The h Bar

12:05 AM

@EmilioPisanty It was about that review. I was just interested in why you voted that way. But when I looked closer I saw that you had already explained your thinking so the question was moot.

I apologize for bothering you unnecessarily.

0celo7

12:41 AM

@BalarkaSen I have a result about negatively curved Riemannian manifolds, I want to lift to the universal cover and somehow get information on the torsion elements

Apparently I'm not supposed to have any

It's not at all clear why this is

I first need to know what the torsion elements represent, geometrically

DanielSank

1:32 AM

Hi everybody!

> science, like art, is thought of as an end in itself, not merely as means to an end.

Again, I dismay of this being posed as fact.

I think the motivations of people doing stuff are not universal.

@ACuriousMind You say "...is thought of..." as if the entire body of scientists and artists share the same inner motivations.

This is, obviously, not true.

I can't tell if I'm misreading your meaning; this could all be an instance of ambiguous meaning due to the inaccuracies of English.

@ACuriousMind I disagree with that :-)

dmckee

1:50 AM

@DanielSank That is in response to a claim that it is done because of the material benefit stated is similarly bald terms.

Of course that is generally one of the major reasons that policy makers and tax-funded agencies give money to the project.

National greatness and staying ahead of the [bad guys of the moment] also being up there.

I suppose that private research money mostly goes to things with nearer term expectations of possible pay off. Certainly the money that kept Dad in research for most of his career did.

But the motivation of the individual and the motivation of the policy maker may be very different.

vzn

@DanielSank hey! are you married now? how about a blog on your trip? :)

@DanielSank this reminds me a lot of cp snowes famous essay on "the two cultures" urge everyone to read it

@ACuriousMind actually it is now scientifically proven that substantial evolution/ selection can occur over fairly short timescales in the human race eg as recently found with milk (in)tolerance... on the order of only ~10K years or so

DanielSank

@dmckee An overgeneralized assertion is not justified by the existence of a contradictory overgeneralized assertion :)

@dmckee The key word is may.

I think @ACuriousMind has this idea that (all) scientists do science for science's sake, which is utterly false.

@vzn Funny you should mention that.

How to make a blog?

I'd love to know.

@vzn Link?

vzn

@DanielSank lol youre literally a quantum physicist :P ... just go to wordpress, think its probably the easiest site... let me think that one owned by yahoo is very easy too watchamacallit

DanielSank

@vzn Mehhhhhhhh wordpress crappy code something something.

vzn

another option, 0celo7 has one, he could make you a guest writer on it

you dont have to install any code for wordpress. is that what youre talking about? its all in the cloud. it also has decent latex fmtting. even code fmtting capability (although really hate it at moment)... etc

hey can you update that meta post with links to the 1st 2 talks sometime? or make it wiki if youre too busy & then can update it myself?

DanielSank

2:05 AM

@vzn Ok will make wiki.

vzn

oh yeah the yahoo blog site acquired was tumblr its probably even easier than wordpress but more limited

so how many wks were you on vac/ travel anyway?

DanielSank

However, I'd like to ask just one thing: some times I've noticed you write somewhat verbose and punctuation-lacking stuff on your posts. I would just ask that we keep that meta post concise and clear :-)

...

I don't see the community wiki checkbox...

vzn

@DanielSank yeah everyones a critic esp you :P ... its your post, will not chg wording, dont worry. planning to do a meta post of my own after your talk & lets see how many ppl downvote it for punctuation o_O

DanielSank

Uh... ok I take it back... can't wiki-ify it.

@vzn No No I just mean sometimes like in comments I've noticed run-on sentences etc.

No big deal!

vzn

why cant it be wikied?

DanielSank

2:07 AM

I just think it's important for that post to be short and clear.

@vzn Don't see the check box :\

vzn

@DanielSank and we all know how important grammar/ punct are... as they all say 1st impressions matter! ...in chat :P

DanielSank

@vzn Haha, for example the comment on that meta post :D

vzn

@DanielSank rats am not thinking its not possible dont see it in another meta either, thats lame

DanielSank

@vzn I've done it before though...

Oh, it's only for answers, maybe.

See here

I have an idea.

I'll post each session's links as an answer.

vzn

actually maybe my problem is just dont have enough rep to edit meta posts yet. see that other sites seem to allow meta editing with my rep

DanielSank

2:11 AM

@vzn No dude, I can't wiki it either.

vzn

@DanielSank not sure thats a great idea. you can edit it right?

DanielSank

But I've wikied other meta answers. See the link I just posted.

@vzn Yeah, I can edit.

vzn

@DanielSank how about just edit it for now & its up to me to get more rep etc

ps have now advertised your talk in 2 other cs sites

DanielSank

LOL are you paying attention to anything I'm saying?

I cannot make it a wiki either but I have made a meta answer a wiki.

vzn

@DanielSank ofc dude. am not saying to change it to wiki (any more). you can edit your own question right?

DanielSank

2:15 AM

@vzn Oh, you don't want wiki any more. Ok.

