his name was the first letter (S), but he called it "Stoachastic" in his paper

people started substituting his name

the ultimate plan

a clever ruse

anyone wanna help me out here real quick?

I know theres a way to move from comments to a question to private chat with the person asking it but i dont know how

@Jaywalker The "move them yourself" option appears after you've posted lots of comments. If you'd like me to move the comments to a chat for you, flag the post or put the link here.

Hello all

In order to use the 1st Born Approximation, we must have a "soft sphere", i.e. the potential is not $\infty$ near the scattering point. Is there another condition that must be met in order to use this approximation? The book I'm following says "low energy scattering", does this mean that the incoming particle must have an energy below some threshold??

@loltospoon uhh, Born is better for high energy...

see page 403 of Sakurai.

The index of Schwartz lists 9 different chapters for the LSZ formula

@0celouvskyopoulo7 see this screenshot of Griffiths: imgur.com/a/Q8yPj

I do not have a copy of Sakurai

@loltospoon that's not the Born approximation

that's the low energy approximation of the Born approximation

@0celouvskyopoulo7 lol for real? I'm so lost...

Born approximation is $f\propto\mathcal F(V)$

Can you explain the terms on the right?

I see that sentence there

It says there's an expoonential

@loltospoon Fourier transform

I know that book has the correct expression because he's talking about it

@0celouvskyopoulo7 How about this one? $\to $ imgur.com/a/ZEf5c

Yes

You want $V\ll E$, where $V$ is the "characteristic strength" of the potential and $E$ is the energy of your particle

Sakurai has a more precise condition

What is his condition?

Let $a$ be the range of the potential

(you also want finite range)

Then you need $$\frac{2m}{\hbar^2}\frac{|V_0|a}{k}\ln(ka)\ll 1$$

Hmmm ok

@loltospoon So for $k$ large you get that

@0celouvskyopoulo7 what is $k$ here? Is it the scattering vector?

@loltospoon Since $\lim_{k\to\infty} \ln(ka)/k=0$.

@loltospoon the momentum of the particle

modulo an $\hbar$ perhaps

just the wave number

@BernardoMeurer

Hi all. If a cylindrical permanent magnet of radius R and height H is placed inside and perpendicular to a uniform magnetic field B, how does the torque experienced by the magnet scale with R and H?

It seems a remarkably sudden, and radical, change of direction.

What made you start thinking about this?

I don't think you're unusual in this. For everyone interested in science how a lump of grey jelly manages to achieve so much is an amazing puzzle.

According to Wikipedia (I speak as one who knows absolutely nothing about cognitive neuroscience) it's a highly technical subject and involves physics and maths as well as biology.

So starting from where you are I'd do a physics degree ...

I know people who have done electronics/CS and I have to say I don't think it trains you any better than a physics degree. Physics is a remarkably good training for anything technical because it develops so many skills.

Pretty much any technical degree is more of a training in how to think and approach problems rather than a specific training.

You'll find an engineering degree will also cover lots of physics, and you'll probably find the later years of your course allow you to specialise more towards the physics.

And you'll be doing lots of computer programming as a routine part of the work.

I would guess that you'd still be in a good position to move into an area like cognitive neuroscience if you still felt that way after graduating.

I'd just be cautious about committing to anything too early. I ended up working in an area that I didn't even know existed when I started my degree.

> I've never thought I'd enjoy doing research in Physics

Really?

If you like doing puzzles, e.g. crossword puzzles or Sudoku, then you'll enjoy being a research scientist.

Really all it takes is finding it satisfying to solve problems.

I wouldn't worry about that. A degree, whether it's physics, engineering or whatever, is really a training in how to think about things. That training is valuable whether you decide to be a scientist or go into some completely different career.

Suppose you do indeed end up working in CN e.g. looking at ways to measure how neurons work. Wouldn't that be doing research in physics? My point being that I wonder if you have a misleading view of what it means to be doing resaerch in physics.

It's hard to really grasp what scientists do day to day before you've even started your degree. It's just too alien. But during your degree you'll be exposed to research in the sense of having to do projects, and you'll get the chance to talk to scientists.

Agreed!

not sure about anyone else; but if I was the only person on earth; I'd probably not do anything academically related. I'd go build a hut and walk around some forests or something

@JoshuaLin I bet you'd quickly get bored and start researching the optimal way to construct a hut :-)

@JohnRennie haha maybe

@JoshuaLin humans are instinctive scientists. Whether it's figuring out a unified field theory or working out the best way to construct a spear for killing mammoths it's something we do instinctively.

finally send my email to apply a research assistant

a bit nervous now

@Kaumudi.H thanks!

