# Entanglement question - how to prove that before one of a pair is measured, it's not well-defined?

**URL:** <https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047>\
**Category:** Factual Questions\
**Created:** [July 9, 2010, 1:18am UTC](https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047 "2010-07-09T01:18:56Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\
**Post date:** [July 12, 2010, 7:36pm UTC](https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047/21 "2010-07-12T19:36:27Z")

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> [@Chronos](#):
>
> That’s still enough to destroy the interference.

And does the detector detect that the particle passed through it always?, sometimes?, never?, 50% of the time?.

These are actual experiments right?, not “thought experiments” like Schroedinger’s cat?.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [July 12, 2010, 8:32pm UTC](https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047/22 "2010-07-12T20:32:24Z")

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The detector at the slit says that about half of the particles went through that slit, and presumably the other half went through the other slit. And yes, these are actual experiments that have actually been done, with the results really matching the predictions of quantum mechanics.

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**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\
**Post date:** [July 12, 2010, 8:35pm UTC](https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047/23 "2010-07-12T20:35:31Z")

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Wikipedia is a good place to start for more information about the actual double-slit experiments: [Double-slit experiment - Wikipedia](http://en.wikipedia.org/wiki/Double-slit_experiment)

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**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\
**Post date:** [July 12, 2010, 10:27pm UTC](https://boards.straightdope.com/t/entanglement-question-how-to-prove-that-before-one-of-a-pair-is-measured-its-not-well-defined/546047/24 "2010-07-12T22:27:05Z")

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> [@Chronos](#):
>
> The detector at the slit says that about half of the particles went through that slit, and presumably the other half went through the other slit. And yes, these are actual experiments that have actually been done, with the results really matching the predictions of quantum mechanics.

Cool! 😃

Thanks!.

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