I'll edit it if you can give me the appropriate chat link :)

vzn

oh the answer can be wiki? maybe that is a solution. strange that the question cant be wiki, dont get it

maybe 1 answer with all the talks is an acceptable approach, just dont think separate answers for each talk is a great idea

@DanielSank sigh ok will dig up the exact links

july 12th yuggib chat.stackexchange.com/transcript/71?m=30980294#30980294

june 14th slereah chat.stackexchange.com/transcript/71?m=30352471#30352471

DanielSank

Thanks

done

vzn

:)

already 22v on your intro, the suspense builds meta.physics.stackexchange.com/questions/7783/…

ps did you see that link posted on the reddit AMA on quantum computing? impressive

and was looking up their history on wikipedia, its rather incredible/ astonishing.. once obama did one, etc

DanielSank

@vzn No!

Link?

vzn

Jul 26 at 2:18, by vzn

quantum technology AMA / reddit

am presuming you have heard of that lab some (small world etc)

DanielSank

2:32 AM

We were mentioned there?

vzn

@DanielSank ?

DanielSank

@vzn Oh I misunderstood what you said.

Bernard Meurer

4:55 AM

@ChrisWhite Hai

user54412

hi

user54412

how's them olympics?

user54412

I hear the drama is picking up

Bernard Meurer

Oh it's awesome

Lot's of hot tourists

I couldn't care less if it's all catching fire

user218912

i'm obe again!!!

Bernard Meurer

5:10 AM

@3507?

user218912

yeah this was my old name from last year

user218912

exactly 1 year ago.

user54412

@BernardMeurer wait I thought people went to Brazil because the locals were hot (or at least scantily clad)

user218912

some weirdo neighbour of mine went there to find a wife.

user218912

@BernardMeurer no, 3750... a.k.a. 3075, 3570, 3507, 0537.

user218912

5:24 AM

baiii

GPhys

5:37 AM

are you guys excited for ICHEP?

peterh

@GPhys What has happened?

GPhys

conference hasn't happened yet, no official results

you can find the usual rumor mill online though

peterh

@GPhys Somewhere I've read that the 750GeV big brother of the Higgs was only an early hope :-(

Bernard Meurer

@ChrisWhite The natives aren't my type

I like my women the same way I like my workload: skinny

@obe wtf

@GPhys Are you excited?

GPhys

unfortunately (?) I have access to one of the experiments' internal conference notes so it's probably not appropriate for me to comment on the rumor mill directly (?)

@BernardMeurer Yeah, I always enjoy listening to the talks at the bigger conferences

they're usually slightly less technical than the internal/group discussions so they're easier for me to follow

Bernard Meurer

5:43 AM

@GPhys What are the rumors currently?

Where are you going to grad school again?

GPhys

in New York

I got into Stony Brook & NYU and chose one of those

:P

Bernard Meurer

Cool, (late) congratulations on that!

GPhys

(well, I got into some others, but they were all lower on my list)

(and nothing to write home about)

Bernard Meurer

I've heard good things about NYU

Although I'm not a reference

GPhys

they hosted ATLAS week this summer

but Stony Brook is almost definitely considered the better overall physics school it seems

Bernard Meurer

5:51 AM

Which one did you pick?

GPhys

it's a secret ;)

Bernard Meurer

Oh pleeeease, I only want your organs

GPhys

on a side note, NYU offered nearly 3 times as much money as Stony Brook

but Stony Brook's went way up after you went to RA from TA

Bernard Meurer

The plot thickens

GPhys

Stony Brook's really wasn't great for the TA pay given where they are in long island (expensive area)

Bernard Meurer

5:52 AM

Radical Agnostic and Tactical Argentinian?

GPhys

research assistantship / teaching assistantship

Bernard Meurer

Ah, less cool

user218912

what is a radical agnostic supposed to be?

user218912

agnostics don't have any idea what they're doing.

Bernard Meurer

@obe Someone very intense about not knowing what's going on

GPhys

5:54 AM

I'm gonna lose access to my CERN account/email/internal notes in a couple months :(

Bernard Meurer

@GPhys Why?

user218912

reminds me where did flp disappear to?

user218912

(fenderlespaul)

GPhys

account access is finite, was from my association with a CERN experiment through my undergraduate reserach

I'll probably get it back next summer when I start doing research in grad school (but maybe not, depends what research I'm doing)

Bernard Meurer

@obe He wont reply to my snapchats

user218912

5:56 AM

for how long?

Bernard Meurer

@GPhys Download everything?

GPhys

\o/

although it will stop the torrential downfall of CERN emails I get

user218912

forgot to sign up for frosh week

user218912

oh well

peterh

7:33 AM

558

A: Why is Stack Overflow so negative of late?

Why is Stack Overflow so negative of late? I hate to pile on, but I couldn't resist summarizing the problem. And I apologize ahead of time for any potentially offensive language that I might use. Basically there are 4 camps of users on Stack Overflow: The "caretakers" who want to keep the...

It helped me a lot to understand, how things are going also on the PSE.

