The h Bar

Slereah

8:10 AM

If you don't need to get GR involved try reading Gourghoulon maybe

he goes on a lot about accelerated frames

Avnish Kabaj

You know you're illiterate when science sounds like sorcery to you

Slereah

it is

Avnish Kabaj

The forces of light came together for one final battle against the Gourghoulon, set to change the destiny of the entire universe.

Slereah

Gourghoulon is just the name of the dude

luth.obspm.fr/~luthier/gourgoulhon

undefined

10:13 AM

If one talks about frame of reference, what exactly does one refers to?
Like any room/space one is observing? Or how would you describe to one not familiar with physics what a frame of reference really is?

John Rennie

@undefined that's been asked on the main site in various different forms.

undefined

@JohnRennie yes, I have seen that. But all those questions seem like asking more specific questions (for example "What is a homogeneous and isotropic frame of reference?" and so on. That's whyI thought my question is too simple for the main site

John Rennie

A frame of reference is just a way of labelling points in spacetime. For example the usual Cartesian coordinates assign labels (x,y,z) there x, y and z are the distances of the point from us measured along the three axes.

undefined

10:29 AM

so the space enclosed by x, y, z would be a frame of reference?

John Rennie

If you choose three perpendicular axes, x, y and z, and an origin that's one possible frame of reference.

undefined

I mean, is like "a piece of a map"? Can I take a map, label three point + my own point and could call that a frame of reference?

John Rennie

In mathematics the word map has a specific technical meaning. Did you mean this, or do you just mean the usual sort of map we use for car journeys?

Slereah

"Spaces of constant curvature doesn't use the Einstein notation

shameful

If you really want to learn about moving frames in GR check out MTW

there's a whole chapter on it

undefined

oh I didnt know, I meant more the car journey map type

thanks @Slereah

John Rennie

10:37 AM

@undefined yes, you can take a regular map, stick a pin in it to mark the origin then use distances measured from the pin as your frame of reference.

undefined

that makes it more clear. thank you very much :) @JohnRennie

are there differences in terms/using or meaning of "frame of reference" between SRT and GRT?

John Rennie

No

Though in GR the coordinates are generally curved while in SR they usually aren't. But we can use curved coordinates in SR as well e.g. Rindler or Milne coordinates.

Actually even regular polar coordinates are curved ....

undefined

thanks :)

John Rennie

Now you're going to ask me what curved coordinates means :-)

undefined

I have a slight imagination of it. I think they are curved because the space time is curved too (at least in GR) and if we want to describe a path we have to account the curvature too

John Rennie

10:53 AM

@undefined let's us 2D, x and y, to keep things simple, and use the usual x and y perpendicular axes so we label points as (x,y). OK so far?

undefined

alright

John Rennie

Now consider the difference between the two points (x,y) and (x+1,y).

The distance is just 1 unit. Yes?

undefined

okay

John Rennie

And it doesn't matter what the values of x and y are. This distance is always the same. Yes?

undefined

the distance between x and y or between (x,y) and (x+1,y)?

John Rennie

10:59 AM

the distance between (x,y) and (x+1,y)

undefined

alright, thanks

John Rennie

But when you look at a map the grid drawn on the map is actually degrees of latitude and longitude. If we zoom out and look at the while Earth the grid actually looks like this:

undefined

yes, that looks familiar

John Rennie

Take x to be the degrees of longitude and y the degrees of latitude, then the distance between (x,y) and (x+1,y) is shown by the red lines for two different values of y. One y near the equator and one y near the north pole.

undefined

is it something like geodesic?

John Rennie

11:06 AM

@undefined Make sense so far?

undefined

make sense, yes

John Rennie

But the lengths of the two red lines are obviously different. The red line is bigger near the equator.

undefined

yes

John Rennie

So for this coordinate system the distance between the points (x,y) and (x+1,y) does depend on the values of x and y. Specificially it depends on the value of y.

This is what we mean by curved coordinates.

It means that the distance between (x,y) and (x+a,y+b), where a and b are constants, depends on the values of x and y.

