The h Bar

Ben Sprott

12:08 AM

has anybody here studied causal structure approaches to quantum gravity, or category theory for physics?

tpg2114

12:42 AM

Neat -- photo of light as a wave and particle at the same time:

nature.com/ncomms/2015/150302/ncomms7407/full/ncomms7407.html

Qmechanic

1:02 AM

3

Q: Careers in physics that are related to quantum mechanics or relativity?

Personally I learned Physics in high-school and found it very interesting, I read up a lot about physics in my free time. Personally I am also a programmer which I think is also good when it comes to Physics. Now I want to start studying in a university and my dilemma right is mainly between Ma...

On-topic?

Sean

1:28 AM

That question's 3 years old

It strikes me as a "big list" question. Aren't those generally off-topic?

Sean

2:02 AM

Not to fuel @ACuriousMind 's paranoia any more, but NikolajK posted an odd comment to annav 's answer on this meta question: meta.physics.stackexchange.com/questions/6567/…

Stan Shunpike

3:19 AM

@alarge So going to your point about converting 2nd order ODEs to first order ODEs which one is which in the Hamiltonian vs Lagrangian case? After all, these are partial differential equations, so I don't know if that matters.

I am still confused why the EL equations is one equation but Hamilton's equations of motions involve two separate equations.

I don't follow how the conversion of a higher order differential equation to a first order differential equation applies in this case.

user54412

@StanShunpike Haha. He was one of the first people I met there. He also had a harpsichord in his dorm room. What a weird and awesome place it was.

Stan Shunpike

that's hilarious. I hadn't heard about the harpsichord.

user54412

@StanShunpike About Mars, I feel like one-way trips might be doable (and there would be no shortage of volunteers) in our lifetime, maybe. But right now I'm pretty sure you'd be dead on arrival due to radiation poisoning.

Stan Shunpike

From what? The sun?

user54412

Everyone gets so caught up in the sociology and the difficulty of returning and how to make a self-sustaining colony, but they gloss over the fact that shielding against radiation is not a solved problem.

user54412

3:25 AM

And cosmic rays.

Stan Shunpike

Yeah, because of no magnetic field you mean on mars?

user54412

Yeah, there's no magnetic field, nor is there an atmosphere to speak of -- Mars is essentially vacuum

Stan Shunpike

right, that's why I'm amazed people talk about going there. It's like they think it's going to be Earth Part 2 or something

user54412

Remember, even the shortest path through Earth's atmosphere amounts to the same column density as 10 meters of liquid water or 1 meter of lead.

Stan Shunpike

Wow, that's a convenient way to think about it.

Have they tested the problems with radiation on the moon. Or even just the Space Station?

Like, how would people sustain food?

user54412

3:31 AM

I feel like there must be some experiments, but certainly the space station crew relies on dried food shipped from the ground.

user54412

I was talking with a guy who went to the space station as a Russion "tourist" and he said when new food shipments arrive the astronauts binge on the shrimp cocktails, since they taste best. Then they spend the next couple months eating the lesser foods.

Stan Shunpike

That's funny. Humans are predictable. I wonder how they ration it or if they even do.

I've heard that the lack of gravity affects bones and creates other problems.

Stuff you don't notice because we always have earth's field acting on us.

Jiminion

In theory, you could terraform mars and get the atmosphere thicker, would take maybe 100 years.

There is lots of frozen CO2 and water on mars.

user54412

I've heard that idea before, but I'm skeptical how well it would work

user54412

Mars is also pretty light -- it's easy for gasses to escape its gravity, so without continuous replenishment you'd only have a temporary solution

user54412

3:37 AM

I'm just not sure offhand what the timescale for leakage is

Jiminion

timescale is 50 million years, mostly due to stripping from the solar wind.

So if you could "gas it up" it would be good for quite a while.

user54412

I guess that's considered a long timescale for some applications :)

Jiminion

You get the atmosphere up a bit and then if you can get just a bit of O2 in it, you could support plant life (in theory). Then they'd get you more O2 over time.

I think the lack of magnetic field mostly protects against gamma rays (?) If you had a thick atmosphere, that would help quite a bit.

user54412

If nothing else, we could probably grow tomatoes quite well -- I understand that they are C02 limited here on Earth

Jiminion

Not much nitrogen on Mars though. Maybe we'd have to crash a few comets into it for the ammonia.

user54412

3:43 AM

@Jiminion Magnetic fields protect against charged particles (which have a habit of spawning showers of more particles). Typical gamma rays are really just stopped by matter. It doesn't matter what the matter is, just how much there is along the line of site (really how many electrons, which is proportional to mass).

