> 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.

> And room for retrocausality. If it exists, the presence of conscious
> observers later in history could exert an influence on those
> first moments, shaping the laws of physics to be favourable for life. This
> may seem circular: life exists to make the universe
> suitable for life. If causality works both forwards and backwards,
> however, consistency between the past and the future is all that
> matters. "It offends our common-sense view of the world, but there's
> nothing to prevent causal influences from going both ways in
> time," Davies says. "If the conditions necessary for life are somehow
> written into the universe at the big bang, there must be some
> sort of two-way link."
> --
> Frederick Martin McNeill
> Poway, California, United States of America
> mmcne...@fuzzysys.com
> http://www.fuzzysys.com
> http://members.cox.net/fmmcneill
> *************************
> Phrases of the week :
> "Be happy for this moment. This moment is your life."
> - Omar Khayyam
> "Blank"
> :-))))Snort!)
> **************************************