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5:47 AM
@JohnRennie Hi John
 
@onurcanbektas hi :-)
The idea of a test particle is that it has no charge and is freely moving i.e. nothing is exerting a force on it.
So when you let go of the test particle it moves free from any external influences.
Then by comparing your motion with the test particle you can see if you are also free of external influences or if there is some force acting upon you.
 
@JohnRennie For a test particle, those assumptions can be made (even though in an EMT class, we are explicitly assuming that it has a charge), but since you are saying "an object", the reader really thinks any object
that is one of my main confusion there
 
True, but then this is a question from a beginner so I deliberately kept my answer simple.
 
@JohnRennie I see, but, even though there are only 4 different forces that we know, can't there be any situation where a test particle is accelerated somehow, whereas you are not accelerating.
I mean ı cannot think any example right now, but your claim in there is some general that one thinks that clever experimentalist can find a good counterexample
 
The test particle argument is more conceptual than something you'd actually do. There are easier ways of determining whether you are in an inertial frame than dropping particles.
 
5:54 AM
@JohnRennie such as ?
 
If you were actually doing an experiment then all sorts of issues raise their head. For example how can you be sure your test particle really has no charge?
I guess you'd do that by observing it's motion, but then you're in a circular argument because the whole point of the test particle is to observe its motion to see if any forces are present.
In practice it's usually possible to be fairly sure. For example the Eötvös experiment was essentially observing test particles.
@onurcanbektas use an accelerometer. Like the one in your mobile phone.
 
I haven't heard the Eötvös experiment, so I will check out, but "how can you be sure your test particle really has no charge?" is a good question :)
@JohnRennie how does that accelerometer work ? upon which principle ?
 
@onurcanbektas I don't know to be honest. I would guess it uses piezoelectric effects to measure strain in some sensor.
A quick Google tells me that some accelerometers work by measuring capacitance changes as a sensor moves under the external force.
 
then all of them assumes that we know some particles have charges, but I've stuck at the question about how we do actually determine whether a particle is charged or not
can a particle be chargeless in some ref. frame, and be charged in another ?
 
@onurcanbektas We could compare it to a particle that has a known charge. e.g. compare the motion of our test particle with the motion of an electron and a proton. We know the electron and proton charges so by comparing the motion of the test particle with the electron we can tell whether it has a charge.
@onurcanbektas no, charge is an invariant.
 
6:08 AM
@JohnRennie Then, by definition, we assumed that proton and electron has some property called "charge", and we are measuring whether another particle has charge or not wrt this property, right ? Otherwise, I would need to ask how do we determine whether a proton or an electron has a charge or not.
@JohnRennie if that is the case, your initial argument fails, because of the example that I gave in the comment to the question
 
@onurcanbektas well, yes, we define charge as that property that an electron has.
@onurcanbektas you've lost me. Is this your point that an electric field could be present so if the test particle had a charge it wouldn't really be inertial?
 
@JohnRennie To be honest, I'm lost a bit, too :). My point was that, "even though there are only 4 different forces that we know, can't there be any situation where a test particle is accelerated somehow, whereas you are not accelerating"
 
@onurcanbektas in practice, no. The weak and strong forces only act on nuclear scales so the only two forces that could be affecting the test particle are electrostatic and gravity. Assuming you're prepared to accept that we can measure the charge on our test particle to be zero that only leaves gravity.
(and we do the experiment in a vacuum so there's no air resistance)
 
@JohnRennie I see
then I've no more objections :)
Thanks for the discussion
 
@onurcanbektas you're welcome :-)
 
 
1 hour later…
7:26 AM
I'm in need of some physics assistance :x
If anyone is qualified?
 
@krauser126 hi, it depends what physics you want to ask about :-)
 
I'm struggling with forces induced by magnetic field on a current running wire loop @JohnRennie
Trying to prepare for exams really
 
@krauser126 I should be able to help with that. What's the question?
 
Is there a way I can contact you via private message? Since theyre pretty low-level questions that wouldn't benefit the chat community, I'd rather just contact you directly. @JohnRennie
 
@krauser126 post in the Problem Solving room. That room is specifically for questions like yours.
 
