The h Bar

Secret

01:12

> I DO NOT DEFINE TIME, SPACE, PLACE, AND MOTION, AS BEING WELL KNOWN TO ALL.
-- NEWTON (1642-1726), PRINCIPIA

How is place different from space? Is there such thing as a place without space?

HDE 226868

@SirCumference It's been good. I had a couple fun (challenging? but doable?) physics seminars last semester, and I'm excited for the spring. I'm taking a directed reading on group theory/representation theory in quantum mechanics that seems pretty awesome, as well as couple good physics and astro seminars.

Life's been busy and fun and challenging and not as hectic as it could have been. How about you?

parvin

01:44

0

Q: Does the red object act as a fulcrum (as explained bellow) to make picking up easier?

In picture one I have shown a robot arm (green) rotating to pick up a box, using a fulcrum (red). The arm after rotating is shown with black rectangle. The vertical arm on the left is attached to the horizontal green arm and goes down to make the green arm rotate. In picture two, I have shown a ...

would you please help me with this? this is about robots but also about physics

parvin

02:11

Another question,

Imagine we have a fulcrum, one side is us and one side is a 20kg weight.

Which one is easier, to push our side's plate downwards to make the other side move upwards, or to lay under the plate and pull it downwards?

GodotMisogi

02:24

@Blue Found it: Jadavpur University professor's shocking comment on virginity: 'Will you buy cold drink with broken seal?’ | "A girl is born sealed from birth until it is opened... A virgin girl means many things accompanied with values, culture, & sexual hygiene [sic]," Jadavpur University prof. Kanak Sarkar said
https://www.timesnownews.com/india/article/jadavpur-university-professor-kanak-sarkar-virginit-cold-drink-with-broken-seal-kolkata-west-bengal-facebook/346987

Reminds me of this comic: pbfcomics.com/comics/preserves

Sir Cumference

@HDE226868 Dang, that sound awesome. For me it'll be QM II and Calculus on Manifolds, the latter of which is more or less Diff Geo. Could help with learning general relativity, although I'm spending break trying to catch up on the expected analysis background.

Aside from research I haven't the chance to learn much astronomy. We only have a Physics major here, not Astronomy, so there aren't many non-graduate astro classes I can take :/

HDE 226868

@SirCumference Interesting; I wonder if your quantum mechanics class will be similar to the other one I'm taking this semester.

Also, I took a geometry class last semester that sounds similar to the second class you're talking about. Do you know what the topics will be?

Sir Cumference

@HDE226868 "Maxwell's equations in terms of 4D Lorentz geometry, de Rham theory, Sard's theorem on the density of regular values of smooth functions, the theory of differential forms and integration, and vector bundles."

HDE 226868

@SirCumference Okay, ~~slightly~~ quite different, then.

Sir Cumference

Mostly we'll just be going through this book

Reviews are a little disheartening

@HDE226868 The course description for QM II is oddly the same as in QM I, so not sure what to expect ¯_(ツ)_/¯

Is your rep. theory class for a math major or (astro)physics major?

Physics Meta

02:35

-1

Q: I want to report a user

I want to report a user because he has been the reason why I was banned from the site. I want strict actions to be taken against him and he should suffer to the same extent as me . Please make it sure. I feel insecured because this user always blocks and downvotes my answers. Please ensure that h...

discussion

HDE 226868

It counts for math credit, actually. A student (friend of mine) put it together at the end of last semester and the math department approved it. We're using these notes as our main text. As of three weeks ago, we were aiming to get through three chapters a week. As of today, that's hopefully going to be 1.5-2. At the very best-case scenario.

Sir Cumference

@HDE226868 Oh, spring semester already started for ya?

HDE 226868

@SirCumference No, it starts next week. I'm here early for track practice. Fortunately, so are two of the other four students in the class, including the aforementioned friend.

Sir Cumference

@HDE226868 Nice. Me and a group of friends tried to do something similar, but couldn't find enough people for it to get approved. Math department here is pretty small :/

Gotta ask, what kind of stuff do you guys go over in astro classes? The philosophy at my uni is that you're expected to learn physics first and astro in grad school, so I sorta had to put learning it on hold

HDE 226868

@SirCumference So, we have two intro courses for majors: Astro 16 (stars and the interstellar medium) and Astro 14 (exoplanets and a wee bit of cosmology). This is the textbook for those. For the major, you have to take one of those two, then two out of three seminars: one on stars, one on observational astronomy, and one on the interstellar medium.

I took Astro 16 in fall of 2017, and I'm taking the stars seminar this semester. The seminars are offered erratically; we have two astro professors, and one's the department chair and the other does interdisciplinary stuff and an astro class for non-majors.

@SirCumference I've also heard that that's the conventional wisdom, to be honest.

The thing about the astrophysics major at my school is that it's really just the physics major + three astro classes, so I get the same amount of pure physics. So I don't have too many qualms about that.

bolbteppa

02:54

How do I evaluate $T_{[a,b]}$ in mathematica, it should come out as $T_{[a,b]} = \frac{1}{2}(T_{ab} - T_{ba})$?

reference.wolfram.com/language/ref/Symmetrize.html is something but...

dmckee

03:05

@enumaris Not one I know.

