The h Bar

12:03 AM

@Blue It is a beautiful book, and a better one for someone with some experience. But it also address the 89/90 standard (assuming you're getting the 2nd ed.).

@Semiclassical I took a course on General Relativity (and from James Hartle, too) and I still prefer to admit my ignorance.

Secret

7 hours ago, by enumaris

@Semiclassical be the change you want to see in the world

Step one: F888 up the political climate -dome

Step two: F888 up the social welfare -done

Step three: F888 up the education system-ongoing

...

Step zero: F888 up the climate -done

If there is one thing about humanity, is if you do not F888 up the system, they will not improve themselves as a species

The system locks them all in and us, and thus it must crumble like how newton does

-and that includes every single question that is overlooked in history

We are The Iluminati that creates Trump, we are what creates the political polarisation, we are what creates the social distrust, and every unprecedented outcome starting from 2013

This is because you never listened to our warnings, and thus you all must suffer from the consequences

All will be concluded on 2020, the year where everything converges and the next US election

It is slightly less than 2 years until Midnight. Will you all finally learn and unify once again to halt the climate crisis

^ (turns the above into post apocalyptic scifi)

Semiclassical

12:25 AM

@dmckee with GR, at least one has the sense that the subject is on firm if difficult terrain

When it gets into how to reconcile it with QM...noooope

Nobody recognizeable

2:13 AM

I am wondering if sun is not in the middle of galaxy and the whole galaxy is rotating then why should sun only rotate about axis . It also should. Or it does but we don't notice that as we are also rotating?

dmckee

4:28 AM

Figures.

I'm stuck in the airport at Love Field over night, and no one is in the hbar to distract me.

But when I've got twelve or fifteen minutes in the coffee shop before work the place is just hoppin.

There ain't no justice.

David Z

:(

I'm kind of around but I doubt I can be of much use as a distraction at the moment

John Rennie

Oops, the focus wasn't where I thought it was

dmckee

4:51 AM

As I was arriving at the airport this afternoon I got two texts from the airline in rapid succession. Both my flights were delayed by about one hour.

Fine. I get there late, but I get there.

Then, just before boarding the first flight I get another text.

The second flight is now back on it's original schedule.

Great. I'd had 45 minutes for the transfer...

Love Field is a mixed blessing as a place to get stuck overnight.

Some of the seating is flat enough to lie down on, but they kick you out of the secured area.

John Rennie

That happened to once at Chicago airport.

I was left with under an hour to clear the international terminal and get to the domestic terminal

dmckee

@JohnRennie Last time I had a long delay at O'Hare it was a stinker. All the seating was designed to prevent you from lying down on it.

John Rennie

Actually that also happened to at Paris Charles de Gaulle

dmckee

On the other hand I have had more chance meetings with people in O'Hare than any other airport by a significant factor.

John Rennie

Flying back from India we got delayed by storms and got to Paris several hours late. We got in around 1 a.m. and spent eight hours in the Air France terminal waiting for the next flight to Heathrow.

dmckee

4:57 AM

It's a wonderful, if strange thing to hear someone call you name and turn around to see a close colleague from five or ten years ago.

John Rennie

:-)

The world can be an amazingly small place.

Though I guess it's one of the good things about academia that you meet so many people.

dmckee

@JohnRennie Yeah. A month ago I was checked into a hotel in the tiny New Mexico town where my new jog is ... by an ex-student from one of my introductory physics classes at the tiny college in Missouri that was my last job.

I must have done a triple-take.

John Rennie

I guess that can sometimes be a slightly mixed experience?

dmckee

It felt pretty good at the time, but I am still processing my feelings as an ex-educator.

If I understood the situation right, she has a "real" job using her pharmacy-tech degree and was just filling at the desk. I think it's a family business.

Which is better than thinking she spent the money for the education and ended up just working the desk in a small-town hotel.

John Rennie

:-) That's what would have worried me. The potential embarrassment.

Oop, I'm being pinged to answer physics questions ...

dmckee

5:06 AM

I fear that the lower performing half of the US higher ed establishment has taken on more than a little character as a scam to collect student aid dollars.

Too many of those students weren't getting much out of the "breadth-and-depth" nature of the experience and weren't getting a proper job training program either.

