12:00 AM
In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long signal for a shorter, known feature. It has applications in pattern recognition, single particle analysis, electron tomographic averaging, cryptanalysis, and neurophysiology. For continuous functions f and g, the cross-correlation is defined as: : (f \star g)(t)\ \stackrel{\mathrm{def}}{=} \int_{-\infty}^{\infty} f^*(\tau)\ g(\tau+t)\,d\tau, whe...
That seems like what I need to do, but I don't know how to actually implement it... how wide of a time window is needed for the Y_{t+\tau}? And how on earth do I load all that data at once without it taking forever?
And is there a better or other way to see if shear strain does cause temperature increase, potentially delayed in time

Hmmm...not one of the one I know.
Maybe start dumb: just plot the temperature over some time range starting at the time the strain is applied and let the Mk I eyeball help you figure out what signal you are looking for?
If you can see anything then you know where to start?
Or is there too much data for that to be practical?

My data is 870x870 points and something like 1500 snapshots
I originally just plotted T vs gamma as a scatterplot, but all the high temperature points are at low strain
Which makes me think there's a time delay

Ah. Well, you can just sample the snapshots for the first pass. Say look at 15 of them. Or even just 5-8 depending on how long each one takes.
Is your strain a single pulse? A step function? Periodic?

So it's a block of material flying at 50m/s that hits a wall
So the strain is caused by the pressure wave

user54412
so strain is dynamically computed along with T as a function of time?

12:06 AM
Yup

I think I'm out of my depth here.

(me too)

user54412
well, if it is lagging but local, you don't need all 870x870 pixels to test the hypothesis, right?

Perhaps there should be a time delay. Energy has to spread into multiple degrees of freedom.

True, I could identify a small region that shows both strain and temperature
Which would greatly reduce the dataset

12:08 AM
Good call, Chris.

user54412
perhaps look at shear(t), T(t) averaged over a 10x10 region for all 1500 snapshots

@tpg2114 So, is the data point you have are temperature T(x,y,t) and strain S(x,y,t), and you want to find the correlation?

user54412
I don't know the details of the sim, but I'd worry about trying to do this on a single pixel at a time - seems noisy

@hwlau Yes, but there is a lag in the data
@ChrisWhite That's also possible

0

Link to the question: Learning roadmap for picking up enough mathematical know-how in order to model "shape", "form" and "material properties"? Alternatively, where could I go in order to have such a question answered?

user54412
12:10 AM
besides, heat will diffuse (presumably), so there will be pixel-to-pixel correlations in temperature I should think

@tpg2114 Is there spatial correlation then?

@hwlau I could check for that too, but I don't think there's much of a physical reason why there would be one

@tpg2114 Diffusion of heat

Heat diffusion is not included in the model
At this point

@tpg2114 For reducing data point for calculating time correlation, you can run two exactly the simulation in parallel separated by the time lag dt. Then there is no need to store all snapshot and spatial points.

12:20 AM
@hwlau The sims are done and took many hours each and the snapshots are already stored so this is all post-processing

@tpg2114 i see. That trick is for the case taking many many time sample points.

Now I'm digging through the web to try and figure out how to do it in Python
So I can give it 3D arrays (x,y,t) and it knows to only vary the 3rd dimension holding the first two fixed... I may have to write my own code

1 hour later…
1:39 AM
@DavidZ I wasn't trying to justify it's existence here, just merely pointing out that because there were some numerics questions posted here, some people might think it okay to post more. I still think marking it as a duplicate is a good idea, then probably an historical lock on the others (maybe with a warning that questions like these belong on Comp Sci?)

1:59 AM
I got a code running and data coming out but I don't really know what I'm looking at... hate when that happens

2:46 AM
@tpg2114 Hmm, perhaps you could squint?

@KyleKanos I'm really really baffled by this output

@tpg2114 How is the figure looki like?

@hwlau Let me find somewhere to post it, hang on
Okay, so I'll explain what it is cause it's not designed for other people to look at

@tpg2114 I was just going to ask...

