“You’ll see me there,” said the Cat, and vanished.
Alice was not much surprised at this, she was getting
so well used to queer things happening.
—LEWIS CARROLL, Alice in Wonderland
IN THE 1980s a new generation of high-speed computers will appear with switching devices in the electronic components which are so small they are approaching the molecular microworld in size. Old computers were subject to “hard errors”—a malfunction of a part, like a circuit burning out or a broken wire, which had to be replaced before the computer could work properly. But the new computers are subject to a qualitatively different kind of malfunction called “soft errors” in which a tiny switch fails during only one operation—the next time it works fine again. Engineers cannot repair computers for this kind of malfunction because nothing is actually broken.
What causes the soft errors? They occur because a moderately high-energy quantum particle may fly through one of the microscopic switches, causing it to malfunction—the computer switches are so tiny they are influenced by these particles which don’t disturb larger electronic components. The source of these quantum particles is the natural radioactivity in the material out of which the microchips are made or cosmic rays raining down on earth. The soft errors are part of the indeterminate universe; their location and effect are completely random. Could the God that plays dice trigger a nuclear holocaust by a random error in a military computer? By shielding the new computers and reducing their natural radioactivity the probability for such an event can be made extremely small. But this example raises the question of whether the quantum weirdness of the microscopic world can creep into our macroscopic world and influence us. Can quantum indeterminacy affect our lives?
The answer is yes—as the example of the soft errors in computers shows. Another example is the random combining of DNA molecules at the moment of conception of a child, in which quantum features of the chemical bond play a role. Atomic events which are completely unpredictable deeply influence our lives—we are in the hands of the God that plays dice.
Unquestionably, quantum indeterminacy can influence our lives. But now a puzzle arises if we think about the implications of the two-hole experiment. The standard Copenhagen interpretation of this experiment showed that indeterminacy—Bern’s waves of probability—meant that we had to renounce the objectivity of the world, the idea that the world exists independent of our observing it. For example, the electron exists as a real particle at a point in space only if we observe it directly. The puzzle is that if indeterminacy implies nonobjectivity and if the macroscopic human world is influenced by indeterminant events, does this mean that human-scale events lack objectivity—that they exist only if we observe them directly? Do we have to renounce the objectivity not only of an electron passing through a hole but also of the annihilation of the entire human species?
Remarkably, if we adhere strictly to the Copenhagen interpretation of the quantum theory, then the quantum world’s weirdness can creep out into everyday reality—the whole world, not just the atomic world, loses objectivity. Erwin Schrödinger devised a clever thought experiment he called the cat-in-the-box to show how crazy the Copenhagen interpretation really was and that it required the whole world to possess quantum weirdness. Unfortunately his intent in this experiment, which was to criticize the Copenhagen interpretation, has been more often misunderstood than understood. Some people who want to see the weird reality of the quanta manifested in the ordinary world have used Schrödinger’s experiment to show that this must be so. But they are mistaken. Mathematical physicists have carefully analyzed the cat-in-the-box experiment, especially the physical nature of observation, and arrived at the conclusion that although the macroworld is indeterminate it need not be nonobjective, unlike the microworld. To understand how this is possible, we will first describe a version of Schrödinger’s cat-in-the-box experiment and see how indeed it seems to imply the end of the ordinary world’s objectivity. Then we analyze more closely the physical act of observation and arrive at the alternate view that we need not apply the Copenhagen interpretation to the macroscopic world—quantum weirdness is only in the microworld.
Schrödinger suggested that we imagine that a cat is sealed in a box along with a weak radioactive source and a detector of radioactive particles. The detector is turned on only once for one minute; let us suppose that the probability that the radioactive source will emit a detectable particle during this minute is one out of two = 1/2. Quantum theory does not predict the detection of this radioactive event; it only gives the probability as 1/2. If a particle is detected, a poison gas is released in the box and kills the cat. The well-sealed box is far away on an earth satellite, so we don’t know if the cat is alive or dead.
According to the strict Copenhagen interpretation, even after the crucial minute has passed we cannot speak of the cat as in a definite state—alive or dead—because as earthbound people we have not actually observed if the cat is alive or dead. A way of describing the situation is to assign a probability wave to the physical state of a dead cat and another probability wave to the physical state of the live cat. The cat-in-the-box is then correctly described as a wave superposition state consisting of an equal measure of the wave for the live cat and the wave for the dead cat. This superposition state for the cat in the box is characterized not by actualities but by probabilities—macroscopic quantum weirdness! It is as meaningless to talk about the cat’s being alive or dead as it is to talk about which hole the electrons go through in the two-hole experiment. The statement “The electron goes either through hole 1 or hole 2” is also meaningless. The electron, if you do not observe which hole it goes through, exists in a superposition state of equal measure of a probability wave for going through hole 1 and through hole 2. Maybe you can accept that weirdness for electrons. But here we have the same kind of statement, “The cat either is dead or the cat is alive,” for a cat, not an electron. Cats, like electrons, can be in a quantum never-never land.
Now let us suppose that a space shuttle with a group of scientists goes out to examine the contents of the orbiting cat-in-the-box and when they open the box they are greeted with a loud meow-the cat is alive. The Copenhagen interpretation of this event is that the scientists by opening the box and performing an observation have now put the cat into a definite quantum state—the live cat. This is analogous to examining with light beams the location of the electron at hole 1 or hole 2. For the scientists in the space shuttle, the state of the cat is no longer a superposition of waves for live and dead cat. But because their telecommunications system has broken down the scientists back on earth don’t yet know if the cat is alive or dead. For these earthbound scientists, the cat-in-the-box plus the scientists on board the space shuttle who now know the state of the cat are all still in a probability wave superposition state of live cat and dead cat. The quantum never-never land of the superposition state is getting bigger.
