The h Bar

Nihar Karve

3:52 AM

is it worth learning about S-matrix theory?

specifically, 'The analytic S-matrix'

RewCie

If you are paid well, everything is worth.

user 6232128

would you do anything to be well paid?

Captain Bohemian

4:10 AM

quora.com/…

123

4:20 AM

Hi Guys...!

I have one numerical i don't understand some insight of it. Pls help me out to figure out.

A 0.05Kg bullet is fired into 10Kg block that is suspended by a long cord so that it can swing as a pendulum. If the block is displaced so that its center of gravity rises by 0.1m. what is the speed of bullet?

I can not upload but try to share with LateX. This uses law of conservation of momentum and law of conservation of energy. I have concern of use of KE = PE .

user 6232128

Sir John will be here soon to help you. @JohnRennie

123

$m_1\cdot v_1 + M\cdot0 = (m + M)v_2$

$v_1 = \Big(\frac{m + M}{m}\Big)\cdot v_2$

From law of conservation of energy.

Nihar Karve

@RewCie Unfortunately, I'm not interested in learning for learning's sake - I'm asking about its relevance to modern high energy physics

123

$\frac{1}{2}(m + M)v_2^2 = (m + M)gh$

$v_2 = \sqrt{2\cdot g\cdot h}$

$v_2 = 1.4 \frac{m}{s}$

$v_1 = 281.4 \frac{m}{s}$

My question is that here we took $KE = PE$ as the box swing as pendulum in curve path. As initially object moving along x-axis after collision it move curve both x and y-axis

For $v_2$ our equation only depend on force of gravity not any other force but box follow curve path it should have force against x-axis. Otherwise it won't stop along x-axis.

RewCie

4:55 AM

@NiharKarve If you aren't interested, it has no relievance to anything

If you are interested, even Ptolemy's Model > Kepler's

user 6232128

but, you need some kind of principle to collect interest

and it is that interest which helps you pay attention

RewCie

@user6232128 yes

user 6232128

conclusion: time is money

123

5:50 AM

Hi @JohnRennie .Pls have a look on my question. I try to explain in better way.

John Rennie

@123 after the collision the block+bullet is just a pendulum and it swings to and fro the way a pendulum swings.

The reason there is an x component of the deceleration is because the string is at an angle and the tension in the string has an x component.

123

Yes If it swing like a pendulum does it has $V_x$ and $V_y$??

John Rennie

Yes.

123

That's why i am confusing $V_x$ also under force. Why we don't take it in our calculation.

John Rennie

(assuming you mean $x$ is the horizontal direction and $y$ is the vertical direction)

If you consider a pendulum then it has maximum KE and minimum PE at the centre of its swing. Then it has zero KE and maximum PE at the ends of its swing. OK so far?

123

5:55 AM

@JohnRennie It means $F_x$ is balanced by $T$ tension in the string. Is that correct?

@JohnRennie Yes..

John Rennie

@123 Yes

123

You are superb @JohnRennie Sir. Give me your brain....... :-)

John Rennie

And the length of the string is constant, so the tension does no work. The only energy change is the KE changing to PE and vice versa.

123

@JohnRennie OooKay.... Thank you..

I have question about Atwood's Machine. For an ideal condition

(A) For String:

(1) Non-Stretchable (T is constant)

bolbteppa

@NiharKarve if this looks interesting to you then it is, but from a broad pov it does not seem that relevant except to understand the history that led to strings and qcd, e.g. the wiki says "But in the guise of string theory, S-matrix theory is still a popular approach to the problem of quantum gravity."

If you even skim that book you'll see how hard this stuff is

123

6:00 AM

(2) Massless String (Negligible mass compared to hanging masses)

makes Tension same on both sides.

(B) For Pulley:

(1) Massless pulley (Makes rotational inertia zero). It means no effect of rotational force on pulley all the forces make pulley rotation.

(2) Frictionless pulley (No heat produce)

@JohnRennie My all assumptions true and complete??? Also is there any other assumption which i don't know.

John Rennie

I can't think of anything else.

123

My analogies are correct about conditions??

