These experiments of the Wigner's friend type are very important. The issues raised in the above post are very deep, but according to me, they need further clarifications*).
As a physicist I am looking at reality from the point of view of quantum theory. There are many interpretation of quantum mechanics and there is no consensus about which one is true. "Nobody understands quantum mechanics" said the famous physicist Richard Feynman. We are like blind people investigating an elephant. Somebody touches his legs and says: "It is like a tree". Another one examines its nose and claims: "It is a sort of snake". Etc. In fact all those statements reveal a property of the elephant, in this sense they were all true. The analogous holds for the diverse interpretations of quantum mechanics. Each reveals a part of the true nature of quantum mechanics. What is lacking is an overall view, a crucial insight about what QM is actually describing.
Some people say that the wave function ψ, the central mathematical object of QM, represents only a formal expression of agents' degrees of belief, i.e. their available knowledge, about a certain experimental situation.
This view about the nature of ψ implies that the dynamical process described by the Schrödinger's Equation -the central mathematical equation of QM-- provides solely the evolution of the probabilities in the agent's mind. Hence, there is no motion of quantum objects in spacetime.
To others this is unacceptable, because -as they say--wave function then does not refer to anything real in the world. They insist that QM should describe ---and that it does describe--- the behavior of objectively existing objects moving in a real space.
Does matter exist separate from consciousness?
Some people say yes, others say no. So what does then quantum mechanics describe? Something real or our perception? The question here is "What is real?" Or even: "What is really real?" When we watch a physical object, for instance a table in front of our eyes, we are aware of its existence through our conscious experience of seeing the table. We interpret this experience as meaning that an actual table exists in the space outside there, independently of us. But if we watch by a suitable instrument an object in the microworld, for instance, an electron, it is not so obvious that we can interpret the situation as if at every moment the electron existed at a certain position in space. The experiments show that the electron exists at many positions at once, and only when we observe it, that is, measure its position, the electron is found at a certain place. Before the measurement, it was not certain at which place it will be found --- only the probability was known. The probability is given by the wave function and is spread over many positions---until we perform the measurement of position.
This has been and is still very enigmatic. What does it mean that the electron has no definite position until we measure it? For instance, electron may exist as a wave and move on until it arrives at a fluorescent screen. If we look at the screen, we find a spot on the screen which indicates the place where the electron hit the screen. But what if we did not look? Would the electron hit the screen at that position even if we did not look?
Because the electron, before coming to the screen, existed as a wave and had thus many positions all at once, this means that also when hitting the screen there should be many impact positions at once. Hence there should be a superposition of many screens, each with a spot at a different place. If my friend Alice looks at the screen, she finds the black spot at a definite position. But if I do not look, there should be many possible positions of the black spot and also many possible versions of Alice, each seeing a different spot on the screen. This means that there must be many different worlds, each one containing a different version of Alice.
Such a reasoning leads to the so called many worlds interpretation of quantum mechanics, invented by Hugh Everett in 1957. Is reality a number of possibilities that all exist at the same time, a multiworlds existence, and if so, is there a reality that is more real than another.
In Everett theory there exist a universal wave function, containing many worlds experienced by many observers.
But what about me, namely, about my conscious experience? When I look, then I see a definite spot at a definite place of the screen, and if I ask Alice, she would say that she also found the spot on the same place. My conscious experience when observing the screen is not in a superposition of different versions of me seeing the black spot at different places on the screen. Relative to me, there can be a superposition of different versions of Alice seeing the black spot at different positions, but my awareness cannot be in that sort of superposition.
In the book "The Grand Biocentric Design", BenBella, 2020, (see also the last part of "The Landscape of Theoretical Physic", Kluwer Academic, 2001) one can read about many fascinating implications of quantum mechanics. It is explained how QM can be understood from the many worlds point of view, and yet all
those many worlds (e.g, different versions of Alice seeing the black spot at different locations on the screen), do not exist independently from a conscious observer. A wave function, comprising many different worlds in the above example is relative to me. My conscious perception is that there are many versions of Alice and in that sense there is no collapse of the wave function. Each version of Alice sees a different outcome of a measurement. But from my subjective point of view, when I look at the result of a measurement, wave function does collapse --- no longer many possible outcomes, but only one definite outcome registered in my awareness.
