The h Bar

Secret

14:02

also, 0celo7, what I said back then to your question is not the full story:

Nov 8 '15 at 6:07, by 0celo7

I don't like being ignored by people I like.

Nov 8 '15 at 6:07, by Secret

well, I am similar

The major reason I felt so related to what you said there is because

I don't like my questions being ignored, and I don't like ignoring questions thrown at me

Or abusing physics terms:

Question answer flow must be conserved, no exceptions

ACuriousMind

@SpaceOtter I'm ACM.

Space Otter

0.0 So it seems. And quite revered too

@ACuriousMind

Secret

This is the reason why my ping inbox never get more than 3 alerts

(and yes, I knew I just opened a pandora's box...)

Space Otter

Then from now on I'm throwing my questions at you @secret

Secret

lol but please note I cannot always have the knoelwdge to answer, thus if you ask me something that is not my expertise, I am likely to send you reading or dig a google link

Space Otter

14:11

That's usually what I'm looking for. I ask when I dont know where to look

Secret

in that case like most people, we will be happy to help out

0celo7

@ACuriousMind dun dun DUN

ACuriousMind

Should've said that in the "I'm Batman" voice, shouldn't I? :P

Space Otter

What is your expertise? Theoretical mathematical law constructs? sounds very Platonistic

Secret

Now I guess why I had that nightmare after that magic discussion on that day, because that dream might be foreshadowing the ignore

Space Otter

14:14

@ACuriousMind NOOO! the darth vadar "I am your real father" voice

Secret

@SpaceOtter My expertise is mainly in chemistry. However I can answer some quantum mechanics questions due to I put a lot of effort studying it

Space Otter

My area of expertise is sustainable agriculture.... very useful for Physics SE XD

Secret

My interest however covers a lot of things, and it was found often strung together in weird ways by my free associations goin on in my thinking

this is why some people get confused about me, and I am taking steps to improve my communication

Space Otter

What's your first language. If I can ask

Secret

Cantonese

I also being told by some people, even native english speakers that I often use very difficult english words in my messages thus making it hard to understand. However I don't realise I have use many difficult words and I thought I am talking normally

Space Otter

14:19

There is a vast difference in intellect among english speakers...

Secret

Beginning from the first day on h bar, I have gone from posting a lot of nonsense, dumping uncaptioned figures everywhere and linebreaks everywhere, to a message writing style more readable by most users

and I have reduced a lot of posting of thoughts or at least made them a lot shorter

Space Otter

👏

John Rennie

@SpaceOtter doesn't that apply to all languages? :-)

Space Otter

@JohnRennie Yes, but the english speaking world never ceases to amaze me with it's blatant lack of sense.

Only two things are infinite...

2physics

I've always been thinking ACM is the name of a community or the abbreviation for an organization name or society lol thank goodness you mentioned

2

Secret

14:27

Currently, I communicate by distinguishing high priority posts form low priority posts. The high priority ones are often followed by pinging the user directly (reply pings don't count because it is inbuilt in the chat and hard to circumvert it). Low priority ones are unpinged

However if it is clear that there is a conversation going on between me and another user, I will just post the message directly with no pings

Also, the way I handle the case when so many users talking at the same time, I just post where my last message left off

2physics

Hi S, JR , SO , ACM and 0

John Rennie

@2physics Afternoon

Secret

Thi is easy for me to follow because they are highlighted blue in my window, and the user who is currently talking to me can follow them by the context

2physics

afternoon. 2p is here. you can also call me doubled p. :D

Secret

Reply pings can actually be substituted, but it requires fast typing. for example:

12 secs ago, by 2physics

afternoon. 2p is here. you can also call me doubled p. :D

Afternoon

2physics

14:31

@Secret evening, night?(for you)

Secret

yup

2physics

how do you use reply pings facilities

Secret

normally when you click the small arrow in a message, a window pop up and then you can click reply

This will give you a unique code to label the mssage, such as :32141905

you then type your message after the code

and then the chat will do this for you

@2physics example message

2physics

I do that but my replys aren't like yours

don't have any qoutions

Secret

O, that box is a quote, it is not a reply

it is done by posting the permalink of the chat message, no space and no linebreaks

e.g. chat.stackexchange.com/transcript/message/32141866#32141866

remove the e.g. and you get the box above

I just exploit how consecutive messages tend to glue together in a chat box to make it clear I am replyign to someone

2physics

14:38

"permalink of the chat message"

what is permalink????

