Thursday, July 14, 2011

Thoughts on the Multi-Universe Interpretation of Quantum Mechanics

Just looking around the 'net I see much written about quantum mechanics that isn't, well, sound; I hope I'm not contributing to that genre, and fear that I might be.  I should start by saying I have studied it a bit, and have a degree in Physics from MIT, but I'm certainly not an expert, and many of its intricacies I'm sure lie buried by the detritus of years of other thoughts.  I'd like to lay out the outline of an idea, as much to get it down so it won't be forgotten.
Very briefly and somewhat loosely, the multi-universe interpretation of quantum mechanics says that everything that can happen will happen; when a random event occurs (say one with two outcomes), the universe splits into two universes - in one the first outcome holds, in the second the second outcome holds.  Wikipedia has a fairly decent overview here, worth taking a moment to scan.  At first this explanation sounds a bit extreme, wasteful in universes, so to speak.  But it does address some fundamental issues that are otherwise hard to make sense of, like quantum mechanical wave collapse (when an observation transforms a system properly described by a wave function to one described essentially classically) .  In the multi-universe interpretation, this never needs to happen, because each split universe has its own observer and its own result.

I have a small (and possibly testable) modification to suggest, which I'll get to shortly, after I describe its genesis - or at least the thought that provoked the idea.  It was years ago, and I was driving to work, and pulled into a bank near my office to get some cash - something I'd done many times over the years.  As I pulled in I saw a decent place to park about halfway down the lot, and a thought sprang up unbidden: "If I park there I'll be in an accident."  So, I didn't park there.  I went into the bank, stood in line, got cash, came out - and a car was in that spot, the driver exchanging papers with a second car that had hit it.

This was of course somewhat shocking - it'd never happened before, this isn't the sort of thing that occurs in my life (I can't think of another example, certainly not as striking).  I'm not a mystic, so I began to wonder how it could have happened at all.

I've always wondered about the instant we call "now" - a Euclidean point on the time axis, of zero thickness, has always struck me as somewhat absurd.  Einstein  reportedly said "There is no 'now' in Physics", meaning there is no model of it, no description of it, indeed it doesn't appear in physical theories.  If you give this a few moments of thought it's quite amazing - our only experience of life is in the instant we call 'now', and our physical theories don't consider it at all.
My thought after the bank parking lot episode, many years ago, was that maybe we don't proceed through time linearly moving forwards - on average we do, perhaps for thermodynamic reasons, but maybe we move forwards a little, back a little, oscillating about what we consider to be "now".  (If you like you could take the furthest point into the future we go and label that 'now', so all of this oscillation is entirely in the past - it is only a naming convention).  Moving backwards in time would change the physics of brain processes so that it would be very unlikely that we'd have any coherent memories of the future.  Occasionally these might persist, perhaps with enough internal consistency that they would be recognizable as useful information - so I might have been 'remembering' an accident, in a manner that felt like a strong intuition to avoid the situation.  This thought has been lying dormant for a long time - perhaps appropriately, as there seemed no way to test it or explore it further.
Back to the multi-universe interpretation: all of those splitting universes seem a bit unnatural.  What if, instead of splits, they are different excursions into the future, that is they all occur in this universe.  Let me describe this by analogy.  Imagine a snow-covered football field, with distance along the field analogous to time (think of one goal line being a few minutes ago, the other being a few minutes on, and where you are being "now").  You start walking at the first goal line, go a few yards, walk back, go forwards again, but perhaps not in the same route; you're moving down the field slowly, but making many tracks forwards and backwards as you go.  These tracks represent the excursions into the future mentioned above, each being something like one of the split universes; and, as you walk, you may interact with the tracks you've already laid down, stumbling over earlier footprints (analogous to the sort of effect one sees in the two-slit experiment, when a photon can 'interfere with itself' and seemingly go through both slits at once), or gravitating to well-trodden areas (analogous to high-probability outcomes).  Eventually you get to the final goal line, and can think over what you've done.
Wave function collapse reappears because we're back in one universe, but it might be a tiny bit clearer: Your consciousness holds memories, and those of the future excursions are very weak.  As your mind makes sense of where you are it sees multiple tracks behind it, as perhaps it should (see this interesting paper on the possible literal meaning of the 'sum over histories' technique:
I'm glossing over lots of details, such as the nature interference can have, what provokes the random wandering in time, etc.  Some of these may be fatal to the entire approach.  One implication of this idea is that the movements forwards and backwards in time are discrete (you will walk across a given yard line some integer number of times, indeed an odd number of times); given that interference between the excursions is possible, there might be an experimental way to detect this, how long they might be, etc.