Copenhagen Interpretation of Quantum Mechanics

From the Lecture Series: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

Quantum mechanics was a complex world from the beginning, and many scientists tried to make it more understandable. The Copenhagen interpretation was one of these successful efforts. It started by distinguishing the classical and quantum worlds, and also reality and knowledge, pushing the misconceptions of philosophy and religion away from the science. Read on to find out where it came from and what it did.

Quantum physics type written on a vintage paper.
Quantum mechanics has different interpretations that have tried to make it more tangible. The Copenhagen interpretation is one of the best ones to date. (Image: RAGMA IMAGES/Shutterstock)

Quantum mechanics is a complex and confusing branch of physics, where predictions are not that common or possible. Of course, Schrodinger’s equation was a successful effort in predicting what an electron does, but there is no certainty involved in the whole realm. Thus, there are many misleading writings built upon misunderstood concepts of quantum.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on Wondrium.

Quantum Vs. Classical

In classical views, everything is deterministic. For example, when a ball is thrown up, it can be predicted how and where it will come back down. This is basically every experience in daily life. Even solar eclipses can be predicted with an accuracy of seconds, thousands of years in advance.

In the classical world, knowing the rules of a system of objects and their initial position and velocity makes predicting their motion at any time in the future possible. This is never the case in the quantum realm with its probabilistic nature.

Learn more about untangling how quantum mechanics works.

Schrodinger Equation and the Wave Function

Schrodinger equation uses mathematics to predict the probability of motion and position for an electron. Its answer can be an infinite number of functions, but one of the most famous ones is the wave function. It is another mathematical function that describes what can happen in the future.

Concept of quantum physics duality of a photon
The wave function explains the probability of finding an electron or photon in a state, but it is not considered real based on the Copenhagen Interpretation. (Image: plotplot/Shutterstock)

The main difference is that in the classical world, predictions are deterministic, but in the quantum world, only the probability of an outcome is predicted. In other words, there are two types of truths involved here, and, surprisingly, the probabilistic one is the foundation of the deterministic one.

The Birth of the Copenhagen Interpretation

The mathematics of the quantum world works fine, but the physical meanings are very confusing. For example, the wave function is acceptable in terms of mathematics, but its meaning in physics is not as easy to get. When the actual position of an electron is determined, and it is spotted in a position, the wave function is said to collapse.

The collapse means when the electron is 100% in one position, the probability that it is in other positions at that time is zero. Still, it is a debatable topic as to what exactly happens when it collapses, and that is why it led to many misconceptions.

In 1926 and 1927, Werner Heisenberg was a university lecturer and assistant to professor Niels Bohr. Bohr and Heisenberg were also struggling with the physical meaning of the Schrödinger wave equation. Eventually, they developed the Copenhagen interpretation for understanding quantum mechanics.

Learn more about what the world gets wrong about science.

Bases of the Copenhagen Interpretation

The first foundation of the Copenhagen interpretation was drawing clear lines between the classical and quantum worlds. They dismissed the wave function as a real thing, as real things belonged to the classical world where observation is triggered. They believed it was completely meaningless to ask what was going on before the observation.

Many people, including Einstein, did not like the standpoint where everything depended on observation to be real. Still, the Copenhagen interpretation works much better than many others. They use the terms ontology and epistemology to refer to things that really exist and things that can be known about something.

Here, the wave function is an epistemology since it is knowledge, not what exists as real things do.

The Two States

Another aspect of the Copenhagen interpretation is looking at quantum situations that can have two different states, for example flipping a coin. The wave function can give the probability of finding the coin in each state, i.e., heads or tails. In the Copenhagen interpretation, before the observation, the coin actually is in both states simultaneously.

When the coin flips and an observation is made, the wave function collapses into either heads or tails. Here, one can ask about the nature of observation and how it happens. Schrodinger Cat is a good experiment to explain with.

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Schrodinger Cat

The famous Schrodinger Cat is a hypothetical situation, where a cat is put in a box with a decaying radioactive atom. Once the atom decays, the Geiger counter will click, and the bottle of cyanide will break. Thus, the cat dies.

Illustration of Erwin Schroedinger's (or Schroedinger) thought experiment, where the cat is both alive and dead due to interpretations of quantum mechanics and state known as a quantum superposition.
The Schrodinger Cat is one of the most famous examples for understanding the quantum world and explains the role of observation in reality. (Image: local_doctor/Shutterstock)

The wave function and the Copenhagen interpretation see the situation differently from the classical either/or. The wave function starts with what we know for sure: the cat, the atom, and the bottle are all put in the box. Hence, in the beginning, the wave function says with a 100% probability that the bottle is intact and the cat is alive.

If the half-life of this radioactive material is 10 minutes, after 10 minutes, there is a 50% chance that the atom has decayed, and the cat is dead. Before 10 minutes, the probability is lower, and after that, it is higher. Until somebody opens the box and makes an observation, the wave function has not collapsed, and the cat is simultaneously alive and dead, in terms of probabilities.

This is how the Copenhagen interpretation tried to make distinctions between realities and knowledge, to make quantum mechanics more understandable.

Common Questions about Copenhagen Interpretation

Q: What is the Copenhagen interpretation in simple terms?

The Copenhagen interpretation explains that a quantum particle does not exist in one state or another, but in all of its possible states at the same time. Observation is needed to collapse the wave function and see the reality of the state.

Q: What is Schrodinger’s cat?

Schrodinger Cat is a hypothetical experiment that, using the Copenhagen interpretation, shows how quantum mechanics works in terms of realities and probabilities. The probabilities remain probable until an observation is made, and the situation turns real.

Q: Does the Copenhagen interpretation say things do not exist when not observed?

No. The Copenhagen interpretation describes the role of observation in reality, not the existence of things. All probabilities that the wave function determines remain valid until an observation reveals the reality.

Q: Is the wave function real?

According to the Copenhagen interpretation of quantum mechanics, the wave function is not real as it does not meet the requirements of being real. It is categorized as knowledge, not reality.

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