Secret

click your triangle in any message, see the blue words 'permalink'?

that give you the URL of that message

copy that URL and paste it on chat and it will give you the box

2physics

@Martin Hi, have a question, what is the difference between physics and mathematical physics and do we also have a non-mathematical physics ??

chat.stackexchange.com/transcript/message/32142054#32142054 alright. thanks!

but it didn't work :o

2 mins ago, by Secret

click your triangle in any message, see the blue words 'permalink'?

Martin

@2physics: It's in a way an institutional difference more than a real difference. Mathematical Physics is usually a subdiscipline of mathematics rather than physics. This means that mathematical physicists want to do physics as rigorously as possible.

2physics

it did

Martin

For instance, you'll start out with axioms and derive everything you are interested in from the axioms as theorems.

Secret

14:43

I tend to use the mathematical physicists mindset for worldbuilding purpose a lot

Martin

Usual physicists aren't that careful. If you get a result that seems right although you take a few mathematical shortcuts (for instance you work with distributions as though they are functions) and the result agrees with experiments or gives interesting predictions that can be checked with experiments, then that's good enough. The mathematical physicist disagrees.

Secret

yeah, ordinary physicists tend to misuse distributions a lot, such as not using test functions and trying to multply two dstributions together

2physics

@Martin isn't " working with distributions" kinda mathematical work?

Secret

Dirac delta distribution is the most famous example where it is not treated rigorously

Martin

@2physics: It's a question of rigour. Obviously, physicists use mathematics, but they don't do mathematics. Mathematical physicists do mathematics.

Secret

14:48

People who are preparing technical questions for the AMA, please familarise yourself with the following video:

Roaming Radicals - Supplemental Video 3

►

Caption: This video describes a typical H roaming mechanism obtained from semiclassical trajectory calculations

Blue = hydrogen, black = carbon, red = oxygen

1. The molecule is being excited by some laser, it is now rotating and one of the CH bonds is having a high vibrational energy

2physics

@Martin sorry my knowledge of english is not that good, can you explain alittle more that what is the difference between "using" math and "doing" math?

do you mean what mathematical physics do is not practical?

Martin

@2physics: I'm not sure it's a problem of language ;). Are you familiar with quantum mechanics? Or classical mechanics?

2physics

alittle calassical mechanics

Martin

You could call it "practical", but that's not really the issue. The issue is rigour.

Secret

2. Eventually, the bond elongate to a long distance

Martin

14:51

Okay, then I'll try to make an example of classical mechanics.

2physics

@Martin I'm also not sure ; I only know that I didn't understand the differences lol

thanks

Secret

3. The H atom, now far form the atom, is still unstable to escape as it lack sufficient energy, therefore it is still attracted to the rest of the molecule by van der waal's interactions

4. It then "roams" and then started to enter a region where it felt attracted to the other hydrogen

5. As the roaming H approaches to the other H, the other CH bond breaks, $H_2$ is formed and it quickly move away vibrating rigorously

6. Meanwhile the CO continue to rotate slowly

Martin

Now, as a physicist, you'll take Newton's equations, you'll take things like the work-energy theorem and you'll solve problems. You need to know how much energy you need to lift your table? Oh, F=mg and then work-energy gives you the answer. I think you'd agree that we are using math here.

2physics

btw, I always can search google but as long as I have access to people who shape the information on the internet, I prefer to ask them through a live conversation rather than reading an essay.

Martin

But using F=mg is not really correct, is it? It's an approximation for the surface of the earth. But when does this become accurate? Well, you'll use the better equation F=gamma*m_1*m_2/r^2 and you just have a look at which values of r this is well approximated by F=mg.

Secret

14:56

your gamma should be G

2physics

yup

and?

Martin

one moment, being interrapted

John Rennie

@secret: I'm not sure I see the point of the roaming radical pathway, and the paper referenced by the YouTube video is behind a paywall.

Is the idea that resonant absorption of light from the laser kicks one of the hydrogens into a very weakly bound state? Sort of the molecular equivalent of a Rydberg state?

