1<\/a><\/sup>. (Frauchiger has now left academia.)<\/p>\nWeird world<\/h6>\n
Quantum mechanics underlies nearly all of modern physics, explaining everything from the structure of atoms to why magnets stick to each other. But its conceptual foundations continue to leave researchers grasping for answers. Its equations cannot predict the exact outcome of a measurement \u2014 for example, of the position of an electron \u2014 only the probabilities that it can yield particular values.<\/p>\n
Quantum objects such as electrons therefore live in a cloud of uncertainty, mathematically encoded in a \u2018wavefunction\u2019 that changes shape smoothly, much like ordinary waves in the sea. But when a property such as an electron\u2019s position is measured, it always yields one precise value (and yields the same value again if measured immediately after).<\/p>\n
The most common way of understanding this was formulated in the 1920s by quantum-theory pioneers Niels Bohr and Werner Heisenberg, and is called the Copenhagen interpretation, after the city where Bohr lived. It says that the act of observing a quantum system makes the wavefunction \u2018collapse\u2019 from a spread-out curve to a single data point.<\/p>\n
The Copenhagen interpretation left open the question of why different rules should apply to the quantum world of the atom and the classical world of laboratory measurements (and of everyday experience). But it was also reassuring: although quantum objects live in uncertain states, experimental observation happens in the classical realm and gives unambiguous results.<\/p>\n
Now, Frauchiger and Renner are shaking physicists out of this comforting position. Their theoretical reasoning says that the basic Copenhagen picture \u2014 as well as other interpretations that share some of its basic assumptions \u2014 is not internally consistent.<\/p>\n
What\u2019s in the box?<\/h6>\n
Their scenario is considerably more involved than Schr\u00f6dinger\u2019s cat \u2014 proposed in 1935 \u2014 in which the feline lived in a box with a mechanism that would release a poison on the basis of a random occurrence, such as the decay of an atomic nucleus. In that case, the state of the cat was uncertain until the experimenter opened the box and checked it.<\/p>\n
In 1967, the Hungarian physicist Eugene Wigner proposed a version of the paradox in which he replaced the cat and the poison with a physicist friend who lived inside a box with a measuring device that could return one of two results, such as a coin showing heads or tails. Does the wavefunction collapse when Wigner\u2019s friend becomes aware of the result? One school of thought says that it does, suggesting that consciousness is outside the quantum realm. But if quantum mechanics applies to the physicist, then she should be in an uncertain state that combines both outcomes until Wigner opens the box.<\/p>\n
Frauchiger and Renner have a yet more sophisticated version (see \u2018New cats in town\u2019). They have two Wigners, each doing an experiment on a physicist friend whom they keep in a box. One of the two friends (call her Alice) can toss a coin and \u2014 using her knowledge of quantum physics \u2014 prepare a quantum message to send to the other friend (call him Bob). Using his knowledge of quantum theory, Bob can detect Alice\u2019s message and guess the result of her coin toss. When the two Wigners open their boxes, in some situations they can conclude with certainty which side the coin landed on, Renner says \u2014 but occasionally their conclusions are inconsistent. \u201cOne says, \u2018I\u2019m sure it\u2019s tails,\u2019 and the other one says, \u2018I\u2019m sure it\u2019s heads,\u2019\u201d Renner says.<\/p>\n
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<\/figcaption><\/figure>\n <\/p>\n
<\/p>\n
The experiment cannot be put into practice, because it would require the Wigners to measure all quantum properties of their friends, which includes reading their minds, points out theorist L\u00eddia Del Rio, a colleague of Renner\u2019s at ETH Zurich.<\/p>\n
Yet it might be feasible to make two quantum computers play the parts of Alice and Bob: the logic of the argument requires only that they know the rules of physics and make decisions based on them, and in principle one can detect the complete quantum state of a quantum computer. (Quantum computers sophisticated enough to do this do not yet exist, Renner points out.)<\/p>\n
Duelling interpretations<\/h6>\n
Physicists are still coming to terms with the implications of the result. It has triggered heated responses from experts in the foundations of quantum theory, many of whom tend to be protective of their pet interpretation. \u201cSome get emotional,\u201d Renner says. And different researchers tend to draw different conclusions. \u201cMost people claim that the experiment shows that their interpretation is the only one that is correct.\u201d<\/p>\n
For Leifer, producing inconsistent results should not necessarily be a deal breaker. Some interpretations of quantum mechanics already allow for views of reality that depend on perspective. That could be less unsavoury than having to admit that quantum theory does not apply to complex things such as people, he says.<\/p>\n
Robert Spekkens, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, says that the way out of the paradox could hide in some subtle assumptions in the argument, in particular in the communication between Alice and Bob.<\/p>\n
\u201cTo my mind, there\u2019s a lot of situations where taking somebody\u2019s knowledge on board involves some translation of their knowledge.\u201d Perhaps the inconsistency arises from Bob not interpreting Alice’s message properly, he says. But he admits that he has not found a solution yet.<\/p>\n
For now, physicists are likely to continue debating. \u201cI don\u2019t think we\u2019ve made sense of this,\u201d Leifer says.<\/p>\n
<\/p>\n
<\/p>\n<\/div>\n
doi: 10.1038\/d41586-018-06749-8<\/div>\n
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<\/p>\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
(\uc6d0\ubb38: \uc5ec\uae30\ub97c \ud074\ub9ad\ud558\uc138\uc694~) In a multi-\u2018cat\u2019 experiment, the textbook interpretation of quantum theory seems to lead to contradictory pictures of reality,(more…)<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[34,36,29],"tags":[],"class_list":["post-1821","post","type-post","status-publish","format-standard","hentry","category-lets-do-chemistry","category-lets-do-physics","category-lets-do-science"],"aioseo_notices":[],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack-related-posts":[{"id":1497,"url":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=1497","url_meta":{"origin":1821,"position":0},"title":"\ucc45 \uc18c\uac1c – Schr\u00f6dinger\u2019s cat among biology\u2019s pigeons: 75 years of What Is Life?","author":"biochemistry","date":"August 30, 2018","format":false,"excerpt":"\u00a0 \u00a0 (\uc6d0\ubb38) \u00a0 \u00a0 Philip Ball revisits a book that crystallized key concepts in modern molecular biology. \u00a0 \u00a0 \u00a0 Physicist Erwin Schr\u00f6dinger also probed questions of molecular biology.Credit: Bettmann\/Getty \u00a0 \u00a0 What Is Life? 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