The h Bar

12:00 AM

@peterh Ok, well I strongly disagree.

I went hiking with Rebecca and realized I didn't understand why parallel transport is an isomorphism. I'm obsessed with differential geometry.

DanielSank

Let me show you an example of a question which fits your requirements but is a terrible question:

"What is Newton's second law?"

0celo7

Made it very hard to enjoy the hike.

DanielSank

Here's another one:

ACuriousMind

@0celo7 Not particularly. Like, six beers over six hours.

DanielSank

12:01 AM

"What is the final speed of a ball which has acceleration 2.3 m/s/s for 1 second, starting from rest?"

peterh

@ACuriousMind Don't overcombine

ACuriousMind

I've been on here much more inebriated :P

DanielSank

Those are both not good questions.

@ACuriousMind Who cares?

You are aware that the prime action of living organisms is to reproduce, yes?

ACuriousMind

@DanielSank No, I deny the validity of evolutionary psychology.

peterh

@DanielSank The MathSE has a rule for similar questions: it is allowed to answer them, but not with a result, only with a hint, how can you calculate it.

ACuriousMind

12:03 AM

Even most of our direct genetic makeup is not selected for fitness - most mulations we carry with out are actually neutral.

DanielSank

[citation needed]

peterh

@DanielSank I also think questions of the elementary school should be offtopic. Other similar, numerical calculations should be answered by a hint, and not with a result.

DanielSank

@ACuriousMind Ha! Then you would be a blasphemer in my religion.

@peterh Ok, well I disagree :)

I am not interested in seeing a list of questions like this:

1) What is the final speed of a ball which accelerates for 4.3 seconds at acceleration of 4.32 m/s/s?

2) What is the final speed of a ball which accelerates for 6.2 seconds at acceleration of 19.32 m/s/s?

3) What is the final speed of a ball which accelerates for 14.2 seconds at acceleration of 6.32 m/s/s?

Do you see my point?

peterh

@DanielSank The second would be closed as duplicate to the first, where only a hint would exist how can it be calculated.

DanielSank

It's not a duplicate.

They have different numbers.

This is exactly why we tell people to ask a concept question instead of asking with numbers.

peterh

12:06 AM

@DanielSank I see, but if you ask "how can I multiply 13 with 72" on the MathSE, it wouldn't be a big chance even there.

Sanya

4) How do you solve the Harmonic Oscillator? 5) How do you solve it in QM? 6) How do you calculate the potential of a point charge in front of a conducting plate?

DanielSank

This is actually a very important aspect of physics: we organize things into laws and patterns so that we don't have to handle all these numerical cases separately.

We ask users to respect this in their questions!

peterh

@DanielSank And this is interpreted on a way that many questions are closed as "homework-like", even if they show the concept and some effort.

Sanya

that's also not really fitting questions as they are so STANDARD that they can be found ANYWHERE

DanielSank

@Sanya Actually, those are less bad :)

Sanya

12:07 AM

and they still get asked a LOT

DanielSank

@peterh Ahhhhhh, yes. The homework close reason needs to be fixed. I completely agree.

As I said, he have an ongoing discussion about this.

Sanya

@DanielSank luckily, less bad and acceptable are not synonyms

DanielSank

However, @peterh, I doubt there are questions that ask a concept but get closed. Actually there are some, as you pointed out, and they should be re-opened, and we should educate users about them.

@Sanya The charge in front of the plate could be ok if the person asks something specific, like how to think about method of images in that case, etc.

0celo7

@ACuriousMind What does this mean?

peterh

@DanielSank Yes, but its reason wouldn't be really bad. Maybe... the "ask for a concept", is too harsh, it would be enough if it wouldn't be required, but the OP would get only a hint and not the result.

ACuriousMind

12:08 AM

@DanielSank I'll gladly blaspheme, then. I highly doubt that selection pressure acts so strongly on high-level behaviour that it can be explained as the result of natural selection. Do you have any evidence showing otherwise?