John Rennie

Yes

123

Thank you @JohnRennie Sir. Hope one day i might be physicist like you guys.

How friction force is proportional to Normal force? as both are perpendicular forces.

What idea lead us to define friction as normal force?

bolbteppa

6:17 AM

It's clear that friction will be higher the larger the mass of an object is, the normal force depends on the mass, so it's natural to assume that the friction force is proportional to the normal force

Nihar Karve

@bolbteppa thanks for the link. I was asking since certain, um, characters seem to regard S-matrix theory in general as irrelevant, but I think the analytic structure of the S-matrix has some interesting features

123

@bolbteppa Thanks...

bolbteppa

21

A: What are bootstraps?

This approach most definitely does work, it just doesn't give the fundamental theory of the strong interactions, it gives string theory. String theory was originally defined by Venziano's bootstrap formula for the leading term in an S-matrix expansion, and the rest was worked out order by order t...

Vaguely I think some of the original motivation was things like Landau poles and more general singularities (even present classically) and trying to avoid them by working from first principles, it doesn't seem wrong or like a bad idea, I don't know how it can bypass the issues stemming from the point particle model of physics which I think it was trying to do in saying field theory was dead etc (this is all pre-strings) but it's also super general and I think trying to specialize it was too hard

Nihar Karve

looks like the nLab page is based heavily off this answer too

bolbteppa

This is a great article (start of it anyway) and is the kind of stuff you do down this path scholarpedia.org/article/Froissart_bound

Nihar Karve

6:33 AM

looks good, thanks a lot (from the contents of the Chew book, it looks similar)

Prateek Mourya

6:47 AM

I want to know a mind of question maker

Like how he/she thinks while writing a good question in physics or mathematics

Which source can i seek?

John Rennie

9:11 AM

Apologies for polluting the h Bar with chemistry jokes, but this just made me literally laugh out loud:

8

user 6232128

9:21 AM

I hope this joke wil not Bohr you.

2

Similar apologies apply.

ymmv

Captain Bohemian

2:08 PM

I read "The key feature of SNO (Sudbury Neutrino Observatory) was the use of heavy water, allowing simultaneous measurement of the relative rate of neutrino-deuteron reactions forming two protons (possible only for electron-neutrinos), and neutrino-deuteron reactions resulting in a proton and a neutron (possible for all neutrino flavours). The ratio would indicate if any transformation of solar electron-neutrinos to other types was taking place."
How can neutrino-deuteron reactions form two protons since there is only one unit positive charge in a deuteron ion?

Prateek Mourya

Is there any easy way to derive the result of leaning tower of liar

en.m.wikipedia.org/wiki/….

?

ACuriousMind

2:23 PM

@CaptainBohemian they've left out the leptons produced by the reactions - the first reaction produces an electron in addition to the two protons, the second a neutrino in addition to the proton and the neutron, so lepton number and charge are both conserved

RewCie

check this out

Just In: Volunteers face 'severe hangover', headache and pain after getting Pfizer vaccine shot Via - WION

I'm not gonna be taking this one.

I'll take Russian Sputnik V only.

@PrateekMourya I guess your profile pic has QM formula for rotation of axis of a quantum wavefunction.

Nah, it's classical mech's rotational formula

to get moment of inertia...

Nihar Karve

2:40 PM

@PrateekMourya what do you mean? The harmonic series divergence or the physical basis?

Captain Bohemian

@ACuriousMind so the second reaction is $\nu_\mu+D^-\rightarrow p+n+\nu_\mu$ while the first one is $\nu_e+D^-\rightarrow 2p+e^-$?

ACuriousMind

@CaptainBohemian yes

(but $v_\mu$ in the first one could also be any other neutrino)

RewCie

I played this with full volume and I discovered my whole PC is on fire

Captain Bohemian

@ACuriousMind ok, thank you

sorry, I found I type something wrong. the above $D^-$ should be $D^+$.

