5 :: “Unity Under Pressure”
A hunt for a Theory of Everything that unites all forces during the first moments of the universe.
Particle accelerators use magnets to accelerate subatomic particles to high velocities before smashing them together in a detector. The highest energy accelerator currently operational is at the Fermi national laboratory outside of Chicago. It can achieve energies of one trillion electron Volts. Physicists hope to use this collider-accelerator (it accelerates, then collides) and an even more powerful collider being built in Europe to find the Higgs boson and other elusive particles. To learn more about colliders, go to:
Grand Unified Theories
The history of our understanding of the fundamental forces of nature stretches across several hundred years and is not yet complete. In fact, we might never be able to fully describe our universe in terms of physics theories and mathematics. What has taken humans generations to even partially understand took nature only a few moments to create. At the end of the first three minutes of the universe, forces had evolved from one force to the four with which we interact today — electromagnetism, gravity, weak force, and strong force.
Through our attempts to understand how the forces interrelate, we have learned some things for certain. Forces are carried by bosons, and the distance over which a force can act is related to the mass of the boson carrying the force. The electromagnetic and weak forces are two aspects of the same (electro-weak) force. The standard model of particle physics describes the subatomic world in terms of three generations of leptons and quarks.
Since the days of Sir Isaac Newton, physics has been a game of find-and-piece. Scientists have been found new theories to explain observations — relativity to describe Mercury’s orbit, quantum mechanics to describe spectral lines — then looking for ways to piece these new concepts into the framework of everyday cosmology.
The overriding structural framework cosmologists and particle physicists have been trying to find is a Grand Unified Theory (GUT) that describes everything. Remarkably, it seems that the foundations of a GUT are located in the realm of the very small — particle physics. In this lesson I give you a crash course in particle physics and introduce the problems involved in finding a grand unified theory.
Let’s meet our cast of characters: the subatomic particles (see Figure 5-1). The various aspects of each particle — charge, mass, spin — allow them to play very specific roles. Any GUT that hopes to hold its ground needs to explain why each particle has its specific set of characteristics. As you learned in Lesson 3, there are three types of particles: quarks, leptons, and bosons.
Quarks and leptons can each be divided into three families of increasing mass. For every particle, there is also an anti-particle made of anti-matter. All stable particles are made up of quarks and leptons from the first, lowest-mass, particle family. Because mass and energy are the same thing, individual heavy particles can transform into energy and a lower-mass particle. For instance, a chunky sibling of the electron, the muon, decays into an anti-muon neutrino, an electron, and an electron neutrino. There is evidence that the muon neutrino will further decay into an electron neutrino. In addition to explaining why particles have given characteristics, a good GUT also has to explain these particle decays and the forces behind them.
Figure 5-1: The standard model of particle physics divides the most basic particles into leptons, quarks, and bosons. (Graphic by Pamela L. Gay, copyright 2002)
Particle physicists slam protons and electrons together in particle accelerators to detect these particles (see the sidebar). The collisions produce huge amounts of energy that condense into particles (see Figure 5-2). The heavier the particle, the more energy is required to discover it. For this reason, particle physicists talk about the mass of particles in terms of the energy that is equivalent to the particle’s mass. The heaviest quark, for instance, has an energy of 174 billion electron Volts. (An electron Volt is the energy an electron gains by going through a potential of one Volt. It takes 6.2 x 1020 eV / sec to light a 100-watt light bulb). The quarks that make up protons and neutrons are only 360 million electron volts.
Figure 5-2: Particle accelerators smash high-speed protons together. The high-energy explosion recreates some of the conditions of the big bang and allows rare particles to condense out of the energy. (Graphic by Pamela L. Gay, copyright 2002)
The weak force causes the decay of particles and involves W and Z bosons. Bosons carry all forces and the distance over which a force can act depends on the boson’s mass. Photons (a kind of boson) carry the electromagnetic force and interact with electrons and protons, causing electromagnetic effects, like a zap of static electricity (that flash is made of photons!). Since photons have no mass, the electromagnetic force can act over infinite distance. The graviton, the still undetected boson associated with gravity, is also thought to be massless.
