4 ms·

I know this must be nuts, could someone ELI20?

by feiss 7y ago

human20190310 7y ago
This video by Veritasium covers the entanglement concept at a non-expert level: https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c
indutny 7y ago
In this image: https://advances.sciencemag.org/content/advances/5/7/eaaw2563/F1.large.jpg?width=800&height=600&carousel=1 https://advances.sciencemag.org/content/advances/5/7/eaaw256... they describe their apparatus. The light comes from bottom left, then gets into Beam Splitter (BS) and splits in two. The top-left detector has a pre-selected filter and a measurement device after a delay line. As far as I understand, this device gives binary output: either light received or not. The top-right detector captures whole image of the light ray, but does so only if the left detector has seen light. The result is that the image from the top-right detector is highly correlated with the pre-selected filter of the top-left detector, even though they're placed in different arms of Beam Splitter! https://advances.sciencemag.org/content/advances/5/7/eaaw2563/F2.large.jpg?width=800&height=600&carousel=1 https://advances.sciencemag.org/content/advances/5/7/eaaw256...
- Kovah 7y ago
  Thank you for the setup explanation. I understood what is happening and it is totally fascinating, even if I do not - and probably never can - understand why or how this is happening.
zwkrt 7y ago
A bell inequality is a feature of quantum systems by which they break our understanding of classical probability. A good reference is here [1]. The basic idea is to imagine a Venn diagram where the sum of the parts adds up to more than 100%. Bell inequalities are nifty because they heavily imply that our classical notions of “local realism” must be incomplete. Namely, that either quantum objects experience instantaneous action-at-a-distance (locality), or they do not have actualized measurement values when they aren’t being actively measured (realism). This paper is showing a way to produce a visual image of a bell inequality and explores ways that these inequalities can be used for other visual techniques. [1] https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs
- perl4ever 7y ago
  The point at which I disconnected from understanding physics is not the actual theory, but the stuff people say about locality. I mean, once you have the concept of fields, they're not local, right? So aren't people saying "what is not nonlocal must be local" which doesn't seem like a substantive principle. Every time I read about the struggle between realism and locality, it makes me feel unhappy because I have a choice between assuming it's me that's really dumb or everyone else. I can't understand why it should be a real problem.
  - SiempreViernes 7y ago
    Since you have a finite speed for information to propagate, locality can always be defined as "within the lightcone", and so locality is well defined, even for fields.
  - archgoon 7y ago
    Fields are neither local nor non local. Locality is about how information propagates. Classical Gravitation defines a force field that is nonlocal, as it propagates instantaneously. Move a mass from point a to point b, and the entire universe immediately updates the force they experience. Classical Electromagnetism defines a field that only propagates changes at a finite speed. If you move a charged mass from point a to point b, the change in the electric and magnetic fields radiate outward at the speed of light. One way to think about this is to observe that time update equations for E+M only rely on values infinitesimally near a point. If you need to compute the next time step on a grid, you only need to look at the neighbors of a point to figure out how to update a given point.
    - perl4ever 7y ago
      Ok, but if you think of a field as existing in space at a given time, then essentially all of it is not in your particular spot. Just saying "this field has a value elsewhere" means you are talking about something that is not near your given point. And having different values at different points is what I thought a field was.
kgwgk 7y ago
https://static.scientificamerican.com/sciam/assets/media/pdf/197911_0158.pdf https://static.scientificamerican.com/sciam/assets/media/pdf... https://physics.aps.org/articles/v8/123 https://physics.aps.org/articles/v8/123 The first link gets bonus points for the 40-year-old advertisements included in the scan.

I know this must be nuts, could someone ELI20?

by feiss 7y ago

I know this must be nuts, could someone ELI20?

human20190310 7y ago

This video by Veritasium covers the entanglement concept at a non-expert level: https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c

indutny 7y ago

In this image: https://advances.sciencemag.org/content/advances/5/7/eaaw2563/F1.large.jpg?width=800&height=600&carousel=1 https://advances.sciencemag.org/content/advances/5/7/eaaw256... they describe their apparatus. The light comes from bottom left, then gets into Beam Splitter (BS) and splits in two. The top-left detector has a pre-selected filter and a measurement device after a delay line. As far as I understand, this device gives binary output: either light received or not. The top-right detector captures whole image of the light ray, but does so only if the left detector has seen light. The result is that the image from the top-right detector is highly correlated with the pre-selected filter of the top-left detector, even though they're placed in different arms of Beam Splitter! https://advances.sciencemag.org/content/advances/5/7/eaaw2563/F2.large.jpg?width=800&height=600&carousel=1 https://advances.sciencemag.org/content/advances/5/7/eaaw256...

Kovah 7y ago
Thank you for the setup explanation. I understood what is happening and it is totally fascinating, even if I do not - and probably never can - understand why or how this is happening.

zwkrt 7y ago

A bell inequality is a feature of quantum systems by which they break our understanding of classical probability. A good reference is here [1]. The basic idea is to imagine a Venn diagram where the sum of the parts adds up to more than 100%. Bell inequalities are nifty because they heavily imply that our classical notions of “local realism” must be incomplete. Namely, that either quantum objects experience instantaneous action-at-a-distance (locality), or they do not have actualized measurement values when they aren’t being actively measured (realism). This paper is showing a way to produce a visual image of a bell inequality and explores ways that these inequalities can be used for other visual techniques. [1] https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs

perl4ever 7y ago
The point at which I disconnected from understanding physics is not the actual theory, but the stuff people say about locality. I mean, once you have the concept of fields, they're not local, right? So aren't people saying "what is not nonlocal must be local" which doesn't seem like a substantive principle. Every time I read about the struggle between realism and locality, it makes me feel unhappy because I have a choice between assuming it's me that's really dumb or everyone else. I can't understand why it should be a real problem.
- SiempreViernes 7y ago
  Since you have a finite speed for information to propagate, locality can always be defined as "within the lightcone", and so locality is well defined, even for fields.
- archgoon 7y ago
  Fields are neither local nor non local. Locality is about how information propagates. Classical Gravitation defines a force field that is nonlocal, as it propagates instantaneously. Move a mass from point a to point b, and the entire universe immediately updates the force they experience. Classical Electromagnetism defines a field that only propagates changes at a finite speed. If you move a charged mass from point a to point b, the change in the electric and magnetic fields radiate outward at the speed of light. One way to think about this is to observe that time update equations for E+M only rely on values infinitesimally near a point. If you need to compute the next time step on a grid, you only need to look at the neighbors of a point to figure out how to update a given point.
  - perl4ever 7y ago
    Ok, but if you think of a field as existing in space at a given time, then essentially all of it is not in your particular spot. Just saying "this field has a value elsewhere" means you are talking about something that is not near your given point. And having different values at different points is what I thought a field was.

kgwgk 7y ago

https://static.scientificamerican.com/sciam/assets/media/pdf/197911_0158.pdf https://static.scientificamerican.com/sciam/assets/media/pdf... https://physics.aps.org/articles/v8/123 https://physics.aps.org/articles/v8/123 The first link gets bonus points for the 40-year-old advertisements included in the scan.