Sunday, April 30, 2017

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.

Sunday, April 9, 2017

An Introduction to the Anthropic Principle and Fine Tuning

Most of the readers of this blog have probably heard about the anthropic principle and the fine-tuning of the universe.  However, because future posts will discuss the ramifications and speculations about fine-tuning, I thought it would be prudent to give a brief overview of these topics. Although not identical, the anthropic principle and the fine-tuning are definitely related.

The anthropic principle takes different forms, but is basically the idea that the universe has the necessary conditions for the existence of any conscious being that is able to observe the universe.  These conditions could, in principle, be very narrow or very broad in their scope. Many of the observations about the anthropic nature of our universe were developed beginning in the 1960's and continue to this day. Perhaps the most definitive book on the subject was written in 1986 by John Barrow and Frank Tipler, The Anthropic Cosmological Principle. The authors actually develop four anthropic principles with the first one, the Weak Anthropic Principle, being the most well known and uncontroversial principle, "The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the universe be old enough for it to have already done so"1

Although the parameters required for life to exist could, in theory, span a large or small range, it turns out that many of the parameters necessary for life to exist in our universe must fall within a very narrow region, or the universe would either not exist or not be able to support life. The fact that the conditions for life fall into such a narrow range, plus the many incredible mechanisms that give rise to the needed building blocks of life, constitute the fine-tuning of the universe.

I liken the finely-tuned universe to a panel that controls the parameters of the universe with about 100 knobs that can be set to certain values. If you turn any knob just a little to the right or to the left the result is either a universe that is inhospitable to life or no universe at all.