Balarka Sen

lol

yuggib

7:51 AM

@0celo7 as a lot of very fashionable stuff

B-)

yuggib

8:27 AM

@GPhys I was offered a scholarship for a physics PhD at SUNY stony brook but I did not accept it...Europe rocks

John Rennie

8:59 AM

I watched from the sidelines during your argument with ACM, and I have to say I think you have an undeservedly negative view of this site. As someone born well before the Internet existed it is astonishing to me that teenage physics nerds can come here and get information about physics from real live working physicists. As a teenage physics nerd in the 70s I would have sold my gtandmother for a resource like this.

It has always been the aim of the PSE to be an authoritative source for information about physics. In the early days we hoped it would be a research resource but the reality is t…

John Rennie

9:11 AM

We continually get complaints from people who think we should allow homework questions. Well there have been several attempts to set up Stack Exchange sites for physics or indeed general science homework questions and they have all failed even to reach the beta stage. They failed because it's unrewarding to answer homework questions from people who generally never even say thank you. The sites couldn't attract enough members who actually know what they're talking about.

This site has managed to attract and keep a core of knowledgable people willing to put in the effort to answer questions,…

peterh

9:28 AM

@JohnRennie I didn't link the question, I linked one of its answers. Based on your reaction, I suspect, you probably didn't read the answer, only the title of the question. If you had, you would probably now know, that my post wasn't a critic. Partially, it was an ironic self-critic. Now please, read the answer I linked.

@JohnRennie After I've got your positive feedback, that you read and understood the linked answer, I am ready to continue.

Actually, I am waiting this.

ACuriousMind

9:45 AM

@DanielSank Ah, phrase that the other way around: I have the idea that no scientist is purely motivated by the practical applications. Insert "solely" in front of every "because", are you okay with that statement then?

John Rennie

@peterh I did read the answer

peterh

@JohnRennie o.k. I've laughed as I've read that post, and I've thought you also will.

ACuriousMind

@vzn "Be lactose tolerant or starve/be malnourished" is an extraordinarily strong selection pressure. Strong selection pressures can lead to fast evolution - but a) there's (to me) no evident strong pressure for specific human behaviours, especially since cultures are constantly changing and b) still, for any given aspect of an organism it's a priori doubtful it is the result of selection as opposed to neutral drift.

peterh

@JohnRennie I didn't initiate a debate about the site mod customs now, I only showed an interesting article about the main sociological structure.

ACuriousMind

(And c): How much of human behaviour is genetic and how much "trained" is unknown, as far as I know)

John Rennie

9:50 AM

I didn't think it was particularly funny, though I thought it was a good analysis. You need to bear in mind that when people ask questions on Stack Overflow they are generally being paid to solve the problem they're asking about. That makes it a very different dynamic to the PSE.

And I would note that ending up in state 4 is precisely what we all want to avoid.

ACuriousMind

@DanielSank I would be very interested to see evidence for your claims! I'd be perfectly happy to agree with you that there is a significant evolutionary aspect to human behaviour (the part that's different from fundamental mammalian behaviour) if there's evidence that supports it. But in the absence of evidence, I prefer to err on the side of caution.

peterh

@JohnRennie Which is obviously not popular in the eyes of (2) and (3).

@JohnRennie But they love eachother, it is their game.

@JohnRennie I admit, it is absolutely not so high level game, as the game of (1), but they can't better.

GPhys

@yuggib ? All their physics PhDs are funded

John Rennie

Rep whore is a wonderful phrase, but I doubt many (any?) people here have absolutely no concern for their rep. Your reputation is a proxy for the respect in which you are held, and only the most determined misanthropes genuinely doesn't care what others think of them. So we all have a bit of the rep whore in us.

GPhys

I wish my rep was higher so my downvotes counted

John Rennie

10:02 AM

I don't think it's a bad thing if people were driven to answer by their desite for a higher rep provided their answers contribute positively to the site. And assuming their quest for rep works that means site members think the answers do contribute to the site.

Balarka Sen

I have stopped caring about rep. I'll start picking some up in the next winterbash.

John Rennie

I'm less impressed when people answer blatant homework questions, but I don't know of anyone here who specialises in answering homework, though somewhat embarrassingly I have a gold badge in the home-and-exercises tag :-)

peterh

@JohnRennie From this view there are really big differences between the sites. As I experienced, the SO is full with green rectangle addicts. The SF is a closed, paranoid voting circle. In the case of the PSE, I think you are mainly right, at least in the case of (1).

John Rennie

There are huge differences between sites - they have very different cultures.

ACuriousMind

@JohnRennie Well, the whole point of the gamification through the point system is that there's a direct (mainly positive) feedback loop disjoint from the abstract desire to "help people" whose feedback pipeline is far less reliable, isn't it?

John Rennie

10:05 AM

@ACuriousMind well yes, and I don't consider that a bad thing.

We're both intermittently annoyed by the biasing effect of the HNQ list, but that's a minor aggravation.