Secret

@JohnRennie Roaming is the central topic in my honours thesis, and I think the main topic of my AMA is my honours thesis, as anything else is incoherent. (cont on next post)

2physics

I feel like a roaming radical sometimes

Martin

15:01

ok, sorry.

That's all very well, but as a mathematician, this is unsatisfactory.

Secret

No, resonant absorption of light by the molecule excite the molecule to a the first electronic state

this energy is then partitioned into vibrational and rotational modes

2physics

alright, and then?

Martin

"Doing" mathematics means proving theorems. Proving theorems means: I give you a set of axioms and a set of assumptions and from these assumptions+axioms you can derive the result by pure logic.

2physics

ok, where those axioms come from?

Martin

So, what a mathematical physicist would do, he'd first define a set of axioms. Those axioms must contain everything that is implicit.

John Rennie

15:03

@Secret so this is a mechanism for pumping energy from light into rovibrational modes?

2physics

you mean a mathematical physicist doesn't believe in doing experiments?

Martin

You just choose them. As a mathematical physicist you choose them based on what physicists usually assume.

Let's leave out experiments for a minute.

Secret

@JohnRennie After that, the molecule dissociate into radicals, but the radicals don't have neough energy to escape form each other

2physics

well, was newton a mathematical physicist or a usual one?

Martin

So for instance, the mathematical physicist would choose Newton's laws as axioms + the fact that space is IR^3 and he would give a precise definition

Newton was neither - the disciplines have only developed in recent history.

Secret

15:05

thus they participate in this orbiting motion (at least for H and methyl roaming) until the weakly attracted radical encountered an attractive potential and abstract another atom e.g. H away to from molecular products highly vibrationalyl excited

Martin

he would give a precise definition of "inertial frames", and some more.

Secret

So basically roaming is an intramolecular abstraction reaction

The significance of roaming is that it forms molecular products with very high vibrational energy, which means it can further underone reactions in the atmosphere that are otherwise not possible

Martin

And then the mathematical theorist would say something like this:
Theorem: Given the axioms of classical mechanics and assuming Newton's law of gravity, an object of mass $m$ on the surface of earth (a sphere with radius r=...) experiences a gravitational force according to $F=mg$.
Proof: Plug in the radius and the mass of earth into the equation.

Secret

thus proviiding extra reaction pathways for chemical reactions in the atmospher

which is important in climate modelling

John Rennie

Aaaaah, this seems a good summary - NB it's a PDF

Martin

15:09

Does the mathematical physicist believe in experiments? Of course he does. But if there is an experiment that contradicts his theorem, this means that one of the assumptions or axioms was wrong.

Secret

@JohnRennie Yes, that's one of the review papers. It give you a nice outline of the roaming reaction

Martin

Newton was neither a mathematical physicist nor a mathematician, because mathematics, at his time, was not nearly as rigorous as it is today.

The idea of mathematical physics is the following: You clearly separate the assumptions you make (axioms+other assumptions) from the logic.

A theorem cannot be wrong because of an experiment. It can only be wrong because of logical errors. If the experiment shows a different outcome than the theorem predicted, then the assumptions of the theorem are wrong.

Shing

how exactly do I measure the energy of a particle in a box (infinite well)?

I am getting more and more confused by quantum physics :/

John Rennie

@Shing measure the energy or calculate the energy?

Shing

@JohnRennie measure the energy.

2physics

15:16

@Martin what is considered as an "axiom". what are the specifications of an axiom?

Martin

Another example: When physicists developed quantum field theory, they found out that the outcome was sometimes infinite. Then, they discovered that the ground state energy was already infinite and inf they substracted the two, they'd get a result that they could measure (and it was right). They developed a whole array of methods to do this "infinity-infinity" that always give them a result that agrees with the experiments (this is called "regularisation").

John Rennie

@Shing You can't measure the energy because you can only measure energy differences. So for example you could measure the energy of the photon needed to excite your particle from the ground state to the first excited state.

But this measures the difference between the states not the energy of the states themselves.

Martin

However, "infinity-infinity" is logically ill-defined. It's nonsense. The problem the whole mess occurs is that the physicist worked with a few objects as if they were simple numbers or matrices or other simple mathematical objects, although they aren't. A mathematical physicist will try to get the same result without doing the logically nonsensical step of "infinity-infinity". He's doing the same physics, but trying to sort out the logic more carefully than the usual physicist.

