@Sid Cool! I guessed :-) Well, I guess I just told you what they're about like 5 minutes ago. Not much to it. This would all be slightly more structured if we had prepared topics to discuss.
Basically the point is to get people together in chat every couple weeks.
Oh and if anyone has any quick general questions about the site, this is a fine time to ask them.
@EmilioPisanty I'm sure news of the pictures will be posted on Twitter among other places, whenever they are made public
Anonymous
@DavidZ There is a separate version of chat for the mobile SE app. Quite similar what you see when you click on the "mobile" button at the right bottom of your desktop view of hbar. BTW, how do you get mathjax to work on mobile?
Back in 2016, Ana conducted a couple of experimental "town-hall" chat sessions. They worked pretty well, so we all agreed to keep doing them...
...And then, uh, didn't.
That's a real shame. So, let's revive the idea, with a bit more commitment: we're gonna do a half-hour session every two week...
Also, SR pretty much ensures that any entangled state "collapse" can be interpreted as having a temporal component, so I'm unsure why "time entanglement" is viewed as so debatable. It's pretty hard to get away from it.
so the links I found today is pretty much an instant google and sciencedaily search plus some intuition for the more elusive articles
If you boost to some moving frame, when looking at an entangled state that is completely spacelike in the rest frame, then surely it will gain some time component to it?
misc topic that recently came up on Computer Science by a newcomer inquiring about CAs related to stuff have personally investigated somewhat. is there any mainstream recognition or criticism of 't hoofts ideas on cellular automata/ digital physics/ universe-as-simulation?
@Secret to me it's very hard even to define a quantum state without introducing a time-like element. The simplest definition of a quantum state is that it is one for which a classical information "history" has not yet been created anywhere in the universe. That actually gives a very solid definition of it. Observation creates that classical info.
@TerryBollinger How is that a definition of a quantum state? Also, states with "history" are still quantum states, I'm not sure what you think you're defining there
@EmilioPisanty I like reading his papers, and his co-author Matthew... I forget! But I get very, very frustrated with the complexity. It's like reading overly complex computer code.
@ACuriousMind I know it's not a common definition, but Feynman had some really interesting observations on those lines. The exact quote about neutron absorption got messed up badly in the text version of FLoP, but it's in the tapes.
But anyway, if retrocausality is proven, or shown no-go, I can equally adapt with it, cause my thinking had gone quite nonlocal due to lonog time exposure to quantum mechanics
Terry: I agree, and that's how I think about entanglement, where the nonlocal wavefunction containing the correlation is the quantum information, and then the classical information pops up as the correlation is established by measurement
Emilo have warned about that a nonlocal wavefunction has no operational (forgot term), thus it is not very useful to think about it. For many interpretations, the correlations are …
@EmilioPisanty It does! This is why I respect/like their work! But over-complexity is often a prelude to drastic refactoring, cutting the idea back to some kind of simpler basics.
I had a very interesting conversation with Klaus Mølmer on reading Bohr once. He described it as really tough, but that for any given sentence, he would try to produce a simpler version, and then when compared against an array of test cases, none of the simpler wordings managed to match the original with all tests.
@vzn There's a big difference, for the latter, you still have a nonlocal hiddne variable right at the start, thus the measurement only reveals the outcome associated to it. But retrocausality is saying that you have that outcome established while previously there is nothing there. These spacelike signalling have implication in time travel because a spacelike separation in a boosted frame will have a timelike component, where the order of events can reverse
@TerryBollinger agreed that is the consensus-bordering-on-dogma but suspect the jury is still out, that the full story yet remains. taking minority/ contrarian/ near-fringe/ devils advocate position.
A comment from computer science: You can have delayed binding of states if you assume that "now" has a well-defined, universal meaning... which gets interesting though with SR (block universe arguments etc). I have a paper still in the works, argh...
@vzn the attention to the topic is great, I think it needs this exploration.
@TerryBollinger agreed! its amazing the historical/ ongoing intellectual firepower applied to this area. a bit staggering/ overwhelming at times. it seems to be one of the deep zen questions of the universe. very multifaceted. reminds me of the iceberg image, ~9/10ths underwater.