Particle physicists have turned to quantum mechanics (QM) to explain why each particle has its distinctive characteristics. The length of this course prohibits a detailed introduction to QM; just bear in mind that QM provides a mathematical framework for explaining how particles interact and decay. For a more detailed introduction read the “Rudiments of Quantum Theory” at http://www.physics.about.com/cs/quantummechanics/.
Physicists first used quantum mechanics to explain the energy levels of atomic spectra, later extending it to cover all electromagnetic interactions. In attempting to find a quantum electrodynamic-like theory to describe weak force particle decay, Sheldon Glashow, Abdus Salam, and Steven Weinberg discovered that the weak and electromagnetic forces act the same at high energies and short distances — and won the 1979 Nobel Prize. In the early universe, both these conditions were met and these forces became the electro-weak force.
This discovery opened the doors for other scientists to find ways to use QM to unite the four basic forces into a single force in high-energy/high-density conditions. So far, they have succeeded in creating a standard model to describe the behavior of all known subatomic particles. Gravity alone remains outside the standard model. Unfortunately, just as Kepler’s laws of planetary motion can describe the motions of planets but cannot explain why they moved, the standard model describes particle interactions, but does not explain some fundamental problems — such as why there is a repetition of three families of particles through the groups.
Some GUTs floating around theoretical physics predict that for the first 10-43 seconds after the big bang all forces were united. First gravity separated, then the other three forces fell apart at 10-11 seconds. GUTs also predict that neutrinos have mass — a prediction that was considered false until 1998 when the Super-Kamiokande particle detector in Japan (http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/super-kamiokande.html) detected the minuscule neutrino mass. Unfortunately, these theories can’t explain why different particles have different masses, and they make some predictions that do not match observations. I discuss the flaws in the most common GUTs in the next section and introduce a possible new theoretical solution at the end of this lesson.
If neutrinos have mass, two large mysteries can be solved. The first is dark matter — matter that can be detected only by its gravitational tug and cannot be directly observed (more about this in the next lesson). If neutrinos have mass, they will exert a gravitational tug, explaining at least part of the dark matter. The second mystery is solar neutrino production. The sun produces electron neutrinos during nuclear reactions, but we observe only < ½ the expected number of electron neutrinos. If neutrinos have mass, electron neutrinos can change to another type of neutrino, such as a tau neutrino, that is harder to observe.
To learn more about these problems, go to The Ultimate Neutrino Page at
Aches in the GUT(s)
I once overheard an observational astronomer telling a colleague “If you lock 17 theorists in a room and ask a question, you will get 21 theoretical answers.” This statement, I am sorry to admit, has much truth. The field is littered with theories; some GUTs have already died under the weight of conflicting observations; others are still thrashing about for a data set to cling to. Basically, the problems with these theories can be traced to three things: proton decay, magnetic monopoles, and neutrino mass (Figure 5-3).
Figure 5-3: Unfortunately, GUTs come with problems. (Graphic by Pamela L. Gay, copyright 2002)
GUT-Ache #1: Proton Decay
All generally accepted GUTs predict that the proton is unstable with a predicted lifetime of 1030-33 years. This is a disturbing prediction because protons make up everything — all the atoms that make up your computer, your watch, and you. (Don’t panic; the universe is just ~15 x 109 years old!) Because even the most stalwart scientist cannot observe an individual proton for the necessary 1030-plus years, they instead watch 1030-plus protons. Ideally, if one proton takes 1030 years to decay, there will be one decay in a vat of 1030 protons each year. When it (theoretically) decays, a proton should become a positron and a pion and give off a small, but observable, flash of light.
Particle physicists have gathered together protons (in the form of purified water) in large tanks and looked diligently for these flashes. One detector, the Super-Kamiokande detector in Japan, has had 11,200 sensors trained on 50,000 metric tons of water for five years without observing a single decay — which implies that the proton must have a lifetime longer than 5 x 1032 years. This single piece of evidence, rediscovered in tanks of water around the world, has killed off many GUTs.
GUT-Ache #2: Magnetic Monopoles
GUTs also predict the existence of magnetic monopoles in large numbers. The magnets that we usually deal with are dipoles, with both a north and a south end. Magnetic monopoles lack one of these ends. They are often described as the magnetic equivalent of an individual charge. Monopoles form as topological defects during a phase in the big bang known as “symmetry breaking” (see http://zebu.uoregon.edu/~js/ast123/lectures/lec19.html). Calculations made using prominent theories suggest that there should be more magnetic monopoles in the present universe than there are baryons (particles made up of quarks, like protons and neutrons). This is simply not the case. Not only do we see more baryons than magnetic monopoles, we don’t see any monopoles at all!
