00:00 - 19:0019:00 - 00:00

12:01 AM
I have to say, this is quite annoying
These things might not even be smooth

@dmckee Thanks for bringing him up actually. His merit definitely goes unnoticed more than it should.

it appears things are smooth
@yuggib Under what assumptions do parabolic equations evolve smooth initial data smoothly (for small time)?
I can't understand the jargon in the PDE literature

3 hours later…
2:44 AM
@ACuriousMind Ćech cohomology is pretty amazing
@dmckee (removed)
wonder what you said

I said "latex boxes". Because I meant to type that in the search bar.

latex boxes?

Now you have to decide if I am typesetting a document or into something really kinky.

is that some kink thing
:)

3:17 AM
Probably just $\box$
Btw
For cells I always think either as a biologist or as a physicist (if the cell content does not matter). Also I use bacterial cells more

3:37 AM
What's up with people using $\phi$ for $\emptyset$

3:57 AM
@dmckee ಠ_ಠ

1 hour later…
4:59 AM
@dmckee it's not the usual way of doing things outside the kindergarten, but physics and maths has traditionally attracted people whose emotional maturity is questionable. I should know - I'm one of them :-)
@dmckee the guy from the Kinks? :-)

5:34 AM
Hi CodeRatchet :-)

Hey John
How are you?

@CodeRatchet Sleepy, it's 06:35 in the UK :-)

ha ha, it's 3:35pm here in Melbourne
in regards to what we just discussed on the thread, is it safe for me to post such question here?

Yes, anything goes in the chat room.

ok
Let me just get the question
Good Morning / Afternoon,

I'm looking at a question which has been given to me and I'm trying to find out what the answer / formula would be to it.

The question is as follows:

A student has been asked to move a box of mass 15 kg onto a shelf that is at a height 2 m above the ground level and 10.85 m from where the box was originally kept. The student is moving with the box at a steady speed of 0.123 m/s. As the student approaches the shelf, he notices another student is opening the door adjacent to the shelf area. i. If the student is able to apply sufficient force to stop in 1.2 s, will

5:36 AM
If I understand the question correctly you need to find out how far the box travels in the 1.2 seconds it takes to stop. Yes?

That's correct
As mentioned I've emailed my tutor however this is rattling my brain

These types of problems are described by a set of quations often called the SUVAT equations because they link distance s, initial velocity u, final velocity v, acceleration a and time t.
If you Google suvat equations you'll find lots of articles about them.
In this case we have to link the distance travelled by the box to the acceleration (actually deceleration) of the box.
And the equation we need is:
s = ut + 0.5at^2
where u is the initial velocity 0.123m/s and a is the deceleration. a will be negative because acceleration is positive and deceleration is negative. Does this make sense so far?

@JohnRennie Ok, I'll be reading up on suvat equations soon.

Thanks for the equation, can you tell me how come up with that equation
It does yes

The SUVAT equations are derived from Newton's laws of motion using calculus, but until you learn calculus you just have to memorise them.
But there are only three of them that matter and if you do physics they'll quickly become routine.
Anyhow, we need to know the acceleration a so we can use it in the equation for the distance, and the way to find a is using another SUVAT equation:

v = u + at
where v is the initial velocity, 0.123m/s, u is the final velocity, zero, and t is the time.

5:45 AM
We're told the student takes 1.2 secs to stop the box from an initial velocity v = 0.123m/s to a final velocity u = 0 so we get:

0 = 0.123 + 1.2 a
or:

a = -0.123/1.2
So now we can use our equation:

s = ut + 0.5at^2

to get:

s = 0.123*1.2 + 0.5*(-0.123/1.2)*(1.2)^2

Ok so will the student avoid a collision if the opening door is 0.8 m ahead of the box?

Well from the above I get s = 0.0738m i.e. 7.38cm

avoids collision by 7.38cm
Thank you John

7.38cm is the distance the box travels while the student is stopping it, and obviously that's a lot smaller than the 0.8m or 80cm distance to the door.

So it's safe to say he / she did avoid collision then

5:51 AM
Even without doing the calculation you can get a rough idea. If the box is travelling at 0.123m/s then in 1.2 secs it can't travel farther than 0.123*1.2 = 0.1476m.

I see
Thanks for your help John, I will dive into suvat equations soon. Also thanks for the explanation and break down really do appreciate it.

This sort of calculation will rapidly become routine if you continue to do physics. It may seem a bit hard now, but it won't after you've done a few of them :-)

That's true. Thanks again

Hello everyone.