7:37 AM
Cheers bud @JohnRennie
 
user351417
7:52 AM
should i always right a answer again and again my time too is important — Aditya Garg Mar 24 at 18:16
 
user351417
Legendary stuff.
 
user351417
9:53 AM
> At this point the sophist complains that with that many parameters the physicist can fit anything to his data, and the physicist says he just has made a useful model. The philosopher next to them will complain about assuming unseen abstractions and the problem of induction. Meanwhile the engineer long ago picked the lock on his chains and escaped, none the wiser.
 
user351417
physics.stackexchange.com/a/469130 was weird but not bad.
 
@Chair I've read the 1st sentence of this answer of his (which you commented on) about 5 times. I kind of get what he's trying to say, but it's very confusing.
 
user351417
@PM2Ring I found that one reasonable enough. But when I'm not making a conscious effort, I write really long sentences (though I use humongous volumes of punctuation, primarily semicolons, brackets, and commas), so I didn't find it too difficult to understand what was going there, since I was able to just strip away the trivial bits like the final phrase which mentioned the product rule.
 
10:23 AM
@Chair I also have a tendency to write long sentences, but I make a conscious effort to simplify my rambling, especially when I'm writing for non-native speakers. One trick I use is to break long sentences into parts at conjunctions. Some purists may object that it's bad grammar to start a sentence with a conjunction. But that's a prescriptive grammar rule imported from Latin, there's no good reason for it in English.
 
user351417
@PM2Ring Another thing which I've been trying to implement recently is the practice of replacing adverb+verb combinations with a more specialized verb. Apparently it shortens sentences.
 
10:40 AM
@Chair That can work well, although it may make it harder on ESL people who don't have an extensive vocabulary.
 
 
1 hour later…
12:00 PM
I thought this guy was improving, but then he comes out with drivel like this :facepalm:
 
12:26 PM
@JohnRennie can I ask you for a dupehammer check on this one? physics.stackexchange.com/questions/469072/…
@DanielSank pues ponte buzo, wey
camarón que se duerme se lo lleva la corriente
(were you explicitly asking for a slang-heavy reply in Spanish? yes, yes you were)
 
@EmilioPisanty it is a duplicate, though actually I was contemplating answering it.
 
@JohnRennie huh. What were you going to write?
"Your question is ill-defined."?
 
It's quite an interesting process how we get increasingly accurate calculations of the molecule i.e. first a Hartree Fock, then a CI calculation, then correcting for the Born-Oppenheimer approximation then (in principle) relativistic effects.
 
@JohnRennie eh, I guess
if you think this is narrow enough to have a reasonable answer, one course of action could be to reopen this one and make the other one a duplicate
though it would require mod intervention on the current dupe target, probably
i.e. if you want to reopen, go for it.
 
@EmilioPisanty the earlier question is closed anyway, so I would just reopen this latest question.
@EmilioPisanty Can I reopen in one, or does the original hammerer have to do that?
 
12:39 PM
@JohnRennie no, you can reopen in one
but still
> Are there examples of an "exact" numeric calculation of small molecules? By "exact" I mean [ill-defined notion].
 
@EmilioPisanty I'm just answering a question in the problem solving room, so I'll have to get back to this later.
 
doesn't sound like it makes the narrowness bar in its current form to me.
@JohnRennie a'ight
thanks for the dupehammer check.
the thing still makes me jittery =P.
 
 
1 hour later…
1:44 PM
Please tell me about doppler shift when the source is accelerating.
 
@Qmechanic. please explain how my question concerng iGR and QM is a duplicate. I really don't see an answer the.my queatiin in the cited post.
 
2:10 PM
@Ba'lrocDemos If you want someone to know that you want to talk to them you need to use the @username syntax.
FWIW, I agree that your question is different to the dupe target. The target asks what are the incompatibilities between GR & QM. You ask why do we even need to unify them, if I understand correctly.
 
Anonymous
2:34 PM
@Ba'lrocDemos I've added the @ for you.
 
Anonymous
Remember that you can ping anyone who dupehammers (or edits) your post, in the comment section itself.
 
@EmilioPisanty Ayudame entenderlo.
 
