The h Bar - 2022-10-02

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Relativisticcucumber

12:59 AM

hi - i want to ask if quarks have a specific inherent color charge to them or it is determined by the situation they are in?

It depends on their mood iirc

jk, I'm not sure sorry bud

2 hours later…

si-LV-er_and_b-LA-ck

2:58 AM

the mood could be determined by the situation :P

Relativisticcucumber

i dont get how that can be possible :o is it known how that works ?

si-LV-er_and_b-LA-ck

Sorry, it was a bad joke.

Relativisticcucumber

ah man im a meme but okay if somebody actually know that would be cool

Are there strong waves?

Relativisticcucumber

A rip tide, or riptide, is a strong offshore current that is caused by the tide pulling water through an inlet along a barrier beach, at a lagoon or inland marina where tide water flows steadily out to sea during ebb tide. It is a strong tidal flow of water within estuaries and other enclosed tidal areas. The riptides become the strongest where the flow is constricted. When there is a falling or ebbing tide, the outflow water is strongly flowing through an inlet toward the sea, especially once stabilized by jetties. During these falling and ebbing tides, a riptide can carry a person far offshore...

we really got a bunch of comedians up in here

3:10 AM

XD

2 hours later…

5:31 AM

Anyone know how to write a Lagrangian for a driven damped oscillator?

I found expressions that work for the driven and damped cases independently, but not both at the same time.

si-LV-er_and_b-LA-ck

6:12 AM

@DanielSank Is that position dependent damping?

6:36 AM

@Relativisticcucumber A quark has a fixed colour charge that doesn't change.

We tend to only talk about six quarks, up, down, strange, charm, bottom and top, but actually there are three quarks and three antiquarks of each type. For example there is a red up quark, green up quark and blue up quark and the three anti-up quarks.

A red up quark always remains a red up quark and cannot change its charge. If you see a quark colour charge change (e.g. in gluon emission) what is actually happening is the original quark is annihilated and a new differently coloured quark is created.

7:15 AM

@si-LV-er_and_b-LA-ck Velocity dependent.

1 hour later…

8:45 AM

0

Q: How do you trace posts from inactive members?

I read an interesting post by post by user # xyxy, I tried to reach other posts by same author searching user:xyxy but got no returns, since the account has been deleted. Is it impossible or there is a way?

3 hours later…

11:27 AM

Consider two masses, each connected to a spring attached to a wall as well as a spring between the masses, i.e. coupled oscillators. The distance between the walls is L and the springs have all the same spring constant k. How does one calculate the rest positions of the masses?

It seems like their rest positions depends on the length of the springs, which is unknown…

@Relativisticcucumber color charge is difficult to talk about because it's not gauge-invariant and because there's confinement, see e.g. physics.stackexchange.com/q/176478/50583, physics.stackexchange.com/q/449591/50583, physics.stackexchange.com/q/664566/50583

@DanielSank It's possible that there is no such Lagrangian, but I don't know of a proof of that either

12:41 PM

hi everyone, long time since I last visited the site/chat

1 hour later…

2:05 PM

Hi, I have a question about quadratic integrable functions. If you would take the integrate of it, would you get zero?

3 hours later…

Relativisticcucumber

4:52 PM

thank you @JohnRennie @ACuriousMind that helps and ill read what you linked !

1 hour later…

5:59 PM

yoo i just realized

is the reason animals are taxonomically grouped as a Kingdom because of the phrase "the animal kingdom"?

well I guess it could be the other way around. or neither

actually it's definitely the other way around. i'm gonna get some sleep now

1 hour later…

Relativisticcucumber

7:28 PM

hi i wanted to ask why the higgs boson in one of the particles of the standard model listed as a force carrier? i have read that scientists expect to find a graviton and that this would be the force carrier for gravity, and i have also read that the higgs field is what gives things mass but i am not seeing why this would make it a force carrier and if there is any connection to the concept of a graviton

also, hi @ShikiRyougi : D

also if there is this explanation of how things acquire mass, is there a similar case for how things acquire other fundamental characteristics such as charge, spin, color, etc.?

8:21 PM

@Relativisticcucumber The Higgs is not usually considered a force carrier (see also physics.stackexchange.com/q/1080/50583), where did you see this?

Relativisticcucumber

the image i have been referencing is the one in this Wikipedia introduction on the standard model en.wikipedia.org/wiki/Standard_Model

@Relativisticcucumber ah, see, they've just grouped fermions and bosons together

a boson is not exactly the same as a "force carrier", and that's why the Higgs looks out of place

Relativisticcucumber

okay so if the leptons and quarks are constituents of matter and the force carriers are the fourth column, then what should i consider the higgs boson ?