Secret

what is the mathematical physicist's approach to circumvert regularisation?

Shing

@JohnRennie thanks for answering, may I ask further how the electron goes lower states from an excited states? totally random?

Martin

15:21

An axiom is basically a statement that is taken to be true. You just say that it's true, even if there is no way of knowing if it is (for mathematical physicist, you'd take those things as axioms that are somehow "basic" but can be checked by experiments, for example Newton's laws).

Secret

It depends on how much the two states overlap

this give you the probability of the electron transitioning from one satet to another

John Rennie

@Shing In an atom the transitions between states are subject to selection rules. These arise because the photon acts as an electric dipole and it has an angular momentum.

ACuriousMind

@Shing Yes, the decay of excited states is "random" in the sense that you can't predict when it happens, but you can compute the lifetime/transition probability with techniques like Fermi's golden rule

Martin

@Secret: As far as I know, you don't circumvent regularisation really, but you make it mathematically sound (causal regularization I believe it is called).

ACuriousMind

@Martin Are you using "regularization" and "renormalization" as synonymous here?

Secret

15:23

@ACuriousMind the $\infty -\infty$ example above sounds like renormalisation

Martin

@ACuriousMind: Not really, but to be honest, I don't really care because I just needed an example.

2physics

can u name some famous mathematical physicists?

Martin

I know that you can do regularisation completely rigorous, so that's the example. Renormalisation is a bit of a problem afaik.

ACuriousMind

@Martin Okay, because I think what you're describing is "causal perturbation theory" which is a rigorous approach to renormalization

Shing

Thank you guys for the help, I will look the selection rules and Fermi's golden rule up.

Martin

15:25

@ACuriousMind: arg. Sorry, mixed it up again... It's been a long time...

2physics

was Einstein a mathematical physicist or not?

Martin

@2physics: What is your level of mathematics? Because it's really hard to understand the need of rigorous mathematics if you never had any formal training.

No, Einstein was no mathematical physicist.

Famous mathematical physicists include Barry Simon, Elliott Lieb, Herman Weyl, Henri Poincaré, Vladimir Arnol'd to name just a few.

2physics

@Martin please explain as you want to explain to a high school student

Martin

@2physics: It's hardly possible. The problem is that in school, all you ever do is calculate things. As above: Calculate some energy needed to do a task. Calculate a force.

Shing

um... to make sure at least I get the right big picture: so basely we will have to look at the spectrum of many ensembles (or many ideal particles have no interaction with each other in one single box) of such boxes? (and quantum physics predicts the values will lie in certain few values?)

ACuriousMind

15:29

@Danu I know you have this calculation lying around somewhere, maybe you want to write an answer? ;)

2physics

but I thought it's all about an approach.. their differences I mean

Martin

But that's not mathematics, that's calculations. Mathematics is about the relations of abstract objects (for instance numbers). For calculations, there is no such thing as "rigour" so you cannot really show the difference.

2physics

@Martin do we need both kind of physicists? and what are the privileges and advantages of each being a mathematical physicist or a usual physicist?

ACuriousMind

Oversimplifying: The mathematical physicists will tell you that rigour is important because otherwise you can't be really sure about any argument. The "normal" physicists will tell you that they don't care as long as it works. As usual, the truth lies somewhere in between.

Martin

@2physics: It's a matter of taste, but I believe we need both. There is basically a continuum between experimentalists and mathematicians and we need all of them.

John Rennie

15:35

@Shing generally speaking it's hard to make measurements of the spectrum of a single atom just because the absorption and emission from a single atom are very weak. So if for example we were studying a hydrogen atom spectrum we'd use a gas of many hydrogen atoms.

As long as the gas pressure is very low the atoms don't interact significantly so they behave in the same way a single atom would.

2physics

@ACuriousMind what do you mean by " rigor" can you give an example?

Secret

You need physicists to carry out their experiments without boggled down too much by the rigor, and the mathematical physicists to sort out the details of the models

Shing

@JohnRennie i see, it is much more clear to me now. Thanks for the answering :)

Martin

Those closest to experimentalists will try to give rough sketches of efficient theories that describe what happens and predict numbers. Others will try to develop bigger theories that encompass different simple things and give a broader picture (for example develop classical mechanics),

others will go even deeper and try to see what the "building blocks" of these theories are - and the mathematical physicists will make this rigorous to the point where you can say "see, if you think that the world has this structure, you'll HAVE to end up with the following predictions without using any physcis at all".