I have drafted it for 30 hours +

hope the researcher will give me an opportunity

Hello

@JohnR Hullo o/

Basic kinematics, anyone?

^ On an "ambiguity-scale" from zero to ten, how would you rate this question (and the supplied options)?

Personally, option 3 and 4 seem right ._.

@JohnRennie Slowly starting to get Rust

Weird language

@0celouvskyopoulo7 I can't. I hate Quavo too much

holy crap I hate this guy

Hello, @JohnRennie

How are you even supposed to do QFT in cosmology when your geodesics are past incomplete

@JohnRennie @dmckee I need some algorithm help

@BernardoMeurer Yes? ...

@JohnRennie Oh, good, you're here :P

So, I have two vectors, one has all the files on my source directory, say /home/foo/bar/a.flac, and another has all the files in my destination directory, say /home/bar/bar/a.ogg

And we have a lot of files, >6000

How can I "match" file-names in src and dst (source and destination) to check which one is newer

Now, my question here isn't how to check the metadata, that I know how to do

But how to transverse the vectors finding the correct files

You need to at least sort the destination array, and it's probably quickest to sort both arrays.

@JohnRennie They are both sorted

My function to get the files always returns a sorted array

Easy then.

Start at the first source file and the first destination file.

If they math then bingo.

Otherwise if source > destination move to the next destination file and compare. If source < destination there is no match.

Now move to the next source file and compare starting at the current destination file. No need to start from the first destination file becauise they are sorted.

Ah, I see

Having them sorted makes it really quick and easy.

I guess I can have an outer loop that goes over destinations, and an inner loop that goes over sources

in which case I can do continue/brakes accordingly

@EmilioPisanty thanks for the link. I haven't really done a lot of electromagnetism yet (I will the next couple of weeks), so I will postpone reading this until I've read some of the basics on EM

@BernardoMeurer You have two distinct operations. For copying new files from the source to the destination have the outer loop go through the source.

When you are deleting files from the destination that don't exist on the source have the outer loop go through the destination.

What about when I'm checking for changes on the original in order to re-encode to destination?

So the complete replication takes two passes - the first to create/update files and the second to delete unwanted files.

Yeah, I'm writing simpler functions, in a module

I called the module "probe"

and I wanted to have like probe::changed, probe::new, probe::deleted

In the first loop you start at the first source file. Does a matching destination file exist? No - copy the source. Yes - compare the last updated date and copy the source if it's newer.

Then go to the second source file, rinse and repeat.

Yeah, the copying will be handled elsewhere, I'm making an effort to write reusable code

guys, does anyone know how to turn of "New feed items"? (I use safari). It only appears when I'm on the physics chat. I can't find anything useful online about this either

@ShaVuklia It's only supposed to show up in the chats IIRC

And it's the chat owner's choice to enable it or not

here it's enabled

and I hate it

oh :(

too bad

@BernardoMeurer OK, you can have your loop just set a status flag probe::new or probe::copy and you can go through the statuses later.

because it's incredibly annoying

@JohnRennie I'm more returning vectors with the paths

@ShaVuklia I don't think you can turn it off.

So when I do probe::changed I get back a vector with all the files that were changed

well at least we're in this together:P I thought I was the only one

@ShaVuklia You're not. I hate it with a passion

Actually I quite like having the notification, but then I'm a net answerer so I'm always interested to see new questions.

@JohnRennie I see you did not "Love" my new facebook picture. I thought you were my true friend. I am heartbroken

@BernardoMeurer: you've lost me slightly. If you have a vector of source and destination file names then the algorithm I describe tells you what operations are required to sync the two. You can process this info in lots of ways ...

@BernardoMeurer I don't remember seeing it. Let me have a look ...

Ah, the one of you eating a meal?

@JohnRennie My grandma commented on it saying I am hot, made my day lol

Yes

I'm probably the same age as your gran. Is she hot? :-)

Hello

@JohnRennie How old are you again? 54?

@SBM Hi :-)

@BernardoMeurer 56

Lol my grandma is 70 something

I think

Is there a place where I could find Physics resources?

@SBM what sort of thing are you looking for?

@JohnRennie What's the time complexity of sorting a sorted array?

O(N) I guess. I don't really know.

Yeah, that's what I was thinking

That's good enough

I added the sort routine into the function, this way I can reuse it later in a setting where the input is not sorted

In your app I'd guess the limiting factor is the file copying, so sorting an array isn't going to be time critical.

Yep, I was hoping to do this assynchronously later

Like having 2 threads getting the file list and so on

and the others encoding/copying

Apparently unitary inequivalence happens for both QFT and the thermodynamic limit

everything is a lie