I'm grabbing a bunch of records from a table and there's a condition where I have something like WHERE X=Y
and that only returns 11000 records
as expected
but then I'm grabbing a bunch of records from another table joined on this table where I use WHERE X!=y and that condition is getting rid of a ton of records
Then if I create a different query that just pulls those 11,000 records into a temp table, merge the temp table with all them joins and find the rows where the temp table's ID "is null" I get the right result again...
what do you mean by too big/too little?
confuuusing
with names redacted:
SELECT * FROM TABLE1 WHERE YEAR(X) = YEAR(Y) AND MONTH(X) = MONTH(Y)
SELECT DISTINCT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.Z = TABLE2.Z LEFT JOIN TABLE3 ON TABLE2.W = TABLE3.W WHERE NOT (YEAR(X)=Year(Y) AND MONTH(X)=MONTH(Y))
I dunno. I think the LEFT JOIN is going to duplicate some rows if you don't have a strict 1:1 mapping between the tables. I'm not terribly familiar with how joins work though. I can usually just access the db and poke on it until it's what I want
Maybe you should replace the left joins with inner joins?
Inner will have length $\text{len}(T_1 \cap T_2 \cap T_3)$ while second will have length $\text{len}((T_1 \cup T_2) - T_1 \cap T_2) + \text{len} ((T_2 \cup T_3) - T_2 \cap T_3)$
Actually the second one could be the length of the whole thing with a union in the middle instead of sum of two lengths. I don't really get to think of tables as sets usually
@DanielSank there is an option in your settings to turn off the left menu. That eliminates the wasted space. With the setting enabled the pages look like this:
The hamburger icon at the top left gives you the options that were on the left menu.
The Elitzur–Vaidman bomb-tester is a quantum mechanics thought experiment that uses interaction-free measurements to verify that a bomb is functional without having to detonate it. It was conceived in 1993 by Avshalom Elitzur and Lev Vaidman. Since their publication, real-world experiments have confirmed that their theoretical method works as predicted.The bomb tester takes advantage of two characteristics of elementary particles, such as photons or electrons: nonlocality and wave-particle duality. By placing the particle in a quantum superposition, the experiment can verify that the bomb works...
So, one way to understand superposition is the quantum particle's wavefunctiom travels through all the paths. If one of the path is blocked, then it changes the interference in the other paths and thus result in a 0 or 1 be detected
Counterfactual communication go one step further by ensuring any photon that tries to travel through the channel to either be discarded and failed to reach its destination, or that the interference of the wavefunction makes it failed to even show up in there in the first place
But since it can still show up in other places, the information is still transferred, now encoded in the phase difference between the paths
Thetefore, if the wavefunction is treated as ontic, but contextual to the experiment setup, then what is being "transmitted" at the channel is literally the destructively interfered wavefunction, which ideally will result in no detection of anything
However I suspect that if I place a bomb in the transmission channel, then there is no way to prevent the bomb exploding within margin of error because the presence of the bomb may be enough to make that region no longer destructively interefered
So this method is still not recommended, but still superior when you are trapped in a room surrounded by bombs and you need to send an SOS out without blowing yourself up
because at least (unless the bombs are positioned in a way to take account even that) you push down certain death to a chance of death
now i don't know whether it makes sense to talk about the travelling velocity of the wavefunction, because in practice any attempt to measure it you modify the interference as soon you slot something in
@Abcd I've seen articles like that, though not that particular article, and I don't agree with the sentiments they express. Google provide me with a lot of important benefits for free and I'm happy to provide my online data in exchange.
@JohnRennie I'm not sure what you mean, but if I have to work at all to prevent 1/4 of the front page from being wasted, I'd say we have some improvements to make.
