The Foundations and Applications of Quantum Technologies (FAQ) research group, led by Xavier Oriols, seeks novel explanations to Frequently Asked Questions about quantum mechanics that, even after more than a century, remain without a proper answer.
Why is quantum mechanics still fascinating after more than a century?
We’re often fascinated by things that stay mysterious and hard to fully understand—and the present status of quantum mechanics is a perfect example. One big reason for that is the insistence in explaining quantum phenomena without definite properties until we measure them. Take Schrödinger’s cat: it’s supposed to be both alive and dead until someone checks. Einstein’s famous question about the moon—“Is it there if no one looks?”—shows how uncomfortable is the lack of a property named position.
And it’s not just about cats or moons. Even tiny, simple things are weird in the same way. For example, an electron in a nanotransistor is said to have no specific position until it’s measured. So where is the electron when no one’s looking? Somewhere in the nanotransistor? Nowhere?
Is it mandatory to describe quantum systems without well-defined properties?
Not at all. The absence of definite properties is not a mandatory feature of quantum systems, but an arbitrary choice of the Copenhagen explanation of the quantum phenomena. There are alternative formulations of quantum mechanics in which the observation of a system does not create its properties.
For example, in Bohmian mechanics, the position of a particle (and other properties) is always well-defined. It is important to emphasize that such alternatives—often referred to as "quantum theories without observers"—accurately reproduce all known quantum phenomena and experimental results. Therefore, the rejection of explanations of quantum phenomena involving well-defined properties is not dictated by experimental evidence but rather reflects a deliberate—one will say unfortunate—choice of the Copenhagen school.
Is there any advantage in using an explanation of quantum phenomena with definite properties?
The lack of well-defined properties presents important—foundamental and comuptational—difficulties when explaining quantum phenomena.
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Beyond this conceptual advantage, there are other, more technical and subtle (computational) benefits, such as:
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Beyond this conceptual advantage, there are other, more technical and subtle (computational) benefits, such as:
Weak values post-selected in position to characterize quantum systems.
Conditional wave functions to describe quantum systems mixing degrees of freedom and time-dependent parameters.
Connecting classical and quantum systems with an equivalent formalism.
Describe quantum measurement as any other type of interaction.
A common way to minimize these difficulties is to visualize certain properties when describing quantum systems. For example, one might interpret the wave function as a description of the electron’s probable location—acknowledging that we don’t know exactly where it is until we measure it, but assuming it is “somewhere” within the region defined by the square modulus of the wave function, even when no measurement is made. This view is very common in journals and textbooks.
However, if you feel more comfortable with the idea that electrons exist somewhere—that they have a well-defined position even when not measured—you should know that, perhaps unknowingly, you are more aligned with a Bohmian interpretation of quantum phenomena than with the Copenhagen interpretation. The Bohmian explanation is often considered less fascinating because it renders quantum phenomena more straightforward—even trivial: cats are either alive or dead, not both; the moon is always there, whether observed or not; and electrons have definite positions, even if we don’t know them (i.e. human ignorance).