Finally, the scientists on board the space shuttle manage to open a communication link to a computer down on earth. They communicate the information that the cat is alive to the computer, and this is stored in its magnetic memory. After the computer receives the information but before its memory is read by the earthbound scientists, the computer is part of the superposition state for the earthbound scientists. Finally in reading the computer output the earthbound scientists reduce the superposition state to a definite one. Then they tell their friends in the next room, and so on. Reality springs into being only when we observe it. Otherwise it exists in a superposition state like the electron going through the holes. Even the reality of the macroscopic world does not have objectivity until we observe it according to this scenario.
Weird as it seems, this is the standard Copenhagen interpretation of reality. We see that it requires a definite line between the observed and the observer, a split between object and mind. At first this line was between the cat-in-the-box and the space-shuttle scientists. After they opened the box the line moved to between the space-shuttle scientists and the computer, and so on. As information about the state of the cat propagated from place to place, so did the objective reality of the live cat. The Copenhagen interpretation demands that a distinction be made between the observer and observed; it does not say where the line between them is drawn, only that it must be drawn.
Something unsettles us in this account of the cat-in-the-box experiment. Somehow we may feel that the microworld of atoms lacks standard objectivity. But should this weirdness get out into the ordinary world of tables, chairs, and cats? Do they exist in a definite state only if we observe them as the Copenhagen interpretation would have it? The analysis of the cat-in-the-box experiment suggests that an observation requires consciousness. Some physicists are of the opinion that the Copenhagen view actually implies that consciousness must exist—the idea of material reality without consciousness is unthinkable. But if we examine closely what an observation is, we find that this extreme view of reality —that tables, chairs, and cats lack definite existence until observed by a consciousness—need not be maintained. The Copenhagen view, while necessary for the atomic world, does not have to be applied to the world of ordinary objects. Those who do apply it to the macroworld do so gratuitously. Let us now examine what actually happens when we observe.
If we observe something, our eyes are receiving energy from that object. But the important feature of an observation is that we obtain information—we know something about the world we didn’t know before the observation. In our study of statistical mechanics we learned it is not possible to obtain information without increasing entropy—the measure of disorganization of physical systems. The price we pay for obtaining information is scrambling up the world somewhere else, thus increasing entropy —an inevitable consequence of the second law of thermodynamics. This increase of entropy implies that time has an arrow —there is temporal irreversibility and physical processes exist which can store information; memory is possible. We conclude that irreversibility in time is the principal feature of observation, not consciousness of the observation, although that, of course, also entails irreversibility because it involves memory. Observations can be carried out by dumb machines or computers, provided they have some primitive memory storage. The main point in this analysis of observation is that once information about the quantum world is irreversibly in the macroscopic world, we can safely attribute objective significance to it—it can’t slip back into the quantum never-never land.
In the cat-in-the-box experiment, the information is part of the macroscopic world once the cat is dead or live irrespective of whether or not you actually observe the cat. You can’t erase that information, because death is irreversible. For the two-hole experiment, by contrast, the information as to which hole the electrons pass through becomes part of the macroworld only if we set up light beams for observation. The electron, in contrast to the cat, cannot carry any record or memory of what state it is in—which hole it goes through.
Recall the line we drew in the zoom-lens sequence of a smoking pipe between the microworld of smoke particles and the macroworld of recognizable objects. The irreversibility of time came about because we sacrificed specific information about individual particles in favor of relevant averages. This is also what we are doing when an observation is performed such as with the light beams at the two holes. The detailed knowledge of the individual probability waves that describe the electron is being reduced to a specific one. The line between the macroworld and microworld is the same as the line between the observer and the observed. By examining where an irreversible interaction corresponding to an observation has been made we can in most cases draw the line between quantum weirdness and the macroscopic world quite close to atomic phenomena. We conclude that while it is consistent to talk about the quantum weirdness of waves of probability superimposing in the macroworld as we did in the description of the Schrödinger’s cat experiment, we are not compelled to do so.
The two-hole experiment and Schrödinger’s cat-in-the-box are thought experiments—way stations on the road to quantum reality along which we are traveling. We have learned that quantum theory implies an indeterminate universe not only on the atomic level but also on the level of human events. The Copenhagen interpretation of the two-hole experiment then goes on to imply that we must renounce classical objectivity for quantum particles. If we apply this same interpretation to the cat-in-the-box experiment it seems we must also renounce the objectivity of our familiar world of tables and chairs. But that is carrying the Copenhagen interpretation too far. From our study of the second law of thermodynamics we saw that the difference between the microworld and macroworld was not simply quantitative—a difference in size—but qualitative—time’s irreversible arrow which is evident in the macroworld does not exist in the microworld. We find, in fact, the irreversibility of observation means that the world of electrons and atoms is qualitatively different from the world of tables and chairs. Quantum weirdness does not exist for the macroworld. We must superimpose probability waves for an electron to go through holes 1 and 2, but we need not superimpose live and dead cats.
As we continue along the road to quantum reality, we see other way stations along the road—traveler’s inns where alternatives to the Copenhagen interpretation of quantum weirdness are offered as food for thought. After finishing lunch in one inn we meet a storyteller who begins his story—a quantum mechanical fairy tale.