As a science fiction writer and a devotee of science, although an English major,1 I am a person of immense technological optimism. I believe that, for all the scientific understanding humanity has gained in the past couple of centuries, and all the technical advances that make our lives easier, better, and more powerful than those of previous high civilizations, we’re still only scratching the surface.
I’ve been involved in recent discussions on social media about this. When Facebook friends disagree,2 they point out that we understand a tremendous amount of what’s going on in the physical world, and that past revolutions have not so much been a true overturning of past knowledge as its refinement and deepening.3 In planetary motion—our view of the stellar and interstellar world out there—the work of 17th century geniuses Johannes Kepler and Sir Isaac Newton is still relevant and is only advanced by the work of Albert Einstein. In quantum theory—our understanding of the building blocks of energy and matter—the work of 20th century physicists Max Planck, Niels Bohr, Werner Heisenberg, and many others has yet to be superseded.
While I do not deny this, I’m still uneasy about the state of our knowledge in three basic areas: the nature of gravity, space, and time. We can use these terms in sentences; we understand and can measure their effects quite accurately; we can use the measurements in equations. But as to their essential nature, what they are, we are still in doubt.
I am not one of those who insist that some things humans were not meant to know, either because our minds cannot encompass ultimate reality or because the truth is reserved to a higher order of being. I do believe that our mind—which evolved over time in relation to our physical senses and the nature of the world we can see, hear, taste, smell, and touch—places limits on how we usually think. But many humans can overcome this. Certainly Einstein showed this kind of imagination when he proposed “thought experiments” such as chasing a beam of light.
We know that gravity is some kind of force. It plays well in the basic physics equation governing force (F), where F=ma, or mass multiplied by acceleration. In that equation, gravity is an acceleration of whatever mass you’re dealing with. A pitcher’s arm also exerts force in throwing a fast ball. The measured force is equal to the mass of the baseball (a regulation 5 ounces, or 142 grams) multiplied by whatever acceleration may be required4 to arrive at the ball’s final, measured speed over the plate.
We understand what’s going on in a pitcher’s arm. Muscles are contracting in a way that whips his arm around. His fingers are clenched to cage the ball, then let it go at the point of maximum acceleration. And the interaction between his fingertips and the ball’s covering—which imparts whatever spin he puts on the ball—is governed by the electromagnetic forces in skin and leather that account for friction. We understand the underlying mechanism of a fast pitch very well.
But what’s the underlying mechanism of gravity? We detect no mechanical interaction, no muscles, no arm, no fingertips. In physics, gravity is considered a field force, like electricity and magnetism, which can create “action at a distance” without requiring physical contact between the atoms or particles involved.
But what does the word “field” really explain? To say that a force acts over a field suggests there may be a limit to its effective range. However, the fields governing two of the fundamental physical forces, electromagnetism and gravity, are assigned infinite ranges—that is, there’s no telling how far a photon may travel, or how far away from a galaxy you must travel before you stop feeling its pull, however faintly. You can write equations governing a force field’s strengths and its effects. But the mechanism by which they accomplish this is a lot less obvious than that of an arm hurling a baseball.
Einstein’s theory of General Relativity pictures gravity as a curvature of spacetime. An body with mass m1 exerts a gravity field that curves space around it, so that the path of an body with mass m2 which is traveling near the first object at a distance of d1 is deflected toward it by a distance of d2.5 I think I can understand that.
A planet like the Earth bends local space so that a satellite launched at a certain speed, rather than traveling in what we think would be a straight line, instead travels in a closed curve, an orbit around the planet’s center of mass. A star like the Sun bends local space so that planets travel around it in closed curves called ellipses. Stars create such strong bending force that they can even curve the path of a light beam.6 Galaxies create such strong distortions in spacetime that they can act as lenses of the light from more distant galaxies. I can intuitively understand motion in curved space.
