A book titled Gravity could be about anything, because, it could be argued, gravity is in, of and about everything.  Thankfully, Clegg mainly limits his focus to the history of the metaphysical understanding we have of gravity (in the sense of gravity as it exists throughout the universe), otherwise, the subject would require an encyclopedia to cover it all. 

For the average layman not actively pondering how it is that an apple heads toward the ground and not the sky when its stem loosens and breaks, the concept of gravity seems to occupy roughly the same place in the shadows of their understandings as does the concept of love.  Both are attractive forces.  Both are powerfully determinative of the course their lives will take.  Both have something of a mystical quality.  And not much of anyone understands either one, which is as true about the layman as it is about those who actually have pondered how it is that apples fall towards the earth and not toward the sky. 

We know almost nothing of gravity, except that it exists.  We have quantified a few of its effects, most usefully through Newton.  We know nothing of its causes.  To his credit, Clegg doesn’t try to pretend otherwise. 

Gravity is not a physics book of the celebrity physicist genre (Stephen Hawking, Michio Kaku, et al), written to an audience already awed by the vast capabilities of our very big brains to resolve, in due time and with massive particle-smashing toys generously provided by taxpayers, all the mysteries of the universe.   It seems rather more directed to the average guy who has reached a stage of life where he seeks to resolve some of life’s mysteries, which is to say, it seems like it’s written for a guy like me.  As a primer on the history of how we arrived at our present understanding of gravity, the book is excellent.  Clegg writes in clear, concise, jargon-free prose.  He does not shy away from explanations which involve some mathematics, but understands how to translate mathematical equations to linguistically resolvable relationships.  And he demonstrates a clear mastery of the subject, so far as such a thing is possible, capably relating what we know and what we don’t know about gravity.  He is not afraid to step on some celebrity physicist toes in explaining the latter, as the following passage, concerning whether general relativity can predict the big bang, illustrates: 

Take the big bang.  Go far enough back in time, close enough to the big bang itself, and you are dealing with a purely quantum phenomenon.  Yet it’s one where gravity is an essential component.  Without being able to combine the two [quantum and relativity theory, as Clegg previously explained were incompatible] it is impossible to say anything meaningful about the big bang itself. 

Just think about that statement.  Next time you see a TV special on which a celebrity scientist is pontificating about the big bang as if it were solid fact, you will know better.  The big bang is an idea that emerges from a theory that doesn’t work at that scale.  Without quantum gravity, we can’t say what happened at the big bang or even if it existed at all.  The only certainty is that our current scientific models fail.  Entirely.  The same goes for the singularities at the heart of black holes, not to mention some of the more exotic concepts of cosmology like negative energy and wormholes.

What a breath of fresh air.  The celebrity physicists, all devotees of Einstein, the original celebrity physicist of the modern, mass-communications age, would shriek in horror, rail in derision at the simple idea that without an understanding of the quantum effects of gravity, we don’t have a clue as to what happened at the big bang, or even if it happened.  This is good and powerful stuff.  Too good and powerful, no doubt for the glad-hearted physicists who try to sell the world on how effectively their big brains have resolved the mysteries of the universe, but quite the right perspective for one who is unwashed in the Einstein catechism who simply wants to understand what we know and what we don’t. 

From these two paragraphs expressing Clegg’s flawless logic and command of the subject matter, it should be clear that what we know about gravity is vastly less than what we don’t know, no matter how much speculation disguised as knowledge we’ve been fed by physicists whose celebrity is made possible through the medium of mass communications.  If gravity can’t be explained at its most fundamental level, do we really know much of anything about it?  (Aside perhaps from Newton’s conclusion drawn from observing large, slow planetary objects, that gravity, at least for them, varies in direct proportion to mass and in inverse proportion to distance–the more mass, the more gravitational pull; the further apart are two objects, the less gravitational pull).   Newton’s conclusions, never mind Einstein’s, don’t work at the quantum level, the most basic level of reality. 

Perhaps the gravity of massive objects is an emergent quality, not predictable by simply adding up its effects at the quantum level.  Perhaps there is a synergy to massiveness.   Or perhaps, as is the case in our understanding of living processes, there are so many forces of so many varying magnitudes acting on such various amounts of matter and energy that predicting what might happen at the aggregate level from what is observed of the quantum is beyond the pale of human comprehension.   Gravity is mystical, but not because it is magical, instead because it is so complex. 