ACuriousMind

@2physics As @Martin said, it's difficult to explain if you've never seen actual mathematics. But at its basis, it's about every statement following logically from the axioms, and that you can prove that it follows logically. Most physics arguments don't hold up to that standard - they aren't careful about not introducing additional axioms, and they often make plausible arguments rather than formally logically sound ones.

Martin

15:39

In the end, whether you need mathematical physics or not is also a philosophical question relating to the question of whether you think that mathematics reveals eternal truths or is simply a tool (and that could be broken at any point).

ACuriousMind's quick simplification is a very nice expression of this.

Secret

Our universe seemed to be described rather well by mathematics, thus it will be a while before we get to that point

It is hard to imagine what physics laws are like that are not governed by mathematics, if any

2physics

well, considering your kind explanations, I personally prefer mathematical physicists approach..

Martin

@2physics: Look here: math.stackexchange.com/questions/473733/…

Secret

Same, physicists are too sloppy that sometimes I lost track on what is going on

John Rennie

@2physics do you know what a Feynmann diagram is?

2physics

15:44

but, how do a mathematical physicist know that the axioms are enough and there is no need for more axioms to be taken into account?

Martin

That's sort of a a good explanation of what "rigour" is about. It's about filling in "and as we see"...

2physics

@JohnRennie nope . I just like gell-man more than him lol

ACuriousMind

@2physics They don't. Finding the "right" axioms to start from is the first challenge.

John Rennie

@2physics ah, OK, I was going to use them to illustrate a non-rigorous calculation, but that's a non-starter then!

2physics

and plus, the last (at this point) question: what's the difference between an axiom and a principal when we cant test that principle?

Secret

15:47

Starting to worry about my performance in a few hours given my massive memory fade when trying to answer that roaming question

2physics

@JohnRennie does "rigor" means "hard" here? or it means intensive ?

Martin

@2physics: An "axiom" is a technical term from formal logic. By definition, it's something that we take to be always true. An axiom in Euclidean geometry is for example "Two parallel lines do not meet at infinity" (well, actually it's a sloppy way of writing the axiom).

A "principle" is (often) a physcists saying "well, here is a pattern I observed and believe that it always holds". The mathematical physcist could take this principle as an axiom or he could derive it from other axioms.

2physics

@Martin yup I know that; but for example if the principals of Relativity have been considered as "axioms" at that time?

Space Otter

Is "Physics chat session" not always happening?

Martin

@2physics: You can do it, yes. But a physicists wouldn't really care...

John Rennie

15:53

@SpaceOtter the chat session is always at 16:00 UTC every fortnight.

Space Otter

So what's the rest of the time? @JohnRennie

John Rennie

So there isn't one every week, if that's what you are asking.

ACuriousMind

@SpaceOtter The rest of the time it's just chat, no session :P

John Rennie

Well, the chat session is supposed to be organised. This time we're talking about Secret's work on molecular dissociation pathways.

The rest of the time we just babble incoherently.

Secret

which I am not very sure if I can answer all questions 100% correctly...

Space Otter

15:55

@JohnRennie @ACuriousMind OOoh. Ok. That's the answer. so there's a topic

Has secret published work?

Martin

@2physics: The difference between an axiom and a principle is that you can take anything as an axiom. You could say "Axiom: 1+1=3" and see where you get from there. Calling something an "axiom" just says: "look, for my theory, this is always true", and it gives the impression that you want to present a logical theory.

Secret

@SpaceOtter nope, my honours just ended last year

the group may have just submitted a paper, it is not going to be acepted so quickly

John Rennie

This weeks chat topic is Secret's work on the roaming radical mechanism for molecular disociation and other related issues. Read this for some background.

Martin

Since physicists want to present a theory that predicts reality (never mind that it's logical, if you have an illogical theory that describes reality, so be it!), they call basic building blocks "principles".

