The Copenhagen Interpretation of quantum physics is so named because the main centre where it was developed in the 1920's under the leadership of Neils Bohr was based in the city of Copenhagen.
The Copenhagen Interpretation of all things quantum states that's it is inappropriate to speculate on the existence and properties of any object until such time as the object in question (it's wave function) is actually observed and measured. Then, and only then, has the probability of such-and-such existing and what properties it possesses become certainty. That's termed the collapse of the wave function. The classic example is that an electron might be here, it might be there, it might be anywhere. The electron might be in a spin-up state, or maybe a spin-down state. Only a measurement or observation will resolve the issues. Ultimately, in the Copenhagen Interpretation, the observer (taken as evident to be human) is the be-all-and-end-all; the supreme umpire, judge, jury and executioner.
The counter classic example, proposed by physicist Erwin Schrodinger as a putdown to the Copenhagen Interpretation, illustrating the absurdity of it all, was Schrodinger's cat. The gist of this thought experiment (no animal was ever actually involved, so animal lovers can breathe easy) was that there would be some sort of quantum event (like a radioactive decay event) that had a 50-50 chance of happening in one hour. If the event happened, it would trigger off a chain of events that would cause the demise of a cat trapped inside an opaque sealed box. If the quantum event didn't happen, the cat would be alive inside the sealed box. The question is, after one hour, is the cat alive or dead? Without an observer, the Copenhagen Interpretation says that in lacking an observer, there exists a superposition of states. In one state is the cat is dead; in the other state the cat is alive. In other words, the cat is dead-alive until such time as an observer observes, and the 50-50 probability becomes 100% certainty.
There's an interesting variation on that cat thought experiment. Say the cat-in-the-box is in a room and I'm also in the room, and after one hour I peek in the box and determine the aliveness or deadness of the animal. But, say you are outside the room when I do that. As far as you are concerned, the cat's wave function hasn't collapsed and the cat is still dead-alive. So you have got to look too! But then what about a third party in another room in the house, then the neighbour next door, and hence other residents of the town, then state, hence country and then the entire world. Of course the cat would be in a limbo dead-alive state to extraterrestrials on another planet until they looked, and so on. In fact, taken to a logical extreme, nothing has reality until the entire Universe observes, which is again (IMHO), absurd seeing as how it could take billions of years for that cat observation to reach the farthest regions of the cosmos!
In the quantum world, there can be a superposition of state. Something can both be, and not be, at the same time (like wave-particle duality) - only measurement or observation can decide whether it's to be or not to be. Since the macro (like a cat) is composed of the micro (the realm of the quantum), then until observed, the cat can both be (dead) and not be (dead).
In a similar way, for those who argue that nothing is real unless that something is observed they'd argue for example that the Moon dissolves into quantum uncertainty, the Moon both is and is not, if nobody is actually looking at the Moon! As soon as someone looks at the Moon, it solidifies back into physical reality - it is. The absurdity (again IMHO) of that is that if the Moon faded away into quantum uncertainty that would play havoc with the tides and be noticed. Perhaps observing the tides is sufficient to give the Moon reality without actually observing the Moon! I'll return to that point shortly.
The Copenhagen Interpretation, translated, and taken to its logical conclusion is that if nobody is looking at the Moon, does it exist? Can the Moon be in a superposition of states - having existence and having non-existence simultaneously? That was a quasi-question poised by Albert Einstein when he - also in opposition to the Copenhagen Interpretation - pondered that he'd like to believe or think the Moon existed even if nobody was looking at it. [Presumably the memory of a prior observation doesn't count.] Well Professor Einstein, it does!
That the Moon doesn't exist if nobody is looking at it is nonsense from several points of view. Firstly, common sense: can anyone really doubt that something you're infinitely familiar with, let's say your partner (or the Moon) doesn't exist or doesn't have the properties you associate with him/her (or the Moon) just because you're not observing them/it?