DanielSank

@ACuriousMind No, which is why I consider it my religion. Words were chosen carefully.

@peterh I think it's important that we reject questions that could be asked a hundred different ways with different numbers.

This is an important part of physics, in fact.

There are questions where the numbers are important though!

ACuriousMind

@0celo7 It means that selection pressure on most genes is not strong enough to actually cause "selection", i.e. many properties of present-day organisms are not caused by selection pressure.

peterh

@DanielSank I already said, they should be closed as duplicates to their first versions, where the answers would only contain hints (i.e. how can you calculate it), and not the numerical results.

DanielSank

For example: how do the equations of motion for a particle in a liquid change, qualitatively, as the Reynolds number goes from >>1 to <<1?

0celo7

@ACuriousMind Organisms in general or just humans?

DanielSank

12:11 AM

@peterh Ok, well, I disagree.

I guess we must agree to have different opinions.

0celo7

@ACuriousMind Have you seen this def of topological space before?

Sanya

@DanielSank sure - but usually this is not the case; I've been here for not a long time and the amount of flags I've raised is really not a good sign ...

0celo7

it's the one given in my analysis text, although the prof uses the standard one.

ACuriousMind

@0celo7 Both. My point is that it is invalid to say that just because a currently extant organism carries a partiuclar configuration, that this configuration has arisen because it is evolutionary beneficial.

peterh

@DanielSank Ok. The reason, why I didn't take really part in the HW thing until now, that I see a lot of text, which is not about the real problem. The real problem is, that the VtC voters are irrationally harsh. Even the current hw close reason wouldn't be really bad, what is bad, how is it applied.

0celo7

12:14 AM

@ACuriousMind Yes, I understood your point.

You sound like a conspiracy theorist is all.

peterh

@DanielSank Furthermore. If somebody asks, what is the mass of 1 $m^3$ 20atm air, the answer would be the universal gas formula (in simpler case, for ideal gases, not the van der waals version or more complex), re-ordered to the actual problem. But, without the actually made calculation. This is what the MathSE does, these are the "hints".

kpv

@DavidZ You edited my question, does that not mean you edited it to your satisfaction?

ACuriousMind

@DanielSank Okay...I think we should not discuss this further, then.

0celo7

@ACuriousMind I guess they're just defining it via the basis.

ACuriousMind

@0celo7 I'm just rather critical of evolutionary psychology.

DanielSank

12:17 AM

@ACuriousMind What, you don't like theology?

peterh

@DanielSank And this should be extended with a site similar to the MO. It would be a communication forum for cutting edge-level physicists.

DanielSank

Besides, we can discuss the original issue. It bothers you that people think male/female interaction is always at least a little bit sexual?

kpv

@ACuriousMind what do you want me to change in the question so that you understand it. I have given an example of coin toss, thought that would make it easy to understand.

ACuriousMind

@DanielSank I don't.

DanielSank

@ACuriousMind Hahaha ok.

@peterh Your proposed solution is that all the high rep users here, the very people who answer the most questions, including newbie questions, should leave and go to a different site?

That sounds ill advised.

ACuriousMind

12:21 AM

@DanielSank Ah, yes, I dislike that people thought that just because I had a lively conversation with these women there was some implication that they wanted to get with me. I know for a fact that none of them finds me attractive in that sense.

peterh

@DanielSank No, it would be a suicide. I didn't think a lot on the details, but maybe a "physics enthusisast" or "physics learners" site could be started. The physicist users of the site are far too important to harm them on any way.

DanielSank

@peterh Those already exists.

quora.com

0celo7

@peterh What?

DanielSank

I think people come here because our high quality standards attract good users.

peterh

@DanielSank Yes, and also the PO does exist.

@DanielSank Currently I am thinking in the possible SE-internal solutions.

0celo7

12:25 AM

I wonder, did skillpatrol die?

DanielSank

@peterh Ok I think you are talking about two (related) issues:

1) You think certain posts are closed even though they ask conceptual questions. You think these posts should not be closed.

