The h Bar - 2017-10-04 (page 1 of 4)

« first day (2526 days earlier) ← previous day next day → last day (2408 days later) »

12:45 AM

@Blue I think it works more like a positive-mass matter behaving as if it had negative mass.

@Icemybread Because it contains information in a very concentrated form, just like the textbooks.

Anonymous

@peterh Umm, it actually depends on how you define "positive-mass". "mass" is normally defined in physics as a measure of resistance to acceleration when a force is applied

@Blue On the same reason we could say electrons have positive charge and negative mass?

Anonymous

@peterh Sure. It's just a matter of convention.

Anonymous

If you switch both charge and mass then it would be just a redefinition.

@Icemybread Btw, all the university textbooks are similar. They contain very strongly concentrated information, this type of knowledge and learning seems to me dying out with the age of the internet, which favorizes more like a quick & dirty learning model.

@Blue I think real negative mass should mean that it behaves like negative mass also in gravity.

Anonymous

12:56 AM

@peterh Of course. I was just saying that if you define an electrons mass to be negative, then you would have to change the definition of "negative mass" itself.

Anonymous

Semantics is confusing :P

@BalarkaSen you here?

1:10 AM

@peterh wat

@peterh You can define it like that if you want, but that's not the sense in which the news item at hand is using the term.

This is strictly an inertial-mass phenomenon, i.e. the proportionality between force and acceleration and between velocity and momentum.

The gravitational masses involved are too weak to probe experimentally and there are no expectations that the gravitational mass of that quasiparticle will be negative

if you want to yell at the literature that they're using the term wrong, then... well, good luck =)

1:25 AM

@EmilioPisanty when calculating a cooling rate should one account for blackbody radiation?

@dmckee O, that word is used because of reflecting how last year the physics community have been talking about that LIGO is a hot candidat for the nobel prize, but sure

Anonymous

@0ßelö7 You should I suppose. Why not?

@Blue too much work

Anonymous

Hmm, it's one of the major sources of heat loss.

Anonymous

BTW you need the value of emissivity in case it's not a perfect black body (which it is probably not)

1:32 AM

it's a black hole

Anonymous

Heh....near perfect black body then ;)

@Mithrandir24601 You cannot expect the news article to be not conflating stuff whenever things like "negative mass", "magnetic monopole" is used. People have been spending many decades looking for these things, and they are so weird and so there seemed to be a tendency for the media to go hyper and said a bunch of quasiparticles behaving like said matter to be the actual thing itself.

Interestingly, it seems even in the condensed matter community, the usage of the term "magnetic monopole" to refer to those quasiparticles that behave as such is common use now

2:12 AM

@Secret I'm not sure you fully understand the word and its connotations. dmckee's observation is completely valid.

Bernardo Meurer

2:35 AM

I want a 4K screen

1080p doesn't fit all my stuff

get a second monitor

3:01 AM

@BernardoMeurer should I remind my professor I have a meeting with him tomorrow?

Bernardo Meurer

@0ßelö7 Yeah, that's always nice IMHO

3:20 AM

@BernardoMeurer did you year Post's new song

Bernardo Meurer

4:03 AM

Nope

4:31 AM

@dmckee what's funny is that one of the main aspects of law in the real world is that it really is not immutable; laws are written, laws are repealed, laws are interpreted by the courts

laws change. and laws only apply within a certain jurisdiction/domain

in that sense, things like "Hooke's law" or "Ohm's law" start to seem like far more reasonable uses of the word 'law'

4:46 AM

@Semiclassical and yet are both approximations and there are exceptions

sure, that's more or less my point

5 hours later…

9:36 AM

@EmilioPisanty I mean, I used that word because of the meaning I said, I.e an opinion, which does not necessary correspond to the actual meaning (or something). Perhaps I should have wrote "I use that word because of...". dmckee's observation is also valid to me and I have no problem on that

9:51 AM

@Madhatter nice name hehe

10:25 AM

@Secret You can use whatever words you want to say what you want to tree, but if everyone else understands elephant different by those words, you're going to have to accept that they're going to understand the standard meaning and not whatever monk you hoped they notebook.

so: ¯\ _(ツ)_/¯ to each their own

@EmilioPisanty For a second I thought you'd lost your mind until I saw what you did there :P

sup

@ACuriousMind =P

@EmilioPisanty did you undo the rogue change to the Lorentz gauge Wikipedia article?

