The Measurement Problem

It is fair to say that quantum theory is not exactly intuitive. Perhaps those who understand it well may argue that it’s actually more intuitive than the classical approach, but for the regular person, the concepts and ideas encompassed by quantum theory feel far from natural. There is a problem which plagues quantum theory, as it is a fundamental phenomena of the theory, and yet its reason for occurring is not understood. It’s called “the measurement problem”, and that’s what I’m now going to explore.

The Uncertainty Principle

In classical physics (non quantum), there are no problems with measurements. It is simply the observation of what is the case. At a point in time, a given particle has a definite position and velocity, which we can measure with little difficulty. However, when we get down to the quantum level, we can no longer makes these definite measurements. What’s happening becomes quite blurry, as we can’t see it well. It’s like if you put on some out of focus glasses, and I asked you to determine the speed and location of a rolling ball in front of you. Making the glasses even more out of focus, would make it increasingly harder to determine these properties with accuracy. In other words, the uncertainty in your measurements increases.

You may have heard of a German theoretical physicist called Werner Heisenberg. He managed to quantify this idea of uncertainty into his famous “Heisenberg’s Uncertainty Principle” (see the formula below), which states that the product of the uncertainty in the position and the uncertainty in the momentum must be greater than or equal to h-bar over 2, where h-bar is a simpler way of writing h over 2 pi, where h is Planck’s constant. Now if maths isn’t your thing, that may be a lot to take in. But all you really need to know, is that the value of h-bar over 2 is incredibly small (on the scale of 10^-34), which is why we don’t notice the effects of the uncertainty principle in daily life, but we do on a very small scale.

Given that there is this level of uncertainty that we cannot break, it means a range of things could be happening in that small area which we cannot see. Consequently, we cannot say exactly what is happening, or what is going to happen. Where classical physics is deterministic in nature, quantum physics is indeterminate. However, we can assign probabilities which describe the likelihood of certain occurrences. Think of rolling a dice. You cannot say exactly what it will land on, but you can say that (for an ideal dice) there is a 1/6 probability of it landing on a six, or a two, or any one of the sides.

The Superposition Principle

Another important principle within quantum theory is “The Superposition Principle”. It essentially holds together all of these alternative probabilities, right until the last moment where one arises as the actuality on that occasion. This is often described as the collapse of the wavepacket (which come from the concept that everything has a wave function which describes all of its different probabilities). For example, with an electron, when we do not see it it has a probability of being “here”, “there” and “everywhere”, but when an experiment is conducted to detect it, on that occasion, it turns up “here”, and all the probabilities collapse to this one actuality. A more famous example, is Erwin Schrödinger’s cat. A cat is placed in a box with a radioactive substance and the box is closed. The substance has a 1/2 probability of decaying in the next few minutes. If it decays, it causes a mechanism to release a toxic gas which kills the cat. After the few minutes, is the cat dead or alive? According to quantum theory, it is both dead AND alive, until the moment we observe it, where one option prevails and it becomes dead OR alive. The obvious question is, how is this decided? What chooses which probability prevails? This is the measurement problem.

Proposals

The measurement problem has not been solved, but there have been many proposals which attempt to explain it. Some believe things on the quantum scale are still determinate, and a definite result can be measured, and even predicted (like in classical physics), and the apparent probabilities of events are simply as a consequence of our ignorance to certain details. Others believe that the answers lie in physics that has yet to be discovered. And some notable physicists say that the reason for certain probabilities prevailing is irrelevant, and that as humans we only need to worry about what does happen, not why. As you might expect, some of these proposals are quite controversial.

Although, there have been a few other more exciting ideas. A very popular one is the “many worlds” theory. This is where in fact, the wave function does not simply collapse to one event, and that is what occurs. But rather, ALL the events do occur. The difference is that we no longer have a collapsing wave packet, but instead we have the branching of events into different, separate realities. So the cat is both dead and alive, and when we open the lid of the box, the realities split. There is one reality where the cat is dead, and one where it is alive, and we witness only one of these realities. This is basically the concept of parallel universes!

Another proposal is the effect of consciousness on the situation. The difference between the moment before a measurement, where there are a range of possibilities, and after the measurement, where one has prevailed, is that a human has witnessed it. This single different factor has led to some physicists proposing that consciousness can act almost as a force which has an effect on a system, and can change it. Consciousness is the experience of the interface between the material and the mental. Drugs clearly show that the material can affect the mental, so perhaps the mental could also affect the material?

All the proposals that attempt to explain the measurement problem give rise to more questions than answers, and thus, none have been proven correct. In the same way the photoelectric effect could not be explained until Einstein came along, perhaps the quantum measurement problem will remain unsolved for many decades, until a genius looks at it in a totally unorthodox way, and adds a new layer of knowledge to humanity’s understanding of the universe.

Originally published at http://thephysicsfootprint.wordpress.com on January 9, 2022.