The h Bar

@Kaumudi.H What?

user228700

Apparently, it hasn't even cleared customs yet.

user228700

A lady from TNT called my father just now.

11:28 AM

Aaaaaaah

user228700

^ Frustration?

Well at least they've been in touch

What did they say?

Is there something to pay on it before they'll clear it?

user228700

She said that usually, laptops are not really allowed and that they need to examine it thoroughly. Hm, she said "re-examine".

user228700

She said that it will clear customs in the coming two days and when it does, it will reach me that day and that I must pay about £18 to the delivery man.

user228700

Hopefully, it will clear customs tomorrow itself.

11:33 AM

I'm sure I remember being told you can't import laptops to India - I think I said that to you a while back. But when I rechecked on the customs web site it said laptops were OK.

The trouble is like all bureaucracies the info on their web site is hopelessly confusing.

user228700

Yes, u did. But the lady said that it will clear customs in the coming few days so no worries :-) She seemed very hopeful.

But to be fair it sounds as if they're trying to be helpful i.e. they're letting the laptop through anyway. They're welcome to examine it as there isn't anything on it that will cause problems.

user228700

36℅ tho. Ouch.

user228700

@JohnRennie Yes, exactly. Like I said, she sounded very hopeful and even told us how much we'd have to pay. Even my dad (who knows a little bit about this stuff) assured me that it will clear customs, eventually :-)

Actually £18 is about what I expected you'd end up paying. It could have been worse. If they'd used the original sale price to calculate the duty you'd end up paying much, much more!

user228700

11:38 AM

Oh, okay. Yes, gosh, that wouldn't been terribly costly.

So now we just wait ... :-)

user228700

Yep. (And discard that stupid website) :-)

But on the bright side you know exactly where it is and what happens to it next.

I wonder what re-examine means. I suppose they'll look to check there isn't any subversive material on the laptop, though what exactly they consider subversive who knows.

user228700

Yes. That is a relief :-) She said that if there are any more queries before it can clear customs, they'll get back to us. She and my dad sounds very hopeful. That's reassuring enough for me :-)

user228700

@JohnRennie That's a little scary but all the movies that u've copied onto it (which is literally the only material that they will find, apart from softwares that everybody uses) are played on Indian channels. Well, I don't see why not. So...

11:43 AM

Star Wars is a battle against an evil empire. I hope they don't consider that a comment on Modi's premiership :-)

user246160

@JohnRennie HAHAHAHAHAHAAHAHA XD ;-D

user228700

x'D Gosh, how dyou know all about he who must not be named?!

The British generally take a very international outlook, so we're well aware of foreign events. Our media is very good at covering world news.

user228700

> "The British generally take a very international outlook"

user228700

Of course! x'D

user228700

11:46 AM

But OK, I see :-)

Is that controversial? I suppose our history is, shall we say, somewhat mixed on this, but no-one here under the age of 70 has any memory of the empire and/or the Raj.

user228700

Anyway, I hope they don't find anything they don't like. In any case, they're only movies. So they'll delete whichever ones they don't approve of. Big deal :-)

user228700

@JohnRennie We have learned quite a lot about the British Raj in our history lessons :-) And I have watched the film "Gandhi" many many times.

I find it hard to believe they'll care. My only concern is that the laptop doesn't get damaged or the power supply lost when they unpack it and repack it.

user228700

I don't. Indians behave very irrationally when it comes to subversive material.

11:50 AM

I suspect the majority of Brits know nothing about the Raj and care less. Those of us who do know about it don't consider it one of our finest hours. About the only good thing is that we probably weren't as bad as Aurangzeb.

user228700

Anyhoo. Yes. Let's hope for the best :-)

user228700

@JohnRennie Many of us still mad about the kohinoor :-P

Really? All the bad stuff the Brits did and you care most about a bauble?

Or does it have a symbolic value that I know nothing about?

user246160

@JohnRennie Actually only politicians care. If they bring it back then their reputation increases. Common people don't even think of the Kohinoor :)

user228700

:-) That's true, we don't. It was a joke. Many people throw that around as a joke, sometimes, that's all.

user246160

11:55 AM

@JohnRennie You should watch Shashi Tharoor's speech in the British Parliament. It is a nice one :). (Assuming you didn't watch it yet)

Dr Shashi Tharoor MP - Britain Does Owe Reparations

This?