@CuriousOne Morning

5:54 AM
I've been watching the diphoton disappearance in the news. Sad...

What are you doing up at 7 a.m. on a Saturday? I have to work - what's your excuse? :-)

I have to work, too, but don't know how to plot a crappy set of data for a client in such a way as to make it look less crappy.

log-log scales are the usual strategy
Anything gives a straight line on a log-log plot :-)
2

Doesn't change the fact that the product doesn't measure up to the competition. :-(
It's not even bad... it's just not the "Italian Stallion" that the owner wishes it to be.
Anyway... that's not my question. LHC is on my mind...

@CuriousOne sounds like a Ray Charles song

5:59 AM
Now that the diphoton is gone, is there any possibility for non-standard model physics left in the currently funded runs or do we have to wait and hope that the upgrade post 2018 (is it?) will return something?

It's not looking good for BSM evidence
The only hope is to do detailed measurements and look for differences between scattering probabilities and the SM predictions.
It doesn't seem likely that anything new and startling will be seen now

I was never on the hopeful side... still disappointing.

6:26 AM
Might be something interesting in the exotics (tetra- and penta-quark states and the like) spectrum, but that's not going to set the world on fire even if it leads somewhere eventually.

To me that's still kind of SM physics, even if the details are not understood... probably because I don't understand the details... :-)
No chance in hell for SUSY anymore, right?

@CuriousOne Little chance of direct evidence, but all hope isn't quite lost.

That's the nature of hope... it's never completely lost...

4 hours later…
10:12 AM
@0celo7 for linear equations, there may be some general theory; for nonlinear ones I doubt it
usually even local well-posedness is proved case-by-case in nonlinear PDEs
there are techniques, but not a general resulrt

2 hours later…
12:18 PM
Attempt to modify the double slit experiment in order to have a cylinderical screen act as the measurement device, and the interference pattern being depend on the state prepared
If successful, one can use this in a high school or uni setting to demonstrate the electron's spin (and possibly some mathematical properties of spinors)
I have no idea how to direct the electron in this cylindrical fashion after it entered the tube, but my ideas are as follows (based on discussion abotu spin with h barers some weeks ago)

1. Have one electron always prepared in a spin up state. This then act as the 0 baseline of the spin. (i.e. we call this orientation of the spin vector "zero" and we define its phase here to be zero)
2. The spin state prepared in another electron is controlled by a dial, which the students can freely adjust it in the set up to rotate the spin vector prepared in that electron
Actually, .1 may not be that straightforward. While it is true that one can prepare an electron in a certain state, this is only up to some overall phase factor, which is unphysical until it started interfering with another electron (i.e. $i\lvert 1\rangle$ can give different interference pattern with antoher electron with some state $\lvert s\rangle$ compared to $\lvert 1\rangle$)

12:59 PM
(NB There's some conceptual error in the above post, this will be fixed after the bath)

@JohnRennie Ok, but the new LHC will work at least years long. What if the new particles need a lot of more data to improve their sigmas?

1:32 PM
@yuggib It's "strictly parabolic", does that mean anything?
And yeah, it's horribly nonlinear

1:47 PM
0

The absence of black in the form of a black light does not mean that it does not exist, or it cannot be done. If it does not exist, then make one. I insist, make one for me. My views are simple, we can use the black light to create holograms that does not need ceilings to project, and make new c...

There is no way one can make a "darkness emitting source" (i.e. black colored light). The closest thing to that is the Coherent perfect absorber
https://en.wikipedia.org/wiki/Coherent_perfect_absorber

@Floris: the question seemd silly to me. There was lots of info we didn't need e.g. the weight of the box. Maybe there's a subtlety I've missed, but it seemed pretty straightforward.

average PSE user:

2:40 PM
@JohnRennie clarity vs obfuscation: teaching vs testing. I don't like this kind of question at the elementary level.

@JohnRennie trivial algebra time
What is the $0$th de Rham group of $\Bbb N$

What's $\Bbb N$? The natural numbers?

yeah
Viewed as a manifold
seems like it should be $\Bbb R^\infty$ or something weird

what image

2:46 PM
@0celo7 this

appears just fine for me
are you on a work computer?

@ACuriousMind @Qmechanic I think you should have a look at this. If the question in question is still unclear, I'd like to see an explanation of why.

Wow, Googling de Rham cohomology group leads you rapidly into a maze of twisty passages

like what

2

I would like to enquire about this post I have made on StackExchange Physics- Conformal group in 2D being a subgroup of Diff x Weyl - Polchinksi's 'String Theory' This post was closed very quickly after being posted, for being unclear, though one of those who voted to close it left an answer in ...