Thus, one thing is many worlds and many versions of an observer and his/her conscious experiences, and the other thing is me, my consciousness, being aware of all those worlds existing in an abstract space of possibilities that I can envisage, and only one of them becomes for me the reality upon measurement or observation. At every measurement or observation it is decided into which of those different realities (worlds, in the terminology of many world interpretation) enters or hangs up consciousness
Do we have free will?
Along the lines discussed above, we can understand the famous Libet experiments. In this experiment the brain activity of a test subject was monitored and it was found that before the person was aware of what she intended to do, this was already reflected in her brain activity a fraction of second earlier. This is usually interpreted that there is no free will, because what we perceive as a free decision is already prepared by our brain before we actually make the decision. Such interpretation holds in a deterministic world, but not in the world that is basically quantum and hence indeterministic and which splits at every measurement into new branches. And this branching occurs at the level of consciousness. Thus, if we decide to flip a finger (e.g., in a Libet experiment), at that moment the consciousness hangs up to the branch in which a fraction of a second earlier, there was a certain brain activity (shown in the occurrence of the readiness potential registered by the instrument in the Libet's laboratory). If we decide not to flip the finger, then the conscious me feeling (the "I") remains on the branch in which there was no readiness potential. The occurrence or non occurrence of the readiness potential has nothing to do with free will. This is explained in more detail and more completely in The Grand Biocentric Design.
Is there an ultimate reality?
Consciousness is fundamental. Without consciousness there can be no reality. Reality is consciousness, and consciousness is reality. In that sense an ultimate reality is consciousness.
The enigma that on the one hand there is me, my first person's conscious experience (the assumption of "ego-absolutism" in the post above), and on the other hand there are other observers, like Alice, who are also conscious, is explained in the book The Grand Biocentric Design by introducing the concept of the hierarchical levels of representation. Consciousness of other people, to me is like a picture within a picture.
In the book we explain that such view on quantum mechanics and consciousness is not solipsism. Namely, consciousness, as the first person experience---"me feeling" can be realized in many different ways: it can be realized as the me feeling in my brain of the person born here in Ljubljana , or it can be realized as the me feeling of any other person in the world.
In our intuition we are quite aware of this fact.
Namely, we often say something like: (i) ``If I were in your place, I would have done it differently'' or, (ii) ``If I lived again, I would have done it differently''. The sentence (i) illustrates that consciousness is localized in configuration space (in one's particular brain), whilst the sentence (ii) illustrates that consciousness is localized in the space of all possibilities. Within the framework of quantum mechanics this is the space of all possible quantum states, called Hilbert space, in which wave function and the consciousness associated with it can evolve along many possible paths, but normally it evolves along just one path. At any moment of its subjectively perceived time, consciousness finds itself localized at (or closely around) some point along an Everett branch. Of course there are other brains and other Everett's branches in which consciousness can also be localized, but these are not the brains and Everett's branches in which my consciousness is localized, i.e., ``I am not in your place'' and ``In my life I have experienced just this particular sequence of events, and not some other sequence of events''. This is related to a frequent objection to Everett's theory typically expressed as ``Why my consciousness finds itself in this particular and not some other Everett's branch?''.
There is a crucial distinction between one's own consciousness, and perception of somebody else's consciousness, expressed by the sentence ``I am not in your place''. Everyone experiences his own consciousness (his/her ``I'') as being localized in his/her head, and not in someone else's head. By ``consciousness'' we have in mind our own consciousness. When we speak about the consciousness of somebody else, we mentally put ourselves in his head (``in his place").
