It's basically data analysis. There is a CSV file with some statons for public bicycles in boston and another one with over 1 million registered bike trips done with the service and some data
We have to make an application that allows an user to filter and see the data
@ACurious maybe you should modify it: "uh boss, there's a confused German on the phone who calls on the behalf of an angry Brazilian who wants to know if we sell brushes"
"ISO/IEC regulations for technical writing prescribe using bold italic for matrices (and slanted sans serif for tensors). I find these regulations incoherent, but if you work in some fields you are required to follow those regulations. Why incoherent? From a mathematical point of view, a matrix is a tensor."
@Anonjohn It is in the begining of the chapter about linear response theory (Chapter 6, pg. 86 on the second edition). Mandl-Shawn book also shows that you can get the propagator in a complex scalar theory from the communtator $[\phi(x),\phi(y)]$, so for me it does sound beliavable. — WilhelmM17 mins ago
This question Talks about
retarded greens function in a field theory
can anyone help me understand why there is a commutator
Hello, everyone! I was reading this good old sentence from Wald - "Since spacetime structure itself is singular at the big bang, it doesn't make sense, either physically or mathematically, to ask about the state of the universe "before" the big bang..." and it occurred to me that maybe I don't understand this fully. In the FRW Cosmology, although space becomes singular at the big bang, the metric for purely timelike movements is completely well-behaved - it is simply one.
So, in some sense, time doesn't become singular and we should be able to talk about time beyond the big bang singularity.
The point is that you can't integrate geodesics backwards through the big bang because the geometry is singular.
Your Christoffel symbols become infinite.
As it happens I have the Christoffel symbols to hand, and they are: {t, x, x} -> a a', {t, y, y} -> a a', {t, z, z} -> a a', {x, t, x} -> a'/a, {x, x, t} -> a'/a, {y, t, y} -> a'/a, {y, y, t} -> a'/a, {z, t, z} -> a'/a, {z, z, t} -> a'/a
At the Big Bang $a \rightarrow 0$ and $\dot{a} \rightarrow \infty$
Oh, okay. Thank you for pointing out this - I hadn't thought of the problem this way. But can I interpret it this way? - Since space becomes singular, and time is behaving normally - it doesn't make sense to talk about something traveling through space. Rather the universe becomes one-dimensional traveling only in time. So the only symbol I should be talking about is {ttt} - which would become trivial.
@Dvij the motion is described by the geodesic equation $$ \frac{d^2x^\alpha}{d\tau^2} = - \Gamma^\alpha{}_{\mu\nu}\frac{dx^\mu}{d\tau} \frac{dx^\nu}{d\tau} $$
If we consider a stationary/comoving observer then only $dx^0/dt$ is non-zero and the equation becomes:
It's very easy to forget that the $x$, $y$ and $z$ coordinates in the FLRW metric are not the distances that you and I measure!
@Dvij: I'll have to think about it, but right now it's the end of the day and I'm about to flop into my armchair and read the last two Hellboy in Hell comics.
In case you are wondering what that is, G stands for Good. A=Afternoon, M-Morning, E=evening, N=night. Capitalise whatever is relevant for you(That is whichever time of day it is in your timezone)
Why the moderators close the questions .I think they should just ignore the questions so that other people could answer .When these greats close the questions they leave no possibility of getting answers.
Hey there. If I have a follow up question on an already answered question, how should I ask this? Should I create a new question? My question will broaden the subject in question.
We do not like "solve this problem" posts. We much prefer that the person identifies exactly the point where they are stuck and ask a focused question about that.
@HocusPocus Yes, we could, but if we do that, then people will learn that they can come here and post problems without making any effort at all to identify a specific question.
We don't want that.
One of the functions of this site is to train people how to ask questions well.
This is a vitally important skill for a scientist.
The goal of this site is not "help everyone get the answer to their problems". The goal is to create a generally useful repository of questions and answers.
@MathematicalRain That could work. I tend to prefer something like "In [a previous question](link) I asked how I could froo the zorbnox even though a sample of bob-crystal is in the way. That got me thinking...." (I don't have the energy to make up realistic-sounding fake words today)
@DanielSank Actually I feel that mods unilaterally closing questions isn't too healthy.(A personal opinion) I feel that more number of closures should be done by community rather than mods.