3:45 PM
@0celo7 lol sure its not a self portrait? :P

@vzn I heartily agree with the question

> A presheaf is simply a contravariant functor from $\mathsf{Open}(X)$ to $\mathsf{Ab}$, and a presheaf morphism is a natural transformation of functors.
@Slereah seems obvious now
@vzn what?
@Sanya don't know what I ever did to you

@0celo7 actually, nothing at all - I am just a mean person by nature and I apologise if you are taking me seriously (if I'd prefer you wouldn't in chat)

I think people who are admittedly mean by nature should remove themselves from society. But that's just me.
2

4:19 PM
@0celo7 or maybe its einsteins alter ego :P

[Scifi] The mathematics of Newtonian Temporal Mechanics
(Too much algebra with rational functions, gonna continue tmr)

4:37 PM
I went down the beach today. I swam in the sea.
2
That picture wasn't from today, and it was sunnier than that. But it gives you the idea.

Good for you?

(Newtonian means we temporily ignore the bottleneck we identified earlier, including the explanation on how to change the topology of space$\textit{time}$)

Will see how it goes...

tmr?
are you speaking line vzn now

tmr=tomorrow abbreivated

PS I hate algebra expansion involving fractions

@0celo7 : what's nice is that I have an 8-year-old son. I can play sand castles. He swam too. The wife just sat on a towel pulling a face.

4:45 PM
There's a Duffield Jr.?

Yep.

I guess it's too early to ask if he's interested in physics

He is. Without being pushed by me. Though he's too young to think of it as physics. It's just science for him.

@JohnRennie New 2 Chainz mixtape!

I have a few "toys" like an electric gyroscope and a super-strong magnet and a telescope etc. All good fun.

4:52 PM
Cool

Re DavidZ's post above, I'm not a fan of Mtheory, but I think this question was closed in error. I voted to reopen it.
Does anybody do any drawing? I need to produce some illustrations, and I'm looking for recommendations.
Uhnnn, I have to go I'm afraid. The wife is calling. Sorry, bye.

@Nk
damn, why are delete and enter so close together ...
@JohnDuffield depends a bit on the type of illustations you want to produce, but besides Inkscape and GIMP I've actually often found the publisher included in older versions of MS Office to be quite a useful tool

5:13 PM
Hi guys. I computed the cross section for a 4-fermions scattering d d > d d (d is the d quark) with the exchanging of a spin 2 particles. The cross-section I computed is twice the cross-section simulated with MadGraph. I would love me to check my analytic formula. Do you know a place where to find the computation of a cross section with an exchanging of a spin2-particle?
The vertex is something like $\partial_\nu \bar{d} Gamma[mu] P_R d V_{\mu\nu}$ where P_R is the projector on the right-component of the dquark
*i would like to check

5:28 PM
exchange of a spin 2 particle?
Are you doing gravitational interaction of quarks
One of Feynman's student did a thesis on graviton exchange between electrons, if that helps

@Slereah what's the difference between a weakly and strongly parabolic PDE

Not a clue

Nono.. I'm working with Effective field theories . In order to do simulations with identical 4-fermions vertices, I have to split them using an equivalent ULTRAVIOLET theory... Then, I have to use a spin-2 particles for my purposes
@Slereah can you give me more references?

Lemme see
That's the one
beware though
It's written with a typewriter

@CuriousOne "when you have a hammer everything looks like a nail". have long thought LHC/ particle accelerators while very impressive/ unmatched in many ways are "too many egg$in one basket" & alternative approaches deserve funding esp computational simulations of matter/ energy, & maybe the community will slowly/ finally come to that widespread realization after total desperation (ala Churchillian philosophy quoted by Rennie) 5:43 PM I will check there, thank you @Slereah Also the standard theory on spin 2 QFT is Pauli Fierz, if you wanna look into it I don't want to know about the theory (for the moment), I need just the expression of the cross section Well the vertex of the graviton is known, dunno if that generalizes to all spin 2 particles General relativity is really a sausage fest I can only think of one lady in all the papers and books I've ever read Yvonne 5:55 PM the very same My prof knows her Her book is pretty hard core Well GR Cauchy problem is no easy task These Ricci flow calculations are getting to be lengthy Computing the time derivative of the Riemann curvature is not fun that's what computers are for @vzn: There are no alternative approaches to the spectrum of the vacuum. You can't get this information any other way than with accelerators. As for cost... LHC is very cheap. The US can spend$13 billion before breakfast on total nonsense without even noticing when the politics lines up. HEP is just not one of those things for which the politics lines up well.