Physicists do not like the idea of action at a distance, part of what gives gravity its apparently mystical quality.  Baruch Spinoza, a metaphysical philosopher of the 17th century, and therefore a rough contemporary of Newton (though it is not clear that the two were much aware of each other), though more concerned with the ethical nature of humanity than the intricacies of the physical world, believed that nothing moved except that it was propelled to motion by another body, a view echoing his immediate Rationalist predecessor, Descartes’, sentiments.  Gravity seemed to reach across distances and move without touching—action at a distance—a revolting idea for those attempting to show, as were Spinoza and Descartes, that mysticism, particularly of the religious variety, could not be used to explain the operation of the universe.   

Newton was less concerned with the nature of gravity than with its effects, and by careful observation was able to quantify its effects so capably that NASA still employs Newtonian physics to calculate the trajectory of its space craft.  For Newton, whether or not the effects constituted action at a distance was less important than what the effects were.   He simply assumed there was something physical transmitting the force across the heavens.   Eventually the assumption that gravity must be transmitted through some physical medium gave rise to the notion of an ether, a medium filling space through which gravity, light and everything else propagated, like sound waves.

Einstein did not like the notion of an ether grown up around Newtonian physics, so set about to destroy it, and did so, first with his Special Theory of Relativity, which proposed, among other things, that the imagined ether did not affect the speed of light, which was proved correct.  Next by his General Theory of Relativity, which incorporated gravity (accelerated motion) into the framework of a universe where light speed is the absolute upper limit of velocity.  But the General Theory fashioned the universe into a fabric Einstein called space-time, which gravity bent to cause the effects seen of it.  And when empirical evidence later showed the universe to be expanding at an expanding rate, the effect could only be explained by assuming the existence of an ethereal-like entity, dark energy, driving its expansion. 

By my reckoning, Newton explained the effects of gravity as precisely and usefully as was humanly possible, which is amazing when it is considered that he did so by crude observations with only rudimentary tools.  Einstein tried to extend the understanding of gravity by eliminating the need for an ether, or for the eerie action-at-a-distance mysticism that gravity seemed to imply without it, by making light the yardstick of the universe, combining time with the three dimensions of space to comprise an entity that gravity bends.   (But how can space-time get bent?  To bend something requires that along some plane it is otherwise straight, and both concepts (bent or straight) imply there is a shape, but having a shape requires a formlessness (a background akin to an ether) from which it might arise, and if the formlessness is comprised of the very space-time that gets bent, what is left then to compare and tell that space-time is bent or straight?) 

Einstein’s attempt to explain everything (which he anyway didn’t, as General Relativity does not work at the quantum level) by using light speed as a universal speed limit instead created more questions than answers, even if, theoretical physicists haven’t minded drawing conclusions on the history of the universe, and the very nature of reality, based on his General Theory of Relativity, even where the theory fails, rendering nonsensical answers.   Though Einstein was hardly such a devotee of the physics academy when he devised General Relativity, the physics academy is now devoted to him.  Einstein became the face of the new age of physical awareness and understanding, of the notion that mankind could figure out anything about the world around him by just thinking hard enough about it, which was precisely the method he used in devising the theory.  He had no empirical science upon which to peg his imaginings.  Yet his theory so simply and elegantly (if quite complicatedly mathematically) seemed to explain everything above the quantum layer, and seemed to be proved by dint of some photographic plates of star images taken on a cloudy day during a solar eclipse, that his mad scientist visage soon enough came to represent all that could be accomplished by the simple application of human genius.    

Clegg points out that the most fruitful explanation for gravity may one day arise through investigations at the quantum level, which seems about right.  Quantum-level discoveries have done far more practical good for humanity than anything derived from General Relativity.   Nothing of modern chemistry, biology, electronics, metallurgy, hydrography, geology etc., would be possible without the understanding quantum theory has provided of the very small.  General Relativity helps keep our navigation satellites from losing accuracy too quickly, and that’s about it.  Relativity theory could be abandoned tomorrow and the human experience would not appreciably change, except perhaps in a philosophical sense, as for some of a more materialistic theological bent, it provides an explanation of mankind’s place and purpose in the universe of space and time.