Ah, perhaps I see what you mean: the page fits well on half my monitor.
now i don't know whether it makes sense to talk about the travelling velocity of the wavefunction, because in practice any attempt to measure it you modify the interference as soon you slot something in
Ok, one possible way to test this is take a simple inferometric setup:
The detectors $D_1,D_2$ are counted by the computer to plot a graph of which detector get clicked vs time
The half silver mirror $S$ is retractable with some frequency $f$
If the wave function is something that can be assigned a meaningful propagation velocity, then given an electron beam in the set up, when the mirror is raised, destructive interference occurs at $D_2$ while constructive interference occurred at $D_1$. If the mirror is lowered, then the reverse happens
The idea is to see how the plot changes as will increase $f$. Any delay between the detector signals switching between $1$ and $2$ after account for the velocity of the electron beam and the path length of the arms, will suggest "something" is travelling before the electron beam does
If the wave function velocity is instantaneous, or not applicable, then the changes will not be a function of $f$
Otherwise, it will be and suggested the ontic nature of the wave function
If we have a kilometer long million slit light source and make an observation on a slit at one of the ends, will the interference pattern fully collapse or partially collapse? is there any delay in the propagation of the collapse or is the collapse instant?
Link to multiple slits.
http://hyperp...
And so referring back to this article, if the notion of "velocity of the wave function", or more precisely "How fast does the initial conditions of the quantum state updates throughout the experiment setup when we change the setting" make sense, then the wavefuncton will be the first concrete demonstration that something that by almost all notions of interactions, behaves like nothingness, is not only real, but can propagate
Euclid is said to have reported to Ptolemy at which time he was asked as to whether there was one key point or one set of finite ways of understanding all that was captured in the Elements. Euclid's reply is famous, "Sire, there is no Royal Road to the understanding of geometry."
However, it seems more likely that the wave function is really not a travelling entity, but inherently nonlocal, given how causality can be put into superposition, something that should not be possible if the update of the initial conditions took place at a finite time
Hello, all! :) Would it be true to say that the spectrum of an operator that doesn't commute with the Hamiltonian of the system should be continuous? I have not put much of a rigorous thought into it but it seems that if we measure that operator and thus reduce the state to an eigenstate of the operator then if the operator is not commuting with the Hamiltonian then it would (except in special cases) be a superposition of several energy eigenstates--and thus [...]
[...] it would evolve to a state that is a superposition of more than one eigenstates of the operator (even in an infinitesimal time?). Thus, if the spectrum is discrete then the subsequent measurement would yield a drastically different result for the same operator. But if the spectrum is continuous then in an infinitesimal time, the state would only spread over infinitesimally close eigenstates of the operator and the value spitted by the next measurement would not be drastically different.
By "close" I mean "nearby" when I say "infinitesimally close eigenstates"
@DvijMankad no. The claim is false. The spectrum of a given operator is a function of the operator and nothing else. It does not depend on what the hamiltonian is.
It's always important to keep in mind that QM makes just as much sense on finite-dimensional systems.
If you're making a general claim about QM and one of its consequences is that the system must be infinite-dimensional, like your claim here, then the claim is automatically wrong.
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@EmilioPisanty Thanks! Yes, that makes sense. The check that any generic claim should necessarily not predict infinite dimensions is something I would keep in mind :) I wrote down a few steps and it is clear that the probability of obtaining a different operator eigenstate in a subsequent measurement made after an infinitesimal time is infinitesimally low--so my claim was based on a wrong intuition.
Even if the spectrum is discrete, the vector space is still continuous (in the sense that the field over which the vector space is defined is continuous) and thus, an infinitesimal time-evolution would cause the state to change its direction in the vector space only by an infinitesimal amount and this would reflect in the fact that the newly contributing eigenstates would only have an infinitesimally low contribution in this infinitesimal time duration.
"newly contributing eigenstates (of the operator)"*
By the way, it doesn't affect the falsity of my claim but I was thinking that it would be a restriction on the Hamiltonian that it be such that it commutes with all operators with discrete spectrums. I realize that all the operators would have their spectra dependent purely on themselves--in a straightforward mathematical manner. But I was thinking that the Hamiltonian could be restricted to behave as I described. Anyway, since the basic intuition behind all of this was wrong, it's moot.
