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Physicists demonstrate new way to violate local causality

phys.org

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Re: Physicists demonstrate new way to violate local causality

#61

Post by jmatthews »

roywiggins wrote: post #52
21 wrote: post #21

Earlier quoted context omitted.

> what does "Bell inequality" really mean Some stuff is just hard to understand. You can make analogies, like talking about two people hiding a colored ball in their hand and one of them showing it later, but these are not truly helpful in really understanding the issue. For that you need a bit of math. One physicist explained this really nicely, saying that because we live in the macroscopic world we have a certain…

Our intuitions about macro stuff is really bad, or at least the intuition of people not steeped in Newtonian mechanics is really bad. Gallilean relativity is deeply unintuitive to many physics-naive people, let alone Newtonian mechanics. There's a reason Aristotelian mechanics lasted so long (okay, also the lack of a real scientific method, but still). To demonstrate, ask a random person these questions: If I stand o…

You'd land slightly back because you'd lose contact with your impeller. I also think most everyone understands how a sling and rock work. Not sure I agree withyour premise. Could you shove a camera in people's face and get dumb answers? Sure. Not sure that's endemic.

Re: Physicists demonstrate new way to violate local causality

#62

Post by empath75 »

toxicFork wrote: post #3

Going meta here, speaking from the perspective of a software engineer with a great interest for physics. There are a lot of terms I do not fully understand in this article. Can you recommend an application that would allow me to select for example the words "distant particles can be correlated in ways that are impossible for classical objects" and find out what lead to that discovery? Or what does "Bell inequality" r…

Do you have a few hundred spare hours? Leonard Susskind did an extensive series of lectures, that assumes some familiarity with Calculus and linear algebra, but almost nothing else.

https://www.youtube.com/watch?v=iJfw6lDlTuA

Re: Physicists demonstrate new way to violate local causality

#63

Post by roywiggins »

hutzlibu wrote: post #58
roywiggins wrote: post #54

Earlier quoted context omitted.

I was imprecise: I'm not talking wing-walkers, I mean standing inside a plane, where wind is hopefully nonexistent

Hm ... and then you expect people would say, they would land backwards? I think allmost everybody would get it right ...

This is based on muddled memories of a highschool physics class, but yes- I'd expect a lot of people to get it wrong, based on the sheer number of my classmates who persistently got it wrong. My teacher subscribed to a slightly nuts teaching philosophy which involved polling the class repeatedly, so I had a pretty good sense of how hard people found the ideas.

Example: literally the other reply to my comment. The notion of an "impeller" to continue uniform motion is Aristotelian, and it seems to coincide with a lot of people's intuition:

https://news.ycombinator.com/item?id=14175605

Re: Physicists demonstrate new way to violate local causality

#64

Post by roywiggins »

jmatthews wrote: post #61
roywiggins wrote: post #52

Earlier quoted context omitted.

Our intuitions about macro stuff is really bad, or at least the intuition of people not steeped in Newtonian mechanics is really bad. Gallilean relativity is deeply unintuitive to many physics-naive people, let alone Newtonian mechanics. There's a reason Aristotelian mechanics lasted so long (okay, also the lack of a real scientific method, but still). To demonstrate, ask a random person these questions: If I stand o…

You'd land slightly back because you'd lose contact with your impeller. I also think most everyone understands how a sling and rock work. Not sure I agree withyour premise. Could you shove a camera in people's face and get dumb answers? Sure. Not sure that's endemic.

You begin at rest, relative to the plane. You jump and experience a force vertically. You land. You haven't experienced any forces horizontally, so you land exactly where you started.

Losing contact with the ground has nothing to do with it. You don't need an "impeller" to continue uniform motion- see Newton's first law.

Re: Physicists demonstrate new way to violate local causality

#65

Post by lisper »

amoruso wrote: post #10

If anyone wants to learn more about Bell's inequality, the simplest (real, no bullshit) explanation I've ever seen is by David Griffiths. You can follow it if you know the basics of calculus: http://www.physics.umd.edu/courses/Phys270/Jenkins/Griffiths...

And if you don't know calculus, there's Mermin's version:

http://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/f...

Re: Physicists demonstrate new way to violate local causality

#66

Post by computerex »

roywiggins wrote: post #64
jmatthews wrote: post #61

Earlier quoted context omitted.

You'd land slightly back because you'd lose contact with your impeller. I also think most everyone understands how a sling and rock work. Not sure I agree withyour premise. Could you shove a camera in people's face and get dumb answers? Sure. Not sure that's endemic.

You begin at rest, relative to the plane. You jump and experience a force vertically. You land. You haven't experienced any forces horizontally, so you land exactly where you started. Losing contact with the ground has nothing to do with it. You don't need an "impeller" to continue uniform motion- see Newton's first law.