In regular Cartesian coordinates the distance d is given by the usual Pythagoras' theorem:

d^2 = a^2 + b^2

Yes?

undefined

I'm thinking about it right now and trying to get a visual image of it in my mind

so the lenght of the red line depends (specificially) on the value of y?

why are a and b constants?

John Rennie

11:16 AM

This is the situation with regular Cartesian coordinates

undefined

2d, right?

John Rennie

Yes

If we start at some point (x,y) and move a distance a horizontally and b vertically then the total distance d that we have moved is the hypotenuse of the triangle.

And the length of the side d is given by Pythagoras' theorem d^2 = a^2 + b^2

undefined

I understand this at all but may I ask why is it ^2?

I know that it IS ^2 but I dont know why

John Rennie

^2 means squared

"x^2" means x squared

undefined

yes I know

but why is it squared?

John Rennie

11:20 AM

You mean why is Pythagoras' theorem true?

undefined

maybe the question is too broad. yes, basically I'm asking why the Pythagoras' theorem is how it is.
I feel like its important for me to know why to really understand things

John Rennie

Pythagorean theorem

In mathematics, the Pythagorean theorem, also known as Pythagoras' theorem, is a fundamental relation in Euclidean geometry among the three sides of a right triangle. It states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. The theorem can be written as an equation relating the lengths of the sides a, b and c, often called the "Pythagorean equation": a 2 + b 2 ...

undefined

teachers in school didnt like me because of that and they never answered such questions

Avnish Kabaj

Hmmm

John Rennie

@undefined to be fair, if you wanted a proof of the Pythagorean theorem you could have just Googled for it. You don't need physicists to explain it to you.

undefined

11:23 AM

but for me, this is almost the most intreressting part to know and to understand why all those amazing formulas are how they are

Avnish Kabaj

You need to know your basics first

undefined

@JohnRennie very true, this question just emerged out of our conversation, I'm sorry

@AvnishKabaj yes, also true. I never really learned them, I guess

Avnish Kabaj

What

undefined

the basics

Avnish Kabaj

You seem to be a middle schooler

I suggest you do everything again

Because if you can't understand the alphabet how're you going to read a book

undefined

11:29 AM

I'm a bit ashamed to admit but im already almost in my mid-thirties

Avnish Kabaj

There's nothing to be ashamed of

undefined

thanks

user929304

1:12 PM

@ACuriousMind hi :)

ACuriousMind

@user929304 hey :)

user929304

@ACuriousMind I dont mean to spoil your christmassy mood, but may I ask a follow up qm question?

ACuriousMind

Sure. Nothing more christmassy than spending my days off with physics ;)

user929304

great :)

it's more a literature related one, namely, I m struggling to find a clear discussion of the "backward Heisenberg EOM" that we talked about briefly while ago. Most textbooks for instance, only show the EOM for time independent Hamiltonian and operators, and never for ones that are explicitly time dependent (to then discuss the backward trick).

do you happen to know of any sources I could read up on it? (even if they are in German I could figure it out with enough time and a dictionary)

ChoMedit

Does someone know how much force we need to split our body?

ACuriousMind

1:21 PM

@user929304 Sorry, I don't know of any - I never really dealt with the backwards Heisenberg e.o.m.

user929304

@ACuriousMind ok, no worries, thanks anyway ;)

Slereah

1:50 PM

The moe grebnesieh?

Secret

2:13 PM

Random thing in my life that felt like quantum mechanics: Siblings:

It is well known that siblings arguments are part of the life of being siblings. In addition, you can sort of get some measure on how much your relationship with your sibling changes by doing some statistics on when and where those arguments as clusters and when and where times when siblings share joy with you are

So as the relationship improves as siblings mature, they argue less, but they do not stop arguing. Rather than "classical" in the sense that the intensity of arguments becoming less, you end up with more instance of sharing joy together, which kinda reminds me of the photoelectric effect

(well at least that's how it works between me and my siblings)

MartianCactus

hello

so i got this question while studying this afternoon

and i have googled it but haven't found a suitable answer yet

why does inertia exist?

ACuriousMind

why wouldn't it?

MartianCactus

more like, why does an object keep going with same velocity unless a force accelerates it?

why does something that moves want to keep moving?

