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:
The afterword seems kind of weak as it tries to return to the question of what quantum mechanics "means" and then... sort of doesn't really do that. It goes on to describe an "ethereal" influence distinct from causal influence, which is to say an "influence" that doesn't actually do anything at all. That's pretty disappointing.
The simplest explanation of what measurement actually is, in a quantum sense, is that it's the act of entangling your brain with whatever amplitude configuration represents the experiment we're interested in looking at. Basically the amplitude goes from looking like "brain * (result A + result B)" to "brain A * result A + brain B * result B". Brain A at that point can't communicate in any meaningful way with brain B because there isn't any causal relationship between the two anymore, so in a sense the brain has "split" and this is what we call decoherence. But for brain A to suppose that brain B has vanished in a puff of smoke, that is to say to privilege brain A over brain B, needlessly complicates the theory. It is the same as saying that since you can't see a thing anymore, it must not exist, and trying to build an entire physics based around what happens after you close your eyes.
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.
For a longer exposition that also covers the historical context more deeply, Jim Baggott's "The Meaning of Quantum Theory: A Guide For Students Of Chemistry and Physics" covers Bell's Theorem in Chapter 4 and also requires only basic knowledge of calculus.
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" really mean?
If not, are there recommended methods of understanding articles better?
I am thinking of this process:
- copy the article to a document
- find any words or sentences that you would like to understand better, highlight them in some colour e.g. red
- do some googling / wiki surfing or ask in HN or some kind of forums about them
- add sections after the paragraphs with highlights to explain the words or sentences
- recursion
- once satisfied, highlight the words in a different colour?
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 kind of experience of it, and our gut feelings and intuitions are just wrong when applied to the microscopic world, and using math is just a way of avoiding these misconceptions, and after a while of doing that you will get a new "math intuition" of physics.
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 on an airplane and jump, do I land where I started, further back, or further forward, relative to the plane?
if I whirl a ball on a string clockwise around my head and let go, does (neglecting vertical motion) the ball travel
A) curving counterclockwise
B) curving clockwise
C) in a straight line
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.
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.
Well yes, standing inside the plane is a very important distinction. If you are standing inside and decide to jump, there is no atmospheric drag inside the plane to slow down your horizontal speed because any atmosphere in the plane is moving at the same speed you are and so the relative speed is zero.
"If I stand on an airplane and jump, do I land where I started, further back, or further forward, relative to the plane?"
I would say, that most people would answer correctly, that you would not land on the same place, but probably be blown away far backwards from air friction (even though they would just call it wind).
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:
> The notion of an "impeller" to continue uniform motion is Aristotelian, and it seems to coincide with a lot of people's intuition:
It also coincides with the kinds of experiment that people using Aristotelian physics are in a position to perform. See my comment about Carlo Rovelli's paper.
>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" really mean? If not, are there recommended methods of understanding articles better?
Venkman: Ray, pretend for a moment that I don't know anything about metallurgy, engineering, or physics, and just tell me what the hell is going on.
Stantz: You never studied.
(Ghostbusters)
Just studying the principles -- e.g. reading an entry level Physics/QM book, would be better and more solid than surfing around, or asking for opinions on forums (which you couldn't evaluate anyway -- tons of bogus opinions in any kind of forum), or googling stuff.
Stuff like "Bell's inequality" are so basic that merely surfing and skimming wouldn't help (it would be all a kind of concept soup in the end). You can do that with more advanced notions.
You can also look at some Susskind talks. He is a very good explainer. They don't skim on the math, but it's followable for somebody with a high-school level of math, at least for the basic concepts like spin or bell inequality.
I'm in the same boat as you, but started looking a while ago when quantum computers started showing promise.
I found:
- PBS Spacetime on YouTube is awesome
- Check out the double split experiment. It seems simple, but when you think about what getting interference patterns when only sending one particle at a time really means..
- Then when ready, the quantum eraser
- Bells inequality is a simple idea that proved the strange interpretation of the double split experiment was true, rather than a more mundane interpretation
If you like the double slit experiment, you'll probably like Feynmann's "motivation" of the path integral in quantum field theory:
Suppose you take the double slit (plate with two holes in it), and you start adding more holes. Of course the particle will go through all of them, but with less probability the further they are off-axis. We can also add more than one plate, and the particle will go through all possible combinations of holes with some probability. Now suppose we let the number of plates go to infinity, and then let the number of holes in each plate go to infinity. The particle still goes through all of the infinite combinations of holes, with less probability if the path is unlikely. But really, there is nothing left anymore between the particle source and the screen! There is just free space! So this path integral with the particle going sort-of-everyhere weighted by probability actually describes propagation through free space.
