We replace the first sheet of foil with another sheet which has a very thin slit just below the level of the emitter. Looking at the screen we see what we might expect, which is almost the same as we saw for the first slit. Most of the screen is dark but there is a glowing band behind the slit where the electrons are getting through and hitting the phosphorescent dots. As the glowing band, slit and emitter are all in direct line of sight the band is at a slightly lower position than for the first slit.
Both slits with a stream of particles - expected results
We now replace the sheet of foil with one containing two slits, of exactly the same size and exactly the same positions as before. Common-sense tells us that we should see an additive effect of the two individual slits. There should be two glowing bands, one at each of the previous positions.
But common-sense is wrong - this doesn't happen!
...
Both slits with a stream of particles - actual results
Instead we see a number of glowing bands at different positions from those seen with either of the two individual slits. Regions which were dark in both previous experiments have become light, and vice versa. In fact the electrons are showing interference effects, which are typical of waves. Waves which converge after travelling two different paths show a pattern of high energies at places where troughs and peaks converge simultaneously, and zero energies where troughs coincide with and cancel peaks.
Stretching common-sense a little we conclude that introducing the second slit has somehow forced the electrons to behave as waves rather than particles.
One of the characteristics of waves is that they spread out. But if we observe the screen closely we notice that the glow isn't spread out. Individual dots are still momentarily glowing while their neighbours may remain dark. The electrons are arriving as particles. So we may conclude that the electrons are travelling as waves, and interfering with one another, but as soon as they meet a detector they immediately resume particle behavior.
Two slits, one particle at a time
One obvious way to get rid of the interference effects is to ensure that only one electron is travelling at any one time. If we do this then each electron will have an unobstructed run and, over the course of time we should see a pattern build up which is the same as for two single slits added together.
To do this we reduce the power of the emitter so that it does not release an electron until the previous one has hit the screen, so removing any possibility of interference. We could actually sit and watch each individual electron arrive at the screen but this would be time consuming. Instead we stick the photographic paper on the screen and leave it for a while.
But when we develop the photographic paper, we find the same interference pattern that we saw when many electrons were passing through the apparatus simultaneously! The same areas which were dark in the two slit experiment remain dark, despite their being light in the single slit experiment.
So our original ideas of electrons interfering with one another by cancelling and reinforcing is wrong. Each electron cancels and reinforces itself when two slits are open, but does not do so when only one slit is open. The only logical explanation left is that a single electron must split and pass through both slits simultaneously. We can install detectors behind the slits to confirm this.
Check both slits
We place extremely sensitive particle detectors behind each slit and then set the emitter to release electrons singly. We wait to observe the simultaneous arrival of two bits of electrons at both particles detectors. And we wait ... and wait ... and wait. But all we ever see is that either one particle detector registers an electron or the other does, but never both simultaneously. Each electron travels through either one slit or the other.
So if it does not traverse both routes, how does the electron 'know' that the other slit is present. Well obviously a thing as simple as an electron can't know anything. And yet knowledge of the existence of a second slit is involved at the deepest level of these series of experiments. Knowledge of possibilities rather than any actual particle trajectory , or other physical event, seems to be determining the properties of material objects. But if the electron has no knowledge of its environment, then the only other place where such knowledge could reside is in the mind of the observer. Therefore the observer's mind is in some way determining the outcome of the observations.
If the experimenter's observational set-up imputes the concept 'wave', then he will see wave-like behaviour. If he imputes the concept 'particle' then he will see particle-like behaviour. Even placing a particle detector behind only one of the slits destroys the interference pattern, because the experimenter has in so doing imputed the concept 'particles' over the electrons despite both slits remaining open and one route being unobstructed. More detailed descriptions of the two-slit paradox are given in The Emperor's New Mind by Roger Penrose [PENROSE 1990b ] and Where Does the Weirdness Go? by David Lindley [LINDLEY 1997a].
Stern and Gerlach's magnets
One of the earliest demonstrations that the choice of observation imputes qualities on a quantum system (rather than merely observes what is already there) is due to Stern and Gerlach.
Many subatomic particles are tiny magnets with north and south poles of equal strength. If we obtain a stream of particles from a random source, such as a hot wire, then we would expect them to be randomly aligned. The north south axis might run up-to-down, left-to-right, back-to-front or vice versa or any intermediate orientation. In fact, we would expect only a small proportion to by aligned exactly up/down, the vast majority will be somewhere in between.
Stern and Gerlach set up a special type of magnetic field where the strength of the poles declines rapidly with distance. In certain areas of the magnetic field this would deflect the particles according to their orientation.
The mechanism is as follows: Assume that particles pass by the equipment's north pole which is at the top. A particle with its north pole facing directly upwards would be expected to be deflected strongly downwards because the repulsion due to its north pole would be stronger than the attraction due to its south pole (because the particle's south pole is further away from the apparatus' north pole and so in a weaker part of the field). Conversely a particle with its south pole upwards would be expected to be deflected upwards.
However the vast majority of particles would not be aligned directly upwards or downwards but somewhere in between. These would be deflected less strongly, and the large number aligned more or less a right angles to the field would undergo very little deflection at all .
Stern Gerlach - expected results
If we examined the beam after passing through the magnetic field (by placing a photographic screen in the way) we would expect it to have assumed an elongated shape, with the brightest areas (most particles ) being in the central undeflected area.
THIS DOES NOT OCCUR!
Stern Gerlach - actual results
All particles are deflected either equally upwards or equally downwards in a 50:50 ratio. There are no intermediate positions.
