I posted a relatively simple question regarding syntax that I didn't understand:

What does self-closing bra-ket mean in Robetson-Schrodinger Uncertainty Relation?

I received 2 one-word responses which completely missed my expected data type for a response.

I attempted to interact with the users to clarify that it wasn't the anti-commutator but a specific piece of syntax that I was confused about.

I am given no constructive response to my comments, attempting to tag the responders results in an attempt at humor? (or sarcastic insult, I can't tell). And my conclusion is that this question must be really terrible [since back at Math.Stackexchange if someone were to comment the way I did I would try to work out, what made sense, clearly to most users here this question is so bad, it doesn't even warrant or deserve any further attention]

How can I figure why this is a dumb question to even ask? I would like to avoid this happening in the future.

I tried to do some diligence with research, I feel like my English is clear, and it feels like its a pointed enough question (enough to make sense to me?). Clearly one of those 3 things must be wrong.

  • 4
    $\begingroup$ It's got one downvote. It's a little early to conclude that there is necessarily something wrong with it, don't you think? I mean, maybe someone just got up on the wrong side of the bed. $\endgroup$ Commented Mar 28, 2018 at 16:45
  • $\begingroup$ I guess your right. 3rd poster was very civil and answered the question clearly. Its just the speed and confidence with which I was hit with the first 2, made me assume I had done something egregious. $\endgroup$ Commented Mar 28, 2018 at 16:51
  • $\begingroup$ @frogeyedpeas No, your question was just easy to answer. Would you prefer that answers come slower instead? $\endgroup$
    – knzhou
    Commented Mar 29, 2018 at 10:22

2 Answers 2


It's not the worst question I've seen on this site, but it's not good either. For one thing, the notation $\langle \mathcal O\rangle$ is explained in the wikipedia page you linked yourself. Indeed, if you read the very first equation, you'll see the sentence

... where the brackets $\langle \mathcal O\rangle$ indicate an expectation value.

If that explanation is not clear enough, the words "expectation value" contain a link to the wikipedia page Expectation value, whose very first equation is $$ \langle A\rangle=\langle\psi|A|\psi\rangle $$

Furthermore, the notation $\{A,B\}$ is also explained in your first link (and if you click on show proof, you will also find an explanation for the notation $\langle A\rangle$). In essence, your question is answered in detail in your own wikipedia link. The downvote you got is not mine, but it is deserved, on grounds of "This question does not show any research effort" (which is the message you get if hover over the downvote button).


Actually your question is not clear at all since it seemed to me at first you were asking for an interpretation of $\{A,B\}$. It was only at the second pass that I realized you were worried about the angle brackets and that part wasn’t immediately clear either since you have brackets around your vectors, which are themselves inside brackets, i.e. $\langle <vector > \vert <vector>\rangle$. If anything, I though you wanted to use $\langle\langle$ and $\rangle\rangle$ and had made a typesetting error.

I didn’t downvote either (I think the downvote is excessive but I’m amazed it got an upvote) but I also think the question could have been answered with minimal search effort, as already indicated by someone else. The notation $\langle A\rangle$ for the average value (or expectation value) is extremely common throughout physics and mathematics (if anything $\langle x\rangle$ is very common in QM anyways).


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