Jiminion

How bad are charged particles? Isn't that solar wind stuff?

Mars is farther away than the Earth, so the solar wind is much less intense (so is the sunlight....)

user54412

Here's an example of how charged particles can be nasty even (especially) when you stop them:

user54412

2

Q: Leakage of X-ray radiation

Suppose a sample of strontium-90 is stored in a lead container with lead walls. It is know that X-ray radiation may be detected outside the lead container. After some discussion with my peers, it seems that we have differing theories on how the X-ray radiation is formed. Beta particles emitted ...

quantum-mechanics nuclear-physics radiation

user54412

In short, they can induce lots of secondary radiation

user54412

In some sense, insufficient shielding can be worse than no shielding at all

Jiminion

3:52 AM

I think a lot of tough lessons would be learned by trying to live on Mars, but it's the second most habitable region in the solar system -- hard to ignore the prospect.

user54412

Our solar system does have something of a dearth of liquid water

user54412

Actually, some astronomers have turned their detection toward "exomoons." The idea is that even if the only planet at a nice distance from a star is a gas giant, it could have lots of moons with solid surfaces and atmospheres.

user54412

That's actually a cool sci-fi premise I wonder if anyone has explored -- a civilization on such a habitable moon might have equally habitable moons right next door.

Jiminion

That was basically the premise of Firefly; we trashed our planet and moved to a system that had something like 70 habitable moons in it.

NeuroFuzzy

4:22 AM

Speaking of which, has anyone heard that one thing of Lawrence Krauss talking about Interstellar?

He hates it! Absolute buzz-kill!

it had its "moments", sure...

Also I did not know that was the premise of Firefly @Jiminion

David Z

5:20 AM

@Qmechanic I don't think so

@DaveCoffman (1) try to find something else that can be improved which will boost the edit up to 6 characters (2) if that doesn't work, leave a comment pointing out the change you think needs to be made (3) if the comment doesn't get a response after a few days, post it in chat and hope a higher-rep user sees it and makes the edit

I'd point out for the record that anything that can be fixed with an edit is generally not a valid use of a flag.

user215721

5:56 AM

6

Q: How would changes in the Earth's magnetic field affect space exploration?

It is well-known that the Earth's magnetic field may flip (geomagnetic reversal), and some data suggests it is weakening or distorting. The following shows the Earth's magnetic field at its surface in June 2014 (by ESA) which shows some distortion. My question is, can such changes in the Earth...

artificial-satellite orbit magnetic-field earth geomagnetic-reversal

Stan Shunpike

8:52 AM

I'm trying to learn more about Yang Mills theory. I am very confused by the $F$. Is this the Faraday tensor or is this some other thing that involves a commutator with guage covariant derivatives?

That's nor intentionally supposed to be vague, I just assume if you (the answerer) are familiar with Yang Mills you will know what I am referring to.

Not*

David Z

9:30 AM

@StanShunpike let me suggest that that is (almost) never a good assumption to make. Even when communicating with experts you often have to define your notation.

That being said, I'd guess the $F$ you're referring to is the analogue of the Faraday tensor.

Yang–Mills theory

Yang–Mills theory is a gauge theory based on the SU(N) group, or more generally any compact, semi-simple Lie group. Yang–Mills theory seeks to describe the behavior of elementary particles using these non-Abelian Lie groups and is at the core of the unification of the electromagnetic and weak forces (i.e. U(1) × SU(2)) as well as quantum chromodynamics, the theory of the strong force (based on SU(3)). Thus it forms the basis of our understanding of particle physics, the Standard Model. == History and theoretical description == In a private correspondence, Wolfgang Pauli formulated in 1953 a six...

Stan Shunpike

10:02 AM

@DavidZ Point taken. I will remember in future.

@DavidZ the equation I have in mind reads $\mathcal{L}_{\mathrm{gf}} = -\frac{1}{4} F^{a\mu\nu}F^{a}_{\mu\nu}$. Assuming this is the Faraday tensor, why does this particular Yang Mills equation include the Faraday tensor? I thought Yang Mills was used in QCD.

And I thought QCD had not been formally unified yet with electroweak interaction.

JamalS

10:26 AM

@StanShunpike In general, the $F$ corresponds to the curvature of a $\mathfrak{g}$ Lie algebra valued form. This is particularly evident when you look at the commutator of covariant derivatives. For Yang-Mills, the Lie group is $SU(N)$.

Stan Shunpike

@JamalS what kind of form?

JamalS

@StanShunpike Well, for example, in Maxwell theory $U(1)$ is abelian, so $F$ is just $dA$, where $A$ is the $4$-potential.

