This is the second part of a two-part review of the latest book from Stephen Hawking, co-authored by Leonard Mlodinow. Part 1 can be read here.
I just want to make something clear before I go on. The Grand Design is a good book. If I spend a lot of time explaining the points I make about it, it is because what bothers me is rather subtle, and to some extent, not too important. My goal is not to discourage anyone to read this book: quite the contrary. I just want raise awareness on a few things that I think should not be taken too seriously in spite of Hawking’s authority.
The Multiverse and the anthropic principle
For those of you who are not familiar with the anthropic principle, it is an argument to explain why we find ourselves on this planet, in this solar system, in this Universe, with these laws of physics, etc. It addresses the following question: Why is our environment so friendly to our existence? The anthropic principle has two variants: the weak and the strong principle. For the weak principle to work, two assumptions are necessary:
- There is a sufficiently large number of planets with varying conditions (different orbits, different suns, different distances to the sun, different atmospheres, etc.).
- The conditions allowing for our existence occur with non-zero probability, no matter how small that probability is.
Given these two assumptions, it follows that a planet with the correct conditions for us will necessarily exist. Notice that the second assumption is confirmed, simply because we exist. The first assumption is on the edge of being fully confirmed, as we start to observe smaller and smaller extrasolar planets. We are just about to be able to detect Earth-like planets. Even if it turns out there is only one planet in the whole Universe capable of fostering human life, this is where we will find ourselves. This conveniently explains why we are just the right distance from the sun to have liquid water, why we have only one sun, why we appeared after more than 10 billion years of cosmic evolution (for sufficient quantities of carbon to form), etc. There is no need for a God to have prepared such an environment for us: it was bound to exist somewhere.
However, this doesn’t explain why the laws of nature allowed for the correct chemistry for life as we know it to emerge. Given that there is only one possible Universe with one set of laws, this remains a mystery, a gap for God to hide. However, there are reasons in string theory to postulate that our Universe is not unique. In fact, there may be an infinite number of Universes, each with their own laws of physics. If this is the case, then it is no surprise that there is at least one Universe with the correct laws of nature for our kind of life to arise, and that we find ourselves in this Universe. This is the strong anthropic principle.
Assuming that there is a Multiverse is however as unwarranted as assuming the reality of M-theory. It is just the simplest way we have of getting rid of God in our physical theories. I guess Hawking’s point is: at the very least, we have one, and we only need one to do away with God, as long as it is not proven false.
Observations in quantum mechanics
Hawking and Mlodinow go to great lengths to explain the Feynman path integral approach to quantum mechanics. It leads to an explanation of the nature of the Multiverse. They eventually say (Chapter 6):
We create history by our observation, rather than history creating us.
This sentence can be so easily misinterpreted that I feel the need to convey my understanding of it. It is easy to think that what is said is that conscious observers have a special role to play in our Universe’s existence. But I believe this is not what Hawking and Mlodinow meant. They could have put it more clearly.
First, the path integral formulation of quantum mechanics implies that anything that can happen will happen. It also implies that all the possibilities coexist at any given moment in time. Among these possibilities is what actually happens in our Universe, but there is also a great many possibilities that cancel out in our own Universe, but contribute to other Universes. In such a view, even the laws of physics as we know it could change momentarily. At any moment in our Universe, reality may branch into an infinity of different histories, each having different outcomes for quantum events, or simply having a change of heart as to the laws these events subscribe to. The statement that we create history by our observation could simply mean that we, observers of reality, can only continue to exist in a Universe where the laws are specified as we observe them and where they stay the same with the passage of time. If the laws change, we will most likely cease to exist, thus we can’t be in a Universe where the laws change. Therefore, we will find ourselves in a Universe with a history where the laws are unchanging. This is another variant of the anthropic principle. Our continued consciousness requires that we follow a history where our consciousness will be allowed to exist.
I must say that I am not sure this is what the authors meant. It just seems to me the most rational way of interpreting the statement, which I must admit, I find very seductive. I sincerely hope that Hawking and Mlodinow didn’t imply that conscious observers are the cause of the collapse of the wave-function of the Universe.
The truth is, we don’t know why or how a quantum system can never be measured as being in a superposition of states. We don’t know if the superposition is destroyed at every interaction with the system or if the interaction needs something special for the collapse to occur. This is a deeply fascinating mystery, and it is at the heart of the counter-intuitiveness of quantum mechanics. To say that all outcomes of the measurement of the state of a quantum system exist at once is just an attempt to make sense of this mystery. This is usually called the many-worlds interpretation of quantum mechanics, and I don’t see any differences between this interpretation and what Hawking and Mlodinow advanced when they introduced Feynman’s path integrals. There are many interpretations of quantum mechanics, and which of these is the real one cannot be tested experimentally (they can’t be distinguished) and therefore much disagreement remains among physicists of the question.
Go and read this book. It is very educative, and it offers an astonishing picture of the reality we live in. My only regret is that this picture is only one among many (Smolin‘s books are excellent sources of information of these other approaches, and he has one approach of his own) that can do without God, and it is still very speculative (although it is highly informed speculation). I must admit, this picture is probably the most well-developed, which makes it worth presenting instead of others. I just wish Hawking and Mlodinow had shown some more skepticism toward it, as I believe it deserves. But it does not change at all the fact that we can answer the great questions of existence with science, and without God.