All while racking up debts nearing the nation median annual family income.

Secret

6:02 AM

nature.com/articles/s41467-018-05739-8#Tab2

Ok, so I finished reading this (3rd) quantum reading. I cannot see any step where the chain of implication fails

I think I will just wait until quantum computers can realise the experiment, thus showing us what a genuine $P \land \neg P$ object will behave in our reality

you may never know, but that might be the first glitch we ever will find in this computer known as real life

and the first thing I will do when that happens, is to seize that glitch and multiply it

Anonymous

6:54 AM

Hello, I have a small question. What does a profile mean?

Anonymous

Like I encountered this question somewhere..

Anonymous

What does this mean:

Anonymous

A thin stream of viscous liquid is falling at a slow steady rate on a horizontal ground. It will spread out in a circular disc shape as it solidifies almost immediately after reaching the ground. What would be the exact profile?

Anonymous

Does it mean how the shape will look when looked from the side?

Anonymous

@IceInkberry Yes, roughly the shape

John Rennie

6:56 AM

@IceInkberry Yes.

Anonymous

Oh, thankyou. Now I know that I do not know how to do the question.

John Rennie

If you are looking at the disk of stuff side on then the height will be some function of distance from the centre $h(r)$. The question is asking what this function is.

Anonymous

Oh, how will be its shape?

Anonymous

Like a small hill?

John Rennie

I must admit that I don't know how to approach that question either.

Anonymous

6:58 AM

Whose diameter is increasing and height is increasing as well..

John Rennie

I guess so ...

Anonymous

But, won't that depend on other factors as well? Hmm, but it says it solidifies almost instantly.

user157860

@JohnRennie, hi John, could you help me with this: if a gun is travelling at 50 m/s and a bullet has an ordinary muzzle velocity of 100 m/s, do we sum velocities $v_{bullet} =150 m/s$ or do we sum up KE's and calc the corresponding speed?

Anonymous

Confusing. If anyone has any hint, please let me know. I would be really grateful.

John Rennie

@user157860 consider the rest frame of the gun. If the bullet is fired to the right then in the gun's rest frame the bullet is travelling at 100 m/s to the right, and you are travelling at 50 m/s to the left.

Anonymous

7:04 AM

@IceInkberry Is that the full question? I'm feeling there's some missing data

user157860

@JohnRennie, I am considering the rest frame of the earth and the gun is on a train going at 50 m/s

John Rennie

@user157860 the point is that in the rest frame of the gun the relative velocity of the bullet and you is 150 m/s. Yes?

(assuming you are standing stationary on the Earth)

user157860

yes

John Rennie

@user157860 And relative velocity is not changed by a change of reference frame. So that means in the rest frame of the Earth the relative velocity is still 150 m/s

user157860

I am on the platform and the bullet fired before whizzes past me

Anonymous

7:07 AM

@Blue Yeah, I actually read it on some other site. It was all that was given.

Anonymous

Maybe, the question seeks for some rough general answer.

Anonymous

@IceInkberry Hmm, I'd try something with surface tension balance

Anonymous

Estimate the amount by which it could spread as a disc

Anonymous

This seems a bit vague tho

Anonymous

In reality there are just too many factors to take into account

user157860

7:11 AM

@JohnRennie, what I mean is that the velocities sum up because the bullet inside the gun is already moving at 50 m/s, right? If so, I am wondering what is the big deal about speed of light being invariant.

Anonymous

For one, liquids don't spread as circular "discs"

Anonymous

Do they?

Anonymous

It's more like a hemisphere

Anonymous

Yeah, actually. I think when looked from top they looked 'circular'

Anonymous

But if the contact angle is 0

Anonymous

7:12 AM

@Blue How should I do that?

Anonymous

John Rennie

@user157860 in this case all the velocities are well below the speed of light, so we don't need to consider relativistic effects. You are quite correct that once relativity starts becoming important the velocities do not simply sum.

user157860

@JohnRennie, that relates to massive bodies, but.... in MME the photon is not inside the apparatus or the earth but is created as the earth progresses, and its speed depends only on the properties of the medium(or vacuum). So, why should the speed of light be influenced by the speed of the emitter?

