The equations of physics are generally time-symmetric, with no difference in function between the future and past. For example the theory of electrodynamics is based on waves travelling backwards and forwards in time.
Several experiments have been devised to test the notion of reverse or retro-causality including a variation of the classical quantum mechanics wave/particle experiment, delaying measurement until after the of photons had passed through the double slit. However this was not is not deemed sufficiently rigorous as a proof, as the path the photon took before the measurement could not be verified.
Physicist John Cramer has recently proposed a more rigorous test using the ‘transactional interpretation’ of quantum mechanics, which proposes that particles interact by sending and receiving physical waves that travel backwards and forwards through time. He has devised an experiment based on the entanglement of photons and their properties such as momentum, which then share the same wave or particle behaviour (7) (8) .
Pairs of photons from a laser beam are entangled before being sent along two different paths. One stream is delayed by several microseconds by sending it through a 10k optical fibre cable. A moveable detector can then be used to sense a photon in two positions, as either a wave or particle.
Choosing to measure the delayed photons as waves or particles forces the photons in the other beam to be detected the same way, because they are entangled. This choice therefore influences the measurement of the entangled photon even though it is made earlier.
If retro-causality is proved, it could solve the major enigma of quantum entanglement and non-locality; already defined within the context of the D-net model, but at the same time would require a complete reformulation of the laws of causality.
Measuring one entangled particle could send a wave backwards through time to the moment at which the pair was created without being limited by the speed of light. Retro-causality might also help explain why we live in a universe so finely tuned to the existence of life as Paul Davies has speculated, The universe may be able to retrofit its parameters using reverse causality to ensure the emergence of life as we know it.
D-Net provides a possible mechanism to achieve such a reverse engineering process, by allowing retracing of the pathways encoded as evolutionary histories. The parameters of the big bang could then be adjusted by a reverse evolutionary process to provide a way of accelerating the selection of a form of life capable of evolving into a more efficient information processor. The presence of intelligent observers later in history could therefore exert an influence on the big bang to reverse engineer the most appropriate conditions for the emergence of an optimally efficient form of life.
The D-Net model would allow this possibility because a causal network would allow both reverse or forward pathways to be established as the most efficient history.
Biologically it has already been shown that life can be reverse engineered, for example to produce chickens with teeth and snakes with limbs, by reversing evolutionary pathways.
In addition the latest interpretation of the Anthropic Principle suggests a much broader range of physical settings governing the creation of a universe which could lead to the evolution of life as we know it(9). For example an alternate pathway has recently been formulated allowing life to evolve without the existence of the weak force. It may be possible to define many other evolutionary pathways that could result in an alternate intelligent life form, capable of efficiently processing information.
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