Secret

As a security measure I have my thesis and presentation slides opened up in my window

David Z

15:57

@JohnRennie You want to handle the chat session today? i.e. doing the introduction and stuff

Secret

and if my cosupervisor Kelvin is on, he is liekly to spot all those crazy mistakes I made in answering some molecular specotroscopy questions

Let's hope all goes well...

user116211

@Secret all the best.

John Rennie

@DavidZ err, I can do. I suppose as an (ex) physical chemist I'm probably best qualified ...

Space Otter

Oh dear. Chemistryesk stuff

John Rennie

@SpaceOtter physical chemistry.

Space Otter

15:58

@jhonh note edit

John Rennie

that's chemistry where we beat you up if you disagree :-)

Secret

johnrennie might be able to help in case I slipped into misconceptions

David Z

@JohnRennie Sure. Simply because I don't feel like it :-P but also it is nice if there is someone else able to handle chat sessions when I'm not around

Secret

he has the same chemistry field, as i am back in honours

Martin

@2physics: I guess this all sounds a bit vague to you. If you really want to understand more, I'd adivse you to do two things: a) have a look at Euclid's "Elements". That's a treaty trying to do Euclidean geometry "rigorously"; b) have a look at an introductory logics book. You don't have to read it all. Just try to think about how YOU would do the whole thing and what the "rigorous" way is doing better.

John Rennie

16:00

The chat session has officially begun. Please refrain from unrelated chatter for the next hour.

@Secret: would you like to introduce yourself?

2physics

@Martin thanks for your patience and good explanations

Secret

I am secret, I worked in my honours project last year on molecular dissocation pathways investigations

the title is "Investigation of hydrogen production in the photodissociation of acetaldehyde

So to begin

The molecule we have investigated is acetaldehyde

It is an intermediate formed after hydrocarbon released from plants and industrial process were beign oxidsed

Acetaldehyde can then undergone 3 different fates in the atmosphere

It can either react with NO2, OH and other radicals to form peroxyacylnitrates, which is a major component of photochemical smog

It can condense with other organic compounds to form aerosols

Or it can absorb light and be phtolysed

Acetaldehyde has many pathways it can dissociates to

For example:

As seen in the figure, aceltadheyde can dissocaite into radicals (pathway 2 and 1b)

or into molecules (pathways 3 and 4)

My project is to investigate pathway 4, the hydrogen production pathway

and to determine whether it exists, and if so determine its mechanisms

The next pics show the device we use for the epxeimrent

We have a sample of acetaldehyde being dispersed in Helium gas, which is then injected into the vacuum chamber from the right

The sample then is being coooled by the skimer and form a molecular beam.

Foloowing that, a laser then phtoloyse the acetaldehyde into hydrogen and ketene

Antoher laseer then ionise the hydrogen fragment,which is then accelerated through the flight tube to the phosphor screen

A camera then photographed the lit screen to obtain the image of the velocity distribtuion of the H2 fragments

vzn

hi secret thx for agreeing to session! your dedication to learning physics fits in well with the site! you have an honours chemistry paper and have worked in a lab doing advanced research in chemistry frontiers. congratulations on starting the phd venture also! heres secrets meta intro

Secret

@vzn You are late, we are midway through

if anyone have any questions,feel free to interrupt me

The laser we use to ionise the H2 is state selective.

2physics

can we ask any questions?

Secret

16:12

I said, feel free to interrupt me with questions

just post any question in and the reply function of the chat will ensure all messages are beign correctly acknoewledged and followed

Constantine Black

Hi. Does the motive for the experiment come from theoretic considerations and are there any practical motivations? Thanks.

vzn

did you work your entire senior year on the acetaldehyde prj? how many pages is your honors thesis? is it online?

Secret

@ConstantineBlack The primary motive is fundemental interest on molecular mechanisms, but a possible practical motivationo is to iprove prediction of atmoshepric chemsitry hence improve climate models

Terry Bollinger

(Hello-goodbye, back to reading a fascinating 1949 Born paper on "Reciprocity"...)

Secret

@vzn The honours year takes 1 year, there are 42 pages (not including reference) in my thesis, I have not put it online yet

Constantine Black

16:17

Could we have a short description of the current theory you worked on for dissociation? Thanks.

vzn

did your senior honors prj play some role in your phd admission? can you comment on your phd program entry, are you accepted yet, have you started? will it be physics or chemistry focused, building on your prior work?