@JohnRennie @ACuriousMind did

10:38 AM

We'll see how long it lasts :-)

Presumably there must be a paper of Lorentz's somewhere that defines the gauge

@EmilioPisanty Ah, no, our "friend" returned and undid it again.

@EmilioPisanty Understood. I will make that intention clearer next time in order to avoid misinterpretations

@ACuriousMind seriously?

@EmilioPisanty I informed the WikiProject Physics, however, and one of their people undid that again.

@ACuriousMind ah, good

I see he's done that twice since

en.wikipedia.org/w/…

once straight away and one yesterday

10:43 AM

So now at least there are active Wikipedians aware of the issue, someone also took the time to dig out proper citations for the signs.

(>_> also h bar, are you telling me because of the misinterpreted "finally" that that item get so much stars, but anyway, its past)

@Secret I would interpret the stars as on the "Nobel prize" bit and not the "finally"

@ACuriousMind yeah, that's all we needed

ball's on their court now

Yeah I guess that makes a lot more sense

@ACuriousMind so, for future reference, how does one go about "informing the WikiProject Physics"?

@EmilioPisanty I did it like this

10:45 AM

it sounds nice in the abstract, but as we saw with "informing Wikipedia"... =P

NB: The nobel prize on chemistry sounds pretty physic-y

nobelprize.org/nobel_prizes/chemistry/laureates/2017

10:58 AM

@ACuriousMind @EmilioPisanty It turns out the term Lorentz gauge, initially called the Lorentz condition, was first defined by Heitler in Quantum Theory of Radiation.

Equation 10 on page 3.

Source here

No servers to fix this morning, eh? ;)

Ah, the dangers of too much free time (again) :-)

I'm avoiding doing any real work since I went out last night and I'm not in tip top condition right now.

I'm currently more likely to break servers than fix them. Oh well :-)

Hehe. Yesterday wasn't a holiday in the UK, was it?

No, just a random meet up with friends. These days family commitments tend to occupy them at weekends so we meet up on random days during the week.

How they explain to their wives why they came staggering in past midnight I leave up to them :-)

@JohnRennie I wouldn't marry anyone who didn't accept "I was drinking with my friends" as a reason in the first place ;P

11:09 AM

That's why everyone should just live alone. No family commitments, nothing. Just enjoy and go out whenever you want...

You don't have to answer to anyone either

@JohnRennie so long as you didn't all dress up in matching animal onesies, it's all good

@JohnRennie ah, good ole Jackson, rip

did I ever show you his "we also eat humble pie" paper?

> The problem of the linear charge density on a long straight conducting wire was considered and solved using a variational technique over 123 years ago. We describe the who and where and summarize his results. We also eat humble pie.

1 hour later…

12:23 PM

So anyway who is mister Ligo and what did he do to win a nobel prize

2

@ACuriousMind who?

Prathyush Poduval

@0ßelö7 Its there here en.wikipedia.org/wiki/… Apparently someone called stephen wynn

https://everestphyl.wordpress.com/2016/03/04/14/
Some chinese poem by some guy on gravitational waves. Below is my attempt in translating it in english:

I think the breakdown of the spacetime database elements will be like

Topology, metrics and coordinates, tensor quantities, equations, stress energy tensor, energy conditions, causal structure, asymptotic structure, limits, bibliography

Maybe some "misc" section, too

For whatever else doesn't fit

and maybe a section for curves, too

Since there's important curves that aren't geodesics

12:42 PM

Today
I received a letter from 13 million light years away
There was no address on the letter
Instead, elegant
and intensifying curves
As I felt, listened and calculated
Slowly that unforgettable feeling
That ancient moment when spacetime was being ruptured and the surroundings trembled

The excitement of the heart
Tears of happiness cried
Turning back, an old man shook his head
He smiled gently

"I am Albert Einstein, is this letter intended for me?"

Fuck

What happens to a warp bubble if the curve considered is arbitrary

Like if it's just a circle

Does the observer just go on a closed timelike curve

Isn't the space inside the warp bubble just (locally) flat spacetime?

the point is the transport of the bubble

Not what's inside

So you are pondering about some dynamic warp bubble metric where the bubble itself "moves" in some curved trajectory in spacetime?