Stan Shunpike

Oh so differential forms then.

JamalS

Oh, yes, by form I mean differential form.

Stan Shunpike

@JamalS Okay. I didn't realize forms could be Lie Algebra valued.

That's an interesting property

JamalS

10:33 AM

Yes, that is, it takes values in the Lie algebra, the vector space equipped with the Lie bracket.

Stan Shunpike

Hmmm...okay that's all very helpful. I guess knowing this, I need to spend some more time getting acquainted with Yang-Mills. But these were all useful pieces of info. Grazie

JamalS

@StanShunpike arxiv.org/abs/0803.1340

That paper may be useful to you, if you're interested in the historical development and 'physical idea' behind the theory.

David Z

11:18 AM

@StanShunpike it's not actually the Faraday tensor, as I said; it's the analogue of the Faraday tensor for a nonabelian gauge group.

The definition is in the Wikipedia page: $F^a_{\mu\nu} = \partial_\mu A^a_\nu - \partial_\nu A^a_\mu + gf^{abc}A^b_\mu A^c_\nu$

For an abelian theory, like QED, the last term (specifically the $f^{abc}$s) is zero, and in that case $F$ is the Faraday tensor. For a nonabelian theory, a.k.a. a Yang-Mills theory, the last term is not zero, and $F$ is what I'm calling the analogue of the Faraday tensor.

Danu

11:40 AM

@DavidZ Does one only call it Yang-Mills in the case of a nonabelian gauge group? I thought electrodynamics was the simplest example of Y-M.

David Z

In my experience, Yang-Mills means a nonabelian gauge group

Danu

Interesting, thanks

David Z

The reason being that Yang & Mills' contribution to the field was extending the formalism of gauge theories to the nonabelian case. Before that people had only used abelian gauge theories.

(or so I've heard)

Danu

Wikipedia seems to agree with you

Danu

2:14 PM

Formatting question: With standard paragraph indents in LaTeX, one sometimes gets really awkward indentation after an equation. What is you guys' favorite way to 'do' paragraphs in LaTeX?

alarge

@Danu Don't smash enter after equations, comment out the empty lines.

Danu

@alarge Okay, this was my solution too. The thing is, I'm editing a 83-page document that doesn't do this, so it can get kinda annoying

alarge

Well you could just search&replace \noindent after each \end{align} or something. not a pretty solution, but hey. Or while you're at it, you could just erase the empty lines after equations with some regex magic. Yes, there will probably be a couple of places where you actually wanted the new paragraph, so... have fun.

Danu

@alarge Yeah... :P

It's nice to do something like this though, it forces you to think about these issues.

For instance, I learned the difference between -, -- and ---

...and discovered I had some pre-existing opinions on how to use them, too :)

alarge

Have you not learned that in class? I mean the different lines are used even in hand-written (and newspaper) text, right.

Danu

2:34 PM

@alarge Nah, not really

I didn't know the difference between -- and ---

...and it's totally unimportant :P

But I like to know it and correctly apply them anyways

Also, is there any reason for leaving two spaces after a period in a .tex file?

apart from 'because I can'?

Jimdalf the Grey

So I'd just like to say that as of today, my union is on strike

Danu

2:51 PM

@JimdalftheGrey Union of... what exactly?

$\operatorname{Hom}$ is supposed to denote homomorphisms, right?

Jiminion

York U? Grad students?

Jimdalf the Grey

@Jiminion Actually, I'm contract faculty, but yes

Jiminion

How are people going to get their equations renormalized? They need that every day!

thestar.com/news/gta/2015/03/02/…

Jimdalf the Grey

UToronto is on strike too

Danu

why?

Jimdalf the Grey

2:56 PM

stupid reasons

the union asked for too much in bargaining and wasn't willing to negotiate. Same old story

Danu

Hey, you're supposed to be on their side, no?

Jimdalf the Grey

I voted for the strike, but that doesn't mean I agree with why they wanted it

Danu

What does ~ in front of something in a latex equation do?

Jimdalf the Grey

extra space I think

Danu

mkay

Kyle Kanos

3:12 PM

@tpg2114: You'll be happy. I've decided to give a quick ~20 minute lecture on colormaps for our weekly Astro seminar.

dmckee

3:43 PM

@Danu In text mode a tilde is an non-breaking space. It prints as a space, but tell the layout engine to treat the two bits as a single word for layout purposes.

Use it in place where you don't want a line break. Like between a title and a name: "Dr.~Smith".

I'm not sure what it's significance is in math mode.

