The h Bar

@FenderLesPaul Nawh. Girlfriends (well, romantic interests of the apposite sex) keep you from doing as much physics as you would otherwise have accomplished.

FenderLesPaul

@0celo7 I'm pressing the play button but nothing is happening

@dmckee haha true

dmckee

@obe It's not actually impossible, but it takes a pretty special kind of person to make it work.

And probably a lack of the aforementioned romantic interest.

FenderLesPaul

3:21 AM

@0celo7 is it different on a mac?

0celo7

3:32 AM

it's streaming for me

so IDK what's up with your PC @FenderLesPaul

FenderLesPaul

idk it didn't do this on my old laptop

0celo7

Well I got pissed off by metro

I'm done

FenderLesPaul

:(

Andrew G

Given the recent claims Hawking has made about one of our problems with black holes, I've been thinking about conservation of information and reversibility. Specifically, if there are any truly irreversible processes that go on in the universe.

Put simply: If I was laplace's demon, would I be able to reverse-engineer the universe all the way back to the big bang with complete certainty, or are there some processes where the final state of a system could have come about from two/more different states? Matter-antimatter annhilation comes to mind…

NeuroFuzzy

3:54 AM

@AndrewG Something I've heard is that in quantum mechanics the past is stochastic in the same way the future is.

Andrew G

@NeuroFuzzy Interesting. Sounds many-worldsy.

NeuroFuzzy

@AndrewG It sort of is. I'm thinking from a Schrodinger's equation POV here; if you knew "the" wavefunction of the universe, which only someone sitting outside of the universe having prepared its initial state and having not observed it since [but calculating its future state exactly through Schrodinger's equation]; then you could derive the initial state of the universe

Andrew G

...but not if I was given the entire state of the universe sometime after it's creation, and asked to derive the initial state.

NeuroFuzzy

Yeah, then it would just be probabilistic, I believe.

but 1. the world doesn't follow Schrodinger's equation, and 2. observing a quantum system while sitting inside a quantum system is zany (or if you're boring 'subtle' :) )

Andrew G

Why wouldn't the world follow the Schrodinger Equation? If I was given the current state of the universe (exact location and velocity of every particle), could I not extrapolate the future state of the universe with certainty?

NeuroFuzzy

4:02 AM

@AndrewG oh I just mean that, like...

Andrew G

(Given infinite computational power.)

NeuroFuzzy

the world isn't classical, it's quantum. But it's not "schrodinger quantum" because that violates SR

and it's better to say it's "QFT quantum" because that satisfies SR, but that can't be quite right either (I think it suffices though)

That was just a disclaimer that

I'm only familiar with schrodinger quantum and can only talk about things to that approximation!

So that all my sentences are prefaced with an implicit "in a schrodinger universe [...]"

FenderLesPaul

States still evolve under the Schrodinger equation in QFT

Andrew G

@NeuroFuzzy Ahh, understood.

NeuroFuzzy

@FenderLesPaul $\hat{H} |\psi \rangle=i \hbar \frac{d}{dt} |\psi \rangle$?

FenderLesPaul

4:06 AM

yes

0celo7

@AndrewG well, if spacetime has a cauchy horizon, GR tells us that knowing everything in the universe at one "instant" doe not enable us to determine everything that ever was and everything that ever will be

NeuroFuzzy

@FenderLesPaul waaaaaat. Where's the catch though?

FenderLesPaul

There's no catch

The schrodinger equation is literally just the statement that the Hamiltonian generates time translation

this is the same in both QM and QFT

so states in QFT also evolve under the Schrodinger equation

don't confuse fields with states of fields

fields propagate according to various relativistic field equations like Klein-Gordon or Maxwell

but the states of these fields still evolve under Schrodinger

0celo7

what about the state $\hat\phi|\phi\rangle=\phi|\phi\rangle$

NeuroFuzzy

@FenderLesPaul so for Dirac/whatever other weird things, you just have a specific space $\psi$ lives in and a specific $\hat{H}$?

Oh boy. Yknow, I'll get there eventually. I'm going to eat and sleep.

