The Quantum Universe Chapter 2 The Fundamental Nature of Matter, Energy, Space and Time
Current and future experiments in particle physics around the world put us in a position to address a range of clear questions about the fundamental physical laws that govern the universe. These questions, both familiar and profoundly revolutionary, define the path of particle physics in the 21st century.
Einstein's "unified field" theory
Since Einstein, physicists have sought a unified theory to explain all the fundamental forces and particles in the universe. The result was a spectacularly successful theory that reduced the complexity of microscopic physics to a set of concise laws. But these quantum ideas failed when applied to the physics of the universe. Some fundamental parts were left out; gravity, dark matter and dark energy had to have a quantum explanation. A new theoretical vision was needed, one that encompassed both the Standard Model and general relativity, while solving the mystery of dark energy. Particle accelerators provide a means of reaching a unified theoretical view in experiments characterised by four clear thrusts of knowledge.
1. Is there an undiscovered principle of nature: a new symmetry, a new law of physics?
Our quest to discover the fundamental laws of nature has led to the revelation that the laws of physics, and the particles they govern, exist because of the underlying symmetries of nature, some of which have been lost since the Big Bang. One of such lost symmetries may be supersymmetry. Just as there is an antiparticle for every particle, supersymmetry predicts that there is also a superpartner particle for every known particle. As an important part of string theory, part of the powerful theoretical appeal of supersymmetry is that it may be related to dark energy and that it provides a natural candidate for dark matter - neutrals.
Discovering supersymmetry is an immediate experimental challenge for particle physics, followed by exploring its structure and the properties of superpartner particles. Particle accelerator experiments will unravel the role of supersymmetry in a unified theory and reveal whether neutrino superpartners account for dark matter.
2. How can the mystery of dark energy be solved?
Recent measurements with telescopes and space probes have shown that a mysterious force - dark energy - fills the empty vacuum and accelerates the expansion of the universe. We don't know what dark energy is or why it exists. On the other hand, particle theory tells us that, at the microscopic level, quantum particles appear even in a perfect vacuum, and that these particles are a natural source of dark energy. However, a naive calculation of the dark energy produced in a vacuum yields a value 10,120 times larger than the amount we observe. Some unknown physical process is needed to remove most, but not all, of the vacuum energy, leaving enough energy to drive the accelerated expansion of the universe. A new particle physics theory is needed to explain this physical process.
The particle physics data point to another mysterious component of empty space, the Higgs field, which gives particles their mass properties. Without the Higgs field, electrons would travel at the speed of light, and atoms would immediately disintegrate. Is there a connection between dark energy and the Higgs field? The discovery of supersymmetry will provide key evidence for a possible connection. Supersymmetry provides both the natural background for the Higgs field and a possible explanation for the small but finite value of dark energy.
3. Is there another dimension of space?
The revolutionary concept of string theory boldly fulfils Einstein's dream of an ultimate explanation for everything from the tiniest quanta of particle physics to the universe itself. String theory unifies physics by generating all known forces and particles as different vibrations of a single substance called a superstring. String theory brings quantum coherence to physics in an elegant mathematical construct that seems to be unique.
Do superstrings exist? Strings themselves may be too small to observe directly, but string theory makes some testable predictions. It implies supersymmetry and predicts seven undiscovered dimensions of space that would give rise to much of the mysterious complexity of particle physics. Testing the validity of string theory requires finding additional dimensions and exploring their properties. How many are there? What are their shapes and sizes? How and why are they hidden? And what are the new particles associated with the extra dimensions?
4. Can all forces be unified into one?
At the most fundamental level, particles and forces may converge, either through a hidden principle like grand unification or through radical physics like superstrings. We already know that very similar mathematical laws and principles describe all known forces except gravity. Perhaps all forces are different manifestations of a single grand unified force that would link quarks and leptons and predict new ways of converting one particle into another. Such a force might eventually decay protons and make ordinary matter unstable.
Particle World
Physicists have identified 57 different elementary particles and have made precise measurements of many of their properties. What do they do? When we find them all, how will we know? Perhaps these particles are just different notes on a superstring. Perhaps they are linked in ways we have not yet deciphered, through grand unification or other hidden symmetries. Unification may provide the key, the simple principle that gives particles their complex identity.
5. Why are there so many particles?
We have discovered three families of quarks and leptons, and the elementary particles in these families differ only in mass, ranging from less than a millionth of the mass of an electron to the mass of a golden atom. Just as quantum mechanics has led to an understanding of the organisation of the periodic table, we look to new theories to explain the patterns of elementary particles. Why do three series of particles exist, and why do their masses vary so much?