But in addition, I think that it would be impossible to restrict the Hamiltonian in such a way because I guess there would arise an infinite number of independent restrictions that the Hamiltonian would have to follow. Right? @EmilioPisanty
1 hour later…
Anonymous
11:33 AM
@Mithrandir24601 Autocompletion of user names in comment replies is an existing feature on both the Android and iOS Stack Exchange apps.
1/3 of the page on the left filled with irrelevant stuff making it harder to read the post on the site, another 1/3 on the right with more irrelevant stuff (less irrelevant but still irrelevant), terrible
Anonymous
As for why they did not enable it on the mobile browsers, I do not know.
Anonymous
To be pedantic, however, it's not right to call it autocomplete. It's more like an equivalent of the "reply" button which is there in the chat.
@bolbteppa go into your profile, then the Edit Profile & Settings tab, then click Preferences on the left then tick the option Hide Left Navigation. That removes the wasted space on the left. You can still get the left menu by clicking on the hamburger icon.
I don't want the electron beam to split when $S$ is lowered. Current setup will result in a 50:50 chance of D1 and D2 to click when S is lowered, and that probability will prevent us from measuring the response time of the wave function when the experiment setup is changed due to raising and lowering S
@EmilioPisanty I am probably missing the point but don't we put restrictions on the Hamiltonian all the time? Like it should be Hermitian (well, maybe that doesn't count) or that it should respect certain symmetries and so on? Or is it something completely different? In particular, can you explain a bit why it is a bad thing to impose restrictions on Hamiltonian? I thought that Hamiltonian is simply about laws of evolution and we can put all the restrictions in the world on laws of evolution.
@user1732 it's not doing too badly, thanks :) We're not getting much in the way of questions, but that's normal for this stage. We have some very good questions though, so that does make me happy :)
@Blue ah, I don't use the app :P
@EmilioPisanty I potentially disagree with the bit about it having to be Hermitian (in principle, anyway)
@TheDarkSide ::chuckles:: Math is such a powerful and necessary tool that we must teach a lot of physics in mathematical style.
So we need to issue the occasional reminder that physics appeals to how things really behave and not to pure logic for the final arbiter of correctness.
Yes. But you see, I was looking for some sort of a derivation of this Bethe-Bloch equation. A simple treatment that I found was in Turner - "Atoms, Radiation, and Radiation Protection".
It kind of derives the equation for the most part.
But it is just that, beyond the classical Bohr derivation, I am unable to understand how they generate the $\beta^2$ correction term.
As it stands, all references I have found follow the same pattern. Bohr derivation exactly, and then, (boom) the correction appears out of the blue. No motivation. Just Bethe said so.
@Nobodyrecognizeable Testing it to high precision is a picky and expensive business and I don't believe that anyone has done a definitive measurements (they hadn't as of ten years ago for sure).
But the general expectation of scientists is that anti-matter falls the same direction as pro-matter.
@Nobodyrecognizeable No. Accelerators with antimatter beams are old hat (e.g. the tevatron), but doing a reliable measurement with a charged beam is prohibitively difficult.
Another question that comes in my mind that how antimatter like antihydrogen be isolated . They are neutral . How can they be isolated with magnetic field and electric field?
@user2236 I miss being part of that culture. My new work and new colleagues are similar in terms of interest and challenge and social comfort, but it is a different world.
@dmckee Another question that comes in my mind that how antimatter like antihydrogen be isolated . They are neutral . How can they be isolated with magnetic field and electric field?
Not sure. Never looked into it. But magnetic fields can exert forces on neutral particles that have a magnetic moment (the mechanism of Sterm-Gerlach separation, right?).
@Nobodyrecognizeable antihydrogen (and hydrogen) have a magnetic dipole moment due to their spin. This is used to trap them ina magnetic field. See this paper for the details.
@TheDarkSide That's a hard one. I think there is a discussion of Bethe-Bloch in Leo, but I "lent" my copy to a student some years ago and never got it back.
I also lost my copies of Perkins and Kernighan-and-Ritchie that way.
@JohnRennie hmmmmm. I spoke with an ALPHA member not long ago. I seem to recall that he said that the trapping wasn't being used in measurements. But that may be inaccurate? Possibly he said that they have cooling ready but it's not being used for measurements.
