Warped Passages - Lisa Randall [149]
This reasoning has a name: the anthropic principle. The anthropic principle diverges substantially from the original string theory goal of predicting all the features of the universe. It says that we don’t have to explain the small energy. Disconnected universes with many possible values of the vacuum energy exist, but we live in one of the few where structure can form. The value of the energy in this universe is ridiculously small, and only exceptional versions of string theory would predict this minuscule value, but we could exist only in a universe with minuscule energy. This principle might be discredited by future advances, or it could be vindicated by more thorough investigations. Unfortunately, however, it will be difficult (if not impossible) to test. A world in which the anthropic principle is the answer would certainly be a disappointing and not very satisfying scenario.
In any case, string theory in its current state of development certainly does not predict the features of the world, even though it is a single theory in its underlying formulation. Once again, we are faced with the question of how to connect a beautiful symmetric theory to the physical realities of our universe. The simplest formulation of the theory is too symmetric: many dimensions and many particles and forces that we know must be different appear to be on the same footing. And to make the connection to the Standard Model, and the world we see, this huge order must be disturbed. After symmetry breaking, the single string theory can manifest itself in many different guises, according to which of the symmetries get broken, which particles become heavy, and which dimensions distinguish themselves.
It is as if string theory is a beautifully designed suit that doesn’t quite fit. In its current state, you can hang it on a rack and admire its fine stitching and intricately woven pattern—it really is beautiful—but you can’t wear it until you make the necessary adjustments. We’d like string theory to accommodate everything we know about the world. But “one size fits all” rarely looks good on anybody. Right now, we don’t even know whether we have the right tools to tailor string theory correctly.
Since we don’t really know all of the theory’s implications, and it is not clear that we ever will, some physicists simply define string theory as whatever resolves the paradox of quantum mechanics and general relativity at small distances. Certainly most string theorists believe that string theory and the correct theory are the same, or at least very closely connected.
There’s clearly a lot left to learn. It is still too early to decide the ultimate merits of a string theory description of the world. Perhaps more elaborate mathematical machinery will permit physicists to truly understand string theory, or perhaps physical insights garnered from applying string theory’s implications to the surrounding universe will provide the critical clues. Addressing the unresolved problems of string theory appears to require a fundamentally new approach that goes well beyond the tools that mathematicians and physicists have so far developed.
Nonetheless, string theory is a remarkable theory. It has already led to important insights into gravity, dimensions, and quantum field theory and it’s the best candidate we know of for a consistent theory of quantum gravity. Furthermore, string theory has led to incredibly beautiful mathematical advances. But string theorists have yet to make good on the promises they made in the 1980s to connect string theory to the world. We still don’t know most of string theory’s implications.
In fairness, questions in particle physics were not immediately answered either. Many of the particle physics problems that were known in the