►

user246160

Yes that one :)

The thing is that Britain owes reparations to about 90% of the world.

user228700

:-P

user246160

@JohnRennie XD

11:57 AM

We spent most of the empire years stealing things!

But the worst, by far the worst, that Britain did to India was to foster the social divisions that eventually caused the partition. Stealing a few diamonds pales into insignificance next to that.

But this is getting a bit heavy, and anyway all this happened before I was born.

Why do electrons lose energy in an orbit?

@DHMO They don't

user228700

^ That is very very true. And I agree that it is getting very heavy. I was just going to say that I dunno what/how to add to that.

I dont mean a quantum orbit

Ah, a classical orbit?

12:00 PM

yes

Any accelerating charge radiates EM waves, and anything performing circular motion is accelerating inwards. So a charge in circular motion radiates EM.

why the first premise?

The energy of the radiation comes from the energy of the charge, so the orbit decays.

Because of the Lorentz force

Well

Maxwell equation

Q: How and why do accelerating charges radiate electromagnetic radiation?

25

Let's consider it case by case: Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property. Case 2: Charged particle started moving (it's accelerating). We were told that it starts radiating EM radiation. Why? What happened to it? What made it do t...

Q: Behaviour of Fluids under strong gravitational field

12:04 PM

I think this question becomes "what the hell is a photon?".

Xasel

hey John since your deal with fluids can you share your expertise on this quesion

2

We were being given intro to fluid dynamics at our school So the thing that sparked me up is $ g$ in expression of: Static pressure of fluid Bernoulli Principle I am curious how fluids(especially liquids) will behave when they are near or at strong gravitational fields and when they approach...

general-relativity fluid-dynamics gravity

Q: What exactly is a quantum of light?

@JohnRennie any distrbance of the electric field is a photon?

user246160

@DHMO Photon is an elementary particle.

user246160

20

I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet. What exactly is a quantum of light? More precisely, is a quantum of light meant to be just a certai...

quantum-mechanics visible-light electromagnetic-radiation photons quantum-electrodynamics

user246160

Also see physics.stackexchange.com/questions/41680/…

user246160

12:13 PM

38

Q: Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?

How can light be called electromagnetic if it doesn't appear to be electric nor magnetic? If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of electric charge/electrons (as there is in AC/DC current in space). In particular, the photons (whic...

electromagnetic-radiation visible-light

@Doraemonドラえもん my question is basically "so every shape can be called a photon?".

user246160

@DHMO What do you mean by every shape ?

user246160

That sounds really strange ^

@Doraemonドラえもん look at the link Mr Rennie sent

it says photon is generated by that change in the electric field

but its shape looks nothing like a localized wave

@DHMO photons don't exist at all in classical electrodynamics.

12:20 PM

well it looks nothing like waves either

@DHMO can you elaborate on that? I'm not sure what you mean ...

hello everyone

Morning Heather :-)

hy heaher

@Kaumudi.H @Doraemonドラえもん do you mind if I cancel the stars on the comments about Modi? They may be a bit controversial for a physics chat room. I won't delete the comments, just remove the stars.

user246160

12:24 PM

@JohnRennie Sure, no probs :)

@JohnRennie well look at the link you gave me

is that the mechanism of generating EM waves?

user246160

@DHMO At what level are you studying Physics? Maybe it will help us to provide an appropriate answer.

@DHMO That is a mechanism for generating EM waves. There are other ways EM can be generated.

well i'm just commenting on the link

@DHMO it's just that I'm not sure what you mean to say. The lines on the diagram are field lines so they don't look like waves because they aren't waves. Is that a possible source of confusion?

12:27 PM

@JohnRennie hence what i said, "they look nothing like waves"

then where is the wave?

