What is the meaning of superposition and wave/particle duality? What do quantum particles really look like, when we are NOT looking?
All quantum particles exist in a state of superposition prior to any interaction. This does not mean that they exist in multiple states at the same time. So what does superposition really mean?
First know that the math of quantum mechanics does not describe the universe. It describes what we might get if we make a measurement. The math describes the possibility of a quantum object being at any of several positions, at some future time. This does not mean that object is in all those positions now.
Quantum theory describes the potential outcomes of our measurements. Before a measurement is made, there is no outcome. It does not describe the reality of the present. Once you make a measurement, the object is in only a single location. Actual measurements have never found it at a second location or multiple locations at the same time.
The quantum object is in only one place, but that place can be anywhere. Until we measure where it is, the only thing we know is the probability of where it is likely to be.
So what happens when we measure things? Why is the object no longer in superposition when a measurement happens?
The precise mechanism of what happens when a measurement is made, or how the object comes out of a superposed state is not well understood. This is known as the “measurement problem of quantum mechanics.” But we do know measurement is really an interaction. We sometimes also call this interaction an observation, but it is purely a mechanical interaction, and does not rely on anyone having to look at the object. It has nothing to do with consciousness.

An interaction is simply an irreversible exchange of energy with another object. Once an interaction of sufficient magnitude takes place, the superposed object is no longer in superposition, and we observe the particle in only one location.

How do we even know whether a particle is in a superposition, if we only know something about the particle AFTER it is measured? To understand what the particle looks like before we measure it, we look what the particle does BEFORE we measure it.

When we send single electrons through the double slit one at a time, it forms an interference pattern. This can only happen if the electron goes through both slits at the same time and interferes with itself. The proof of superposition is that if we measure which slit the electron goes through, the interference pattern disappears. So this shows that something happens to the electron when a measurement happens. It no longer is in a superposed state and acts like an individual particle. This can be done with any quantum object.

The way to think of this intuitively is not to think of the electron as a point-like particle before it is measured, but as a wave before it is measured. The wave, like any other wave going through two slits will indeed interfere with itself. This is why quantum objects are said to have wave/particle duality. It’s because they exhibit wave-like behavior prior to measurement. But exhibit particle-like behavior, after measurement.

Now, there are several things I want to qualify on this animation. First, quantum objects like an electron are not literally like the wave shown.

Where does this idea of probability wave come from? It comes from the Schrodinger equation. This is basically an energy equation showing how the energy of a quantum system changes over time. The equation has a wave function in it, represented by the Greek letter psi, which is a mathematical expression that represents the quantum state of a particle, or isolated quantum system.

An electron prior to measurement, would be such an isolated quantum system. What’s important to understand is that the wave function, is a variable quantity that describes the wave-like characteristics of a particle.

Where an electron shows up after measurement is random, but subject to the calculated probability represented by approximately the square of its wave function.

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