The current investigation is focused on providing a detailed description of the existing patterns in the particle world. Significant progress has been made, particularly in characterising quarks. But why are the patterns of leptons and quarks completely different? A detailed study of quarks and leptons in accelerator experiments will provide the clearest insight into these questions.
6. What is dark matter? How can we make it in the laboratory?
Most of the matter in the universe is dark matter. Without dark matter, galaxies and stars would not form and life would not exist. It holds the universe together. What is it? Although the existence of dark matter was proposed in the 1930s, it is only in the last 10 to 15 years that scientists have made substantial progress in understanding its properties, mainly by determining what it is not. Recent observations of the effects of dark matter on the structure of the universe suggest that it is not like any form of matter we find or measure in the laboratory. At the same time, new theories have emerged that may tell us what dark matter really is. Supersymmetric theories predict new families of particles that interact very weakly with ordinary matter. The lightest supersymmetric particles are likely to be the elusive dark matter particles. We need to study dark matter directly, to detect leftover dark matter particles in underground detectors and to create dark matter particles in accelerators where we can measure their properties and understand how they fit into the picture of the universe.
7. What are neutrinos telling us?
Ubiquitous, elusive and full of surprises, neutrinos are one of the most mysterious of the known particles in the universe. Their interaction with other particles is so weak that trillions of neutrinos pass through us every second without leaving a trace. The sun shines with neutrinos, which arise from the internal nuclear fusion reactions that power it. These reactions produce only one type of neutrino, but they mysteriously metamorphose into two others on their way to Earth. Neutrinos have mass, but the heaviest neutrinos are at least a million times lighter than the lightest charged particles.
The existence of the tiny non-zero mass of neutrinos raises the possibility that neutrinos derive their mass from unknown physics, perhaps in relation to unity. A detailed study of neutrino properties - their mass, how they change from one to the other, and whether neutrinos are their own antiparticles - will tell us whether neutrinos fit the pattern of ordinary matter, or whether they are leading us to new phenomena.
The birth of the universe
What triggered the Big Bang? How did space, time, matter and energy take the form we see today? Can we look backwards and uncover the history of the universe?
After the Big Bang exploded with tremendous energy, the universe began to cool down and continued to do so until our own time. The resulting sequence of events is a cosmic drama with many acts and dramatic transitions, with many actors appearing and disappearing along the way. The early scenes play out at unimaginable temperatures and densities, with the stage set by the fundamental properties of particle physics. These processes had to be fine-tuned to produce a universe capable of forming the galaxies, stars and planets we observe today. Do some undiscovered fundamental laws determine the conditions that allow us to exist?
To reconstruct the story of the universe, telescopes and space probes probe the remnants of the early universe, while particle accelerators recreate and study the extreme physics that characterise the various stages of development and the transitions between them. As we begin to understand the universe's past, we can look to its future and predict its ultimate fate.
8. How did the universe come to be as it is?
According to modern theories of cosmic evolution, the universe began with a single explosion, followed by a burst of inflationary expansion. Understanding inflation requires a breakthrough in our understanding of fundamental physics, quantum gravity and the ultimate unified theory. Although inflationary conditions are too energetic to be reproduced on Earth, we can observe their characteristics, passed on through the long ages by their imprint on the leftover matter of the era that we can still detect.
After expansion, conditions in the early universe were still so extreme that they could combine elementary particles into new phases of matter. As the universe expanded and cooled, transitions of matter from one phase to another occurred, like the condensation of steam into water. Some of these phase transitions are probably the most dramatic events in the history of the Universe, shaping its evolution and leaving a legacy that can be observed today. Cosmic phase transitions can be recreated in high-energy accelerator experiments.
9. What happened to antimatter?
Experiments tell us that there is an antiparticle for every elementary particle. The Big Bang and its aftermath almost certainly produced equal numbers of particles and antiparticles. However, to the extent that we can detect the universe, our observations suggest that we live in a universe made up of matter, not antimatter. What is happening to antimatter? The tiny imbalance between particles and anti-particles must have formed early in the evolution of the universe, otherwise they would both have annihilated, leaving only photons and neutrinos behind. Subtle asymmetries between matter and antimatter, some of which we have observed experimentally in the laboratory, must be responsible for this imbalance. But our current understanding of these asymmetries is incomplete and insufficient to explain the observed dominance of matter.
There must be some other undiscovered phenomenon that makes matter and antimatter behave differently. We may find it in quarks or neutrinos. Its origin may lie in the properties of the Higgs boson, in supersymmetry or even in extra dimensions.