12:44 AM
I like units of measurement because they keep me from dying.
If I'm supposed to make a landing approach at 50 miles per hour and I attempt to make it at 50 kilometers per hour instead, my chances of survival cease to be well approximated by 100%.
Something similar happens if I'm supposed to load my ship with 400 pounds, but I load it with 400 kilograms instead.
Anyway :D
I found a question over on Aviation.SE where somebody wrote: one horsepower is 550 foot pounds-force per second, and the acceleration due to gravity is 32 feet per second, so an ideal helicopter with a one-horsepower engine should be able to carry 550 / 32 = 17 pounds.

@TannerSwett But those things are all just totally subjective. A (messy) landing at 50 kilometers per hour is just a different subjective experience. Same thing with a sinking ship. If I keep using the word subjective we can just not care about it.

I'm writing an answer explaining, no, you're doing the units wrong. The answer you get from the division is actually 17 pound-force seconds.
And the significance of that is... what? There isn't a physical interpretation of that result, is there?
Oh shoot, yes there is.
One pound-force second is the same as... help me out.
One slug foot per second?
Boom, there's my physical interpretation. So one horsepower is enough to lift 17 slugs one foot per second.

Yup.
Except imperial because... well, let's not talk about that. :D

thankfully I studied engineering in Canada so I'm fairly comfortable going back and forth. They like to ask questions with both units, because in practice you run into both.

12:54 AM
@JMac You know, my particular flying club did, in fact, have an occurrence related to flying too slowly on approach.
That particular subjective experience was the last experience that the subject ever experienced.

1:10 AM
My subjective experience of the world is that it's a world in which objective measurements are very important.
Subjectively speaking, of course.
By the way, if you're learning to fly for the first time, and the aircraft you're learning in has an attitude indicator, cover that thing up.

2 hours later…
3:39 AM
So in QM, double slit experiment, the wave function collapse has nothing to do with whether we observe it or not, only whether there's something that can interfere with the particle?
(That's what I understood from Sir Cumfurence's answer earlier)
Does that mean that if we put something that can interfere with the particle, the wavefunction collapses doesn't matter whether we can observe it or not? Or maybe it does matter and we can observe it since we can (in principle) track the hit thing?

7 hours later…
10:35 AM
> If everyone uses a random collection of 4 words
yeah, good luck with that
$\{\text{set of things humans can do}\} \cap \{\text{set of things that are random}\} = \varnothing$

I guess it's the length of the password that is important because the number of possibilities rises exponentially with the length. The four words are just a way to get people to remember a 20 letter password.
My memory is pretty good, but I would struggle to remember a 20 letter password built using random alphanumerics :-)

@JohnRennie aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa is long but probably quite easy to crack
32

This is probably a question about TeX's history. Why there is no command \nothing? If there's no \nothing, why \varnothing is called \varnothing instead of \nothing? If I try to compile a document with \nothing, I get the error "! Undefined control sequence." The command \varnothing is defined...

huh
interesting
nobody seems to know why LaTeX uses \varnothing for the empty set but doesn't have anything defined for plain \nothing

I did say using random alphanumerics though I suppose your password is random in the sense that it is just as likely as any other sequence of 84 alphanumerics :-)

10:58 AM
A question
Do you need to assume fixed boundaries for Lagrangian mechanics
Since that is rarely how we actually solve those problems
Can you just assume fixed initial conditions

So in QM, double slit experiment, the wave function collapse has nothing to do with whether we observe it or not, only whether there's something that can interfere with the particle?
(That's what I understood from Sir Cumfurence's answer earlier)
Does that mean that if we put something that can interfere with the particle, the wavefunction collapses doesn't matter whether we can observe it or not? Or maybe it does matter and we can observe it since we can (in principle) track the hit thing?

11:51 AM
what's a summary of the recent (last 20 years) important advances in fluid mechanics?

1 hour later…
12:57 PM
@RyanUnger Can you scan that thesis
I tried asking the local library and that is not within their power tl get it
Most of the librarians weren't even quite sure if interlibrary loans wdre still a thing that existed

1:34 PM
Is Energy conservation, violated ever

Depends how you define it

1:58 PM
I didn't, t get, I thought energy conservation is universal to all situation.

@yuvrajsingh every conservation law corresponds to a symmetry.
This is Noether's theorem.
Noether's theorem states that every differentiable symmetry of the action of a physical system has a corresponding conservation law. The theorem was proven by mathematician Emmy Noether in 1915 and published in 1918, after a special case was proven by E. Cosserat & F. Cosserat in 1909. The action of a physical system is the integral over time of a Lagrangian function (which may or may not be an integral over space of a Lagrangian density function), from which the system's behavior can be determined by the principle of least action. This theorem only applies to continuous and smooth symmetries over...
So conservation of energy must correspond to a basic symmetry, and in the case of energy that symmetry is called time shift symmetry.

OK sir do you have any example

This basically means the laws of physics don't change with time, so if I do an experiment today then do exactly the same experiment tomorrow I will get the same result.
As long as time shift symmetry holds energy will be conserved.