It's reasonable that anything that's biological and alive observes itself. So, even if you're not observing your partner and therefore could deny your partner's existence, your partner is observing him/herself and therefore would argue very strenuously that s/he exists. That renders that point mute.
No one has defined exactly what constitutes an observer. Can it be anything that's alive like a plant or bacteria, or does it have to have a sophisticated nervous system (higher sensory capacity)? Maybe there has to be a complex brain within. Maybe an observer is only a bona-fide observer if it has intelligence, but what degree of intelligence? A one day old baby or someone who is brain damaged might look in the box and see Schrodinger's cat but has no capacity to understand what they are seeing. Does it have to be human? What about an artificial intelligence? So, what's an observer? Can it be less than human, but still an organic life form - say your dog - or what about a fish; or an insect; or a microbe? Can something inorganic be a bona fide observer? What about an inorganic things like a Geiger counter or thermometer? I'd argue that an electron can be as bona fide an observer as a human being.
For example, say a photon is emitted by the Sun, heading our way. Unfortunately, the Moon is in the way (solar eclipse) and so the photon is thwarted, it's pathway to Earth blocked. From the point of view of that photon, the Moon exists, even if nobody on Earth were observing the solar eclipse (rather unlikely, but possible for sake of argument).
The curveball in all of this is that there's more than one way to observe. The assumption is that by observing we're using one or more of our five senses - usually sight. I suggest that in fact, every particle/object in the observable universe is observing every other particle/object in the observable universe, 24/7. Translated, even when your partner (or the Moon) is out of sight (and hearing, etc.) you are still observing him/her (or it), even though you're not aware of it!
Why? How? Does the Moon exist if you're not observing the Moon, but you are observing the rising (or falling) tide? There's causality between the two. So, observing the tides is in effect observing the Moon, or at least one of its properties - gravity. And therein lays the solution to Einstein's quasi-question.
You have mass. The Moon has mass. Any two masses attract each other via the gravitational force. Therefore, you 'feel' the gravitational attraction of the Moon; the Moon 'feels' the gravitational attraction of you. Therefore, even as you read these words, you are 'observing' the Moon. You have observed the Moon 24/7 since the moment you were conceived.
The observer problem, the Copenhagen Interpretation, is a furphy since everything that has mass observes everything else that has mass, all the time - gravitationally.
This explains how the Universe got along and evolved very nicely, thank you very much, for all those eons before biological observers happened upon the scene. The Universe existed and had properties prior to the origin of any life, anywhere, since observers don't have to be living! However, there are those who believe and would argue that the entire Universe exists (has reality) only because there are observers to observe or measure it. Clearly the Universe was in a lifeless state and evolved in a lifeless state from Day One (the Big Bang event) through several billions of years at least. That is, there were no biological observers at all. The Universe had to exist in a pre-biological observers stage in order to evolve the complexity required to produce biological observers. An early Universe consisting of only hydrogen, helium and radiation doesn't hack it as far as being a suitable environment for biological observers. So, in terms of this chicken-or-the-egg question, the Universe-or-the-biological-observer question, the answer must clearly come down on the side of the Universe. The Universe can exist either with or without biological observers; biological observers exist only because there is a Universe.
So, the resolution to Schrodinger's cat is as follows (and no human observers are required). While the cat is alive, the cat observes itself. If the cat is dead, those bits that make up the cat's body are observers in noting that the organism is now dead because the interactions they participate in have altered. Further, interactions between the dead cat and all the particles that surround the dead animal can also be considered bona-fide observers.
Conclusion: The ultimate observer, that be-all-and-end-all; that ultimate umpire, judge, jury and executioner reside within that abstract phrase, 'Mother Nature' - that is anything and everything, anywhere and everywhere, at anytime and every time; at all times. I believe that observers and measurements have bugger-all to do with reality, existence and how things work on either a macro or micro scale. The proof of that pudding, if any were necessary, is that radioactive substances decay with a measured half-life. The entire science of radioactive dating depends on this. And radioactive elements decay whether or not observers are present - they have; they do; they will.