FenderLesPaul

4:17 AM

night

fuck these cookies are too sugary

0celo7

my sister in law made me some awesome chocolate chip cookies

and I have to sleep now, night

Secret

4:54 AM

I read this

https://en.wikipedia.org/wiki/Principle_of_least_action

After reading the following (which is related to my honours)

http://scitation.aip.org/content/aip/journal/jcp/126/16/10.1063/1.2720838

specifically equation (4). I cannot help but have this possibly screwy picture in mind:

That there's a curve (which its length is the action) in configuration space which is like a higher dimensional version of a hilly landscape, the curve slides until it reached a valley thus it cannot change its shape anymore (since it is stuck). Then it is easy to see that the action is relatively un…

Secret

5:07 AM

objects that are free falling follows geodesics, which is like a generalisation of straight lines on manifolds (which in the case of GR, it is the spacetime manifold),

The previous post's interpretation of mine cause me tempted to think that the trajectory in phase space of which its length is action, then the principle of least action is the same as saying to find a geodesic in phase space?

I remember that phase space is a symplectic space, but is it also a symplectic manifold?

$$\delta \int_{t_1}^{t_2}L(\mathbf{q},\dot{q},t)dt=0$$
$$\dot{\phi}=0=-\nabla V(\phi)^{\perp}-\lambda \hat{\tau}$$
$$\ddot{x}^{\lambda}+\Gamma^{\lambda}_{\mu \nu}\dot{x}^{\mu}\dot{x}^{\nu}=0$$

They seemed to based on the same principle: To minimise the arc length of a curve??

typo: They seemed= the 3 equations seemed

John Rennie

@ChrisWhite Thanks Chris. I think I'll delete my answer. I've obviously misunderstood what was being asked.

Secret

5:24 AM

Is there any significant meaning when the classical lagrangian vanishes, other than it means the total kinetic energy has the same value as its total potential energy?

If I have a lagrangian that vanishes for all times, then the action of that system in question will be zero, what will that mean?

classic because I still yet to wrap my head around the relativistic version

*for all times in an interval [t1,t2]

*action of that system between t1 and t2

Phonics The Hedgehog

6:29 AM

For certain physics problem you find t, which stands for time. In this example, $100 = 30*x-(1/2)*(-3.8)*x^2$. When you try to solve x in that problem, you get either -18.6166, or 2.282713. Since we are trying to find time, and since time cannot be negative, we only use 2.28 seconds. But that -18.6 can't be there for no reason at all. Is there any physics reason why there is -18.6?

Secret

7:05 AM

@PhonicsTheHedgehog The -18.6 is a consequence of solving the maths equation (which does not know that t > 0 if you don't explicitly state that as a constraint to the equation to be solved)

If one insists on giving a physical meaning, then for the case where x in the above equation represents time, it means whatever phenomenon that the above equation is describing, it occurs 2.28 seconds later and -18.6 seconds ago

then you gave the physical meaning to the - sign in your answer. However it is not always interesting to consider what happens in the past of some given phenomenon since most p…

Slereah

8:38 AM

@Secret : Try to cut up your posts a bit btw

I often have to click on "see full text" on your posts

Rigor

9:20 AM

I think there already is a text limit built in...

Slereah

12:12 PM

imgur.com/t/infographic/oNo9IvT

Ugh

The fusion doesn't power the car in back to the future

That's the whole point of the third movie

Secret

12:30 PM

https://www.youtube.com/watch?v=VylZJrep1MM
I wonder what happens when the top slows to a stop, will it just drop down since the magnetic filed there is basically constant and uniform?

0celo7

@Secret I think so

skill patrol

@0celo7 who's your fav NFL team?

0celo7

probably Redskins but that's embarrassing to say in public

skill patrol

hmmm...

John Duffield

@0celo7 : just try to imagine what happens when you make the torus fatter and fatter.

0celo7

12:38 PM

@JohnDuffield It will never become a sphere.

This is a topological fact.

FenderLesPaul

check your thin privilege

0celo7

wtf fuck off SJW

there's enough at my school

which is crazy btw

the student body is crazy diverse opinion-wise

FenderLesPaul

wow patriarchy

0celo7

there are anarchists and full on communists in my dorm

btw I'm not even thin

skill patrol

real commies?

