Time Travel

Can the Future Change the Past?

TIME TRAVEL is an idea that has challenged people from all walks of life, be they scientists, philosophers, the educated and uneducated.

With his Theory of Relativity, Einstein demonstrated that the time dimension is affected by the relative speed between two observers or by the warping of the space-time fabric by massive bodies, such as black holes. This variation in observed time, however, does not allow for the reversal of the direction of time.

The theoretical laws of physics do not impose a time direction on massless bodies, such as photons; beyond that we enter the realm of theoretical speculation.

In his most recent efforts to dislodge God as the creator of the universe, Steven Hawking has put forward the hypothesis that the universe may have existed in non-temporal form ‘prior’ to the big bang (meaning since-forever, where forever is measured without time). There is some philosophical similarity between Hawking’s hypothesis and the theological understanding of non-temporal time in God’s realm, and therefore in the world to come. Christianity (and Judaism) recognises that with God there is no distinction between the past, present and the future; God referred to Himself as I AM. Although this concept has been discussed by theologians, and philosophers, over millennia, when pressed for clarification, it becomes clear that none of the experts has any idea what this means.

A major philosophical barrier to time travel is that it opens up a ‘can of worms’, such as by travelling into the past, you can change the circumstances by which your parents met, thereby preventing your own birth. The fact that we witness historical stability suggests that time travel is not possible.

So it may come as a surprise that scientists are now planning to perform an experiment where they propose to send photons into the past. Details were published in New Scientist, 30 September 2006.

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 retocausality’s feet to the fire by attempting to send a signal to the past.

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.

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.

In the mid-1980s, working at the University of Washington, Seattle, [John Cramer] proposed the transactional interpretation of quantum mechanics, one of many attempts to relate the mathematics of quantum theory to the real world. 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 [the] 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. 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. [It seems feasible] 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. 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.

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?

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. 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.

If this experiment proves successful what will be the impact on theology? The author of this post considers that retrocausality is not an uncommon phenomenon (be it an infrequent one) in everyday life. It comes in the form of premonitions experienced by those people who are receptive to God’s help, and in circumstances when such help is required. Understanding the actual mechanism of the premonition has no relevance on its validity, the same as the total lack of knowledge concerning the mechanisms of quantum events has no bearing on their scientific status.

Let us use a basic example. A person has a premonition of impending danger, and he does not board an aeroplane that is subsequently hijacked and blown up. Sceptics would attribute this to coincidence. The survivor, being a God-loving Christian, is convinced that his guardian angel came to the rescue. Allowing the Christian version to be reality, whether it was an angel or not is not the issue; the fact is that a future event was made known before its occurrence. In other words, at the macroscopic level, knowledge of a future event was brought forward in time (the arrow of time was reversed), and acted upon by the recipient. This happened within nature, and is therefore a natural process. It is no different to God upholding the rest of His creation, and an experiment at the subatomic level only serves to confirm this fact.

This is somewhat similar to the Orthodox experiences with saints appearing in several places concurrently. To the non-believer this is nonsensical and simply cannot be true because it cannot be confirmed by science. The problem (for them) is that science indeed does confirm multi-apparitions. Again at the quantum level, subatomic particles can, and do, appear in two different places at the same time; and this has been demonstrated experimentally.

Thus quantum physics is the non-spiritual gateway connecting the material and spiritual worlds.

Science and Religion

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