Three Roads to Quantum Gravity by Lee Smolin Page A

the scale at which the effects of gravity and quantum phenomena will be equally important. For larger things, we can happily forget about quantum theory and relativity. But when we get down to the Planck scale we have no choice but to take it all into account. To describe the universe at this scale we need the quantum theory of gravity.
    The Planck scale can be established in terms of known fundamental principles. It is calculated by putting together in appropriate combinations the constants that come into the fundamental laws. These are Planck’s constant, from quantum theory; the speed of light, from special relativity; and the gravitational constant, from Newton’s law of gravitation. In terms of the Planck scale, we are absolutely huge. The Planck length is 10 -33 centimetres, which is 20 orders of magnitude smaller than an atomic nucleus. On the scale of the fundamental time, everything we experience is incredibly slow. The Planck time, which must be roughly the time it takes for something truly fundamental to happen, is 10 -43 of a second. That is, the quickest thing we can experience still takes more than 10 40 fundamental moments. A blink of an eye has more fundamental moments than there are atoms in Mount Everest. Even the fastest collision ever observed between two elementary particles fills more elementary moments than there are
neurons in the brains of all the people now alive. It is hard to avoid the conclusion that everything we observe may still be incredibly complicated on the fundamental Planck scale.
    We can go on like this. There is a fundamental Planck temperature, which is likely to be the hottest anything can get. Compared with it, everything in our experience, even the interiors of stars, is barely above absolute zero. This means that, in terms of fundamental things the universe we observe is frozen. We begin to get the feeling that we know as much about nature and its potential phenomena as a penguin knows of the effects of forest fire, or of nuclear fusion. This is not just an analogy - it is our real situation. We know that all materials melt when raised to a high enough temperature. If a region of the world were raised to the Planck temperature, the very structure of the geometry of space would melt. The only hope we have of experiencing such an event is by peering into our past, for what is usually called the big bang is, in fundamental terms, the big freeze. What caused our world to exist was probably not so much an explosion as an event that caused a region of the universe to cool drastically and freeze. To understand space and time in their natural terms, we have to imagine what was there before everything around us froze.
    So, our world is incredibly big, slow and cold compared with the fundamental world. Our job is to remove the prejudices and blinkers imposed by our parochial perspective and imagine space and time in their own terms, on their natural scale. We do have a very powerful toolkit that enables us to do this, consisting of the theories we have so far developed. We must take the theories that we trust the most, and tune them as best we can to give us a picture of the Planck scale. The story I am telling in this book is based on what we have learned by doing this.
    In the earlier chapters I argued that our world cannot be understood as a collection of independent entities living in a fixed, static background of space and time. Instead, it is a network of relationships the properties of every part of which are determined by its relationships to the other parts. In this chapter we have learned that the relations that make up the world are causal relations. This means that the world is not
made of stuff, but of processes by which things happen. Elementary particles are not static objects just sitting there, but processes carrying little bits of information between events at which they interact, giving rise to new processes. They are much more like the elementary operations in a computer