1. Text 2. Towards a Quantum Theory of Gravity

The current theories do not really explain the workings of gravity at scales below that of the atom. Physicists have been expecting a significant amendment to the theoretical conception that would place gravity, like the other three fundamental forces of the universe, in the context of quantum theory - i.e., in the modern picture of the subatomic realm. According to quantum theory, matter and forces are made of minute particles that spin and jump and absorb and release in tiny bundles known as quanta. During the past fifty years, physicists have identified numerous particles: quarks and leptons make up matter; photons carry electromagnetism, gluons, the strong force, and intermediate vector bosons, the weak force. It is expected that because gravity is a force, it also must operate at the subatomic scale, and hence it should have its "gravitation“⁴. It may be a while before physicists can observe a gravitational force carrier because to do so will require much more sophisticated equipment than is now available.

Physicists hope to discern, once a quantum theory of gravity is developed, how all four quantum forces are related. They wish, eventually, to develop a so-called grand unifying theory describing the single, fundamental phenomenon from which the four forces are believed to be derived. So far, however, they have been frustrated in all attempts to come up with such a theory in large part because, unlike the other three forces, gravity is inextricably bound up in the geometry of space-time, and correctly accounting for the complex role of this geometry has proved to be an almost overwhelming obstacle.

Now, though the obstacle has seemingly been overcome, thanks to a mathematical breakthrough that has vastly simplified the quantum equations for gravity, making them, for the first time, solvable. And though researchers are still at work interpreting what these mathematical "hieroglyphics" mean for the physical world, the solutions have already enabled them to begin peering into the minute structure of space-time. It turns out to be even stranger than the continuous elastic fabric that Einstein imagined.

III Text 3. The Task Ahead

Nothing so captures the spirit of quantum theory as the way it describes particles. Indeed, word particle is somewhat of a misnomer, since the behavior of subatomic objects is radically different from that of ordinary, visible objects. Scientists predict the movement of, say, a baseball flight by referring to its position and momentum at particular instants. But in the quantum realm, it’s impossible to measure both variables at once - a dilemma known as the uncertainty principle – i.e., in some ways, subatomic particles do not act as ordinary objects do. Although they can dislodge en electron from an atom, like one billiard ball hitting another, they ripple like radio waves through space, as if they are not confined to a single point.

Because of the uncertainty principle, physicists cannot predict exactly how subatomic particles will behave. "Physics", the late Richard Feyman observed, " a science of great exactitude, has been reduced to calculating only the probability of an event." However alien this idea might be, the picture that quantum theory draws of the fundamental nature of the world has been born out sometime and again in such experiments as those using high-energy particle accelerators which destroy subatomic targets by bombarding them with beams of charged particles. By examining the results of such collisions, physicists have been able to fill in the details of how the building blocks of matter interact via the four basic forces.

The four forces possess different strengths and manifest themselves at scales of magnitude, from the nuclear to the cosmic. Gravity, the weakest of the four, ranges across all scales but is most evident between large bodies - planets and stars, for instance - since it is proportional to the mass of the objects involved. Electromagnetism, expressed as an attraction between dissimilarly charged particles and a repulsion between similarly charged ones, also acts over all distances, but is most evident at the small end of the scale: it pulls one magnet toward another while, in an atom, it holds negatively charged electrons in orbit around the positively charged nucleus. Inside the nucleus, the strong force, with a range of only 10^-13 centimeters, binds together quarks, thus overcoming the comparatively weak electromagnetic force, which would otherwise cause the positively charged protons to fly apart. The weak force, so named because it is a hundred thousand times weaker than the strong force, is responsible for beta decay, a certain kind of radioactivity.

Because there are such formidable obstacles to reaching a quantum theory of gravity, some scientists have sought to redefine the problem; a few had even wanted to abandon general relativity.

However, just a few years ago, University of Syracuse (New York) physicist Abhay Ashtekar developed the framework for a new theory of quantum gravity which simplifies Einstein's equation. Ashtekar's theory cannot be verified by experimentation, hence the only way of corroborating it is to point to its internal consistency, and to its conformity with general physical principles. To date, the results have been encouraging and a picture of the microstructure of space-time is emerging.

Since Ashtekar's description of gravity has the same basic structure as the quantum theories of electromagnetism and of the strong and weak nuclear forces, it also could take physicists a step further toward achieving a unified field theory, in which all particles - both of matter and of forces - derive from one basic constituent. Besides revising our image of space and of time, the new theory may make the search for a coherent picture of nature much easier.