Is there a flag for answers that are non-mainstream? Eg, answers that use scripture instead of science, like this one
 
3:15 PM
@DanielSank literally, "the shrimp that dozes off gets whisked away by the current"
"If you snooze, you lose", you'd say in English
(roughly translated)
as for the first one this is a reasonable take on 'ponerse buzo'
the other bit....
Güey (Spanish pronunciation: [ˈwei]; also spelled guey, wey or we) is a word in colloquial Mexican Spanish which is commonly used to refer to any person without using their name. Though more often, and originally only applied to males, it can be used equally for males and females; although women would more commonly use another slang word to refer to another unnamed female person, such as "chava" (young woman) or "vieja" (old lady). It is used roughly the same way "dude" is used in modern American English. It is derived from the term buey, which refers to an ox, used for meat, sacrifice, or labor...
..... is complicated ;-)
 
user351417
I wrote something about "estoy en la edad del pavo" in one of my tests... I was trying to explain why I don't talk much (which is downright false but we get some freedom). It didn't go too well.
 
@Chair indeed, it wouldn't
"the age of the turkey"?
 
user351417
We do this thing where we search for idioms online and stuff sentences with them without indicating a clear understanding of their usages just to be funny.
 
user351417
@EmilioPisanty Huh? the internet told me that it's an idiom for puberty and shyness.
 
user351417
I know that's the literal translation though. I used it because we'd just learned some words for animals.
 
3:30 PM
it does sound like it might mean "puberty" somewhere or other
probably heavily skewed towards Spain
 
user351417
@EmilioPisanty Apparently either spain or latin amreica has an aversion to "usted".
 
user351417
I keep getting confused about that one.
 
@Chair not quite
 
user351417
@EmilioPisanty Wait, are you from spain or latin america?
 
there are some significant differences in the handling of the plural second person
the singular second person is treated vaguely uniformly across the board
there's some variations in the degree of formality, but they're changes of degree and not of kind
and there are changes to the informal second person in Argentina, Uruguay and Chile
but other than that, 'usted' is used much the same way everywhere
 
user351417
3:35 PM
@EmilioPisanty So people don't very consciously distinguish between 'tu' and 'usted'?
 
'ustedes', on the other hand...
no
@Chair oh, no, we very much do
it's just that it's the same distinction regardless of where you are
what does change is that only Spain uses an informal plural second person, 'vosotros'
which does not exist in Latin America
 
user351417
Interesting. I should find out some more about latin american spanish sometime... I'm given to understand that we religiously stick to the spanish variant, because they emphasize the usage of vosotros.
 
@Chair that's a valid choice
particularly if you plan to use your Spanish in a Spain-centric environment
though it's generally a good idea to watch out if you're learning a variant that's only spoken by a smallish minority of the speakers of the language you're learning
so e.g. Spain has about 10% of the world's spanish-speaking population
everybody else will look at you funny if you say "vosotros"
it's not a bad idea to learn how to use it
i.e. it's better to have the verbal person in your arsenal than to not have it when you do need it
but it might be a good idea to keep in mind, when practicing, in which contexts it will sound natural and in which contexts it will sound as if you were using "thee" and "thou" in a conversation in English
 
user351417
@EmilioPisanty considering the fact that I can't speak spanish very fast, I don't know how hard it would be to cut out the 'vosotros's and use usted. It's technically a bad thing that I can't speak that fast and fluently, and the language doesn't just flow (probably due to a lack of practice), but I imagine this may be an advantage. That being said, I've never tried speaking without the vosotros, so I can't be sure if that's how it works.
 
3:58 PM
@Chair you just use the formal register for all of the plural second person
i.e. you just use 'ustedes' every time you'd use 'vosotros'
(and the associated verbal tenses, as well)
if you're just starting out and you don't have much fluidity, don't worry about it too much
yet
just keep it in mind
think about what you want to use the language for, and learn it accordingly
so, for instance, if you're gunning for Spain, then by all means learn the s vs z pronunciation difference
just know that this is what you'll sound like to the other 90% of spanish speakers
 
@EmilioPisanty my brother speaks Castillian Spanish.
(I have no idea how good his pronunciation is since I speak not a word of Spanish)
 
@JohnRennie lotth of people thpeak cathtilian thpanish, yeth. They all thound weird to me.
it'th thlightly problematic given my current rethidenthe
 
:-)
My brother worked in the wine trade for ten years and he specialised in Spanish wines. I would guess that his Spanish is quite posh, given that the wine trade is quite posh.
(one might even exchange posh for pretentious, though I wouldn't say that to my brother :-)
 
4:14 PM
it's not so much the poshness, there are regional differences in the pronunciation that are specific to Spain
 
@EmilioPisanty that's true of the UK as well of course, but regardless of what part of the UK you pick the posh end (or those who consider themselves such) tend to speak BBC English.
 
