Last week I presented my layman’s understanding of gravity—or rather, the great gaping holes at the center of our two competing theories about physics, General Relativity and Quantum Mechanics, where a complete explanation of this force should be. We can write grammatical sentences using the word “gravity,” measure it and predict its effect on objects, and use those measurements in practical equations. But we still don’t know how it works.

Perhaps that’s because we still don’t have a complete definition of two other aspects of nature that we can talk about, quantify, and solve for but don’t really understand. These are the silent partners of gravity: space and time.

**Space**

For most of us, space is the definition of nothingness. Go out among the stars, or further out among the galaxies, corral a cubic meter of space, capture and exclude the handful of atoms representing stray gases and dust fragments—and what you have left is “empty space.” Photons may pass through that cube. Gravity fields may affect those photons and anything else that passes through it. But what’s left after your extraction is pure nothing.

Early physicists—people who now rank with alchemists on the scale of seriousness—posited that since light acted like a wave, empty space must contain some invisible material that was being agitated, in the same way that an ocean wave propagates in water. They called this invisible, conjectural substance “luminiferous ether.” That died in 1887 when two men, Albert Michelson and Edward Morley, conducted an experiment that measured the speed of light using an array of mirrors mounted on a turntable. Since the Earth is moving around the Sun, and the Sun is moving around the galaxy, then the Earth and the turntable must be moving through this natural element, the ether, in a certain direction. So, when the speed of light was measured with the turntable in a certain position, it ought to be moving faster than when the table was turned 180 degrees—like a wave going up or down in a moving stream. And when the table was turned by 90 degrees to either side of that apparent flow, then the light ought to move at an intermediate speed. What they found instead was no change in speed at all. This led to understanding light as having properties of both a particle and a wave, or a particle moving with a wavelike oscillation.^{1}

The Michelson-Morley experiment tended to prove that space was really, really empty. But Einstein maintains that gravity can curve space (or, more properly, a four-dimensional continuum known as “spacetime”). And physicists may suppose even higher-order dimensions. As you can fold and crumple a two-dimensional sheet of paper in the three dimensions of the space that we commonly experience, so our three dimensional space may be presumed to fold and crumple at even higher dimensions. This allows for faster-than-light travel by punching between two nearby folds. It also allows for the supposed existence of naturally enduring “wormholes” through these higher dimensions.

So, apparently, “empty” space can have structure. And this structure can be complex at dimensions we do not ordinarily experience, except through thought experiments and mathematics.

More than that, the universe we can detect appears to be expanding. It is not only expanding under the impulse of the Big Bang some 13 billion years ago, but accelerating even faster, so that eventually nearby galaxies will vanish, then our own galaxy’s stars, then even our next-door neighbors, and finally the molecules of our brains—all ripped apart and carried away by expanding space. To account for this situation—where all the visible matter and dark matter of the universe is not enough to close it through mutual attraction—physicists propose a new characteristic of empty space: dark energy, sometimes called “vacuum energy” or “zero-point energy.” This energy supposedly manifests as hypothetical particles, virtual particles that always appear as pairs of identical yet opposing bits of matter and antimatter. These virtual particles come into existence and instantaneously annihilate each other before they can be detected. Space is not just empty, it’s teeming with bursts of energy that no one can see.^{2}

So space has structure, it can be warped, curved, bent, folded, and punctured. It’s multiplying faster than we can measure it. In fact, we can only measure it because our rulers and other measuring systems are expanding (and curving and folding) right along with the underlying “fabric” of space.

I don’t think we understand “space” yet.

**Time**

Everyone knows about time. Our watches click off seconds, hours, days and on to the calendar and then to the millennia of history. We can set a clock by the predictable oscillations in the electromagnetic energy of certain atoms. We can measure infinity by the fact that no one has ever detected a proton naturally decaying into two or more lighter particles. All the protons that condensed out of the Big Bang just short of 13 billion years ago are still with us and will remain with us until the universe fades out in the Big Sigh.

We think of time as a fourth dimension added to the three that we can observe from one isolated point of view: left-and-right, expressed by mathematicians as *x*; up-and-down, expressed as *y*; and in-and-out, expressed as *z*. So there should also be a before-and-after dimension, expressed as *t*. If I stand utterly still at a certain point *x, y, z*, I am still moving along the dimension *t* from the past into the future.

