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Is Gravity Feeble? No, in Theory

Gravity is a universal force. It shapes the basic structure of space and time. It is fundamental. So we should use gravity as the measure of other things, and not use other things as the measure of gravity. Gravity, therefore, can’t be feeble in an absolute sense—it just is what it is. The fact that gravity appears feeble is confounding to theory. It also puts a major hurdle on the path to unification.

 

 

EINSTEIN’S THEORY OF GRAVITY, general relativity, ties the existence of gravity to the structure of space and time. The effect we see as the force of gravity, according to this theory, is simply bodies doing their best to travel in straight lines through the curved landscape of space-time. Bodies also cause space-time to curve. The curvature caused by body B affects the motion of body A to produce what in Newtonian language we would call the “force of gravity.”⁴⁴

A far-reaching consequence of Einstein’s picture of gravity is the universality of that force. Any body, doing its best to travel in a straight line through curved space-time, will follow the same path any other body would. The best path is determined by the curvature of space-time, not by any specific property of the body.

In fact the observed universality of gravity was a big part of what led Einstein to his theory. In Newton’s account of gravity, the universality was an unexplained coincidence (or rather an infinite number of coincidences, one for each body). On the one hand, the gravitational force felt by a body is proportional to its mass. On the other, the acceleration a body feels in response to a given force is inversely proportional to mass. (That’s Newton’s second second law of motion. His original second law of motion is F = ma; this is a = F/m.) Clapping those two hands together, we discover that the gravitational acceleration of a body—the actual disturbance of its motion—doesn’t depend on its mass at all!

And that is what’s observed: motion independent of mass. The observed behavior is universal: all bodies accelerate in the same way under gravity. But in Newton’s account there’s no reason why it had to occur. It’s another of those things that works in practice but not in theory. The gravitational force on a body didn’t have to be proportional to the body’s mass. We certainly know of forces, such as electric forces, that aren’t proportional to mass.

Einstein’s theory explains the gravitational “coincidence.” Or rather, transcends it: we don’t have to speak separately about a force and a response to force, that happen to depend on mass in opposite ways. We just have bodies doing their best to keep going straight through curved space-time. This is profound simplicity at its best.

Universality and Unification

When we come to seek a unified theory including all the forces of nature, the combination of gravity’s universality and its (apparent) feebleness poses great difficulties. Here are the alternatives:

• Gravity might be derived from the other fundamental forces. Because it is a small (feeble) effect, maybe gravity is a byproduct, a small residual after the near-cancellation of effects of opposite electric or color charges, or something more exotic. But then why should it be universal? The other forces are definitely not universal: quarks but not electrons feel the strong force; electrons and quarks, but not photons or color gluons, feel the electromagnetic force. It is hard to imagine a simple universal force that has the same consequences for all particles arising from such lopsided ingredients.

• The other forces might be derived from gravity. It’s easy to imagine how nonuniversal forces might arise out of a universal one. There could be several different solutions of the universal equations with energy concentrated in small regions of space; we’d interpret those solutions as particles with different properties. (Apparently Einstein himself had hopes of constructing a theory of matter along these lines.) But it is hard to see how an extravagantly feeble force can spin off much larger ones.

• All the forces might appear on the same footing, as different aspects—perhaps related by symmetry—of a single whole, like different sides of a die. But again, it is hard to make this idea consistent with gravity being much feebler than the other forces.

 

Turning it around: Faith in the possibility of unification drives us into a state of denial. We can’t accept that gravity really is feeble, even though it appears that way. Appearances—or rather, our interpretation of them—must be deceptive.