@JohnRennie Do we not expect all physical theories to be continuously symmetric in time due to the conservation of energy?

@JohnRennie so, does time-shift symmetry hold, in all situations?
(sir)

2:05 PM
@yuvrajsingh For all practical purposes, yes, we expect the laws of physics not to change with time. However the universe as a whole is expanding, and what's more the rate of expansion is changing. And this breaks time shift symmetry. That means when you look on the sort of timescales relevant to the expansion of the universe we would expect to see conservation of energy violated.

Please.sirCorrect me if I am wrong I can say that in physics the pragmatic (and only) view of time is that is that which is measured by a clock. lots of laws are time invariant. So a film of a bouncing ball if done right, will not provide a direction in time, it looks the same if you run it backwards. I haven't the expertise, but at the micro level there is a conflict that involves spacetime
@JohnRennie

The fundamental laws of physics, i.e. interactions between elementary particles, are almost time symmetric.

Feynman states in his lectures that the laws of physics are not unchanged under a change of scale.
3

There is a very small breaking of time symmetry that is seen in only a very few special cases. This is related to CP violation.
In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry (parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry) while its spatial coordinates are inverted ("mirror" or P symmetry). The discovery of CP violation in 1964 in the decays of neutral kaons resulted in the Nobel Prize in Physics in 1980 for its discoverers James Cronin and Val Fitch. It plays an important role both in the attempts...
CP violation implies there must also be time symmetry violation.

I can, t read wiki articles. @JohnRennie can explain it in short.

@yuvrajsingh there isn't a short explanation as it's complicated. If you Google CP violation you'll find lots of articles on sites other than Wikipedia.

I got one more question @JohnRennie

Yes?

Whatever I study in physics till now explain me one thing only, the world of symmetry, whether it is planet or star, or any physical forces, is symmetry can be the word, which can define physics. @JohnRennie

@JohnRennie T symmetry isn't the same as time invariance symmetry!

2:21 PM
@yuvrajsingh symmetry is important in math also, right?

@JohnRennie Are symmetry in the wave functions of particles are the reason for the existence of multiple dimensions?

@AbhasKumarSinha not as far as I know.

@JohnRennie So, how dimensions are defined?
I'm getting more and more confused when I see multiple different definitions on internet (without maths) I can relate them both to GR and QM...

Do you mean "what is a dimension"?

@JohnRennie I rather mean How are they defined? Because when you see theories in string theory and stuff, they say that a large acceleration can move objects from one dimension to another (not sure if I understood that part properly), so it first seems related to GR, but when you hear the name string theory associated with it, then it more seems relatable to QM....

2:32 PM
24

How do you exactly define what is and isn't a dimension? I heard somewhere that it is "anything you can move through" but if that is right, why wasn't time and space considered a dimension before Einstein?

@skull petrol yes.
Can you comment on my last question @JohnRennie

13 mins ago, by yuvraj singh
Whatever I study in physics till now explain me one thing only, the world of symmetry, whether it is planet or star, or any physical forces, is symmetry can be the word, which can define physics. @JohnRennie

@yuvrajsingh I'm eating lunch now. Back in a few minutes.

enjoy :-)

Oops, have a lunch.

2:36 PM
@JohnRennie that's time reversal symmetry, not time-shift symmetry

3:35 PM
Have you finish @JohnRennie

@yuvrajsingh yes.

Question is already in front of you.

Re your question, symmetry is a very important consideration in physics, but it is not the only consideration. That is not all of physics can be derived just from symmetry.
For example the symmetry underlying general relativity is diffeomorphism invariance, but general relativity is just one of many theories can have this symmetry.
So the symmetry alone is not enough to determine that general relativity is the correct theory.

The GR symmetry is tetrad rotation
Not diffeomorphism invariance

Re the time symmetry: when I was talking about the universe expanding the symmetry I was talking about was time shift symmetry i.e. the laws of physics don't change with time.
In:
2 hours ago, by yuvraj singh
Please.sirCorrect me if I am wrong I can say that in physics the pragmatic (and only) view of time is that is that which is measured by a clock. lots of laws are time invariant. So a film of a bouncing ball if done right, will not provide a direction in time, it looks the same if you run it backwards. I haven't the expertise, but at the micro level there is a conflict that involves spacetime

3:40 PM
I think that is time reversal symmetry @JohnRennie

I think you're referring to time reversal symmetry i.e. it makes no difference whether time runs forward or backward.

Yup

Emilio was just saying be careful not to confuse the two symmetries because they are different.
@Slereah have you got a link for that? Googling doesn't find anything that seems useful at a first glance.

@JohnRennie well it's gauge gravity, as it is called
for instance

@JohnRennie does time-shift symmetry hold, in all situations?

3:46 PM
Diffeomorphism invariance is certainly a gauge, in the sense that leaves the action invariant and the EoM underdetermined
But it's not a gauge field

besides translation and reversal, are there other kinds of time symmetries?

@Secret The most general one is reparametrization
$t \to f(t)$
For $f$ some diffeomorphism

right, makes sense

You can make the EoM of point particles invariant under reparametrization
though it is more unpleasant

@yuvrajsingh it doesn't hold in the expansion of the universe, and that means the total energy of the universe isn't conserved, but then the total energy of the universe is a rather hard quantity to define.