The x axis is the index in the array -- so I have 200 time series
Each one is equally spaced, 1e-9 seconds apart
The black line is \frac{d T}{d t} and doesn't have an axis -- I don't care what the values are
The solid blue line is the abs(shear strain) and is valued on the right axis
The dashed blue line is the result from scipy.signal.correlate
And is valued on the left axis
So what I don't understand: 1) Why is the correlation value negative when they look pretty positively correlated to me? 2) Why is the result from the correlation function 400 time steps long? 3) How do I find the lead/lag between the signals? Wikipedia says the argmin or argmax of the result will tell me that, but I don't know how

2:54 AM
Hmm. Going back to your thinking it was lagging, it does appear that shear strain is lagging behind the temperature change
Or is it the other way....give me a moment.

Yeah, I'm confused too :)

Wait, yes. Seems I had that backwards. First the shear spikes, then a moment later the dT/dt spikes
Wait.
I'm confused now about how it's being plotted temporally.
Is x=0 really t=0 or is it t=t_{end}?

The whole thing confuses the heck out of me
It is t = 0, at least for the solid lines
For the dashed correlation line, I have no idea what the time index corresponds to

Okay...then it seems that the temperature spike comes before the shear strain spike

If it is convolution that the indexx ~200 should be t=0
@tpg2114 and how can you get 200 time series? Cross correlation is supposed to be the value between two time series only

2:57 AM
@hwlau Sorry, I meant to say I have 200 time snapshots

do you mean 200 time point at a particular location?

Yeah

Then it really seems they have correlation from your figure

Yeah, I don't do any correlations, so I'm of little help
Chi-by-eye is good enough for me

@tpg2114 1) convolution of +ve and -ve, give you -ve? 3) seems temperature (black line) lead shear strain (blue line)

3:02 AM
@hwlau 1) Both are positive functions but I get a negative correlation value
3) I agree. But I need to find the time between the two signals, the offset that makes them line up
Which wikipedia says the argmax or argmin will tell me
But I don't understand how
In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long signal for a shorter, known feature. It has applications in pattern recognition, single particle analysis, electron tomographic averaging, cryptanalysis, and neurophysiology. For continuous functions f and g, the cross-correlation is defined as: : (f \star g)(t)\ \stackrel{\mathrm{def}}{=} \int_{-\infty}^{\infty} f^*(\tau)\ g(\tau+t)\,d\tau, whe...
Because I don't know how the result is indexed in time

Crazy idea: Take Fourier transform of data, then perform correlation?
2nd Crazy Idea: Call up a statistician at your Uni and give them your data and ask them to do this stuff for you.

Slides due for my boss in the morning to justify me continuing past this month...

Hmm
Nevermind, crazy ideas usually aren't worth much effort unless you've got time to kill.

@KyleKanos That is excellent idea

So I can buy the premise that halfway through the resulting series is tau = 0

3:08 AM
Can't you present it as is (sans the odd correlation values) and say, "It seems that Temperature change occurs before shear strain"??

@tpg2114 It is very likely that it is from t=-199 to t=199, total 401 points

I could, I may have to

just check that yourself

It's 399 points actually

sor it is from t=-198 to t=198
because of the zero index of the array

3:10 AM
Yeah

Science often progresses by promises of something more much later down the line, so hopefully it'll work in your favor this time

sorry, looking at the figure, I guess I am wrong somehow
the time lag should be around t~10
which function are you using?

How are you determining that?

The peak to peak location

Oh, yeah
And that's pretty close to what I get now:

3:15 AM
to have strongest correlation, the shape of two peaks should be almost overlapped
that is the definition

-2.7e-08
Each time index is 1e-9
So it's about 27 between the peaks

ya, differe by 2.7 times, same order of magnitude

Yup. So for x < 200, is the time negative or positive?
On the result function

how do you calculate it?

That's what I'm asking -- the correlation has 399 points in it. If time = 0 at point 200
Then is point 100 negative time or positive time

3:21 AM
you use the scipy function?

