JUST because we perceive time flowing in one direction, does that mean there "really is" a difference between the past and future? The old philosophical question has been re-examined by Huw Price, of the University of Sydney, in the context of quantum mechanics. He concludes that the idea that the past is not influenced by the future is an anthropocentric illusion, a "projection of our own temporal asymmetry". By allowing signals from the future to play a part in determining the outcome of quantum experiments, he can resolve all the puzzles and paradoxes of the quantum world.
This approach has a long (if not entirely respectable) history, but the implications have never been spelled out as clearly as Price does in an article to be published in the journal Mind. It is one of the curiosities of Maxwell's equations, for example, that they allow two sets of solutions for the effect of a moving electric charge, one describing an electromagnetic wave moving out from the particle into the future at the speed of light (a retarded wave) and the other describing waves from the future converging on the particle at the speed of light (advanced waves). The advanced wave solutions have been largely ignored since Maxwell developed his equations in the 19th century, but a few researchers, including Richard Feynman and Fred Hoyle, have considered the implications of taking such waves to be physically real.
More recently, the idea has been investigated in a quantum context by the American researcher John Cramer. He envisages a quantum entity such as an electron that is about to be involved in an interaction (from the everyday point of view) sending out an "offer" wave into the future. The particle that the electron is about to interact with picks up the offer wave, and sends a response echoing backwards in time to the electron. The advanced and retarded waves combine to create a "handshake" between the two particles which, in a sense atemporally, determines the outcome of the interaction at the instant the electron starts to make the offer .
As Price discusses, this kind of approach solves the classic quantum puzzles, such as the electron faced with two holes in a screen, "deciding" which hole to go through. Experiments show that, even though an individual electron can only go through one hole, its behaviour is affected by whether or not the second hole is open or closed. The offer wave goes out through both holes, but the echo comes back only through one hole, the one the electron then goes through. So the handshake process does take account of the presence of both holes, even though the electron only goes through one of them.
Many physicists find such ideas abhorrent, because they run counter to "common sense". They would, for example, encourage speculations like those of Henry Stapp (see Science, XX August), that our own minds can influence things that have already happened. The power of Price's approach, though, is that it offers a framework for understanding how the world can include both forward and backward causation at a fundamental level, but appear to have a unique direction of time from a human perspective.
His argument is complex, but in words it boils down to an argument that the reason why the things we do in the present do not seem to have altered the past is that the past has already taken account of what we are doing! If we decide to do something different, the past already knows -- so "to say that if we suppose the present to be different, while the past remains the same, it will follow that the past is different . . . is untrue, of course, but simply on logical grounds. No physical asymmetry is required to explain it".
For the more mathematically inclined, Price offers a discussion of John Bell's famous inequality, in which two widely separated quantum systems seem to be connected by what Albert Einstein called a "spooky action at a distance". The action at a distance is real, on this picture, and is essentially Cramer's handshaking process. But there is no limitation on free will, according to Price. We are free to make any decisions we please, and to take any actions we choose. The past already knows what those decisions will be, but that does not affect our freedom in making them, and "we shouldn't expect to 'see' backward influence in action," which may be bad news for Stapp, after all. "It is time," says Price, "that this neglected approach [to quantum mechanics] received the attention it so richly deserves."