What is Wigner's friend: Definition and 20 Discussions

Wigner's friend is a thought experiment in theoretical quantum physics, first conceived by the physicist Eugene Wigner in 1961, and developed into a thought experiment by David Deutsch in 1985. The scenario involves an indirect observation of a quantum measurement: An observer W observes another observer F who performs a quantum measurement on a physical system. The two observers then formulate a statement about the physical system's state after the measurement according to the laws of quantum theory. However, in most of the interpretations of quantum theory, the resulting statements of the two observers contradict each other. This reflects a seeming incompatibility of two laws in quantum theory: the deterministic and continuous time evolution of the state of a closed system and the nondeterministic, discontinuous collapse of the state of a system upon measurement. Wigner's friend is therefore directly linked to the measurement problem in quantum mechanics with its famous Schrödinger's cat paradox.
Generalizations and extensions of Wigner's friend have been proposed. Two such scenarios involving multiple friends have been implemented in a laboratory, using photons to stand in for the friends.

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  1. H

    A Is there still a Wigner's friend paradox?

    Happy new year to all the forumers. The Wigner's friend paradox appeared in the early years of quantum mechanics: Wigner knows that his friend looked inside the box to see if the cat is dead or alive. He has two cells in his mind to put what his friend saw. As he ignores the result, he may...
  2. Marco211298

    I Doubts regarding my interpretation of Wigner’s Friend Scenario

    I have some doubts regarding my personal interpretation that i was contemplating about in the context of Wigner's friend experiment (also tested in the laboratory recently). Could it be that a system is always in a superposition, and when we perform a measurement, we obtain a definite value due...
  3. StevieTNZ

    I Another Wigner's Friend Thought Experiment Paper

    https://phys.org/news/2021-05-future.html The paper in question can be found at https://www.nature.com/articles/s42005-021-00589-1 This thread is for discussion of the said paper.
  4. SEYED2001

    I Why does "control" mean "coherency" in Wigner's friend paradox?

    There we read: "note that if Wigner did not know this phase due to the lack of control of it, he would describe the “spin + friend’s laboratory” in an incoherent mixture of the two possibilities". Why is this the case? Given that the author has propoede neither a citation nor a proof for this...
  5. J

    I Is this a valid Wigner's friend experiment?

    1. Wigner's friend is inside a lab, and has a pair of entangled photons. 2. He measures one in the vertical axis. 3. He sends the other photon to Wigner. 4. Wigner can: a) measure it in the vertical axis (if so, he will get the same result as Wigner's friend). 5. Or Wigner can: b) send the...
  6. allisrelative

    I Does Decoherence get rid of all Quantumness?

    The answer is no and even when decoherence occurs for Wigner's Friend in the lab, quantum coherence remains. Let's start with the paper that illustrates this. Assisted Macroscopic Quantumness CONT. https://arxiv.org/abs/1711.10498 Wow, I recently read this paper and the results are simply...
  7. allisrelative

    Doesn't Wigner's Friend Experiment solve the measurement problem?

    If you look at the recent Wigner's Friend experiment, it seems to support Carlo Rovelli's Relational Interpretation which says there's no real measurement. Wiger's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical...
  8. S

    Wigner's Friend and algorithmic information

    I'm working on a novel involving QM ideas. What I'm most excited about is the prospect of taking a view on Markus Muller's propositions involving reality and algorithmic information theory. I'm trying to make it work with the Wigner's Friend thought experiment. Are there any ideas on how...
  9. S

    A How Can Wigner's Friend Experiment Influence Novel Plots in Quantum Mechanics?

    I'm working on a novel involving QM ideas. What I'm most excited about is the prospect of taking a view on Markus Muller's propositions involving reality and algorithmic information theory. I'm trying to make it work with the Wigner's Friend thought experiment. Are there any ideas on how...
  10. Quantum Alchemy

    I Wigner's Friend shows the difference between Observer and Measurement

    You often hear this debate about the role of the observer in Quantum Mechanics. How you view this role is usually dictates the interpretation you prefer. If it's Copenhagen, then the observer is more robust and plays a crucial role in wave function collapse. If it's Many Worlds, then the...
  11. S

    A Could different outcomes have different physics in Wigner's friend?

    Summary: Could different outcomes have different physics in Wigner's friend? Physicist Eugene Wigner said that consciousness was fundamental for physics and that laws of physics existed because of it. He said that "consciousness can change the usual laws of physics" He also proposed the...
  12. DarMM

    A Wigner's Friend and Incompatibility

    Summary: Wigner's friend seems to lead to certainty in two complimentary contexts This is probably pretty dumb, but I was just thinking about Wigner's friend and wondering about the two contexts involved. The basic set up I'm wondering about is as follows: The friend does a spin measurement...
  13. M

    I Qbism vs Copenhagen: What is the Difference?

    I read a few articles about Qbism, but feel like I don't understand the difference from Copenhagen. One author claims the difference is that each observer has his own view of the world (and own probabilities in qbism). I thought that was also the case in Copenhagen. E.g. the old Wigner's...
  14. J

    B Proietti 2019 experiment: Doubt in measuring A1(B1)

    Hi everyone! Sorry for the bad english! Paper: https://arxiv.org/abs/1902.05080 I guess I understood the experiment until the moment Alice and Bob chooses to measure A0(B0) or A1(B1). I guess it's kind of straightforward that without a bell state measurement with photon alfa(beta) and photon...
  15. I

    I Very simple explanation of Wigner's friend needed

    Hi, just browsed through some news stories and saw a story about a group of researchers who seemed to prove we can experience very different realities. I was just wondering if anyone could provide a simple explanation of their experiment and what they did? Do we really live in different realities?
  16. tomdodd4598

    I Frauchiger & Renner: There's a Mistake, Right?

    Hey there! I was recently pointed to this thought experiment, claiming an apparent 'contradiction' involving the various predictions of the observers. Now, this has been discussed on PF quite recently, but I found the discussion rather hard to follow. I've read the paper, the PF discussion...
  17. PeterDonis

    A A Realization of a Basic Wigner's Friend Type Experiment

    See this recent paper: https://arxiv.org/abs/1902.05080
  18. RUTA

    Insights The Quantum Mystery of Wigner's Friend - Comments

    Greg Bernhardt submitted a new blog post Wigner's Friend Continue reading the Original Blog Post.
  19. G

    B Exploring Wigner's Friend: The Debate on Wave Function Collapse

    Wigner's Friend was supposed to be the arguments that wave function collapse can't be really occurring, but there seemed to be counterarguments that refuted the illustration of the Wigner's Friend. What is it? I can't seem to find them at google. Thank you.
  20. bremsstrahlung

    A Consciousness and the Wigner's friend problem

    Consciousness and the Wigner's friend problem by Bernard d’Espagnat. Information and fundamental elements of the structure of quantum theory by Caslav Brukner and Anton Zeilinger So Quantum Mechanics is nothing but footnotes to Plato. Is Philosophy dead?
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