2) You think we should support questions like "What is the acceleration of a mass 3kg subjected to a force 4N"?

I agree with 1, but I disagree with 2.

I think to address 1, please participate in our revision of the homework policy.

I think for 2, forget it. It will never happen.

I have to go for now.

Goodbye.

peterh

@DanielSank 1) agreed 2) As I already explained a) I don't think questions belonging to the elementary education should be ontopic b) they should get only conceptual answers (hints) 3) their dupes on the future should be closed as dupes.

@DanielSank I said (2) already twice in our current talk, it was the third time. I can only hope, you had the possibility to understand it before your leave.

@kpv Are you still here?

kpv

yes watching

@peterh yes

Sanya

reallife problem

there is a fly sitting on my laptop which doesn't want to move, so I can't close it

gna >_<

vzn

12:58 AM

@ACuriousMind lol and what is that based on?

CUORE almost ready for first cool-down / symmetry mag

2

peterh

1:23 AM

@vzn Wow! 1 $m^3$ mK monocrystall :)

vzn

@peterh yeah Big Physics™ is truly amazing sometimes, its almost like out of scifi movies or even better... yet more/ lots of evidence/ indications we are in a golden age of physics both applied/ theoretical...

peterh

@vzn As I understood, they try to detect neutrinoless beta decay. But, how will they do this?

vzn

@peterh (roughly) extremely subtle neutrino events could cause extremely small temperature fluctuations (increases). its an energy detector for extremely small energy bursts. not sure of the theory that would definitely ascribe it to neutrinos.

peterh

1:44 AM

@vzn Ok, but if a neutrinoless double beta happens, compared to a normal double beta, the only difference is the two neutrinos.

@vzn ...maybe these two neutrinos don't take away energy.

vzn

1:59 AM

@peterh so you have some understanding of neutrino decay? its all conjectured and could turn up a null result. afaik we can barely even measure neutrinos let alone neutrino decay. its possible it hasnt turned up anything "anomalous" so far after ~~vast~~ very substantial sums. that seems to be a theme with Big Physics™ these days, but sssh dont dare comment on the emperor wardrobe, dont rock the boat, just marvel at the awesome scale like modern 21st century cathedrals :P

2physics

@peterh @vzn good morning

peterh

@vzn I only think, Occam's razor would delete the right-handed neutrinos from the picture

DanielSank

5:38 AM

Anyone here?

I need to talk about how scientists are supposed to fit into the scientific literature review process.

Secret

6:25 AM

What aspect? The issue of open publishing, the peer review process, the editor, the community or the integrity?

David Z

@kpv I improved everything I could find to improve, within the bounds of what constitutes an acceptable edit (given that I'm not the poster of the question). It doesn't mean I now think it's on topic.

Secret

7:03 AM

[Reading classical mechanics the theoretical minimum] It is easy to see the universe is a closed system, but is it also a clopen system...?

Secret

7:29 AM

sciencedaily.com/releases/2016/09/160902142132.htm

> With this disruption now documented, Newman and colleagues are currently focused on studying both its causes and potential implications. They have two hypotheses for what could have triggered it -- the particularly
strong El Niño in 2015-16 or the long-term trend of rising global temperatures. Newman said the scientists are conducting further research now to figure out if the event was a "black swan," a once-in-a-generation event, or a "canary in the coal mine," a shift with unforeseen circumstances, caused by climate change.

black swan = Significant and/or suprise
once in a generation = almost untestable hypothesis
canary in a coal mine = a warning of a tipping point

Secret

7:43 AM

Start of the school year = all SE chats are pretty much filled with neutrinos

actually not just all SE chat, but pretty much everywhere I frequented

Exceptions: Some MMORPGs

Secret

cosmosmagazine.com/space/…

The north pole looked really turbulent

space.com/…

Juno Listens to Jupiter's Auroras

►

Secret

8:09 AM

sciencedaily.com/releases/2016/09/160901093331.htm

What acuriousmind, Danu, Slereah, my honours research group warned about: Putting a picture without explaining it, people are nto going to get what you are trying to say. But there's more: You need to transform the picture in a way that is familar to the way the audience think, in order to lt them better understand

