The h Bar

I swear I was freaking out looking at $Q$ in other books, he just says how you get it in a few sentences no problem, I don't know why you do what he does fully but this is a step up

Secret

2:19 AM

Last night dream involve a windows program which is some kind of wormhole flight simulator. It talks about how there's a wormhole near earth that will go to Jupiter and another wormhole near Saturn that leads to Neptune. Followed by that is another wormhole somewhere in the Kuiper belt that leads to somewhere in deep space, plus another wormhole that is located at a vortex in a purple space storm

All wormhole interior are rendered very similar to that in Interstellar, except with a purplish highlighted rim and the distortion inside is circular instead of spherical

There is also some weird spiritual thing going on if one moved into the space storm, which over time will split your avatar, which is some kind of giant buddha statue into 4 quadrants

along with the narrator mentioning about how we humans are made up of the broken pieces of that buddha thing

(Actually typo, the wormhole at the vortex looks like a purple swirl feature on the purple cloud, probably because it is obscured by the space storm)

Rumplestillskin

6:58 AM

Can anyone provide me with sanity check regarding a Hamiltonian?
I’ve got a Hamiltonian that is of the following form

$H = 1 + 1/c^2*(other terms)$

Is it legal to define a new Hamiltonian $H_new = (H - 1)/c^2 = other terms$???

I mean H_new = c^2(H-1)*****

David Z

@Rumplestillskin you can edit

But anyway, Hamiltonians have units of energy, so you can't just subtract 1.

You could subtract some fixed constant amount of energy, and that would be fine. I don't think it really changes anything though.

Jarek Duda

9:58 AM

Fresh discussion about the problem of infinite energy of point electron - there is a looking competent person explaining that QED modifies electric field below Compton wavelength (~2pm): reddit.com/r/Physics/comments/86z0ct/…

John Duffield

10:22 AM

@SirCumference : yes, I care for cosmology a great deal.

@JarekDuda : I posted this "You can't really repair the problem using vacuum polarization, you have to repair it by knowing that the electron isn't a point particle!" Can you see it on the reddit page?

Secret

The following are facts:

$$\int f(x) dx$$ has no closed form in general

$$\int^{\int} f(x) dx = \sum_{n=0}^{\infty}\frac{C^n}{n!}=e^C-1$$

$$\int^{\int^{\int}}f(x) dx = 0$$

and more generally

$$\left .^{n}\int f(x) dx =0\right .$$

Proof in math chat

Slereah

10:39 AM

mornin

John Duffield

10:51 AM

@Slereah : Mornin. But I can't stop to chat I'm afraid. I've got to do some gardening, then I've got people arriving at 2pm. Busy busy busy.

John Rennie

Balls. I've just realised a question I've just answered is a duplicate. Indeed mine is the accepted answer to the duplicate :-) Oh well.

Slereah

just don't say it to anyone, maybe you'll get more points!

John Rennie

:-)

Qmechanic spotted the duplicate and posted a comment to that effect, so my room for manoeuvre is rather limited.

In the end I dupehammered it and deleted my answer

Jarek Duda

11:06 AM

@JohnDuffield, I don't see it is on reddit ... QED also need to modify electric field near electron, but it seems don't allow for asking details about this modification ...

Secret

The wormhole dream might be inspired from seeing this diagram in Slereah's book (which the english version I once own before I threw it away due to outdated information)

John Rennie

@JarekDuda electrons aren't localised unless you interact with the electron in such a way as to localise it. To localise an electron to the point where the field self energy is comparable to the rest mass of the electron requires an interaction energy comparable to the rest mass of the electron. Any such interaction is inevitably going to result in particle production.

It doesn't make sense to consider an electron as a point charge then ask how QED modifies the field near the point. Real electrons never are point charges.

Kenshin

Hello people

Hello?

Jarek Duda

11:31 AM

@JohnRennie, imagine empty universe with single resting electron - what is electric field configuration of such situation?

If electric field is not physical enough, what is (electric) potential e.g. if considering Schrodinger equation?

John Rennie

That's an unphysical situation. In a universe with a single electron that electron would be competely delocalised i.e. it would spread out over the entire universe (given enough time)

Kenshin

@JohnRennie, you say "given enough time", but what if we analyse the situation before "enough time" has elapsed to allow it to spread out?