Yeah, and I call it with scipy.signal.correlate(gradT, shear)

so you use the "full" mode
have you ever tried the "same" mode

I did, it gives me an answer but I don't know how to get the lead/lag time from it
That's where I'm stuck now

user54412
hmm

@tpg2114 So, if you use the "same" mode, do you see a peak/trough at index 25 (t=25e-9)?
There are always the problem of the discrete correlation
they are not the same as the continuous case
and you need some kind of padding of zero before and after it, so the problem.

3:32 AM
I'll have to run again with same and see what comes out

user54412

user54412
#!/usr/bin/python

import numpy as np
import scipy.signal as signal
import matplotlib.pyplot as plt

t = np.linspace(-10.0, 10.0, 100)
tt = np.linspace(-20.0, 20.0, 199)
signal_1 = np.exp(-t**2)
signal_2 = np.exp(-(t-5.0)**2)
correlation = signal.correlate(signal_1, signal_2)

plt.plot(t, signal_1, 'r--', t, signal_2, 'b:', tt, correlation, 'k-')
plt.savefig('correlation.png')

user54412
I don't know anything about scipy.signal.correlate(), but it seems to output the correlation on double the time interval

user54412
which makes sense - if both signals run from -10 to +10, you can ask about offsets anywhere from -20 to +20

I think you know how it works

3:48 AM
0

Is there a word or phrase for when someone is testing for a certain result thereby scewing their findings?

Off topic?

Yup, I think so

I think so too

I concur as well

user54412
me four

Who's #5

3:52 AM
The next one is taking a nap, lets wait

Naps? Who does that?

On the list, I think @dmckee

Yeah, off-topic.

1 hour later…
5:00 AM
Finally making progress on understanding this stuff... finally!

1 hour later…
6:28 AM
Quick question that I don't think merits a question. A vacuum isn't a perfect insulator right?

7:19 AM
@lswim A vacuum stops conduction and convection but not radiation.

Sorry, I should have been clearer. Electrical conduction. Like isn't the permitivity of free space basically saying it can conduct at some point?

@lswim Ooh well I looked this up some time ago.
At some point the electric field can be so great that it tears electrons away and they fly right through the vacuum
I think it's called the vacuum breakdown voltage.

Ok that's what I thought. A teacher said otherwise so I got super confused.

well it's perfect under reasonable conditions
the breakdown voltage really is absurdly high

Mhm. Yeah. That makes sense. Thanks for the help!

7 hours later…
2:17 PM
Love is in the air for physics. :) http://t.co/aqzmeI0o24

3:14 PM
Chat session in about an hour!

@Waffle'sCrazyPeanut ohai!

Hey! :D

Is $x = A \sin^3(\omega t)$ simple harmonic?

1

. For 1(a), is the speed $\dfrac{(R+x)}{\sqrt{2R/g}}$? For 1(b), is the answer $R+x$?

I wonder why people upvoted that?

@ShuklaSannidhya I think "No"
@DavidZ Weird... Downvoted! ;-)

3:31 PM
@DavidZ My guess is because some people are crazy

@KyleKanos Crazy is gone... It's now called "Waffle's" :D
Though there's a "crazy" in the middle for old sentimental reasons, this crazy isn't that crazy :P

@Waffle'sCrazyPeanut I didn't mean you, so stop being so egocentric! :D~

LOL... :D

3:48 PM
@Waffle'sCrazyPeanut Yeah... I thought so... but...
I've got a question
> A particle moves along the X-axis according to the equation $x = 10sin^3(\pi t)$. The amplitudes and the frequencies of component SHMs are...
??

@ShuklaSannidhya Have you plotted this?

@KyleKanos yeah? but how does that help me?

Amplitude is the value of the peak, so...

@ShuklaSannidhya This one is a better look at it

3:58 PM
...
but how did they do it?

Chat session time?

inb4, boring meta discussions...

well nobody else seems to be here for boring meta discussions just yet

@DavidZ Might be too difficult, but one of my theachers said if we could bring more high level physicsts to the site
like Weinberg and such

That would be nice, but difficult

4:09 PM
I'm here
But like the last 2, I still don't know what's discussed

OK, well I think we have a discussion on homework policy that's needed to be had for a couple months

I think that at least a rewording of the policy is a must

Yeah, I've been thinking about that. There are several areas for improvement but first we have to resolve the issues that have been brought up on meta
6

Related: Why don't we just ban homework altogether? Banning homework: vote and documentation We're having some more recent discussions on the homework tag. A month ago, there was a flurry of activity involving a tightening up of the policy. Unfortunately, I was really busy after th...

