The h Bar

12:00 AM

@G.Bergeron I've had a very eventful evening :)

12:51 AM

Are there applications of characteristic classes other than the first Chern class in physics?

(I mean physical results like Dirac's proof that there are no magnetic monopoles, which uses the first Chern class, I don't mean something like ''it's a useful tool in gauge theory'').

heather

hello =)

Meow

Hello

David Z

@heather hey, welcome to modship

5

heather

@DavidZ thanks.

Balarka Sen

jesus god too many Be Nice police members

David Z

1:06 AM

Hey now that wasn't very nice

vzn

dailymail.co.uk/news/article-5613645/…

Dawood ibn Kareem

1:22 AM

@heather Wow, are you a mod now?

(I'm always a bit slow to the party)

heather

@DawoodibnKareem yep, on the new Quantum Computing site.

and you're not really slow; it just got announced today.

David Z

@vzn could you explain how that's relevant to the room?

Kenshin

@vzn, nothing new there, of course one shouldn't run fro the police

Dawood ibn Kareem

@heather Well congratulations.

I'm going to have to start hanging out there.

heather

@DawoodibnKareem you'd be very much welcome. The more users on the site the better!

Dawood ibn Kareem

1:40 AM

Does it matter that I know nothing about quantum computing?

heather

then you can ask questions =)

nitsua60

1:55 AM

@heather congrats! And good luck. Diamond on a new site is a particularly engaging, challenging, and busy project.

heather

@nitsua60 thank you =)

nitsua60

Lots of reading and rereading to do, and just make sure to leave yourself lots of notes in annotations, mod-room, ask questions of mods and staff.

vzn

2:23 AM

with great power comes great responsibility™

← runs o_O

nitsua60

@vzn running because of the mis-quote?

=)

Cows

@EmilioPisanty oh I was able to power through that hump. hehe taking tiny steps :P

Kenshin

Breaking news, US has struck syria

Cows

@Kenshin yeah watching it live on cnn

Cows

2:54 AM

ok so cool

$(\omega - \frac{p^2}{2m}) G_0 (\omega, p ) = 1$

then the inverse plus a prescription covers the signularity

cool

that is . . . when one solves for $G_0$

Let me now look at the interacting propagator

. . . . . then look at the electron after that

Cows

3:27 AM

Let me do some integrals ..... contour integrals really quickly. presumably this should be fairly straight forward

Cows

3:42 AM

ok nice

ok quick break. Would put on some hip hop and dance

then return and slap the electron propagator

Some Cardi B for motivation

Cardi B, Bad Bunny & J Balvin - I Like It [Official Audio]

►

Electrons are next

Let me dance for a bit

ok electrons I'm coming for you

eulB

livestream it

Cows

hehe :P

hmm

ok fine

I might live stream the dance

hehe

eulB

yes do it

Cows

3:57 AM

lolz probably not a good idea. I'd stream some physics soon though :p

I'm going to go hardcore on electrons getting hyped

eulB

lol

kinky

Cows

hehe

John Rennie

5:05 AM

@dmckee Done!

0

Q: Does light really travel more slowly near a massive body?

It is a routine problem for beginners in general relativity to calculate the coordinate velocity of light for the Schwarzschild metric. Starting from the metric: $$ ds^2 = -\left(1-\frac{r_s}{r}\right)c^2dt^2 + \frac{dr^2}{1-\frac{r_s}{r}} + d\Omega^2 $$ We use the fact that light travels on a ...

general-relativity

Yuzuriha Inori

5:23 AM

Hey, anybody can give me a small hand with a doubt? The question is as follows : Assuming all kinds of energy in a system (thermal, radiation, kinetic, potential whatever), and the momentum of each and every constituent particles (radiation, atoms, everything), then does conservation of total energy imply conservation of total momentum?

John, I can only see you. Could you please help a bit? I am literally stuck...

David Z

No, it doesn't. For example, think about what happens to momentum and energy if you rotate the entire system.

John Rennie

@DavidZ I assume Yuzuriha means an isolated system, so applying an external torque is cheating :-)

Yuzuriha Inori

xD

Yes, obviously isolated.

David Z

But in that case the conservation of momentum comes from the isolation, not from the conservation of energy

Yuzuriha Inori

Then? Do one imply the other? (Btw, we are talking in Galilean frames, so no special relativity)

John Rennie

5:30 AM

In an isolated system total energy and total momentum are obviously conserved, but does one conservation law imply the other. I think so ...

Yuzuriha Inori

@DavidZ Yes that's true. But if momentum is conserved, will that imply that energy will be conserved?

David Z

Well now you're asking a different question than what you were asking before

Yuzuriha Inori

I just rotated the sequence of implication. Will that actually matter?

