Status

Study continues - not as well as hoped, but possibly better than expected. After a month and a half of revision I actually feel fairly comfortable regarding about half of the material on the exams, so hopefully in another ten days I'll be comfortable with the rest? :|

The papers are May 29th/May 30th/June 1st/June 2nd... 12 hours of exams in 4 days. Why oh why they can't put some breaks between the things, I have no idea. I'm beginning to wonder if my fingers are up to writing for three hours straight like that - I haven't really done more than a few minutes of handwriting in the last two years. I don't really have much time to practice now either, although I'll probably do one or two exam-style essays this week just to check how long an hour is in 'writing time'...

After the last paper I plan to spend the first week of June doing very little at all... meditate, draw, write, and generally unwind. I'm looking forward to that. I think it's much more beneficial to look forward to the post-exam relaxation than to stress too much over the exams ;)

Free Will and Retrocausality

I discussed yesterday's post on retrocausality with a few people and thought I'd make a quick post to explore an analogy that shows what a successful demonstration of the phenomenon would really mean for free will.

Some of you may be geeky enough to remember the Fighting Fantasy books - one incarnation of the 'choose your own adventure' concept. For those who aren't familiar with them, the books are divided into paragraphs (usually a few hundred) which are numbered; after reading one, you're offered a choice of which paragraph to read next to continue the story. Some choices lead to an ignoble death, some lead to the eventual solution.

When you read a paragraph (let's say #150), you take the information in it, plus any previous information you know, and make a decision on which paragraph to read next (we'll say #200 or #60). In fact it's probably fair to say that you pick the option that seems most sensible - given what you know (i.e. the past). That's analogous to any other decision-making moment. If you'd taken a different route through the story to reach #150, you might know that paragraph #60 means certain death - so you'd pick the other option. You do make a 'real decision'. (In reality, unlike Fighting Fantasy, you can't go back and start again, so you only make one decision in practice - this is really what determinism means.)

Now let's introduce retrocausality. This is very easy to include in our Fighting Fantasy analogy - you can just cheat. You glance ahead, read #60, realise it'll kill you, and go back and pick #200 instead.

What may not be so obvious is that back-in-time signalling has exactly the same effect. The signal from the future changes the information you have, so your decision changes. There's no paradox, because at the time you received your signal from the future, it accurately described what the future would be. However, now knowing that, you can deliberately pick a 'different future'. (It's not even a problem that the future where you sent the signal back is now probably not going to happen - although the explanation of that is more complicated.)

This is really the key distinction between determinism and fatalism, so I'm quite excited about it - if we can really send retrocausal signals, we should be able to thoroughly put the latter to rest.

Retrocausation?

It says something about me that I take breaks from revising for my Biochemistry finals by reading about quantum mechanics. Exactly what it says I'm not sure. :|

John Cramer has published an update on his progress to try to demonstrate retrocausal signalling. I'll summarise in a few sentences for those who don't have time to read the article or prefer a less technical phrasing:

Quantum mechanics allows non-local signalling between entangled particles. This essentially means that an interaction with one particle that changes its state can change the property of its entangled partner at arbitrary distance, instantly (not just at light speed). The 'reality' of this phenomenon is disputed but there is a great deal of experimental evidence that it exists.

If we take two entangled photons and pass one into a long optical fibre, we can delay the arrival of the second photon at its destination by a few microseconds. We detect both photons, but the delayed photon is detected in a specific fashion that changes its state - the non-delayed photon is detected without forcing it to take any particular state.

If the quantum prediction holds, the two photons will always be in the state induced by the measurement of the delayed photon, even though several microseconds passed between the first 'free' detection and the delayed detection.

So what?

Cramer's paper is a progress report and doesn't speculate about applications of the phenomenon if it's demonstrated to be possible, but any number of science fiction authors have considered possible 'future-scope' devices, some more credible than others. I have a feeling Greg Egan wrote one such story, but I don't recall in which collection.

The most obvious application is a device for 'signalling back in time'; by delaying one entangled particle for longer than a few microseconds (this is hard, of course, without breaking the entanglement - the same problem is encountered in quantum computing - but not impossible), i.e. for minutes or hours, we can then 'immediately' receive information on what the state of the future will be. This throws up all sorts of interesting potential paradoxes (although it may simply deal a final decisive blow to the concept of free will).

Spooky Sensing at a Distance

Slightly less obvious is the idea of using the phenomenon as a sensor - fire one photon at a distant object (like an extrasolar planet), and observe how the entangled partner changes. This should in principle reveal information about the 'target', again 'instantly' - even though the sensor photon takes subjective time to reach the target. Only certain kinds of information could be retrieved, but astronomers are very good at making sense of sparse data.

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More obscurely - and perhaps most interestingly, although I have a niggling feeling it may prove to be impossible - one could in theory perform long computations 'without actually performing them, by 'sending the answer back' to the beginning of the computation. If that's really true, then a suitable computer can perform any finite-length computation 'instantly'.