ACuriousMind

it was clear what you meant anyway, don't worry

RewCie

2:48 PM

I want to become Eminem of Comtupar Science Engynaring!

fqq

3:21 PM

@bolbteppa @NiharKarve not sure if you are interested, but note that there has been significant work on bootstrap after this answer was poster (and after the scholarpedia article)

I'm more interested in the statmech side so I am not sure how it fits with the rest of hep-th, see bootstrapcollaboration.com

bolbteppa

Yeah I'm not sure at all how that stuff fits into the general picture as a lot of it is cft right

fqq

3:42 PM

yes, a lot of it is CFT (and that's the part I care about) but they had the ambition to go well beyond that. I don't know what they managed to do on general QFT/SUSY etc

Emilio Pisanty

4:06 PM

6

Q: Is there a way to increase the speed of light itself?

When light enters materials it slows down due to its refractive index (due to absorbing and re-emission of photons). But, is there a way to increase the speed of light itself? Can there be some material which would increase rather than decrease the speed of light?

↑ could use attention from 20k'ers

@JohnRennie @ZeroTheHero

John Rennie

@EmilioPisanty thanks, I've voted to delete the rogue answer. I wonder what motivates some people.

user 6232128

4:23 PM

@JohnRennie Wow, your joke is beating mine 4 stars to 0!

John Rennie

@user6232128 :-)

user 6232128

Shashank V M

@JohnRennie, you marked my question as 'Homework and exercises'. The fact is that I am not a student of Physics. Also the question I asked was my own.

user 6232128

Link please

ZeroTheHero

@EmilioPisanty Looks like @JohnRennie beat me to it.

Shashank V M

4:31 PM

0

Q: Gravitational mass vs inertial mass: A problem in classical mechanics

There are 2 boxes wooden boxes of the same size and shape. One is empty and another is filled with a heavy material, like Gold. These boxes are placed on an open car of a roller coaster ride and fastened to the car. When the car goes over the top of the roller coaster and starts its descent downw...

homework-and-exercises newtonian-mechanics mass

user 6232128

Thanks :-)

ACuriousMind

@ShashankVM Whether or not you are actually a student or it is actual homework is irrelevant for the homework-and-exercises close reason. Please see physics.meta.stackexchange.com/q/714/50583 and physics.meta.stackexchange.com/q/6093/50583 (which are also linked in the close reason)

Nihar Karve

@fqq I did a double-take there because I thought for a sec it said the 'Simmons' foundation as in David Simmons-Duffin, whose lecture notes are, notwithstanding, sublime (although not focused on the stat-mech side)

And the other Simons Collabs look really interesting too

Shashank V M

@ACuriousMind I tagged my question with Newtonian mechanics. If I tagged 'Quantum mechanics' or 'Special relativity', can I expect different answers?

ACuriousMind

@ShashankVM ...your question is not about quantum mechanics, why would you tag it with that? We don't close questions for having or not having specific tags.

Shashank V M

4:44 PM

@ACuriousMind It's very much connected to Quantum mechanics. Friction arises from quantum interactions

@ACuriousMind, I feel I forced other users to predict the wrong outcome because I tagged 'Classical mechanics', since classical mechanics does not describe the real world. Perhaps if I tagged all the physical theories that are out there, they might be able to calculate the right outcome in the real world, without conducting an experiment.

ACuriousMind

@ShashankVM Tags are a way to describe what the question is about, not a way to invite specific kinds of answers. If you're interested in some sort of quantum mechanics explanation, say so in the question. As it stands, your question is about the behaviour of ordinary objects usually well-approximated by classical mechanics, so the tags are correct as they are now.

Shashank V M

@ACuriousMind, I never found a useful theory for friction in classical mechanics.

ACuriousMind

...but your question isn't about a "theory of friction"

I agree that if you want a "deep" explanation of what friction is you probably need QM, but what does that have to do with your question?

user 6232128

What about rubbing two sticks together to make fire?

ACuriousMind

@user6232128 what about it?

user 6232128

4:58 PM

How is that not useful?

ACuriousMind

how is that a theory?

user 6232128

Someone had to think about doing it first.

