apples don’t fall off apple trees because space-time assumesnthe undistorted, or “flat” form of Minkowski space-time.nWhen Einstein proposed the general theory of relativitynin 1916, science, technology, and mankind’s exploration ofnthe universe were still so primitive that there was almost nonoverlap between the theory’s predictions and observation. Innfact the only point of contact was a peculiarity in the orbit ofnthe planet Mercury.nMercury, Earth, and the other planets all pursue ellipticalnorbits around the Sun. Thus, as a planet orbits the Sun, itnmoves from a point of closest approach to the Sun, callednperihelion, out to a point of greatest distance, callednaphelion, and then back to perihelion. In the case ofnMercury the difference between its perihelion and aphelionndistances from the Sun — 29 million versus 43 millionnmiles — is nontrivial and is a result of the little planet’snpronouncedly elliptical orbit. Somewhat surprisingly.nMercury’s orbit is not fixed.nAlthough the planets are all much less massive than thenSun and therefore have correspondingly tiny gravitationalninfluences, each planet exerts its own small gravitationalneffect on the others, and each planet, in its turn, responds tonthe combined gravitational effects of all the other planets.nThus a planet’s orbit is calculated as if it is alone in orbitnaround the Sun — the gravitational master of the solarnsystem — and then the small gradual changes due to thenpresence of the other planets are calculated as departures, orn”perturbations,” from the simplified one-planet-on-its-ownnpicture. One effect of the other planets’ gravitation is thatnMercury’s orbit slowly rotates in its own plane. This meansnthat successive orbits of Mercury do not exactly repeat eachnother but build-up a rosette-like pattern. Astronomers, whonuse the direction from the Sun to a planet’s perihelion asnone of the measures that fix the orientation of the planet’snorbit, see this as a progressive change of Mercury’s perihelion,nwhich they label “precession of perihelion.”nIn the second half of the 19th century it became apparentnthat Mercury’s perihelion was precessing slightly faster thannexpected, and by the start of this century thorough analysisnof Mercury’s orbit had shown that its anomalous perihelionnprecession was 43 arcseconds per century. (A circle isndivided into 360 degrees; a degree into 60 arcminutes; andnan arcminute into 60 arcseconds.) The effect, whichncorresponds to completion of a full circle in three millionnyears, was of minute proportions. Not only did the 19thcenturyncelestial mechanics notice the discrepancy, but theynmeasured it accurately — a tribute to the keenness of theirninstruments and calculations (all done by hand). As theninnermost planet of the solar system. Mercury lies closest tonthe Sun and therefore, of all the planets, it experiences thengreatest curvature of space-time that the solar system has tonoffer, so it was no coincidence that it was the first planet tonbetray noticeable non-Newtonian behavior.nIn spite of the slightness of Mercury’s deviation fromngood Newtonian form, it had prompted astronomers tonadvance a variety of explanations for the puzzle. None ofnthem, however, had much observational basis or any generalnacceptance.nIt was at this point that Einstein and his general theory ofnrelativity arrived on the scene. The physicist Clifford Willndescribes things as follows:nIn November, 1915, while struggling to put thenfinishing touches on the general theory, Einsteinnwas well aware of the problem of Mercury, and itnwas one of the first calculations he carried out usingnthe new theory. To his delight, he found anprecession of 43 arcseconds per century! He laternwrote, “for a few days I was beside myself withnjoyous excitement,” and told a colleague that thendiscovery had given him palpitations of the heart.nSince then, the development of science and mankind’snexploration beyond the Earth have provided new tests ofnEinstein’s theory. The most demanding of these experimentsninvolved the American Viking soft-lander on Mars.nAccurate measurements of the time taken for radio signalsnto travel from Earth to the Viking space-probe on thenMartian surface and back to Earth have shown that thenSun’s curvature of space-time in the inner solar systemnagrees with the general relativity prediction to one part in anthousand.nThis success was a success not only for Einstein but alsonfor Minkowski. Einstein’s theory presupposed Minkowski’snflat space-time—because gravity curves space and timentogether, not separately, any description of gravity as ancurvature of space-time can be successful only if Minkowskinput space and time together successfully in the first place.nEinstein’s theories of relativity are, at their core, theoriesnofinvariance, not relativity. “Invariance” in his theories saysnthat the particular situation of the observer should not affectnhis description of physical laws. The laws of physics, asnperceived by an observer cruising along the highway in anlimousine and another observer sitting at the side of thenhighway, should be the same. This simple but very powerfulnidea guided Einstein toward both his theories of so-callednrelativity. Arnold Sommerfield, the distinguished physicistnand colleague of Einstein, remarked that: “Relativity theorynis accordingly an Invariantentheorie of the Lorentz group.nThe name relativity theory was an unfortunate choice: thenrelativity of space and time is not the essential thing, which isnthe independence of laws of nature from the viewpoint ofnthe observer.”nIn 1987, and in the same vein, physicist David Jacksonnwrote: “The whole worldview of modern theoretical physicsncan be traced back to the fundamental postulate or idea thatnphysical phenomena do not change just because you happennto be moving by, instead of standing still, when observingnthem.”nThe misnomer of Einstein’s theories has caused thenwidespread impression that they somehow support a universalnrelativism. They do not. The parallel between the worldsnof physical science and human conduct is a very dubiousnone, and, if it is made, it argues in favor of absolutism, notnrelativism.nBut what about time? Although science has successfullynunified space and time, nonetheless time remains utterlyndifferent from space. It flows. Time can be multiplied by anynconstant you choose, but it won’t stop flowing.nWhere does this flow come from? Is it somethingnhappening in the external world, or is it something imposednon the world by the mind? Is time a river or is life a journey?nScience gives a clear answer. Life is an immense journeynnnSEPTEMBER 1988/19n