ing quantum fluctuations. This is the “reaction” of fields corresponding to their “action” on charged bodies. Similarly, the strength of the metric field is affected by all bodies that have energy and momentum (as all known forms of matter do). Thus the presence of a body A affects the metric field, which in turn affects the trajectory of another body B. This is how general relativity accounts for the phenomenon formerly known as the gravitational force exerted by one body on another. It vindicates Newton’s intuitive rejection of action at a distance, even as it dethrones his theory.

Mass has traditionally been regarded as the defining property of matter—the feature that gives substance to substance. So the recent astronomical discovery that space weighs—that the entity we perceive as empty space has a universal, non-zero density—crowns the case for its physical reality.

The concept of space density is essentially the same as Einstein’s cosmological term, which is essentially the same as “dark energy.” In 1917 Einstein introduced a modification of the equations he originally proposed for general relativity two years earlier. His motivation was cosmology. Einstein thought that the universe had constant density, both in time and (on average) in space, so he wanted to find a solution with those properties. But when he applied his original equations to the universe as a whole, he could not find such a solution. The underlying problem is easy to grasp. Simply put: gravity is a universal attraction, so it is not content to leave things separate. Gravity is always trying to bring things together. So it’s not terribly surprising that you can’t find a solution where the universe maintains a constant density.

To get the kind of solution he wanted, Einstein changed the equations. But he changed them in a very particular way that doesn’t spoil their best feature, namely that they describe gravity in a way consistent with special relativity. There is basically only one way to do that. Einstein called the new term that he added to the equations for gravity the “cosmological term.” He didn’t really offer a physical interpretation of it, but modern physics supplies a compelling one, which I’ll describe momentarily.

The cosmological term can be viewed in two ways. One way, the way Einstein viewed it, is as a modification of the law of gravity. Alternatively, the term can also be viewed as the effect of having a constant density of mass and also a constant pressure everywhere in space and for all time. Since this mass-density and pressure have the same value everywhere, they can be regarded as intrinsic properties of space itself. That's the Grid viewpoint. If we take it as given that space has these properties, and focus exclusively on the gravitational consequences, we arrive back at Einstein's viewpoint.

A key relationship governing the physics of the cosmological term relates its density ρ to the pressure p it exerts, using the speed of light c. It reads

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