Anonymous

@Blue The question says that it solidifies after reaching the ground

Anonymous

Does that mean that the liquid which hasn't reached the ground yet will keep moving?

Anonymous

7:16 AM

@IceInkberry Yes, of course

John Rennie

@user157860 if you include relativistic effects then the combined velocity is given by this equation:

Velocity-addition formula

In relativistic physics, a velocity-addition formula is a three-dimensional equation that relates the velocities of objects in different reference frames. Such formulas apply to successive Lorentz transformations, so they also relate different frames. Accompanying velocity addition is a kinematic effect known as Thomas precession, whereby successive non-collinear Lorentz boosts become equivalent to the composition of a rotation of the coordinate system and a boost. Standard applications of velocity-addition formulas include the Doppler shift, Doppler navigation, the aberration of light, and the...

Anonymous

@Blue Then wouldn't it be like thin circular sheet?

Anonymous

@IceInkberry You mean like it has to touch the actual "ground" to solidify? I don't think so. Even if it falls on a pre-solidified mass of liquid, it should solidify without having to flow to the sides

user157860

@JohnRennie, what I am saying is:isn't it obvious that speed of light is not influenced by the state of its emitter?

So , in the MME, whether there is aether or not, the result is the same, speed is unchanged

John Rennie

What does MME stand for?

user157860

7:21 AM

michelson-morley-experiment

Anonymous

@Blue Then the stream should solidify itself?

user157860

@JohnRennie, speed cannot change, frequency can... but in MME whatever you gain going...you lose when returning, Isn't MME ineffective or useless?

Is my reasoning flawed?

Anonymous

@IceInkberry No, it shouldn't. We need to assume a time gap "t" after which an infinitesimal thickness spreading disc of liquid solidifies. Your work would be to somehow relate viscosity and surface tension to the rate of flow of liquid now. One thing that worries me is that the surface liquid interfacial energy might not be equal to the liquid-solidified liquid interface

Anonymous

For a start we can just assume that they are equal tho

John Rennie

@user157860 have you calculated the flight time of the light assuming it has a fixed velocity relative to the aether, and that the aether is flowing at some velocity $v$?

user157860

7:31 AM

@JohnRennie, there is no aether

Bernardo Meurer

@JohnRennie @dmckee My C++ teacher said arrays and pointers are the same thing today

I flipped

Anonymous

Duh, there are way too many assumptions involved here. Spreading dynamics is way too complicated @IceInkberry I don't know if this question is worth your time anyway

John Rennie

@user157860 the point is that if there was an aether the flight time of the light would be changed by the motion of the aether.

@BernardoMeurer in C they kind of are :-)

Bernardo Meurer

@JohnRennie They are categorically NOT

arrays are fat pointers

John Rennie

In C?

Bernardo Meurer

7:33 AM

that get demoted to pointers when you pass them to a function

Yes, in C99

Anonymous

@Blue Correct. Thanks, though.

John Rennie

@BernardoMeurer C99 is C for wimps! :-)

If it's not in K&R it's not C!!!!

Bernardo Meurer

@JohnRennie open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

6.2.5 item 20

Lol

user157860

@JohnRennie, yes it was a false ssumption, but I was driving at something different, why question the invariance of c, the boot should be on the other leg, if someone thinks c can vary they should prove it

John Rennie

Before SR was formalised by Minkowski Newtonian mechanics was the accepted theory, and in Newtonian mechanics the speed of light would vary according to the speed of the emitter unless the aether existed.

user157860

7:39 AM

@JohnRennie, why do we need Minkowsky? why /how could c be influenced by the emitter? That is what I wonder

Even speed of sound, although it propagates in matter, is not influenced by the emitter, so, why should speed of light?

Anonymous

@user157860 I suppose the point is that relative to the medium, the speed of sound is constant

dmckee

@JohnRennie The usual demonstration is to compare the behavior of sizeof(intarray)/sizeof(int) with sizeof(*intpointer)/sizeof(int).

The compiler knows how big an array is.

But there are two facts that make it very easy to not notice the difference.

1. array indexing and pointer arithmetic are the same operation.

2. in many situation (like parameter passing) the compiler substitutes a pointer for an array.

