How to change the past

Ever wish you could reach back in time and change the past? Maybe you'd like to take back an unfortunate voicemail message, or
rephrase what you just said to your boss. Or perhaps you've even dreamed of tweaking the outcome of yesterday's lottery to make
yourself the winner.

Common sense tells us that influencing the past is impossible - what's done is done, right? Even if it were possible, think of the
mind-bending paradoxes it would create. While tinkering with the past, you might change the circumstances by which your parents met,
derailing the key event that led to your birth.

Such are the perils of retrocausality, the idea that the present can affect the past, and the future can affect the present. Strange
as it sounds, retrocausality is perfectly permissible within the known laws of nature. It has been debated for decades, mostly in
the realm of philosophy and quantum physics. Trouble is, nobody has done the experiment to show it happens in the real world, so the
door remains wide open for a demonstration.

It might even happen soon. Researchers are on the verge of experiments that will finally hold retrocausality's feet to the fire by
attempting to send a signal to the past. What's more, they need not invoke black holes, wormholes, extra dimensions or other exotic
implements of time travel. It should all be doable with the help of a state-of-the-art optics workbench and the bizarre yet familiar
tricks of quantum particles. If retrocausality is confirmed - and that is a huge if - it would overturn our most cherished notions
about the nature of cause and effect and how the universe works.

Dating back to Newton's laws of motion, the equations of physics are generally "time symmetric" - they work as well for processes
running backwards through time as forwards. The situation got really strange in the early 20th century when Einstein devised his
theory of relativity, with its four-dimensional fabric of space-time. In this model, our sense that history is unfolding is an
illusion: the past, present and future all exist seamlessly in an unchanging "block" universe. "If you have the block universe view,
the future and the past are not any different, so there's no reason why you can't have causes from the future just as you have
causes from the past," says David Miller of the Centre for Time at the University of Sydney in Australia.

With the advent of quantum mechanics in the 1920s, the relative timing of particles and events became even less relevant. "Real
temporal order in general, for quantum mechanics, is not important," says Caslav Brukner, a physicist at the University of Vienna,
Austria. By the 1940s, researchers were exploring the possibility of time-reversed phenomena. Richard Feynman lent credibility to
the idea by proposing that particles such as positrons, the antimatter equivalent of electrons, are simply normal particles
travelling backwards in time. Feynman later expanded this idea with his mentor, John Wheeler of Princeton University. Together they
worked out a theory of electrodynamics based on waves travelling forwards and backwards in time. Any proof of reverse causality,
however, remained elusive.

Fast forward to 1978, when Wheeler proposed a variation on the classic double-slit experiment of quantum mechanics. Send photons
through a barrier with two slits in it, and choose whether to detect the photons as waves or particles. If you put up a screen
behind the slits, you will get a pattern of light and dark bands, as if each photon travels through both slits and interferes with
itself, like a wave. If, on the other hand, you take a snapshot of the slits themselves, you will find each photon passes through
one slit or the other: it is forced to pick a path, like a particle. But, Wheeler asked, what if you wait until just after the
photon has passed the slits to make your choice? In theory, you could suddenly raise the screen to expose two cameras behind it, one
trained on each slit. It would seem that you can affect where the photon went, and whether it behaved like a wave or particle, after
the fact.

In 1986, Carroll Alley at the University of Maryland, College Park, found a way to test this idea using a more practical set-up: an
interferometer which lets a photon take either one path or two after passing through a beam splitter. Sure enough, the photon's path
depended on a choice made after the photon had to "make up its mind". Other groups have confirmed similar results, and at first
blush this appears to show the present affecting the past. Most physicists, however, take the view that you can't say which path the
photon took before the measurement is made. In other words, still no unambiguous evidence for retrocausality.

That's where John Cramer comes in. In the mid-1980s, working at the University of Washington, Seattle, he proposed the
"transactional interpretation" of quantum mechanics, one of many attempts to relate the mathematics of quantum theory to the real
world (New Scientist, 24 July 2004, p 30). It says particles interact by sending and receiving physical waves that travel forwards
and backwards through time. This June, at a conference of the American Association for the Advancement of Science, Cramer proposed
an experiment that can at last test for this sort of retrocausal influence. It combines the wave-particle effects of double slits
with other mysterious quantum properties in an all-out effort to send signals to the past.

The experiment builds on work done in the late 1990s in Anton Zeilinger's lab, when he was at the University of Innsbruck, Austria.
Researcher Birgit Dopfer found that photons that were "entangled", or linked by their properties such as momentum, showed the same
wave-or-particle behaviour as one another. Using a crystal, Dopfer converted one laser beam into two so that photons in one beam
were entangled with those in the other, and each pair was matched up by a circuit known as a coincidence detector. One beam passed
through a double slit to a photon detector, while the other passed through a lens to a movable detector which could sense a photon
in two different positions.

The movable detector is key, because in one position it effectively images the slits and measures each photon as a particle, while
in the other it captures only a wave-like interference pattern. Dopfer showed that measuring a photon as a wave or a particle forced
its twin in the other beam to be measured in the same way.