Qmechanic

@JohnRennie : That was an unselfish act. If everyone acts similarly Phys.SE may become easier to navigate :)

John Rennie

@Qmechanic 10 rep points that cost me. Ten rep points!!! :-)

@Kenshin A perfectly localised electron would have an infinite uncertainty in momentum (and an infinite energy).

Kenshin

@JohnRennie, what if it isn't perfectly localised (e.g. say some Gaussian distribution around a particular point)

John Rennie

11:38 AM

You could arbitrarily impose a very highly localised initial state for your electron, but this state would evolve rapidly with time to spread out, and more highly localised it was the faster it would spread out.

Kenshin

but before it spreads out, what is electric field configuration?

Anonymous

@Kenshin Isn't that an imaginary situation anyway? Universe with single electron, without even any initial/boundary conditions specified. How could one possibly give an answer to your question?

Kenshin

@Blue, the initial conditions are specified in my example, the initial condition is the electron in a specified guassian distribution

Anonymous

@Kenshin That doesn't seem sufficient to me.

Anonymous

Anyway JR would know better

Kenshin

11:43 AM

the situation might not match the physical world

but the situation isn't inconsistent withour theory of QM

John Rennie

Fair enough I guess. If you impose those initial conditions there must be a solution for what the field looks like.

In that sense I guess you can talk about the field of an (almost) point charge.

OK I withdraw my objection :-)

Kenshin

@JohnRennie seen Kaumu lately?

@KaumudiH

Tanuj

11:59 AM

@JohnRennie Hi ! Good morning !

John Rennie

@Tanuj Afternoon! The clocks went forward last night, so it's 1 p.m. here now.

(That also meant I had to get up at 4 a.m. physiological time - not so entertaining :-)

Tanuj

@JohnRennie How did that even happen ?

ACuriousMind

Ohhh, right, the clocks went forward

In this world where all of my clocks adjust themselves automatically, I actually forgot about that

But it explains why it was 4 am so soon last night :P

Tanuj

Well , I didn't know about that ! Ah , so that's the reason I was always confused about the English time !

John Rennie

Daylight saving time

Daylight saving time (abbreviated DST), sometimes referred to as daylight savings time in US, Canadian and Australian speech, and known as British Summer Time (BST) in the UK and just summer time in some countries, is the practice of advancing clocks during summer months so that evening daylight lasts longer, while sacrificing normal sunrise times. Typically, regions that use daylight saving time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time. George Hudson proposed the idea of daylight saving in 1895. The German Empire and Austria...

Tanuj

12:06 PM

@JohnRennie I'm 18 and didn't know about it ! WOW , where have I been living !

John Rennie

@Tanuj India?

Tanuj

@JohnRennie I meant I've been living unaware of this fact . Like on some other planet

ACuriousMind

@Kenshin The question is ill-defined, because "the electric field configuration" is a classical concept. It doesn't make sense to ask for "the" electric field of an electron if you are treating the electron quantum-mechanically, just like it doesn't make sense to ask for "the" position of the delocalized electron

Tanuj

@JohnRennie That's sounds cool , may or may not be .

John Rennie

@Tanuj it's nice when the clocks go back in Autumn because you get an extra hour in bed. The problem is in Spring you have to get up an hour earlier.

Tanuj

12:08 PM

@JohnRennie not really though

John Rennie

Anyhow, I'm off to sort out lunch. Schnitzel!!

Anonymous

@ACuriousMind I assumed they meant position probability distribution rather than electric field. But I agree that even then the question looks ill defined

Tanuj

12:36 PM

in Mathematics, 15 mins ago, by Tanuj

How do I eliminate $t$ from these two expressions to get an expression solely in $x$ and $y$ . $$y=\dfrac{3t}{t^2-1}$$ and $$x=\dfrac{3+3t^2}{t^2-1}$$

Slereah

$$t^2y - 3t - y = 0$$

Solve for $t$

berrygreen

Can anyone help me with some very elementary representation theory? physics.stackexchange.com/questions/395573/… It should be really simple. I must be missing out something very elementary

Shing

1:09 PM

hi everyone

Tanuj

@Slereah That get's too tedious , any other way ?