I guess that you're saying Alfred Centauri's answer doesn't leave you any clearer about why accelerating charges produce EM waves?

yes

The trouble is that trying to give an intuitive picture for why it happens is hard. There is a derivation on Wikipedia or Googling will find you more examples, but the maths is a bit hairy.

hmm...i need to use LaTeX in a google slides presentation I'm making, but I can't seem to find a plug in for that...anyone know of one?

@heather This possibly ... ?

12:40 PM

@JohnRennie, oh, I have that. It works only for google docs. I found a workaround, and a google thread that said it wasn't possible. So I put in a feedback thing requesting LaTeX/plugin/something for slides.

codecogs.com/latex/eqneditor.php

^allows you to just type it in and download an image

and then you can just upload the image.

Better than nothing I suppose, though obviously it's a pain to make changes.

yeah. i like google docs/slides, but sometimes they can be a pain. my mom hates them with a passion =P

People can be irrational above their preferred apps. I still use Office 2003, thirteen years (!!) after it was released :-)

=)

How is the rest mass of photon found?

12:48 PM

the photon has no mass

this does not answer my question

@DHMO To understand what photon's are requires quantum field theory. In classical EM photons don't exist, and in non-relativistic quantum mechaics they are just put in by hand.

@JohnRennie "rest mass" doesn't exist in classical theory either

@DHMO sorry?

@JohnRennie my question contains "rest mass" so we are not talking about classical theory

12:51 PM

Well mass, and therefore rest mass, certainly exists in classical mechanics, but I take your point. The same applies to non-relativistic QM of course, so you need to resort to quantum field theory for an explanation of what photons are and how they behave.

Then how is the rest mass of photon found to be zero in QFT and relativity?

The QFT that describes photons is quantum electrodynamics.

i've always thought it was because nothing with mass can go faster than light, is that wrong?

QFTs are generally defined by the Lagrangian, and this is the case for QED. In particular the QED Lagrangian is constructed to have a symmetry called local gauge invariance.

This symmetry places strong restraints on the properties of the quantum fields, and in particular it forbids a mass for the photon field.

ok thanks

12:54 PM

It's important to emphasise that the local gauge symmetry is an assumption. We make this assumption because the resulting theory matches experiment.

However you get a low but continuous level of suggestions that the assumption might be wrong and that photons could have a very, very small but non-zero mass.

Xasel

hey John can you take a looka t this :physics.stackexchange.com/questions/298052/…

But the mass would have to be ridiculously small for it not to cause contradictions with experiment, and common snse suggests it really is zero.

@JohnRennie and I don't trust common sense

@DHMO OK, and we have no proof that the photon mass isn't zero though we do have proof that if it is non-zero it must be very, very small.

IIRC the current constraint is that if the photon has a mass, it must be $< 10^{-17}eV$

12:57 PM

Is it correct that mc^2 = E = sqrt((m_0c^2)^2 + (pc)^2)?

@Slereah eV is not a unit of mass

Crickey

it is if you use natural units

$\hbar = c = 1$

@DHMO it's commonly used as a unit of mass by physicists.

ok, sorry for being picky

@Slereah are you an Aussie?

Objecting that eV isn't a unit of mass will make you look a bit silly in any circles involving physicists. Like, for example, this chat room :-)

wow, 1e-17 eV is quite small

@JohnRennie well I did not know

12:59 PM

@DHMO I did warn you :-)

Am I correct that mc^2 = E = sqrt((m_0c^2)^2 + (pc)^2)?

No, not really.

^possibly me

@JohnRennie then what is it?

You're using the concept of relativistic mass and no physicist uses that these days.

3

The only mass we consider is the rest mass, which we call just $m$ not $m_0$.

1:00 PM

well I ain't no physicist

then is the relativistic mass E/c^2 ?

But you are talking to physicists and presumably wish to be taken seriously.

@JohnRennie so the momentum is intrinsic and not quite dependent on m0 and v?

by "not quite dependent" I really mean it isn't m0v

maybe it is $\gamma m_0 v$?

Massless particles can have a momentum, so the momentum is not dependent soley on $m$. For massive particles the momentum is indeed $p = \gamma m v$.

1:04 PM

where $m$ is the rest mass?

Generally $p = h/\lambda$ where $\lambda$ is the de Broglie wavelength.

and what is $p$ for massless particles?

