Outer Limits of Reason (40 page)

We are not done yet. A large polarization filter can be placed between the other polarization filters and the screen, as in
figure 7.21
. This polarization filter is set in the diagonal direction.

Let us see what happens as the photon goes on its journey. If it goes through the top slit, it will pass through the horizontal filter and come out horizontal. It will then have to go through the diagonal filter, and if it comes out, it will come out diagonal. Similarly, if the photon goes through the bottom slit, it will also go through the vertical filter, and if it passes through the diagonal filter, it will again come out diagonal. Either way, the diagonal filter “erases” the tagging information of which slit the photon passed through. Without that information or our ability to get that information, the photon reverts to its superposition status and will interfere with itself. Amazingly enough, this is exactly what happens when the experiments are carried out: there is an interference pattern.

Already we have an extremely interesting scenario. When the photon approaches the slits, it has to determine if it should go through one or both slits. This depends on whether there will be a diagonal polarization filter on the right side of the slits. Somehow the photon “knows” what it will find on the other side of the slits. If the filter is not there, it will go through one of the slits and if it is there, it will go through both slits. How does the photon know what will be on the other side of the barrier? This again conforms to the Wholeness Postulate. Here we see that the outcome depends on the setup of the
entire
experiment, including whether there is an eraser filter on the right side of the barrier.

Physicists take this experiment one step further with something called a
delayed-choice quantum erasure
. Imagine that the diagonal polarization filter is far away from the slits and is on rollers so that it can be moved away from the screen quickly.
²⁴
To recap: if we leave the eraser in place we will get an interference pattern and if we take the eraser away there will be no interference. However, the experiment can be set up so that the eraser can be left in place or pulled away after we know that the photon has passed through the slits in the barrier. The diagonal polarization filter can be in place and we can let the photon go. Because the filter is in place, we know that the photon will go into a superposition and will go through both slits. Once it has passed through the slits, we can then pull the diagonal filter away and the photon will be in a position and not form an interference pattern. In contrast, if the diagonal polarization filter is not in place, then the photon will be in a position and go through one of the slits. Once the single photon has passed through the single slit, we can push the filter back in place. Then the photon somehow goes back into a superposition and creates interference.

There are two crazy ways of looking at this: (a) After the photons pass through the slits, by moving the diagonal polarization filter the experimenter is changing what the photons did in the past before they got to the slits. Or (b) somehow before the photons come to the slits, they “know” whether the observer will pull away the filter. In short, either (a) the experimenter changes the past or (b) the photon “knows” the future. Both options are mindboggling.

It is hard to understand what it means to change the past. Such a concept violates our notions of cause and effect and all of science. In contrast, option (b), where the photon acts as if it “knows” the future, fits in well with our Wholeness Postulate. The outcome depends on the whole experiment, including what the experimenter will do while the experiment is in progress. Here
whole
emphasizes that the experiment takes place in time and the outcome depends on the experiment from start to finish. The outcome takes into account whether the experimenter is going to pull away the diagonal filter. As Yogi Berra says, “It ain't over till it's over.”

How can the photon “know” what the experimenter will do? What about the experimenter's free will?
^25
,
26
Doesn't the experimenter have free will to decide whether to pull the eraser away?
²⁷
Let us be careful with our language. A photon does not have consciousness or “know” anything. What we mean is that whatever physical law is controlling the actions of the photon must take into account all the actions of the experimenter. The reason this is amazing is that the laws that govern the actions of the photon must take into account the
future
actions of the experimenter even though such actions do not exist yet. That is, the laws controlling the actions of the photon must take into account the laws controlling the actions of the experimenter. If we are going to assert that the experimenter has free will and there is nothing controlling the actions of the experimenter, then there is nothing controlling the actions of the particle either. In other words,

human beings have free-will ⇒ particles have free-will.

Do particles really have free will? Can we believe such a thing? They do not seem to show any freewill action. There is another way of looking at this: What if the particle does not have free will and its actions are totally determined by laws? Well, then a human being also has no free will. Hence the contrapositive:

particles do not have free will ⇒ human beings do not have free will.

From a scientific perspective this is not strange at all. After all, human beings are made out of particles. Abiding by the usual dictum of reductionism, scientists would have to say that the tendency of particles to follow the habitual laws of physics implies that humans must follow the habitual laws of physics. If they believe that particles have no free will, then they are forced to believe that humans, made out of particles, also have no free will.
²⁸

Other Strange Aspects of Quantum Mechanics

There are at least three other aspects of quantum mechanics that are strange and worth mentioning. First, quantum mechanics uses the mathematics of complex numbers. Such numbers are usually written as
a + bi
, where
a
and
b
are real numbers and
i
is the imaginary square root of negative one. This seems shocking because most other physical laws use simple real numbers. After all, the measurements that we make are real numbers. A rod is 18.63 inches long. The temperature of an object is measured at 46.168 degrees Celsius. The projectile is going 265.643 miles per hour. Since we measure properties with real numbers, we expect the laws of physics to be stated with real numbers. They are not! Rather, complex numbers are used in a most fundamental way. It would be very hard to perform any calculations in quantum mechanics without complex numbers.

