To make sure, we do the experiment again, but this time we leave both of the slits open. Just as we thought, there are no photons recorded now in the area where the photon hit in our first experiment. When both slits are open and interference is present, this area is in the middle of a dark band.
The question is, How did the photon in the first experiment know that the second slit was not open? Think about it. If both slits are open, there are always alternating bands of illuminated and dark areas. This means that there are always areas where the photons never go (otherwise there would not be any dark areas). If one of the slits is closed, there is no interference and the dark bands disappear; the whole wall becomes illuminated, including those areas which previously were dark when both slits were open.
When we fired our photon and it went through the first slit, how did it “know” that it could go to an area that must be dark if the other slit were open? In other words, how did the photon know that the other slit was closed?
“The central mystery of quantum theory,” wrote Henry Stapp, is ‘How does information get around so quick?’” How does the particle know that there are two slits? How does the information about what is happening everywhere else get collected to determine what is likely to happen here?3
There is no definitive answer to this question. Some physicists, like E. H. Walker, speculate that photons may be conscious!
Consciousness may be associated with all quantum mechanical processes…since everything that occurs is ultimately the result of one or more quantum mechanical events, the universe is “inhabited” by an almost unlimited number of rather discrete conscious, usually nonthinking entities that are responsible for the detailed working of the universe.4
Whether Walker is correct or not, it appears that if there really are photons (and the photoelectric effect “proves” that there are), then it also appears that the photons in the double-slit experiment somehow “know” whether or not both slits are open and that they act accordingly.*
This brings us back to where we started: Something is “organic” if it has the ability to process information and to act accordingly. We have little choice but to acknowledge that photons, which are energy, do appear to process information and to act accordingly, and that therefore, strange as it may sound, they seem to be organic. Since we are also organic, there is a possibility that by studying photons (and other energy quanta) we may learn something about us.
The wave-particle duality was the end of the line for classical causality. According to that way of thinking, if we know certain initial conditions, we can predict the future of events because we know the laws that govern them. In double-slit experiments we know all that we can know about initial conditions and we still can’t predict correctly what happens to single photons.
In experiment one, for example (only one slit open), we know the origin of the photon (the lamp), its velocity (186,000 miles per second), and its direction just prior to passing through the open slit. Using Newton’s laws of motion, we can predict where the photon will land on the photographic plate. Let us suppose that we make these calculations.
Now let us consider experiment two (both slits open). Again we know the origin of the photon, it velocity, and its direction just prior to passing through the open slit. The initial conditions of the photon in experiment one are the same as those of the photon in experiment two. They both start from the same place, travel at the same speed, go to the same place, and therefore, are moving in the same direction just prior to passing through slit number one. The only difference is that in the second experiment, the second slit also is open. Again, using Newton’s laws of motion, let us calculate where the photon will land on the photographic plate.
Since we used the same figures and the same formulas in both cases, we get identical answers indicating that the photon in experiment one will impact in exactly the same place as the photon in experiment two. That is the problem. The photon in experiment two will not impact in the same area as the photon in experiment one because the photon in experiment one landed in an area that is a dark band in experiment two. In other words, the two photons do not go to the same place even though the initial conditions pertaining to both of them are identical and known to us.
We cannot determine the paths of individual photons. We can determine what the wave pattern on the wall will be, but in this case we are interested in a single photon, not waves of them. In other words, we know the pattern that large groups of photons will make, and their distribution in the pattern, but we have no way of knowing which photons will go where. All that we can say about a single photon is the probability of finding it in a given place.
The wave-particle duality was (is) one of the thorniest problems in quantum mechanics. Physicists like to have tidy theories which explain everything, and if they are not able to do that, they like to have tidy theories about why they can’t. The wave-particle duality is not a tidy situation. In fact, its untidiness has forced physicists into radical new ways of perceiving physical reality. These new perceptual frames are considerably more compatible with the nature of personal experience than were the old.
For most of us, life is seldom black and white. The wave-particle duality marked the end of the “Either-Or” way of looking at the world. Physicists no longer could accept the proposition that light is either a particle or a wave because they had “proved” to themselves that it was both, depending on how they looked at it.
Of course, Einstein was aware of the fact that his photon theory contradicted Young’s wave theory without disproving it. He speculated that photons were guided by “ghost waves.” Ghost waves were mathematical entities which had no actual existence. The photons seemed to follow paths which had all the mathematical characteristics of waves, but which in reality did not exist. Some physicists still view the wave-particle paradox this way, but for most physicists, this explanation seems too contrived. It is an answer which appears to make sense, but somehow doesn’t explain anything.