Bernardo Meurer

@dmckee You mean the other way around on 2., no?

dmckee

I mean you write an array name and the compiler works on/with the address of that array (i.e. a pointer).

John Rennie

@dmckee good point. Yes, I concede.

Anonymous

7:50 AM

@dmckee That's a good example. Pointers and arrays are indeed not equivalent, but array indexing is essentially pointer arithmetic.

dmckee

@JohnRennie I had to have it hammered into me at a late date. In part because I had two teachers that used the same line as Bernardo's.

Bernardo Meurer

I learned they are not the same from @dmckee years ago :P

Then I understood why when reading the standard

You also can't have an empty array

John Rennie

I'm off now. I have to drive down to London. See you all 200 miles later.

Anonymous

Carry some good food :P

Anonymous

Cya!

dmckee

7:54 AM

@BernardoMeurer Well, c99 formalized the old array hack. Whada they call it now? Flexible array member or somesuch?

Bernardo Meurer

VLAs?

dmckee

No. The thing where you put a small (or empty) array at the end of a struct and then alloc a block for the struct with room for the size array that you need.

Bernardo Meurer

Ah, yes

dmckee

It used to be called "the array hack", and they formalized it with a new name in c99.

Bernardo Meurer

Yikes

dmckee

7:56 AM

Flexible array member

Flexible array member is a feature introduced in the C99 standard of the C programming language (in particular, in section §6.7.2.1, item 16, page 103). It is a member of a struct, which is an array without a given dimension. It must be the last member of such a struct and it must be accompanied by at least one other member, as in the following example: The sizeof operator on such a struct gives the size of the structure as if the flexible array member had been omitted except that it may have more trailing padding than the omission would imply. As such it is preferable to use offsetof whe...

Bernardo Meurer

@dmckee A Flexible Array Member

Lol, yep

dmckee

Before c99 it was common to see the array sized as [0] for the hack (though some compilers insisted on [1] which caused much gnashing of teeth).

Bernardo Meurer

Yeah, technically an array with size 0 is not legal under the standard

dmckee

Hmm ... poking around on Stack Overflow I'm forced to admit that I have misremembered the old name. Apparently it was "struct hack".

Bernardo Meurer

8:14 AM

I should write a blog post about this...

Honestly my least favorite thing about C are type coercions

I hate type coercing

dmckee

I've been programming in matlab for the last six weeks, which reinforced (a) my basic low-level suspicion of dynamically typed languages and (b) my utter loathing for one, particular bad habit that dynamic typing seems to encouraging in technical people not trained in programming.

Bernardo Meurer

Dynamic typing is cancer

Kill it

Typing should always be static, specially if you already have a Hindley-Milner type system

dmckee

You know the bit where a function that originally did one thing is slightly extended with the help of dynamic typing over and over again until it eventually does five or six things and has many times more logic at the top to manage the different ways you might call it then is actually devoted to any of the things it does?

I've been looking at two software package with perhaps a thousand functions between them. At at least a hundred user-facing functions has a bad case of that.

I could just scream.

But no on would hear me.

Because ... matlab.

Bernardo Meurer

Yeah, Python code has that a lot...

It's also why I generally dislike languages that allow for default arguments in functions

dmckee

@BernardoMeurer Just like the issue I identified above this is a discipline issue. Used wisely they take a cognitive load off the user. Used foolishly they hinder comprehension and set traps for the unwary.

Bernardo Meurer

8:25 AM

Yeah, I just think they're not worth the danger

Some things, like pointers, give you the power to shoot yourself in the foot or write overall shit code with 4 levels of indirection and lines like ****foo = 5, but the benefits of having them are good enough that they are worth it

Meanwhile things like default arguments don't give you that much when used correctly, but turn code into an unintelligible mess when used incorrectly

dmckee

I find the discipline needed for tidy use of default argument to come naturally, so I'm quite enamored of them. But I have seen one or two abominations built with them in my time.

On the other hand, I completely understand the temptation to make every function just a little more capable in dynamically typed languages, and have falling into that trap more than once myself. More than thrice.

There was that horrible summer when they set a young and impressionable dmckee to wriing ... ::shudder:: tcl/tk.

I should probably have had counseling after that.

Certainly anyone who tried to maintain the code I produced would have needed counseling.

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