Secret

sciencedaily.com/releases/2016/08/160831224020.htm

As mentioned in my profile, I like cooling technologies. It seems this functions like bundling two heat engines together, and using the atmosphere as a heat sink

Secret

sciencedaily.com/releases/2016/08/160831162953.htm

I wonder, if part of our habits can be explained by a certain composition of microbes

colonyb.com

Apps become a very convenient platform to collect experimental data

I had a feeling this game is just teaching computers how to identify a cluster of dots so that it can be used in the data

Secret

sciencedaily.com/releases/2016/08/160831111000.htm

A new game in town in the quest for invisbility. Metamaterials are the old stuff

Secret

8:25 AM

sciencedaily.com/releases/2016/08/160831103023.htm

For a toxic metal, mercury sure has an interesting role in the ecosystem

Secret

sciencedaily.com/releases/2016/08/160830130826.htm

This is why I prefer face to face communications, especially because of my communication habit often relies so much on gesture and affect in order for someone to understood me

This is slowly improving, thanks to the advice by acuriousmind on how to make a sentence sound more like talking rather than thinking

(modulo me tend to misread sources...)

sciencedaily.com/releases/2016/08/160830091625.htm

Symbols are multidimensional, thus they have more flexibility in storing a meaning

sciencedaily.com/releases/2016/08/160830101344.htm

Some old things are quite useful because they are simple to implement and thus less chance to go wrong

Secret

8:56 AM

scitation.aip.org/content/aip/journal/jcp/144/21/10.1063/…

Hmm, a dynamic density of states...

Secret

And finally, for those who are wondering, another reason why I tend to post like the news informing style of vzn as shown above (minus the comments) is because most of the time when I am active in this chat room, people are doing stuff in their afternoon in the northern hemisphere, thus there is basically no one I can talk to

as such, one can imagine the response are actually delayed to until the h barers get onto the chat again. While a mentoned by ACM and many others this is not realyl communicating, I do know a couple of people do read these posts and comment.

Once people started getting on, I switched back to the new communication style learnt from my interaction with ACM, danielsank and 0celo7, and thus readers will find my posts beocme shorter again

Sanya

@Secret good morning :)

Secret

hi

Sanya

what's a clopen system?

Secret

9:12 AM

I am not sure if it makes sense, but since in maths we can have sets that are both open and closed (hence clopen), and the universe is an interesting system that is in some sense consist of everything (hence nothing external to it) but unbounded (hence whether we can actually say it has something external to it is (I have no idea)), then perhaps the universe, in the traditional sense, might be both an open and closed system at the same time

(For the sake of discussion, I am ignoring the issue of multiverses and use the universe as a word to refer to everything)

Sanya

hmm, ok, I get your point

Secret

Actually another more philosophical question will be, is the notion of "something external" well defined for an infinite unbounded object?

Sanya

even though, coming from classical thermodynamics, an open system can exchange energy and particles with its environment - and for me, that is the opposite to a closed system, so I wonder about the mathematical open&closed applying here. It's an interesting question and I'm just thinking

I liked that article about text interpretation regarding emotions by the way

Secret

I think for that, we need to figure out how to define a notion of something external on an object of infinite extent, though my gut feeling and learning in classical mechanics suggest that the universe should be a closed system since it contains everything

Sanya

even though I'd find it interesting to have a look at the age group of "digital natives" or look specifically for people who spent lots of time in IRC/Instant messaging to search for differences

Secret

9:18 AM

For text, there is simply too many ways to guess the emotions convey behind it, I don't think it is in general possible

Sanya

@Secret well, I'd tend to agree with the last part of that

Secret

and this study also disprove what I previously said about that with time and epxerience, one can guess the emotions and tone of speech used in a typed message

