The standard interpretation of quantum mechanics places a lot of emphasis on the act of measurement. Before measurement, quantum systems exist in many states at once. After measurement, the system "collapses" into a specific value, so it's natural to ask what's really going on when measurements don't take place. There isn't a clear answer, and different ideas can go in some really wild directions.
One of the first lessons that physicists learned when they started examining subatomic systems in the early 20th century was that we do not live in a deterministic universe. In other words, we cannot precisely predict the outcome of every experiment.
For example, if you shoot a beam of electrons through a magnetic field, half of the electrons will curve in one direction while the other half will curve in the opposite direction. While we can build mathematical descriptions of where the electrons go as a group, we cannot say which direction each electron will take until we actually perform the experiment.
In quantum mechanics, this is known as superposition. For any experiment that can result in many random outcomes, before we make a measurement, the system is said to be in a superposition of all possible states simultaneously. When we make a measurement, the system "collapses" into a single state that we observe.
The tools of quantum mechanics are there to make some sense out of this chaos. Instead of giving precise predictions for how a system will evolve, quantum mechanics tells us how superposition (which represents all the various outcomes) will evolve. When we make a measurement, quantum mechanics tells us the probabilities of getting one outcome over another.
And that's it. Standard quantum mechanics is silent as to how this superposition actually works and how measurement does the job of collapsing the superposition into a single result.
If we take this line of thinking to its logical conclusion, then measurement is the most important act in the universe. It transforms fuzzy probabilities into concrete results and changes an exotic quantum system into verifiable results that we can interpret with our senses.
But what does that mean for quantum systems when we're not measuring them? What does the universe really look like? Does everything exist but we are simply unaware of it, or does it not really have a defined state until measurement takes place?
Ironically, Erwin Schrödinger, one of the founders of quantum theory (it's his equation that tells us how the superposition will evolve in time), railed against this line of thinking. He developed his famous cat-in-a-box thought experiment, now known as Schrödinger's cat, to show how ridiculous quantum mechanics was.
Here's a highly simplified version. Put a (live) cat in a box. Also put in the box some sort of radioactive element that is tied to the release of a poisonous gas. It doesn't matter how you do it; the point is to introduce some ingredient of quantum uncertainty into the situation. If you wait awhile, you won't know for sure if the element has decayed, so you won't know if the poison has been released and thus if the cat is alive or dead.
In a strict reading of quantum mechanics, the cat is neither alive nor dead at this stage; it exists in a quantum superposition of both alive and dead. Only when we open the box will we know for sure, and it's also the act of opening the box that allows that superposition to collapse and the cat to (suddenly) exist in one state or the other.
Schrödinger used this argument to express his astonishment that this could be a coherent theory of the universe. Are we really to believe that until we open the box that the cat doesn't really "exist" — at least in the normal sense that things are always definitely alive or dead, not both at the same time? To Schrödinger, this was too far, and he quit working on quantum mechanics shortly thereafter.
One response to this bizarre state of affairs is to point out that the macroscopic world does not obey quantum mechanics. After all, quantum theory was developed to explain the subatomic world. Before we had experiments that revealed how atoms worked, we had no need for superposition, probabilities, measurement or anything else quantum-related. We just had normal physics.
So it doesn't make sense to apply quantum rules where they don't belong. Niels Bohr, another founder of quantum mechanics, proposed the idea of 'decoherence" to explain why subatomic systems obey quantum mechanics but macroscopic systems do not.
In this view, what we understand as quantum mechanics is true and complete for subatomic systems. In other words, things like superposition really do happen for tiny particles. But something like a cat in a box is most definitely not a subatomic system; the cat is made of trillions of individual particles, all constantly wiggling, colliding and jostling.
Every time two of those particles bump into each other and interact, we can use quantum mechanics to understand what goes on. But once a thousand, or a billion, or trillions upon trillions of particles enter the mix, quantum mechanics loses its meaning — or "decoheres" — and regular macroscopic physics takes its place.
In this view, a single electron — but not a cat — in a box can exist in an exotic superposition.
However, this story does have limitations. Most important, we have no known mechanism for translating quantum mechanics into macroscopic physics, and we can't point to a specific scale or situation where the switch takes place. So, even though it sounds good on paper, this model of decoherence doesn't have a lot of firm backing.
So does reality exist when we're not looking? The ultimate answer is that it appears to be a matter of interpretation.
https://www.researchgate.net/publication/364306784_Existence_reality_and_perception_in_classical_and_modern_physicsThis goes into more detail about much of your subject here. The reference to semiclassical physics refers to work done at Los Alamos in the late 1980's in calculations for the bright source laser project, calculations that involved realistic atomic models driven by intense laser fields into regions of state space that were nearly continuous, where the classical response became dominant in spite of the fact that the atomic calculation was purely quantum mechanical.
The term "measurement" is a leftover from the early days when it was their only contact with their experimental results. I agree that it's now an outdated term with misleading connotations too often taken far beyond its actual meaning. In a modern sense, "measurement" of a system has to include any interaction with another one, since that seems to be the only way around the ridiculously egocentric attitude it otherwise implies.
Our CERTAINTY about the cat's survival is open. But, that doesn't make the cat both dead & alive.
Put enough food & water in the box. Don't open the box. If after a week it stinks, the cat is dead. It isn't dead 'suddenly' because of anything we do.
It seems ego- (homo-) centric to think that everything happens because of our actions. When I die, things will keep happening -- clearly; some cats will die, some will not, without my opening anything. Let humans become extinct. The remaining cats will live or die. We who have shuffled off this mortal coil may not know it. But our failure to open the cat's box has nothing to do with its survival.
How do we define "reality"? We believe what we perceive to be real and define what we perceive by sets of characteristics and their behaviors.
This assignment of characteristics exists by perception. If no one ever observed an unknown phenomena either directly or by theoretical inference; how can it be said that phenomena exists?
Even though unicorns only exist through the imaginary, the concept of exists just like flight and other inventions did. They are measured by the power of observation even if they exist purely theoretical. We understand purely imaginary concepts like philosophy, stock markets and national boundaries to be real?
So there's the universe. We're looking at it. Because we're looking at it we can say it's there.
We can imagine if all humanity and life on earth were extinct that the universe would be there.
We can only imagine that because we are here observing it right now.
You can define consciousness? If the universe is one giant quantum computer, could it itself have consciousness? Sean Carroll thinks that all there is, is the wave function. Well... how can the wave function cause itself to collapse through observing itself, if its all that is ? How can you be an observer, when phenomena had to be observed in the first place for you to exist and observe? I tend to like Max Tegmark's thoughts on the matter. Existence is pure math, and we experience one of many existances.
When he sees that this tree
Continues to be When there's no one about in the quad
Sir, your astonishment is odd
I am always about in the quad
Which is why this tree Continues to be
Since observed by yours faithfully, God
"For example, if you shoot a beam of electrons through a magnetic field, half of the electrons will curve in one direction while the other half will curve in the opposite direction. While we can build mathematical descriptions of where the electrons go as a group, we cannot say which direction each electron will take until we actually perform the experiment."
Doubly wrong. Charged particles like electrons will curve just one way - in accordance with the classical Lorentz force law. What the confused writer probably meant was neutral particles having an intrinsic magnetic moment. Then - IF the applied magnetic field has a particular divergent geometry - and the initial trajectory follows an appropriate relative orientation - then a Stern-Gerlach experiment type of spin dependent 'integer' path splitting follows.
I agree with the total inapplicability of Schrodinger's cat as a conceivable alive-dead superposition 'paradox'.