In Spanish dialectology, the realization of coronal fricatives is one of the most prominent features distinguishing various dialect regions. The main three realizations are the phonemic distinction between /θ/ and /s/ (distinción), the presence of only an alveolar [s] (seseo), or, less commonly, the presence of only a denti-alveolar [s̟] that is similar to /θ/ (ceceo). While an urban legend attributes the presence of the dental fricative to a Spanish king with a lisp, the various realizations of these coronal fricatives are actually a result of historical processes that date back to the 15th century...
there it is
of course it has its own full Wikipedia page
"Ceceo" is how Biggus Dickus speaks
"distinción" is what most Spain speakers use, and it's basically like sounding like Biggus Dickus 50% of the time
¯\ _(ツ)_/¯ they reckon it sounds good
 
There was an article in (I think) New Scientist recently claiming that language varies with diet. The claim is that diet affects jaw development and that affects the ability to pronounce various sounds.
 
yeah, I saw references to it somewhere
it sounded... like more research is needed to see if that's indeed the case
 
It's the sort of thing that sounds temptingly plausible but I'm not sure how you'd prove it.
 
4:20 PM
Does anyone have a better dupe target for physics.stackexchange.com/questions/469173/… ?
 
This is a tongue twister if I've ever seen one
 
'Let the axiom of countable choice be accepted' reeee
 
"The sixth sick sheik's sixth sheep's sick" leaves it for dead. ;)
 
@bolbteppa Yeah that part bugs me...
Guess it can't be avoided tho
 
@EmilioPisanty Aren't you in Barcelona? How's your Catalan?
At least countable choice seems intuitively reasonable, but of course intuition is rarely reliable when it comes to transfinite sets.
And (of course) omega inconsistency blows it out of the water.
 
@vzn "One option is that they are truly made of five quarks, with all of them mixed together evenly within a single hadron. Another possibility is that the pentaquarks are really a baryon and a meson stuck together to form a loosely bound molecule" Although a genuine pentaquark would be kind of cool, I'm betting on option b.
 
5:09 PM
@JohnRennie how’s humour grasp on reversible processes?
 
@JakeRose speelchuck error?
To be honest my grasp of thermodynamics in general is a bit hit and miss. The thermodynamics courses at uni seemed designed only to allow the construction of especially sadistic exam questions and I didn't find that especially motivating.
 
Was that physics or chem?
was John Ellis lecturing back in them days?
 
I suffered through thermodynamics questions in physics, chemistry and crystalline materials. Of all of these the crystalline materials was the most enjoyable since it was interested in what thermodynamics can do for you. Actually 1A crystals was a good course (I got a first in it :-).
@JakeRose I don't remember ever getting lectures from John Ellis. But to be honest I've forgotten most of the lecturers.
 
Do you think you could give me some advice on reversible processes? Just the concept of them
I find it nearly impossible to wrap my head around
 
In thermodynamics?
 
5:16 PM
(I have a specific example to work through conceptually if you’ll allow it)
yes
 
@JakeRose yes, go ahead, though I can't promise to help
 
One of the common definitions of ‘reversible’ is that if we take an infinitesimal step one way, for example putting some heat into a system from then surroundings, and then reversed this step the system and then surroundings would be in the same states as before
why does this not occur when it’s a finite step?
 
vzn
5:30 PM
@PM2Ring viva la reductionism, maybe something yet left to be squeezed out of LHC )( o_O
 
@JakeRose the fundamental property of a reversible process is that the system is in equilibrium all the way through the process.
This matters because properties like pressure, temperature, etc are only defined when the system is at equilibrium.
 
How so?
as in why can’t you define a temperature when it’s not at equilibrium?
 
Because a temperature, like a pressure, is a statistical property. A temperature is based on the velocity distribution of gas molecules, and that profile only matches a Maxwell-Boltzmann distribution if we have equipartition on energy.
 
Not exactly sure what that means
oh I suppose I do
but you can have equilibrium processes that aren’t reversible
 
@JakeRose can you?
(genuine question, offhand I can't think of any)
 
5:45 PM
Piston with friction
operated infinitely slowly
 
I think introducing non-conservative forces is cheating :-)
 
ahh fair
its just something that comes up in a lot of the thermo books you see :)
 
The point is that in a reversible process from A to B the state of the system is well defined at every point along the path from A to B.
If you have irreversible processes then the pressure etc are no longer well defined.
 