Terry: This question, https://physics.stackexchange.com/questions/338208/simple-entanglement-experiment-probabilistic-entanglement is meant to explore the role of time played on measuring something, e.g. we expect there should be no correlated outcomes if the screen is positioned before the device that entangle the positions of the two electron beams and correlation otherwise.
However this question will remain closed and deleted unless I can figure out how to find an experimental device that entangle positions of two electrons on a screen
@vzn Consensus obtained by careful experimentation (i.e. Bell inequality tests) is very different from dogma. Please substantiate your scepticism of "the jury is still out" with references - or, if you can't, stop claiming it is.
Strangely, delayed "collapse" can be a positive computationally, not a negative. This is counter to common perceptions that quantum uncertainty is a "bad" thing. Point particles with infinitely precise trajectories, now that is bad computationally.
@vzn There are many questions regarding quantum foundations that are still worth investigating indeed and where no consensus exists, but "are there local hidden variables?" is not one of them, since the Bell tests rule that one out.
@Shing I would certainly say so, because over-complexity means that folks aren't afraid of broad exploration of possibilities. Sticking to "just this" is a great way not to solve hard problems.
@vzn It is possible to differentiate consensus from dogma, you just have to actually look carefully at the evidence people use to substantiate their position.
@TerryBollinger Bohm is an example of a nonlocal hidden variable theory. The pilot wave guides particles with well defined and determinstic trajectories, is like a given potential in space thus it is a nonlocal entity
@vzn Precisely. And your claims that parts of the scientific community act on dogma are essentially equivalent to saying that those parts of the community act in unscientific ways.
@Secret that's an interesting and insightful way of expressing Bohm... but I stay by my assertion that you need to be very, very careful about concepts like that as possibly re-introducing the very concept you are nominally trying to eliminate.
@vzn Yes, and never have you presented any evidence except these silly loopholes, i.e. you're saying the standard position is "dogmatic" because the experiments carried out are imperfect compared to an ideal thought experiment. I mean, that's true, but many experiments are imperfect but we still tend to believe their results (with error bars).
The way I think about nonlocality without signalling is that there's an arena, and then things are already specified without caring about whether they are spacelike separated
@vzn oh, it's plenty easy: try "OK, yeah, I agree that claims that others act dogmatically are overblown and I will refrain from making them until I can back them up with concrete evidence"
I mean, if it is "dogmatic" to say that there are no hidden local variables because every experiment ever done so far has ruled them out, then it is equally dogmatic to say that the earth's gravitational acceleration is $g=9.81$. If that's your definition of "dogmatic", then yes, science is dogmatic, but it's not a very useful definition.
Here's an interesting problem: Familiarity engenders low prioritization. Thus I would say there is still important physics within spin 1/2 = anti-symmetric, but it's so well-known and familiar that most people treat it as fully solved. It's "sort of" quantified, but there is no deep insight there that I am aware of.
guys, dont want to get into a fight! my main ideas on this are about scientific revolutions. we are not done with them. some still remain. some are right in front of our noses. lets change the topic, maybe dont feel emotional or adversarial enough today to continue this thread :|
@EmilioPisanty my pattern is to challenge the status quo. aka "question authority" the pioneers are the ones with the arrows in their backs etc... its nothing more meaningful than a bumper sticker. sigh
@vzn Calling the standard position dogmatic is an accusation of the people holding that position being unscientific. Can you really not see how that's an accusation?
@ACuriousMind @EmilioPisanty Here's a very watered down version of that hopelessly unclear question I tried to ask: Consider an experiment with two particle beams such that as they past through a device (something like a cavity) in the middle, they become entangled, before they hit the screen. Now if I move the screen to before the beams hit the device, should I get no correlations? What happens if my screen is positioned right at where the device is, do I get 50% chance of correlated outcomes?
@vzn Challenging the status quo with evidence is good. Challenging the status quo with empty insinuations of "dogma" and vague insinuations that we don't know everything yet is not.
@TerryBollinger agreed that is the consensus-bordering-on-dogma but suspect the jury is still out, that the full story yet remains. taking minority/ contrarian/ near-fringe/ devils advocate position.
@vzn No, you just accused everyone disbelieving hidden variables of being dogmatic. Accusing a large group of people of being unscientific is no better than accusing any specific one.
