What does the term "measurement" mean in quantum mechanics?

I’m trying to get a better understanding of some of the classic experiments in quantum mechanics, and I’m having a hard time understanding what one exactly means by “measurement” or “observation”. From what I’ve read, any particle’s properties are indeterminate until observed or measured. Not just unknown, but genuinely has no specific properties (like position or momentum)… and this is all well and good because as soon as you try to extract information from the particle it will instantly have these properties. My question is, what is it that causes the particle to go from indeterminate to known? Is it simply just interacting with another particle (that just happens to be the measurement device that can extract information)? If the particle interacts with say, a stray air molecule, and nothing is there to extract any information from it, does the wavefunction still collapse?

Quantu mehcanics tells you what happens when a measurement is made, but it doesn’t tell you what a measurement is, and that’s basically what is called the quantum mechanical measurement problem.

Now there’s something called decoherence which explains what happens when a very large quantum system interacts with another system such that the classical limit is always obtained (and I apologise to whoever it was who asked for explanation of decoherence), which most physicists agree should take placdee when a measurement occurs, thoguh decoherence in itself cannot answer the QM measurment problem.

“Measurement” or “observation” in a quantum mechanics context are really just other ways of saying that the observer is interacting with the quantum system and measuring the result in toto. Although it is common (and not typically very accurate) idea that one cannot observe an experiment or event without participating in it or effecting the outcome, this is literally true with quantum mechanics. While many illustrative examples and thought experiments in quantum mechanics are presented in the form of a discrete quantum event observed by a purely classical observer, the truth is that the observer is also a quantum system and influences (and is influenced by, albeit typically in an immaterial fashion) by the event in question.

It should be emphasized the whole “waveform collapse” concept of an observer measuring or observing a quantum event is really just a mathematical formalism. Quantum theory is a wave theory, and events are distributed along a probability distribution, but it interacts at a discrete point in time and space, so we never see the “waviness” of the particles, only that the mechanics are best modeled as waves in fields. That the waveform literally collapses is but one interpretation (the Copenhagen interpretation) but other interpretations where the waveform doesn’t collapse are functionally equivalent in terms of a falsifiable theory. The concept of wavefunction collapse seems legitimate when you think of a system of only a small number of interactions in which one event or observation nucleates the collapse, but doesn’t seem feasible when you start considering that real world systems of particles would be collapsing into single points all the time.

The o.p.'s questions about when and how the particle goes from indeterminate to locally “real” (i.e. existing at a point with specific momentum and positional properties) are valid, but are rendered nonsensical our inability to peer past the veil of indeterminacy to see the state of the particle before we see the state of the particle, or as Heller wrote, “How can he see he’s got flies in his eyes if he’s got flies in his eyes?”

These are my own pants, your explanation of decoherence is a reasonable, if abbreviated description. Essentially, when you have a single, low mass-energy particle like a photon or an isolated electron, its position-momentum distribution is much larger than the “size” of the particle. (I put size in quotes because quantum particles, being spread out fields of energy, don’t actually have a physical diameter. However, they have an effective size about their locus of action, so thinking of an electron as having a diameter is functionally perverse but useful in analogy.) Because the particle is much smaller than the total distribution of where it might be, its indeterminacy is very high and it acts in a statistical fashion. However, if the particle is very heavy, like a proton or neutron, or glued together into a larger composite quantum system, like the nucleus of an atom, the wavefunction is much smaller compared to the size of the particle. If the particle is actually significantly larger than the scale of the wavefunction then for all practical purposes the particle behaves as a classical particle with a measurable position and momentum and is considered to be decohered. This naturally happens whenever you use a large quantum system to measure a single quantum event or particle. This is true for pretty much any system on a scale that we can work directly with, such as chemical, mechanical, or solid state electrical systems, with a few extreme exceptions like Bose-Einstein condensates.

Jim Al-Khalili’s Quantum: A Guide for the Perplexed is an excellent (and beautifully illustrated) explanation of quantum mechanics for the layman that avoids going into any bizarre mystical interpretations. He makes it very plain that many of the seeming paradoxes in quantum mechanics theory arise not from internal conflicts in the theory but from the inconsistencies of fitting that set of consistent rules into our everyday intuition.


Thanks for the explanations. Stranger, that is the first time I’ve seen the mass of a particle or system being related to its probability distribution, but that makes sense and helps explain why we don’t observe quantum mechanical effects on macro objects.

At work, I sit next to a system that uses the quantum hall effect to produce an intrinsic standard resistance value, and have been a little curious lately about the physics behind it, and wanted to start by understanding quantum mechanics… at least to the point where I could explain it to someone else without being misleading. I’ll try out your book recommendation.

I’m not so sure about most physicists, I’d be more comfortable with many or some.

I think I’d like to see an imo in front of this statement. The math of decoherence is not even complete, and it’s also not really fully understood or accepted.

Decoherence is not about the mass of the particles. Yes decoherence does explain the classical limit (with a sutiable interpretation) and the classical limit of single particle system can be recovered by increasing the particles mass, but these two classical limits of QM should not be conflated.

You need to be very careful about trying to remove wavefunction collapse from the theory of quantum mechanics, removing it brings up other issues. For example the Many World (no-collapse) Interpretation plus decoherence can explain what a measurement is, but what it can’t explain why we only get one of the possible results for our measurement or what the probabilty of obtaining a result means. That’s why the quantum mechanical measurment problem is still a big issue in QM.

Griffiths says this on wavefunction collapse:

I’m not sure I’d describe QM as a wave theory.Sure there’s waves there, but it’s completely unlike any classical wave theory. I’d also note that genreally speaking the position operator has continous eigenvalues which means practically the psotion of a partcile is not discrete and (in plain old vanilla QM at least) time is a pretty much the same as it is in classical phsyics i.e. a continious variable. And wavefunction collapse is not collapse in the physical sense, the ‘collapse’ described in mathematical.