Why does that mean it can be reversed though?
 
heyo
 
5:49 PM
i dont see the connection to reversible
howdy @Danu
 
sup
 
Take your example of a piston. If you yank out the piston at speeds approaching the speed of the gas molecules then the gas can't equilibrate on the timescale of the motion. Likewise if you shove in the piston at those speeds.
The result is that the force on the piston becomes dependent on the speed, and is also different during the compression and expansion stages.
 
hi John! :)
 
@Danu Hi :-)
 
So the reversing the process has actually left a permanent change
 
5:52 PM
Yes. The system won't go back to the state it started in.
In the example I gave I think (though I wouldn't swear to it) that you end up with a raised temperature because more work is done on the compression than on the expansion.
@JakeRose Actually, now I think about it this is quite straightforward. If the piston is moving outwards at a velocity v then it reduces the collision speeds of the gas molecules with the piston. If the piston is moving inwards with velocity v then it increases the collision speeds of the gas molecules with the piston.
So the force on the piston is always lower on the expansion than on the compression.
To get reversibility you need the piston speed v to be negligibly small compared to the speed of the gas molecules. Under those circumstances the pressure at at particular volume is the same whether we're expanding or compressing the gas.
 
6:13 PM
physics.stackexchange.com/questions/469212/… New OP with 2 closed homework questions within an hour. Let's see if they go for the hattrick. ;)
 
 
1 hour later…
7:17 PM
So I'm no physics expert, but I'm curious. In GR energy-momentum isn't conserved due to cosmological redshifting. Are there any other examples of violations of conservation of energy in physics (not necessarily cosmology)?
 
@SirCumference Depends on what you count as a violation. You might well count non-conservative forces (friction) in classical mechanics as violations, since classical mechanics is usually not concerned with the heat that friction generates at all. Or consider that conservation laws like that only hold as laws for expectation values in quantum mechanics.
The way GR violates conversation of energy is still somewhat special though in that it says energy is not conserved even if you consider the whole universe, while energy usually becomes conserved in the other cases when you consider everything you left out
 
@ACuriousMind Hmm, well let's consider a static universe for the moment. Are there any phenomena where, even if we consider the entire universe, the energy will not be conserved?
 
@SirCumference No, because energy conservation and time translation invariance are inevitably linked through Noether's theorem, and when you declared the universe to be static, you declared it to be time translation invariant!
 
Huh, fair. But just to make sure, in an expanding universe we could never observe an increase in total energy right?
Granted I have exactly zero experience with GR so I'm hesitant how much my classical intuition, e.g. with Noether's theorem, apply in the same way
 
7:34 PM
I'm not convinced the phrase "total energy" is well-defined in the case where energy is not conserved. In the end, energy is an abstract notion, not some sort of "stuff" that inhabits the world. When it is not conserved, it is not very useful to think about it
I mean, energy is not even Lorentz-invariant - different observers disagree on its actual value. A Lorentz-variant value that isn't even conserved per-frame is a not a quantity you want to look at in GR: It just doesn't mean anything
 
I mean yes it's not, but at least in SR it is necessarily nonzero. My question is, e.g. if the universe were expanding extremely slowly (and therefore cosmological redshift happens slowly), then after some process, could we observe an increase in energy over time even if we consider the entire universe?
 
If the universe is not finite in spatial extent "total energy" isn't necessarily a meaningful concept, either!
 
Then what is meant by "energy-momentum is not conserved"?
If we could argue that the concept is meaningless, that is
 
@SirCumference Noether's theorem is not only a statement about the global value of some quantity, it also establishes a local continuity equation for its conserved charge - the charge inside a volume can only change through a current leaving it.
 
Ah, right, now I think I'm starting to get it
Well sort of at least...I need to think for a little
 
7:49 PM
@SirCumference Note that, if you're willing to say that "the gravitational field" has energy, you can define the Landau-Lifshitz pseudotensor that then obeys a local continuity law. But this is not a unique choice for attributing such an energy, and there are others in some cases like ADM or Bondi energy
The core problem really is that the notion of energy is only both useful and unique in the rest of physics because of time translation invariance. I'm convinced that many people only keep trying to shoehorn it into GR because all the intro physics long before Noether's theorem treat energy conservation as this sacrosanct law that no physical theory may ever dare to violate. :P
 

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