Wigner's Friend
Retrieved April 6, 2025, from Information Philosopher
Web site https://informationphilosopher.com/solutions/experiments/wigners_friend/
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Wigner's Friend
Eugene Wigner made quantum physics even more subjective than had John von Neumann or even Erwin Schrödinger with his famous Cat Paradox. Wigner claimed that a quantum measurement requires a conscious observer, without which nothing ever happens in the universe.
In 1961 he complicated the problem of the von Neumann or Heisenberg"Schnitt" (or the "shifty split" of John Bell) that forms the dividing line between the quantum world and the classical measurement apparatus. Wigner moved it farther into the conscious mind of the observer. Wigner is often said to have extended the problem of Schrödinger's Cat, by adding a second observer inside the laboratory who is commonly known as Wigner's Friend. Popular treatments of Wigner's Friend usually describe it in terms of a live and dead cat. Actually, Wigner's example was a photon and whether its wave function collapsed to cause a flash on a screen or not. Wigner's goal was to show that only consciousness can collapse a wave function. The cat example is more vivid. You can see it on Wigner's page. Here we give Wigner's original photon example The physicist friend inside the lab either sees a photon flash or not. (In a footnote Wigner notes that the human eye can perceive as few as three quanta.) But Wigner is outside the lab and does not know the outcome. Wigner says that without human consciousness, this leaves the world in a superposition of states." Wigner says that any inanimate material measuring device will leave both himself and his friend in a superposition of states. The only thing he sees that could change this is human consciousness, He resolves the paradox by saying that his friend's consciousness collapses the wave function inside the laboratory. Here is Wigner's complete argument:
Given any object, all the possible knowledge concerning that object can be given as its wave function. This is a mathematical concept the exact nature of which need not concern us here—it is composed of a (countable) infinity of numbers. If one knows these numbers, one can foresee the behavior of the object as far as it can be foreseen. More precisely, the wave function permits one to foretell with what probabilities the object will make one or another impression on us if we let it interact with us either directly, or indirectly. The object may be a radiation field, and its wave function will tell us with what probability we shall see a flash if we put our eyes at certain points, with what probability it will leave a dark spot on a photographic plate if this is placed at certain positions. In many cases the probability for one definite sensation will be so high that it amounts to a certainty—this is always so if classical mechanics provides a close enough approximation to the quantum laws. The information given by the wave function is communicable. If someone else somehow determines the wave function of a system, he can tell me about it and, according to the theory, the probabilities for the possible different impressions (or "sensations") will be equally large, no matter whether he or I interact with the system in a given fashion. In this sense, the wave function "exists." The information interpretation of quantum mechanics helps to resolve this paradox as follows,
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