Knocking on Heaven's Door - Lisa Randall [62]
The hierarchy problem of particle physics poses one of the biggest challenges to the underlying description of matter. We want to know why the masses are so different from what we would have expected. Quantum mechanical calculations would lead us to believe they should be much bigger than the weak energy scale that determines their masses. Our inability to understand the weak energy scale in the superficially simplest version of the Standard Model is a real stumbling block to a fully complete theory.
The likely possibility is that a more interesting, more subtle theory subsumes the most naive model—a possibility we physicists find much more compelling than a fine-tuned theory of nature. Despite the ambitious scope of the question of what theory solves the hierarchy problem, the Large Hadron Collider is likely to shed light on it. Quantum mechanics and relativity dictate not only contributions to masses, but also the energy at which new phenomena must appear. That energy scale is the one the LHC will probe.
We anticipate that at the LHC a more interesting theory will emerge. This theory, which will address these mysteries about masses, should reveal itself when new particles and forces or symmetries show up. It’s one of the big secrets we hope LHC experiments will unmask.
The answer is interesting in itself. But it is likely to be the key to deep insights into other aspects of nature as well. Two of the most compelling suggested answers to the problem involve either extensions of symmetries of space and time, or revisions of our notion of space itself.
Scenarios that are further explained in Chapter 17 tell us that space might contain more than the three dimensions we know about: up-down, forward-backward, and left-right. In particular, it could contain entirely unseen dimensions that hold the key to understanding particle properties and masses. If that’s the case, the LHC will provide evidence of these dimensions in the form of particles known as Kaluza-Klein particles that travel throughout the full higher-dimensional spacetime.
No matter what theory solves the hierarchy problem, it should provide experimentally accessible evidence at the weak energy scale. A train of theoretical logic will connect what we find at the LHC to whatever ultimately resolves this problem. It might be something we anticipate or it might be unforeseen, but it should be spectacular either way.
DARK MATTER
In addition to these particle physics issues, the LHC could also help illuminate the nature of the dark matter of the universe, the matter that exerts gravitational influence but does not absorb or emit light. Everything we see—the Earth, the chair you’re sitting on, your pet parakeet—is made up of Standard Model particles that interact with light. But visible stuff that interacts with light and whose interactions we understand constitutes only about four percent of the energy density of the universe. About 23 percent of its energy is carried by something known as dark matter that has yet to be positively ID’d.
Dark matter is indeed matter. That is, it clumps together through gravity’s influence and thereby (along with ordinary matter) contributes to structures—galaxies, for example. However, unlike familiar matter such as the stuff we’re made of and the stars in the sky, it doesn’t emit or absorb light. Because we generally see things through light that is emitted or absorbed, dark matter is hard to “see.”
Really, the term “dark matter” is a misnomer. So-called dark matter isn’t exactly dark. Dark stuff absorbs light. We can actually see dark stuff where