3:49 PM
\begin{eqnarray}
\left[ - \frac{\ddot{x}^\mu(\tau) \dot{x}_\mu(\tau)}{(\dot{x}^\mu(\tau) \dot{x}_\mu(\tau))} \dot{x}_\mu(\tau) + \ddot{x}_\mu(\tau) \right] = 0
\end{eqnarray}
Is the proper reparametrization-invariant EoM

@yuvrajsingh You will find physicists disagree about whether conservation of energy is or isn't violated as the universe expands, but this is mainly because they disagree about how they define energy.

The most important part of learning physics is realizing that the terms are just words
They don't have any fundamental meaning
You have to work with them as they are defined in the theory

@yuvrajsingh But for all practical purposes energy is always conserved.

As you said it is the point of debate, '' with different glasses have different views''. @JohnRennie

@yuvrajsingh Indeed :-)

3:57 PM
Have you ever saw an test match at lords @JohnRennie

No. As a youngster I sometimes went to watch my county team, Somerset, but I have never been to a Test match.

Do you have time for a match. @JohnRennie

I generally have better things to do than sit for eight hours watching a game that will probably end in a draw anyway :-)

Lol

Have you ever had a pint of scrumpy

4:03 PM
@Slereah back in the 1970s lots of the local farms in Somerset made scrumpy that they would sell to you for a reasonable price and the Inland Revenue need be none the wiser. It was alcoholic but otherwise execrable.

Look at the city slicker here

I grew up in the country. I now live in a (small) city. The city is better.

Do you drink, I mean beer, or any other liquid which contain alcohol @JohnRennie

@yuvrajsingh I don't drink very much. Every few weeks I go out with friends and we drink beer, but I do it more for the pleasure of chatting with friends than out of any great desire to drink beer.

You can order something other than beer
Get a tall glass of apple juice

4:08 PM
I could, but the others seem to enjoy their beer drinking and I don't want to be the one who stands out. Anyhow I have nothing against beer. It has to be conceded that jokes seem funnier after a few pints. Indeed it is the only occasion anyone laughs at my jokes :-)

Lol, I ask so many question, do you have any joke. @JohnRennie

An ion goes into a bar and says to the barman "I've lost an electron."
The barman replies "Are you sure".
The ion replies "Yes I'm positive".

Haha, one more please please.

The trouble is that most of my jokes are puerile, obscene, or both :-)

Please. Please @JohnRennie
Jokes are jokes, one who have good sense of humor can laugh, that what I believe.

4:40 PM
... can you calculate the average velocity of molecules just using temperature? You need to know the mass of the particles too, right?

Yes. The average KE is $\tfrac32kT$ but you need the mass to get a velocity from this.

Cool, just making sure I'm not crazy when responding to stupid comments online

@JMac Someone is wrong on the Internet? :-)

@JohnRennie That's why I had to check. It seemed so unlikely that I was really starting to doubt myself. And it's literally not possible for me to be wrong on the internet; so I wanted to make sure I wasn't destroying the structure of reality.

@JMac I suppose you should really point them to the Maxwell-Boltzmann distribution. This includes the mass in the equation for the velocity distribution.

4:52 PM
I think the misunderstanding is actually more trivial then that, which is nice. They just assumed I didn't know the relationship between KE and temperature, and they were assuming mass was a known in the situation, while I wasn't.
So it made me think there was some extra relationship I didn't know, but really it was just different assumptions.

5:06 PM
ugh, this took me 5 hours!
It is almost impossible to find this again without knowing the exact name

5:19 PM
However rereading this again, it is not an entirely new object, rather, it is more like attractors and bohmian trajectories. They might be related and if they can be relativitised, might provide a field theory without involving fields

1 hour later…
6:26 PM
@JMac I think it is impossible for anyone to be wrong on the internet

@AaronStevens Well you're wrong, and I can't be. So checkmate.

@JMac Ah, foiled again

@AaronStevens Unwavering arrogance is the strongest form of logic.

@JMac Ah yes, I love reading a good proof by unwavering arrogance. Oddly enough I never saw these types of proofs in my text books though...

Big Science is afraid everyone will see the truth

6:31 PM
Maybe that's what's going on with flat-Eathers. Although they use proof by unwavering ignorance as well

You know what, I bet Big Math and Big Philosophy are in on it too... or else we would still hear about it. It's a full blown conspiracy. I'm going to make you a co-author on my paper that blows this wide open. Thanks a bunch.
and yeah I was totally channeling my flat earth satire with that one

@JMac Nothing warms my heart more than oppressive math and science. But I guess since I am a white male I was in on it the entire time.

2 hours later…
8:56 PM
Hi, everybody.

@DanielSank hi Dan

Hi guys, is anyone familiar with Killing vectors in Schwarzschild and Kruskal-Szekeres coordinates? Because the space times are related through a coordinate transformation. Is there a way to show the Killing vectors are the same? Or are they the same? All help appreciated!