What do you mean, "some dynamic warp bubble"

That's the basic mechanism of a warp bubble

It moves along some curve

12:50 PM

How do you make one

I am thinking about having a warp bubble in the middle of flat spacetime, such that spacetime contract and expand in a way so the bubble appears moving in a circle in the flat background

But from what you said I think I misinterpreted you

@0ßelö7 : Consider the curve defined by $(x^2 + t^2) = 1$

With the metric $$-dt^2 + (dx - \frac{dx_s(t)}{dt} f(\sqrt{x - x_s(t)}) dt)^2$$

Where $x_s$ is the curve

Hm, wait, if it's a circle then at some point the speed diverges

user image

and it's not a proper metric

I think

The usual warp bubble metric basically involves the bubble moving in a straight line in effectively a flat spacetime, I think..

12:57 PM

constant speed warp bubble is easy to prove causal but arbitrary warp bubble not so

No, the usual warp bubble has an arbitrary movement

It is often considered of constant speed for calculations

1:17 PM

@Secret you're right in previous years it always sounded more bio-y

hmmm just checked the list for previous years again and it looks a lot less bio-y than i remember

false memory i guess

@0ßelö7 post malone suckity sucks

1:46 PM

@BalarkaSen you just wish you could pop pills like him

lol

1:58 PM

random SR question

Given a spacetime which can be foilated, I have an observer O at a certain spacetime coordinate

Now somewhere roughly 0.0000005 light seconds away in front of the observer (some small enough separation such that the observer is on a tangent space almost parallel to that of the event p), some event p happened. Now since any manifold is locally Minkowski spacetime, I can draw a small light cone there to represent the light cone locally at p

@BalarkaSen does this bullet look large? i.gyazo.com/e3a77aea5efea87ccf69cbb7058122fa.png

Now, I am in O's frame, and O is interested in using his clock to work out how the impact of p spread out in spacetime. Is it true that as we increment the proper time, the light cone representing the impacts of p spreading outwards just get bigger with a flat cross section?

Actually I made a mistake, let me clarify:

Suppose in minkowski spacetime I have O at the origin. Now at the coordinate (t,x), p occurs. The events that can be causally related to p are given by its future light cone.

Now O is measuring the proper time with its clock. As the proper time increment, and assuming p and O are at rest wrt each other, do the future light cone of p first begin as a point and then grow as the proper time increments?

user image

More precisely, suppose p is an explosion and the future light cone gives all the possible paths where the debris will be spreading, do we see the whole cone growing with a horizontal cross section (the frontier of the cone) as we increment the proper time in O's frame?

and will the frontier of the cone become slanted accordingly if O is moving relative to p?

2:15 PM

Hello!

This is the hamiltonian of a solid (many electrons-nuclei). My question is why doesn't the '1/2' appear in the electrons-nuclei attraction?!

user image

@Samà Because otherwise you double count things

Since you do the attraction of (a,b) and (b,a)

but this is the same thing

^ that. with the nuclei and electrons, each pair appears as both $(a,b)$ and $(b,a)$, so the sum is double-counting.

with the nuclear-electronic cross term, each term only shows up once so there's no need to divide.

@Slereah I know that! The '1/2' appears in electrons-electrons repulsion and nuclei-nuclei repulsion. But it doesn't in electrons-nuclei attraction and that confuses me.

That's because they're not the same thing, so you can't add the same interaction twice

(electron1, nuclei2) isn't the same as (electron2, nuclei1)

I might need to think more about that light cone question and post it on main later, cause I forgot that some explosion can be powerful enough to have debris moving at near light speeds, thus time dilation might distort the shape of the spreading debris hence the shape of the cone in a given inertial frame at different proper times

2:28 PM

@rob Maybe I misjudged our Polish friend. He just sent me an email letting me know the seminar time was moved. Wonder how he got my email.

But a pretty nice thing to do.

I might be able to approximate the cone's profile with enough number of debris randomly distributed and with some velocity distribution

therefore the SR question basically reduces to the spacetime coordinate of all debris wrt a given inertial frame parametrised by proper time of said frame

That is, an ensemble of worldlines parametrised by one parameter, the proper time of O's frame

amazon.com/…

Hmm...

@0ßelö7 's fine

Folks, any idea why these users don't show up when I search in the standard user page?

2:46 PM

That's weird, cause at least the 107K rep one does for me

@Secret yeah, but why don't the others turn up?

those pages said (unregistered), perhaps those accounts are disabled or something?