But what about two objects standing still in relation to each other? When I stand on the Earth’s surface, I may be flying through space with the planet’s rotation and its revolution about the Sun, but with respect to the center of the planet, the two of us are not moving. Yet, according to the measurement of gravity, I am continuously accelerating toward that center at a rate of 32 feet per second per second (32 ft/s2, or 9.8 m/s2). I am accelerating without a change in relative speed. It might make perfect mathematical sense, but it leaves something to be desired in terms of common sense. In fact, if you think about it, gravity in this particular case really messes up your sense of time and distance.7
Quantum Mechanics has a different interpretation of gravity. According to its Standard Model, all forces and fields are represented by an associated particle. Electricity and magnetism are associated with the photon. The strong nuclear force—which holds protons and neutrons together in an atomic nucleus—is associated with the gluon. The weak nuclear force—which accounts for the decay of subatomic particles—is associated with emission or absorption of W and Z bosons. These particles have all been measured and detected. But the particle associated with gravity, called the “graviton,” remains hypothetical. According to the Standard Model, it should have an infinite range, a mass of 0, and a spin of 2—but it just hasn’t been seen.
Another particle, the Higgs boson, associated with mass, has also remained unseen—until last July. This particle is so massive that it supposedly hasn’t been around since the Big Bang. But scientists at the European Organization for Nuclear Research (CERN), after many experiments involving highly energetic particle collisions and comparison of the data, believe they see trails of particles already decaying from a more massive particle that may indeed be the Higgs. Although the Higgs particle no longer exists naturally in the universe, its associated field accounts for how the various particles acquire mass. When we understand that trick—as opposed to simply being able to use the word “mass” in a sentence or use its value in an equation—we may be closer to understanding gravity.
Until then, while we can manipulate electricity and magnetism and observe the interaction of particles in a nuclear explosion, we are powerless over gravity. We know it exists and can measure its effects. But our understanding of the “pitcher’s arm” remains highly theoretical, mathematical, and abstract. In fact, until we can deal with gravity on the same terms as electromagnetism and the strong and weak nuclear forces, our physics remains broken between General Relativity’s spacetime and Quantum Mechanics’ array of particles.
I expect great things when we finally have a working definition of gravity. If that understanding follows the pattern of our knowledge of electromagnetism and the strong and weak nuclear forces, then soon after we define gravity we will be able to build technical applications that either capture and sequester live gravitons or manipulate spacetime in closed gravity curves. We will fly without wings and float up to heaven.
Gravity is, in my view, the first of the three things we don’t yet understand. Next week, I’ll tackle the other two—which seem to be intimately related to gravity: the structure of space and the nature of time.
1. Truth in advertising: I studied English literature at the university and my liberal arts math requirement was fulfilled by Philosophy 1, Introduction to Logic. But since then I’ve consistently worked in technical organizations—pharmaceuticals, applied biotechnology, engineering and construction, an electric and gas utility, and an oil refinery—and have had to learn a lot of math and science to keep up with the engineers and scientists. I’m an aficionado if not a practitioner of science.
2. Yes, you can have interesting discussions on Facebook. It’s not all pictures of kittens and ironic motivational posters.
3. One Facebook friend, an entrepreneur and expert in rocketry, aerospace, and orbital solar energy, has posted that he believes we know about 80% of what’s going on in the physical world, leaving 20% to be discovered. My response has been that those proportions depend on what you think constitutes the 100% of what there is to know—and hence the subject of this meditation.
4. Acceleration may be the hardest part of all this to understand for the mathematically challenged, because acceleration makes two references to time. Acceleration (a) is the change in velocity (v) over a measured period of time (t). That first component—your velocity or speed—is expressed as distance traveled divided by time elapsed (d/t, as in “so many feet per second,” or ft/s). The change in velocity during acceleration represents progress from the object moving at one speed, its initial velocity (vi, which may also be zero), to moving at a higher speed at its final velocity (vf). All of this is expressed as a fraction, a=(vf–vi)/t. Solving the equation and accounting for the “per-second” of that final velocity and the “per-second” of the acceleration yields the result in distance per-second-squared.
5. By extension, one imagines that all forces which work through fields must somehow alter the properties of supposedly “empty space” over their effective range.
6. Yes, the photon is supposed to be without mass, and therefore not subject to gravity’s effects. But that’s the “rest mass,” as if a photon ever stood still. The photon’s momentum at the speed of light gives it “apparent mass” and lets it participate in gravity equations.
7. I had fun with this in my recent novel The Children of Possibility when describing one possible method of traveling through time.