Hello

I wanted to use perturbation theory in solving a problem

I just want to know

What are the rules

Where it can't be used etc

Can anyone help?

@Korra perturbation theory is always an approximation

as with most approximations, there are no "hard rules" on when you can use it - it always depends on what margin of error to the "true" result you can tolerate for the application at hand

in general, the relevant question here is how "small" the perturbation is, but what exactly "small" means depends on the concrete situation

@Korra That's tricky to answer and depends on the specific problem at hand. The 2 most general rules are to ensure that the background solution is recovered when the perturbation parameter is put to zero, and that successive perturbations (of whatever quantity you're computing) are smaller than the previous ones.

@ACuriousMind @Avantgarde thank you so much. That was great help

@JohnR twitter.com/TikTokInvestors/status/1404935219682107396

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Hi! Anyone here?

Guess no

can anyone help me with a particle physics related problem? I am stuck at a certain point in my solution, and even google can't help me atm xD

As a high school student, I (most probably) won't be able to help, but why not post your question/problem? Someone might read it later.

I did, but there are further things which I fail to undestand, and you cannot make a post about every single one of them.

@imbAF What's the question? Particle physics isn't my thing, but I'll help if I can.

John thx for considering

Well..I will link the question and then explain to you what I fail to understand

I'll give you a moment, so you can read it and have an idea on what's it about

Ah, I can't help. Sorry :-(

Damn it

There are people who know this sort of stuff, but the room is always quiet on a Friday evening. Give it a day or so and hopefully someone who knows this stuff will see your question.

I hope someone comes. The problem is that particle physics is being explained to us, without having knowledge about Isospin, SU3 and other symetries, etc etc, and it's like walking in the dark and hoping you can find something to grab on. I've been struggling for like 3 days now. And without further knowledge I am simply wasting my time, with these homeworks

Can phonons with an energy below the superconducting gap propagate through superconducting films? Or are they not allowed in?

If I have a fixed target collision and I find the threshold energy, how can I then find the momentum of the accelerated particle for a give particle collision/ production process ?

@imbAF dont you just subtract the threshold energy from the energy in the collision

besides the nuance of an actual experiment the principle is energy is conserved

@user129412 I don't know much about films so I don't know if it's different from standard (bulk?) superconductors

in normal superconductors the gap only refers to the electrons. The gap means that there are no excitations of the electrons below some energy. The lattice phonons exists and propagate normally, but they do not interfere with the superconducting electrons.

I did it. I found out how to find the momentum once i know the E-thre