Sanya

@Secret in general from unknown people, no, I agree. For people you've been knowing a long time and only via chat it'd be interesting to have a deeper look

Secret

For my case is very extreme, because based on experience and ACM's observations, it seems my habit of 'communication' caused me to have more than 70% is actually in the form of affect, which might also explain why people have a hard time understand what I am writing in my draft 1 of my honours thesis and my messages in general

Sanya

" it seems my habit of 'communication' caused me to have more than 70% is actually in the form of affect" I don't understand that part

Secret

9:22 AM

Basically, when I communicate face to face, I use a lot of aids besides my speech, such as gestures, diagrams, emotions and tones

If you ever saw me talking, I am like moving my whole body throughout the chat

and moving about

Sanya

ah ok ... yeah well, that part is obviously missing

Secret

However, in any written media, the only thing I can use is text, and as a result, a lot of information is missing

Sanya

but on the other hand, that should be one of the reasons written language is usually different in style from spoken language

Secret

most importantly, are all the cues that other people have noticed that allow them to connect the dots between seemly disconnected points in my chats

@Sanya Yes, written language is a lot more descriptive, for journalism, a lot of emotional words and adjectives were used. For academics, things are presented impersonally and in a logical fashion step by step

Sanya

yeah, in general you'll need to try harder to make your concept or line of thought visible in written forms like here because you can't look into peoples eyes and see whether confusion arises or not

a bit like the difference between writing a script/text book and holding a lecture :D

Secret

9:28 AM

ACM and my honours co-supervisior Kelvin are examples of a group of people that often rely on me elaborating those cues in order to understand the flow of logic in my thought process

Via talking with them, I have came to become more aware of the various ways I communicate differently from most people thus allowing me to slowly adjust that issue on making the connection between the points more visible in text form

As ACM said

Sanya

well, that is a process we all need to go through to some extent - but I'm positive it is getting better judging by what I can read from you here today :)

Secret

Aug 15 at 16:20, by ACuriousMind

@Secret In physics, text communication is the most efficient communication there is. You can't convey a precise physical idea through "body language". The reason I often don't understand what you're saying is simply that you tend to create very long posts that have erratic line breaks and don't seem to possess a clear thread of reasoning holding them together.

Aug 24 at 12:54, by ACuriousMind

Communication takes effort. Communicating your ideas to other people is different and harder than just preserving them for yourself.

Sanya

ACM can be a bit harsh ... real German

Secret

Kelvin's case is more extreme than ACM's, because he can be confused by me even when face to face. However this might have a lot to do with me being inexperienced in the field, said a lot of things that are actually nonsense while I don't realise

Kelvin's analoguous case in ACM and yuggib's is when we discuss anything about groups, and other mathematical formalism, ofteb because I use the wrong terminology, they become confused on what I am talkign about

Sanya

well, that is part of the supervision process, that's what it is for

but yeah, especially when talking about math, using the right terminology is essential to make people understand what objects you actually mean

2physics

9:34 AM

fellas I have a question, may I ask?

Secret

shoot

2physics

what does gravitational length mean?

Secret

what context you saw that term?

2physics

more precisely:what does it mean "gravitational length of electron/proton" is smaller than Planck length

Secret

@Sanya Also another interesting to point out: Often it is the cosupervisor that I have issue getting along with. One reason is that they, not the supervisors are the ones who have the longest contact hours with me, and hence very likely to experience whatever weird personality traits I have that result in a communication gap (as well my disorganisation)

but at the same time, they are the ones who can see deep into my personality and not become sidetraked by the chrisma that is a side effect of my passion in science

Sanya

9:39 AM

@Secret a good cosupervisor would however know that this is part of the supervision process :)

Secret

and he does

https://www.physicsforums.com/threads/how-can-the-planck-length-be-claimed-to-be-the-smallest-length.233448/

I think it is the length scale when quantum graivty becomes important, otherwise I never heard of that terminology before

Sanya

but well, the skills you learn about presentation of your ideas and line of thought while writing your thesis are almost as important as the content after all - especially if you leave science afterwards

Secret

Indeed

Sanya

actually, what exactly are you working on?