So, things we need to decide (but not necessarily today): (1) do we implement John Rennie's suggestion of having the mods not close homework questions for a month (2) do we reword the homework policy, and how (3) do we get rid of the tag
2

4:19 PM
IMO, 3) should be the last thing to do. Even if we ban homework that tag could be for a while.

I think (1) would be a decent option if we had >5 3k+ voters online at any one time to do the small-time moderating. Between the HW being posted and (finally) being closed, there's usually some <1k poster who answers the question
It'd be better if we could do it quick enough that no answers get posted until the question is clarified to satisfy the current HW policy
(by "do it", I mean close)

Yeah, that's why (at the moment) I think moderator closing is useful to keep around.
At least... a month is a long time. I wouldn't be quite so opposed to a trial run of, say, a week. Maybe two.

I'd approve of that. I mean, I'm in my office 7 days a week (trying to finish up furiously for a May graduation), so I'm online quite a lot

I think I know how it will go (badly), but people seemed to want it.

Likely. But, as dmckee suggested in the comments to John's post, Mods could cast the 5th vote (but it would require you paying attention to the question)

4:25 PM
Right.

Maybe we can ask the person who casts the 4th vote to flag a mod to cast that 5th?
Which would be terribly hard to enforce

So in this period we're supposed to be more strict with homework?

I don't know about more strict but definitely more on top of flagging for closures (i.e., making sure we check the review queue a few times)

@KyleKanos I think the only essential thing would be that all five of us moderators are aware of what's going on, so we know not to cast binding close votes
@jinawee no, you vote to close homework as you always would (or should), just bear in mind that moderators aren't going to be closing those questions unilaterally

And what's the purpose? To see how healthy the >3k community is?

4:30 PM
I think the purpose was to develop a sense of what counts as a homework question, based only on what the community decides to close
although I do have to admit that wasn't entirely clear to me

I'll have to look a bit more deeply at the current policy for possible re-wording. If I come up with a few things, should I post each as an answer on a meta question?
Do it as a poll-type thing?

I guess you could, if you have isolated ideas
I was actually going to go over the current policy, rewrite it entirely, and then post it as a question so people could offer their feedback as answers
but who knows when I'll get to do that... you could do it if you want

I guess I'm viewing it as requiring to write up a pair of extremes (e.g., let all questions and no problem-like questions whatsoever) and then a few middle-ground options

Ah... well, I guess that could be somewhat useful, but I think it's more important to solicit specific comments, not just do a poll

Okay. Maybe I'll try it your way and write a good one and ask for feedback

4:37 PM
For example, I though that most people wanted a stricter policy, which would close some questions which are currently allowed

