How come in mechanics a damping force for a spring is given as a multiple of velocity and not another acceleration

Like an EM force applied to the spring could be damping it

And what is a damping force?

nvm i got it

@Obliv isn't this due to friction / drag which is velocity dependent?:

There was an answer by ACuriousMind explaining why Fourier transformation and mode decomposition of fields are different. I can't find it...Can somebody help me find it?

@ManasDogra physics.stackexchange.com/a/700596/50583?

Thanks

Hi

Is the CFT from AdS/CFT a non perturbation formulation of string theory?

I think this CFT is sort-of different from the other CFT we use in the worldsheet feynman diagram stuff

Becuz this CFT is just a QFT. We don't need to introduce Feynman diagrams in an ad-hoc way

Why is planck length considered to be the smallest possible length ? Im curious and also how did they come up with that value ?

experimental means is what im guessing but is there a way to derive it ?

@NaveenV it's not, this is a myth. See physics.stackexchange.com/q/185939/50583 or physics.stackexchange.com/q/9720/50583

@ManasDogra i also hav a post relevant to this : physics.stackexchange.com/questions/734021/… . It covers the connection with 4D Fourier transforms

@RyderRude The string theory side of AdS/CFT is the AdS side, not the CFT side!

The other answer there (other from mine) has explained it more clearly than me @ManasDogra .

@ACuriousMind oooh its a myth ! I really did think planck length to be the smallest length and with those magnitude of videos saying its the smallest length possible

@ACuriousMind yeah. I mean the predictions can b mapped

All calculations of string theory can b carried out on the othr side

if the conjectured holographic principle held in the general sense that indeed "string theory in the bulk" is fully equivalent to "QFT on the boundary", then you could believe that the boundary QFT is a non-perturbative formulation of string theory

but no claims of this strength have been established

@RyderRude Thanks...I got that answer in the first place, then remembered I had seen ACuriousMind's somewhat different reasoning someday...so I asked for it.

@ManasDogra ACM's answer covers the connection with 3D Fourier transformers. Both posts are relevant :)

They cover different things

@ACuriousMind yeah, the connection remains to b proved

But it's still good for practical calculations i guess

"practical" "calculations" in string theory?

So why do people take Ads/CFT on faith? There must be some calculations that agree on both theories?

Hello, to the attention of users or some member of moderator team (Hi @ACuriousMind) I would like to know if I can to add comments (myself ideas), here in this chat room, as a discussion of a video of YouTube Is science about to end? | Sabine Hossenfelder edited two weeks ago from the official channel Big Think. Many thanks.

Is the CFT from Ads/CFT a candidate for M theory then

If true, it wud b a non-peturbative theory of everything

But idk. This CFT reduces the number of dimensions from 10D string theory, becuz it's on the boundary.

While M-theory is speculated to hav one extra dimension

@RyderRude I mean, it's not faith - AdS/CFT in the narrow sense is proven

but "there are certain AdS backgrounds on which we have that the stringy amplitudes on it are equivalent to the amplitudes of a SCFT on the boundary" is not the general holographic principle you would need for this to be a statement about the formulation of "string theory" rather than the very specific AdS backgrounds

Oh

So it's the proof for the general case that's lacking

Anyone hav Lucid Dreams?

Energy of states in SUSY are strictly positive in the real world. How does that not imply that all particles in SUSY are massive?

Why only SUSY breaking scale and not other symmetry breaking scales like Electroweak symmetry-breaking scale considered measurable in the sense that it can't be set to zero by "shift of origin"?

@ManasDogra because the SUSY theory is usually a theory of gravity

So it can't be set to zero for the same reasons the cosmological constant cant be set to zero?

yes, it's not because of SUSY but because of SUGRA

In QM, how is a bound state formed i.e. if the state of the system is a vacuum and I send a momentum plane wave from the LHS, how do I describe the formation of a bound state?

The trouble is don't we make the assumption that T+R=1 therefore meaning that the particle won't become bounded

Can I start rigourous EnM studies if i haven't learnt lagrangian / Hamiltonian stuff

@DIRAC1930 I don't see a fundamental difference between "system goes from excited bound state to lower bound state" and "system goes from unbound state to lower bound state"

i.e. I would expect you can treat this exactly the same way you treat the decay of excited states - the unbound state "decays" into the bound state

Sorry, I meant how to do it in the QM way e.g. constructing regions similarly to this quantummechanics.ucsd.edu/ph130a/130_notes/node160.html

how is that the QM way?

why do you expect a particle that you fire at another particle to become bound?

the point of these scattering computations is usually that the energy of the incident object is conserved

If I fire an electron at a proton, isn't there a chance I get Hydrogen?

but obviously you can't conserve energy when you go from an unbound to a bound state, the whole point of being bound is that the spectrum of discrete bound states lies below the spectrum of continuous unbound states

@DIRAC1930 not unless the electron gets rid of the energy difference between the free state and the bound hydrogen state somehow, which again I would consider exactly analogous to how an excited state drops to a lower state

see physics.stackexchange.com/q/240062/50583

in any case regardless of what you do you need either radiative mechanisms or something else that can take energy/momentum, this really isn't like elastic scattering at all

Ah okay thanks

I'm confused. A bound state has a higher energy than the ground state and so does a scattering state

Nevermind ignore that

Is it correct to say that in the region of the potential well, the energy of a bound state is lower than that of the free states in the asymptotic regions at $X=\pm \infty$?

For a thin vertical column we have $EI\frac{d^2y}{dx^2} = -Py$ the buckling under a compressive axial load

I don't understand the forcing function $-Py$

Shouldn't that be a force? So a 2nd derivative

and it's applied perpendicular to the buckling sort of..

