The h Bar

Firstoff, in his general relativity Einstein showed that dimensions could bend, curve, and move. This is an experimentally proven fact. Dimensions can, and do, move. In an earlier post we logically prove that the photon remains stationary in the fourth dimension: According to Einstein & Bria...

Blatant report of:

-5

Q: After proving that the photon remains stationary in the fourth dimension, must we conclude that the fourth dimension is moving at c?

After proving that the photon remains stationary in the fourth dimension, we must conclude that the fourth dimension is moving at c. In an earlier post we logically prove that the photon remains stationary in the fourth dimension: According to Einstein & Brian Greene, does the photon remain sta...

quantum-mechanics special-relativity speed-of-light relativity

It appears some people just won't give up.

AccidentalFourierTransform

4:26 PM

I think I just answered a question by adding a tag to it :/

is that bad? :P

dmckee

@JohnRennie Well, he's ready for our advanced class. Unless the poster is a native German speaker, the capitalization of abstraction is a powerful channel marker.

Obliv

I don't get why people re-post questions instead of just bumping them..

Slereah

I am hungry

Hungry for burgers

John Rennie

@AccidentalFourierTransform Depends on the tag. As long as it wasn't the egregious-nonsense tag :-)

Slereah

Aw, no post with that

Does MSE have an abstract nonsense tag?

John Rennie

4:41 PM

@Obliv to be fair the original question was closed

@Slereah slereah ? :-)

Slereah

Hey I'm not the math guy here

I only do down to earth physics!

John Rennie

A feeble attempt at humour. Just smile pityingly and ignore me.

Danu

8

Q: How did the universe shift from "dark matter dominated" to "dark energy dominated"?

In order to get dark energy to dominate, wouldn't you first need another form of energy to push the expansion until dark energy could dominate? Otherwise I don't understand how the universe could shift from having a decelerating expansion to an accelerating expansion. Is there any analogy that co...

cosmology space-expansion dark-matter dark-energy

Do you guys think this is a duplicate of:

4

Q: Was the matter-energy content of our universe always distributed in the same ratios?

Currently, Dark energy (68.3%) and Dark matter (26.8%) together constitute about 95.1% total matter-energy content of the universe while only 4.9% is ordinary baryonic matter. Was this always the case? Are these ratios constant since the big bang or did they change over time?

cosmology dark-matter dark-energy

ACuriousMind

I was just about to say "Yes" :P

Danu

@ACuriousMind Hahaha

Bernard Meurer

4:47 PM

@ACuriousMind Heya

ACuriousMind

@Slereah Sadly, they are boring and call it category-theory ;(

Obliv

anyone know how to show that $|G| = 12$ where $G$ is the group of rigid motions in $\mathbb{R}^3$ of a tetrahedron?it's not as easy as in $\mathbb{R}^2$ to show rotations and reflections..

ACuriousMind

@Danu No, it's not. The first asks for the mechanisms by which the ratios shifted, the second for the fact if and by how much they shifted.

Obliv

is it like $r$ is the rotations clockwise of a face, then $a$ is the rotations of the faces? 4 different faces and 3 different rotations of each face so $4*3 = 12$?

ACuriousMind

@Obliv That doesn't make sense. What's a "rotation of a face"? You always rotate the entire thing, not individual parts.

Obliv

4:52 PM

oh true

what is a rigid motion defined as?

dmckee

@Obliv Well, they may not know how Stack Exchange works.

Obliv

@dmckee true but most forums work the same way. people get mad at you for reposting your thread when you could just bump them. i guess if they're new to forums in general it makes sense

ACuriousMind

@Obliv Um, why are you attempting to show something about "rigid motions" if you don't know their definition?

dmckee

@Obliv A "rigid body" is one in which each particle or mass element remains the same distance from all the others. Rigid motion is motion of a rigid body.

The chapter on the subject in a mechanics texts is generally about general rotational motion, because the translational part is pretty boring.

Obliv

under that definition shouldn't there exist infinitely many rigid motions?

John Rennie

4:56 PM

@Obliv I've offered to explain if the OP wants to come to the chat room, but we just get insults. That makes me suspect it isn't a serious question.

dmckee

@Obliv All rigid motions can be written in terms of a translational motion of the center of mass and a rotational motion around the CoM.