Jimdalf the Grey

as I cosmologist, I take offense to that statement. We are not opposed to accepting new ideas. You are simply number 2158 of the people that have approached me with a new, better theory that explains everything in the universe. I am now serving number 112. I will get to your theory when I have exhausted or disproven the ones who got here first. If you could show me some credentials that merit it (like a degree in cosmology), I will certainly move you to the front of the list. But if you can't, then I have to treat your theory like the other 2045 waiting on the list — Jimdalf the Grey 20 secs ago

^ I'm proud of that one

dmckee

@JimdalftheGrey ::snorfle:: Nice. Or rather, "nicely done" because it's not really that nice.

Jimdalf the Grey

It's not nice, but neither is him saying that cosmologists are close-minded and arrogant

we simply deal with many more "revolutionary" theories brought to us

Kyle Kanos

@JimdalftheGrey To be fair, don't most people think that all scientists are close-minded and arrogant?

Jimdalf the Grey

probably

Kyle Kanos

3:55 PM

So cosmologists, being scientists, are just a subset he's picking on

dmckee

In fact many scientists are close minded, just like many non-scientists. It's the process that makes science special, not necessarily the people. And yeah, he's being a twit.

Jimdalf the Grey

@KyleKanos Yes, but nobody picks on Jim without getting a hot blast of snark shot back in their face

Kyle Kanos

@JimdalftheGrey That is reasonable.

Jiminion

@JimdalftheGrey What statement? String Theorists seem opposed to accepting new ideas for the last 30 years.... (not saying you are one of them....)

Danu

@dmckee Hmmm... I was aware of the text mode usage. I encountered it in the document that I'm currently editing. Oh well, I ended up removing it and saw no change ;)

Jimdalf the Grey

3:59 PM

@Jiminion bah, string theorists

love to stay and chat, but I'm due at the picket lines today. Ciao

Kyle Kanos

Serious?

Jimdalf the Grey

yes

Kyle Kanos

Pics or it didn't happen ;)

dmckee

@Jiminion The problem with string theory right now is that the end of the process where ideas are put to the ultimate test is flapping around loose. There is no mechanism for forcing the field to confront any problems that they might have.

Jimdalf the Grey

@KyleKanos And let you see my face? Yeah, right

Kyle Kanos

4:01 PM

@JimdalftheGrey You could take pictures of other people, not just you, you know

Jimdalf the Grey

Who is this "other people"?

Kyle Kanos

Other picketers

dmckee

Perhaps is face is particularly memorable...

Jimdalf the Grey

There are other people out there!?

dmckee

... and of course I mean more comely than us mere, flawed mortals.

Kyle Kanos

4:02 PM

Yes, you solipsist

:D

Danu

@KyleKanos Good point: Just today I was reading Snow's speech on 'The Two Cultures' where he voices exactly the same opinions (not his own, but he is paraphrasing colleagues from the 'humanities culture')

Jimdalf the Grey

Wish I could stay, but picketing=money

Danu

@JimdalftheGrey wut, really?

Jimdalf the Grey

yes

that's why I voted for the strike

Danu

@dmckee I really like the idea that I heard Witten express in an interview once: Maybe string theory will turn out to be a new branch of geometry.

Jimdalf the Grey

4:04 PM

I'm off

Danu

Loving it

Have fun!

Kyle Kanos

Does anyone know of a truly awful looking 1D plot?

Characteristics I'm looking for: too many data sources, bad color choices, too busy (many lines)

Danu

google.nl/…

Lol, 'One Direction' is dominating the results

Kyle Kanos

Yes

That was the hard part

Danu

Can't you make one yourself?

Kyle Kanos

4:06 PM

I'd rather draw from published works, but I could make one I guess

Error bars would be nice in there too

I found a really good one but can't use it because the guy who made it is going to be in the audience and I don't really want to insult him

Danu

LOL

share it!

Kyle Kanos

Danu

Holy shit, dat legend

Kyle Kanos

Yeah

Danu

all the similar-looking symbols

Kyle Kanos

4:10 PM

That's the big eye catcher

The symbols are reused, though filled vs not-filled

But your error bars make it hard to discern

Danu

Really bad

Kyle Kanos

Yep

It'd be a good one to use, but I can't :(

Jiminion

@dmckee The Theory of Anything....

alarge

@KyleKanos I'm sure Tufte rips several apart.

dmckee

BTW -- Question with net scores lower than -3 no longer show up on physics.stackexchange.com, so that is one way to get them to disappear without needing to muster a crowd or a moderator for deletion.

3

I believe that those with scores lower than -8 don't even show up on physics.stackexchange.com/questions

ACuriousMind

4:58 PM

@Danu It, uh, depends, but in general, it'll be the set of morphisms, yes.

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