The standard example used to explain how gravity acts under General Relativity is to liken space to a trampoline, with a large bowling ball at its center, representing a massive object like a star, and then to observe the action of a small object, like a BB, placed on the trampoline’s surface.  Obviously, the BB will roll towards the bowling ball.  The curvature in the trampoline’s surface caused by the bowling ball is something how space-time is supposed to bend in the presence of a massive object.  But there is no analogous surface to a trampoline, flat and bounded all around, in space-time.  Massive objects in the universe are nearly always spherical, and according to Newton’s observations, emanate a gravitational pull in direct proportion to their size, which rapidly fades with distance.  How does all the space-time around them get curved?  If one particular massive object is curving space-time around itself, how does that affect the curvature of space-time around other objects?   How does the fabric of the universe not end up a jumbled mess, like a pile of dirty towels in a heap on the floor? 

Imagining that time is intricately related to space is not, however, all that hard, though Clegg claims some difficulty with the notion on his first exposure to the concept.  Simply put, variations in space define time; without them, there would be no time, there would be just eternity, which is not infinite time, but the absence of time altogether.  Every single moment is different from the next because every single moment the entire universe of spatial configuration changes, if only just a little bit.  As I type this sentence, the earth is spinning on its axis, the moon is circling the earth, the earth is traversing its orbit, the sun is spinning and also orbiting the Milky Way, and etc.  Nothing about the spatial arrangements of matter and energy that obtained in the moments I just spent typing can ever be completely and identically recreated.  Some aspect will be different—everything is in constant flux—spatial relationships are inherently transitory, giving rise to time.   

Consider one of those working models of the solar system you see in high school science classes, where turning a crank makes the planets, through mechanical linkages, orbit the sun, a more or less proper representation of how things actually happen.  Except that “time” in the model continually cycles back to the same place.  Start rotating the crank, and don’t stop until the planets are back in the place where you started, and that particular spatial configuration will appear to be identical to the original configuration from where you started.  To someone who observed the arrangement before and after the crank was turned, but unwittingly dozed off during the turning, time for the miniature solar system stood still.  They could imagine there had been no passage of time, or an eternity of it, because nothing of the spatial relationships changed.  But the real solar system doesn’t operate quite like that.  There is no time when the real solar system isn’t itself traveling through space.  There is no time when the planets of the real solar system travel in exactly the same orbits at exactly the same speed with exactly the same rotational velocity around their axes as a time before.  The real solar system never recreates the exact spatial arrangements.  Every spatial arrangement of every single moment, is different.  Time is a product of spatial relationships.  We could time travel if we could identically recreate spatial relationships, but we can’t, and the impossibility has nothing to do with the speed of light.  Every point in time is unique because every point in space is only ever visited once.

(It seems therefore that time is an analog concept.  But that may not be the case at is tiniest scales.  Planck’s constant, the smallest divisible space of which we are aware, is perhaps the foundational unit of a fully digitized time that only appears analog from our perspective, a matter for another day. )

It is clear that gravity affects time, as time is a by-product of ever-fluctuating spatial relationships, and gravity, understood as a force, shapes those spatial relationships.   But that’s about all I am willing to admit is conclusively known of the relationship.

It seems to me, though Clegg does not specifically articulate as much, that the main impediment to furthering our understanding of gravity, and by implication, the entire universe, is General Relativity.  It has been proved true in some limited domains, but we know that it is not capable of explaining things at their most fundamental level.  And it is possible that some other “hidden variable” captured by General Relativity explains its ability to correctly predict some outcomes.  Even so, the premises upon which it is founded are considered sacrosanct. 

For instance, there can be no questioning of whether light truly is the fastest thing in the universe, traveling at the same speed for all observers all the time, though nothing about the premise is conclusively provable, not even if nothing can be shown to have exceeded it.  In a universe of infinite possibilities, proving that something does not exist is impossible.  Gravity might not “travel” at all, being in all places at all times, knowing all things about the massiveness of every bit of matter and energy at once.  But as we know its effects to decline according to the inverse square rule, there is no way to test the speed of gravity—its effects would decline so quickly with distance as to be too negligible to detect.  You can’t simply place a massive object somewhere to study how long it took for its gravitational pull to arrive at another location. 

Ironically, it will take someone who has the same brilliance of insight and disregard for the academy as Einstein to usurp Einstein and bring forth a keener human understanding of gravity.  Newton was an empiricist in his science and a mystic in his theology.  Einstein was a mystic in his science and an empiricist in his theology.  Whoever finally extends the understanding of gravity in a way that can account for its apparent action at a distance, and its apparent immediacy and constancy in the manner with which it shapes space and therefore time, will need to be both an empiricist and mystic in their science, which will by extension make of them the greatest of natural philosophers and theologians the world has ever seen.