Yet I Still Want to be a Catholic: post 8 – Quantum (2)

Yet I Still Want to be a Catholic Post 8.  Quantum (2) It Gets Weirder Still.

Ever since Bohr, faced with the paradox of how matter can be both particle and wave, had proposed that real things do not exist until they are measured in some way, scientists have sought other explanations, although since these are even more bizarre than Bohr’s many still accept Bohr’s Copenhagen thesis.  But there are other explanations, chiefly two.  One is the many worlds interpretation first proposed by Hugh Everett III in 1957.  According to Bohr, if there is no detector a photon passes through both slits as a wave.  But if there is a detector or observer the detector collapses the wave function and the photon passes through only one slit as a particle.  Everett proposed that even if there is a detector the photon passes through both slits as a wave, but enters our universe as a particle as if it had only passed through one slit and as it passes through the other slit enters a parallel universe where it might behave quite differently.  There could be an infinity of these parallel universes.  Alles klar?  Well hardly.


Another explanation is that of David Bohm’s hidden variables.  In Bohm’s view, there is an underlying and hidden dimension of reality that he calls the implicate order.  Real things behave as real things at all times in the way common sense assumes.  But they are guided by a hidden pilot wave that only becomes manifest at the quantum level.  It is the pilot wave that guides a photon through both slits as a wave.   But it is also the pilot wave that detects the presence of a detector and instantaneously collapses the wave function.  It is the pilot wave that does the collapsing, not the detector itself.  Bohm thinks that the universe is filled with an infinity of pilot waves connecting all matter into an undivided whole.


Do these explanations really explain anything though?  Who has ever seen an infinity of parallel universes or Bohm’s hidden implicate order?  The plot had thickened still further when Heisenberg had proposed his uncertainly principle.  This states that (1) at the quantum level there is complementarity.  Some aspects of reaiity, position/ momentum and time/energy for example, are intimately connected with each other and dependent on each other. (2) at the quantum level there is radical uncertainty, the more precisely you measure momentum the less you can know about position, the more you know about the energy of an entity the less you know about the elapse of its time.  The best you can do is make an estimate of probability. This explained one of the deepest problems that had puzzled physicists ever since Planck.  Under some circumstances electrons lose the energy which is necessary to keep them in their orbits.  Why, then, as their energy drains away do they not collapse into the nucleus of the atom?  Heisenberg at last explained this.  Because the nucleus of an atom is so small, as the electron dwindles into the nucleus its position could hardly be more precisely defined.  But because of complementarity its momentum is correspondingly enormous, so great it immediately flies out of the nucleus, until, as the momentum lessens with increased distance, the attraction of the positive nucleus to the negative electron matches the momentum and the electron is kept in a stable orbit.

Heisenberg: a leading member of the second generation of great quantum physicists; at the heart of quantum physics there is radical uncertainty

All this uncertainty was too much for Einstein. The whole point of science is to establish certainty.  With two colleagues, he proposed what became known as the EPR thought experiment.  Suppose, imagined Einstein, you fired two photons at exactly the same time at exactly the same speed from a point X in two precisely opposite directions, Y and Z.  According to Heisenbergian uncertainty, if you were able to measure the position of one of them at a certain point, you would have no precise idea of its momentum, and vice versa, and so with the other.  If you measured that one’s momentum you would not know its position. But since they were moving at the same speed, if you knew the position of one of them you would be able to calculate that of the other. So you could know precisely both momentum and position.

In the 1960’s an Irishman called John Bell produced an equation showing that if you could send two particles in opposite directions and change one of either their position or their momentum during its flight, if, because of complementarity, the other corresponded instantaneously then Heisenberg would be right and Einstein wrong.  By the 1980’s it was possible to actually do the experiment and it was done by Roger Aspect in Paris over a distance of 11 metres.  In fact, Aspect changed not the position or momentum but the spin of one of the particles (needless to say at the quantum level particles don’t spin in the way you or I would mean) and abracadabra! the other immediately corresponded, in accordance with the complementarity of quantum physics.  It was Heisenberg who was right.  Then a few years later the same experiment was done in Geneva over 11 kilometres and the same thing happened.  The scientists concluded that even if two linked particles were separated by the whole universe, they would be in this kind of instantaneous communication.  They call this phenomenon nonlocality.  It would be hard to exaggerate the importance of this discovery.  It bursts the bounds of all previous science that we have known, it abrogates the very cornerstone of Einsteinian physics that nothing can travel faster than the speed of light.  Answers on a postcard please.

If you thought that quantum physics had already gone beyond all bounds of credibility, it was about to get weirder still.  Richard Feynman noted that if you drop a teaspoon to the ground the electrons in the spoon travel straight to the ground, well of course they do they are in the spoon.  But according to Feynman, on the way they visit everywhere in the universe, popping randomly out of virtual reality into actual reality, and back again, in all sorts of odd places, Alpha Centauri, next door’s washing up basin,  the Pope’s tiara, Orion’s rings.   This explains why a particle passes through one slit but also in so far as it is a wave two, because it’s passing through everywhere else as well.  The electrons in the spoon go straight to the ground in a straight line because since all the other directions in which it is going cancel each other out, the straight line is the only direction which doesn’t have a cancelling partner.  Complementarity is everywhere in quantum physics.

Pius V.  Did an electron look in on his tiara before popping up in your dishwasher?

In 1978 and then again in 1984 John Archibald Wheeler conducted his delayed choice experiments.  Wheeler fired photons through the two slit screen with two detectors trained on the two slits.  In accordance with the accepted doctrine of quantum physics, when the detectors were turned off the  photons went through   as waves, and when they were turned on the wave function was collapsed and they went through only one slit one at a time as particles.  But then Wheeler left the detectors off and switched them on during the flight of the particles between the two screens.    Since the particles had gone through undetected as waves they presumably would arrive at the back screen as waves.  But they didn’t.  They arrived as particles.  Somehow it was as if they knew what Wheeler was going to do before he did it.    He called it backward causation.


Wheeler’s backwards causation: he switched the detectors on  after the photons had left the slits but before they arrived at the back screen.

What conclusions, if any, can we draw from all this?

Of course, I’m only a fascinated amateur so if I have grossly misunderstood any of this, which doubtless I have, I hope some more knowledgable person will correct me.  How compelling it all is though.








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