Slereah

you fell for the classic trap

Physics isn't metaphysics

We're not too worried about why the universe has the laws it has

just that we describe them correctly

ACuriousMind

@MartianCactus That's just the way the world is. If I told you "Oh, it's because of X.", wouldn't you just ask, "Yeah, but why is X true?".

MartianCactus

2:24 PM

yeah, the infinite why loop

ACuriousMind

If you think that there needs to be a reason for everything that is, then you'll never find a single reason that satisfies you.

Slereah

Munchausen's trilema

as it is called

Secret

Another similar question is "why is there charge"

MartianCactus

i have been trying not to dwell to much in metaphysics as even if you dedicate your whole life into it, I will just be another philosopher just like so many more. I want to get more into the practical side of application.

but at the same time can't let go off this question..

ACuriousMind

"Why?" is a manifestation of a human tendency to seek purpose. But physics doesn't know about purpose, it's like asking a colorblind what color the sky is.

Slereah

2:26 PM

There's no really satisfying answers to that question.

I mean I could tell you "Because physical laws involve second derivatives"

but that's not really great

ACuriousMind

There are two questions you can ask in physics and that are often also expressed by "Why is this?" - "How do we know this is true?" and "From what axioms can this be derived?". A lot of confusion is generated by people asking the metaphysical "Why?" and getting an answer to one of the latter two questions.

MartianCactus

hm..i guess..but we DO need to understand the universe at its core and understand every "why" after say we become a really technologically advanced universe colonizing species, if we do..

cuz what more will be there to do after that?

Slereah

Baking

Secret

Philosophically speaking, it might be impossible for us to understand, let alone even perceive every "why"

MartianCactus

yeah..and thats kinda sad..

Secret

2:30 PM

We are born with brains wired in ways determined by the rules of biology

there are simply things this brain cannot even perceive and is basically indistinguishable from noise

The best we can do as a species is to get as close to this boundary of what we can comprehend as humans

MartianCactus

and "why inertia exists" is on the other side of this boundary?

Secret

This is why I am excited that one day we can meet extraterrestrials, because they would have a completely different domain which too us will be something that lies in the other side of the boundary

MartianCactus

cant we expand our domains by modifying our brains artificially?

undefined

I for myself like "why questions". They drive me on to read and learn more

Anonymous

@MartianCactus Why would we need the answer to every "why" to colonize the universe? :P

MartianCactus

2:35 PM

i want to go into the field of space exploration as according to me thats the best thing our species can do to develop at this point..

ACuriousMind

@MartianCactus As I said above, you need to tell us what sort of "why" you mean. A physics answer would be to show a derivation of it from the prinicple of least action, cf. physics.stackexchange.com/q/196068/50583

MartianCactus

@Blue im saying after we colonize the universe and find and study everything we have to in this existence(bit too far fetched?), all that would be left to do is answer the fundamental why?

Slereah

I'm pretty sure that in most cases people really want us to tell them that everything is made of springs or magic gnomes

they want to understand the universe in a way that makes sense to our everyday experience

MartianCactus

as that will probably be the only thing we probably wouldnt have done

ACuriousMind

@MartianCactus Why do you think the "fundamental why" is even a meaningful question? Not every syntactically correct question is meaningful.

Slereah

2:36 PM

that's basically what cranks usually do when they try to make a model

everything is make of point particles or something simple

@ACuriousMind that's a furious green idea

MartianCactus

whats principle of least action

Anonymous

@MartianCactus Yes, it is far fetched

Secret

(I am a bit behind in the chat atm, I am going to respond in a few minutes)

Slereah

here's the physicist secret : it's gnomes

little gnomes just keep pushing matter

MartianCactus

but again @ACuriousMind then the next why would be "why the thing you used to prove inertia" exists..

which is again the infinite why

Slereah

2:40 PM

yes

also the real question is

what kind of answer would satisfy you

ACuriousMind

@MartianCactus It doesn't exist. The principle of least action is a theoretical axiom. You can grind the universe into fine dust and sieve it and you will not find a single trace of this "principle" anywhere.

MartianCactus

i guess the "everyday life" one haha

yeah, guess i shouldnt be dwelling into metaphysics, its just a big maze leading nowhere

although it would probably be a good thing to dwell into when im old and (hopefully) have accomplished my goal..