If the answer you haven't isn't it, wait for John R to show up. He knows a lot of old SF stories.

bolbteppa

Mathematica is terrible

dmckee

I mean, I like to think that I know a lot of old SF stories, but John's story-identification score is three times mine.

Sir Cumference

@HDE226868 I see then. We do have a few (irregularly offered) introductory astro classes as electives, but it's not really what I expected going in. I get the reasoning behind it tho.

Oh speaking of astro, nice idea with the blog. Hoping it'll get more attention, though astro SE seems pretty inactive lately :/

HDE 226868

@SirCumference Thanks. called2voyage and I are hoping it goes somewhere. We've got a bunch of folks working on some posts; we're just letting that write and edit and polish before going forward.

bolbteppa

03:50

Wow! cadabra.science/manual/asym.html

'Cadabra
a field-theory motivated approach to computer algebra'

traducerad

05:41

Hello

I encountered the following expression of acceleration due to gravity: https://imgur.com/a/VP1nquz

I have some difficulties to understand from where this comes. This is AFAIK quite different from what eg is written on Wikipedia or from what I have seen so far. So I'd like to know how this model is called so I can look it up and try to understand how they came up with those formulas.

Could somebody redirect me to some sources or something which explains and ideally proves this?

John Rennie

06:13

@traducerad it's just te usual gravitational force written in a weird way.

dmckee

Egad but the typesetting is atrocious enough to make parsing out waht is meant harder than it needs to be.

John Rennie

Indeed :-)

dmckee

The latitude variation (if indeed that is what is meant by $lat$), suggests the author might be trying to fold in a tidal effect, but ... if so they seem to have mistaken the geometry of the Earth-moon or Earth-sum system a but and overlooked the longitudinal effects off the line between the bodies.

All and all, without knowing more I'm reluctant to make any assumption about what is going on there.

Slereah

07:52

hello

enumaris

08:03

@dmckee doing a google search appears to bring up a subgenre of dragon shapeshifting....erotica....that was...surprising...but it makes my search much more difficult lol

John Rennie

08:15

@enumaris I had a look but it doesn't ring any bells. But then I'm not good on YA books since the genre didn't exist when I was a young adult.

Can you think of nything specific like names of people of places that might act as a memory jogger?

Shing

09:55

consider two indistuguishable particles, where $\psi_A(x)$ is the wave function for particle $A$; $\psi_s(x)$ is for particle $B$

$$\Phi(x_1,x_2)=\frac{1}{\sqrt{2}}\bigg(\psi_A(x_1)\psi_B(x_2)\pm\psi_A(x_2)\psi‌_B(x_1)\bigg)$$

I want to see when they are distinguishable

say, after measurement, they will be localized

then both the probability density of $\psi_B(x_{\boldsymbol{1}})$ and $\psi_A(x_{\boldsymbol{2}}) $ approach to zero; therefore, the probability density $|\Phi(x_1,x_2)|^2$ of two particles $A$ and $B$ becomes:

$$|\Phi(x_1,x_2)|^2 \approx \frac{1}{2}\bigg( |\psi_A(x_1)\psi_B(x_2)|^2 \bigg) $$

I am confused by the "$\frac{1}{2}$"

that seem to me not making any physical sense

it should be "1"

but I am not sure where is wrong with my reasoning

Slereah

10:31

The probability is

\begin{eqnarray}
|\Phi(x_1,x_2)|^2&=&\frac{1}{2}\bigg(\psi_A(x_1)\psi_B(x_2)\pm\psi_A(x_2)\psi‌_B(x_1)\bigg)^* \bigg(\psi_A(x_1)\psi_B(x_2)\pm\psi_A(x_2)\psi‌_B(x_1)\bigg) \\
&=&\frac{1}{2}(\psi^*_A(x_1)\psi^*_B(x_2)\pm\psi^*_A(x_2)\psi‌^*_B(x_1))^*
\end{eqnarray}

Ran out of time to edit

Man that's too many terms to do it in chat

My eyes are not liking it

Shing

lol

I hate the "editing time limit"

Slereah

I can't even delete it!

Damn the man

John Rennie

11:11

@Slereah I can delete it if you want. Which posts specifically?

Slereah

11:27

@JohnRennie eh no matter

Secret

Dec 13 '17 at 8:49, by Secret

He then made a footnote saying that "physicists tend made the mistake by equating indeterminacy (the measurement/observation produces an indeterministic outcome) with indeterminable (the idea that even if there is a process that is deterministic, it is fundamentally unknowable and inaccessible to all our attempt in interrogating the system (as if the probability distribution does not even exist))"

Got it figured out, there does exists a mathematical object that has the above properties:

Consider some event $E_n$ for $n \in \Bbb{N}$ such that the following is true:

$\text{Pr}(E_n)=1$

$\text{Pr}(E_n|E_m)=1$ for all $m<n$

Then identify all the $E_n$s as the same kind of phenomenon. Now you have a phenomenon that is recurring, but no amount of statistical modelling can fit it, and hence indeterminable and a pdf does not exists

Now that is short and sweet, and does not clog up the chat much. I should stop saying things now lol

kartik c.p