ACuriousMind

@JohnRennie It isn't - it seems to work amazingly well on some level, judging by the constant expansion of the SE network

peterh

@ACuriousMind If you read the linked post, please don't forget: I didn't link the question (and particularly not its title), I linked only one of its answers.

John Rennie

@peterh bear in mind I wasn't just responding to your link to that answer. I was responding to your earlier argument with ACM.

ACuriousMind

@JohnRennie Unfortunately, negative feedback is difficult to reconcile with the idea of gamification, I think. If the positive feedback can be nullified or even reverted quickly, it becomes less of a game and more of a critical evaluation - something less appealing to that primal part of us that the instant gratification of a green +10 speaks to.

There's a reason they don't show red -2 when you're downvoted - you have to go actively looking for that

peterh

@JohnRennie Ok. One of the comments points that eliminating (1) would be also a viable solution.

@JohnRennie In the case of the MO-MathSE split, they are actually on different sites. The result: the MO has active Big Names, while the MathSE is in top3 of the network.

John Rennie

10:12 AM

The psychology of the MathSE escapes me. It's wildly successful but I don't understand why.

2

peterh

@JohnRennie But it is only possible on a contract between the SE and volks of the MO. The SE seems trying to minimize its sites, and probably believes in the indirect control through a group of CMs.

@JohnRennie Because they allow (2) and (3) to play.

John Rennie

There have been several attempts to set up science homework SEs and all have foundered because they couldn't get enough good people to answer.

Though it would be an interesting experiment to see what would happen if we refocussed the PSE to be homework oriented. NB interesting \ne good!

Balarka Sen

@JohnRennie We're "rarely if ever expressible as a ratio of integers"

Physics Meta

-1

Q: raise the minimum reputation required for downvote

As the title suggests, I would like to hear everyone's opinion on who should be allowed to downvote. I have seen youngsters downvote good answers simply because they could not understand it. They should instead request a clearer explanation rather than downvoting. What do you think ? Should the r...

ACuriousMind

@JohnRennie A bunch of users would leave and a giant influx of homework questions would happen. The only interesting thing I see is whether the people willing to answer them could keep up

Balarka Sen

10:18 AM

@PhysicsMeta lol at the downvote on that

peterh

@JohnRennie I've toke part a lot in the area51 as well. On my impression, the actually started sites mirror more like the preferences of the SE Inc, as the community support behind the initiatives.

John Rennie

@ACuriousMind I'd leave too! Still, I'd be curious if the site would have a long term future in that form. The MathSE does.

peterh

@JohnRennie And the SE Inc won't many sites. The MO-MathSE split is a special exception.

John Rennie

There was a Theoretical Physics site that was the physics equivalent of the Math Overflow. It closed because there simply wasn't the traffic to support it. That wan't the SE Inc being snarky, it genuinely didn't get the traffic.

2

I suspect that at research level maths is a nicer place than physics in terms of researchers sniping each others work, and that's why maths researchers are more willing to post on the MO.

ACuriousMind

@JohnRennie MO has a different history - it wasn't started by SE, it was started by mathematicians. That populated it with rather different users at the beginning.

The SE area51 process means that new SE sites are mostly populated by users from other already existing SE sites

And only after these sites manage to get a few questions in that rank highly on Google, they start to attract outside users

John Rennie

10:27 AM

Well we started with a fair few big name physicists. However they wouldn't discuss their own work and they quickly got bored with answering questions from students so they drifted away.

peterh

@ACuriousMind A similar site could be started by physicians. I would be happy to read their posts and they try to understand them with others on a MathSE-like site.

John Rennie

@peterh the attempt was made and has largely failed.

physicsoverflow.org

peterh

@JohnRennie Maybe a next try?

John Rennie

@peterh there's no point trying the same thing as Physics Overflow since that failed. So what would you try instead?

peterh

@JohnRennie I don't know the PO enough well, if they are professional site, I think it is better if I let them in peace. As I let the MO also in peace.

John Rennie

10:30 AM

I just don't think the culture amongst physics researchers lends itself to a site like MO.

peterh

@JohnRennie But, on my opinion, the Theoretical Physics shouldn't have been killed.

ACuriousMind

@JohnRennie I'm not sure who you're thinking of - the only really big names I've seen are 't Hooft and Shor, and the former seems to have left after that strange thread that was actually discussing his work, the latter is still around, but after all more on cstheory than here. And we don't know how many big names are among us anonymously, after all

John Rennie

@peterh Theoretical Physics was dying in the same way that Physics Overflow is dying. I can't find it in my heart to criticise SE for pulling the plug. After all they are a commercial organisation and they have to make a profit. If they don't then none of the SE sites exist and we all lose.

peterh

@JohnRennie Maybe because physics is important? It is high science, in the objective reality. Everybody knows it and it causes often... not enough cooperative social structures to born.

John Rennie

Which bit of After all they are a commercial organisation and they have to make a profit did I fail to make clear? :-)

peterh

10:36 AM

@JohnRennie Every site starts hardly. I think, with time it had been better. They committed some mistakes, too, as I know. A low-traffic SE site doesn't mean real cost for the SE Inc, on my opinion.

