Unit 3 Life in the future教學設計與反思

       four dimensions are difficult to imagine, so physicists usually suggest you think of space-time as a rubber sheet stretched out flat. if there are no large masses around, the sheet stays flat, and so any object placed on it will move around in straight lines. but a large mass, such as the sun, makes a dip in the sheet because it actually warps space-time. now any other object with smaller mass, like our earth, moving about in space-time rolls into the dip as it comes past the sun. it appears “attracted” to the large mass. this effect of warping space-time is what gives rise to gravity.       the universe is full of heavy objects exerting gravitational effects and the net result is that space-time is not flat at all but curved. everything, including light, has to follow curved paths in space-time. we know einstein was right about this because astronomers can sometimes see distant stars that ought to be masked by nearer objects such as the sun. instead of traveling in straight lines and hence being blocked, the light from the stars bends round the obstruction.       when a star reaches the end of its life it mat collapse inwards under the influence of its own gravity to such an extent that all its matter becomes concentrated into an extremely dense object, a fraction of its original size. this is a black hole. black holes have such a huge gravitational pull that nothing can escape from them, not even light. we cannot see them but we have good evidence that they are being pulled about by a nearby invisible object with enormous mass.      what does a black hole do to space-time? relativity predicts that at the center of a black hole is an infinitely dense point, called a singularity, within which all the normal laws of physics no longer apply. time, space, matter and energy no longer have any well-defined meaning. einstein’s equations show that such a singularity doesn’t just make a dip in the imaginary rubber sheet of space- time, it makes a tunnel that goes right through and momentarily opens out on the other side. where is “the other side”? it could be somewhere else in space-time, either in the future or in the past, or it could even be in another universe! supposedly it was derwood’s time lords who first harnessed the power of a black hole to begin their experiments in time travel. if you could take a space trip through such a tunnel, or wormhole, you would have discovered the secret of time travel. this is of course impossible with today’s technology. but in the future, who knows?