::shrug:: the thing that gives some of the other particles mass

Relativisticcucumber

also is it expected that scientists will discover more "higgs-like" particles? such as bosons to give things charge and color and whatnot? or is the higgs predicted to be the only one of its kind

8:25 PM

no, the Higgs mechanism is very specifically for fermion mass

it's not some instance of a generic idea that could also grant other properties to particles, it's about mass specifically

If we have two waves, expressed in exponential form, how can we find what the phase shift is between the two?

Relativisticcucumber

hm interesting - so the standard model should include any fundamental particle, right? that is why the higgs boson is there ?

@Relativisticcucumber I'm not really sure what you're asking

the standard model is intended to be a full theory of our world (excluding gravity), and we have enough evidence to conclude that the Higgs exists, so the SM includes it

Relativisticcucumber

i mean this diagram that i linked that shows the components of the standard model, the thing that unites these all is that they are fundamental particles without smaller constituents, correct?

ah, yes

Relativisticcucumber

8:27 PM

okay got it thanks very much !

@imbAF unless you're using "phase shift" in an unusual way, that only makes sense when the waves have the same frequency, and then it should be very simple; can you be more explicit about what you want to know?

Yes I can

We have 3 plane waves (3 monochromatic EM-waves or matter waves), and each has a certain wavenumber value, and those values are:

$k_0$,$k_0+\frac{\Delta k}{2}$,$k_0-\frac{\Delta k}{2}$.

As a result of the superposition, we get a wave packet, with a final wave function equation:

$\psi(x)=const.e^{ik_0}[1+cos(\frac{\Delta k}{2})]$

2 things to note: 1. I am considering the simple 1D case, and 2, the case in which we observe this packet at a fixed moment in time, and that is for t=0, to simplify things

you can easily tell that for x=0, you $|\psi(x)|$ has the largest value

If you would equalize the part in [...]

you can find that for $x=+ \frac{2\pi}{\Delta k}$ and $x=- \frac{2\pi}{\Delta k}$ , $|\psi(x)|$=0

I understand this much

what I don't understand is this commentary:

The relative phases change so that for increasing x values (written as $x\rightarrow \infty$ which doesn't make much sense since, the packet reappears again,as you tend to move further left or right in the direction of propagation) $|\psi(x)|$ becomes smaller and ultimately becomes zero when the phase change between $e^{ik_0}$ and $e^{ik_0+\frac{\Delta k}{2}}$ and $e^{ik_0-\frac{\Delta k}{2}}$ is $\pi$

Where does he get this from? this last part about $\pi$

I'm given a question regarding how the human ear can differentiate sound just .34ms apart. The example is that a low frequency source is placed and the right and left ear of a person is 18cm apart. The wave is going 340 m/s and I'm asked how long the interval is from going to the right ear to the left ear. My question is, shouldn't the left ear have to listen to the reflected sound wave?

Like, how could a sound wave traveling from the right to the left be heard in your left ear without a reflection?

the source is placed to the right of the person forgot to add that in.

8:47 PM

@imbAF they're just saying that the points $x=\pm \frac{2\pi}{\Delta k}$ where $\psi = 0$ are the points where the "phase difference" between the waves is $\pi$ in the following sense: We "start" all three waves in sync at $x=0$ and then they oscillate with different periods, meaning we can "count" how many periods ahead one is of the other (with one period being "$2\pi$ phase difference", meaning one has completed exactly one full oscillation more than the other).

When that difference is $\pi$, that's the points $\psi = 0$ that are the "boundary" of the wavepacket.

@Obliv diffraction

That's what I was thinking too, but not sure how the anatomy of the ear works.

nothing to do with the anatomy of the ear, the sound wave just diffracts around your head and then into the ear

doesn't really matter how exactly that looks anatomically

Now that I think about it, the decibel threshold is extremely low so even if it isn't a direct wavefront it's probably not an issue.

ACM, I paraphrased your city pigeon quote on my instagram to bring about awareness :D That is a cool fact I didn't know about.

I used to view them as just pests like rats but I guess that was not the case.

I mean...they absolutely are pests :P

they're just also tragic

but it's nice you found it interesting

@ACuriousMind since the phase difference changes, while we move, then it's not a constant but somehow dependable from position?

8:52 PM

@imbAF yes - at each position you can say how much one of the waves is "ahead" of the other

Right but it's different when WE were the ones who put them there :P like a fungus will grow uncontrollably but if we developed the fungus it feels like our responsibility.

since propagation speed is the same, for all 3, the only reason why we get destructive interference is because of the wavelength difference, aka wavenumber difference

is it correct to say so?

