Sunday, 25 September 2005

Quantum Gravity

There are four known fundamental interactions in our universe: strong, weak, electromagnetic and gravitational. When quantum mechanics was found to be the correct framework to formulate our physical theories, a program to describe these four interaction in a quantum way started. Successful quantum descriptions of the first three (strong, weak and EM) were found. EM was the most simple to be quantized, since Maxwell wrote his equations in such a general way that they didn´t need to be modified (they even didn´t need to be modified to adapt to relativity too! I´ll write a post about this in the future, if I remember…). The resulting theory is known as QED, or Quantum Electrodynamics and is largely confirmed by experiments. The quantum theory of strong interactions is known as QCD, or Quantum Chromodynamics and is also confirmed by experiments. The weak interaction is described by Electroweak Theory, that has the additional bonus of unifying the weak force and EM interactions in a single framework. Together, these three quantum theories form theoretical framework named the Standard Model that explain all the known microscopic physics that our today technology has access without being contradicted by any experiment.

This seems to be very nice, but there is a hole in all of this: gravity. In the microscopic world, gravity is so weak that we can ignore gravitational interactions and the results of experiments will not change in a perceptible way. But we have strong arguments supporting that we must quantize gravity in the same way as we did to the other forces. We tried it, but we failed miserably. Technically, if we try to quantize gravity using the same techniques that succeeded in quantizing the other three forces, we discover that gravity is nonrenormalizable. This is a technical word that means that in our calculations we find a lot of infinities that we cannot make disappear and so, we cannot calculate things with our theory and cannot make predictions. We are lost!

Be calm, not everything is lost. Our failure only means that gravity is more complex than we though and we need to be a little smarter to find the correct theory. This is good, because we have a real tough challenge and we physicists like challenges! Well, let´s get back to gravity. Since our simplest tentative didn´t work, we had to try other approaches. The first and most widely known by the general public approach which gave us some hope of finding a quantum gravity (QG) theory was String Theory, but its complexity reached such enormous proportions and after decades it didn´t yet provided a correct testable QG theory. Strings are an ambitious theory, because it not only tries to quantize gravity, it tries to unify all the four forces of Nature in a single unified theory, as people usually call it, a theory of everything (TOE).

But in the last two decades, a lot of theories alternative to strings appeared. The most popular now is Loop Quantum Gravity (LQG). These theories are not so ambitious as strings, because they only try to quantize gravity, not to unify all forces. Other approaches include: causal sets, causal dynamical triangulations (CDT), twistors, spin foam models and others. Remember that these are TENTATIVE theories and none was tested yet.

This is the big problem of QG in physics today. It is one of the most challenging problems of physics and a very active area of research. With this post, I´m starting a series of posts that will explain the existing approaches to QG with more detail. I hope that you enjoy this journey. But be patient, it is long and tortuous. Prepare yourself but remember that the most important is ALWAYS to have fun.

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