The wave-particle duality prompted the first real step in understanding the newly unfolding quantum theory. In 1924, Bohr and two of his colleagues, H. A. Kramers and John Slater, suggested that the waves in question were probability waves. Probability waves were mathematical entities by which physicists could predict the probability of certain events occurring or not occurring. Their mathematics did not prove correct, but their idea, which was unlike anything that had been proposed before, was sound. Later, with a different formalism (mathematical structure), the idea of probability waves developed into one of the distinguishing characteristics of quantum mechanics.
Probability waves, as Bohr, Kramers, and Slater thought of them, was an entirely new idea. Probability itself was not new, but this type of probability was. It referred to what somehow already was happening, but had not yet been actualized. It referred to a tendency to happen, a tendency that in an undefined way existed of itself, even if it never became an event. Probability waves were mathematical catalogues of these tendencies.
This was something quite different from classical probability. If we throw a die in a casino, we know, using classical probability, that the chances of getting the number that we want is one in six. The probability wave of Bohr, Kramers, and Slater meant much more than that.
According to Heisenberg:
It meant a tendency for something. It was a quantitative version of the old concept of “potentia” in Aristotelian philosophy. It introduced something standing in the middle between the idea of an event and the actual event, a strange kind of physical reality just in the middle between possibility and reality.5
By 1924, Planck’s discovery of the quantum was producing seismic effects in physics. It enabled Einstein to discover the photon, which caused the wave-particle duality, which led to probability waves. The physics of Newton was a thing of the past.
Physicists found themselves dealing with energy that somehow processed information (which made it organic), and unaccountably presented itself in pattern
s (waves). In short, physicists found themselves dealing with Wu Li—patterns of organic energy.
1
What Happens
Quantum mechanics is a procedure. It is a specific way of looking at a specific part of reality. The only people who use it are physicists. The advantage of following the procedure of quantum mechanics is that it allows us to predict the probabilities of certain results provided our experiment is performed in a certain way. The purpose of quantum mechanics is not to predict what actually will happen, but only to predict the probabilities of various possible results. Physicists would like to be able to predict subatomic events more accurately, but, at present, quantum mechanics is the only workable theory of subatomic phenomena that they have been able to construct.
Probabilities follow deterministic laws in the same way that macroscopic events follow deterministic laws. There is a direct parallel. If we know enough about the initial conditions of an experiment, we can calculate, using rigid laws of development, exactly what the probability is for a certain result to occur.
For example, there is no way that we can calculate where a single photon in a double-slit experiment will strike the photographic plate. However, we can calculate with precision the probability that it will strike it at a certain place, provided that the experiment has been prepared properly and that the results are measured properly.
Suppose that we calculate a 60 percent probability for the photon to land in area A. Does that mean that it can land somewhere else? Yes. In fact, there is a 40 percent probability that it will.
In that case (asking the question for Jim de Wit), what determines where the photon will land? The answer given by quantum theory: pure chance.
This pure-chance aspect was another objection that Einstein had about quantum mechanics. It is one of the reasons that he never accepted it as the fundamental physical theory. “Quantum mechanics is very impressive,” he wrote in a letter to Max Born, “…but I am convinced that God does not play dice.”1
Two generations later, J. S. Bell, a Scotish physicist, proved that he may have been right, but that is another story, which we will come to later.
The first step in the procedure of quantum mechanics is to prepare a physical system (the experimental apparatus) according to certain specifications, in an area called the region of preparation.
The second step in the procedure of quantum mechanics is to prepare another physical system to measure the results of the experiment. This measuring system is located in an area called the region of measurement. Ideally, the region of measurement is far away from the region of preparation. Of course, to a subatomic particle, even a small macroscopic distance is a long way.
Now let us perform the double-slit experiment using this procedure. First, we set a light source on a table, and then, a short distance away, we place a screen with two vertical slits in it. The area where these apparatuses are located is the region of preparation. Next we fix an unexposed photographic plate on the opposite side of the screen from the light source. This area is the region of measurement.
The third step in the procedure of quantum mechanics is to translate what we know about the apparatus in the region of preparation (the light and the screen) into mathematical terms which represent it, and to do likewise for the apparatus that is located in the region of measurement (the photographic plate).
To do this we need to know the specifications of the apparatus. In practice, this means that we give the technician who sets up the equipment precise instructions. We tell him, for example, the exact distance to place the double-slit screen from the light source, the frequency and intensity of the light that we will use, the dimensions of the two slits and their position relative to each other and to the light source, etc. We also give him equally explicit instructions concerning the measuring apparatus, such as where to put it, the type of photographic film that we will use, how to develop it, etc.