I kind of get it like $Py$ is a function of $y$ so it increases as the buckling increases

and $P$ itself is a force I guess

@DIRAC1930 states aren't local to regions

the unbound states of definite energy are plane waves in position space, i.e. completely delocalized

(1/4) I'm going to add some ideas about the mentioned video, but it is not my intention discuss the scientific speecha of professor since I'm not a scientist (I'm not a physicist) The talk of the professor is the video of YouTube Is science about to end? | Sabine Hossenfelder edited two weeks ago from the official channel Big Think. If I can edit some comments today and tomorrow. I can to expand more such ideas (the why of my words). I wait your replies.

So what is the correct statement?

(2/4) In my opinion for the achievement of quantum gravity which physicists need is to think (mainly) thought experiments (and maybe experimentum crucis), more than in tabletop experiments. The idea is to get the alchemy of general relativity and quantum mechanics in a simple diagram similar than the System of the World by Newton,

see the diagram from the section Other appearances from en.wikipedia.org/wiki/Newton%27s_cannonball(other inspiring diagram are en.wikipedia.org/wiki/Equivalence_principle and a figure that I can to refer from a book by professor Jim Al-Khalili of quantum biology).

(3/4) My belief is that in the diagram for the new System of the World we must to consider different ingredients a gravitational geon instead of the body "planet", to avoid to be Flat-earthers, and with the intention to probe the measurement problem, an open minded idea could be to consider agents as the hydra, octopus or a small lizard (all these animals have mechanisms of regeneration, see Wikipedia) instead the anthropocentric astronaut.

(4/4) The devices/artifacts were closely related to shipwrecks or journeys (navigation) in some way: the gnomon, the Antikythera mechanism, an astrolabe, the GPS... In my opinion we must to think the tree of Life as a system/device that probed our solar system. Many thanks.

@DIRAC1930 just that the energy of a bound state is lower than that of a free state

I'm not sure why you want to bring regions into this

@RyderRude Feynman wrote an interesting bit about them you know. He didn't call them Lucid IIRC but it was clear from his description that they were. He was doing some personal research on dreams I think.

@user250478 I think the main problem today is that we have a lot more speculations and many highly developed competing theories without sufficient experimental observations that can either corroborate or refute them. I think the main area where new ideas are needed is actually in how to gather new experimental data in clever ways that require the minimum amount of resources, so that all of the "hungry" theories out there which are waiting for feedback, will have something to chew on.

@user250478 Thought experiments can be a good starting point in that regard, if they can later be implemented, even if only in a limited way

@Amit He mentions something about that in Surely you're joking, Mr. Feynman!

Yes, that's probably where I remember it from. I always avoid specifying without being sure 'cause there's also the second Anecdote book... :)

Which I haven't read yet. Taking the intersection proves the statement :P

lol, indeed

I see that you @Amit are very respectful although my speculations are very very very poor. In which I believe is in how do work the Nature the rafts of vegetation in Oceanic dispersal (Wikipedia) instead of humans dreams to travel as is showed in science fiction.

One crucial thing is that we don't know (please allow the plural) what our solar system was like and what happened in remote time

I refer regeneration (and all worlds of DNA/RNA, LUCA, biofilm/LECA, I think the stable structures as were explained professor Donella Meadows in her book Thinking in Systems) from the suspicion of the paradox en.wikipedia.org/wiki/Ship_of_Theseus

A project that was failed is the Project Orion (nuclear propulsion). Please feel free to add more replies, you or your colleagues of the site, I can to add more speculations/ideas, in particular involving prime numbers as arithmetical billiards.

@user250478 Every science has an historical counterpart. Biology has Evolution, Geology has Paleontology, Physics has Cosmology, etc. If the methods and practices are sufficiently similar, the historical subject can be seen as just a branch of the main subject, I think in Physics that's the case. But as you point out by definition, so long as we can't time travel, there remains the distinction that we have no direct experience of the past.

In cosmology we can directly measure past event by virtue of the duration it takes for light and radiation to travel to our telescopes and detectors from distant stars. On the other hand, we could learn a lot more if we could jumpstart our own experimental universe and learn whatever happened at the point now called "Big Bang"

Many thanks for your replies, you're incredible, The library (I have not internet at home) closes soon, tomorrow I'll add a good morning message for you. My belief is that the more important thing is to know our Solar System, and what happened in the past @Amit

@Amit what's the history of chemistry?

@Slereah No chemistry! It's just Physics!

lol

grrr

I'm only kidding, I don't know if chemistry has an analogue

@user250478 Cheers

@Slereah Actually the question is whether there is a chemistry of history :) Cosmology is the Physics of history, the history of Physics is something else -- with stories about Newton, Galileo, etc. :)

@ACuriousMind Yes I wasn't thinking properly

are there any issues in the foundations of general relativity

nah, it's all good

perhaps it is because i haven't taken GR but you hear quite a bit about foundations of quantum, but not foundations of gr

oh

what do you mean, there's plenty of talking about it

Every GR paper starts out with it

i guess it is since i haven't taken GR yet :P

Take $M$ to be a smooth manifold of dimension $\geq 2$ equipped with a metric $g$ of signature $(-,+,+,+,\ldots)$ and a compatible torsion-free connection $\nabla$

That is technically all you need for GR

that and the Hilbert action I guess

XD well i'd need quite the body of text to understand what just a manifold is

@SillyGoose I'm working through Dirac's book on GR now... but only after studying quite a bit about the more geometric modern approaches... what I like about it is how concise it is

@SillyGoose Classical theories like GR, in general have only one serious foundational issue: that they're not Quantized :) In other words, we don't know how to apply all of the known foundational issues we have normally because they're all QM related lol

That's why Physics is crazy I guess, we want to solve a single problem so that we'll be able to inherit loads of new ones :)