This applies to all particles, massive and massless.

For light the de Broglie wavlength is just, well, the wavelength.

why don't physicists use relativistic mass?

@JohnRennie what is the wavelength

1:05 PM

Because it doesn't really generalize well

AFAIK, photons' shape is nothing like a wave

how do you find the wavelength from the wavefunction

@heather it's an inconsistent concept. For example a very, very fast moving object should become very massive so why doesn't it turn into a black hole?

Because a black hole depends on the rest mass

Obviously

@JohnRennie i always thought it does turn into a black hole....

it doesn't

1:06 PM

@JohnRennie everything is a blackhole

oh, wait, nvm, I know what you mean, i mixed that up

It wouldn't be Lorentz invariant otherwise

black hole depends on rest mass, okay, but why does that rule out the relativistic mass?

Q: If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?

37

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since. From the show transcript: Arunus Gidgowdusk from Lithuania asks: "If you took a one kilog...

black-holes speed-of-light relativity mass-energy

thank you @JohnRennie

1:08 PM

@DHMO I think QFT is an absolutely fascinating subject and I'm happy to talk about for hours, but statements like everything is a blackhole suggest you don't share my wish to understand physics.

@JohnRennie well I guess I might have some misconceptions

but I do wish to understand physics

to my mind, if you are Schwartzchild radius away from a point object, then you're basically gone

so maybe you're not talking about point objects

Point objects don't exist

Elementary particles don't have a size because size is a classical concept that doesn't exist in the quantum world.

could you answer my earlier question?

Elementary particles can behave like point objects, as far as we can tell given current collider energies, but there is no sense if which they really are a point object.

5 mins ago, by DHMO

AFAIK, photons' shape is nothing like a wave

5 mins ago, by DHMO

how do you find the wavelength from the wavefunction

1:12 PM

Photons are far weirder things than most people realise, and that includes most physicists.

then how on earth do they have a wavelength?

I guess you don't find the momentum by finding the wavelength, but rather the other way round?

If so, then how do you find the momentum in the first place?

To describe photons we start with a field called the electromagnetic four potential and we quantise it.

When we do this we find that the field has states with a characteristic energy and momentum, and those states are (kind of) the photons.

So a photon is a state of a quantum field.

It behaves like a particle in that the state has a well defined momentum and a well defined energy, and it can transfer that momentum and energy to other objects as if it were a particle bashing into that other object.

But a photon is not literally a little ball as commonly believed.

Is "inertial mass" a thing among physicists?

@DHMO That's an old concept too. It dates from the time when we distinguished between inertial mass and gravitational mass. These days I think most of us would say these are the same and just call it mass.

@JohnRennie you mean inertial mass = rest mass?

1:19 PM

inertial mass = gravitational mass = rest mass

How is momentum related to linear velocity?

inertial mass=grav. mass??

We've done that haven't we? $p = \gamma m v$ (for a massive object).

@Hey-men-whatsup yes

How do you measure the momentum of a photon without knowing its wavefunction?

like, I just shoot a photon at you

Eötvös experiment

The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy. The earliest experiments were done by Isaac Newton (1642–1727) and improved upon by Friedrich Wilhelm Bessel (1784–1846). A much more accurate experiment using a torsion balance was carried out by Loránd Eötvös starting around 1885, with further improvements in a lengthy run between 1906 and 1909. Eötvös's team followed this with a series...

1:21 PM

I think it would be E/c ?

@JohnRennie Sorry for asking two questions at once, but if rest mass = gravitational mass, then how does photon attract any objects at all?

@DHMO yes. The equation for the relativistic energy $$E^2 = p^2c^2 + m^2c^4$$ applies to photons as well with the mass set to zero.

So for a photon $E= pc$

thanks

oh ya, the guy tell it here "The interesting thing is that, physically, no difference has been found between gravitational and inertial mass. Many experiments have been performed to check the values and the experiments always agree to within the margin of error for the experiment. Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass and that the acceleration of gravity…

physlink.com/education/askexperts/ae305.cfm

Eötvös experiment

The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy. The earliest experiments were done by Isaac Newton (1642–1727) and improved upon by Friedrich Wilhelm Bessel (1784–1846). A much more accurate experiment using a torsion balance was carried out by Loránd Eötvös starting around 1885, with further improvements in a lengthy run between 1906 and 1909. Eötvös's team followed this with a series...