@vzn it's very simple. It's not trigger words, it's a continued pattern of accusations that large parts of the scientific community act in unscientific ways.
@vzn I'm afraid you'll have to explain what "consensus-bordering-on-dogma" is supposed to mean if it's not an accusation of the scientific community being dogmatic.
@vzn It's not about "anybody here". Even if you're so kind to grant me that I'm not dogmatic I'm still disturbed that you run around and denounce scientific consensus as "dogma".
Hmm. Trying to change focus, and for whatever it's worth, I think the universe is fundamentally quantum, and that classical is the emergent and very weird part of it. It's essentially the emergence of information that persists and time for it to persist in. That also means that any model that begins with classical will have some problems.
@ACuriousMind said that consensus and dogma are sometimes hard to separate. its just a bumper sticker idea. its not a personal accusation. feel like its a slow news day around here or something lol
@vzn I do like devil's advocate positions for exploration of broader spaces, and use then consciously in exploration algorithms. They are an important tool if used with care.
@TerryBollinger Ánd yet quantization procedure works amazingly well - so much so that we get lots of people asking how to derive quantum mechanics from classical mechanics, but much fewer questions the other way around :P
BTW, the US geology community as a whole pretty adamantly resisted plate tectonics for about a decade longer than the rest of the world. Would that be an experimentally verifiable case of community-wide "dogma"?
@Sid O btw, all SE chat rooms have this property: whenever there are 2-3 parties get locked into an argument, they are very hard to respond to other users
@TerryBollinger exactly. there are a bazillion cases of scientific consensus blown away. enjoy collecting them. everyone here is both aware of them, and yet forgets them in a way. its a kind of weird cognitive dissonance in a way... actually plate tectonics itself is an excellent case study of heavy worldwide skepticism/ resistance of a valid idea etc...
@TerryBollinger It takes time, for me, while as acuriousmind have found out I still kinda suck at quantum, I am comfortable with its weirdness because I like weirdness and only complain if the world is not weird enough
By the way our scheduled time for the chat session is up, although the discussion can continue if people like. See everyone for the next one in two weeks!
@ACuriousMind The above diagram is the closed and community deleted question: Simple entanglement experiment. Probabilistic entanglement in a nutshell
(The question cannot be reopened since the only unclear point is I have no idea what experimental device will entangle electron beam positions. I thought I can get away with the explicit details of the device since it is not really the important thing in that question). But the gist of that question is presented there. I am especially interested in the case on the right, cause that might be related to Terry's argument that measurement has a time factor
@Secret The question is not unclear because you can't exactly say what experimental device you'd use. It's unclear because you haven't really specified what the device is supposed to do in the first place. Correlation of what are you talking about?
@ACuriousMind Positions of the two electron beams on the screen such that given the area that the two beams hits are two circular region each, then if an electron is detected at $(r,\theta)$ away from the centre of one of the circles, then the other electron must land in the corresponding location of the other circle. I am nto sure how to write this more clearly other than as $(r_1,\theta_1)=(r_2,\theta_2)$
if they are not correlated, then the positions on where electron 1 land on the screen has no relationship with that of electron 2
@Secret So, you have some weird device that takes two electrons as input and produces a superposition $\int f(r,\theta) \lvert r,\theta\rangle_1 \otimes \lvert r,\theta\rangle_2 \mathrm{d}r\mathrm{d}\theta$, where $f$ is the distribution of the particles in thebeam and by $\lvert \cdot\rangle_i$ we denote a position state of the $i$ particle.
The question is, if the screen is placed before the beams passed through the device, then I should expect no correlation as the beams have not been entangled yet; the electrons will show correlation if the screen is positioned after the device since the beams would have been entangled by the device before hitting the screen (and there are no erasing setups to remove the coherence)
So the question is then, what happens if my screen is positioned right where the deivce is, do I get 50% of the electrons entangled (and hence showing correlation on the screen) and 50% of the electron not entang…
That sounds like "what happens if I put my screen right where the beam splitter is?", to which the answer is of course "you can't without changing how the beam splitter works"
yeah, you'd need to know how the device is actualyl built to answer that
@ACuriousMind hmm I see..., btw is my deduction on the first paragraph correct? This closely tied to Terry's idea because, well the difference between those two cases means we can use distance of the screen like a time stamp thus we can then pinpoint where the entanglement took place, or is it still not valid to think about measurement being able to have a time stamp associated to it?