5:59 PM
Just because it's relatively cheap does not excuse it
It's still a complete waste of money

What is?

HEP
We are talking about the LHC, right?

Is it?

how have we benefitted?

You can make that argument about anything in science. We might as well still be living in the caves.
Yes, I have greatly benefited.

6:00 PM
No, there is science with clear goals and applicability

0

Since I am only caught up in text book type problems and calculations, I rarely use a computer to help out with a computation. The one exception is to use Mathematica for mundane things that just help get to the answer (Integration, plotting graphs, and so on). But, I read that real-world probl...

The LHC has no near-term applicability
Unless I'm missing something

Science only has one goal: to explain nature. What you are talking about is engineering.

Then I guess science is a waste of money

The LHC isn't all standard model and Higgs boson

6:02 PM
So why are you here?

There's also a lot of nuclear physics done there

@CuriousOne I ask myself that all the time

Nuclear physics? Not really.
Sounds more like someone is having a crisis. Do you need an intervention?

No, I need physicists to stop spending tax money on their toy projects

@Qn
@Qmechanic yep

6:03 PM
Why do you need that?

To increase military spending, of course.

Are you a soldier?

No

So then you don't need better guns.

Sure I do
Gotta prepare for the UN invasion

6:05 PM
For what? Low self esteem?

Nah

Guns don't help with that.
LHC is a pretty good job creation program, by the way. Tons of metal fabrication.

lol

Very little of that has been outsourced to China. :-)

Keynesian economics at its finest

6:07 PM
Whatever works.

Pump tax money into the economy and call it jobs

Military spending is 100% Keynesian, by the way. That's why I am all for it.
The US couldn't do without. Too many unemployed...

Well why not do it on something more pleasant

6:08 PM
Pyramids are stupid.

Tell that to Ramses

Pyramids are African @Slereah

The only true form is a cube.

Tell that to the Aztecs

@Slereah ok, Mexican

6:09 PM
They were all losers.

@CuriousOne Low energy, too

That will create jobs

Everybody who has ever built pyramids is gone.

Yup

the jews are still there

6:10 PM
You mean the lizards

It's a good start to eternal looserdom... once you build a pyramid, you are finished.

The Jews didn't build the pyramids.

Did they build the LHC, though

That's much closer to the truth. :-)
I wouldn't mind a Titan colonization program.

6:11 PM
Maybe start a bit smaller

Don't know why everybody wants to go to Mars...

Moon bases
Newt Gingrich wants moon bases
Vote for him

Some are even hoping to find a pyramid there...

Are the martians losers?

Which would explain why the Martians are such losers, though.
Of course, they lost all their water and atmosphere.
Must have been the pyramid construction programs.

6:13 PM
This guy gets it

0

The Wikipedia article on wormholes claims: Researchers have some observational evidence for wormholes, and the equations of the theory of general relativity have valid solutions that contain wormholes. But there doesn't seem to be any details on the 'observational evidence' in the artic...

Pyramid schemes

I tried to answer "we do not" but apparently that's too short an answer

Ay!

6:14 PM
Shoot!

HE HAS A GUN

Virtual guns don't scare me.

If he had a computer virus...

Ok so is soft questions on-topic here ?

6:15 PM
Sure
As long as it's not dumb

Soft tofu? Fine with me.

what is a soft question for you exactly?

The common trap of soft questions is that there aren't that many of them

6:18 PM
i cant find a good one but this one has 8 upvotes but closed :(

debatable ... I'd bet that it will get closed as opinion-based here

1

Does one have to be an amazing problem solver to become a mathematician or its the passion and dedication? Can every mathematician solve the IMO problems? What is required to solve IMO problems.

but it is certainly a most fitting subject for the chat here - though you might still want to give it a short to see whether it goes through
*give it a short
*shot
argh -.-

A very special brain. If you have to ask, you most likely don't have it?

will this one be closed here ?

6:20 PM
Yes.

All you need is to find the Mathematician Stone
And lay your hands on it

Anyone here know about Uhlenbeck's trick?

Never heard of it. Tell me!

@ritwiksinha the problem is that "the physicist" does not exist and that there are no obvious criteria to predict what a person can achieve

Yes, there are.

