Theories of the Universe: Attempts at Unification

Attempts at Unification

At one time in the history of the universe, the four fundamental forces, along with the particles that carry these forces, and all the myriad elementary particles we covered in “Forces, Particles, and Some Cosmological Glue,” came from a singularity. The big bang is the ultimate unifier, the one thing from which everything else has come. So the search for a theory that unifies everything back to this one point might reveal the reason why it happened in the first place. This is one possible reason why the search is on. Or is it simply reductionism, the method that science is known for? It could also be that sense of a search for beauty and elegance that we discussed in “The Accelerating Universe,” that drives physicists and cosmologists to find the theory that will unify everything. Whatever the reason, it lays at the core of present-day cosmology.

Einstein devoted a large part of the last thirty years of his life in an attempt to unify electromagnetism and gravity. He wanted to show that these two forces were different manifestations of the same force. His valiant attempts proved fruitless because he hadn't known about the other two forces, the weak force and the strong force. So without the knowledge of these other forces, any attempt at unification would of course fail. But even in today's search for unification, trying to combine gravity with the other three forces is what still eludes solution. As we'll see shortly, gravity is the main force that poses the most problems for unification.

There have been some remarkable achievements toward unification of the forces. In the late 1960s, physicists Steven Weinberg, Abdus Salam, and Sheldon Glascow showed that the nuclear weak force and the electromagnetic force were different aspects of one force, now called the electroweak force. Since then, particle accelerators have been able to recreate the energies needed to combine these two forces. When the universe was about 10^-11 seconds old, particles had kinetic energies of about 100 times the rest-mass energy of the proton.

Since such energies can be created in particle accelerators, the theory has been confirmed by experimentation. The carriers or messenger particles are those that carry the four forces (you learned about those in Forces, Particles, and Some Cosmological Glue) and the theory linked the carrier of electromagnetism, the photon, with three previously undiscovered particles, the carriers of the weak force. The existence of these predicted particles, the W⁺, the W^- , and the Z⁰, were finally discovered in 1983.

The general idea behind unification is very simple, regardless of the difficulty encountered in trying to make it happen. All the basic interactions were unified in earlier stages of the universe when it was, as you know, much, much hotter. The universe was highly symmetrical, in the sense that interchanging any of the forces or particles among themselves would have resulted in no change. This understanding of symmetry has led to trying to unite the four forces under the theories of supersymmetry, a theory that we'll also be covering shortly. But as the universe expanded and cooled down, at certain critical temperatures, symmetry breaks occurred, and eventually the four interactions gained their distinct identities. These certain critical temperatures are called phase transition temperatures, and knowing exactly what the degree of heat is at these temperatures is another key ingredient in knowing when the four forces differentiated. The fact that we observe four distinct forces now simply reveals the fact that the universe is already much colder than it was.

The unification of the strong nuclear force with the electroweak force occurs, according to the GUTs, or grand unified theories, only at energies of about 10¹⁵ times the rest-mass of a proton. That's an incredibly higher amount of energy than was required to show the connection between the electromagnetic and weak force. That was only 100 times the rest-mass of a proton, or 10².

Particles in the universe had such energies as theorized by the GUT's when the universe was less than 10^-35 seconds old and the temperature was above 10²⁸ degrees. The high energies that are required to unite the strong force and the electroweak force can't be achieved by any accelerator. Current accelerators can peak out at 1TeV, or one trillion electron volts. These energies have been created at the four-mile circular accelerator at Fermilab, but even that level of energy is only 10¹². Another three powers of 10 need to be produced, and unfortunately, each power of 10 becomes incredibly harder to attain.