Copenhagen Interpretation
author: Mara Krause, 13.07.2025
The Copenhagen Interpretation is the fundamental feature of the quantum world. It challenges our common sense and contradicts the way we assume the world works.
Many people believe that physical properties are somehow fixed. The moon is still in the sky even when no one is looking at it, right?
The Copenhagen Interpretation questions this everyday intuition. It argues that objects don‘t have definite properties until we observe. They exist in in-between states with probabilities.
„The concept of reality is not a fixed or well-defined concept, but rather it is a complex and multifaceted notion that is shaped by our understanding of the physical world.” – Niels Bohr
Summary
- Copenhagen Interpretation emerged as a solution to the quatum crisis
- Before measured, a system is only described by its probability wave-function, existing in many quantum states simultaneously
- Observation collapses the wave-function, giving it definite properties
- Its formulation leaves key terms – like ‚observation‘ – undefined, making interpretation difficult
Background
So, when did our confusion about reality and certainties begin?
Up until the nineteenth century, Newtonian mechanics and Maxwell’s equations dominated everyday life. Classical physics seemed to explain all phenomena. It worked perfectly for our macroscopic world:
- Motion of Sun, Earth, Satellites
- Smartphones and Touchscreens
- Driving to work or school with public transportation (though punctual trains seem to defy Newton’s laws more often than not)
However, by the end of the 19th century more and more phenomena and experiments gave physicists a headache.
Quantum Breakthroughs
- 1900: Max Planck suggested that energy is quantised and exists in discrete, rather than continuous energy amounts. One of the first approaches to quantum mechanics.
- 1905: Albert Einstein explained the photoelectric effect with the photon theory of light. Classical wave theory cannot explain why electrons are released from a material when it absorbs electromagnetic radiation. Einstein assumed that light is composed of wave-particles or photons, each of energy equal to hf (Planck’s constant times frequency).
- 1913: Niels Bohr proposed a model of the atom with electrons in stationary states and discrete energy levels. For an atom to absorb radiation, the energy difference between two stationary states must be hf.
These early breakthroughs laid the foundation, but they also raised more questions than answers.
The Quantum Crisis
Many people began to wonder whether classical mechanics needed an update. But, soon they realised that an update was not enough. A whole new theory is required.
- 1924: Louis de Broglie introduced his theory of electron waves: electrons can be thought of as particles and waves. The wave-particle duality is nearly impossible for our brain to comprehend, but proven by various experiments.
- 1926: Erwin Schrödinger developed the wave-function or Schrödinger equation. This wave-function describes how the state of a system changes over time by giving probabilities rather than certainties.
- 1927: Werner Heisenberg proposed his uncertainty principle. He showed that the product of the uncertainties in position and momentum (mass x velocity) must always be bigger than Planck’s constant (h).
In classical mechanics, predicting the eclipse of the moon just required measurement of motion and calculations for a certain outcome. In quantum mechanics, predicting the trajectory of an electron suddenly required consideration of Heisenberg’s uncertainty, and the wave-particle duality, only to get a probability wave-function.
Yet, Schrödinger’s wave-function left a crucial question unanswered.
How does a possibility become reality?
After all, it is not just likely that this fly is sitting on my laptop – it is. So, what causes the wave-function to collapse into a single outcome?
The quantum revolution had begun but needed an interpretation.
Solution: Copenhagen Interpretation
Bohr and Heisenberg took on the challenge and postulated the Copenhagen Interpretation, still one of the most accepted ones of quantum mechanics:
Before a system is measured, it is described by probabilities, its wave-function. The measurement of the system changes the state and forces a single eigenvalue: the observed reality.
In other words, our observation forces the wave-function to collapse and „decide“ for one event.
John Gribbin points out in his book [1]. „In classical physics we imagine a system of interacting particles to function, like clockwork, regardless of whether or not they are observed. In quantum physics, the observer interacts with the system to such an extent that the system cannot be thought of as having independent existence.“
Double-Slit Experiment
Supporting the Copenhagen Interpretation, Claus Jönsson, in1961, performed the first double-slit experiment. His results were unbelievable. It is one thing to have a theory and build mathematical functions around it. But another to actually see that it works and what it means.
Electrons are fired, one at one, at a barrier with two narrow slits. Behind the barrier is a screen that records where they land.
When no one observes which slit the electron goes through, the screen records an interference pattern: a spatial distribution just like when waves pass through the double slit and overlap (like ripples on water). Even if we only send one electron through the double slit, it somehow goes through both, like a wave.
Okay, we already knew that electrons can be both, particle and wave. Apparently, they choose to behave like waves when passing through a double slit.
Now, you may wonder, what happens if we try to trick the electrons. We just place a detector at each slit to see which slit the electron takes.
The interference pattern vanishes. The emerging pattern is similar to how classical particles behave.
Yes, the outcome of the experiment just completely changed without changing anything in the setup except the detectors of the slits. Suddenly, the electrons behave like particles going through one random slit.
To summarise: As long as we don‘t look, electrons go through both slits simultaneously like waves. But the moment we watch, they behave like particles, changing the detected pattern.