6:21 PM
@CuriousOne You put Ricci flow on a pullback bundle and the induced connected can be used to simplify equations

http://math.stackexchange.com/questions/1883563/is-being-good-at-mathematic-contests-necessary-to-pursue-a-career-in-mathematics

http://math.stackexchange.com/questions/1637562/can-a-mathematical-theorem-be-proved-in-infinite-ways

@CuriousOne you are a wizard :o

But they keep saying "without loss of generality, assume the vector fields are constant in time"

@Sanya i dont want to ask that question, i am just curious

And I don't see why that's possible

6:23 PM
@ritwiksinha: These kinds of questions are always nonsense. Either you want to do something and then you do it, or you don't.

@ritwiksinha the second one is fine even though I'm not sure what the counterpart in physics would be, the first one is in my opinion off topic here ... In general, simple career advice and opion based stuff is frowned upon; you still need to consider however that in the end it is never one single person that closes a thread so opinions might differ

Ah
Stuff cancels
That's a crappy way of putting it though
Maybe there's a better way of seeing it.

@CuriousOne i was just curious because there are many such things on MSE but not found single one here

The question is just as nonsensical there as it is here.

Ah, I bet any time varying vector field is a time constant one multiplied by a function of time

6:29 PM
What somebody wants to do is a personal decision.

By tensoriality, it works

@CuriousOne maybe those questions are nonsensical but they are generally well received there

No

@ritwiksinha: More importantly, many of these questions seem to be coming from Asians, particularly Indians. That's more an expression of culture than an honest interest in science/math.

No, that's wrong

6:31 PM
@CuriousOne I can disagree there

@ritwiksinha: That's your right. The data seems to suggest otherwise. A lot of these questions seem to come from students who are being pushed by parents and environment to do things they are not really interested in and that they are not really gifted for.

Indians have a fascination with Olympiad stuff
It's really weird

@CuriousOne Maybe because people here don't consider many streams other than medical or engineering

I'd never heard about it until mentioned by an Indian here

@ritwiksinha: One should only consider stuff that one likes to do. I was constantly being pushed into medicine by my environment. If I had done that, I would have killed many people.

6:34 PM
@CuriousOne I was wondering something
It seems like Indians are super zealous about math and physics education
But you rarely hear about Indian mathematicians or physicists

@0celo7: Seems like it.

Where is the disconnect?

@0celo7 ohh, you don't know how much those streams are popular , every second person would ask you what you want to pursue medi or engi

The Japanese and Chinese are pretty prominent

@0celo7: They do have great both, though, but physics requires state funding. If your country doesn't have it, you have to move.

6:36 PM
@CuriousOne actually i have talked to many Phd students, they get funding

@ritwiksinha: Physics needs facilities. India doesn't have facilities.

@CuriousOne Of course not and everyone can't afford aboard education

@ritwiksinha: That's a real problem, but you can't do physics without machines. That's just a fact.

@CuriousOne Don't know if it is only prominent here or not but teachers here will teach you how to do it not why it is done that way, especially in physics and chemistry.

@ritwiksinha: Physics is a game for rich nations.
@ritwiksinha: Done what way?

6:40 PM
@CuriousOne i mean the derivation or procedure behind a result

@ritwiksinha: There is a well known way how to teach and do physics, and either your teachers know it or don't. If they don't, you need to study it somewhere else.

@CuriousOne For eg : i have been told that we need to check dimensional homogeneity of argument inside trig function to know that of the whole equation, but did not told why to do it

Oh, I think this was trivial :/

there's plenty of well known indian physicists

@ritwiksinha: You mean how people came up with the derivations? That's intuition. One can't teach that... either you have it or you don't.

6:42 PM
Even just in GR

@CuriousOne how?
@Slereah proof?

Chandrasekar and Raychaudhuri, for one

@CuriousOne re "no alternative approaches" [citation needed] ... "absence of evidence ≠ evidence of absence" ... am not as hard on LHC as 0celo7 (lol) but do detect some massive groupthink at work

@Slereah i haven't heard many

Chandra worked in America
I think.
And they're old-school

6:43 PM
How many new school GR guys do you know

Sanchez
Visser
...

isn't chandrasekar died in 1995 ?

Both are people I told you about
You copycat

Ricci flow is pretty nice
On page 30 of this book and I've only died a few times

Ricci flow won't increase the GDP

6:50 PM
So?

No point in doing it
you should be doing condensed matter physics
That's where the GDP is at

That's what my job entails @Slereah
Working on graphene right now

I had to read some graphene papers to prepare for the PhD I never did

Why

Apparently there are supersymmetric methods you can use for graphene

6:55 PM
What do Kahler manifolds have to do with GDP

Which involve the Atiyah Singer theorem

Huh