A second oddity that needs to be reiterated is the total nondeterminism of quantum mechanics. As mentioned in the previous section, every other part of the physical sciences is deterministic. There are formulas that describe the actions of the system. These formulas might not be computable. They might not even be known to us. Nevertheless, the systems follow rigid laws of physics. This is in stark contrast to quantum mechanics, which seems, at its heart, totally random. Why should that be?

And finally, the last strange aspect of quantum mechanics that I will discuss was actually the first one discovered by researchers. In the early twentieth century, Max Planck (1858–1947) found that certain types of energy had only discrete values. Whereas you can turn your thermostat to any value between 72.4 and 72.5, quantum mechanical systems had energy being released in certain units and could not have energy between these units. As time went on, the founders of quantum theory realized that not only did energy have this discreteness, but many other properties of quantum mechanics had this characteristic as well. They found that particles had discrete spinning states, space was discrete, and time was also discrete. Electrons jump from shell to shell but do not cover the intermediate distance. Such jumps are called “quantum leaps.”

For me, it is hard to see how these three aspects of quantum mechanics fall under the purview of the Wholeness Postulate. Perhaps they do not.

 

We have finished our little tour of quantum mechanics. What a strange trip! What have we learned?

• Properties of objects have more than one value at a time.

• There is no way to determine which value will be observed when the property is measured.

• There are pairs of properties for which there is an inherent limitation of our ability to know their values.

• Reality depends on how it is measured.

• Distant parts of our universe are strangely interconnected.

• Experimenters and their free will cannot be separated from their experiments.

What are we to make of this psychedelic world? Quantum mechanics has been around for more than a century and researchers have been busy making it more palatable. This theory has done such violence to our usual intuitions of the physical world that there is no way we can ignore it. I will highlight four major schools of interpretation.

Interpretation of Quantum Mechanics

The Copenhagen School

The most popular school of thought was developed by the founders of quantum mechanics Niels Bohr, and Werner Heisenberg. This school represents the orthodox viewpoint of most physicists and has influenced our presentation. Basically, it says that there is really no underlying physical universe. Values do not exist until a conscious observer measures the property. The value is not there beforehand, rather the measurement causes the value to come into being.

To Bohr & Co. there is no quantum world. There is only an abstract quantum mechanical description. It is wrong to think that the task of physics is to find out what nature is. Rather, physics concerns itself with what we can say about nature. The Copenhagen interpretation does not explain why quantum mechanics is nondeterministic or how the superposition collapses. In fact, they go further and say that such questions are not scientific and must be considered meaningless.

It is important to realize that the Copenhagen interpretation is not some opinion held by a few crazy people on the outskirts of scientific discussion. Rather, this interpretation is considered the mainstream view among people who study quantum mechanics. In fact, it is the other opinions that are considered unorthodox and far-fetched.

There are both positive and negative aspects to the Copenhagen interpretation. The positive side is that if all questions about the meaning of quantum mechanics are deemed illegitimate, then there are no questions. One can go on and simply work with the equations. While this is unsatisfying to me, most physicists who do not want to think about foundational issues like this freedom. They believe that all the problems of quantum mechanics are simply pseudoproblems. The physicist Murray Gell-Mann said in his Nobel Prize acceptance speech that “Niels Bohr brainwashed a whole generation of physicists into believing that the problem [of the interpretation of quantum mechanics] had been solved fifty years ago.” Good for them.

The negative aspects of the Copenhagen interpretation are obvious. Someone is, literally, a lunatic if they think that the moon is only there when people are looking at it. How can it be that a subatomic particle does not exist until it is observed, and yet we each seem to have a coherent notion of existence? Also, this interpretation does not really explain how or why consciousness brings about a collapse of a superposition. The worst part of the Copenhagen interpretation is that there is a feeling of unquestioned dogma: “This is the way it is and one should not ask about it.” There is something unsatisfying when scientists tell you there is no real explanation for phenomena and the question does not make sense.

Multiverse

The main reason superposition is so strange to us is that we have never seen a superposition. When we look at the universe we see every object in one place with only one value for every property. The measurement problem asks: Why should an object collapse simply by measuring it? Furthermore, why should it seem to randomly collapse to one position and not another? In 1955, Hugh Everett III (1930–1982), a brilliant young physicist, proposed a radical solution called the
many-worlds hypothesis
or
multiverse
. He said that when a measurement takes place, rather than a superposition collapsing to one position, the entire universe splits into many different universes where each universe has one of the possible outcomes of the measurement. For example, when Schrödinger's cat experiment is carried out, the universe splits into two. In one universe you open the box and happily find the cat alive, while in another universe you open the box and are sad and regretful about using animals in your scientific experiments. These two universes do not have any connection and are totally separate. They each contain a version of you that does not know about any other universe.