The Reports of the Death of String Theory may be Greatly Exaggerated

In a recent podcast, Dr. William Lane Craig discusses my blog post about the book The Grand Design by Stephen Hawking and Leonard Mlodinow. In his remarks, Dr. Craig says that he is surprised by Hawking's reference to String Theory because String Theory seems to have some fatal flaws. I have heard from others that String Theory has not lived up to the promise it seemed to show about 10 or 20 years ago. In this post, I will describe what String Theory is, why some people think it may be in trouble, and some theological implications of String Theory.

String Theory is a theoretical idea in physics that proposes that the most fundamental objects in the universe are one-dimensional objects called strings. In String Theory, the fundamental particles that we currently know make up the structure of the universe including leptons, quarks, and bosons, are composed of one-dimensional objects that are described as vibrating strings. Just as different vibrational modes of a guitar string will give different musical tones, so the different vibrating modes of the strings give rise to different particles.

Some Thoughts on a Multiverse

     

It may sound more like science fiction than serious science, but many scientists have recently embraced the idea that there may be many more universes than just our own.  Such an idea can produce a great science fiction story, such as that in the original Star Trek episode "Mirror Mirror" where a transporter accident causes Captain Kirk to move to an alternative universe that is more brutal and savage aboard its version of the Starship Enterprise.

There are a few different scenarios for producing many universes (or a "multiverse") within proposed scientific theories.  One mechanism is the probabilistic nature of quantum mechanics. The "many worlds" interpretation of quantum mechanics proposes that every time a quantum event occurs that has multiple possible outcomes, all outcomes are produced in some universe. This is similar to saying something like every time I flip a coin, if a "heads" shows up, then an additional universe branches off from ours in which everything is the same except the coin shows "tails." Not many scientists actually believe that quantum mechanics produces these universes, but some do.

A different scenario that produces many universes is called "eternal inflation" in which our universe is somewhat like one bubble in a boiling pot of water, but there are many other bubble universes that are also produced. This theory of eternal inflation¹ is an extension of the inflationary big bang model in which our universe had a period of rapid expansion in the first 10^-35 seconds. In the eternal inflation model, the rapid expansion may have stopped in our universe but continued in others producing many other universes.
     
Perhaps the most popular theory that includes a multiverse is string theory, that postulates the fundamental particles we know of, like quarks and leptons, are composed of "vibrating strings of energy."  String theory requires that the universe is actually composed of 10 dimensions, rather than just the three dimensions of space and one dimension of time that we are familiar with.  Different string theories can be derived from an even more exotic theory, called "M-theory" which requires an 11 dimensional universe.  I'm not an expert on string theory so I don't understand all of the calculations that produce the string theory landscape, but my understanding is that there are many different minimum energy states (vacua) within string theory and any universe could occupy one of those states.  Depending on certain assumptions the number of possible vacua is often estimated at something like 10⁵⁰⁰. This means that there are 10⁵⁰⁰ different kinds of possible universes, but since every one of these universes could be duplicated many times within string theory, there could even be many more universes than that unimaginably large number.