Slereah

That's a quadratic equation

It's fairly straightforward

Tanuj

@Slereah yea but once I get $t$ in terms of $y$ , putting it in $x$ really gets messy.

Slereah

You asked to eliminate the terms, not make it nice :p

Tanuj

@Slereah that the only way ?

Slereah

1:24 PM

Also please note that you can rewrite $x$ as $$x = \frac{1}{t} \frac{3t}{t^2 - 1} + t \frac{3t}{t^2 - 1}$$

Or in other words, $$x = y(\frac{1}{t} + t)$$

or $$x = y \frac{1 + t^2}{t}$$

that should simplify things somewhat

Tanuj

hmm okay. Let me ask on main. Hope someone has another way.

Slereah

And $$1 + t^2 = \frac{3t + 2y}{y}$$ so $$x = y \frac{3t + 2y}{yt}$$

So $$x = y( \frac{3}{y} + \frac{2}{t})$$

Not simplified enough yet? :p

Abcd

2:04 PM

What does the energy of electromagnetic radiation depend on?

According to me:
frequency of wave

reason: $E=h\nu$

ACuriousMind

@Abcd That $E$ on the l.h.s. there is not the energy of a "wave", but of a single photon.

Abcd

That's sad :"(

But energy of wave should then be $E= nh\nu$

Where $n$ is number of photons

ACuriousMind

Indeed (assuming for now that a wave has a definite number of photons)

Abcd

So today they asked this question in my exam:

The energy of an electromagnetic waves is due to their:
a) Frequency

b) Wavelength

c) electric and magnetic field

d) none of these.

I marked frequency on the basis of my reasoning above.

But then the answer key says option c.

ACuriousMind

Well, the answer key is correct - when you have a classical electromagnetic wave, its energy density is $\frac{1}{2}\left(\lvert E\rvert^2 + \lvert B \rvert^2\right) $ (modulo constants)

Abcd

2:11 PM

But question doesn't say "classical" EM wave

0celo7

constants = 1, clearly

ACuriousMind

@Abcd Your reasoning just shows that for a given number of photons, the energy depends on the frequency. But in general, a EM wave does not have a specific number of photons - you can send a wave with (e.g.) 1 billion photons or one with 10 billion, it's still an EM wave of the same frequency, but it has 10 times more energy - and its electric and magnetic fields will be higher accordingly, too.

Abcd

I see. Thanks.

Slereah

2:49 PM

It will change the amplitude

That is

Physics Meta

0

Q: Is it possible to include invisible code in a post?

I wanted to include a diagram in an answer, but Mathjax doesn't support TikZ. I therefore wrote the TikZ code in a local .tex file in my PC, compiled it, and included the diagram as a .png file. The problem with this procedure is that it prevents me from sharing the code (in case someone wants to...

support

Secret

@PhysicsMeta $\phantom$

what? It works in maths chat

John Rennie

Schnitzel! :-)

Slereah

Making crepes myself

John Rennie

3:09 PM

Last time I tossed a pancake it almost fell on my head. I nearly creped myself!

0celo7

@ACuriousMind help

@JohnRennie looks gut

I haven't had Schnitzel in ages

I'll see what the price of pork is later and maybe I'll get something going

ACuriousMind

@0celo7 What's up?

0celo7

@ACuriousMind do you think a notation section is good at the beginning of the thesis?

ACuriousMind

@0celo7 I don't think they do any harm, but I can't remember ever actually having read/used such a section if it was there

0celo7

3:25 PM

@ACuriousMind What I've found is that if it's properly written, I will never use it

But if I should have use for such a thing, it will be absent

Slereah

3:36 PM

If it's a thesis maybe not useful

A notation section is good if you don't expect your reader to read it chronologically

at least for the people who will judge it

Slereah

3:58 PM

I did put a tiny notation section in my master thesis

like a quarter page

0celo7

I took it out

I think it'll be fine

writing the preface right now

0celo7

4:14 PM

wtf

I don't need this garbage

0celo7

4:40 PM

@ACuriousMind umm

latex is doing a derp

ACuriousMind

I'm afraid the finer aspects of the line break algorithm are a mystery to me

Loong

That "3.1-3.5" looks also wrong. It probably should read "3.1–3.5".