@JohnRennie Based on our data, we can exclude the possibility that the gravitiational mass of antihydrogen is more than 110 times its inertial mass, or that it falls upwards with a gravitational mass more than 65 times its inertial mass it is what alpha says.
@Nobodyrecognizeable everyone expects the gravitational and inertial mass of antihydrogen to be the same. It's just that the Alpha measurement was at such an early stage that the error bounds were huge.
@Nobodyrecognizeable no. The expt did not prove or rule out that the gravitational and inertial masses are the same. All it did was place upper bounds on any possible difference.
@Nobodyrecognizeable It depends what you mean by gravity being repulsive. Dark energy is causing the universe to expand, so that's a sort of repulsion.
Is anti-gravity (i.e. repulsive gravity) possible in string theory?
I have read some articles about scientists making assumptions about the existence of anti-gravity, but is it possible in string theory?
@DvijMankad It's not that your formalism puts some restriction on a hamiltonian, it's that it would restrict all conceivable hamiltonians.
Generally speaking, we expect the universe as a whole to be symmetric, but that doesn't mean that all objects in it need to be symmetric. That means that, unless you're doing fundamental physics, it is perfectly reasonable to consider hamiltonians that do not have any well-defined symmetry.
@Mithrandir24601 You can write Schrödinger equations for all the hamiltonians you want, but if your hamiltonian is not hermitian, then you're absolutely going to need to jump through some tall hoops to justify any claim that it is a meaningful model of reality.
It's not entirely ridiculous if you know what you're doing. Say, a decaying particle where the total population decreases over time, and you explicitly write off from your books the particles that go away. Or you're doing PT-symmetric QM, which isn't a good model of the mechanics of particles, but which can be implemented in optical simulators.
But if you don't know what you're doing, non-hermitian hamiltonians do have a huge Stay Away sign on top.
@EmilioPisanty yeah, although there are examples of classical 'effective' Hamiltonians that are physical. Usually, the argument involves redefining the norm, but many physicists don't like that for some reason
In a recent review, one reviewer said we must rework the paper in light of two other papers. One paper was a highly valuable suggestion, and we've enthusiastically taken it on board. However, the second paper seems to be an ultra-specific paper (one example among thousands) and not applicable t...
Basically what's documented here:
What formula should be used to determine "hot" questions?
We have a few tweaks:
Succeeding questions from the same site are penalized by increasing amounts. So, the first question from SO in the list gets multiplied by 1.0, the second by 0.98, the third ...
@ACuriousMind hmmm. alternative sources for the same fact?
@ACuriousMind I can't say I've noticed, but I freely concede that I'm not terribly sensitive to visual design and details tend to pass me by. But as far as functionality goes I like it.
@JohnRennie E.g. the comment box you type your comment in is now sans serif, but the posted comments are serif. I'm pretty sure that wasn't like that before because I find it pretty jarring
hmmmm. That AnswerCount * QScore part is interesting. For the two examples we were looking at earlier (five answers within the first hour), each vote on the question really counts
@EmilioPisanty Yeah. I think that AnswerScore part is responsible for it often being rather opinion-based or vague questions that make the HNQ - questions where there is one correct answer and someone already gave it near-perfectly have AnswerScore = 1 :P
First off, you haven’t committed a crime (assuming she was above the age of consent) and you haven’t engaged in academic misconduct (since there is only an indirect academic relationship between you). However, the fact that she is threatening/blackmailing you is very concerning. The fact that she...
I've been working with this company for a quite some time, we don't have any formal agreement, I haven't signed anything. At the same time among our clients were some really big and well known companies, who gave us some documents and asked for NDA.
My employer shared with me those documents, so...
@ACuriousMind yeah. Cutting it down to AnswerScore/AnswerCount is probably a bit too draconian, but maybe something like AnswerScore/AnswerCount^0.5 might help with that?
Rationalise it anyway you want, but it's still blackmail.
You have plenty of legal recourses specifically made to protect your rights to be paid that don't include playing games with other people's property.