I'll try to getting to that before the end of the week, but no promises

1

For the SHO, our teacher told us to scale $$p\rightarrow \sqrt{m\omega\hbar} ~p$$ $$x\rightarrow \sqrt{\frac{\hbar}{m\omega}}~x$$ And then define the following $$K_1=\frac 14 (p^2-q^2)$$ $$K_2=\frac 14 (pq+qp)$$ $$J_3=\frac{H}{2\hbar\omega}=\frac 14(p^2+q^2)$$ The first part is to show that $$Q \... Like this one @jinawee That's what I thought too @KyleKanos if I have time, I'll do it too and we can compare notes @jinawee yeah, but nobody voted to close that @jinawee And I think this part of the problem: everyone has a different opinion about what is a homework question. We need to work on writing one that makes it far more clear about what should be allowed and what should not be allowed It's kinda like the American definition of porn: I can't define it, but I know it when I see it. 2 What might be useful, as a precursor to editing the current homework policy, is to post a meta question asking for feedback on what is not clear about it. We could do that in conjunction with a week of no mod HW closures (if the other moderators get on board with that) 4:40 PM @DavidZ This is probably a good idea @DavidZ But I think the only people who interact with meta questions are the ones who don't need to explain what's not clear... We need to folks posting the bad homework questions Most of them are brand new posters. Realistically I don't think there's any way to make them read something before posting. This would be for the benefit of people deciding whether or not to vote to close a particular question. And is it possible not to allow just homework tag? Okay. I guess we'll have to see what people say but my guess is the unclear part is what constitutes homework itself. We've had discussions where some people equate it to the level of the question and not the content, or where "where is my mistake in the math" is okay if it's advanced topics but not for mechanics @jinawee It's possible, sure, but the consensus seems to be that we should change the homework policy before we decide whether to get rid of the tag @tpg2114 yeah, that's the sort of thing we need to figure out. 4:44 PM @tpg2114 Which the latter seems to be hypocritical/biased I meant the system to allow only [homework]+[other-tag], no just [homework] Part of my motivation for wanting to write a revised homework policy is to make explicit that any question asking "Where did I go wrong?" or "Is this the right equation to use?" (without further clarification) or "Any feedback would be appreciated" is not okay @jinawee oh, that I don't think will happen. In any case that would be an indication that is a meta tag, i.e. a tag that we shouldn't have. @DavidZ I don't think anybody disputes that it's a meta tag :) Yeah, which implies that we shouldn't have it. And yet we do. :-/ I am pretty sure everyone uses it as if it were a meta-tag 4:52 PM We added other meta tags since then, that whole pop-sci thing Yeah... for now, we should just enforce through editing that shouldn't be the only tag on a question But that's another discussion @tpg2114 I don't think that ever really took off yeah let's just worry about homework for now So anyway, I think suggestions for things that need to be clarified -- what is homework and what is "conceptual." Ie. is it conceptual to be stuck when deriving the distribution of microstates cause somebody doesn't know what Stirling's Approximation is Some have argued that is on topic even though there's nothing really physical about it just because it's 'graduate level' Others would argue it's not on topic because it's not conceptual I think that some no effort graduate questions should be closed, like What is the proof of exercise 7.8 Wald or 5.2 Jackson? 4:58 PM yeah, definitely OK, well our hour is up for today. I'm sure we'll come back to this at our next chat session in two weeks. 5:34 PM 1 How can one prove that$$ \operatorname{Tr} \log \cal{A} = \int_{\epsilon}^\infty \frac{\mathrm{d}s}{s} \operatorname{Tr} e^{-s \mathcal{A}}, for a sufficiently well-behaved operator $\cal{A}?$ How (mathematically) rigorous is the expression? I'm looking at the $d=2$ Euclidean case, as discuss...

Off-topic? Looks like a math-only question to me

I've noticed that there is a remarkable difference between me in a selfie and me in the mirror. Left-right reversal might be part of it, but I wonder what is the r-e-a-l reason. Too bad the question got closed.
And what about selfies in the mirror? (I didn't try yet.)

6:22 PM
@KyleKanos @jinawee @DavidZ @tpg2114 So my take is that we should probably do the "mods only 5th vote"-- I've already been doing that for a while, except for that occasional time when I just wipe the queue clean.
Additionally, what we can do instead is go through the closed questions and delete the homework ones as quickly as possible, as mods.
Or maybe that can be a second step.
If we can reduce visibility of HW, then the tag becomes less of a bone of contention

6:49 PM
@ManishEarth yeah, that might be good. Though perhaps only older than a certain threshold?
For the only 5th vote thing we just need the whole mod crew on board with it.

yep

People not wanting to tag graduate level questions as homework is another reason to get rid of the tag. No tag should be some kind of stigma.

@jinawee Yep
Also, everyone has a different meaning for the tag. Which sucks, because some people try to bend the tag meaning to fit their own preferences

7:12 PM
Just a reminder to everybody when thinking about the homework issue: please keep this and this in mind too.

