In my musing about the Higgs particle on Monday, I considered the odd case of the graviton. If there is such a thing, it has proved mighty hard to find. Particle accelerators of higher and higher energy don't seem the right approach, because it would have no mass anyway. I recall a science fiction novel from many years ago (50 or more) in which control of gravity was attained, and forces were developed called "electrogravitic" and "gravitomagnetic" and so forth. All kinds of magical effects were found.
So far, though, the only way to produce gravity waves, if they do exist, is to shake something very massive, and the only way to detect them is with a long, massive "antenna". The longest "antenna" yet produced has yet to detect anything, even though we expect things like the collision of two neutron stars, or of two black holes, to occur from time to time. Such events ought to radiate copious amounts of gravity waves. We have only indirect evidence that gravity waves might exist: A pair of neutron stars that orbit one another with a period of about 8 hours. The period is getting shorter, which indicates that the orbit is getting smaller. It is inferred that gravity waves are leaking away some amount of energy. But who knows? Maybe there is a thin gas dragging on them.
So what is gravity? If you put the question to Brian Clegg, author of Gravity: How the Weakest Force in the Universe Shaped our Lives, he'll most likely quote Richard Feynman, who urged us not to fear not knowing. In other words, we have mathematical descriptions of how gravity acts, but what it is? Nobody knows. Fortunately for the full employment of physicists, not knowing is a spur towards finding out!
Gravity as a concept seems to have begun with Galileo, and was first described mathematically by Newton. In earlier times, at least in the West, people thought things moved because angels pushed them, and heavy stuff moved downward because it was their purpose to do so, a completely teleological explanation. The "elements" were understood to be earth, air, fire and water. Earth received heavy, "earthy" things, and water, having a lesser downward purpose, stayed upon the earth but above it. Fire's purpose was to rise, and air's was to rest upon water or earth, having a neutral purpose (It didn't occur to people that they needed air to breathe. The function of the lungs was discerned quite late).
The focus of the book is the development of the general theory of relativity by Albert Einstein. He first thought of the equivalence principle, that acceleration and gravity mimic one another, in 1907, but it took him until 1915 to learn and apply the mathematics needed to describe the warping of space that is its most useful feature. Having this theory, which has been verified to great accuracy by a number of methods in the following century, it drive physicists crazy that it cannot be reconciled with quantum physics, which describes everything else!
Thus the hunt for gravity waves and the graviton. The ideas getting the most attention all relate to "string theory", which has been sardonically called, "a theory of everything, because everything can happen". I read in another book that there are at least 10500 versions of string theory that are possible, and so far there is no way to determine if any of them is more correct than any other. It sounds like it'll be a long search. But string theories are not the only choice. A number of alternative theories are being studied, including some that would sever time from space, thus scrapping relativity altogether. Both theories of relativity rely on unified spacetime. It is thought that the effects of special and general relativity might be better described by a better meta-theory.
This is not as far-fetched as it might sound. Theories based on strings, for example, get complicated fast, and they remind me of the epicycles used prior to Kepler's work to describe planetary orbits. Even Copernicus needed epicycles to make his earth-centered cosmology work, because he thought the orbits were based on circles. Kepler showed they are ellipses, and later work showed that the ellipses are "perturbed" by the gravitational effects of all the other planets. Maybe someday an über-Kepler will discover the hyper-elliptical math required to reconcile gravity and quantum mechanics, or maybe it will be a different approach entirely.
I was hoping for an extended discussion of the graviton. I puzzle over whether it will itself feel gravitational effects. I tentatively conclude, probably not—or not very much—or they could never escape from black holes. Instead, in the discussion of general relativity I learned that one term describes the self-energy of gravitation, which amounts to the same thing. It is an enfeebled effect, lest gravity's self-energy cause it to collapse into itself (and stay inside black holes). It is kind of like the self-energy of the electron, which classically becomes infinite at zero radius. Thus, the electron cannot be a point particle (and this might be a hint that space is actually quantized, in support of one of the alternative theories).
I have also wondered whether gravity's effects, or gravitons, necessarily move at the speed of light. Gravity is not light. The common understanding that information cannot travel faster than c (repeated on p 224), may have a spurious basis. We derive c from Maxwell's equations, based on the strength of interaction between electrical and magnetic energy. Gravity seems to stand alone. Why should it be subject to Maxwell? Besides, g may be less than c! If so, could that explain the seeming orbital anomalies in galaxies that led to the theory of dark matter?
The book entire is a masterful and accessible survey of the field. I find that I have read another book by Brian Clegg, and that another one is near the bottom of my current pile of reading material, so I'll get to it in a week or two. This book's frontispiece lists ten books, so this is his eleventh. It looks like he has caught the bug to explain everything. I like that.
I do have to point out a few signs of hurried production. In general, I find numerous indications that publishers tend to scrimp on proofreading. This book is much better than most. However:
- At the bottom of p 130, concerning a star that appears near the limb of the sun during an eclipse, we find, "…the star should appear shifted slightly toward the Sun…"; the bending of the light is toward the sun, so the star should appear shifted away from the Sun. Draw a picture to see what I mean.
- On p 143 the GPS satellites are described as moving "around 87,000 miles (14,000 kilometers) per hour". The actual mph speed is 8,700. A good item to remember is that escape velocity from Earth is about 25,000 mph or 40,000 kph.
- This is a more picky point: In the middle of p 215 the term "vast difference" ought to be "vast distance", describing whether light is refracted by the quantum nature of empty space. It takes a lot of distance to see a difference. An easy slip by a fast typist.