That connection between slits in the infinite limit and path integrals is really fascinating. I've read of lot of Feynman, but never that. Where does he talk about it?
I'm actually not sure, off the top of my head, but I think it's from his book "Quantum Mechanics and Path Integrals"; he also used it in lectures IIRC.
The two slit experiment has been done with buckyballs, which would indicate all matter is capable of exhibiting quantum nature.
Keep in mind the double slit experiment is designed to illustrate the "effect" we observe, when we observe, is at least partially governed by the observation. Given certain interpretations, it can also have philosophical ramifications[1][2] for the existence of reality, self, and choice.
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.
It does not feel good enough since it would be ~opening new tabs and~ make me go through a lot more effort than necessary, and it feels like I would be overwhelmed and give up eventually. If there is something that would make "understanding articles as easy as possible" then that would be amazing. I'll probably end up developing this myself...
>It does not feel good enough since it would be ~opening new tabs and~ make me go through a lot more effort than necessary
While you don't need new tabs to google a term with some addons, actual knowledge of the subject, even at a basic level, takes a lot more effort than opening tabs.
I was thinking a Chrome extension to automate this process would be a great way to turn "inaccessible science gibberish" into "Eli5" for a mass of people (I'd like to read that but I'm not willing to do any work other than install a Chrome xtension)
Not sure if that would really work. Understanding science, math, and engineering using analogies leads to misconceptions when one tries to apply them. The math is true understanding. If you understand the equation, then you not only understand the subject, but often what would happen if you tweaked the inputs...etc. Although it would be nice if Wikipedia had an ELI5 section on its physics articles to go through first, so I agree with you there.
Legitimate scientists have rejected local causality as the mechanism to explain spooky quantum actions at a distance for some time now. Of course when something is false there will be multiple ways to prove it, but giving a new way doesn't really teach us anything new
Ugh. Please don't give more life to the meme of "spooky" quantum mechanics. It's mostly propagated by popular science outlets trying to make physics seem weirder than it is.
Einstein, in the famous EPR paradox paper[1] used quantum mechanics (then fairly new) to show that it predicted some truly unexpected behaviour that we now call entanglement, but which Einstein referred to as "spooky action at a distance" (This is the source of the word "spooky" in this context, which has been picked up on by new-age nuts). Einstein didn't think entanglement was real at this time, but that QM's prediction of entanglement showed that QM was broken or incomplete in some way. No method was then proposed of testing whether or not Einstein was right (it was likely beyond the technical capabilities of the time).
John Stewart Bell came up with with a theorem[1] that would be developed into experimental tests that showed what Einstein thought was an error in QM was actually observable. It's worth noting that closing all possible loopholes in experimental tests of Bell inequalities is difficult and elusive. The consensus is that QM is valid and entanglement does indeed exist, but there remains room for a tiny amount of doubt. Note: This is doubt the way scientists see it. i.e. A reason to perform further experiments. Theories can never be proven true, but teasing out every nuance and conceivable way in which they can be wrong, and testing them, is what makes theories accepted and useful. QM has stood up to enough tests that it's now a very well established theory. However, continuing to test QM is not useless. Searching for new ways in which well established theories can be wrong is how we found QM in the first place!
Note that QM does not say what, philosophically, entanglement is or explain the mechanism for how non-local correlations propagate. There are other theories that have attempted to explain this, but none have been successful so far in being both correct and, more importantly, testable.
There have been other inequalities experimentally tested in addition to Bell inequalities, such as Leggett inequalities[3]. To your understanding, these would not be interesting because they don't "teach us anything new". This is false. The question about QM and entanglement isn't as straight forward as a binary "yes it exists" vs "no it doesn't". There exist subtleties. Different inequalities have indeed enhanced our understanding of entanglement and proven useful in practical applications such as quantum computing and quantum cryptography. I haven't had time to dig into the work linked to here, but to dismiss it out of hand is not reasonable. The press release (to be taken with a grain of salt given the usual failings of science journalism) does suggest a novel approach that could be of practical use.
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. [...]
It's shocking to me no one has written a physics game engine based on observation and waveform collapse.
I believe there's a 5000 line genre buster just waiting to be written, using a simple SDF renderer and the wave equation. Please, someone beat me to it I have other things to write first.
