Wednesday, October 26, 2022

An Electron is in Probabilistic Limbo

Science journalist John Horgan writes in SciAm about the recent Nobel Prize for Bell experiments:
Electrons possess a quantum property called spin, which is unlike the spin of a planet or top. Quantum spin is binary; it is either up or down, to use a common notation. Imagine if planets could only spin clockwise, or counterclockwise, with their axes pointed only at the North Star, and in no other direction, and you’re getting the gist of spin. Although quantum spin, like entanglement, makes no sense, it has been verified countless times over the past century.
No, that is not quite right. Quantum spin is measured by a Stern-Gerlach device, and that measures spin in a particular direction. Spin could be in another direction, but if you only measure North-South, you will only see North-South spin.

The concept makes sense. The most confusing thing is that the uncertainty principle prevents knowing the spin in different directions at the same time.

Okay, now you let the electrons fly apart from each other. Then you measure the spin of electron A and find that its spin is up. At that moment, the wave function for both electrons collapses, instantaneously predicting the spin of electron B, even if it is a light-year away. How can that be? How can your measurement of A tell you something about B instantaneously? Entanglement seems to violate special relativity, which says that effects cannot propagate faster than the speed of light. Entanglement also implies that the two electrons, before you measure them, do not have a fixed spin; they exist in a probabilistic limbo.
Horgan has been taking a QM class, but he has been led astray. Spin is not so confusing.

Even one electron by itself is in a probabilistic limbo. Even if you prepare it to have definite spin in a particular direction, then measuring spin in other directions will be governed by a probability forumula.

He complains about entanglement, but a similar thing happens classically. Suppose you had a two-planet system, and knew the total angular momentum. Then the planets got separated, and you measured the spin of one of them. You would immediately know the spin of the distant planet.

Nobody thinks that is spooky, or violates special relativity. So why do people get so excited by Einstein EPR and quantum spin entanglement?

You might say the quantum spin is more mysterious because of the uncertainty principle, and because Bell proved that the correlations are somewhat higher that what you would expect from a local hidden variable theory.

Okay, I agree, those are mysterious. So talk about them! Those mysteries can be understood.

Instead, these article use smoke and mirrors to exaggerate what is mysterious. I cannot even blame Horgan for this, as he is just parroting popular explanations that are designed to confuse.


  1. A probability is a second hand calculation dependent entirely on measurements in order to exist even abstractly. Not knowing where something is does not consign it to an existential limbo, that's a small child's wishful thinking.

    Math does not and never has carried forces, or initiated motion, or informed reality. Math is what you use to say something about something else using logical operations, the math is not the cause of anything.

    If you describe something that conforms closely to what the actuality does in order to predict something, it is not because the actuality informs itself with your description that the model works, it is because your description attempts to inform itself with the actuality.

    1. Dear Roger,

      > "these article[s] use smoke and mirrors to exaggerate what is mysterious."

      True. But to be honest, I enjoyed going through this piece by Horgan.

      [PS: BTW, my above comment to CFT's reply was made sincerely, not in sarcasm. [Such are the days we live in, even such things have to be clarified.]]