John Rennie

The SE aren't a charity and it's unfair to expect them to be. You're criticising them for not being what you want them to be, but that isn't what their shareholders want them to be.

Good grief!

0

Q: Does astrophysics work?

Is there a scientific explanation for planetary and star formation? If you don't give details in your response, you only prove that you don't know what you're talking about.

astrophysics

ACuriousMind

@JohnRennie I'm not sure that there are SE shares, but you're right about their for-profit aspect nevertheless.

@JohnRennie I was highly tempted to VTC with "I'm voting to close that question because I don't know what I'm talking about".

John Rennie

@ACuriousMind I would guess they're venture capital funded.

ACuriousMind

It's difficult to search for because all I get are money.SE questions :D

John Rennie

@ACuriousMind :-) I was also tempted to say something sarcastic, but I doubt the OP cares.

Balarka Sen

10:43 AM

@JohnRennie It surprises me that PSE is full of ~~trolls~~ (I mean, orcs @ACM) like that. The amount I think is far greater than MSE comparatively, given the fact that MSE gets more traffic.

peterh

@BalarkaSen It is because physics is important.

John Rennie

@BalarkaSen yes I agree. I think it's because maths has a high barrier to entry while many people have watched a physics documentary on the Discovery Channel.

Balarka Sen

Yeah.

John Rennie

Actually I don't criticise the MathSE for allowing homework. It seems to me (as someone who didn't do a maths degree) that worked exercises are a much bigger part of a mathematicians education than they are for a physicist.

Balarka Sen

I hope they won't broadcast Numberphile in near future.

ACuriousMind

10:47 AM

@BalarkaSen But...in this case "trolls" was correct! Aaaaaaaaaaargh! ::goes into orcish rage and searches for new picture::

Balarka Sen

:P

ACuriousMind

Sometimes we get insulted after we post answers. Insulting us before we post our answers is something new. Off to help someone else then! — David Wallace 58 secs ago

lol

Balarka Sen

"Watches The Big Bang Theory. Thinks he knows astrophysics. Realizes astrophysicists don't know crap about what they are doing."

peterh

@JohnRennie This is true, but the reason of their success is that they allow (2) and (3) to play.

John Rennie

Well doing worked exercises is a good way to learn, and we do get questions where the OP is genuinely trying to learning by doing an exercise. I don't mind questions like that, but the problem is reliably distinguishing them from homework cheats.

NB my view on worked examples is on the liberal side of the distribution. How many sigmas out I am I don't know :-)

@JohnRennie now deleted. Shame really :-)

peterh

10:59 AM

@JohnRennie If somebody really wants, he can be a repwhore also here, but it is much harder here. Answering MathSE questions, is like a dream. You go after the questions, and answer, answer, answer. On the PSE it doesn't work, because 1) most of the answers can be much better answered by HQ users. A 2nd grade equation can be solved by anybody, and it will be an O.K. solution. A trivial question like "how planets form" will have significantly different quality answers from me, and from you.

ACuriousMind

One of the stranger comment threads I've seen on our meta...

peterh

@JohnRennie If somebody really wants, he can be a repwhore also here, but it is much harder here. Answering MathSE questions, is like a dream. You go after the questions, and answer, answer, answer. On the PSE it doesn't work, because 1) most of the answers can be much better answered by HQ users. A 2nd grade equation can be solved by anybody, and it will be an O.K. solution. A trivial question like "how planets form" will have significantly different quality answers from me, and from you.

John Rennie

I heard you the first time :-)

Balarka Sen

@ACuriousMind lolwut

ACuriousMind

@BalarkaSen I think rob is having fun, but I can't tell for sure, he's being so wordy!

John Rennie

11:05 AM

I've just been bitch slapped by ACM!! :-)

peterh

@JohnRennie 2) On the PSE, you take away the meat from the repwhores. And also from the vampires. While they are working hardily to produce content ( ;-) ) their questions are closed and this makes impossible to answer them. The result: both (2) and (3) goes away. And (1) starts to elevate the quality limit, because there is always something to moderate (= delete)

ACuriousMind

@JohnRennie hmmmm?

John Rennie

-3

Q: Validity of Newton's second law of motion

If F = ma, F/a = m If acceleration is 0, does it mean that mass is infinite ? Note : I am a applying a force of very less magnitude - so less that the force is absorbed by the particles of the object and the object does not accelerate. If F = 0 = a, Does it mean that mass is not defined ...

newtonian-mechanics forces

ACuriousMind

I just thought you misread the question trying to make it more meaningful than it is - and I thought I'd let you know. One of us has a non-standard meaning of "bitch slapped", I think

John Rennie

@peterh which is good! But where I came in was you complaining that we were trying to turn the site into something like the Math Overflow.

Balarka Sen

11:08 AM

Irrelevant, but isn't photon massless?