David Z

Yes, it does. It's possible that one implies the other but the other does not imply the one.

John Rennie

Ultimately conservation of momentum comes from a symmetry called spatial shift symmetry and conservation of energy comes from a different symmetry called time shift symmetry.

So they do have different origins.

But if you challenged me to think of an isolated system where conservation of momentum applies but conservation of energy doesn't, or vice verse, nothing springs to mind.

Yuzuriha Inori

5:36 AM

Noether's theorem, that's also true. Hmm... What's wrong with this math then? $E=p^2/2m \rightarrow dE/dt=\frac{p}{m}dp/dt$. If $dE/dt=0$, then $dp/dt=0$ or $p$ is a constant. (Neglecting p=0).

David Z

@YuzurihaInori In general, $p$ is a vector and $E$ is a scalar (assuming nonrelativistic physics), so even if $\mathrm{d}E/\mathrm{d}t = 0$, it could just mean $\vec{p}$ and $\mathrm{d}\vec{p}/\mathrm{d}t$ are perpendicular to each other at all times.

That'd be basically circular motion. Energy is constant but momentum is not.

Of course isolated systems don't move around in circles, at least not under normal physics.

Yuzuriha Inori

Exactly :"), So what can we conclude under normal physics?

David Z

Under "normal physics" we can conclude that both energy and momentum are conserved, but neither one of those is necessarily because of the other.

Yuzuriha Inori

But the maths? Doesn't it imply that conservation of one conserves the other?

David Z

Which math(s)? You mean the $E = p^2/2m$ reasoning you posted above?

John Rennie

5:59 AM

@YuzurihaInori I think you need to be a bit cautious with that approach. Consider two particles approaching each other then total momentum is zero but the energy is non-zero. Your equation should be something like $$ E = \sum_i p_i^2/2m_i $$

where you sum over all the $i$ particles present.

But then when you have to include potential energy there will be contributions to $E$ that don't have a corresponding term in that sum.

Yuzuriha Inori

Yes, I think that's the thing I am missing

Yuzuriha Inori

6:18 AM

I was actually experimenting on a few ideas and saw that if energy conservation implies momentum conservation or vice versa, then Galilean relativity is self-inconsistent. Has there been any work on these lines?

John Rennie

@YuzurihaInori Galilean relativity is self-inconsistent? Really?

Yuzuriha Inori

If the condition I mentioned above is satisfied.

John Rennie

Can you show how the inconsistency arises?

Yuzuriha Inori

If one implies the other, then the space-time transformation matrix should hold for momentum-energy transformation, and that implies momentum to stay the same in any inertial reference frame you choose.

There is the inconsistency. Special relativity avoids this because 1. the condition is satisfird and 2. the transformation matrix holds without conflict.

John Rennie

At a first glance I'm not sure what that means. Sadly I have to work now for a bit so I'll have to come back to this.

Yuzuriha Inori

6:31 AM

Okay sure. There's no need to be in a hurry. I may even be wrong... :P

Dawood ibn Kareem

7:52 AM

@YuzurihaInori No, energy conservation and momentum conservation are two entirely different things. Neither implies the other.

For a single body, sure energy and momentum are related. As soon as you have more than one body, you can no longer work out one from the other. So either one could be conserved, without the other. It just so happens that both seem to be conserved in our universe, but the two conservation laws don't appear to be related.

Balarka Sen

8:41 AM

Scary things are happening in Syria

Slereah

Aren't they always

John Rennie

Why is everyone so keen to start world war 3?

Yuzuriha Inori

8:57 AM

@DawoodibnKareem They don't appear... But do we have any proof that they aren't related?

Dawood ibn Kareem

@YuzurihaInori I believe there are some advanced theories in which they are related.

Yuzuriha Inori

@JohnRennie Mostly because people think that world wars are(were) cool, and they would like to try out one.

Dawood ibn Kareem

@JohnRennie Do you really want me to answer?

John Rennie

@DawoodibnKareem it was a rhetorical question. I already know the answer :-)

Yuzuriha Inori

@DawoodibnKareem In SR, they are. Can we prove that in an universe which follows Galilean Relativity, these two are separate and one does not follow from the other? One counterexample would do...

Dawood ibn Kareem

9:00 AM

Sure. Two bodies. The maths shows they're independent.

An inelastic conversion doesn't conserve kinetic energy (it turns into heat), but momentum is conserved.

Balarka Sen

@YuzurihaInori idt anyone but 13 year olds think world wars were cool. there have been several extremist groups (of activists and intellectuals alike) prior to both world wars who wanted to break through the pre-war era of stagnation and escalate a vision of cyberpunk though

Dawood ibn Kareem

@JohnRennie Nobody else does.

