Warped Passages - Lisa Randall [7]
We will not know right away which of the theories gets it right about our place in the universe. For some of them, we might never know. But, incredibly, that is not true for all extra-dimensional theories. The most exciting feature of any extra-dimensional theory that explains the weakness of gravity is that if it is correct, we will soon find out. Experiments that study very energetic particles could discover evidence supporting these proposals and the extra dimensions they contain within the next five years—as soon as the Large Hadron Collider (LHC), a very high energy particle collider near Geneva, is up and running.
This collider, which turns on in 2007, will bang together tremendously energetic particles that could turn into new types of matter we have never seen before. If any of these extra-dimensional theories is right, it could leave visible signs at the LHC. The evidence would include particles called Kaluza-Klein modes, which travel in the extra dimensions yet leave traces of their existence here in the familiar three dimensions. Kaluza-Klein modes would be fingerprints of extra dimensions in our three-dimensional world. And if we’re very lucky, experiments will register other clues as well, perhaps even higher-dimensional black holes.
The detectors that will record these objects will be large and impressive—so much so that working on them will require climbing gear like harnesses and helmets. In fact, I once took advantage of this gear when I went glacier hiking in Switzerland close to the European Organization for Particle Research (CERN), the physics center that will house the LHC. These enormous detectors will record particle properties that physicists will use to reconstruct what passed through.
Admittedly, the evidence for extra dimensions will be somewhat indirect, and we will have to piece together various clues. But that is true of almost all recent physics discoveries. As physics evolved in the twentieth century, it moved away from things that can be directly observed with the naked eye to things that can be “seen” only through measurements coupled with a theoretical train of logic. For example, quarks, components of the proton and neutron familiar from high-school physics, never appear in isolation; we find them by following the trail of evidence they leave behind them as they influence other particles. It’s the same with the intriguing kinds of stuff known as dark energy and dark matter. We don’t know where most of the energy in the universe comes from or the nature of most of the matter that the universe contains. Yet we know that dark matter and dark energy exist in the universe, not because we’ve detected them directly, but because they have noticeable effects on matter that surrounds them. Like quarks or dark matter and dark energy, whose existence we only indirectly ascertain, extra dimensions will not appear to us directly. Nonetheless, signatures of extra dimensions, even when indirect, could ultimately reveal their existence.
Let me say at the outset that obviously not all new ideas prove correct, and that many physicists are skeptical about any new theories. The theories I present here are no exception. But speculation is the only way to make progress in our understanding. Even if it turns out that the details don’t all align with reality, a new theoretical idea can still illuminate physical principles at work in the true theory of the cosmos. I’m fairly certain that the ideas about extra dimensions we’ll encounter in this book contain more than a germ of truth.
When engaging with the unknown and working with speculative ideas, I find it comforting to recall that the discovery of fundamental structure has always come as a surprise and been met with skepticism and resistance. Oddly enough, not just the general populace, but sometimes even the very people who suggest underlying structures have been