What happens? Do they know that we observe them?
Explanation
According to the Copenhagen Interpretation the probability wave-function of the electron collapses as soon as we observe it. The act of observing a system forces it to select one of its options, which then becomes real.
- Gribbin [1] says:„Nothing is real unless we look at it, and it ceases to be real as soon as we stop looking.“
Complications with the Copenhagen Interpretation
Not only Einstein literally hated the Copenhagen Interpretation. Many physicists disapprove of it. Einstein once said: „God doesn’t play dice“. In other words, he didn’t like its randomness.
The Copenhagen Interpretation describes the world in mathematical probabilities without looking for where those probabilites originated. What does it actually mean to „collapse“ a wave-function? Where do all those states exist? The strategy of the theory seems to not ask too many questions. Or maybe or brain is cognitively not able to understand?
Also, it is formulated very vaguely. What exactly is meant with measurement or observation? Where is the measurer and does he play by the rules of quantum mechanics?
Too often, theories are accepted on authority. If the professor says that it’s true it must be, right? Simple answer: no.
It is important to question quantum concepts and not just follow professors, YouTube videos, or articles. Otherwise, the progression of physics is in danger. How do we know we are not on the wrong track without questioning? How can we drive innovation without new thoughts?
Conclusion
The Copenhagen interpretation highlights our role in the experiment. Observations change the outcome. The unobserved state of a particle isn’t one certain state, but many simultaneously.
Although its formulations are imprecise, it aligns with experimental results.
Richard Feynman summed up the situation in his lectures: „The ‚Paradox‘ is only a conflict between reality and your feeling of what reality ‚ought to be‘.“
Why necessary?
Advanced technology like semiconductors, lasers, and GPS all rely on quantum mechanics and the Copenhagen Interpretation. Only hard work and sharp discussions about quantum mechanics enable us to follow GPS and use technology as we are used to.
Not to be neglected, the Copenhagen interpretation evokes philosophical questions. To what extent can humans shape reality if nothing is fixed? Can we trust observations? Can we alter reality?
Most of the future seems determined in our everyday life. Everything follows clear rules and laws. But in quantum mechanics, there is no certainty. No fixed history. Maybe God does play the dice.
Taking it further
Do objects exist before we observe them?
Let’s take the neutrino. In 1930, Wolfgang Pauli proposed this elusive particle to conserve momentum and spin during the beta decay. A neutron transforms into a proton and emits an electron plus the something else. This „something“ was invisible, uncharged, and almost massless.
Enrico Fermi named it neutrino in 1933. But it wasn’t until 1956 that Frederick Reines and Clyde Cowan detected it.
According to the Copenhagen interpretation, a particle has no definite properties until it is measured. So… can we really say the neutrino existed before 1956? Or did it become real through the act of observation?
And what about history? When we measure a particle today, the wave-function collapses, defining not just its present – but also its past. Does that imply we are rewriting history every time we observe?
Many-Worlds Interpretation
Another interpretation of quantum mechanics is the Many-Worlds interpretation. It suggests that each time we collapse a wave-function our universe splits.
While Copenhagen limits reality to what we observe, Many-Worlds states that all possibilities exist and we are just stuck in one of them.
Interpreting the double slit experiment with this theory states that the electron also passes both simultaneously, but in different universes. When the electron reaches the double slit, the universe splits. In our universe we only measure the electron passing through one seemingly random slit. But in another, we detect it passing through the other slit.
This means for every decision you ever made, you made a different one in another universe. You might be the Noble Laureate in one of the parallel universes. Or an olympic swimmer. Or a teacher. Actually, the possibilities seem infinite.
Sources
[1] J. Gribbin, In Search of Schrödinger’s Cat: Quantum Physics and Reality, Black Swan, London (1984).
[2] IN2P3/CNRS, Neutrino History, https://neutrino-history.in2p3.fr (accessed June 30, 2025).
[3] A. Ney and D. Z. Albert (eds.), Copenhagen Interpretation of Quantum Mechanics, Stanford Encyclopedia of Philosophy, https://plato.stanford.edu/entries/qm-copenhagen/ (accessed June 30, 2025).
[4] ThisScience1, The Copenhagen Interpretation is Wrong — Here’s Why, Medium, https://medium.com/@thisscience1/the-copenhagen-interpretation-is-wrong-heres-why-de526a8a3471 (accessed July 01, 2025).
[5] W. Heisenberg, The Development of the Interpretation of the Quantum Theory, 1955, https://www.astrophys-neunhof.de/serv/Heisenberg1955.pdf (accessed June 27, 2025).
[6] R. Shankar, Fundamentals of Quantum Mechanics, 3rd ed., Wiley-VCH, Chapter 1, https://application.wiley-vch.de/books/sample/3527347925_c01.pdf (accessed July 02, 2025).
[7] Number Analytics, Philosophical Analysis of the Copenhagen Interpretation, https://www.numberanalytics.com/blog/philosophical-analysis-copenhagen-interpretation (accessed June 30, 2025).