ACuriousMind

@BalarkaSen it is

John Rennie

@BalarkaSen photons are surprisingly elusive objects. You need to be very careful when making broad brush statements about photons.

peterh

@JohnRennie No, I WANT a MO-like PSE. And also I WANT a MathSE-like PSE. I want BOTH. But I don't get any of them.

@JohnRennie Well, on a MO-like PSE I would be only a visitor.

Balarka Sen

Yeah, it doesn't make a lot of sense to me. If they are massless (@ACM confirms, so I guess it is), photolectric effect then becomes hard to interpret.

John Rennie

But I don't get any of them - unsurprisingly really since those demands are mutually contradictory :-)

Balarka Sen

11:10 AM

But I don't know any physics, so...

John Rennie

@BalarkaSen photolectric effect then becomes hard to interpret - eh?

peterh

@JohnRennie This is my big problem. And probably I am not alone with it.

John Rennie

@BalarkaSen are you finding yourself puzzled how a massless particle can carry momentum?

Balarka Sen

Right.

peterh

@BalarkaSen What if they have rest mass, but it is $<10^{-18} eV$?

Balarka Sen

11:12 AM

I was writing essentially that in a long-winded way in response, good that you brought that up in a short way.

ACuriousMind

91

Q: If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass and velocity, so, by this defini...

special-relativity electromagnetic-radiation photons mass momentum

hehehehehe

John Rennie

@ACuriousMind oh you bugger!!

ACuriousMind

:)

Balarka Sen

I mean, I "know" $p = E/c$, but I don't understand it. I'll read the answers though.

peterh

@JohnRennie And, if a MO-like PSE is impossible, then the rational optimum would be the MathSE-like PSE. And it means, (2) and (3) should be allowed to love.

John Rennie

11:15 AM

Perhaps I didn't make clear just how FRAKKING AWESOME I think the PSE is in its current form.

in its current form

Balarka Sen

OK, I went though the first two answers and it doesn't seem to address my particular confusion. I am "aware" of $E^2 = p^2 c^2 + m^2 c^4$ and how one formally manipulates this to $p = E/c$ for $m = 0$. But given $E/c$, $E$ being the photon's energy, how do I interpret that - directly - as the momentum?

Why should it be the momentum? What's an obvious physical - not symbolic - reason?

John Rennie

@ACuriousMind damn, I did misread the question trying to make it more meaningful than it is

ACuriousMind

@BalarkaSen I'm not sure what you mean by "directly" - are you familiar with the classical concept of momentum for the electromagnetic field?

this thing with "momentum" being unintuitive is not a quantum phenomenon - already in classical EM one has to ascribe momentum - both linear and angular - to the electromagnetic field to have momentum conservation

Balarka Sen

@ACuriousMind I was asking how to intuitively recognize that as momentum. No to the second question.

ACuriousMind

You can refuse to assign such a momentum on ontological grounds (Like "that's unintuitive"!), of course, but that just breaks momentum conservation and has no benefit.

John Rennie

11:19 AM

@BalarkaSen I suspect the problem is that intuition is born of everyday experience, and everyday experience tells us the impact of a moving object is related to its mass and should go to zero as the mass goes to zero.

But relativistic mechanics isn't an everyday experience so we have no intuition for it.

ACuriousMind

In essence, we have decided that for both energy and momentum, defining them to be conserved is far more useful than defining them to conform to our intuition.

John Rennie

That intuition is only gained by much blood, sweat and tears.

Balarka Sen

Fair enough. Admittedly I don't know anything about physics; I do mathematics and there I mostly interpret everything on intuitive grounds rather than formally. I have found it harder to do for the few random things I have heard of physics.

John Rennie

The main barrier for students new to QM and relativity is abandoning their preconceptions. It's hard!

But then I remember a time (in primary school) when I knew that multiplying numbers together made them bigger, and I couldn't understand how multiplying something by a fraction could possibly make it smaller.

Balarka Sen

heh

John Rennie

11:27 AM

Oh well, I have to go sort out lunch ...

Physics Meta

12:03 PM

0

Q: Explicit links: should I edit to fix them?

On the meta question What does everyone mean by "insufficient research effort"?, in the third note below the question, user Chris White says Explicit mobile links (e.g. en.m.wikipedia.org) are a scourge on the internet and should be fixed. Later, in the comments, user Emilio Pisanty says ...

discussion editing hyperlinks

bolbteppa

@BalarkaSen the main way to see $E/c$ is momentum is dimensional analysis, the units are those of momentum, to see it more clearly - if this was classical mechanics, $E = \frac{1}{2}mv^2$ and $c = v$ so $E/c = \frac{mv^2}{2v} = \frac{1}{2}mv = \frac{1}{2}p$

Balarka Sen

Eh yes, but just an equality of unit doesn't feel satisfying.

I have seen this sort of explanations before though, admittedly in different contexts

bolbteppa

But why not? You clearly just have a scalar multiple of momentum when the units are those of momentum

Balarka Sen

Because this, after all, is no more than a formal or symbolic manipulation. This doesn't convey any physical meaning, to me.