ACuriousMind

you don't need an understanding of what friction is to discover that it produces heat

I'm not sure what you're going for here

Prateek Mourya

@RewCie its the equation representing forces in rotating frames

fqq

@NiharKarve yeah it's nice to have a lot of funding for basic research, which is getting harder and harder to come by. I'm not a fan of privately funded science in general but at least in this case it looks like the PIs actually have scientific freedom

ACuriousMind

5:00 PM

the practical knowledge that friction produces heat certainly predates any concept of modern physics or physical theories

Prateek Mourya

@NiharKarve i mean that the overlapp forms harmonic series

Using elementary physics or mathematics

John Rennie

Classically friction is described by two processes:
1. adhesion
2. viscoelastic damping

user 6232128

"3." heated discussion

John Rennie

You could argue that you need QM for a fundamental understanding of adhesion, but typically we construct an effective theory where we just take the strength of the adhesion as an input parameter.

user 6232128

:-)

John Rennie

5:06 PM

Discussions often get heated when there is friction between the participants :-)

user 6232128

Were you on any debating teams, sir?

John Rennie

No. The problem with debates is that the arguments are always based on assumptions that have no obvious validity. You end up with the two sides disagreeing because they have made different assumptions, and it all becomes rather futile.

At least with science you can do the experiment and test your assumptions.

user 6232128

Indeed.

John Rennie

I went to the Cambridge debating society (the Cambridge Union) a couple of times but it was just people trying to show off how clever they were and I didn't find that entertaining.

user 6232128

Yeah, it's more about how skilled you are with language.

Mathematics takes us into the region of absolute necessity, to which not only the actual word, but every possible word, must conform.
-Bertrand Russell

Emilio Pisanty

5:26 PM

@JohnRennie @ZeroTheHero the rogue answer? ;-P

John Rennie

Well, the most recent rogue answer anyway.

Prateek Mourya

Sir how is the weather in chester

Currently in india all people are celebrating diwali

Lots of noise due to fireworks

I mean week of diwali

John Rennie

@PrateekMourya it's cold - about 12°C today - but it has been a nice sunny day. I went out for a long cycle ride, which was especially enjoyable as the lockdown has drastically reduced traffic.

user 6232128

Did "the region of absolute certainty" turn out to be unattainable due to Godel @JohnRennie?

John Rennie

What is the region of absolute certainty ?

user 6232128

5:31 PM

13 mins ago, by user 6232128

Mathematics takes us into the region of absolute necessity, to which not only the actual word, but every possible word, must conform.
-Bertrand Russell

John Rennie

@PrateekMourya it's very quiet in the UK as we are in the middle of a one month lockdown to try and control covid.

@user6232128 I don't know to be honest. I think it was Hilbert's programme of axiomatisation that was the main casualty of Gödel's theorems.

Nihar Karve

I don't mean to pry or anything, but why isn't Luboš active here anymore?

ACuriousMind

@user6232128 What is that even supposed to mean?

John Rennie

@NiharKarve I don't think he works as a physicist any more.

user 6232128

@ACuriousMind That is what I'm asking?

ACuriousMind

5:36 PM

@user6232128 It's likely a misquote - there's plenty of places quoting this with "world" instead of "word"

Nihar Karve

@JohnRennie well his blog currently seems to be characteristically rife with political posts, but there is the odd physics one too

user 6232128

@ACuriousMind does it make sense either way?

ACuriousMind

@user6232128 i.e. Russell is talking about how "all possible worlds/objects" in math is usually tightly constrained by the formal definition of "world" or "object" in contrast to the colloquial use of "all possible" meaning anything imaginable

I'm not sure why one would choose this strange phrase as particularly quotable, but that's what I get from it

@NiharKarve if there isn't anything in the profile and there aren't any "interesting" posts by the user on Physics Meta around the time of their last activity there's usually no publicly known reason

Nihar Karve

That's unfortunate, I really did like his answers

ACuriousMind

Users come and go, life changes and priorities shift

RewCie

5:41 PM

:'-(

Nihar Karve

Unexpected life advice from ACM :P

ACuriousMind

e.g. there's plenty of users where you can see their activity dropping sharply after graduating

Prateek Mourya

Till last year i used to burn crackers

But now i understand the pain of jee aspirant

Nobody can stop Indians from burning crackers

RewCie

It's already banned in my state :(

user 6232128

by modi?

