Time and again:
From time to time physicists notice with apparent surprise that there is something off about time, cause and effect.  My best attempt to resolve the difficulties can be found at http://www.nobabies.net/Making%20a%20Universe.html .  But lets go over the basics.

Suppose you have a match so dim that when you strike it it only gives off one photon before dying.  When you do so, the location of that photon is not fixed but it distributed according to a probability function over a sphere that expands with the speed of light.  When the photon hits something, the probability function collapses and the entire essence of the photon, it’s energy, arrives at the point of impact.  You can demonstrate that. 

Imagine a laser that strikes a partially reflective object – a beam splitter.  A photon can continue through unaffected or it can be reflected.  This produces two beams.  Send the beams along courses that bring them together some distance away.  Place a lens where they cross, and expand the beams, and you will get a pretty pattern of perfectly parallel, evenly spaced lines of light.  You can take my word on that one – I’ve done it many times – or get the bits of equipment and do it yourself.  The lines are called an “interference pattern” and are due to the wave-like properties of light. 

Now take a bunch of your punk matches and strike them in exactly the same place a bunch of times; you’ll need a photographic plate or some way of recording the result since you can’t really see a single photon well, but the result is the same interference pattern.  I’ve not done that one, but others have.

The reason you get the interference pattern is that each lonely photon expands such that a bit of it probability of its presence goes through each path and then the two bits interfere just as the flood of photons from the laser did.

So it seems inescapable (unless you follow that link above) that the location of any photon is not defined until the photon is absorbed somewhere. 

People who study such things have rung the changes on making bits of stuff with undefined properties, and now there is something kind of new,  (Philip Ball “A world without cause and effect” Nature vol. 546 no. 7660 June 29, 2017 page 590)

They set up a particle at A and sent it to place D such that it could go from A to B to D or from A to C to D.  It’s path was determined by a particle with some property like up spin or down spin but with some tweaking was placed in a state such that the up/down state was undefined.  So when the moving particle arrived at D it could have gone either path, so sequence was undefined.  Oh, squeal, giggle; there goes causality. 

I fail to see the difficulty.  Sure enough, when the peripatetic particle is caught it is impossible to tell what route it took (just like a photon in that interference pattern … you can’t say which path it took).  But hang on.  That up/down particle is still there.  Measure that, and all becomes clear.

At least all becomes as clear at quantum physics ever is; there continues to be something mystifying about that interference pattern as far as I am concerned, even though the bulk of the paradoxen seem to be to be resolved by that link.

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