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4.2 Quantum Experiments

It is not the place here to describe in detail all the experiments that manifest the peculiar phenomena of quantum physics, so I will have to assume some previous acquaintance with the quantum problems.     There are a great many semi-popular introductory books (e.g. Herbert [1985] or Rae [1986]) which describe the basic experiments demonstrating quantum peculiarities. I presume some acquaintance with this material, so here I need merely point out the existence of experiments such as:
  • The photo-electric effect, where light interacts with electrons in discrete units called quanta as if it was made of particles. These particles (the quanta of the light field) are called photons.    
  • The two-slit interference experiment, where both light, electrons, and neutrons (and even composite atoms) produce oscillatory interference effects as if they were waves.      
  • Interferometer experiments (see e.g. Zeilinger [1986]) which verify the wave nature of light and neutrons to high accuracy. Both light and neutrons beams are here split up into several components that are observed to remain coherent with each other even after several reflections from mirrors. The mirrors of course are composed of many atoms -- the beams are reflected coherently from all these atoms.    
  • Einstein-Podolsky-Rosen (EPR) types of experiments4.1, where two particles emitted in single event retain a correlation as if they were continually connected even when a long way apart. These non-local correlations have been quantified in terms of Bell's       Inequalities (Bell [1964]), and have been experimentally verified in a large number of experiments (see Clauser & Shimony [1978]). Aspect et al. [1982] have observed these non-local connections persisting even when any communication between the particles would have to travel faster than light.    
  • `Quantum Beat' experiments (see Andrä [1970]), where a hydrogen atom is excited into a superposition of two excited states, and oscillatory beats are observed when the two states decay together coherently.    
  • Tunnelling phenomena, such as the slow decay rate of radioactive nuclei. Here, a particle (or cluster of particles) is confined to move inside the nucleus by nuclear potentials that stop it from getting too far from the centre. However, it can slowly `leak out' by tunnelling through the potential barrier.        
  • Aharanov-Bohm effect (Aharanov & Bohm [1959]), where an electron beam is passed around both sides of a long thin solenoid containing a magnetic field. Although the magnetic field can be completely confined within the solenoid, there are observed interference oscillations between the parts of the beam that pass on the two sides. These oscillations depend on the strength of the magnetic field, even though the electrons never enter a region of space where the magnetic field is non-zero.    
  • Scattering experiments between (say) two electrons show oscillatory fluctuations in the scattering rate that can only be explained if the two electrons are strictly indistinguishable.

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Next: 4.3 Quantum Ontologies Up: 4. The Peculiarities of Previous: 4.1 Classical and Quantum
Prof Ian Thompson


Author: I.J. Thompson (except as stated)