Balarka Sen

Eg futurists and vorticists before the first world war.

Dawood ibn Kareem

Very simply, if nobody deters al-Assad from massacring his own people, he's going to keep massacring his own people. It's not a difficult equation.

Balarka Sen

@DawoodibnKareem The Russian position strikes me as strange here though

Dawood ibn Kareem

9:03 AM

@BalarkaSen I suspect you've been exposed to Western media, not Russian media. I would suggest reading both.

John Rennie

@DawoodibnKareem really? It's because having a enemy to rage at produces an adrenalin surge that feels good. It feels especially good if you're stressed or depressed, which is why social deprivation invariably leads to increased violence.

Dawood ibn Kareem

I don't agree with the Russian position, but there is a kind of logic to it.

Balarka Sen

Hmm.

Dawood ibn Kareem

@JohnRennie Do you think Trump, May or Macron feel good about having had to bomb Syria?

John Rennie

@DawoodibnKareem I think they want to stay in power and can see which way the wind is blowing.

Balarka Sen

9:06 AM

Power demonstration does give a collective seratonin boost, tbh

Dawood ibn Kareem

@JohnRennie That's probably true of Trump and Macron; less so for May. I don't think this will have been a popular move in Britain. Or maybe I don't understand the British people.

Something like this was inevitable with the decline of the so-called Islamic State.

John Rennie

NB I am not condoning Assad's use of chemical weapons, I'm just not sure this was the best response. But then I'm not sure what a good response would be. Probably to not be at odds with Russia thereby turning it into a proxy conflict.

Dawood ibn Kareem

The other participants in the Syrian crisis no longer had a common enemy to unite them.

@JohnRennie Looking just at Syria, it was an excellent response. The question was to what extent this will have poked the Bear. Russia have already promised recrimination, and we don't really know what that might mean.

China, of course, are also aligned with Syria, but less strongly than Russia.

Some would argue that angering a nation with nuclear weapons and a slightly unstable president is unwise.

Balarka Sen

yeah that Putin talk was scary

Dawood ibn Kareem

Putin is getting to the point where he realises that his power is close to unlimited.

John Rennie

9:10 AM

Putin isn't unstable. He is the single best leader in the world right now (best for Russia not best for us). He knows exactly what he is doing. When the west have gone up against Putin he's always come out on top.

Dawood ibn Kareem

He saw how easy it was to get rid of anyone who might stand against him in an election. Just like al-Sisi.

Balarka Sen

He successfully eliminated opposing candidates in the elections, yup

Dawood ibn Kareem

I fear that the power is going to his head.

Emilio Pisanty

@JohnRennie I feel that that question would benefit from a deeper explanation of what a "coordinate speed" is

Dawood ibn Kareem

And I disagree with John R about his stability.

Balarka Sen

9:11 AM

It doesn't seem like most presidents of the world right now are super stable so I dunno about that :p

Emilio Pisanty

And of how to "fix" it so that it becomes the invariant object we were promised

Dawood ibn Kareem

There's a really ugly power block forming, with USA, Israel, Saudi Arabia, Egypt and UAE. The world should be very frightened of it.

John Rennie

@EmilioPisanty ah, yes, well, erm, the four-velocity of light is ... erm ... a bit of a problem :-)

Emilio Pisanty

Keep in mind who your audience is - non-sciency types that might otherwise be misled by incorrect explanations elsewhere, and who need a clear telling of what's what

@JohnRennie how so?

John Rennie

@EmilioPisanty it's undefined.

Dawood ibn Kareem

9:16 AM

And I have no evidence of the sanity of the leaders of any of those five countries.

John Rennie

@EmilioPisanty basically that Q/A is my sticking a marker in the sand. Specificially aimed at Duffield of course. I'm hoping that people will add more sophisticated answers, and I might even add a second answer that goes into more detail about what is meant by coordinates. But I think that would have detracted from my headline if I put it in the main answer.

Dawood ibn Kareem

9:27 AM

Are you expecting Duffield to offer a poor answer?

John Rennie

@DawoodibnKareem He has a standard rant that he posts to any even remotely similar questions. Just read back through his last few answers to get an idea of this. I fully expect him to post something similar then get heavily downvoted.

Dawood ibn Kareem

My experience is that he is not deterred by downvotes. Therefore, I wonder what you are trying to achieve.

John Rennie

I am posting a clear, cogent and accessible explanation that can be referred to in future when we need to explain to site members that they shouldn't take him seriously.

A canonical question

Dawood ibn Kareem

I see. And if Duffield tells those same people that they shouldn't take you seriously?

Balarka Sen

Lol

The Periodic Table

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