(Possible) GUT-Ache #3: Neutrino Mass
The search for neutrino mass has been one of the most controversial areas of science for the past two decades. All initial attempts to weigh a neutrino failed and the standard model — which, unlike most GUTs, actually works — does not require neutrinos to have mass. Even though no early weigh-in produced weight, scientists keep trying to detect a neutrino mass because it can solve a number of problems (see the sidebar).
In 1998, scientists from the Super-Kamiokande presented evidence of neutrinos changing types — something they can do only if they have mass to convert into energy. Since then, more experiments have indicated that neutrinos have a very small mass — only 2.2 electron Volts to ~20 million electron Volts. This makes them too small to account for all the dark matter, but does fix the solar neutrino problem, and prevents some GUTs from facing an early demise (see the sidebar).
After looking around the universe and comparing GUTs to observations, many cosmologists find themselves on unstable ground. The quest for a single cohesive theory to unite all forces and explain the universe is not going well. All hope has not yet been abandoned, however, and a solution might lie in looking at the universe in 10 dimensions instead of just four.
String theory is a complex subject that defies easy explanation. If you truly want to make your brain hurt, you can find a good layperson’s explanation of the basics at The Official String Theory Web site:
String Theory Saves the Day
Many of the major breakthroughs in physics have come from geometry. Kepler applied ellipses to orbits, becoming the first to understand how planets move. Einstein transformed time into a spatial dimension, becoming the first to understand motion in high gravity/high velocity environments. We also need a geometric leap of faith to understand how the four forces came a single force in the early universe.
String theory describes the universe as consisting of 10 dimensions (or perhaps 26, according to one version of the theory). What we see as particles are actually strings that exist in all the dimensions. We experience only the four dimensions of space and time because the other dimensions folded up on themselves during the first second of the universe. String theory predicts that every force-carrying boson has a series of fermions (matter-carrying particles) associated with it, called supersymmetric particles. If this theory is correct, we should begin to find these particles as increasingly powerful particle colliders come on line in the next five years.
To understand string theory (which no one understands completely) you must find a way to deal with 10-dimensional space, and visualize what 10-dimensional objects look like in our four-dimensional world. For example, imagine a three-dimensional basketball in one dimension. First represent the basketball in two dimensions, which gives you a circular picture of a basketball. The sphere reduces to a circle with the same diameter as the sphere. Reducing the circle to one dimension gives you a line with the same length as the original basketball’s diameter (see Figure 5-4). When 10-dimensional strings are reduced to four dimensions, their characteristics in the unseen dimensions get reflected through their particle characteristics. Just as the basketball’s diameter becomes the length of a line, higher dimensional attributes of strings come out as particle characteristics. Currently, the greatest factor preventing the advance of string theory is the complexity of the theory. It is hard to advance a subject that takes a lifetime of study to understand.
Figure 5-4: A three-dimensional basketball reduces to a line when “flattened” into one dimension. (Graphic by Pamela L. Gay, copyright 2002)
Unfortunately, there is no GUT that works. At this time, two major predictions of GUTs — proton decay and magnetic monopoles — seem to be incorrect. Scientists have turned to string theory to try to tie the forces together and bind together the particle world.
One of the greatest enigmas arises in trying to unite general relativity and quantum mechanics. So far, a QM description of gravity has eluded theorists. Before we continue further on our journey to find a universal framework, we’ll return in the next lesson to relativity, the cosmological constant, and introduce in detail the greatest mystery of all: dark matter.
Assignment: All Theories are One, and One Theory for All
Read the Chapter 8 of The Whole Shebang: A State-of-the-Universe(s) Report by Timothy Ferris. In this lesson we start to encounter science’s struggles with string theory and quantum mechanics — two extremely complex theories that defy complete understanding for all but a rare few geniuses. This raises the prospect that we live in a universe is ultimately beyond the complete understanding of even the most brilliant geniuses. Do you think men and women will eventually find a mathematical description of the heavens? Come to the Message Board and discuss your thoughts.