@EmilioPisanty because they aren't registered users?

user image

@JohnRennie huh

Follow the links in your query. Only Ján's main account is a registered user.

2:48 PM

makes sense

@BalarkaSen I'm mentioning the A roof genus but I don't think anyone but my advisor will know what that is. I'm not sure I want to tell anyone what it is, either. Should I just omit the theorem then?

anyways, raised a flag, mods will merge or not depending on reasons, I guess.

@Mithrandir24601 @blue @0ßelö7 thanks for the help on negative mass. from research the largest problem it seems is that a massive particle will pull on a negative mass particle while at the same time the negative mass particle pushes on it, causing a translation of the entire system but with zero momentum. which is weird but i dont think prohibited. it would just seem like a system of massless particles

As slereah will said, the major issue of negative mass is when you try to consider them in QFT, you have unbound from below states, thus they will create divergence in energy

@BalarkaSen explaining what it is involves scary things like seymmetric polynomials and stuff I don't want to write down

2:52 PM

@secret can you explain that?

I am not really good at the path integral explanation of it, but the intuitive idea is that you can imagine the negative mass will decay indefinitely down the energy levels (since there is no ground state), thus theoretically releasing infinite amount of energy in the process

interesting, why then does negative mass seem to be allowed for defined energy levels in $E^2=m^2c^4+p^2c^2$

The existence of that equation doesn't imply negative mass is allowed.

Also spoiler

In relativity, the sign of the mass isn't important

it's the sign of the Casimir invariant, $m^2$, which is important

The physics doesn't change if you use $m$ or $-m$

i'm considering a parameterization given by $m=cos(t)/c^2$ and $v=csin(t)$, i'm trying to figure out the domain

normalize c to 1 and you get $e^2=m^2+(1-v^2)m^2v^2$, and $m=cos(t)$ $v=sin(t)$ and $e^2=cos^2(t)+sin^2(t)$ so its defining a system of constant energy

3:03 PM

The rest mass is a constant. I don't understand what you mean by making it a function of time.

The rest mass is a constant if the connection is metric, of course

"casimir"

that's one of those physics math things

@BernardoMeurer I need Linux help getting a public key or some shit

the TAs are garbage

i should say that i am mostly a mathmatician. for me The existence of that equation doesn't imply negative mass is allowed. is a very poor argument, there needs to be a reason why the math suddenly stops working

@shaihorowitz There is no reason why math should represent the real world.

You should be very concerned with the fact that our arbitrary notions of real numbers, etc. seem to model reality.

i can think of a few

reason one our math comes from intuition based on the real world.

3:07 PM

Nah.

Not if you talk to a logician

reason 2 experiments keep confirming that no matter how abstract= math we try to do its always somehow useful.

A nonzero amount of math is based upon motivations coming from physical reality. But the abstract reasoning itself brings up various facts and properties which are not consistent with our physical intuition.

It's borderline false to say "math comes from intuition based on the real world".

For example, anything to do with infinite sets

how do we get our axioms then?

3:10 PM

Not all axioms are based upon physical intuitions. But even then, that doesn't itself mean all the conclusions of the axioms will be consistent with the physical reality.

I can give you ten million mathematical theorems which will break your mind

Abstract reasoning being inconsistent with intuition is abundant in mathematics.

i doubt it, but none of this is good motivation for no that parameterization is wrong. the statement that we have never observed something with changing rest mass would be a useful statement. but i doubt its validity because nuclear reaction seem to change rest mass all the time

No they don't. They create new particles so the total rest mass increases, but the rest mass of any type of particle is always constant.

@0ßelö7 I learnt dynamics of solutions of ODEs today

fun shit

Get pretty slope diagrams?

Or are you thinking of the mass deficit in nuclei?

3:14 PM

Not sure what slope diagrams are, but we drew a lot of phase spaces

the system lost total rest mass and replaced it with energy is what im saying, one manifestation of this is mass deficit nuclei after a reaction

tutorial.math.lamar.edu/Classes/DE/DirectionFields.aspx

O sorry wrong terminology, I mean direction fields, though they are basically phase plots anyway

The most interesting dynamics often happens at the critical points of these phase diagrams

@BalarkaSen Ew I hated those.

Too graphical

analyst

@BalarkaSen aww thanks

3:22 PM

Random set theory stuff:

Task: find a set theory that has uncountable number of axioms

any 2nd order logic in a first order system?

