• Choose a category
• All
• Classic-Fiction

By Root 1995 0

him of general relativity. Don’t get me wrong. It’s understandable that, given only five minutes (one of which was spent explaining baseball), you focused on curved spacetime as the source of gravity. Newton himself had called attention to the fact that there was no known mechanism for transmitting gravity, and he always viewed that as a yawning hole in his own theory. Naturally, you wanted to show him Einstein’s resolution. But Einstein’s theory of gravity did much more than merely fill a gap in Newtonian physics. Gravity in general relativity differs in its essence from gravity in Newton’s physics, and in the present context, there is one feature that cries out for emphasis.

In Newton’s theory, gravity arises solely from an object’s mass. The bigger the mass, the bigger the object’s gravitational pull. In Einstein’s theory, gravity arises from an object’s mass (and energy) but also from its pressure. Weigh a sealed bag of potato chips. Weigh it again, but this time squeeze the bag so that the air inside is under higher pressure. According to Newton, the weight will be the same, because there’s been no change in mass. According to Einstein, the squeezed bag will weigh slightly more, because although the mass is the same there’s been an increase in pressure.4 In everyday circumstances we’re not aware of it, because for ordinary objects the effect is fantastically tiny. Even so, general relativity, and the experiments that have shown it to be correct, makes it perfectly clear that pressure contributes to gravity.

This deviation from Newton’s theory is critical. Air pressure, whether the air is in a bag of potato chips, an inflated balloon, or the room where you’re now reading, is positive, meaning that the air pushes outward. In general relativity, positive pressure, like positive mass, contributes positively to gravity, resulting in increased weight. But whereas mass is always positive, there are situations in which pressure can be negative. Think of a stretched rubber band. Rather than pushing outward, the rubber band’s straining molecules pull inward, exerting what physicists call negative pressure (or, equivalently, tension). And much as general relativity shows that positive pressure gives rise to attractive gravity, it shows that negative pressure gives rise to the opposite: repulsive gravity.

Repulsive gravity?

This would blow Newton’s mind. For him, gravity was only attractive. But your mind should remain intact: you’ve already encountered this strange clause in general relativity’s contract with gravity. Remember Einstein’s cosmological constant, discussed in the previous chapter? I declared there that by infusing space with a uniform energy, a cosmological constant generates repulsive gravity. But in that earlier encounter, I didn’t explain why this happens. Now I can. A cosmological constant not only endows the spatial fabric with a uniform energy determined by the constant’s value (the number on the third line of the apocryphal relativity tax form), but it also fills space with a uniform negative pressure (we will see why in a moment). And, as above, when it comes to the gravitational force each produces, negative pressure does the opposite of positive mass and positive pressure. It yields repulsive gravity.*

In Einstein’s hands, repulsive gravity was used for a single erroneous purpose. He proposed finely adjusting the amount of negative pressure that permeates space to ensure that the repulsive gravity produced would exactly counter the attractive gravity exerted by the universe’s more familiar material contents, yielding a static universe. As we’ve seen, he subsequently renounced this move. Six decades later, the developers of the inflationary theory proposed a kind of repulsive gravity that differed from Einstein’s version much as the finale of Mahler’s Eighth differs from the drone of a tuning fork. Rather than a moderate and steady outward push that would stabilize the universe, the inflationary theory envisions a gargantuan surge of repulsive gravity that’s astoundingly short and thunderingly intense.