ACuriousMind

@bolbteppa That doesn't work. Torque and energy have the same unit, but neither is a multiple of the other.

Secret

12:28 PM

@vzn

Baidu AI composer

►

bolbteppa

@ACuriousMind That is incredibly incredibly misleading, torque is a vector quantity, energy is a scalar quantity, in the strict SI system where angle is not given a dimension answers.yahoo.com/question/index?qid=20090427104719AA7HhMI to turn it into work as a line integral you integrate it against a dimensionless parameter physics.stackexchange.com/a/36085/25851 so it is entirely consistent,

if you actually include radians as a dimension torque is more accurately a bivector physics.stackexchange.com/a/37890/25851 and the line integral (scalar) still produces the correct energy quantity

John Duffield

@peterh : as Danu said, no field is the same in the whole universe.

ACuriousMind

@bolbteppa Right back at you: E/c is a scalar quantity, momentum p is a vector quantity. If the argument doesn't work for torque and energy (it doesn't!), then it doesn't work for E/c and p, either.

Physics Meta

1

Q: Mistaken block on asking questions

The site says we are no longer accepting questions from this account when I try to submit a question in physics section. I don't even have a bad reputation. All I asked was one single question which got me two upvotes. So I don't even know why I'm being targeted or something like that.

support status-bydesign questions

bolbteppa

@BalarkaSen All I can say is to try to figure out why it makes sense and to find out why it should be intuitive, a lot of things can be derived by dimensional analysis instantly, the Bohr radius instantly following from the Hydrogen hamiltonian via dimensional analysis being a cool example

John Duffield

12:38 PM

@BalarkaSen : what we're dealing with is energy-momentum. Forget about scalars and vectors, and think about a cannonball coming at you at 1000 m/s. It has considerable kinetic energy, and momentum. But these are merely two sides of the same coin. One is effectively a measure of stopping distance, the other is a measure of stopping time. You divide by c to go from one to the other. And you cannot reduce the cannonball's kinetic energy without reducing its momentum.

peterh

@JohnDuffield And the Higgs field?

John Duffield

@BalarkaSen : a photon has energy-momentum but it has no rest mass because it has an E=hf wave nature. It's never at rest, and you cannot make it go faster or slower like that cannonball, so mass does not apply. That might sound odd, but think about a wave in the sea. It conveys energy-momentum and it can knock you over. But it has no mass, because the water has the mass.

@peterh : sorry, what about it? Guidice said it's only responsible for circa 1% of the mass of matter. E=mc² accounts for the rest. For example, when you trap a massless photon in the gedanken mirror-box, you increase the mass of that system. But the Higgs field is not responsible for this mass increase. Matt Strassler said it isn't responsible for the Higgs boson mass either:

"Now it is true that the W and Z particles, the quarks, the charged leptons and the neutrinos must get their mass from a Higgs field. It’s not possible for them to have masses any other way. But this is not true of the Higgs particle itself."

Danu

2:36 PM

@JohnDuffield you misread the quote by Strassler: He is saying that it is in principle possible to give mass to the Higgs in different ways. Nevertheless, the mass of the Higgs does come from the Higgs mechanism.

Note the word "must" in the quote.

ACuriousMind

@Danu oO

What do you mean, "the mass of the Higgs does come from the Higgs mechanism"?

To me, the Higgs mechanism is how a non-zero scalar VEV induces masses of massless fields that would ordinarily be protected from acquiring mass

user685272

Oo

Danu

2:56 PM

@ACuriousMind...and the field parametrizing fluctuations around the minimum (the field usually called $h$) also acquires a mass.

...I just mean that its mass also comes from the Higgs potential (which is kind of a tautology)

ACuriousMind

Well, but it had a mass to begin with (it was tachyonic) - it just shifted. I wouldn't call that "getting a mass from the Higgs mechanism", but I suppose that's a matter of taste

Danu

As opposed to its mass coming from any additional input. This is why we could predict the Higgs mass.

I would, because it's a normal mass, not a tachyonic one

Also the main point is the above: the Higgs mass is not due to something different from the quartic potential---it's part of it.

John Rennie

I'm back. Lunch now successfully prepared and consumed!

what did you have?

3:15 PM

looks yummy

That's a pizza with chopped Polish sausage and fried eggs on top, and it was delicious :-)

user685272

:-)

four eggs is a bit high on the cholesterol count, no?

John Rennie

I eat healthily during the week, so I can afford a cholesterol binge at the weekend :-)

0celo7

Four eggs is standard

user685272

3:24 PM

for you?

0celo7

Sure

user685272

:-/

do you exercise it off?

0celo7

What's that

bolbteppa

@ACuriousMind what I meant by scalar multiple is the $\frac{1}{2}$ when you do $E/c = \frac{mv^2}{2v} = \frac{mv}{2}$, for the magnitude of torque you can do dimensional analysis too, I don't understand what the issue is

John Rennie

@user685272 four eggs is about 300 calories, which is about 12% of the daily dietary requirement. You don't need to exercise off.

