Other Worlds by Paul Davies

Author:Paul Davies [Davies, Paul]
Language: eng
Format: epub
Tags: Non-Fiction, Science
ISBN: 9780140138771
Google: qiA8PwAACAAJ
Amazon: 0140138773
Goodreads: 524497
Publisher: Libre Dionysia
Published: 1980-01-01T05:00:00+00:00

Figure 14 Spin correlation

When the neutral pi-meson decays into two photons, the spin of one must be opposite to that of the other, so if the spin of, say, the right hand photon is measured, that of the left hand photon is deduced immediately. However, paradoxes arise when it is realized that the spin direction is intrinsically indeterminate until the measurement actually occurs.

which way one of them is spinning immediately gives the information about which way the other is spinning.

The essential feature of this example is that after the disintegration of the parent body, the two product particles may move a great distance apart. Indeed if the explosion occurs in outer space, the particles might continue flying apart until separated by millions of light years. If the spin measurement is now made, a local observation of the spin direction of one particle gives immediate information about the other particle, which is far outside the galaxy. Now according to the theory of relativity, information cannot travel faster than light, so the idea of the instantaneous acquisition of knowledge about a particle in a very distant place might appear to violate a fundamental principle. In the case of the gun and bullet the commonsense picture is that, long before the observation of the spin direction was made, the bullet was’re ally’ spinning, say, clockwise and the gun anticlockwise, and the only effect of the measurement is to make this knowledge available to an observer. This does not really amount to sending a signal faster than light as no actual physical influence travels between the two bodies. So provided we assume the existence of a real world, independent of our knowledge or our intention to make an observation, containing real objects (guns, bullets) with fixed and meaningful attributes (spinning, separating), there is no conflict with the principles of relativity and the inability to send signals faster than light.

It is natural to extend this picture to the subatomic domain also, and to suppose that the two particles are’re ally’ spinning in such-and-such a way, irrespective of whether we intend to find out by performing an experiment. Thus Einstein hoped to establish the independent reality of the physical world but any straightforward attempt to claim that such entities are’re ally’ behaving in a certain way before we observe them has been demolished by recent experiments.

Let us choose as the two separating particles photons of light. Rather than discuss their spins, as above, it is easier to deal with a related property called polarization, for this is familiar in daily life and is also the quality which physicists have actually measured to experimentally check what will be described. Modern sunglasses often include polarizing glass, and an understanding of their operation is basically all that is needed to comprehend why the world is not as real as it may seem. Light is an electromagnetic vibration and we can ask in which direction the electromagnetic field vibrates. A mathematical study, or some simple experiments, show that if the

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