To use this set-up to send a signal, it needs to work without a coincidence circuit. Inspired by Raymond Jensen at Notre Dame
University in Indiana, Cramer then proposed passing each beam through a double slit, not only to give the experimenter the choice of
measuring photons as waves or particles, but also to help track photon pairs. The double slits should filter out most unentangled
photons and either block or let pass both members of an entangled pair, at least in theory. So a photon arriving at one detector
should have its twin appear at the other. As before, the way you measure one should affect the other. Jensen suggested that such a
set-up might let you send a signal from one detector to another instantaneously - a highly controversial claim, since it would seem
to demonstrate faster-than-light travel.

If you can do that, says Cramer, why not push it to be better-than-instantaneous, and try to make the signal arrive before it was
sent? His extra twist is to run the photons you choose how to measure through several kilometres of coiled-up fibre-optic cable,
thereby delaying them by microseconds (see Diagram). This delay means that the other beam will arrive at its detector before you
make your choice. However, since the rules of quantum mechanics are indifferent to the timing of measurements, the state of the
other beam should correspond to how you choose to measure the delayed beam. The effect of your choice can be seen, in principle,
before you have even made it.

That's the idea anyway. What will the experimenters actually see? Cramer says they could control the movable detector so that it
alternates between measuring wave-like and particle-like behaviour over time. They could compare that to the pattern from the beam
that wasn't delayed and was recorded on a sensor from a digital camera. If this consistently shifts between an interference pattern
and a smooth single-particle pattern a few microseconds before the respective choice is made on the delayed photons, that would
support the concept of retrocausality. If not, it would be back to the drawing board.

Cramer says the plan is to do the instantaneous signalling experiment first, to iron out technical glitches from noise or errors in
photon tracking, which would wreck the retrocausality experiment. Only after performing that would they add in the delay cables.
"This experiment, if successful, would bring retrocausality into the macroscopic realm," says Cramer.

Other experts are supportive of the idea but sceptical of what it might mean. "It would be important to perform such an experiment
just because of curiosity about interpretations," says Brukner. "If you accept the transactional interpretation, then this
experiment would show a retrocausal influence." Cramer agrees it is speculative, but says the experiment is our best shot at seeing
retrocausality in action. Because of the implications he is cautious, but still positive. "I don't see any show stopper yet," he
says.

If the experiment does show evidence for retrocausation, it would open the door to some troubling paradoxes. If you could see the
effects of your choice before you make it, could you then make the opposite choice and subvert the laws of nature? Some researchers
have suggested retrocausality can only occur in limited circumstances in which not enough information is available for you to
contradict the results of an experiment.

Another way to resolve this is to say that even if the present can influence the past, it cannot change it. The fact that your hair
is shorter today has as much influence on your going to the barber yesterday as the other way around, yet you can't change that
decision. "You wouldn't be able to talk about altering, but you could talk about causing or affecting," says Phil Dowe, an expert on
causation at the University of Queensland in Australia. While it would mean we cannot change the past, it also implies that we
cannot change the future.

If all that gives you a headache, then consider this: if retrocausality does exist, it says something profound about how the
universe works. "It has the potential to solve what is one of the biggest problems in modern physics," says Huw Price, head of
Sydney's Centre for Time. It goes back to quantum entanglement and "nonlocality" - one particle instantaneously affecting another,
even from the other side of the galaxy. That doesn't sit well with relativity, which states that nothing can travel faster than
light. Still, the latest experiments confirm that one particle can indeed instantaneously affect the other (New Scientist, 18 June
2005, p 32). Physicists argue that no information is transmitted this way: whether the spin of a particle is up or down, for
instance, is random and can't be controlled, and thus relativity is not violated.

Retrocausality offers an alternative explanation. Measuring one entangled particle could send a wave backwards through time to the
moment at which the pair was created. The signal would not need to move faster than light; it could simply retrace the first
particle's path through space-time, arriving back at the spot where the two particles were emitted. There, the wave can interact
with the second particle without violating relativity. "Retrocausation is a nice, simple, classical explanation for all this," Dowe
says.

While the jury is out awaiting the results of Cramer's experiment, some researchers expect reverse causality will play an
increasingly important role in our understanding of the universe. "I'm going with my gut here," says Avshalom Elitzur, a physicist
and philosopher at Bar-Ilan University in Israel, "but I believe that when we finally find the theory we're all looking for, a
theory that unifies quantum mechanics and relativity, it will involve retrocausality." If it also involves winning yesterday's
lottery, Cramer won't be telling.

“When we finally find the theory that unifies quantum mechanics and relativity, it will involve retrocausality”From issue 2571 of
New Scientist magazine, 28 September 2006, page 36-39
Why we are here
If retrocausality is real, it might even explain why life exists in the universe - exactly why the universe is so "finely tuned" for
human habitation. Some physicists search for deeper laws to explain this fine-tuning, while others say there are millions of
universes, each with different laws, so one universe could quite easily have the right laws by chance and, of course, that's the one
we're in.

Paul Davies, a theoretical physicist at the Australian Centre for Astrobiology at Macquarie University in Sydney, suggests another
possibility: the universe might actually be able to fine-tune itself. If you assume the laws of physics do not reside outside the
physical universe, but rather are part of it, they can only be as precise as can be calculated from the total information content of
the universe. The universe's information content is limited by its size, so just after the big bang, while the universe was still
infinitesimally small, there may have been wiggle room, or imprecision, in the laws of nature.