Although those eggs were fried in butter, which adds a calorie or two :-)

ACuriousMind

3:36 PM

@bolbteppa My issue is that just like dimensional analysis doesn't tell you that torque is energy, it doesn't tell you that E/c is momentum, either. That E/c is half the momentum of a massive body moving at the speed of light in Newtonian mechanics where the speed of light is nothing special doesn't to anything to convince me that E/c should be the momentum I associate to a massless particle in relativity.

0celo7

And they're probably small ~~European~~ British eggs

European civil war when?

When the south tried to pull a UKIP they got #rekt

user685272

well, 4 ostrich eggs would definitely be over 12% of the daily dietary requirement :-)

John Rennie

They were large eggs, where in the UK large means between 63 and 73g. In the USA that would make them Very Large/Extra-Large.

@ACuriousMind do you mind, we're talking about eggs here :-)

user685272

Ya! and the chicken, which came first?

bolbteppa

@ACuriousMind dimensional analysis does tell you (the magnitude of) torque is energy (per angle) en.wikipedia.org/wiki/Torque#Units Dimensional analysis does tell you $E/c$ has dimensions of momentum (also dimensional analysis ignores the $\frac{1}{2}$ factor...) How do you prove that $E/c$ has dimensions of momentum?

John Rennie

3:46 PM

Speaking of large things, I've been doing my weekly blog reading session and I see the 750GeV particle has now been ruled out. Shame.

ACuriousMind

@bolbteppa I'm not debating the statement about dimensions. Yes, E/c has units of momentum. That in itself doesn't mean it has any simple relation to the actual momentum of a particle with energy E moving at c.

You have to do more to establish that E/c really is momentum to avoid being this guy

4

Balarka Sen

lol

that xkcd is gold

user685272

pure

bolbteppa

I don't understand what that means, I don't think I am doing the same thing as that pic

ACuriousMind

@bolbteppa Then I must have misunderstood your argument. From where I'm reading, you appeared to be saying that because E/c has units of momentum and checks out to be momentum in a different circumstance, it should also be the momentum of a massless particle in special relativity.

bolbteppa

3:51 PM

You said $E/c$ has units of momentum, well as long as you don't deny $E$ has units of energy, then you seem to be denying that $[p] = [E/c] = [E]/[c]$ is allowable

ACuriousMind

So, to clear this up, tell me what the proper exegesis of

4 hours ago, by bolbteppa

@BalarkaSen the main way to see $E/c$ is momentum is dimensional analysis, the units are those of momentum, to see it more clearly - if this was classical mechanics, $E = \frac{1}{2}mv^2$ and $c = v$ so $E/c = \frac{mv^2}{2v} = \frac{1}{2}mv = \frac{1}{2}p$

is, if not what I just said.

bolbteppa

Is $[p] = [E/c] = [E]/[c]$ correct or incorrect, it's as simple as that, all I did was write this non-relativistically in what you just quoted, literally as simple as that

ACuriousMind

It's correct. But it doesn't tell you that $p\propto E/c$.

bolbteppa

Obviously I wrote it non-relativistically to try and make it more physically intuitive, technically $c \rightarrow \infty$ non-relativistically

I agree, I never said that

ACuriousMind

@bolbteppa In response to

4 hours ago, by Balarka Sen

Eh yes, but just an equality of unit doesn't feel satisfying.

you said

4 hours ago, by bolbteppa

But why not? You clearly just have a scalar multiple of momentum when the units are those of momentum

If you didn't mean that equal units imply $p\propto E/c$, I'll stop saying that now - but then you sure didn't address @BalarkaSen's actual question at all.

bolbteppa

3:55 PM

The way to see that $E/c$ is momentum is because it's units are momentum, whatever scalar multiple it actually is is another question, I said "scalar multiple" to say that just because you have $\frac{1}{2}$ $mv$, that does not detract from the fact that it is a momentum

That is unbelievable

@ACuriousMind "I was asking how to intuitively recognize that as momentum", by showing that it is in units of momentum I completely answered how it is intuitively a momentum, what's your problem?

Balarka Sen

Recognizing the units to be that of momentum is not sufficient to recognize it as momentum, is the point.

bolbteppa

Well it should be, welcome to real physics

Balarka Sen

Sorry, I don't care about real physics when it gives garbage logic.

bolbteppa

I can give you a better explanation using Lagrangians if you want it

Balarka Sen

Sure, I will be happy to listen to it. Ping your stuff with my name, I'll see it in the morning - I have to leave in a bit.

ACuriousMind

3:58 PM

@bolbteppa What? Not everything that has units of momentum is momentum that was my whole point with torque and energy!

In particular not everything that has units of momentum participates in momentum conservation, which we really should take as the definition of being a momentum.

bolbteppa

The magnitude of torque is energy per angle, as a vector quantity it is a completely different concent and that rings true in the fact that it is really a pseudovector not a vector, sloppy thinking to conflate torque with concepts like momentum!

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