Knocking on Heaven's Door - Lisa Randall [163]
Ultimately, if string theory is correct, all the models that describe real-world phenomena should be derivable from its fundamental premises. But its initial formulation is abstract, and its connection to observable phenomena is remote. We would have to be very lucky to find all the correct physical principles that will make string theoretical predictions match our world. That is string theory’s ultimate goal, but it is a daunting task.
Although elegance and simplicity can be the hallmarks of a correct theory, we can only really judge a theory’s beauty when we have a reasonably comprehensive understanding of how it works. Discovering how and why nature hides string theory’s extra dimensions would be a stunning achievement. Physicists want to figure out how this occurs.
THE LANDSCAPE
As I joked in Warped Passages, most attempts to make string theory realistic have had something of the flavor of cosmetic surgery. In order to make string theory conform to our world, theorists have to find ways to hide the pieces that shouldn’t be there, removing particles from view and tucking dimensions demurely away. But although the resulting sets of particles come tantalizingly close to the correct set, you can nonetheless tell that they aren’t quite right.
More recent attempts to make string theory realistic have something of the flavor of a casting call. Although most ingenues can’t act very well and some have frozen faces that don’t let them emote, with enough auditions, a beautiful talented actor might show up.
Similarly, some ideas about string theory also rely on our universe being the rare but ideal configuration of its ingredients. Even if string theory does ultimately unify all the known forces and particles, it might contain a single stable basin representing a particular set of particles, forces, and interactions, or more likely, a more complicated landscape with many possible hills and valleys and a variety of possible implications.
According to recent research, string theory can manifest itself in many possible universes in a scenario corresponding to a multiverse. The different universes can be so far apart that they never interact—even through gravity—over their lifetimes. In that case, completely different evolution can occur in each of these universes, and we would end up in only one of them.
If these universes existed and there were no way of populating them, we would be justified in ignoring all but our own. But cosmological evolution provides ways to create all of them. And the different universes can have significantly different properties, with different matter, forces, or energy.
Some physicists employ the idea of the landscape in conjunction with the anthropic principle to try to address the particularly thorny questions in string theory and particle physics. The anthropic principle tells us that since we live in a universe that permits galaxies and life, certain parameters must take values at or near the values they do—or we would never be here to ask the question. For example, the universe couldn’t have so much energy that it would expand at a rate too quickly for matter to collapse into cosmic structures.
If this is the case, we need to determine what physical features, if any, favor one configuration of particles and forces and energy over another. We don’t even know which properties should be predictable and which are simply necessary for us to be sitting around discussing science in the first place. Which properties have fundamental explanations and which are an accident of location?
Personally, I believe a landscape of many possible configurations where we might reside is reasonably likely since there are many possible solutions to any set of equations for gravity we write down, and