Secret

you mean my studies or my personal projects due to hobbies?

or do you mean my honours thesis I have done last year?

Sanya

9:44 AM

ah, that was last year? I thought you were still at it

but yeah, I was thinking of your thesis

(honours is ... Master?)

Secret

honours is something before masters. Like a 4th year undergraduate

it is there to give you a taste of research,and thus helping you to decide whether you want to do a research masters or PhD

Sanya

ah ok ... here the system is a bit different, but I get an idea

Secret

The title of my honours thesis is "Investigation of the hydrogen production in the photoldissociation of acetaldehyde". Basically the group have found a new photolysis channel that the acetaldehyde molecule can fragment to. Acetaldehyde

Sanya

that sounds interesting. It also sounds like a bit of experimental work?

Secret

is an important intermediate formed from the oxidation of hydrocarbons from industrial processes and plant aerosols. Its ultimate fate in the troposphere is important in climate models and some important cycles

2physics

9:50 AM

@Secret well actually it was the exact question I was trying to know about lol

Secret

The project is a joint epxerimental and theoretical process. On the experimental side we have a vacuum chamber to photolyse a beam of acetaldehyde molecules cooled to 4 kelvin

This is the imaging device we are using

(and yes I made that in powerpoint and sketchup)

this beam, after being photlysed, creates molecular fragments

The photolysiss is done by a laser tuned at a wavelength corresponding to a certain photolysis channel

Another laser is then being used to ionise a fragment we are intertested in . In this case its hydrogen

These ionised hydrogen is then accelerated through a tube by an electric potential, which then strikes a phosphoresent screen

The camera then relay the light emitted from the screen to produce an image that showed the velocity distribution of the fragments

The idea is that the velocity and the positions are correlated. Thus the further away the dot is from the centre of the image, the faster they are actually travelling and hence higher kinetic energy

This distribution thus give us information on what happened in the photolysis, that is the mechanism of the reaction

Chemical reactions typically have two broad type of mechanisms: Transition state and non transition states. For transition state reactions, our moelcule once receiving enough energy, will rearrange its atomic configuration into some high energy transient state (transition state)

It then quickly roll downhill in energy into reaction products

The energy difference between the height of the energy barrier to the transition state, and the energy pumped by some external process such as the laser, will reflect as the kinetic energy, rotation and virbations of the fragments

For the theoretical side, without getting too technical, we basically perform quanutm chemistry calculations to work out the energy of the molecules and the transition state, and find out the geometry taken by these states. The geometry provide informationo on how the fragments are formed hence we have a specific distribution of rotational, virbtational and translation energies

Sanya

@Secret it looks very professional

And you actually got me very jealous of your research environment :) Thanks for the detailed outline

Secret

And when combining the informstion of the distribution of the energy being partitioned into the fragments (product state distributions) between the calculations and the imaging experiment, one can reconstruct the reaction mechanism in the gaseous phase

so that's basically the aim

Sanya

10:06 AM

yeah, that sounds like a thought-through project with a really reasonable concept, it's very interesting

MonaLisaOverdrive

11:11 AM

Holy crap textbooks are expensive. The Sakurai recommended a few weeks ago is $95

Sanya

that's not very expensive for a textbook even ;) but I advise you to go to your local university library if you can

2physics

@MonaLisaOverdrive Can't you find a pdf version online?

Sanya

that is the slightly less legal version :D

2physics

@Sanya yup, less legal :D

Sanya

but indeed it is easily found as pdf online

just checked :D

2physics

11:22 AM

@MonaLisaOverdrive anyway you can borrow some of QM books from here and delete them after you read :D : pnufizik.blogfa.com/page/quantum.aspx

@Sanya paper versions are illegal, and pdf/djvu versions are a little less illegal maybe :D

Sanya

@2physics Oo

2physics

I have a question about natural units in physics; anybody here can help?