John Duffield

5:16 PM

@JarekDuda : that's because I was shadowbanned on reddit, for no good reason.

vzn

@JohnDuffield cant see it suggest you get a new reddit acct. do they use IP addresses in the shadowbanning? dont see how that could work exactly. did just cite the Close 2015 ref you mentioned. very nice stuff. am going to cite it in a future blog too :)

John Duffield

@vzn : I won't get a new reddit account. Some would say that's dishonest, and I have my reputation to think about.

vzn

@JohnDuffield afaik afaict there is no official reddit policy about that. there will always be ~~trolls~~ "naysayers" esp on reddit & other social media. understand a sense of honor but what does it mean if others dont have it? o_O

John Duffield

@vzn : it means I'll have some harsh words to say about reddit when the time comes.

vzn

@0celo7 actually long suspect that may be regarded as a (la)tex defect...! have seen it not uncommonly...

@JohnDuffield think if you just log out of your acct & look at the page "anonymously" you should see the "actual state". as for harsh words against reddit lol "join the club™". just ran across this it might amuse you, an intellectual/ erudite deconstruction newyorker.com/magazine/2018/03/19/…

dmckee

5:29 PM

@JohnRennie Set-your-clocks-wrong-time (by any name) is an abomination.

bolbteppa

BRST quantization

In theoretical physics, the BRST formalism, or BRST quantization (where the BRST refers to Becchi, Rouet, Stora and Tyutin) denotes a relatively rigorous mathematical approach to quantizing a field theory with a gauge symmetry. Quantization rules in earlier quantum field theory (QFT) frameworks resembled "prescriptions" or "heuristics" more than proofs, especially in non-abelian QFT, where the use of "ghost fields" with superficially bizarre properties is almost unavoidable for technical reasons related to renormalization and anomaly cancellation. The BRST global supersymmetry introduced in the...

He gives the form of $Q = c^i L_i - \frac{1}{2}\dots$ you see in string books in the Yang-Mills context, Peskin doesn't give it, I guess $Q$ is not just some magic operator it needs an explicit form

I don't know why $Q$ takes that form, e.g. wtf is $c^i L_i A$, should be part of $D c$ but em...

(Always easier to blame others, e.g. reddit, than looking within...)

John Duffield

6:26 PM

@vzn : thanks, but I can't see anything about the "actual state" and IMHO that long tedious article was a puff piece.

@JarekDuda : you repair the problem by appreciating that the electron doesn't have an electric field, it has an electromagnetic field, or should I say it is electromagnetic field. It isn't some point particle surrounded by a field of force. The linear electric force only appears when you add a second particle, because it takes two to tango. But if you throw that second particle such that it has some motion that is not towards the electron, you also see a rotational magnetic force.

Unfortunately it sometimes feels as if Maxwell's electromagnetic unification never happened.

Rick

I have a circuit like this and at a particular instant, the inductor has a current $\frac{CE}{\sqrt{LC}}$ flowing through it and one capacitor has a charge CE, the other is uncharged, what will maximum current that can pass through the inductor?

Rick

7:01 PM

This was the original question, I'm not getting the last part,the answer key says (C)

John Duffield

8:14 PM

Who wants to see an optical illusion? The right-pointing arrow is horizontal.

Meh, it doesn't work so well in chat. Hello, hello, is there anybody there?

vzn

8:49 PM

@JohnDuffield the (long) article was critical of the CEO of reddit etc., not sure how that makes it a "puff piece". re reddit, it seems it should be easy for any user to figure out if theyve been "shadow banned" by just logging out and looking at the site & the state of the site (on any pg) should be the same for everyone who views it "anonymously".

knzhou

@0celo7 package 'microtype' was invented to fix this!

John Duffield

@vzn : IMHO it was mildly critical, and not nearly critical enough. Yes, it's fairly easy to suss a shadowban. Here's a screenshot. See my comment? When I log out it doesn't appear. Like I said, I have a special place in physics hell reserved for reddit.