Knocking on Heaven's Door - Lisa Randall [162]
Although this problem doesn’t spoil any calculations for observable phenomena—certainly not those at the LHC—it does mean theoretical physics is incomplete. Physicists don’t yet know how to consistently include quantum mechanics and gravity at extremely high energies or short distances where both have comparable importance for predictions and neither can be neglected. This important gap in our understanding could potentially point the way forward. Many think string theory could be the resolution.
The name “string theory” derives from the fundamental oscillating string that formed the core of the initial formulation. Particles exist in string theory, but they arise from the vibrations of a string. Different particles correspond to different oscillations, much as different notes arise from a vibrating violin string. In principle, experimental evidence for string theory should consist of new particles that would correspond to the many additional vibrational modes that a string can produce.
However, most such particles are likely to be much too heavy to ever observe, and that’s why it’s so difficult to experimentally verify whether string theory is realized in nature. String theory’s equations describe objects that are so incredibly tiny and that possess such extraordinarily high energy that any detector we could even imagine would be unlikely to ever see them. It is defined at an energy scale that is about 10 million billion times larger than those we can experimentally explore with current instruments. At present, we still don’t even know what will happen when the energy of particle colliders increases by a factor of 10.
String theorists can’t uniquely predict what happens at experimentally accessible energies since the particle content and other properties depends on the as yet undetermined configuration of fundamental ingredients in the theory. String theory’s consequences in nature depend on how the elements arrange themselves. As it is currently formulated, string theory contains more particles, more forces, and more dimensions than we see in our world. What is it that distinguishes those particles, forces, and dimensions that are visible from those that are not?
For example, space in string theory is not necessarily the space we see around us—space with three dimensions. Instead, string theory’s gravity describes six or seven additional dimensions of space. A workable version of string theory has to explain how the invisible extra dimensions are different from the three we know. As fascinating and remarkable as string theory is, puzzling features like its extra dimensions obscure its connection to the visible universe.
To get from the high energy at which string theory is defined to predictions about measurable energies, we need to deduce what the original theory will look like with the heavier particles removed. However, there are many possible manifestations of string theory at accessible energies, and we don’t yet know how to distinguish among the enormous range of possibilities, or even how to find the one that looks like our world. The problem is that we don’t yet understand string theory sufficiently well to derive its consequences at the energies we see. The theory’s predictions are hindered by its complexity. Not only is the challenge mathematically difficult, it is not even always clear how to organize string theory’s ingredients and determine which mathematical problem to solve.
On top of that, we now know that string theory is much more complex than physicists originally thought and involves many other ingredients with different dimensionalities—notably branes. The name string theory still generally survives, but physicists also talk about M-theory, although no one really knows what the “M” stands for.
String