Sanya

Î'll try

Secret

11:38 AM

http://physics.stackexchange.com/questions/278165/where-in-universe-does-time-move-faster-than-it-moves-on-earth

pretty much anywhere in space will does it, but not too a very large extent

2physics

I've noticed that in some texts, one has considered $[c = \frac{h}{{2\pi }} = G = {k_e} = {k_B} = {\rm{ }}1]$ as the basis for natural units.

Sanya

what's k_e?

2physics

but in some other sources only $c = \frac{h}{{2\pi }} = {\rm{ }}1$ has been taken into account , which one is the standard form?

Sanya

for me, coming from QFT, the latter, but I'd be prepared for both

2physics

@Sanya $[{k_e} = \frac{1}{{4\pi {\varepsilon _0}}}]$

@Sanya well which one is known as the natural unit system normally ?

how do you know you have to gravitation constant equal to $1$ or $6.7*10^-57$

Sanya

11:47 AM

from whether it appears in equations or not

2physics

in this case, which is claimed to be presented in natural units: ($\sqrt G \le \Delta x$) how do you know $G=1$ or $6.67*10^-57$

Sanya

which unit is \Delta x in? I mean, as long as you use unit systems in a consistent manner and constants are actually written out, the expression is valid in any unit system - it only becomes bothersome if constants are set to 1 and then left out

because then you need to use those units to obtain right numerical results

peterh

@Secret I suspect he is asking for a place where it is more significant.

2physics

12:02 PM

@Sanya it doesn't talk about the unit of $x$ it only said that the natural system is used and we have this relation; and it hasn't mentioned $\hbar$ and $c$ because they have been normalized in natural units calculations.

so my question is how do I have to know if the $G$ is equal to $1$ or not?

Sanya

eh ... I think if it was 1, they wouldn't write it down

but if they don't state it ... ...

2physics

ok, but I don't know when we talk about natural units, which one should be used..

Natural units

In physics, natural units are physical units of measurement based only on universal physical constants. For example, the elementary charge e is a natural unit of electric charge, and the speed of light c is a natural unit of speed. A purely natural system of units has all of its units defined in this way, and usually such that the numerical values of the selected physical constants in terms of these units are exactly 1. These constants are then typically omitted from mathematical expressions of physical laws, and while this has the apparent advantage of simplicity, it may entail a loss of clarity...

MonaLisaOverdrive

I'd feel (slightly) guilty for grabbing a pdf online

But man…spin is killing my brain

2physics

@MonaLisaOverdrive you can read it online, like borrowing a movie

Sanya

@MonaLisaOverdrive this is the way to slow research success

peterh

12:10 PM

@2physics Planck units are the most popular.

John Duffield

@2physics : the problem with that, is that some "universal physical constants" aren't constant. Which means that your "natural" system of units isn't natural.

2physics

@MonaLisaOverdrive you also can invent the QM theory from begining :D

@JohnDuffield like which ones?

John Duffield

See for example the fine structure constant: "Thus α depends upon the energy at which it is measured".

2physics

@peterh if Planck's natural units consider $G=1$?

MonaLisaOverdrive

@2physics: No. Just…no. I'd rather not melt my brain

Off to work on an Android port of powershell :-/

peterh

12:16 PM

@2physics Yes, and this is the reason, why they aren't used more widely. We can't measure G enough accurate, the current best results are only exact until around 4 digits.

@2physics If G could be measured until 12 digits, it would be a funny shift to rewire the whole SI system to the Planck units.

John Duffield

@2physics : note that α = e²/4πεₒħc. So if α varies, some of the other terms have to vary too.

2physics

@JohnDuffield well it talks about fine structure const, not merely $c$,$\hbar$,$G$, and $[\epsilon ]$

killed me to type it lol

@JohnDuffield maybe there are some other variables which can affect $a$ or maybe it's only $e$'s problem

@peterh do we need to measure them?? we consider $c$ equal to $1$ , with no need to measure it. We just need to define what velocity is.

