The Fabric of Reality: The Science of Parallel Universes--and Its Implications

by David Deutsch

In short, the result is that if pathways into the past do exist, travellers on them are free to interact with their environment in just the same way as they could if the pathways did not lead into the past. In no case does time travel become inconsistent, or impose special constraints on time travellers’ behaviour. That leaves us with the question whether it is physically possible for pathways into the past to exist. This question has been the subject of much research, and is still highly controversial.

The general theory of relativity predates quantum theory and is not wholly compatible with it. No one has yet succeeded in formulating a satisfactory quantum version — a quantum theory of gravity.

Past-directed time travel, which requires the manipulation of black holes, or some similarly violent gravitational disruption of the fabric of space and time, will be practicable only in the remote future, if at all. At present we know of nothing in the laws of physics that rules out past-directed time travel; on the contrary, they make it plausible that time travel is possible. Future discoveries in fundamental physics may change this.

Time travel may be achieved one day, or it may not. But if it is, it should not require any fundamental change in world-view, at least for those who broadly share the world-view I am presenting in this book.

Most of the questions from the floor will have the form of criticisms which, if valid, would diminish or destroy the value of the professor’s life’s work. But bringing vigorous and diverse criticism to bear on accepted truths is one of the very purposes of the seminar. Everyone takes it for granted that the truth is not obvious, and that the obvious need not be true; that ideas are to be accepted or rejected according to their content and not their origin; that the greatest minds can easily make mistakes; and that the most trivial-seeming objection may be the key to a great new discovery.

Neither Penrose nor anyone else has yet actually proposed any viable rival to the Turing principle, so it remains the prevailing fundamental theory of computation.

good explanations are not to be discarded lightly.

I believe that there is some truth in the gut feeling behind these criticisms.

Now, as I have said, my guess is that the brain is a classical computer and not a quantum computer, so I do not expect the explanation of consciousness to be that it is any sort of quantum-computational phenomenon. Nevertheless, I expect the unification of computation and quantum physics, and probably the wider unification of all four strands, to be essential to the fundamental philosophical advances from which an understanding of consciousness will one day flow.

Although we do not know what consciousness is, it is clearly intimately related to the growth and representation of knowledge within the brain.

knowledge can be understood as complexity that extends across large numbers of universes.

Popper even said that ‘no theory of knowledge should attempt to explain why we are successful in our attempts to explain things’ (Objective Knowledge p. 23). But, once we understand that the growth of human knowledge is a physical process, we see that it cannot be illegitimate to try to explain how and why it occurs. Epistemology is a theory of (emergent) physics.

When, in the course of my research on the foundations of quantum theory, I was first becoming aware of the links between quantum physics, computation and epistemology, I regarded these links as evidence of the historical tendency for physics to swallow up subjects that had previously seemed unrelated to it. Astronomy, for example, was linked with terrestrial physics by Newton’s laws, and over the next few centuries much of it was absorbed and became astrophysics. Chemistry began to be subsumed into physics by Faraday’s discoveries in electrochemistry, and quantum theory has made a remarkable proportion of basic chemistry directly predictable from the laws of physics alone. Einstein’s general relativity swallowed geometry, and rescued both cosmology and the theory of time from their former purely philosophical status, making them into fully integrated branches of physics. Recently, as I have discussed, the theory of time travel has been integrated as well.

the Turing principle, which was already, under the name of the Church — Turing conjecture,

But if knowledge is of fundamental significance, we may ask what sort of role now seems natural for knowledge-creating beings such as ourselves in the unified fabric of reality.

cosmologist Frank Tipler. His answer, the omega-point theory, is an excellent example of a theory which is, in the sense of this book, about the fabric of reality as a whole. It is not framed within any one strand, but belongs irreducibly to all four.

To understand our best theories, we must take them seriously as explanations of reality, and not regard them as mere summaries of existing observations.

The Turing principle is our best theory of the foundations of computation. Of course we know only a finite number of instances confirming it — but that is true of every theory in science.

It is not only scientific knowledge that informs people’s preferences and determines how they choose to behave. There are also, for instance, moral criteria, which assign attributes such as ‘right’ and ‘wrong’ to possible actions. Such values have been notoriously difficult to accommodate in the scientific world-view. They seem to form a closed explanatory structure of their own, disconnected from that of the physical world. As David Hume pointed out, it is impossible logically to derive an ‘ought’ from an ‘is’. Yet we use such values both to explain and to determine our physical actions.

James P. Hogan, The Proteus Operation, Baen Books, 1986, Century Publishing, 1986. [Fiction!]

Karl Popper, Conjectures and Refutations, Routledge, 1963, HarperCollins, 1995. Karl Popper, The Myth of the Framework, Routledge, 1992.

John Barrow and Frank Tipler, The Anthropic Cosmological Principle, Clarendon Press, 1986.