Given that quantum physics, and quantum field theory in particular, seems to be the language spoken by the universe, it is essential to understand what it implies for the way we perceive reality.
We perceive our surroundings by interacting with them. We may observe light emitted by an object, or probe it in some other way, but in every case information has to be transferred from what is observed to whoever, or whatever, observes it. Measurement is therefore not something outside physics. It is itself a physical process.
In quantum physics, this process does not generally produce one single classical outcome from the start. It produces a larger quantum state in which different possible outcomes are correlated with different states of the detector, and eventually with different states of the observer.
This is where much of the confusion begins.
It is often said that a measurement somehow collapses the wavefunction onto one outcome. In reality, no such physical collapse needs to be invoked. The underlying quantum evolution does not suddenly stop and hand its job over to something outside the laws of nature. A state that was once a superposition evolves into another state, which may contain several correlated alternatives.
In some situations, however, these alternatives cease to interfere for all practical purposes. This happens especially when each branch involves a large number of particles and already exhibits classical features. In that regime, the different branches evolve almost independently, as if there were no superposition at all.
For simplicity, imagine an unstable particle placed between two observers, Alice and Bob. Suppose the particle can decay into two photons, one traveling to Alice and the other to Bob. Let us also imagine, for simplicity, that Alice and Bob are positioned symmetrically, so that whenever the decay happens, they both register it at the same moment. The only uncertainty is when the decay occurs.
A particle of this kind is characterized by a decay rate, which tells us its typical lifetime. If we observed many such particles, most would decay within some characteristic interval, though some would decay earlier and some later. For a single particle, these different possibilities are not represented by one unknown but definite decay time hidden from us in the classical sense. They are represented in the quantum state itself.
So the state evolves from one in which the particle has not yet decayed into a superposition of alternatives: one in which Alice and Bob register the decay at one time, another in which they register it later, and so on, each with its own weight.
The important point is that Alice and Bob are not outside this story. They become part of it. After the interaction, the quantum state no longer describes only the particle. It describes the particle, the detectors, and the observers together.
This naturally raises a question. If the full state contains several alternatives, does an observer in one of them somehow experience the others?
For macroscopic observers, the answer is effectively no. Alice and Bob are not elementary particles. They are made of enormous numbers of quantum constituents and are already well described, to an excellent approximation, by classical physics. Once different detection outcomes become imprinted in such macroscopic systems, the corresponding branches of the state cease to interfere in any appreciable way. They become, for all practical purposes, separate records.
This is why observation feels definite.
The point is not that quantum physics stops applying when a conscious being enters the story. The point is that consciousness, detectors, notebooks, screens, and nervous systems are themselves parts of the quantum world, but parts whose large-scale behavior is extremely well approximated by classical descriptions. That is what allows one outcome to appear as a definite experience rather than as an explicitly visible superposition.
There is more to say about measurement than this. But the central point is already here. Measurement is not an exception to quantum physics. It is one of the processes quantum physics describes.
The tension appears only when we try to force this description into a classical picture, in which a measurement is expected to uncover one definite pre-existing reality.
See also:
What quantum physics changed
The states the universe could be in