John Duffield

@2physics : I'm happy with conservation of charge, which would suggest that e doesn't vary. And I'm happy that π doesn't vary. I'm not sure about ħ or whether Planck's constant varies. But I know that c = √(1/εₒμₒ), and I know about the second paragraph here.

Einstein said "the curvature of light rays occurs only in spaces where the speed of light is spatially variable".
But as you said, "we consider c equal to 1". Or at least, some of us do.

I don't.

And nor do I talk about fine-tuned constants and the Goldilocks multiverse.

2physics

12:37 PM

@JohnDuffield I'm not sure, please correct me if I'm wrong: suppose we want to set some units to measure phenomena

we want to make it precise , such that any scientist from mars or any other planet when comes to earth, can understand our calculations using our units. means he can convert our units to his units on earth

therefore we consider some natural facts to use

for example , first we know what distance is, and define time as the period between to transitions of the cesium atom

peterh

@2physics Ok, but then we need to measure what is 1m. Just as now. The question is, to what we measure, what is 1m.

2physics

then there is an other natural fact: the distance which light can pass between the period of occuring two transitions of the atom of for example cesium

and we set it to 1, which means we set the $c=1$

we just used two natural facts: 1-the amount of time between two transitions of an atom, and 2-the distance which light can pass while occurring the aforementioned transitions

peterh

13

Q: Are we a basically friendly lot?

A question was asked in a recent chat whether the Physics SE was getting ruder and/or unfriendlier. My immediate reaction was that it wasn't but then I was surprised to be criticised (in a constructive way) for being unfriendly in a comment. I had suggested I thought the question was a homework p...

discussion

It seems, 4 years wasn't enough to solve the problem. Maybe now :-)

2physics

and other facts like $h$ and $G$.. can be taken to account to create the unit system. well, so we can claim that our units system doesn't depend on our measurements , and it is independent from the measurements errors. is it true or not?

peterh

12:53 PM

@2physics Units are for practical needs, i.e. you measure something by comparing it to a unit.

2physics

@peterh well when you can't find the precise value of $\pi$ it doesn't mean you shouldn't use it in your equasions

peterh

@2physics Some tens of millions of digits is enough for practical use.

2physics

"practical use" depends on the period of time we live in. and besides many of today's practical uses are the results of theoretical works of early times.

I didn't understand what @JohnDuffield means.. facts are facts; they are independent of our measurements, aren't they?? does the distance which light passes in a period of time now can differ from what it passes 1000 years later ??

John Duffield

1:09 PM

@2physics : so we define the second as "the duration 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". That's like sitting there counting nine billion microwaves pass you by, then jumping up and saying "that's a second". Then we use that second to measure the speed of microwaves.

Duh.

John Duffield

1:24 PM

See arxiv.org/abs/0705.4507

ACuriousMind

Why is there a question that was last edited on Feb 9 '14 in the reopen queue as "edited after it was closed"?

John Duffield

1:44 PM

Oooh, that's interesting. I've likened the Einstein-de Haas effect to my garden hose reel, and there it is in a book called quirky sides of scientists.

peterh

@ACuriousMind I suspect old db + new query side-effect

2physics

2:11 PM

I disagree with this.

John Duffield

2:26 PM

The constancy of the speed of light is a tautology. It's to do with the wave nature of matter. For an analogy, chuck a clockwork clock into an oil bath, and watch it go slower. But then watch the clockwork man jump in after it, and claim that it doesn't.

2physics

@JohnDuffield I don't know about the constancy of light . I only disagree with the way they have rejected the claim

I really can't believe this. it's annoying

John Duffield

@2physics : Magueijo and Moffat are right. Think about what I said above. We define the second using the motion of light, then we use it to measure the motion of light.

2physics

it was from one of OSA journals. I'm really tired of revising and modifying it and preparing comprehensive answers

Transcript for