The so called ``hard problem of consciousness'' arises when we attempt to understand (describe in scientific terms) how consciousness arises from the brain's activity. If a scientist, say Bob, inspects the brain activity of somebody else, say Alice, then Alice's brain and its functioning is in fact represented in Bob's brain, that is in Bob's consciousness. The outside world, including Alice, is like being painted in Bob's consciousness. When inspecting, for instance by monitoring functioning of Alice's brain, Bob tries to figure out how Alice perceives the world, how she is conscious about the world around her. From Bob's perspective this is just like a picture within picture, or novel within novel, a play within play, movie within movie. However hard Bob tries to understand Alice's consciousness and her perception of the outer world, this is just a picture represented (``painted'') in Alice's brain, which in turn is a picture in Bob's consciousness.
We see that there are different levels of representation. Relative to me, on the highest level there is a representation, a ``picture'', of the world as perceived by consciousness. Within such the highest level ``picture'' there are lower level ``pictures'' associated with other observers. If we do not take this into account and do not distinguish between different levels of representation, then we have the ``hard problem of consciousness''. The problem is in our failure to recognize that the lower level representation of the world (a ``picture'') within a third person's brain under our scientific investigation cannot be identified with the higher level of representation (associated with the experimenter's "consciousness"). And the experimenter's "consciousness", is just a representation (a picture) in my consciousness.
The highest level of representation of the experienced world is associated with consciousness. On the other hand, the world is described by a wavefunction. This means that there is close relationship between consciousness and wavefunction. The lower lever representation of the world in another person's brain is not consciousness, and if we wish to understand how consciousness can arise in that person's brain we have ``the hard problem of consciousness''. Consciousness and the associated wave function are the highest level concepts, and cannot be derived from the lower level concepts.
Solipsism is avoided by postulating that wave function (consciousness) can be localized in any brain (either within a particular Everett's world or somewhere else in the multiverse). This can be illustrated by turning the sentences (i) mentioned above, namely, ``If I were at your place, I would have done it differently'' into `` I could have been at your place''. Wavefunction is a mathematical object whose evolution is determined by its initial value, which can be either such, or others. When having in mind the wave function of the entire universe, it can be such that the ``I'' ("me feeling") is in Bob's brain, seeing Alice as a representation (a ``picture'') in his brain. Or alternatively, the wave function of the universe can be such that the ``I'' is in Alice's brain seeing Bob as a representation (a ``picture'') in her brain. In other words, the wave function of the universe can be localized in or associated with Bob's brain, or it can be localized in Alice's brain. Other possible forms of the wave function can exist in principle, for instance, a wave function not sharply localized in one's particular brain at all, or within one particular Everett branch, but spread over a larger range . Mystical experiences reported by many people can be understood as being associated with such wave functions. Ultimately, all possibilities embracing state (wave function), containing many diverse individual conscious experiences ("me feeling") could be, if one wishes, understood as being associated or even identified with God. In that sense, God is the consciousness, that is the quantum state (wave function) embracing all possibilities, all possible worlds.
Where does consciousness reside within an individual? Do we think with our brain?
From the point of view of an individual, his brain, as everything he/she perceives, is the awareness in his consciousness. Thus saying that my consciousness resides in my brain, is a step from the highest level of representation to a lower one, because the talk is here about a picture within a picture. Because when think of my brain, this is a circular process, it is a like a snake eating its own tail.
Does consciousness end with death?
From the point of such reasoning (explained in more detail in The Grand Biocentric Design), consciousness does not end with death. We explain why from the first person point of view, consciousness does not cease to exist. Technically, the explanation is based on an extension/generalization of the famous "quantum suicide" thought experiment. Such topics were also discussed in the last, non technical part of my book The Landscape of Theoretical Physics, mentioned above.
Does time exist? Is it linear?
The time in the sense of a flow of events exists only in consciousness. Normally, in normal states of awareness it is linear, sequential, but in altered states of consciousness it need not be linear. Just think of dreams.
*)The text of this post is adapted from my interviews
https://www.google.com/search?client=firefox-b-d&q=mattej+pavsic#fpstate=ive&vld=cid:4c62735b,vid:3LgGv4jb3sU
https://www.thekarlfeldtcenter.com/the-biocentric-universe-with-physicist-matej-pavsic/