Obliv

@johnR i don't think he has the rep required to talk, does he? says he has 20. you need 50 i think

AccidentalFourierTransform

@JohnRennie damn it! brb ive got to revert an edit

Obliv

@dmckee I am told to show that the amount of rigid motions of a tetrahedron in $\mathbb{R}^3$ is only 12. So I need a more specific/constrained definition of rigid motion I think

dmckee

@Obliv I suspect that they are talking about the symmetry group for the form, but then I don't know why the description would be written that way.

20 rep is sufficient to talk in chat

Obliv

5:04 PM

well I found the solution here crazyproject.wordpress.com/2010/01/08/… it's just a matter of understanding what the notation means. I was thinking there was another way of writing this. Something I'm more familiar with.

dmckee

Of course, at this point that poster no longer has 20 rep.

Obliv

I don't get what they mean by 'corresponding' face with $\theta (X) \theta (Y) \theta (Z)$

@dmckee whelp. now he really can't join the chatroom for discussion lol

ACuriousMind

@Obliv I have no clue what a "rigid motion" is, but the number of rotations are that symmetries of the tetrahedron is indeed 12.

John Rennie

@Obliv Oh well, I tried. I find myself disinclined to believe it's a great loss.

user54412

@JohnRennie You gave him far more leeway than I would have. Considering every other comment he makes begins by mentioning his PhD in physics and Ivy League education, it's pretty clear he's not a particularly intelligent troll.

John Rennie

5:17 PM

To be fair it is a hard concept for beginners to SR. I wonder if we need a definitive canonical Q/A for it as the current duplicates are somewhat scattered.

Danu

@ACuriousMind Thanks :)

DanielSank

5:43 PM

I think this comment thread needs some clean up, since it turned into a ridiculous discussion about some political issue.

I'd also note that one of the participants has a very long consistent history of posting rude and unconstructive comments.

@dmckee

dmckee

Done.

DanielSank

@dmckee Thanks.

dmckee

I have to wonder if that poster actually believes anyone will be impressed by his claims to have a Ph.D.

John Rennie

@DanielSank there is a distinct odour of troll around Astrophysics Math.

It staggers me that people who have potential access to real physicists choose to reject their help. In my days as a teenage physics nerd (long before the Internet existed) I would have given my right arm for the sort of access the PSE offers.

dmckee

6:06 PM

Just noticed the existence of Beer, Wine & Spirits on the network. They are still feeling their way as yet. I wonder what kind of site it will mature into.

DanielSank

@JohnRennie Astrophysics Math?

@JohnRennie The internet is the great Equalizer. Anyone can publish their meanest thoughts to the world at large with a click of a button. In your nerd days these people were mostly confined to their desk and whatever souls they could corner in the library.

John Rennie

Astrophysics Math

1 5

special-relativity relativity spacetime photons speed-of-light time time-dilation

@dmckee Oooohhh :-)

@dmckee It doesn't have that effect on me, honest!

7

Q: Beer, sexual desire and potency

I've heard that drinking beer lowers sexual desire and potency, and that's the physiological effect of hops. Is it true? Does drinking a lot of beer have negative long-term effects on sexual desire or potency? Or is it just an urban myth, based on negative impacts of drinking too much?

health

user54412

@dmckee Homebrewing has been... brewing... in beta for 6 and a half years

David Z

6:33 PM

@DanielSank I saw your ping from a couple days ago - I'm on vacation this week, hence the lack of response.

DanielSank

6:52 PM

@DavidZ don't recall what it was about.

user116211

@DanielSank: o/

Bernard Meurer

@JohnRennie That's true, it's amazing the kind of nawlidge one can get here

David Z

7:08 PM

@DanielSank it was when you were showing 0celo7 how to get feedback from the mods

Seems that he's not here now, otherwise I'd ping him too

Danu

7:34 PM

@EmilioPisanty What's up with the fighting on TeX meta? :P

Emilio Pisanty

7:51 PM

@Danu I don't know

I'm pretty upset about it.

To be honest.

Bernard Meurer

@EmilioPisanty Which fight?

Emilio Pisanty

@BernardMeurer If you haven't seen it then it's honestly not that important.

Bernard Meurer

@EmilioPisanty Ah, I was just curious, TeX doesn't seem like a very controversial thing for people to be fighting over it

AccidentalFourierTransform

@BernardMeurer I would murder anyone who claims that \rightarrow is in any way superior to \to

Bernard Meurer

@AccidentalFourierTransform Okay, I take back what I just said

ACuriousMind

7:59 PM

@AccidentalFourierTransform It is superior in the rare cases where it does not denote a map.

