This is my interpretation of chapter 2 of Leonard Susskind's book

The Cosmic Landscape: String Theory and the Illusion of Intelligent Design.

Susskind uses this chapter to introduce the problem that physics has with correctly predicting the value of the cosmological constant. What we naively call the "vacuum" actually contains a seething mass of virtual particles (or vacuum energy), because that is what is implied by our most fundamental theory of how physics operates (i.e. quantum field theory). This mass will affect the gravitational field and thus have observable consequences.

Unfortunately, our fundamental physics theory predicts an

*infinite* (or, at least, very large) vacuum energy, which is

*not* what we observe experimentally. The discrepency is about 120 orders of magnitude!

Susskind then describes the cosmology that was introduced by Einstein (i.e. a static universe governed by the equations of general relativity), by introducing the analogy that space is like the surface of a balloon, where the tension in the ballon is analogous to a gravitational attraction that causes all objects in space (e.g. galaxies) to attract one another. Thus an additional repulsive force is needed to hold the galaxies apart from each other. Einstein noticed this problem, and introduced a suitable term into his equations to provide the necessary repulsive force; this was his famous cosmological constant.

It turned out that Einstein's cosmology was wrong, because the astronomer Edwin Hubble observed that galaxies actually

*recede* from each other, so the universe was

*not* static, so Einstein had

*no need for* a non-zero cosmological constant.

Now, Einstein's cosmological constant is physically the same thing as the vacuum energy mentioned above, so we know that it is non-zero, and in fact we predict (by adding up the effects of all of the virtual particles) that it has a value that is enormously too large. We know the value is too large because this vacuum energy implies the existence of a gravitational field that is

*not* actually observed.

Even calculating this vacuum energy is fraught with problems because a naive calculation gives an infinite result, so we have to make some adjustments to the calculation in order to eliminate the infinity in a "natural" way. The method used is to ignore the contributions from all virtual particles that are so energetic that they would form a black hole if they collided.

One possible way of predicting a zero vacuum energy is to notice that virtual bosons (e.g. photons) have

*positive* vacuum energy, whereas virtual fermions (e.g. electrons) have a

*negative* vacuum energy, so there is the possibility that they produce

*cancelling* effects. Unfortunately, such a cancellation does

*not* materialise for the known types of bosons and fermions. However, if the universe were such that bosons and fermions existed in

*matched pairs* (which they don't!), then cancellation

*would* occur; this is what happens in the so-called supersymmetric theories.

Finally, Susskind points out:

- Cosmologists regard the cosmological constant as something with a small non-zero value that needs to be measured.

- Physicists (especially string theorists) regard it as something with an exactly zero value that is produced by an as-yet-not-understood cancellation in their theory.

However, there is a Third Way! It is called the Anthropic Principle: some property of the universe must be the way it is, because if it wasn't then we wouldn't be here to observe it. Can this approach be used to explain the values that fundamental physical constants? Should this approach be used, or is it not even science?

Generally, physicists hate the Anthropic Principle for various reasons:

- It abandons their cherished hope of deriving everything from first principles. It questions the very root of what science is, and if it turns out to be true it would certainly be a "new kind of science". This possibility
*cannot* be discounted.

- It smells suspiciously like an "intelligent design" argument, where the properties of the universe are tuned so that we can exist. In fact, this suspicion is ill-founded, because it fails to recognise the difference between the
*weak* and *strong* forms of the Anthropic Principle (see the last paragraph below). This possibility is based on a misunderstanding, and can thus be *discounted*.

Steven Weinberg has looked closely at the Anthropic Principle, and he concluded that a value that is *not much larger* than the observed upper bound would prevent galaxies, stars and planets from forming from the original very small density fluctuations that existed in the matter distributed throughout the universe. These sorts of arguments can also be used to place a bound on how *negative* the cosmological constant can be, and again its magnitude has to be extremely small.

Steven Weinberg has thus used the Anthropic Principle to place bounds on the magnitude of the cosmological constant that are consistent with observations. This work has been largely ignored because physicists mistakenly assume that it is necessarily based on an "intelligent design" argument. This is *not* the case if there is an *ensemble* of universes each with its own laws of physics, one of which is the fortuitously tuned version that we observe around us

Susskind then describes a thought experiment in which he rediscovers the "natural" (i.e. Planck) units of length, time and mass. These units turn out to be fundamentally important in the unification of quantum theory and Einstein's general theory of relativity, because they tell us where the naive picture of space as being analogous to a smooth rubber sheet breaks down (i.e. Einstein's gravity) to be replaced by a deeply wrinkled and knotted surface (quantum gravity).

The most important thing to understand in this chapter is the Anthropic Principle, which is *not* an "intelligent design" argument. None of Susskind's book will make sense until it is appreciated that Susskind is using the *weak* form of the Anthropic Principle, in which there is no *prior* tuning of the laws of physics, but rather there is an *ensemble* of universes each with its own laws of physics, one of which is the fortuitously tuned version that we observe around us.