Interpretations of Quantum Mechanics

How do we deal with the strange quantum phenomena we observe? To help make sense of things, physicists have come up with varying interpretations of quantum mechanics. Most interpretations make identical predictions about the world— they’re just different ways of thinking about why or how the observed behavior of QM can be true. Some interpretations are less clearly defined than others, and only a few can really be falsified.

What you’ll see, however, is that different interpretations make vastly different statements about the world we live in, and there are serious impacts on things like philosophy. Even if we can’t currently “prove” one interpretation to be correct, these implications are still important to think about and discuss.

A Valid Interpretation?

For an interpretation to be valid, it needs to answer questions like:

1. Does the interpretation make predictions that match the outcomes of experiments we’ve already done?
2. Is the wave function a complete description of reality, or just a mathematical tool?
3. The measurement problem: What happens when you measure a wave function? Does it change?
4. In order to satisfy Bell’s Theorem, which is violated: locality, or realism? (or, it the theorem escaped somehow?)
5. Does the interpretation make sense of superposition, entanglement, the Schrödinger equation, etc.?
6. Is the interpretation compatible with existing theories (e.g. General and Special Relativity)?

Let’s discuss five common interpretations, and how they answer some of these difficult questions.

The Copenhagen Interpretation

Prior to measurement, wave functions are truly probabilistic. During measurement, a quantum system “collapses” from a probability wave into an eigenstate.

The Copenhagen interpretation states that before a wave function “collapses”, a wave functions system exist only as a probability wave. In other words, prior to being measured, a quantum system truly does not have physical properties, thus violating realism. Something special occurs during measurement, causing the wave function collapse. Why this occurs is unknown— this is known as the measurement problem.

Arguably, the Copenhagen interpretation is the most straightforward, as it essentially says “we don’t know why quantum mechanics behave probabilistically, so it must be fundamental.” It remains the traditional interpretation of quantum mechanics. According to a 2025 survey of ~1,100 physicists,¹ the Copenhagen interpretation is the most popular view, sitting at 36%.

Because the Copenhagen interpretation involves truly random behavior, it violates determinism. There is no “cause” for, e.g. an electron to be “spin up” instead of “spin down”.

Pilot Wave Interpretation

Individual quantum systems (e.g. an electron’s movement) only seem random and wave-like; they are really just particles. The motion of every particle is deterministically guided by a single, fundamental, universal “pilot wave”.

The standard Pilot Wave model, known as the de Broglie–Bohm theory, includes two equations:

1. The standard Schrödinger equation, which describes how a wave function evolves over time
2. The guiding equation, which uses the wave function and the position of every particle in the system to determine how a particle will move over time

Our universe is encompassed by one massive wave function, known as the Pilot Wave. It’s impossible for us to calculate the Pilot Wave, but it works with the guiding equation to determine the position of every particle, all the time. This is a type of global hidden variable theory.

The issue here is that the guiding equation requires knowledge of the position of every particle in the system, all the time. The only way this would be possible is with information traveling faster-than-light, which violates locality and special relativity. This is a serious problem, and a major reason the interpretation remains a minority view. Some pilot wave theories have been created to solve this problem, but none are widely accepted.

Many-Worlds Interpretation (MWI)

For each quantum possibility, a “world” exists where that outcome is realized. Measurement isn’t special, it just confirms which “world” you’re in.

The Many-Worlds Interpretation treats the entire universe as a single wave function. Just like any other wave function, it evolves according to the Schrödinger equation. When a measurement occurs in a world, that world “branches” into multiple new worlds, each corresponding to a different outcome. MWI solves the measurement problem, eliminates the concept of wave function collapse, and adds nothing to the Schrödinger equation; it is arguably the most intuitive model.

To help understand how the MWI works, imagine the following scenario:

1. A qubit exists in superposition.
2. You observe the qubit, thus entangling yourself with it.
3. Your interaction affects your world, resulting in your entire world also becoming entangled with this qubit. In other words, the world’s state depends on what you measured: up, or down.
4. Because your entire world is now in superposition, it can be described as a single wave function with two possible outcomes.
5. Each world, as a wave function, evolves unitarily according to the Schrödinger equation. Unitarity means that all possible values of the wave function always exist— their individual probabilities can change, but they will never disappear. So, both worlds are equally, permanently real.
6. If a 2nd qubit is observed later in both worlds, the process repeats.