ACuriousMind

Ontology is hard. Inertia, too, is no a thing that "exists". It is the observation that bodies only accelerate due to forces. If I gave the observation that I like chololate the name "Flurb", would "Why does Flurb exist?" be a meaningful question?

Slereah

Because chocolate is delicious

ACuriousMind

Also, while Flurb is certainly very important to me, I do not feel lessened by the fact that I do not understand "why" it is. I just enjoy it.

MartianCactus

2:44 PM

probably because your brain's chemistry works that way?

Our brains like sweet, it gives us lots of glucose.

but again, i get the point

ACuriousMind

@MartianCactus I could point out that there are people who don't like chocolate. Or I could point out that you're giving me exactly the kind of reductionist "non-answer" that you find dissatisfying about physics answers to "Why inertia?".

MartianCactus

physics dwells into the study of the universe, and to a certain extent the question of "why" too but its mostly considered with the study of "how" and not "why"

@ACuriousMind yeah..

Slereah

ever read some epistemology and ontology for physics?

It's some awful shit man

Like Reichenback or Jammer

I hope you like reading about formal systems and congruence

MartianCactus

i dont even know what that means

anyway, something makes me believe that one day, as a species, we will understand the origin of existence and that would clear up all the "why"s

ACuriousMind

@Slereah Well, that sounds pretty firmly in the tradition of analytic philosophy

MartianCactus

2:49 PM

because technically, nothing should exist, if everything has a reason

existence shouldn't be there, nothing should be there..

Slereah

@ACuriousMind it is quite fun

especially compared to the other kind of philosophy

You know what I mean

ACuriousMind

@MartianCactus So since something does exist, some things don't have a reason.

MartianCactus

i believe "why existence is there" will answer all the "why"s

but again, its just a feeling

@ACuriousMind yes..probably..

Slereah

(it's gnomes)

ACuriousMind

"If A, then B" implies "If not B, then not A". It's simple :P

MartianCactus

2:50 PM

yeah..

soo this started with the question of inertia

is inertia a fundamental property?

Slereah

@ACuriousMind you're assuming aristotelian logic though

ACuriousMind

@Slereah Hm?

Slereah

What if we have to use intuitionist or paraconsistent logic???

ACuriousMind

@MartianCactus That's...not really a better question than "why?" :D

"fundamental" is a word with similarily unclear and contentious meanings

MartianCactus

i believe there are some "fundamental" things in physics that cannot be explained and we just have to accept them, and everything else derives from them?

Anonymous

2:54 PM

@MartianCactus Yes, physics is like mathematics in that sense

Anonymous

You have some axioms and you try to explain everything else with those

Slereah

Except axiomatizations aren't unique!

Hell you can even do the worst axiomatization possible

and still get physics

the Craig axiomatization!

or do that naturalness bullshit thing

MartianCactus

so the question to the fundamental "why"s leads you to metaphysics?

but every "why" after the fundamental whys is possible to answer and every explanation of those lead to the fundamental whys

so the question again is, is inertia fundamental?

Anonymous

@MartianCactus Define fundamental :P

Slereah

" Let me provide you with an unnatural theory that I will call Leo Vuyk 2020 theory. He hasn't invented this precise theory yet but he's free to plagiarize me. The theory says that the world is a giant strawberry whose vital characteristic, the Vyukness, is a real number, a parameter known as 𝐿𝑉. It defines the ratio of the diameters of the strawberry and its pit. Strawberries usually don't have pits but Leo Vyuk's strawberry has one.

Now, the parameter 𝐿𝑉 contains the answer to all questions in the world. It's approximately equal to 42.05121973…. Fourty-two was explained in the Hitchh…

The best theory

MartianCactus

2:58 PM

that which cannot be explained or derived..

1+1 =2 is fundamental

you cant go behind it

Slereah

It's not

Anonymous

Is inertia an axiom (in the sense of modern physics)? No

MartianCactus

if you add 2 things the answer is 2

Slereah

You can very much go behind it

I mean you can put it as an axiom

but you can also find axiomatizations where you can derive it

MartianCactus

hm..

so its not fundamental..

but those axioms from which we derive it are fundamental..