We are therefore left with three possible conclusions:
(1) The apparatus somehow forces the particles to align parallel to its magnetic field before it deflects them.
OR
(2) The particles are not emitted with random orientation but are produced either up or down.
OR
(3) The particles have no orientation until it is observed. The act of observation produces the orientation.
Alternative (1) - forced alignment - can be rejected because there is no known two-step mechanism whereby a magnetic field would wait until it had aligned all the miniature magnets before it decided to turn on the deflection. Also, progressively weakening and shortening the magnetic field would be expected to allow some particles to escape the alignment process. But this does not happen. Particles are, within the limits of experimental measurement, all deflected to exactly the same extent either up or down.
Alternative (2) - non random orientation - can be disproved by observing what happens when the incoming beam is left unchanged and the the Stern-Gerlach magnet is rotated through 90 degrees. The particles are then either deflected left or right with nothing in between. In fact the orientation is totally arbitrary. If the Stern Gerlach magnets are aligned at orientations corresponding to any axis (one o'clock/seven o'clock or two o'clock/eight o'clock) etc then the original beam will split into two beams with all particles showing an equal deflection towards the one'clock or seven o'clock position.
So we are left with alternative (3) - the orientation has no inherent existence. The attribute of orientation is utterly meaningless in the absence of an observer. The meaning of the orientation is projected by the observer's mind. If the observer projects the up/down axis of orientation on a stream of particles then that is the way that they will all be sorted. If any other direction is chosen then they will be sorted along that axis. Quantum theory does not appear to allow any fundamental distinction between the mind of the observer and what is being observed.
Full details of the Stern-Gerlach experiments are given in Where Does the Weirdness Go? by David Lindley [LINDLEY 1997b]
Spooky action at a distance - EPR
One of the most vivid illustrations of the interactions of the mind of the observer with a quantum system is given by EPR - the 'Einstein Podolsky Rosen Paradox', or 'Spooky action at a distance' as it is sometimes known. The experimental evidence seems to show that the observer's mind goes to its object unobstructedly and instantaneously, for example through ten kilometres of intervening Geneva city-scape (walls, buildings, railway stations, the lot!) at speeds exceeding that of light.
Nor does the effect diminish with distance. According to the Copenhagen interpretation of quantum theory, the 'spooky action' can affect a particle instantaneously whether it is a metre away from the observer or halfway across the universe.
The observation of 'spooky action' relies on the concept of entanglement. It is possible to obtain pairs of fundamental particles where it is known that their properties will always cancel one another out, even when those properties have not been defined. These pairs are said to be 'entangled' . However the entanglement is conceptual rather than physical and the particles are free to move far apart.
Consider an experiment where we create an entangled pair of magnetic particles. Their polar alignments will always be opposite. We allow them to move far apart. We then place a Stern-Gerlach magnet in the path of one of the particles and observe what happens when it passes through. If it is defected upwards then, according to the 'spooky action' hypothesis, its distant partner would be deflected downwards by a similar magnet. By making the nearby observation we have instantaneously defined the properties of the distant particle.
Note that this is not the same thing as saying 'The near particle was always up but we didn't know until we decided to observe it. So the distant particle must always have been down even though we didn't know at the the time.'
The reason the statement above is incompatible with quantum theory is that we could have equally well decided to align the Stern-Gerlach magnet on a left/right axis instead of up/down. In which case we would have fixed the near particle as, say, left-deflected and the distant particle would instantaneously be known to be right-deflected.
For many years both theoretical and technical difficulties stood in the way of determining whether 'spooky action' does indeed take place. However as a result of the theoretical work of John Bell and the ingenious experimental designs of Alain Aspect strong evidence was obtained that the effect occurred over distances of a few metres. The act of making a decision of what attributes of one member of an entangled pair were to be observed immediately determined what could be observed of the other member.
Since then 'spooky action' has been demonstrated over increasing distances. The current record is 10 km obtained by Nicolas Gisin and his team at the University of Geneva [BUCHANAN 1997]. Starting from near Geneva railway station they sent entangled photons along optical fibres through the city to destinations separated by 10km. They showed that observing the state of one member of the pair instantaneously determined the state of the other.
Quantum sunyata
Basically, what quantum theory says is that fundamental particles are empty of inherent existence and exist in an undefined state of potentialities. They have no inherent existence from their own side and do not become 'real' until a mind interacts with them and gives them meaning. Whenever and wherever there is no mind there is no meaning and no reality. This is a similar conclusion to the Mahayana Buddhist teachings on sunyata.
The ultimate manifestation of quantum sunyata is when quantum theory is applied to the entire universe. According to some cosmologists, the universe began as a quantum fluctuation in the limitless Void (Hartle-Hawking hypothesis). The universe remained as a huge quantum superposition of all possible states until the first primordial mind observed it, causing it to collapse into one actuality. This fascinating theory is discussed in
The Participatory Anthropic Principle.
- Sean Robsville
REFS
[LINDLEY 1997a] Lindley, David, Where does the Weirdness Go? page 39ff. (London: Vintage 1997, ISBN 0 09 974751 0 )
[LINDLEY 1997b] ibid, page 8 ff.
[BUCHANAN 1997] Buchanan, Mark , Light's Spooky Connections Set Distance Record , New Scientist, 28 June 1997, p 16.
[PENROSE 1990a] Penrose, Roger, The Emperor's New Mind, page 385 (London: Vintage, 1990, ISBN 0 09 977170 5)
[PENROSE 1990b] ibid page 299ff}
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The Sleep of Reason brings forth Monsters