@Obliv Yeah, I agree (but that's really the case with a lot of stuff we've done to our environment, sadly)

@imbAF sure

@ACuriousMind 2 final things.

1. Is there a mathematical way, to find out that $\psi = 0$ for phase difference of

$\pi$

Right, admittedly my stomach is quite weak for the details of the entire situation at hand. Makes me want to give up when I consider all of the global problems that we have caused and are forced to face now.. (Also my day to day interactions with people make me have less hope because of the way people behave at least in my part of the world)

8:56 PM

because i found out the distance for which I get that

@Obliv it's ok man. If humans aim to just stay on earth, we are as good as dead. So probably in the future we will go from planet to planet, and the earth will be a place we used to live. So not that important

As if it were that easy, to just pack up and leave an entire planet lol.

well it will and it has to

@Obliv Saving the world is a Sisyphean task, but we must imagine Sisyphus happy ;)

Yeah sisyphus except the boulder will crush him and everything he loves if he doesn't push it up :D

many resources that we use are finite, so harvesting minerals etc from other planets will be of at most importance, and after that, with increasing population, we will need to provide more food, so we will need to spread in the galaxy

9:00 PM

@Obliv all the more reason to push!

I still think the best odds is to just stay on earth and continue to adapt/survive. It's not easy to leave the planet, you ever watched interstellar lol @imbaF

And my second thing is: if $\psi(\vec r,t)$ is the wave function, which is the probability amplitude, and,$|\psi(\vec r,t)|^2$ is the probability density function, then what does $|\psi(\vec r,t)|$ represent?

Carl Sagan said a long time ago in his Pale Blue Dot speech (https://www.goodreads.com/quotes/230027-look-again-at-that-dot-that-s-here-that-s-home-that-s): "The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world.…

@Obliv I did. And it's not realistic to think we can survive by staying on earth for million and million of years. And it shouldn't

@imbAF nothing

@imbAF you did when you determined the values of $x$ for which $\psi(x) =0$!

9:03 PM

how did I do that?

I only found the x value

not the phase shift

you just need to think a bit about what the wavenumber $\Delta k$ represents and what a value of $x = \frac{2\pi}{\text{wavenumber}}$ means

(actually, are you sure the 2 is there?)

@ACuriousMind Wait, if i substitute that x value i am left with e^ik_0 and the pi

got it

But if this $|\psi(\vec r,t)|$ means nothing, then why in the lecture the professor took this into consideration ? And not the square of it ?

well, if it is zero, then so is its square

but other than that I got nothing :P

Ah, ok

I initially was looking at this : $\psi(x)=const.e^{ik_0}[1+cos(\frac{\Delta k}{2})]$ and trying to figure out how it changes for increasing x values, it looks like it increases, and that threw me off

@ACuriousMind what do you mean with what $\Delta k$ means? Yes, the 2 should be there

I mean the wavenumber counts how many periods of a wave are within a certain distance

9:13 PM

in physics, (in difference from crystallography), it is k=2pi/ \lambda, which i take it as the number of wavelengths within 2pi radian

that you can "fit" in 2pi radian

9:26 PM

I have a supposedly silly question. Consider a massless rope wounded around a circle and a point mass at its end. If you push the mass, the rope unravels drawing an involute. The problem is that tangent vector to the involute is zero at the initial point (zero velocity), which is weird as the point mass moves

Am I talking nonsense?

@Feynman_00 what's wrong with it having zero velocity at the start?

anything that starts to move from rest starts with zero velocity

That I can't start with any velocity

I'm not following

I'm elaborating an answer, I'm very confused :P

9:48 PM

Due to the situation described above, the mass is constrained on a curve such that $\dot{P}=(r\theta\dot{\theta}\cos\theta, r\theta\dot{\theta}\sin\theta)$. From this I can write $L=T$ and E-L and solve with some initial conditions. If $\theta(0)=0$ then the initial velocity cannot be an arbitrary $v_0$ because $v(0)=r\theta\dot{\theta}=0$

Sure: If you start with the rope coiled, the center of the rope cannot move relative to the rope.

Can you expand on that? I don't understand what you mean

oh wait

I think I misunderstood the situation

I might also have talked nonsense, I started this thing at night and I'm kinda dummy :)

okay, new attempt: If the point mass started with non-zero velocity, then you could reverse time and the mass would move into the circle

but one of the constraints of your situation is that the circle is impenetrable, i.e. that initial condition would violate your constraints

9:59 PM

The the text of this thing must have some mistakes

10:38 PM

At the end of the day (literally lol) it was a mistake in the EoM lol

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