You can use time in grammatical sentences governed by verb tenses. You can tell stories about a hypothetical past beyond your own experience, as well as a future that has not yet occurred. You can use *t* in an equation—most often represented in physics equations as that “/s” for “per second,” the second being the universal reduction of all physicists’ time in International System (SI) units—and you can even assign to *t* positive and negative values.^{3}

But time is not like a dimension. Although I can stand still in reference to point *x, y, z*, I cannot stand still with respect to *t*—unless, that is, I move at the speed of light and give up all pretense to a static *x, y, z*. I cannot experience the past except in memory, which is encoded in chemicals in my brain, or in stories, which are encoded in words interpreted by my brain. I can anticipate and think about the future, but I cannot rush into it. I can only experience the instant we call “now.”

Einstein maintains that time is relative to speed, but there’s a catch: time is always subjective and related to a particular frame of reference. If I move faster on a scale that approaches the speed of light, my personal time would seem to slow—but only from the viewpoint of an outside observer moving at a different speed in a different frame of reference. When I travel exactly at the speed of light, anyone looking through the window of my spaceship sees me frozen in mid-gesture, but from my own point of view I’m acting normally and still experience time at the same steady crawl as if I were standing still. For Einstein and modern physicists, any absolute measure of time does not exist: it’s all subjective, all based on your frame of reference. “Standard clocks” cannot exist, because the rate at which the mechanism ticks, or the atomic nuclei oscillate, depends on the speed at which the clock is traveling.^{4}

So, for right now, our understanding of the most common aspects of both our science and our daily experience—time, space, and gravity—remains highly theoretical, conjectural, and mathematical. Most physicists would maintain that a common-sense understanding of these things is impossible because we humans inhabit a single viewpoint in space and time, that real understanding requires the flexibility and pervasiveness of mathematics. They would say that a person who does not understand the mathematics of Quantum Mechanics or General Relativity cannot grasp the true meaning of these things. But perhaps that’s because the underlying nature as we understand it only exists as conjectures in theory and descriptions in mathematics.

I take a different tack. I think the nature of existence, expressed in what we can see of time, space, and gravity, can be ultimately understood—but we need new ideas. The conceptions we’ve used to date have stretched about as far as we can push and pull them—like trying to cover too much bed with too little sheet. We don’t have the right words to adequately describe what we can sense about nature because we don’t yet have the right concepts. We don’t have the right mathematics yet, either.^{5}

The fit of our minds and imaginations, of our language and mathematics—and ultimately of our clever fingers and the machines they can create—to the world that we can now see and experience is still inexact. We can measure space, time, and gravity. We can use those words in meaningful sentences and those measurements in equations. But we are still a long way from understanding exactly what’s going on out there.

1. The water in an ocean wave, on the other hand, does not move over long distances. Individual water molecules may rise and fall with each wave, moving in short ellipses from the surface to some point below it and back up again, but the molecules themselves do not travel forward along with the wave crest. The mass of water itself is not traveling from the deep ocean to the shore.

2. You’re wondering, at this point, whatever became of high-school physics, where “matter and energy can neither be created nor destroyed.” Well, that’s true only inside a closed system. Open your imagination and your mathematics to a larger system composed of multiple, unseen dimensions, and anything is possible.

3. And yet physicists still puzzle over “time’s arrow.” Time only moves forward. Time has never been observed, even in the strange mechanics of Einsteinian spacetime, to move backward. The arrow does not appear to be a function of speed or frame of reference: you can’t outrun time’s arrow, and no manipulation of field forces will reverse it.

4. And yet physicists still accept the second as the universal measurement of time—but only for events, experiments, and observers who are standing still in relation to one another.

5. Mathematics is not a closed system, and our understanding and use of it is not yet complete. The Romans functioned perfectly well with their counting system based on concatenated symbols (I, II, III, IV, V, VI …) in base ten and their fractions in base twelve. But their technology only got so far with it. The Arabs—and then western civilization as a whole—got farther by using a more complex system involving ten symbols and a series of decimal placeholders.

Then thoughtful analysis of that tenth placeholder, the symbol “0,” as also standing at the starting place below “1” took arithmetic beyond mere finger counting into the realm of accounting for and manipulating absence or nothing. From that starting point, we next discovered negative numbers, which can be used to express hypothetical losses.

Our current mathematics uses calculus to deal with advanced concepts like sums and fractions of undefined quantities (e.g., discrete segments under a continuous curve) and uses geometric functions called tensors (e.g., scalars and vectors) to quantify elements and motions in a hypothetical space. Each level of mathematical discourse opens up new concepts. My faith is that, first, further levels remain to be explored, and second, those concepts can ultimately be explained to the rest of us in words, even if those words cannot be manipulated as easily as the mathematical symbols they describe.