Because the universe always contains a world for each outcome, it’s said to be deterministic. However, wave functions still appear probabilistic to any observer who can access only one branch (like you).

For example, if you observe a qubit, there will be two worlds: one in which you measure spin up, and another in which you measure spin down. Prior to the measurement, you exist as a consciousness with a specific set of memories. After the measurement, there are two almost identical consciousnesses, one in each world, each with a distinct memory of what was observed. The “you” that existed pre-measurement now exists in two places at the same time.

Whether MWI violates realism is debatable. Critics argue that because a wave function in the MWI maps to multiple possible future values, it violates realism. Supporters argue that the wave function always resolves to every possible outcome, so realism is not violated.

Quantum Suicide Experiment

There are some interesting games you can play if the many-worlds interpretation is correct. For example, the quantum suicide experiment goes like this:

1. You obtain a qubit in superposition.
2. You hook up the qubit to some sort of killing contraption (e.g. a perfectly lethal gun pointed at your head), such that if the qubit is observed to be spin “up”, you are killed, and if spin “down”, nothing happens.
3. You measure the qubit’s spin. In one world, you die. In the other world, you survive.
4. You repeat this experiment a large number of times (say, 1 billion times). Since every quantum probability maps to a world that exists, there will always be at least 1 world where you survive.

Is has been argued that the quantum suicide experiment can be used to experimentally confirm the many-worlds interpretation. In a classical world, there is an almost certain chance of death, but if the many-worlds interpretation is true, there is always at least one world where you survive. By repeating the experiment countless times, you can “prove” that you are immortal. If you end up being wrong, you die, but some other version of you lives, moving to the next iteration, and so on.

But in my view, this experiment proves nothing. If many-worlds is correct, or if it’s wrong, the odds of “this you” surviving are always . If you survive after the experiment is done 100 times, you will think to yourself:

Wow. I just measured 100 qubits and got spin-down every single time. That was extremely lucky, and I’m still alive. Did I die in other worlds, or did I just get a lucky streak in this one? I can’t tell.

Spontaneous Collapse

Wave functions are real, and they experience random “localizations” wherein an outcome is temporarily realized. As a system grows in size, the chance of localization increases.

In Spontaneous Collapse, there is an additional, stochastic variable in the Schrödinger equation. This random variable determines when a wave function becomes momentarily “localized”. Most spontaneous collapse theories include an “amplification mechanism” which increases the rate of localization as the size of the system increases. This helps explain why large-scale objects behave differently than tiny quantum systems.

When you measure a qubit, you become entangled with it, thus creating a very large superposition of many particles. Since measurement necessarily involves this entanglement of many particles, it will almost certainly lead to a localization. This solves the measurement problem.

If the spontaneous collapse interpretation is true, we can expect to see tiny heating during one of these “kicks”. However, experiments have failed to detect this so far.²

QBism

The wave function isn’t real, it’s just an observer’s subjective knowledge. When you measure a quantum system and see a result, it’s personal to you.

Up to this point, we’ve assumed that when you describe a quantum system using a wave function, you’re doing so objectively— the wave function is the system, and everyone measures the same wave function.

But what if this wasn’t the case? According to QBism,³ the “wave function” is just something a person comes up with to describe their “best guess” about how the wave will behave. When that person observes the wave function, they just update their “best guess”.

For example, say you measure a qubit’s spin. Before measurement, you may think “50/50 up/down”. After measurement, you think “ok, it was spin up, so I should update my guess. Now, I’d reason that the odds are 100/0”. Nothing about the system actually changed. No wave function collapse actually occurred, you just updated your predictions.

Conclusion

Over time, scientists experiment with quantum mechanics, generating a corpus of scientific data, theorems, and equations. The various interpretations of quantum mechanics are just that— interpretations. They make very similar predictions about known experiments, so they’re difficult to falsify.

In the meantime, we can quibble about which interpretation most accurately reflects the universe we live in, but it’s largely a guess (with large philosophical implications). So, pick the one that’s most appealing to you, and wait for further experiments!⁴ You can even take a quiz in this article to see which interpretation best matches your intuitions. Fortunately, there is some hope⁵ that serious progress will be made in the next 100 years.

Here’s a graph of some interpretations, and their acceptance from ~1,100 physicists:

I’m not a physicist, so you shouldn’t put any merit on my opinion, but I find the Pilot Wave interpretation to be the most intuitive and appealing, even if it has some issues. I like the idea that our universe is deterministic.