The h Bar

General chat for Physics SE (physics.stackexchange.com). For M...
Tue 17:49
Well. I've been swimming more these days.
Tue 16:59
Hi, everybody.
Jul 18 19:11
:-)
Jul 18 19:08
Hi, everybody.
Jul 14 01:34
@qwerty Thanks!
Jul 13 04:32
@controlgroup Would you happen to know what tone?
Jul 13 03:25
@naturallyInconsistent What do the ~ mean?
Jul 13 01:38
Fight me.
Jul 13 01:38
Rabi oscillations aren't quantum: physics.stackexchange.com/questions/705175
Jul 8 16:18
@ACuriousMind Yes, I see your point.
Jul 8 04:27
@qwerty It's one the best books I've ever read.
Jul 7 23:21
Good point about Schutz. Carroll's GR book is good too.
Jul 7 23:19
@Allie The only book I've ever read that had a not-horrible introductio to tensors was Analysis on Manifolds by Munkres.
Jul 7 23:18
@naturallyInconsistent Interestingly, it seems that when $f$ is bimodal, it is possible to have multiple solutions.
Jul 7 23:18
$f$ is a probability density, i.e. it is positive and integrates to 1.
Jul 7 23:17
where N and A are both constants.
Jul 7 23:17
$$N(A - x)f(x) = \int_{-\infty}^x dy \, f(y)$$
Jul 7 23:17
@naturallyInconsistent I plotted the left and right hand sides of the equation to get a feel for the solutions (which are interesting). I asked about an analytic solution to see if it happens to have one. Yes, looking for solutions near A would be of interest. In that case, allow me to pose the full problem
Jul 7 23:02
@TobiasFünke @ACuriousMind I shoudl clarify: the problem is to find $x$ for given $f$.
Jul 7 22:56
@TobiasFünke I was trying to calculate the best value to bid in a silent (i.e. bids are private), first price auction, assuming the goal to be maximization of the expected earnings, where "earnings" means "value of product minus amount bid in the auction."
Jul 7 20:07
@ACuriousMind you have 5 starred posts. This is unacceptably high.
Jul 7 20:06
Is this equation solvable: $$(A - x)f(x) = \int_{-\infty}^x dy \, f(y) \, ?$$
Jul 3 23:16
@ACuriousMind ooooh this could be interesting. Perhaps someone with nefarious intent finds out and yes, wants to control the numbers to create outcomes beneficial to themself. But then it turns out they need quantum computation to do that... which can't be done without the random number ticker doing its thing.
Jul 2 16:59
Scifi comic/movie idea: Someone accidentally finds the random number generator that fuels the quantum mechanical universe.
3
Jun 30 02:09
@SillyGoose Yeah. I like that so much that I've written a ~40 page handbook on that viewpoint for students.
Jun 30 02:08
What physicists should probably say is this:
"This list of numbers, which describe a thing in a particular basis, represents a vector iff those numbers transform in a particular way when I change to a different basis".
Jun 30 02:07
The definitions are equivalent though.
Jun 30 02:06
It was very helpful for me, for example, to realize that a function and its Fourier transform can be viewed as different coefficients for the same vector.
Jun 30 02:05
@SillyGoose Sure but isn't it a lot easier to understand the basic definition of a vector as a thing that exists without any specific representation before getting into that?
Jun 30 02:04
The same problem happens in thermodynamics once the physics books start pushing around $dQ$, $dV$, etc. That stuff is much more clear if you focus on the maps themselves as opposed to treating $d\text{whatever}$ as some kind of pseudo-value.
Jun 30 02:03
It's useful once you understand the algebraic definition of tensors and vectors but it's a terrible definition.
Jun 30 02:02
Yes
Jun 30 02:02
When I read that, I finally realized that an $(n, k)$ tensor is a linear function that maps $n$ vectors and $k$ co-vectors to a scalar. Period. No transformation nonsense in the definition. But of course you can investigate how to represent a tensor via coefficients and investigate how those coefficients change with changing basis.
Jun 30 02:00
There again, they are misidentifying the values of the elements of a tensor expressed in various bases with the underlying tensor itself. it's horrible. I only understood tensors by reading Munkres's Analysis on Manifolds where the idea of an antisymmetric tensor is introduced as a function $T: V^n \rightarrow \mathbb{R}$ that maps $n$ vectors to a real number, is linear in each argument, and has the antisymmetry property that swapping any two arguments inverts the result.
Jun 30 01:58
This confusion oblitarates students' chances of understanding tensors because physicists write insane phrases like "A tensor is a set of numbers $g_{ij}$ that transform like a tensor".
Jun 30 01:57
This is most clearly evidenced when physicists write e.g. "...the function $f(x)$", which is almost nonsense. It seems what they mean is "the function whose values are $f(x)$", which as I said, is a sort of confusion between functions and values.
Jun 30 01:56
The fundamental problem with so much of physicsl literature is a confusion between functions and values.
Jun 30 01:55
This notation is completely unambiguous while using fewer symbols than the usual $\int_a^b dx \, f(x)$.
Jun 30 01:54
Consider a function $f: \mathbb{R} \rightarrow \mathbb{R}$ and a subset of the real line $X$. Then there's an integral $\int_X f$. Notice that there is no $dx$ anywhere and there's no $f(x)$ anywhere.
Jun 30 01:53
@ACuriousMind At some point during undergraduate studies I started to understand the value in writing things like this:
Jun 29 21:21
@ACuriousMind Yeah... I'll link the Math post so you can see exactly why I'm confused.
Jun 29 21:12
ok. I'm still stuck to find the particular solution, which is why I fall back on the formal equation with mu.
Jun 29 21:10
That makes some sense.
 

 Mathematics

Associated with Math.SE; for both general discussion & math qu...
Jul 7 23:23
Are you sure you can put that denominator inside the integral?
Jul 7 23:14
Constant.
Jul 7 23:05
I'm unfamiliar with integral equations. General directions/advice would likely be enough for me to make progress.
Jul 7 23:05
How does one solve the equation: $$(A - x)f(x) = \int_{-\infty}^x dy\,f(y)$$ for $x$, if we know the function $f$?
Jul 1 20:27
$$\dot{x}(t) = f(t)x(t) + J(t) \, ?$$
Jul 1 20:27
Is it possible to express the solution of this equation as an integral