Danu

@AccidentalFourierTransform lol

DanielSank

@DavidZ Oh, well it's none of my business really.

Bernard Meurer

@ACuriousMind I'd hide if I were you

DanielSank

@BernardMeurer So young and naive.

Danu

Also that^

ACuriousMind

7:59 PM

@BernardMeurer What's he gonna do, Fourier transform me?

Bernard Meurer

$\to$ or $\rightarrow$? I see no difference

AccidentalFourierTransform

@ACuriousMind and make it look like an accident ;-)

Bernard Meurer

#Allarrowsmatter

ACuriousMind

@AccidentalFourierTransform :O

Danu

None of my friends are into tex and I routinely find myself almost shouting in excitement about what I've recently been doing

Bernard Meurer

8:00 PM

@DanielSank Teach me master

Danu

You know what's an annoying arrow? \curvearrowright

ACuriousMind

@Danu You...might have a problem ;P

Danu

Why would anyone put "right" at the end?!?!?!

I just redefined it as \action in the end...

ACuriousMind

@Danu It's obviously the opposite of \curvearrowwrong

3

Danu

@ACuriousMind Ba-dum-tsssss

0celo7

8:03 PM

Which arrow is that

Danu

The one doing actions of groups

AccidentalFourierTransform

@Danu for some reason I read "sex" there and thought "way to go, Danu!"

Danu

@AccidentalFourierTransform Well, I try to keep my voice down about that stuff ;)

ACuriousMind

lol

0celo7

@Danu hmm, never seen that one

Danu

8:04 PM

@0celo7 You know how $S^1\cong \Bbb R/ \Bbb Z$? That's dividing out the standard group action of $\Bbb Z$ on $\Bbb R$: $\Bbb Z\curvearrowright \Bbb R$

0celo7

I know what a group action is...

Danu

Ah.

0celo7

In saying I've never seen that arrow.

*im

Also on a phone and god damn I hate PSE chat on the phone

Is it squiggly?

AccidentalFourierTransform

@Danu \stackrel{#1}{#2};\loop\insert;\noquiet #IfSexWereTex

Danu

@0celo7 No, it curves to the right (predicably).

0celo7

8:12 PM

@Danu how is that predictable

Danu

Curve-arrow-right, y'know.

0celo7

Is it like the injection arrow

Danu

No, that is hookrightarrow.

(it has a hook, and goes to the right)

But I agree that the names are similar ;)

AccidentalFourierTransform

"it curves to the right" #IfSexWereTex

DanielSank

@BernardMeurer Teach you what?

Bernard Meurer

8:19 PM

@DanielSank Everything

DanielSank

@BernardMeurer Cannot.

Busy right now.

Bernard Meurer

@DanielSank I mean, over the years

0celo7

@Danu I know what hookrightarrow is >:(

DanielSank

@BernardMeurer Go learn linear algebra, then come talk to me and I'll explain all the important stuff they forgot to say.

0celo7

Can someone please draw a picture of this arrow

I've never seen anyone use this fancy thing

DanielSank

8:24 PM

@BernardMeurer remember yesterday when I pointed out that functions are vectors and then integration is a linear operation on functions? Don't forget that.

Bernard Meurer

@DanielSank I will after I finish getting Python to run on this microchip

@DanielSank I'll tattoo that on my right arm

DanielSank

@BernardMeurer Good.

@BernardMeurer I'm curious: what math have you done?

Bernard Meurer

@DanielSank In school you mean?

DanielSank

@BernardMeurer Or otherwise.

Bernard Meurer

@DanielSank Okay, bear with me while I remember, and ask if something is vague (which a lot will be)

We had 2D and 3D geometry, last thing I recall was calculating surface areas, volumes and that stuff

Trigonometry

DanielSank

8:34 PM

@BernardMeurer What do you mean calculating surfaces?

Bernard Meurer

Just surface area

DanielSank

@BernardMeurer Surface area of 3D objects?

Bernard Meurer

Yeah, like balls, cubes, and the other ones idk the name in english

DanielSank

@BernardMeurer I see. Using calculus or not?

I guess not.

Bernard Meurer

Not

DanielSank

8:35 PM

ok

So you did trigonometry.

Bernard Meurer

Yeah, although I'm rusty in it by now

DanielSank

So for example, you could write the real and imaginary parts of exp(i pi / 4)?

Bernard Meurer

what's exp in $exp(\frac{i\pi}{4})$?

