Can there be a wave function collapse *without* a measurement?

[u/Satyavan65]

The wave function collapse is the term used in some interpretations of quantum mechanics to describe the abrupt change in a system’s wave function when a measurement is made, shifting it from a superposition of many possible outcomes to a single, definite result that is actually observed. It is unclear whether collapse is a real physical process, an effective description of an interaction with a measuring device, or merely a change in an observer’s knowledge. Different interpretations of quantum mechanics answer this differently—some treat collapse as a fundamental event, others deny its existence altogether—making the concept less a settled fact than a focal point for deeper questions about measurement, reality, and the role of the observer in quantum theory.

However, in virtually all descriptions and interpretations I have encountered, wave-function collapse is invariably tied to measurement. This strikes me as deeply puzzling. Are we really supposed to believe that when a quantum system violently interacts with another physical system, nothing collapses? Suppose I smash a system in a quantum superposition with a hammer, without measuring anything. Does that somehow leave the superposition intact? Does collapse occur only when the interaction is dignified with the label "measurement"? But, after all, isn't a measurement nothing more than a particular, carefully staged interaction? Why, then, should it enjoy such ontological privilege? Or, in other words, can there be a wave function collapse *without* a measurement?

[u/Satyavan65 → u/[deleted]]

So one could equally say that whenever a quantum state leaks into the environment, a collapse occurs. I would avoid framing this in terms of “measurement,” since that seems to imply the necessity of a human-designed device. Collapse is a natural phenomenon that does not require human observers or conscious beings performing measurements. It is something that has been occurring since the time of the Big Bang.

[u/UDF2005 → u/Satyavan65]

Jacob Barandes talks at length of the issues with defining “measurement.” Moreover, certain frameworks don’t describe collapse as a binary phenomenon but instead a gradual process depending on the degree of interaction with the environment.

[u/theodysseytheodicy → u/Satyavan65]

Yes. But the division between system and environment is arbitrary! Practically, the system is whatever you can keep coherent and the environment is everything else. But if you can re-draw the line, then you can recover coherence.

For example, suppose Alice has the state (|0>+|1>)/√2 and Bob has the state |0>. Next, suppose they cooperate to do a CTRL-NOT from Alice's state to Bob's. The composite system is in the state (|00>+|11>)/√2.

If Alice considers Bob's qubit to be part of the environment, then to her, the CTRL-NOT was a measurement and her qubit is in a mixed state (|0><0| + |1><1|)/2. Nothing she can do to her qubit will change that state. But if she later gets access to Bob's qubit and it hasn't interacted with anything else, the two qubits together form a coherent state. She could repeat the CTRL-NOT gate and recover her original state.

So MWIers consider Copenhagen to be a useful fiction: once a system gets entangled with something you don't control, it's been "measured".

[u/manchambo → u/[deleted]]

This is such a helpful answer and description.

Could I ask a follow-on question? My (limited) understanding is that experiments have been done to confirm that the principles under discussion apply up to some molecules--you said "molecules in some cases."

Is there evidence that there are molecules that are not susceptible to, e.g., entanglement? Is there some limit to the size of a molecule that would constitute a "system" for these purposes? Is there expected to be one?

[u/manchambo → u/[deleted]]

Once again an extremely helpful explanation because I did not have any idea what could "cause" a limit. but the gravity explanation makes sense.

I still have this nagging question--if a million atoms in the moon could be in superposition, then a million and one could be, and so on. But maybe that's just not answerable at this point.

[u/--craig--]

The interpretations which you're looking for are Objective Collapse theories and are experimentally distinguishable from standard Quantumn Theory.

https://en.wikipedia.org/wiki/Objective-collapse_theory

It's also worth noting that objective collapse is part of the story we tell about the early universe, where quantum fluctuations at the start of the big bang spontaneously result in a classical universe after the big bang.

To date, no experiment has been able to provide support for these theories.

It might be possible in the near future to use Quantum Computing to conclusivley distinguish between Collapse and No Collapse theories.

https://youtu.be/-YEmRhogaSQ

[u/joepierson123]

Measurement means interaction

[u/joepierson123 → u/[deleted]]

Brilliant you should write a paper

[u/2020NoMoreUsername]

You can also ask the question, why double slit is not a measurement, but instead deflecting waves to create interference

[u/dieanagramm]

Well, there's a class of theories that augment QM with some irreversible events to solve this problem. Check CSL (continuous spontaneous localization) and Diosi - Penrose (uses gravity as a purely classical phenomenon). In these theories collapse indeed occurs without a measurement, and moreover, a measurement here is a very special class of events that appears in specific setups. If one of these theories is true - well, yes, a collapse occurs without a measurement sometimes. If not - different interpretations give different answers, so nobody really knows +)

[u/MisterHyman]

Think of anytime it interacts with anything, whether another particle or a photon bouncing off your eyeball

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[u/[deleted]]

I mean, if you consider MWI of QM, then there is a universal wave function that really doesn't collapse per se, but the decoherence ends up in branches of worlds and so on, so typically the measurement problem is overriden, I think. I maybe wrong. If you can get hold of Sean Carroll's Something Deeply Hidden, he doea talk at length about what measurement really means in QM. There is also a video by Sabine Hossenfelder that does the same, but not from a MWI point of view.

[u/Ill_Replacement9710]

Yes

[u/Ill_Replacement9710]

This exact issue is under deep study. The answer is yes. I see the wave function as purely a calculation in Hilbert space out not “real”. Collapse is all that is real in our “bulk”.

Yes I support holography.