@DHMO an excellent question! And the answer is that the gravitational field is not just due to mass. The source of the gravitational field is an object called the stress energy tensor.

@JohnRennie then what good is the gravitational mass?

1:25 PM

The stress energy tensor includes mass, and in most cases the mass is the dominant contribution to it. However it also includes momentum and pressure.

@DHMO The mass term that goes into the stress energy tensor is the gravitational mass, and it is equal to the inertialmass.

thanks

I have to go I'm afraid. I need to go to the shops. Physics is more fun than shopping but it won't feed me :-)

ok, see you

See you, if you buy any chocolate it may good to share

@DHMO, isn't it great SE has many community sites, it feels like travelling to many places....

@Hey-men-whatsup indeed

Q: If time dilation can slow time down, is there away to speed time up?

1:33 PM

I was like in animes site asking about newests movies in chat, now I'm in physic

Secret

3:16 PM

1

Okay, I know the title is really confusing but I couldn't find words to explain it sorry. Pretty much what I mean is, if I can get in a lightspeed spaceship moving away from earth, time slows down for me. So one year for me will be 20 earth years or what ever. But is there away were I can reverse...

time-dilation

don't think it is possible

The Speed of Light is NOT About Light | Space Time | PBS Digital Studios

3:27 PM

►

Jim

@Secret easy response is that a geodesic outlines the path of maximal proper time between two events

4:05 PM

@ACuriousMind Are there any other structure you can construct the Hilbert space out of other than $\mathcal{L}^2$

4:37 PM

[Division by zero] The case for n=3 is now closed: There are NO zero divisor inverse of any kind for n=3 for any associative algebras

Hopefully to be able to generalise to all finite dimensiosn n, and then tryign to figure how to generalise to infintie dimensional cases

For n=3 the conclusion is that in an attempt to implement a zero divisor and an inverse, at least one associative law will fail as not all multiplication by rows maps to a row in the cayley table

Tmr might also try to catch Tobias and see if this mapping can be formulated nicely, cause as it currently stands, it is quite powerful for finite algebras as nonassociativity can be easily check using just 3 lines of writing plus inspection and common properties of the elements (e.g. idempotent must fix there corresponding row when it multiplied to that row)

5:56 PM

Bleem

2

koolman

Hii @heather

Could you again visit

@JohnRennie Rattus Norvegicus is great!

@BernardMeurer I think it's their best album. The followup, No More Heros, has a few good tracks but that's all. And after that they drifted off into a more mellow vibe that didn't really suit me.

I'm really liking the synth work

6:12 PM

@BernardMeurer yes, although the Stranglers were part of the punk rock movement they were always a lot more sophisticated than that.

Good stuff man

7:26 PM

@John Rennie : we care about getting the physics right instead of peddling tautological nonsense and promoting an ersatz incorrect version of general relativity that flatly contradicts Einstein. Or at least I do, as your resident GR expert. — John Duffield 2 hours ago

That's me told then :-)

First the naked physicist then John Duffield. It's always great to be surrounded by my adoring fans :-)

5

7:54 PM

Talking about GR in differential geometry language inhibits those misunderstandings...

@JohnRennie Think about what happened when I tried answering in a handy-wavy way to a question and then went back and properly thought about the issue...

8:13 PM

@BernardMeurer hi

Sorry, had meeting.

8:38 PM

More and more snow here...

Shovelling!

I like snow!

I miss real seasons living in Southern California.

I like it too, but at some point in February it gets COLD! Now it's just fine

It didn't go under -10 C yet...

user246160

@G.Bergeron -10 C :O....where do you live ?

@Doraemonドラえもん That's really usual over here in winter. We have a few days below -25 C

Montreal in Canada

user246160

@G.Bergeron My gosh! I would surely faint at such temperatures or freeze :P...here the temperature is always >25 degrees

user246160

8:44 PM

Do you guys have heaters installed everywhere ?

user246160

Or do you just wear a lot of sweaters and jackets ?