After we translate these specifications of the experimental arrangement into the mathematical language of quantum theory, we feed these mathematical quantities into an equation that expresses the form of natural causal development. Notice that this last sentence doesn’t say anything about what is developing. That is because nobody knows. The Copenhagen Interpretation of Quantum Mechanics says that quantum theory is a complete theory because it works (correlates experience) in every possible experimental situation, not because it explains in detail what is going on.* (Einstein’s complaint was that quantum theory doesn’t fully explain things because it deals with group behavior and not with individual events.)
However, when it comes to predicting group behavior, quantum theory works as advertised. In a double-slit experiment, for example, it can predict exactly the probabilities of a photon being recorded in region A, in region B, in region C, and so forth.
Of course, the last step in the procedure of quantum mechanics is actually to do the experiment and get a result.
To apply quantum theory, the physical world must be divided into two parts. These parts are the observed system and the observing system. The observed system and the observing system are not the same as the region of preparation and the region of measurement. “Region of preparation” and “region of measurement” are terms which describe the physical organization of the experimental apparatus. “Observed system” and “observing system” are terms which pertain to the way that physicists analyze the experiment. (The “observed” system, by the way, cannot be observed until it interacts with the observing system, and even then all that we can observe are its effects on a measuring device.)
The observed system in the double-slit experiment is a photon. It is pictured as traveling between the region of preparation and the region of measurement. The observing system in all quantum mechanical experiments is the environment which surrounds the observed system—including the physicists who are studying the experiment. While the observed system is traveling undisturbed (“propagating in isolation”), it develops according to a natural causal law. This law of causal development is called the Schrödinger wave equation. The information that we put into the Schrödinger wave equation is the data about the experimental apparatuses that we have transcribed into the mathematical language of quantum theory.
Each set of these experimental specifications that we transcribed into the mathematical language of quantum theory corresponds to what physicists call an “observable.” Observables are the features of the experiment and nature that are considered to be fixed, or determined, when and if the experimental specifications that we have transcribed actually are met. We may have transcribed into mathematical language several experimental specifications for the region of measurement, each one corresponding to a different possible result (the possibility that the photon will land in region A, the possibility that the photon will land in region B, the possibility that the photon will land in region C, etc.).
In the world of mathematics, the experimental specifications of each of these possible situations in the region of measurement and in the region of preparation corresponds to an observable.* In the world of experience, an observable is the possible occurrence (coming into our experience) of one of these sets of specifications.
In other words, what happens to the observed system between the region of preparation and the region of measurement is expressed mathematically as a correlation between two observables (production and detection). Yet we know that the observed system is a particle—a photon. Said another way, the photon is a relationship between two observables. This is a long, long way from the building-brick concept of elementary particles. For centuries scientists have tried to reduce reality to indivisible entities. Imagine how surprising and frustrating it is for them to come so close (a photon is very “elementary”), only to discover that elementary particles don’t have an existence of their own!
As Stapp wrote for the Atomic Energy Commission:
…an elementary particle is not an independently existing, unanalyzable entity. It is, in essence, a
set of relationships that reach outward to other things.2
Furthermore, the mathematical picture which physicists have constructed of this “set of relationships” is very similar to the mathematical picture of a real (physical) moving particle.* The motion of such a set of relationships is governed by exactly the same equation which governs the motion of a real moving particle.
Wrote Stapp:
A long-range correlation between observables has the interesting property that the equation of motion which governs the propagation of this effect is precisely the equation of motion of a freely moving particle.3
Things are not “correlated” in nature. In nature, things are as they are. Period. “Correlation” is a concept which we use to describe connections which we perceive. There is no word, “correlation,” apart from people. There is no concept, “correlation,” apart from people. This is because only people use words and concepts.
“Correlation” is a concept. Subatomic particles are correlations. If we weren’t here to make them, there would not be any concepts, including the concept of “correlation.” In short, if we weren’t here to make them, there wouldn’t be any particles!*
Quantum mechanics is based on the development in isolation of an observed system. “Development in isolation” refers to the isolation that we create by separating the region of preparation from the region of measurement. We call this situation “isolation,” but in reality, nothing is completely isolated, except, perhaps, the universe as a whole. (What would it be isolated from?)
The “isolation” that we create is an idealization, and one point of view is that quantum mechanics allows us to idealize a photon from the fundamental unbroken unity so that we can study it. In fact, a “photon” seems to become isolated from the fundamental unbroken unity because we are studying it.