Time travel

Gary Lachman’s article on precognition [ FT415:32-38] was an interesting read, but a couple of his assertions caused me to raise an eyebrow. For one thing I’m not aware of anyone suggesting that positrons travel backwards in time. ‘Tachyons’ would – but they are hypothetical, and even if they did exist in some fashion they couldn’t be exploited to send information into the past unless they had a detectable

interaction with the slower-than-light world that we know about.

My other issue is that Lachman notes that there are logical problems with ‘time travel’, but fails to note that the problematic implications of being able to see the future are much the same as those for travelling backwards in time, i.e. that – on the face of it – they permit causality to be defied through the exploitation of information that arrived at a point prior to that information being created. The two pillars of modern physics – quantum mechanics and General Relativity – are both notable for not providing any particularly compelling explanation for the arrow of time; that is, we usually perceive it to flow in one direction, but we don’t really know why that is.

Quantum mechanics – by some interpretations at least – suggests that events do not take a definite path until they are observed to do so. For example, a single photon passing through a beam splitter and given two possible paths to impact on a photographic plate will create an interference pattern consistent with taking both paths simultaneously. Trying to ‘catch it out’ by putting detectors in one or more paths while the photon is in flight doesn’t work – suggesting a lack of respect for the arrow of time in unobserved quantum systems. I would surmise that observing an event collapses the wavefunction to create a definite outcome, irrespective of whether that observation was made after the fact or via some as-yet not understood precognition; once observed it can’t be changed.

General Relativity also has something to say about the arrow of time. The Universe is considered to be a four-dimensional structure that can be distorted by gravity. In principle it can be bent into closed time-like curves, where the ‘time-like’ dimension is looped back on itself and allows you to reconnect with your own past. However, all events that occur within the closed timeline have to be self-consistent (they can’t change over time because time is part of the structure), which implies that there are no paradoxes because paradoxinducing events have a probability of zero (the Novikov Self-Consistency Principle).

Both ways of understanding the world suggest that either our choices are illusory, or that if they aren’t then they imply the involvement of further additional dimensions of reality (i.e. some kind of multiverse that allows us to navigate between possibilities). In either case, exceptions to time’s arrow don’t create any insurmountable difficulties for our understanding of physics. Were it to be established that our memories sometimes work in reverse then we would in some respects be closer to reconciling theory with actual experience.

A follow-up question then occurs: How sure are we that we don’t habitually remember future events? When I toss a coin and it comes up heads I retain a vague conceptual recollection of the fact that I was looking at a coin with a head on it. Before I toss a coin, I don’t know what it will do, but I have in my head a concept of a coin ‘heads up’ and a coin ‘heads down’. If the coin was double-headed then I would have in my head ‘heads up’ only, and I would be proved correct. The thing about past events is that there are only a limited number of possible prior sequences of events that can logically lead up to the state of affairs we call ‘the present’, whereas future events can diverge into a (possibly) infinite number of possible courses. So is the difference between our memory of the past and our memory of the future only that the limited possible ‘past’ event pathways that are consistent with our present give us a greater perception of certainty than we can have when considering possible future events? How could we tell?

For that matter, is the past always certain? Many times I have found an item missing that has subsequently turned up somewhere I know I didn’t put it. Am I senile or is my recollection of past events simply a memory of the most probable sequence that’s consistent with the most probable present – while not always being perfectly consistent with the actually observed present?

If I have any overarching point it is that we intuitively perceive time and causality in ways that aren’t necessarily consistent with what is revealed by careful observation.

Ian I’Anson

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