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It is known that the concept of an observer plays important role in quantum mechanics. In interpretations with collapse only the observer can trigger the wavefunction collapse. From this naturally arises a question, whether the laws of nature were different before the observer appeared. It seems there can be at least two theories on this matter.

  • First theory is that before the observer appeared the universe was in a mixed state and underwent unitary evolution following the universal wave function. With the first observation (when he experiences the first ever qualia) the universal wave function collapsed to a random (but probable) state consistent with the observer's existence.

Since before the first observation the universe was undergoing unitary evolution, it acted as a well-isolated from the external world quantum computer.

This means that the observer in principle may be able to spot relic interferences that distort the statistical behavior of the past, such as quantum jumps in biological evolutionary progress, similar to how quantum annealing allows jumps from one local minimum to another in an adiabatic quantum computer.

Unfortunately such differences can be spotted only by statistical methods, but this idea possibly completely compromised by the much greater influence of the anthropic principle on the observed statistics of the past. It is not evident whether the anthropic principle itself can be explained in terms of relic interferences.

Another major problem with detection of relic interferences is that with the increase of the complexity of a quantum system, the chances of interferences between different evolutionary paths and their eventual reconciliation quickly decreases. This fact has been proven by Thomas Breuer who showed that the increase of the complexity of a system (i.e. its entropy), its evolution under objective decoherence becomes virtually indistinguishable form its evolution under both objective and subjective decoherences (see below) with the contribution of the latter exponentially decreases (indeed it is quite more probable to see an interference of two states of an electron than two states of a Schroedinger's cat).

This suggests that relic interferences may be only observable at the very first moments after Big Bang when the entropy of the Universe was still very low.

  • The second theory is that the laws of nature never changed, and wavefunction collapse happened always. This means that the observer effectively existed always and also always existed subjective decoherence. So the differentiation between these two theories lies in the existence of subjective decoherence before the birth of the observer.

A problem with this theory that postulating existence of the observer forever (even if unconscious) allows us to trace him (by the parental lineage of his cells for example) only up to 4 billion years back from now and then we only can refuge to the first theory.

That said I should note that Breuer published an article titled "Ignorance of the Own Past" that seemingly supports the second theory, but the result was obtained only with an assertion of determinism and the author notes that he does not know whether the result still holds in an indeterministic world.

So my question is whether this result was since improved to include indeterministic case so we could decide which of the two theories to prefer?

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Physics does not depend on a "conscious" observer - qualia are a purely philosophical construct that has nothing to do with physics. If anything, observation happens all the time whenever a quantum system interacts with a larger system with many degrees of freedom such that the different possible quantum possibilities decohere with each other.

In other words, I think the only quantum interpretation that really makes sense from a physics point of view is the Everett many worlds interpretation (MWI). In MWI the universe is always following the unitary evolution of the wavefunction of the entire universe. After all every observer is part of the universe and must also be described by a unitary quantum wave function. In all the interpretations where wave functions collapse there is no mathematical description of how this collapse occurs - therefore there is really no theory there.

The Copenhagen interpretation with wave function collapse is a useful tool that permits calculations of the outcomes of experiments, but this cannot be the way the world really works. There must be a coherent mathematical theory that explains all of quantum mechanics without any handwaving when it comes to the observer. See this for more information.

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    $\begingroup$ "observation happens all the time whenever a quantum system interacts with a larger system with many degrees of freedom" - you possibly know the Schroedinger's cat experiment in which observation does not happen when the atom interacts with the cat. Anyway this your statement contradicts with your claim that you believe in MWI. And finally if you want to get some predictions from MWI you have to operate not only with universal wave function (this is fruitless for making predictions) but with particular wave functions corresponding to certain observers. $\endgroup$ Commented Feb 24, 2012 at 0:50
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    $\begingroup$ Schrodinger's cat is a thought experiment, it doesn't prove anything. If it shows anything at all it shows that the "wavefunction collapse" interpretations of quantum mechanics is problematical. My statement that "observation happens all the time" is exactly compatible with MWI - whenever a quantum system interacts with a system with many degrees of freedom, the decoherence effectively causes the universal wavefunction to branch into the "multiple worlds" branches. $\endgroup$ Commented Feb 24, 2012 at 3:19
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There were always observers around, at least since very shortly after the big bang.
An observer is, from the quantum field theoretical perspective, any object with two properties:
(i) It is large enough that it can be well described by statistical mechanics.
(ii) It interacts with the degrees of freedom of the system to be observed in a way giving a well-defined macroscopic response.
Thus in our universe now, essentially any macroscopic object observes all the objects it interacts with significantly.

In any case, it doesn't need a conscious observer. Otherwise we wouldn't find that events far back in time satisfy the same dissipative physics as what we observe now since conscious observers are around.

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  • $\begingroup$ What you said contradicts the Schroedinger's cat paradox where the cat although macroscopic and complex, still is not the observer. Anyway the apparent continuity in time of statistical laws can be explained in different ways, even without claiming that there was always the observer. For example if the observer emerged somewhere in time, his contact with the environment makes him the "observer" of the events far before his appearance by medium of the environment and thus collapsing the wavefuntions for the whole history in one moment. $\endgroup$ Commented Mar 25, 2012 at 21:19
  • $\begingroup$ This does not mean that subtle relic interferences still could not be observed for far away objects which did not have contact with the observer's environment. $\endgroup$ Commented Mar 25, 2012 at 21:21
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    $\begingroup$ Schroedinger's cat and its collapse is pure fiction. What can be realized experimentally are S/C states with a microscopic cat consisting of one particle only; maybe a few particles in the near future. - Experimentally verifiable collapse is also restricted to very tiny systems. $\endgroup$ Commented Mar 26, 2012 at 8:11
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    $\begingroup$ Note that I am talking asbout real observers, i.e., quantum subsystems of the universe, the biggest quantum system of all. Not about ficticious observers that live in a classical world to do quantum experiments on arbitrarily large systems, pretending that Born's rule holds for these. $\endgroup$ Commented Mar 26, 2012 at 15:21
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Physics is about measurement outcomes. Assuming an objective reality without measurement is literally a metaphysical assumption. Considering the loss-of-unitarity related to the act of observation/measurement, one can resolve the problem in terms of superposed observers. Rovelli's (and others') Relational Quantum Mechanics is built along these lines. As any issue in physics, this one too will be resolved by experiments once macroscopic-enough superpositions will be detected. PS "As always, I hear people murmur the dreaded word “decoherence”. But I claim that this is a major red herring ...".

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Your question should really have been closed on the grounds that it invites answers based on opinions, but since it is still open I will venture to say that it is nonsense to suppose that quantum theory depends on the existence of an 'observer' in the sense of the word you adopt. The so-called 'collapse' of the wave-function just means that the form of the wave-function associated with an object changes when the object interacts with something else. There is no need for the process to be 'observed'. As for Schroedinger's experiment- that was thought experiment designed to show that it was nonsense to assume a fundamental role for an 'observer'.

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    $\begingroup$ This depends on quantum interpretation: either you have the distnguished observer or some other philosophical consequesces, such as "many worlds". "the form of the wave-function associated with an object changes when the object interacts with something else" - no, without the obsever the quantum system always undergoes unitary evolution, no collapse, regardless of interractions. This is basics. $\endgroup$ Commented Mar 23, 2022 at 12:29

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