To someone who doesn't actually know many of the terms here, it sounds like they didn't accomplish much. They defined a stronger form of locality called "bi-locality" and found a way to violate it.
The structure of the argument is, "We stood up a strawman and knocked it down," which sounds pretty weak. Is bi-locality an inherently interesting property? Does all classical computation follow it? I don't know enough to know if I care, but press releases tend to oversell so my default stance has to be "No, I don't really care."
>The structure of the argument is, "We stood up a strawman and knocked it down," which sounds pretty weak.
Weaker than "I don't understand what the paper says, only read a high level layman summary, not sure even what what the experiment was for, but I'm gonna piss on it anyway"?
Even assuming what you already describe, the ability to show bi-locality as a new lower bound above which locality constraints apply, is very interesting, and nothing like a strawman.
It would only be a strawman if they claimed they violated locality in general.
It's amazing how so many people on the internet feel so confident in their own intellectual abilities that they can read something that they don't understand and dismiss it as if it's non-important.
That's funny, I read his comment as I don't understand this, it sounds lame, can someone please explain the import for me as I trust this source wouldn't be pushing lame crap.
That is, he was giving credit as respect to the researchers.
A lot of science news is sensationalist nonsense spun out of university press departments and that the most overhyped stuff also tends to get the most coverage. I think it's actually fair to look at something you may or may not understand fully and if your gut feeling says "this doesn't seem important", discount its importance significantly. In this case, OP was even explicitly asking whether or not someone more knowledgable agreed with their asssessment.
> Weaker than "I don't understand what the paper says, only read a high level layman summary, not sure even what what the experiment was for, but I'm gonna piss on it anyway"?
Come on. Parent was saying what it looked like to a layman, and then asking for someone to address that and ask what was actually going on. No need to "piss on [him]".
Well, the parent also wrote "it sounds like they didn't accomplish much" and that the structure of their argument is a strawman -- without qualifications.
The way I read it, the qualifications were implicit through the questions he followed up with; and the whole point of the 'strawman' was that he wanted help to get a better understanding than the obviously wrong impression he got from his 'layman' reading of the article.
That isn't the structure of the argument at all. If you don't understand the terms, how did you manage to conclude it was a strawman?
They took an prediction of existing theory that violates a weaker form of locality (i.e. demonstrated stronger non-locality), and realized it experimentally. They also didn't invent bilocality in any case.
Your take makes it sound like "we had this awesome idea! but we tested it and we were wrong." I have no idea if you're correct, but that is indeed how science should work.
I think the actual argument is the converse: "We had this awesome idea! We tested it and we were right!" (In this context "violations" are the desired experimental result.)
It rests on whether bi-locality is an empirically interesting property. And as it turns out there are some well-cited theoretical physics papers on the interest in this property for quantum networks [1], so it is likely that I am not the right audience for this press release, and a physicist might find this significantly more interesting.
Nah, locality is expected to be violated. Bell test experiments have proven this for like forever, and rather recently a very accurate test with very few loopholes was conducted too.
They used a new kind of locality and proved it was broken too. Interesting, but not groundbreaking.
Software developers are not physicists. Software developers aren't even mathematicians, although many are enamoured with related concepts.
To grasp the difference between the hard academic sciences and software development community, consider the differences between a telephone company and an astronomical observatory with a large telescope on a mountain top.
Many people here might be amateur enthusiasts, mixed with a handful of professional academic participants. Any award winning physicists hanging out here would be surprising.
That said, feel free to distinguish yourself by outshining everyone else. It would be a welcome addition.
Be nice. We all make mistakes in subjects we don't know. I have a few examples in my comment history ...
Try to pick one or two comments that are wrong but not very wrong. Reply to them explaining the error(s). I like to add a Wikipedia link when available.
Ignore the very wrong comments. It's usually impossible to give a coherent reply. You only will get upset.
Also, to encourage a civil discussion try to upvote the grey comments unless they are offensive or extremely wrong.
well how about writing a series of blog posts to enlighten us lesser mortals. In any case somebody should be doing that.
I am an engineering grad with decade and a half of gap in physics education despite of my constant high level interest/familiarity. I can honestly say I have no idea what bi-local causality is which this article handwaves around in the first para. how can you expect people to debate this when they cant even cut open this basic 'black-box'.
http://www.physics.umd.edu/courses/Phys270/Jenkins/Griffiths...