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eeply for a moment and replied： “i am trying to think who the thirdperson is。” in fact； the problem with relativity wasn’t that it involved a lot of differentialequations； lorentz transformations； and other plicated mathematics （though it did—eveneinstein needed help with some of it）； but that it was just so thoroughly nonintuitive。
in essence what relativity says is that space and time are not absolute； but relative to boththe observer and to the thing being observed； and the faster one moves the more pronouncedthese effects bee。 we can never accelerate ourselves to the speed of light； and the harderwe try （and faster we go） the more distorted we will bee； relative to an outside observer。
almost at once popularizers of science tried to e up with ways to make these conceptsaccessible to a general audience。 one of the more successful attempts—mercially at least—was the abc of relativity by the mathematician and philosopher bertrand russell。 init； russell employed an image that has been used many times since。 he asked the reader toenvision a train one hundred yards long moving at 60 percent of the speed of light。 tosomeone standing on a platform watching it pass； the train would appear to be only eightyyards long and everything on it would be similarly pressed。 if we could hear thepassengers on the train speak； their voices would sound slurred and sluggish； like a recordplayed at too slow a speed； and their movements would appear similarly ponderous。 even theclocks on the train would seem to be running at only four…fifths of their normal speed。
however—and here’s the thing—people on the train would have no sense of thesedistortions。 to them； everything on the train would seem quite normal。 it would be we on theplatform who looked weirdly pressed and slowed down。 it is all to do； you see； with yourposition relative to the moving object。
this effect actually happens every time you move。 fly across the united states； and youwill step from the plane a quinzillionth of a second； or something； younger than those you leftbehind。 even in walking across the room you will very slightly alter your own experience oftime and space。 it has been calculated that a baseball thrown at a hundred miles an hour willpick up 0。000000000002 grams of mass on its way to home plate。 so the effects of relativityare real and have been measured。 the problem is that such changes are much too small tomake the tiniest detectable difference to us。 but for other things in the universe—light；gravity； the universe itself—these are matters of consequence。
so if the ideas of relativity seem weird； it is only because we don’t experience these sorts ofinteractions in normal life。 however； to turn to bodanis again； we all monly encounterother kinds of relativity—for instance with regard to sound。 if you are in a park and someoneis playing annoying music； you know that if you move to a more distant spot the music willseem quieter。 that’s not because the musicis quieter； of course； but simply that your positionrelative to it has changed。 to something too small or sluggish to duplicate this experience—asnail； say—the idea that a boom box could seem to two observers to produce two differentvolumes of music simultaneously might seem incredible。
the most challenging and nonintuitive of all the concepts in the general theory of relativityis the idea that time is part of space。 our instinct is to regard time as eternal； absolute；immutable—nothing can disturb its steady tick。 in fact； according to einstein； time is variableand ever changing。 it even has shape。 it is bound up—“inextricably interconnected；” instephen hawking’s expression—with the three dimensions of space in a curious dimensionknown as spacetime。
spacetime is usually explained by asking you to imagine something flat but pliant—amattress； say； or a sheet of stretched rubber—on which is resting a heavy round object； suchas an iron ball。 the weight of the iron ball causes the material on which it is sitting to stretchand sag slightly。 this is roughly analogous to the effect that a massive object such as the sun（the iron ball） has on spacetime （the material）： it stretches and curves and warps it。 now ifyou roll a smaller ball across the sheet； it tries to go in a straight line as required by newton’slaws of motion； but as it nears the massive object and the slope of the sagging fabric； it rollsdownward； ineluctably drawn to the more massive object。 this is gravity—a product of thebending of spacetime。
every object that has mass creates a little depression in the fabric of the cosmos。 thus theuniverse； as dennis overbye has put it； is “the ultimate sagging mattress。” gravity on this view is no longer so much a thing as an oute—“not a ‘force’ but a byproduct of thewarping of spacetime；” in the words of the physicist michio kaku； who goes on： “in somesense； gravity does not exist； what moves the planets and stars is the distortion of space andtime。”
of course the sagging mattress analogy can take us only so far because it doesn’tincorporate the effect of time。 but then our brains can take us only so far because it is sonearly impossible to envision a dimension prising three parts space to one part time； allinterwoven like the threads in a plaid fabric。 at all events； i think we can agree that this wasan awfully big thought for a young man staring out the window of a patent office in thecapital of switzerland。
among much else； einstein’s general theory of relativity suggested that the universe mustbe either expanding or contracting。 but einstein was not a cosmologist； and he accepted theprevailing wisdom that the universe was fixed and eternal。 more or less reflexively； hedropped into his equations something called the cosmological constant； which arbitrarilycounterbalanced the effects of gravity； serving as a kind of mathematical pause button。 bookson the history of science always forgive einstein this lapse； but it was actually a fairlyappalling piece of science and he knew it。 he called it “the biggest blunder of my life。”
coincidentally； at about the time that einstein was affixing a cosmological constant to histheory； at the lowell observatory in