Gravity in reverse: the tale of Albert Einstein's "greatest blunder"

Natural History, Dec, 2003 by Neil deGrasse Tyson

Sung to the tune of "The Times They Are A-Changin'":

   Come gather 'round, math phobes,
   Wherever you roam
   And admit that the cosmos
   Around you has grown
   And accept it that soon
   You won't know what's worth knowin'
   Until Einstein to you
   Becomes clearer.
   So you'd better start listenin'
   Or you'll drift cold and lone
   For the cosmos is weird, gettin' weirder.
   --The Editors (with apologies to Bob Dylan)

Cosmology has always been weird. Worlds resting on the backs of turtles, matter and energy coming into existence out of much less than thin air. And now, just when you'd gotten familiar, if hot really comfortable, with the idea of a big bang, along comes something new to worry about. A mysterious and universal pressure pervades all of space and acts against the cosmic gravity that has tried to drag the universe back together ever since the big bang. On top of that, "negative gravity" has forced the expansion of the universe to accelerate exponentially, and cosmic gravity is losing the tug-of-war.

For these and similarly mind-warping ideas in twentieth-century physics, just blame Albert Einstein.

Einstein hardly ever set foot in the laboratory; he didn't test phenomena or use elaborate equipment. He was a theorist who perfected the "thought experiment," in which you engage nature through your imagination, inventing a situation or a model and then working out the consequences of some physical principle.

If--as was the case for Einstein--a physicist's model is intended to represent the entire universe, then manipulating the model should be tantamount to manipulating the universe itself. Observers and experimentalists can then go out and look for the phenomena predicted by that model. If the model is flawed, or if the theorists make a mistake in their calculations, the observers will detect a mismatch between the model's predictions and the way things happen in the real universe. That's the first cue to try again, either by adjusting the old model or by creating a new one.

One of the most powerful and far-reaching theoretical models ever devised is Einstein's theory of general relativity, published in 1916 as "The Foundation of the General Theory of Relativity" and refined in 1917 in "Cosmological Considerations in the General Theory of Relativity." Together, the papers outline the relevant mathematical details of how everything in the universe moves under the influence of gravity. Every few years, laboratory scientists devise ever more precise experiments to test the theory, only to extend the envelope of its accuracy.

Most scientific models are only hall baked, and have some wiggle room for the adjustment of parameters to fit the known universe. In the heliocentric universe conceived by the sixteenth-century astronomer Nicolaus Copernicus, for example, planets orbited the Sun in perfect circles. The orbit-the-Sun part was correct, but the perfect-circle part turned out to be a bit off. Making the orbits elliptical made the Copernican system more accurate.

Yet, in the case of Einstein's relativity, the founding principles of the entire theory require that everything take place exactly as predicted. Einstein had, in effect, built a house of cards, with only two or three simple postulates holding up the entire structure. (Indeed, on learning of a 1931 book titled 100 Authors Against Einstein, he responded, "Why one hundred? If I am incorrect, one would have been enough.")

That unassailable structure--the fact that the theory is fully baked--is the source of one of the most fascinating blunders in the history of science. Einstein's 1917 refinement of his equations of gravity included a new term--denoted by the Greek letter lambda--in which his model universe neither expands nor contracts. Because lambda served to oppose gravity within Einstein's model, it could keep the universe in balance, resisting gravity's natural tendency to pull the whole cosmos into one giant mass. Einstein's universe was indeed balanced, but, as the Russian physicist Alexsandr Friedmann showed mathematically in 1922, it was in a precarious state--like a ball at the top of a hill, ready to roll down in one direction or another at the slightest provocation. Moreover, giving something a name does not make it real, and Einstein knew of no counterpart in the physical universe to the lambda in his equations.

Einstein's general theory of relativity--called GR by verbally lazy cognoscenti--radically departed from all previous thinking about the attraction of gravity. Instead of settling for Sir Isaac Newton's view of gravity as "action at a distance" (a conclusion that discomfited Newton himself), GR regards gravity as the response of a mass to the local curvature of space and time caused by some other mass. In other words, concentrations of mass cause distortions--dimples, really-in the fabric of space and time. Those distortions guide the moving masses along straight-line geodesics, which look like the curved trajectories that physicists call orbits. John Archibald "Wheeler, a physicist at Princeton University, put it best when he summed up Einstein's concept this way: "Matter tells space how to curve; space tells matter how to move."