ACuriousMind

12:43 PM

...isn't a "crazy diverse opinion-wise" student body normal?

0celo7

it's not what I expected from an old southern school

skill patrol

^

ACuriousMind

Ah, ok

John Duffield

@0celo7 : the electron is a 511keV electromagnetic wave in a Dirac's belt configuration. See this water-wave gif and try to imagine what you'd have if the wave was going round and round.

Slereah

2

skill patrol

12:46 PM

:D

John Duffield

@0celo7 : it isn't a sphere. It's a torus that's so fat it looks like a sphere.

ACuriousMind

I think tori die of clogged arteria before they can get so fat.

0celo7

@JohnDuffield Does anyone in a professional journal support this claim?

@JohnDuffield Lol what does that even mean?

@ACuriousMind They're hardy American tori. They can take it.

@JohnDuffield Hmm, how does QED work in this picture?

What's the Lagrangian?

ACuriousMind

@0celo7 I thought the "American torus" is merely a donut :P

2

0celo7

Haha

John Duffield

12:49 PM

@0celo7 The guy who wrote this.

Slereah

Could you link the paper he wrote on the topic

0celo7

...

Please.

Slereah

And point the page where the claim is made

0celo7

Wait, do you mean the equation or the Wiki article?

Slereah

(peer reviewed paper)

0celo7

12:51 PM

I thought the electron was the fermionic excitation of a string...

So is the string in a Dirac belt configuration?

@ACuriousMind Remind me, are the superstring fermions excitations of the open or closed string?

Also are there clopen strings.

Slereah

Open I suppose

From what I recall closed strings behave like gravitons

ACuriousMind

@0celo7 How could it? The "Dirac belt configuration" would be merely one worldsheet topology over which is summed.

Slereah

SUSY analogs are gravitinos, which are... spin 1.5?

ACuriousMind

@0celo7 Uhhhhhhhhhhhhhhhhhhhhhhhhh...dunno :/

0celo7

@ACuriousMind Damn your poor sarcasm meter...

@Slereah Also dilaton and Kalb-Ramond.

Although I have no clue what the Kalb-Ramon actually does.

Slereah

12:54 PM

Aren't dilatons spin 0

ACuriousMind

@0celo7 It's still being repaired. They have to imports some rare parts.

0celo7

Dilaton has something to do with the...vacuum?

Slereah

Oh wait

0celo7

@Slereah Yes.

Slereah

Are strings like

Spin 0 + spin 2 thing

ACuriousMind

12:55 PM

Kalb-Ramond is a 2-gauge field, iirc

It belongs...to 3-branes, I think, but that could be wrong

Slereah

I vaguely recall that you could have both modes, maybe?

0celo7

Uh you have the massless multiplet that splits into Kalb-Ramond, dilaton and graviton.

ACuriousMind

Or maybe 6-branes

0celo7

lol

Did Weigand even talk about that?

I haven't touched string theory since the summer began.

John Duffield

@Slereah : see The Quantum Theory of the electron. He doesn't spell it out, but he makes it clear the electron has a wave nature, and this wave is not propagating linearly at c. Doubtless somebody will now announce that the electron is a point particle.

0celo7

12:56 PM

No, it's a string.

@JohnDuffield Wave nature does not imply it is literally a wave.

@ACuriousMind Someone needs some Heisenberg formalism.

John Duffield

@0celo7 : it works the way it works, but now we know why it works. Because the electron field is just a reconfiguration of the photon field, and the messenger particles are more fundamental than the field they're supposed to mediate.

ACuriousMind

@0celo7 Yes, at least we did in the course, briefly. Hmm, now I think it was 5-branes: 2-gauge field -> 3-curvature, canonical kinetic term is a 6-form, which is naturally integrated over a 6d worldvolume

0celo7

Decide already :P

ACuriousMind

That was one part I found actually interesting because it was not perturbative

0celo7

Are you sure?

John Duffield

12:59 PM

@0celo7 : no, electron diffraction implies it literally is a wave.

2

ACuriousMind

@0celo7 That I found it interesting? Yes. :D