In part 1 of this review I discussed the main issues raised by the book and in part 2 I said that the book by Hawking and Mlodinow argued that M-theory and the no boundary condition can provide answers to the three big questions: Why is there something rather than nothing? Why do we exist? Why this particular set of laws and not some other?
To understand what lies at the basis of M-theory, we need to appreciate a key difference between classical physics (which describes the large-scale structure of the everyday world we live in and from which we draw our intuitions about how the world works) and quantum mechanics (which describes the microscopic atomic and subatomic world).
What classical physics says is that if we release an object at some point A, it will subsequently wander off on some trajectory (or path) that depends on its initial state of motion and the forces that act on it. This is what enables good football quarterbacks to throw passes to receivers with such accuracy. If the ball is poorly thrown on a windy day and/or we stop observing the ball, we may not know or be able to predict which path the ball will take or where it will land but our classical intuition tells us that it will go along some specific path that is determined by the initial throw and the wind conditions.
But quantum mechanics has this counter-intuitive idea that once we stop observing the object, the object takes every conceivable path simultaneously. This means that there is no unique location for the object at any given time, that it is everywhere at the same time and could eventually end up anywhere at all. Another way to say it is that an object has many different histories. This is what boggles most people’s (including scientists’) minds about quantum theory but we have to learn to live and work with it (i.e., develop ‘quantum intuition’, so to speak) because this theory is phenomenally successful and there seems to be no getting around it at this time. Some people are working on developing alternative theories that do not have its strange features but have not had much success so far.
Now if we detect the object at some later time to be at some point B, this eliminates some of the potential paths we started with because they would not have resulted in the object ending up where we detected it. So the act of detection picks out a subset of the initial set of possible histories, limiting the ones of interest to those that began at point A at the specified time and ended at B at the later time, which still includes an infinite number of paths or histories. An elaborate mathematical machinery (called the ‘sum over histories’ or more technically ‘path integrals’) has been created to add up all the possible paths the particle could have taken in going from A to B. The calculated results correctly predict the empirical observations, which is why scientists have confidence in quantum theory despite its counter-intuitive features.
What M-theory does is take this key idea of quantum mechanics and apply the ‘sum over histories’ approach to the universe as a whole. Building on the idea of the inflationary universe (see part 9 and part 13 of my series Big Bang for Beginners for more details), since the net energy of the universe is zero, there is no restriction on the number of new universes that can ‘pinch’ off from previously existing universes. Since the Heisenberg uncertainty principle states that you can never have truly empty and inert space (p. 113) but that space constantly has particles coming into existence and disappearing again, any one of those fluctuations in space could form the seed of a quantum fluctuation that triggers the birth of a new universe.
So universes are being created all the time and there are a vast number of possible histories of the universe, of the order of 10500. They each have their own forms of matter and their own laws. According to the ‘sum over histories’ in quantum mechanics, all these universes exist simultaneously, giving rise to the name ‘multiverse theory’. When we observe our universe, we are picking out just those histories that could produce the present state we see. As Hawking and Mlodinow state:
Quantum physics tells us that no matter how thorough our observation of the present, the (unobserved) past, like the future, is indefinite and exists only as a spectrum of possibilities. The universe, according to quantum physics, has no single past, or history. (p. 82)
We seem to be at a critical point in the history of science, in which we must alter our conception of goals and of what makes a physical theory acceptable. It appears that the fundamental numbers, and even the form, of the apparent laws of nature are not determined by logic or physical principle. The parameters are free to take on many values and the laws to take on any form that leads to a self-consistent mathematical theory, and they do take on different values and forms in different universes. (p. 143)
Given the staggeringly large number of possible histories, it was almost inevitable that one of those universes would have the properties that ours has. It is like rain. If you pick a point on the ground, the probability of it being hit by a raindrop is infinitesimally small. But in a rainstorm, there is such a huge number of drops that it is inevitable that at least one will hit the ground there.
Hawking and Mlodinow’s book does not shy away from making strong claims, such as that the theory they describe has to be the right one. “M-theory is the only candidate for a complete theory of the universe… M-theory is the unified theory Einstein was hoping to find.” (p. 181, emphasis in original.)
That seems hubristic to me. If the history of science teaches us anything it is that theories, however successful at any given time, tend to be later replaced by other theories as the questions that need to be addressed change. However obviously important they may seem, is usually a mistake to think that the questions that concern us now will be the same questions that future generations care about. Also the theory of supersymmetry, which is central to M-theory though not necessarily to the idea of multiverses, has been around since 1970 or so, with none of the exotic partner particles it predicts having been detected as yet. The theory’s supporters are pinning their hopes on the Large Hadron Collider that has just started operations, hoping that its energies will be sufficient to produce these particles.
In the last part of this review, I will look at the implications of M-theory for religion and give some of my reactions to other features of the book.