How probable is the past changing without us noticing? Practically zero in the physical sense — objective past events cannot change — but our collective and personal memories, records, and narratives can be misremembered, selectively recorded, or revised (through misinformation, archival loss, or reinterpretation), so perceptions of the past shift; cognitive biases (e.g., Mandela effects) make subjective unnoticed changes plausible, but the historical record (primary sources) remains the anchor.

In the Wigner-friend thought experiment, Wigner is up to change the past of his friend locked in his isolated laboratory without that friend being up to notice any change of his past. That's due due to Wigner's friend absence of irreversibly recorded trace of his past after his initial state but prior to his returning to that initial state thanks to Wigner quantum manipulation of his friend's laboratory quantum state (and all its content). Of course, that thought experiment is practically totally impossible to achieve as we aren't up to manipulate, in a sufficiently detailed manner, the quantum state of a macroscopic system.

Our impossibility to get memories of the future as well as the impossibility to keep tracks of a previous past (a past prior to its change) can be linked to Maccone remark in his articlePreprint A quantum solution to the arrow-of-time dilemma

"Within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having

happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to a mere tautology: physics cannot study those processes where entropy has decreased, even if they were commonplace."

That issue can also be linked to Wheeler's statement in

Chapter Delayed-Choice Experiments and Bohr's Elementary Quantum Phenomenon

"No elementary quantum phenomenon is a phenomenon until it is a registered ('observed', 'indelibly recorded') phenomenon, 'brought to a close' by 'an irreversible act of amplification'."

Note : I share the Wheeler's view stated above with the proviso that I don't like the term 'observed' used in his statement. In my opinion, that word suggests (wrongly) the need for the physical presence of an observer for the Schrödinger cat to die. Of course, the cat is intersubjectively dead as soon as there is an irreversible record of his death, that's to say an irreversibly recorded trace of that past event (a macroscopic quantity intersubjectively and reproducibly readable by distinct macroscopic observers, be there present of not).

Without our thermodynamic statistical grid of reading of macroscopic observers=living beings (defined by irreversibly recorded macroscopic quantities) there would be:

• no traces of the past (hence no memories),
• no distinction between past and future events,
• no irreversible flow of time,
• no causality principle (and more generally no time asymmetries),
• and no physical laws, constants and properties we ascribe to our (macroscopic interactions with) the universe.

More on that issue can be found in: Preprint The arrow of time issue, an overview