Anonymous

3:00 PM

Yeah, I mean you can of course define it as an axiom. But we tend to prefer a minimal set of axioms in physics

Slereah

You can prove 1+1 = 2 using Russell's Principia Mathematica

it takes a few hundred pages

MartianCactus

i guess axioms are fundamental, they just are and you cant question them

Anonymous

@MartianCactus 1+1=2 is not considered to be an axiom (again, in modern mathematics)

Slereah

MartianCactus

so Russel's principle mathematica is an axiom?

Slereah

3:01 PM

your big mistake is to imagine that there exist the list of axioms

there's nothing fundamental about axioms

different axioms can give rise to the same theory

cf the LV 2020 theory

MartianCactus

oh..

Slereah

I'm afraid Jesus didn't write down a big list of axioms for the universe

from which all derive

MartianCactus

so am i right when i say "fundamental cannot be questioned. It just is."?

Slereah

Not really

Anonymous

5 mins ago, by MartianCactus

that which cannot be explained or derived..

Slereah

3:03 PM

I mean if you really want to go that way you can go ask the philosophy SE instead

MartianCactus

yeah..

so now, whats your answer when i ask "why does inertia exist" again?

Secret

Yes and no. With some suitable augmentations, we can e.g. learn to perceive UV signals. And if this universe is purely physical in that all phenomenon is the result of particles and other energy and matter, then there may exists some augmentations that allow us to in principle perceive something we never seen before, but we won't be able to recognise it (unless the phenomenon
stood out e.g. as some perculier pattern of something) if we don't have knowledge of it in the first place, and hence how to interpret the signals

Anonymous

@MartianCactus When I ask "what fundamental is?" you say: "it is something which cannot be explained or derived." And then you ask "fundamental cannot be questioned. It just is?"....so I'd have to say "yes" (you defined it that way after all) :P

Secret

and if our universe turns out to have even a tiny bit of supernatural component in it, then there are things we will never be able to perceive no matter how much we physically augment our brains since we will never knew where to look and what to expect

Anonymous

Axioms cannot be questioned within the framework of the model based on those axioms. That is true.

Anonymous

3:06 PM

This reminds...I still need to read up the mathematical logic textbook someday

MartianCactus

i guess the basic thing i get from this is..

Slereah

@Blue then you can read axiomatic frameworks for physics

The worst thing to read

MartianCactus

dont dwell into metaphysics

2

Slereah

Metaphysics is fine, just read about it first

Anonymous

@MartianCactus Golden realization

Anonymous

3:08 PM

:P

Slereah

These are not really new insights

a bunch of jerks have been discussing such topics for 3000 years

MartianCactus

i guess there ARE some proves of inertia but i simply do not know enough to get into them

12th grader here

Anonymous

ACM already gave you a clue

Anonymous

Go read principle of least action

MartianCactus

yeah

i did read it on wiki, didnt get nothing

Anonymous

3:10 PM

@MartianCactus Double negative....intentional?

MartianCactus

wat?

Anonymous

@Slereah lol

Anonymous

@MartianCactus You said "didn't get nothing"

Slereah

including a filthy hobo

Anonymous

I hope that means "did get something" ;)

Anonymous

3:11 PM

And you're not speaking in gangsta rap

MartianCactus

gansta rap it is

Slereah

My name is slereah and I'm here to say
I'm the greatest physicist in the USA

Anonymous

@MartianCactus ....you disappoint me :P

MartianCactus

i mean i dont even know what Langrange L is

Anonymous

Pick up a copy of Goldstein and start reading

Slereah

3:13 PM

@ACuriousMind Silly question

MartianCactus

lol

Slereah

Is there a specific thing if a string in string theory is wrapped around a non-trivial curve that's like

Not a hidden dimension thing

MartianCactus

im pretty sure i wouldnt understand this principle in 12th grade atleast

Slereah

like just a gigantic string

wrapping around the universe

MartianCactus

alright, so the question"why is inertia" is metaphysics, a no go spot for physicists

and my goal has nothing to do with metaphysics anyway

will keep that for old age philosophy

ACuriousMind

3:15 PM

@Slereah Well, then you just have a string wrapping around the universe :P

Anonymous

@MartianCactus Cool, now ~~get lost~~ go study actual physics

Slereah

is there any specific property for such strings, though

I guess they'd be fairly low energy things

MartianCactus

and also, when einstein a lot of times questioned about relativity and stuff, was that metaphysics?