7:28 PM
@tpg2114 I've marked it as such too
@jinawee I think if someone asks, "How do I do Jackson 11.26," it certainly should be marked as homework. But if someone asks, say, "How is source theory different from qft?" it certainly shouldn't be marked as Homework

@Dilaton because that's talking about the tag. And like I said, everyone has a different meaning for the tag, so we'll have to phase it out. There's no need for it if we are able to swiftly handle the main page closeable homework clutter.
@Dilaton also, have a look at the topvoted answers on both.

8:05 PM
Afternoon folks. I tend to ask questions about perturbation methods and asymptotic expansions that arise in my work over on Math.SE, but most of those folks aren't too interested in these kinds of approximate questions. Would posts like this be on topic at Physics.SE? (my initial feeling is no because its really a math question, but I figured I'd ask anyway)

8:42 PM
There have been an unusually high number of questions in the Reopen queue today.

9:00 PM
@KevinDriscoll you're right, that would not be on topic for us. (My opinion, anyway) There's no physics in it.
By the way your integral's upper limit is missing an i

@DavidZ Ya I figured as much. Thanks for the typo catch. Do you know of any other place for questions like this? I spend a lot of time at math.SE and they're really mostly interested in either high-level pure math or recreational math (limits, series, integrals, etc). There doesn't seem to be a good place for the approximate and applied techniques I tend to rely on.

hm... I guess you could check at Computational Science. I wouldn't necessarily expect it to be on topic there either, since that's mostly numerical methods and stuff about scientific software, but it's worth looking into at least.
Or... to be honest, if you were to rephrase your question in a way that makes clear how it's about physics, it might actually be okay on this site. There's a fine line between math and theoretical physics sometimes.

@DavidZ How about MathOverflow? Are these kind of questions allowed there?

I highly doubt it.
MO is for research-level mathematics, not "how do I compute X"

user54412
@KevinDriscoll You could maybe reword to push that question in the direction of another site, but imo as worded it falls squarely in the domain of math.SE - it's just a shame they don't give that kind of question as much attention as, say, explaining why 7 is the only prime followed by a cube

9:11 PM
Yeah I think the level of this question is too low for math overflow

user54412
"Attention graduate students: You may not apply to attend programs, unless you will receive your PhD by the time the program begins and state the name of your supervisor."

user54412
wow the Kavli Institute comes off as uninviting

@DavidZ @ChrisWhite Thanks for the advice. I'll consider it. The link to the physics will be difficult to explain concisely, but its maybe worth the effort

9:31 PM
@ChrisWhite As I understand it, KITP wants big names in the field who will promote crazy ideas with the intent of getting someone else to develop their idea into a reasonable solution (c.f., Hawking's recent paper)

2 hours later…
11:03 PM
how many rep points does one need to comment?
oh it says 50. man this is so annoying, i only have 49

@hadsed I was under the impression that if you linked your SE accounts you'd get +100 rep automatically.

oh. uhh.. so what exactly does this mean

I dunno. I don't remember how I did it.

i have a few SE accounts

Anyways you should be above 50 now, I just upvoted your electron degeneracy pressure question.
I have the same question but I think I understand what's happening which has kept me from actually asking it.

11:14 PM
ah, thanks!!
btw, you were the one who i talked to last time about entanglement?

yeah. im trying to figure out if i can look at the last time i was tagged in the transcript

It was probably me.

i lost a link that you (or someone else) gave me

Some right hand / left hand glove entanglement analogy

11:16 PM
yeah exactly

But with that analogy it's classical and the outcome is predetermined but with entanglement the outcome isn't predetermined.
Which is why entanglement is so strange. And also why it doesn't communicate any information.

i think my issue was that someone had commented that it wasn't very weird that the zillion lightyear away electron knows what the other electron's spin ended up being measured as
i couldn't really understand how that wasn't strange
i think it was lubos. some guy was hounding him for an explanation but i assumed he didn't answer because perhaps it's sometihng he has explained before

user54412
@BrandonEnright cc/ @hadsed Once you have 200 on any one site, you get a +100 bonus on all sites meta.stackoverflow.com/a/7238
2

11:34 PM
oh nice
i should be more active on these sites. i spend most of my time on quora, but SE seems more technical and i like that