Probably cause I vaguely recall you have uncountable models of real numbers for 1st order logic but only one for 2nd order logic

i know its infinite, not sure its countable. thinking about PMI as a 2nd order axiom

can we please never talk about set theory in this room?

O yes I forgot that (google the transcript), sorry about that

ok then

Random analysis:

3:32 PM

@0ßelö7 $$A \cap B$$

what are A and B?

sets

...

what's a set?

rigorously?

oh i thought it as supposed to be a joke on dont talk about set theory

A set is defined by ownership according to some property in the universe of discourse

3:35 PM

with a few more features^

Or it's not defined at all, as in ZFC

that property allows for classes

Sets are just a primitive notion

Depends on which set theory you are using. Assume we are using ZFC, then something is a set if it obeys a certain number of axioms in set theory

ZFC doesn't even have ur-elements

3:36 PM

they have the feature they dont contain themselves for example

You must be a proper class because you're full of yourself

Heyoooo

The ZFC axioms, in english are as follows:
1: Axiom of extensionality: two sets are equal if for all z, "z is in the first set" is equivalent to "z is in the second set".
2: Axiom of empty set: There exists a set with no elements.
3. Axiom of unordered pairs: if x and y are sets, then there is a set z such that for all w, "w is in z" is equivalent to "w=x or w=y", i.e. z is the set containing only x and y.
4. Axiom of union: if x is a set, then there is a set y that is the union of the elements of x.

A proper class, is something that cannot be on the left hand side of the $\in$ symbol

They are, intuitively speaking, "larger" than any set

(Source: Leaky lun and me, Mathworks chat room)

i take back what i said, the axioms of set theory can be defined so that sets contain themselves but weird things happen math.stackexchange.com/questions/1046863/…

"set theory can defined so that...weird things happen"

huzzah for selective ellipses :P

Any set theory basically revolves around the notion of membership

3:46 PM

"A set is a well-defined collection of distinct objects."

-cantor

A set is a gathering together into a whole of definite, distinct objects of our perception [Anschauung] or of our thought—which are called elements of the set.

It's a bunch of stuff

Calling the empty set a set is a bit weird with that definition, though I suppose it comes down to: One possible collection of distinct objects is to pick no objects.

anyway, i'm bored of discussing arbitrary axioms. do you think there is a part of the universe that operates at a higher calculation model then Turing complete?

Would that have any operational/experimental/empirical consequences?

Ugh

3:49 PM

The universe isn't even Turing complete

Delete my account please

I can't take this crap

If it doesn't, then I'm going to invoke Newton's flaming laser sword and dismiss it thereafter.

why is the universe not turing complete?

and yes it would say we could solve the halting problem for one

Because quantum mechanics means no classical computer can simulate it

nah the quantum Turing machine is well defined

3:53 PM

In philosophy, a supertask is a countably infinite sequence of operations that occur sequentially within a finite interval of time. Supertasks are called "hypertasks" when the number of operations becomes uncountably infinite. A hypertask that includes one operation for each ordinal number is called an "ultratask". The term supertask was coined by the philosopher James F. Thomson, who devised Thomson's lamp. The term hypertask derives from Clark and Read in their paper of that name. == History == === Zeno === ==== Motion ==== The origin of the interest in supertasks is normally attrib...

@shaihorowitz not enough memory

also what your saying is that the universe is more sophisticated then a turing computer therefore above turing complete i.e. it can simulate a classical computer but not the other way

Probably one of the observable consequence is we will have supertask

its not clear whether supertask can be performed

Quantum computers have the same power as classical computers

And the universe doesn't even rise to that

Due to cosmological horizons you can't access all matter, even if there's an infinity of it

3:56 PM

thats not so clear

for example en.wikipedia.org/wiki/Malament%E2%80%93Hogarth_spacetime

is a spacetime that allows for supertask

Not really, no

It doesn't have an infinite memory accessible

It would run afoul of the Bekenstein bound

i think you could turn that around and ask: if a given spacetime allows hypercomputation, would that spacetime being physically reasonable?

« first day (2526 days earlier) ← previous day next day → last day (2408 days later) »

Transcript for

Oct '174

The h Bar

General chat for Physics SE (physics.stackexchange.com). For M...

join this room
about this room

00:00

06:00

12:00

18:00

all times are UTC

site design / logo © 2024 Stack Exchange Inc; legal

mobile