@Doraemonドラえもん You get used to it, really. Sweaters and jackets are not enough when the nights are less than -10 C for months...

Everybody have central heating everywhere, not just small heaters

You want something with serious power! :D

And snow is well handled, we are having 15-20 cm of snow coming down today, but streets and sideways are periodically cleaned. Although having a car can be a pain in the ass...

user246160

@G.Bergeron I see :P Walking on the roads at such temps seems really scary to me ! But it looks like a really beautiful place. I am yet to visit a place with real snowfall as I have always lived in tropical countries.

user246160

@G.Bergeron Oh...you don't travel by cars ? Or are there special vehicles for travelling on snow ?

It pretty much looks like this outside my window: google.ca/…

@Doraemonドラえもん No, you just walk ;) and use the subways, it's a big city!

There are special vehicles for snow (snowmobile) but nobody uses them in cities and they are probably illegal to use. Only people living in the countryside use them.

user246160

8:53 PM

Beautiful place, really ! i.cbc.ca/1.3891387.1481425146!/fileImage/httpImage/… Would love to visit Canada someday :)

user246160

So, are you a student or a professional presently ?

I guess I'd say the same for where you live :)

PhD student in mathematical physics

user246160

@G.Bergeron (I'm actually from India and not from Japan :P)

user246160

@G.Bergeron wow..great!

user246160

Nice to meet you. And see you around ! Bye :) Gotta go to sleep!

8:56 PM

Bye gotta go myself!

9:25 PM

hello everyone

Sanya

good evening

Any Russian speakers here?

A Russian theorist just told me a hilarious joke, but it only works in Russian :\

nope, no russian

sadly

9:46 PM

@DanielSank did you just block me since I said magic doesn't exist?! lol

Nope. I didn't block you. I changed the room to gallery mode.

well, i can't talk there anymore... oh well...

@TanMath I'll add you to the room if you will kindly stick to the topic of the room.

It is not for general chat.

It is for Linear Algebra Happy Fun Time, as you can see from the title.

3

@DanielSank sure...

Myabe I will come back also, when you teach QM...

I am totally forgetting my QM...

ah, i want to be there for that lesson =)

9:54 PM

Guys, it's a room for Linear Algebra Happy Fun Time... not quantum mechanics XD

We can make quantum mechanics happy fun time too...

When I reach my QM class

@DanielSank you should... then an SR one, then a GR one, then a QFT one, as well as QCD, and QED, then an ST one, then a...

haha, no time!

gosh, the outer product wikipedia page is to complex than it should be...

Yes, Wikipedia is often crap for math.

10:00 PM

it's good for biology I must say..

Ok for physics...

bad for math!

@DanielSank how long does it take to get into a room once you are added?

I still cannot get into the room...

@DanielSank did you add me? I feel like saying some stuff...

(huh, synchronicity!) hi @TanMath havent seen you in awhile. @heather, awhile back TanMath was working on qm simulation in python, exploring QM biology/ photosynthesis... any news on that TanMath? heather is interested in QM simulation software & has tried out some of the IBM open QM simulations/ demos

10:15 PM

@vzn well... I haven't worked on that in a long time.. I was very busy at college... plus, I seem to be stuck at a dead end... @DanielSank was going to help me but he has been very busy it seems... I might get back to it... I was thinking about programming it from scratch in MATLAB and play around with the constants to see where I am off. MATLAB is easier to program in, and is faster than python...

@TanMath trying to remember were you using qutip?

@vzn yeah, but that sucks... It takes forever to run...

Look at this: stackoverflow.com/a/36647676/4368746

^ the code in that answer runs so much faster...

@TanMath am sure MATLAB has its advantages but wonder if its performance is that much faster. QM simulation is just very "heavy" by nature no matter what pkg used...

@DanielSank your discussion on linear algebra is so enlightening...

@TanMath I'm glad you think so!

10:22 PM

@TanMath so are you working on biology degree? what yr? what country are you in?

@vzn yeah... 3rd year... USA