Read The Physics of Star Trek for Free Online

billion times the total average power presently produced and used by all
     human
    activities on Earth!
    Now, you may suggest (as a bright colleague of mine did the other day when I presented him
     with this argument) that there is a subtle loophole. The argument hinges on the
     requirement that you carry your fuel along with the rocket. What if, however, you harvest
     your fuel as you go along? After all, hydrogen is the most abundant element
    in the universe. Can you not sweep it up as you move through the galaxy? Well, the average
     density of matter in our galaxy is about one hydrogen atom per cubic centimeter. To sweep
     up just one gram of hydrogen per second, even moving at a good fraction of the speed of
     light, would require you to deploy collection panels with a diameter of over 25 miles. And
     even turning all this matter into energy for propulsion would provide only about a
     hundred- millionth of the needed propulsion power!
    To paraphrase the words of the Nobel prizewinning physicist Edward Purcell, whose
     arguments I have adapted and extended here:
    If this sounds preposterous to you, you are right. Its preposterousness follows from the
     elementary laws of classical mechanics and special relativity. The arguments presented
     here are as inescapable as the fact that a ball will fall when you drop it at the Earth's
     surface. Rocket-propelled space travel through the galaxy at near light speed
    
    
     is not physically practical,
    
    
     now or ever!
    So, do I end the book here? Do we send back our Star Trek memorabilia and ask for a
     refund? Well, we are still not done with Einstein. His final, perhaps greatest discovery
     holds out a glimmer of hope after all.
    Fast rewind back to 1908: Einstein's discovery of the relativity of space and time heralds
     one of those “Aha!” experiences that every now and then forever change our picture of the
     universe. It was in the fall of 1908 that the mathematical physicist Hermann Minkowski
     wrote these famous words: “Henceforth, space by itself, and time by itself, are doomed to
     fade away into mere shadows, and only a kind of union of the two will preserve an
     independent reality.”
    What Minkowski realized is that even though space and time are relative for observers in
     relative motionyour clock can tick slower than mine, and my distances can be different
     from yoursif space and time are instead merged as part of a four-dimensional whole (three
     dimensions of space and one of time), an “absolute” objective reality suddenly reappears.
    The leap of insight Minkowski had can be explained by recourse to a world in which
     everyone has monocular vision and thus no direct depth perception. If you were to close
     one eye, so that your depth perception was reduced, and I were to hold a ruler up for you
     to see, and I then told someone else, who was observing from a different angle, to close
     one eye too, the ruler I was holding up would appear to the other observer to be a
     different length than it would appear to be to youas the following bird's-eye view shows.
    Each observer in the example above, without the direct ability to discern depth, will
     label “length” (L or L') to be the two-dimensional projection onto his or her plane of
     vision of the actual three-dimensional length of the ruler. Now, because we know that
     space has three dimensions, we are not fooled by this trick. We know that viewing
     something from a different angle does not change its real length, even if it changes its
     apparent length. Minkowski showed that the same idea can explain the various paradoxes of
     relativity, if we now instead suppose that our perception of space is merely a
     three-dimensional slice of what is actually a four-dimensional manifold in which space and
     time are joined. Two different observers in relative motion perceive
    
    
     different
    
    
     three-dimensional slices of the underlying four-dimensional space