@Secret Typically entanglement is produced by the action of some unitary $U(t)$ over some finite time interval $t\in[0,T]$. If you interrupt the process with some projective measurement at some time $0<t'<T$, then generically that will yield some partially entangled state.
I always thought I can get rid of the retrocausal interpretation of the delayed choice eraser experiment by saying that the state need to be entangled first before it can show correlations after measurement. Now this realisation might complicate my initial thought...
o wait perhaps not..., cause the "screen placed after the entangling device" should be well beyond time $T$ thus it should be fine
It's the "screen placed before the entangling device" that is new to me
so using your logic, it seems the beams can start entangling before they even reach the device
How to understand all of this properly. I don't want to subscribe to the retrocasual interpretation in the delayed choice experiment, but in order to do that, I need time stamps (which are intervals because all process takes time) and showing that the time stamp for the establishment of entanglement must come before the measurement that established the correlated outcomes?
But I think I might be still thinking too classical, thinking that every process can be ascribed an interval where the time stamp will be
In symbols, what I want to show is the following: $[0,T]$ for the time to entangle stuff $[a,0]$ for the case where screen is placed before device $[T,b]$ for the case where screen is placed after device
If I get even a partial correlation before t=0, then that will be very weird
Let me re-read through the posts and see if I misunderstood something...
@Secret think that this line of thinking is on the right track, looking at timing of events, & may reveal something new. reminds me of a (now-old) old bell experiment by weihs that actually did "time stamp" arrival of photons (with high resolution) at the 2 arms and did postprocessing. he even put his data up on the internet at 1 pt.
The only way that you can absolutely guarantee something of the form "at time $t$ subsystems A and B were entangled" is to either make a projective measurement then and there (which will destroy the state if you wanted to use it alter) or to enforce a spacelike separation between the two parts.
@Secret (also, to be clear: it's OK to assume that kind of processes are instantaneous, particularly for timestamping purposes and the like. What you can't do is assume that two incompatible processes are both instantaneous and happen at the same time.)
Ok, this might work, though if we want to perform it one after the other, something more sophisticated might be needed as once projected, we will lose the state
So experimentally, we might be able to pick an observable we want to entangle (e.g. spin or polarisation), and then compare the results between the two cases
though... if we are going to time stamp the area near the region of entanglement, we might need to do a projection measurement (or numerous weak measurements), but perhaps we might be able to avoid that by using the fact that distance of travel will be correlated to the time of travel
uh, wait a sec.., is this what the starlight loophole free bell experiment guys have done..., let me check...
My aim for such set up is simple: I don't care whether there are hidden variables (people who deal with bell experiments will take care of that), the thing I concern the most is retrocausality, as quantum mechanics and QFT said that is not possible without CTCs, since spacelike operators will commute
If within experimental error there is significant correlation for the beams before they enter the entangling region, then we have something need to deal with
https://www.nature.com/articles/srep20603 Hmm, this paper does not said much on how close the subsystems has to be before the wavefunctions will overlap...
https://arxiv.org/pdf/1611.06985v2.pdf Ah, the loophole free bell test is testing something different, they are testing entanglement, not when it took place
No, they're not eigenvalues, they're the probability to find the state $\lvert \psi\rangle$ in that eigenstate. It is a long time since you've done QM, huh? :P
I just received my daughter's copy of hivemill.com/collections/smbc/products/… and recommend to to everyone here with little people in their lives any way that might be.
@ACuriousMind I only know as much beam physics as you get from the one talk they do at each particle physics summer school you attend. I think I've seen three. Maybe four.
@Sid No idea. I certainly does work in the US, and I would guess at higher shipping charges to go very far around the world.
@0celo7 There is more than a little of that going on, yes.
@ACuriousMind @dmckee the problem was mostly communication: beam scientists and geometric analysts have very different views on how ion beams actually work
And when they say things, the other understands the opposite
Back in 2016, Ana conducted a couple of experimental "town-hall" chat sessions. They worked pretty well, so we all agreed to keep doing them...
...And then, uh, didn't.
That's a real shame. So, let's revive the idea, with a bit more commitment: we're gonna do a half-hour session every two week...
