. According to Hawking, some as-yet-unknown law of physics must intervene to prevent time travel. He has no cast-iron evidence of such a law but simply asks: “Where are the tourists from the future?”
Einstein himself did not believe that time travel was possible, despite the fact that his theory of gravity predicted it. He was wrong, however, about two other predictions of his theory. He did not believe that black holes were possible, and today we have compelling evidence that they exist. And he did not believe what his theory was trying to tell him about the origin of the Universe—that it began in a Big Bang.
1 This is not at all obvious on Earth, where frictional forces act to slow a moving body. However, it is apparent in the empty vacuum of space.
2 It is worth pointing out that acceleration does not just mean a change in speed. It can also mean a change in direction. So a car travelling around a bend—even at constant speed—is accelerating.
3 Most people assume that astronauts orbiting Earth are weightless because there is no gravity in space. However, at the 500-kilometre-or-so height of the International Space Station, gravity is only about 15 per cent weaker than on Earth’s surface. The real reason astronauts are weightless is that they and their spacecraft are in free fall just as surely as someone in an elevator when the cable breaks. The difference is that they never hit the ground. Why? Because Earth is round and, as fast as they fall toward the surface, the surface curves away from them. They, therefore, fall forever in a circle.
4 For technical reasons, this effect is known as the gravitational red shift.
5 Or at least a workable theory for the time being, since even general relativity is not thought to be the last word on gravity.
6 The term “black hole” was coined by John Wheeler in 1965. Before 1965 there were very few scientific papers on such objects. Afterward, the field exploded. The term has even entered everyday language. People often talk about things disappearing down a bureaucratic black hole. The term is a perfect illustration of the importance of getting the right words to describe a phenomenon in science. If they paint a vivid picture in people’s minds, researchers are attracted to the subject.
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THE ULTIMATE RABBIT OUT OF A HAT
HOW WE LEARNED THAT THE UNIVERSE HAS NOT EXISTED FOREVER BUT WAS BORN IN A TITANIC EXPLOSION 13.7 BILLION YEARS AGO
A white rabbit is pulled out of a top hat. Because it is an extremely large rabbit, the trick takes billions of years.
Jostein Gaarder
They are high-tech glasses. Merely by twiddling a knob on the frame, you can “tune” them to see all kinds of light normally invisible to the human eye. You take them outside on a cold, starry night and start twiddling.
The first thing you see is the sky in ultraviolet, light pumped out by stars much hotter than the Sun. Some familiar stars have vanished, and some new ones have swum into view, shrouded in misty nebulosity. The most striking