Slereah

are they important for something?

ACuriousMind

I don't ever recall encountering such a thing

Slereah

3:16 PM

Perfect thesis topic I suppose!

Send money my way and I will investigate

the giant string

in a shocking twist 99% of the mass of the universe is just the giant string being useless

MartianCactus

@someone my question?

last question

Slereah

you know what seems like a cool topic though

that you don't really see much about

Supersymmetric point particle stuff

Secret

I am not afraid of dwelling too deep into metaphysics. I like things that are ineffable

Slereah

Green et al briefly talks about it but without going into details

It would probably be a nice intro to superstring stuff

Secret

For example, we can take this issue of "infinite regress" up to eleven and ask about whether there exists a chain of "whys" that just happened to close up into an implication loop and hence does terminate

MartianCactus

3:20 PM

when einstein went into general relativity, was that metaphysics?

Secret

not necessary, since he based his questions from observations and knowledge on how light behaves

MartianCactus

@ACuriousMind

ohh alright, so it was all physics

Slereah

Einstein did write a lot about various philosophy of physics issues connected to GR yes

MartianCactus

so basing something over somethign is physics

Slereah

ontology and whatnot

MartianCactus

3:21 PM

asking something from nothing is metaphysics

Secret

Of course, one can recover the infinite regress by instead "zooming out further" and asked "why is this chain of implication closed into a loop

ACuriousMind

@MartianCactus I don't quite understand the question. Einstein liked to philosophize a lot, but GR itself is clearly physics

MartianCactus

right, cool

k goodbye o/

Secret

and then you will soon end up with "hypertori of whys"

Anonymous

@Chair I remember you wanted a TeX word counter. This looks pretty good actually...it gives a detailed breakdown, which Overleaf does not. I like the web interface

Anonymous

3:29 PM

On the other hand, the Physical Review word counter (meant for RevTeX documents only) seems broken. It shows "Length Check Failed" for all my documents ;_; without giving any clue as to why

Slereah

did you try adding more words

Anonymous

I did

Anonymous

The limit is 3750 words

Anonymous

I tried with 4500

Anonymous

As well as 2500

Anonymous

3:38 PM

Last time the website accepted the document although it didn't pass the length test . But then they rejected it via email...saying it exceeded by around 1000 words

Anonymous

Now I can't be sure how much to remove from the document without their official counter working (I mean even if it fails the length test, they should ideally be showing the word count breakdown of the document...but they're not....which seems like a bug)

Anonymous

Just gotta make some guesswork I suppose....trying to cut down by around 1500 words (and send the rest to supplementary)

Anonymous

Jasper Loy

@MartianCactus I think you can just take axioms to be starting points in a system from which other things follow, no more no less. That's how I like to think about axioms.

Slereah

4:22 PM

one thing that's a bit annoying is that basically awards for theoretical physics are basically awarded for luck

If you have 10 theories that people worked on for years and they all give the same predictions and then one thing is measured and bam

One of the theory turns out to fit that

it doesn't really mean the other 9 theories were bad

enumaris

4:37 PM

kachow

knzhou

4:48 PM

@Slereah Surely actually being right is important.

Tons of people can do hard math. Only a few are right.

And being right matters, because physics is about actually describing our universe. If it doesn't matter, why even bother doing experiments?

Slereah

@knzhou well sure but it's not a question of skills

The other 9 groups who worked on those theories weren't wrong because they were bad at science

Well not necessarily anyway

enumaris

outcome is all that matter >3

22

Q: jumping into water

Two questions: Assuming you dive head first or fall straight with your legs first, what is the maximal height you can jump into water from and not get hurt? In other words, an H meter fall into water is equivalent to how many meters concrete-pavement fall, force wise? (I'm assuming the damage c...

is this question within the scope of PSE...

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