DanielSank

The exponential function, like $e^x$.

Bernard Meurer

is that the same as $e^x(\frac{i\pi}{4})$?

Oh yeah that's what I thought

DanielSank

8:39 PM

@BernardMeurer No I mean $e^{i \pi / 4}$.

Bernard Meurer

I know $e^{\pi i}$ is -1

idk why but I know that happens

but idk how to do what you said

DanielSank

Hmm, ok.

Have you seen $e^{ix} = \cos(x) + i \sin(x)$?

Bernard Meurer

That was written in the board at some point yeah

I've seen it, but I haven't learned it

Brazilian education is very focused on getting you to see everything humanely possible, without really understanding most of those things; and that goes for all subjects

AccidentalFourierTransform

@BernardMeurer you have just described all I know about optics/thermodynamics/astrophysics/electrostatics/...

DanielSank

@BernardMeurer Hmm, that's interesting.

Bernard Meurer

8:45 PM

@AccidentalFourierTransform During high school we have 14 distinct subjects, there's no way in hell people are actually learning stuff :p

@DanielSank But yeah we saw some complex number stuff

AccidentalFourierTransform

@BernardMeurer yeah, I feel you

and college is no better :-(

at least the first couple of years

DanielSank

@BernardMeurer Do you know how to solve something like $\int dx / (1 + x^2)$?

Bernard Meurer

@AccidentalFourierTransform At least in College I'm hoping they won't expect me to know every single bloody philosopher that has ever existed on the face of this god forsaken land

DanielSank

@BernardMeurer In the US that is extremely unlikely.

@AccidentalFourierTransform College is far more up to the student, IMHO.

You can seek out as much help as you're willing to consume.

Bernard Meurer

@DanielSank Nope, we had 0 calculus at school. I picked it up by myself during my last year but I never used it so I just forgot it all except limits and derivatives

Danu

8:50 PM

@BernardMeurer I think Holland has like... 12?

Bernard Meurer

@Danu Simultaneously? Yikes

dmckee

@BernardMeurer With the ideal of a "Liberal Education" (here liberal means wide-ranging rather than a political stance; though on many campuses that also applies) means they will expect you to dabble in a range of subjects, but you get to focus on your major field of study.

Danu

@BernardMeurer I think I had 10 simultaneously in the last 3 years; maybe 11?

DanielSank

@dmckee Yes, this was my experience. I did have to learn history and arts, but it was rarely imposing on my scientific and math studies.

Bernard Meurer

@Danu Yeah its the same here, 11 hours of class a day, couldn't stand it

DanielSank

8:52 PM

Only one semester, as I recall, was stressed by these requirements.

@BernardMeurer That's horrible.

I already think the U.S. school day is too long.

Bernard Meurer

@dmckee Yeah but 'dabbling' is way better than having to be a ninja in everything

I'm barely a ninja in my own area, let alone in organic chemistry and whatnot

@DanielSank It's bad yeah

dmckee

@BernardMeurer Yep. I'd encourage you to work the system so that you can take as many classes that interest you for general education (that's the dabbling) as possible.

DanielSank

@dmckee That's what I did.

That's why I took "Forensic Anthropology" as a humanities course :)

It was awesome.

Bernard Meurer

@dmckee Yeah I'm already looking at that, UCSD has a GE class in "Nature and sexuality" that seems like it'll be useful someday :p

2

DanielSank

The final project was a shoe box full of bones and our assignment was to write up a lab report on what we thought happened to the previous owners of said bones.

They tried to fool us by including bones from non-human animals.

Mine had a pig leg.

dmckee

8:56 PM

@DanielSank Nice. I took "Human Origins" from Brian Fagan. Aad "The History of Technology". And "Great Moments in the History of Public Policy" and made each of them cover a different gen ed. Good times.

DanielSank

I incorporated him into the story.

@dmckee Haha, excellent.

Bernard Meurer

@DanielSank Was the pig the murderer ?

DanielSank

@BernardMeurer Hahaha, I don't remember. I just remember that he was somehow involved. I think I made the story pretty tame. I'm sensitive about murder and violence.

dmckee

@BernardMeurer When I was at UCSB they had a sociology course entitles "Human Sexuality" that covered a couple of gen eds. It was the most popular course on campus and it wasn't the cakewalk that many expected it to be.

DanielSank

I think it was something like the pig was someone's pet and he dug up the other peoples' graves.

@dmckee I think they still have it.

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