John Rennie

Is it also worth mentioning that the usual energy minimisation methods for calculating cross sections in this sort of experiment do not give good predictions?

Secret

@JohnRennie what cross sections are you referring to ,the absorbption cross section of the molecule?

John Rennie

Ratios of the various species produced by the dissociation

But if this is a distraction just ignore me ...

2physics

(okay, sorry I meant if it's an AMA or just questions related to the thesis should be asked. and then I realized that it's an AMA session. thanks; thanks for helping me personally to learn more about physics and thanks for accepting to attend this session)

Secret

16:21

@JohnRennie For that experimentally, we determine the ratio of the hydrogen in different rovibrational states by spectroscopic methods. For the computational part of the project, I only have done geometry optimisation of the transition states and the calculation of energy barriers to the reaction

To calculate the cross sections, the research group use semiclassical tajectories

it is found that trajectories started at transition states give very different cross sections compared to roaming mechanisms

@JohnRennie While for my case it is not sure because we have not run trajecotries for the pathway that is investigated in my hounorus project, generally trajectories start at the groudn state minumum of a moelcule agree better with expeirments that have signs of roaming, and they don't agree well with those from transitiona state caltulations

John Rennie

@Secret which are a better match to experiment?

vzn

can you comment some on role of acetaldehyde in atmosphere? low concentration? any effect on humans/ animals? has it chged over time? is modern industrial age affecting concentrations? any known/ hypothesized connection to climate chg? etc

Secret

@ConstantineBlack Breifly, roaming is a radical dissocation reaction followed by abstraction. That is,

0celo7

Lots of people here...

Secret

you have a moelcule being pumped with energy and it goes to an exctied state, it then fragment into radicals,

these radicals don't have enough energy to escape form each other, thus they orbit around each other until asbtraction occurs

Space Otter

16:28

@0celo7 Shhhh! Secret's lecturing

Secret

forming highly vibrational excited mlecular products

0celo7

@JohnRennie Still just electron shell physics

Secret

@vzn I am currently writng my proposal, it will be started on 1 march. It will be buotl upon my honours proejct for the computational quanutm chemsitry skill I acquired

@vzn I don't remeber the exact numebrs form the top of my head, but the concentration is generally higher in urban areas. Acetaldyehdye is carcinogenic to humans and nimans thus you don't wnat ot sniff it

One of the photoproducts of acetaldehyde is methane, and methane is a well know greenhosue gas, thus it has roles in climate modelling

and I don't remeber the anuual chanfes of it over the courseo f history

Space Otter

Would we want to promote the mechanism taking place then?

Secret

@SpaceOtter We don' have any reason to promote it nor inhibit it, but understnading it better will help us predicting the fate of chemicals in the atmosphere

romaing was found in condensed phase too ,but it is still a bit early on saying how it can help on chemcial synthesis

@vzn my phD planned to do something metal complex related, no more gas phase chemistry so I can get my hadns dirty in the synthesis. However there will be comptational chemistry to help back up the experiment observations, as well charactiersing with things like NMR\

Roaming is signficant because it means traditioanl transitionstate theory need to be modified in order to predict the rate of some chemical reactions, and for some species romaing domianntes the eaction pathways

For example, methane formed by acetaldheyde is mainly via the roaming mechanism

John Rennie

16:36

It would be interesting to hear what is special about the roaming mechanism

i.e. how it differs from what we normally think about the way a molecule splits up.

vzn

so youre very chemistry focused but like to hang out on a physics chat room. any comment on the interplay/ boundaries between chemistry/ physics? would you say that there were physicists on the acetaldehyde prj? what is the attitude toward physics by the members of the team?

Secret

roaming make molecules with very high vibrational energy. Imagine when these molecules collide with atmosheprci moelcules, thsi exneergy can trigger reactions that ae other too slow to hapenn in ordinary conditions

Romaing is not very differetn from a dissocation, because the moelcuel first dissocaites into radicals, which then recombine into new moelcules. It is basicallt an intramoelcular abstraction reaction. Romaing is aalso govern by a saddle point, that is very flat, thus a bit different form ordinsry trsition state

@vzn All people in our group are physical chmists, some spectroscopist and moelcular dynamists. We do have a physicists in our conference, and he asked some questions regarding the spectroscopy

Constantine Black

Very interesting. Is the model mainly quantum mechanical. And may I ask if there is any chaotic behaviour and such studies on these systems and processes? Thanks.