RewCie

5:45 PM

state government

Why would modi ban firecrackers? XD Lmao

user 6232128

because he wants all the attention

like trumpet

et al

RewCie

lmao, I like him :)

Anyone who'd tell the CGI method will get a star!

user 6232128

@ACuriousMind isn't it obviously impossible to put a restriction on "anything imaginable" ?

123

Hi guys.. Yo..

user 6232128

hi

ACuriousMind

5:57 PM

@user6232128 yes, that's why the mathematical and the colloquial meaning of "all possible" differ :P

user 6232128

Ok, thanks.

RewCie

What I didn't know earlier that Russel was a philosopher.

I liked his Philosophy books! :-)

If I were not a CSE/Physicist, I'd have been a philosopher.

user 6232128

He won the Nobel prize for literature.

RewCie

Yes...

His papers in Higher Order Logic were incredible!

ACuriousMind

that's not what he won the prize for

RewCie

6:01 PM

Predicate Logic... HOL wasn't started during that time.

He won in Literature, I guess

Also, knows for Russel's Paradox! $A = [X : X \not \in X]\iff X \in X \iff X \not \in X$

ZeroTheHero

@EmilioPisanty the most recent one yes. Some of them are pretty bad

RewCie

What are bad?

user 6232128

Answers here

RewCie

This one?

> Possibly in the slightly negative energy region of a Casimir mirror pair However, the effect is really small

ZeroTheHero

the worse ones are already deleted... (thankfully)

RewCie

6:12 PM

> May be possible, if the interaction of light happens with strong electromagnetic force Just think upon this

what were they?

ACuriousMind

get 10k rep and you can read them :P

RewCie

Lmao!

user 6232128

lol

:P

RewCie

Ama philosopher, so, wrong answer matters me as much as right ones :P

user 6232128

"Ama"?

Ima or i'm, "Ama" looks too foreign :P

RewCie

6:20 PM

Indian word... Shashi Throor started it on BBC

user 6232128

I see.

may I ask why there are no pawns on your avatar? @ZeroTheHero

> My avatar is a pair of mutually orthogonal Latin squares

Prateek Mourya

6:36 PM

In another approach they took here why did they say that three forces must be coplanar enough to say block is in both rotational and sliding equilibrium?

ZeroTheHero

6:54 PM

@user6232128 well... firstly I took the picture I found. Next, it turns out that most 6x6 Latin squares are discussed in the context of complete sets of Latin squares for which there are no solutions (en.wikipedia.org/wiki/…)

DanielSank

8:03 PM

Hi, everybody.

ZeroTheHero

allo...

Emilio Pisanty

8:19 PM

@DanielSank Hi Dan :-)

@ZeroTheHero honestly, I only see one answer there that shouldn't be deleted

ZeroTheHero

@EmilioPisanty I've already voted to delete one of the remaining 2 rogue answers.

(one is clearly non-sensical)

Stupid question inc

10:45 PM

In the present treatment, the rotation of the Earth is neglected and we regard the Earth as an inertial frame of reference. A more accurate treatment which takes the Earth’s rotation into account is given in Chapter 17. When the system S is the Earth (or some other celestial body) it is convenient to introduce the notion of the local vertical direction. The unit vector k, which has the opposite direction to g, is called the vertically upwards unit vector relative to the Earth.

In terms of k, the force exerted by the Earth on a particle of mass M is given by F =−Mgk, where the gravitational acceleration g is the magnitude of the gravitational acceleration vector g. Both g and k are functions of position on the Earth.

Quite confused here

Wait so F=-Mgk why it there minus and k?

Isn't Mg enough ?

also K and g is vector so this will be dot product ?

giving scalar answer

I remember force is vector quantity

Charlie

11:11 PM

@Stupidquestioninc If $\hat k$ is the unit vector in the $z$ direction then $g$ is being given as a constant, not a vector. The negative sign is there because they are taking the "positive" direction to be away from the Earth, hence the gravitational force vector (which points towards the Earth) is pointing in the negative direction.

So the equation probably reads $\vec F=-mg\hat k$.

Stupid question inc

11:24 PM

ah thanks

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