Maybe I don't understand this thought experiment. Say you are on a plane cruising at 500 mph. The plane is applying constant thrust to counteract atmospheric drag in order to maintain steady 500 mph. At this point your horizontal speed is 500 mph and your vertical speed is zero, as the plane is maintaining level flight. When you decide to jump, you start accelerating towards the ground at roughly 9.8 m/s^2. Your horizontal speed is still close to 500 mph, but it starts decreasing due to atmospheric drag. So you are falling towards the earth, but you are also moving at a considerable horizontal speed relative to the ground due to newton's first law. The plane keeps apply thrust to maintain 500 mph. Eventually you'll land, behind the place but a considerable distance away from where you started.

Re: Physicists demonstrate new way to violate local causality

#67

Post by roywiggins »

computerex wrote: post #66
roywiggins wrote: post #64

Earlier quoted context omitted.

You begin at rest, relative to the plane. You jump and experience a force vertically. You land. You haven't experienced any forces horizontally, so you land exactly where you started. Losing contact with the ground has nothing to do with it. You don't need an "impeller" to continue uniform motion- see Newton's first law.

Maybe I don't understand this thought experiment. Say you are on a plane cruising at 500 mph. The plane is applying constant thrust to counteract atmospheric drag in order to maintain steady 500 mph. At this point your horizontal speed is 500 mph and your vertical speed is zero, as the plane is maintaining level flight. When you decide to jump, you start accelerating towards the ground at roughly 9.8 m/s^2. Your hori…

I may have been imprecise: I'm talking about jumping inside the plane, not wingwalking. There's no drag inside the plane.

If you've ever taken a pee standing up on a plane, you'll notice that your pee doesn't veer rearward. You can juggle balls without losing them (you can try this in the back of a car). If you throw a ball forwards on a plane, it doesn't come back to you, and those carts that are rolled up and down the aisles aren't constantly trying to flee towards the back of the plane.

Re: Physicists demonstrate new way to violate local causality

#68

Post by ttctciyf »

The article seems particularly hermetic, especially wrt "bilocal causality" which doesn't seem to occur as a phrase before Dec 2016[1].

The piece tells us "bilocal causality" is

> a concept that is related to the more standard local causality, except that it accounts for the precise way in which physical systems are initially generated.

and later references

> a new type of Bell inequality that accounts for the fact that the two sources of states used in the experiment are independent, the so-called bilocality assumption.

If, like me, you find this non-obvious, you may like to get some background from an earlier paper[2] (different authors) which has a seemingly less opaque description of bilocality than the phys.org article:

> Nowadays, fast progress towards advanced demonstrations of quantum communication networks, involving quantum repeaters [10] based on entanglement swappings [11] and quantum memories [12], are underway in many labs around the world. In these future quantum networks, several independent sources of entangled qubit pairs will distribute entanglement to partners who will then connect their neighbours by performing joint measurements on two (or more) qubits, each entangled with one neighbouring qubit, as illustrated for the simple case of three partners in Fig. 1. Such experiments have an interesting feature that has so far received little attention in previous works on nonlocality: the multipartite correlations between the measurement results at each site do not originate from a single multipartite entangled state, but from a series of bipartite entangled states that are initially independent and uncorrelated from each other; i.e., there is not a unique initial joint state (the analogue of λ in a locally causal model) that is responsible for the observed correlations, but these are instead created from smaller systems through joint measurements. [...]

1: https://www.google.com/search?q=%22bilocal+causality%22+-%22...

2: https://arxiv.org/pdf/1112.4502.pdf

Re: Physicists demonstrate new way to violate local causality

#69

Post by pducks32 »

ice109 wrote: post #30
amoruso wrote: post #10

If anyone wants to learn more about Bell's inequality, the simplest (real, no bullshit) explanation I've ever seen is by David Griffiths. You can follow it if you know the basics of calculus: http://www.physics.umd.edu/courses/Phys270/Jenkins/Griffiths...

it's funny i read this years ago (at the end of second semester qm) and it never clicked. guess i've gotten smarter since then. thanks.

I'll say that most of my physics classes lasted 2 years. 1 quarter in the class room and a year and a half before it really clicked. Wouldn't it be nice if you could retake classes again.

Re: Physicists demonstrate new way to violate local causality

#70

Post by rayuela »

mrcactu5 wrote: post #11

a lot at stake this year with a discussion of the foundations. Leonard Susskind, Adam R. Brown, Ying Zhao Quantum Complexity and Negative Curvature https://arxiv.org/abs/1608.02612 The Second Law of Quantum Complexity https://arxiv.org/abs/1701.01107

These are some really cool papers. Thanks for sharing them.
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