Secret

@ConstantineBlack Our model is semiclassical, that is, we use trajctories obtained from solving a newton equation of motion to propagate it. But for the treatment of the rovibrational states, and enegry calculations we use quanutm mechanics such as hartree fock and post hartree fock methods

@vzn Chemistry and physics are closely related, 0celo7 is right in a sense that chemsitry is electorn shell physics. Chemistry makes the stuff and the physics describe how the stuff is made

Constantine Black

Is the reason behind semi-classical treatment the difficulty of the calculations or a physical reason establishes such a modelling as good?

Secret

16:45

@ConstantineBlack The major issue is the computational expsnese. solving the schordinger equation for a molecule is very hard (let alone sovling how the state eovlve with time). I have read some groups use wave packets and propagate it on the potential energy surface to do almost fully qunautm mechanical calculations, but our group stick to trajectories

vzn

what is MP2/ CCSD QM chemistry frameworks mentioned in your meta post?

Secret

@vzn MP2 and CCSD are quanutm chemsitry methods in solving the schoridnger equation f a molecule. In hartree fock, we only took account of interaction energy caused by exchanging electrons, in MP2, we include repuslion between electrons by a pertubative series up to 2nd order

Space Otter

What other reactions are likely to occur in the atmosphere due to the highly energised radicles? That otherwise wouldn't have been predicted without your mechanism

Secret

and for CCSD, we consider the energy contributiosn of all possible terms of exciting 2 elctrons to other states to account for the electron repulsions

vzn

do you use simulation software in some way? any standard systems/ packages? any custom code?

Secret

16:50

@SpaceOtter We might expect things such as some oxidaton reactions will occur as if the mixture is in some high temperature

Constantine Black

Any fractal behaviour?
Does dealing with molecular dynamics and chemistry gives a chance of viewing quantum mechanics in a different manner? For example, is there a difference on how one sees time in a system like that? What' s the role of entanglement? Thanks.

Secret

@vzn The standard computational cheimstyr package is gaussian 09. I also spent the frist semester writing a mathlab program on analysing spectras

@ConstantineBlack I don't recall seeing any chaos in any roamign papers I ahve read. But chaotic phenomoen in chemistry do exist based on other journal articls I have read

2physics

well most of these questions can be asked on SE. But let me ask sth sincefree in between .what's your favorite movie? lol

vzn

you mentioned biology as an interest in your meta post. any reaction to an apparent emerging research trend/ program of "QM + biology" aka "quantum biology"?

John Rennie

Gaussian 09 - for anyone who fancies a bit of modelling.

Secret

16:54

Molecular dynamics helps model and follow the chemical reaction with time, thus it contribute massively on understanding how catalysts work and how drugs bind to receptors.

I am not sure about if entanglement can survive in such turbulent system. I am also not sure if roamign mechanism also took place in photosynthesis

Constantine Black

And on a different level, could any "technology" come from these understandings, such as biochemistry applications or pharmaceuticals or any new knowledge on biological systems? Thanks.

Secret

@2physics I don't have any in mind, but I liek movies tht make you think such as inception

@vzn It is an exciting field, beginnnign with the observation of long lived coherence in photosynthesis. I won't say so quickly like some it can explain consciosuness though, more to be learn as it is still a young field

@ConstantineBlack suppose there is another group that followed up on the bromomethane paper and found roaming is ubiquitious in condensed phase, then people might start to think about controlling it or inducing it to make desirable products, such as drugs or catalyst and other materials that otherwise might need high temperatures to make

but I must say this is a very rough guessing

vzn

@Secret spking of consciousness and on a side area, youve mentioned in chat you keep a dream journal & have occasionally even written up dreams in here (some involving chat regulars!). whered you get idea, how long have you been doing it, what is its meaning to you? etc

Secret

@ConstantineBlack We don't see any fractal like behaviour in our reactions, ad none of the roaming papers reported any fractal behaviour so far

@vzn Since a lot of my dreams become complicated especialy due to inception, and often contains random mixtures of real life academic stuff, I tend to extract them and analyse themto see if they can be used

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