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00:10

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Q: Why close an answered question when both answer and question has a positive non zero vote count?

Especially when a user cannot delete their own question.

Physics Meta

00:35

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Q: For close votes, require one downvote and comment

I think close votes and reasons should be public. In the very least make them available to the author so they can change the question. At most make every close voter downvote and state whats wrong with the question. A good middle ground would be to have that for at least the first close vote. Doi...

feature-request

Ronald Villiers

03:46

Is there a reason to build inefficient fusion power plants right now? Is there some sort of experimental gain to this or are we just wasting resources?

William John

05:59

You get free supply for beer for winning a nobel prize?

John Rennie

06:17

@RonaldVilliers We don't yet know how to build an efficient fusion reactor, or even if it's possible to build an efficient fusion reactor. The point of building inefficient ones is as experiments to find out how to build an efficient one.

@WilliamJohn When I was a student my tutor was a Nobel prize winner (Aaron Klug) and as far as I know he did not have an endless supply of free beer. Or at least, if he did he didn't share it with us students :-)

Slereah

07:48

Nazmul Hasan Shipon

Hello, I've a short question.
I was told that quantum jumps/state transitions of an electron are random events. But I also have learned when a photon strikes an atom, a quantum jump occurs between energy levels. (And somewhere I also read that they would instantly emit a photons after absorbing a photon corresponding to their difference in energy level and be back to that original energy level). Then how come these be random? Won't an electron jump between states when it would be struck by a photon?

John Rennie

@NazmulHasanShipon If we have an isolated hydrogen atom then it is described by the hydrogenic wavefunctions 1s, 2s, 2p, etc. Likewise if we have an isolated photon it is described as an infinite plane wave.

However when we bring the two together they start interacting and the way we describe the system changes. Now we have a single wavefunction that describes both the hydrogen atom and the photon.

Furthermore this wave function cannot be split into a hydrogen part and a photon part - the two are mixed together. The atom and the photon have become entangled.

@NazmulHasanShipon OK so far?

Nazmul Hasan Shipon

08:06

OK, then?

John Rennie

This entangled system is time dependent, and if we wait the system will evolve into a superposition of two states. One state has the photon absorbed by the atom and the electron in an excited state like the 2p. In the other state the photon is not absorbed and the atom remains in the ground state. The process by which the entangled system evolves into this superposition is well understood and is called decoherence.

The maths involved in decoherence is a bit scary, but luckily we don't have to worry about the details. All that matters is that our system ends up as a superposition of the two states.

So far there is nothing random about this. The evolution of the system is described by well understood physical laws.

@NazmulHasanShipon Does this still make sense?

Nazmul Hasan Shipon

@JohnRennie Yes sir, amazing.

John Rennie

Now this is where the randomness comes in, because when we observe a system we never observe a superposition. We observe one state or the other.

That is we might observe an excited hydrogen atom, or we might observe that no interaction has taken place and the photon just passed by the atom.

But we never observe a mixture of the two.

This is called the collapse of the superposition i.e. the system starts as a superposition of two states but as soon as we observe it the system collapses to one state or the other.

And I think it's fair to say this collapse process is not understood. Physicists have been arguing about it for as long as quantum mechanics has existed i.e. a hundred years or so!

It is believed to be truly random process, but no-one has ever proved this to be the case.

@NazmulHasanShipon Does this make it clearer?

Nazmul Hasan Shipon

@JohnRennie So, Erwin Schrödinger and Einstein were against this randomness and Einstein said, "God doesn't play dice"?

@JohnRennie It's clear sir.

John Rennie

Yes, the famous argument between Einstein and Bohr was about this subject, though they didn't understand it as well as we do now e.g. they did not understand the decoherence process.

Nazmul Hasan Shipon

08:20

@JohnRennie Sir, is decoherence somehow related to Pauli's Exclusion Principle?

John Rennie

Also note that the entangled system takes time to evolve into the superposition of the two states, so the process is not instant. In fact physicists have been able to measure how long it takes.

It is typically very fast, e.g. on the order of picoseconds, but it is not instant.

@NazmulHasanShipon No, they are unrelated.

Nazmul Hasan Shipon

@JohnRennie Thank you sir.

John Rennie

You're welcome :-)

RayPalmer

08:47

What's the difference between the calorimetry equation Q=mc dT, and the heat equation T_{t}=kT_{xx} ? If we know, the final temperature using the calorimetry equation, what exactly does the heat differential equation tell us ?

Slereah

12:02

You know for something so important to GR not a lot of books talk about cone geometry

ACuriousMind

and why is cone geometry important? :P

Slereah

12:20

Tricky question because it leads to the more sinister question of "Why is anything in GR important"

John Rennie

Cone defects crop up in GR. As I recall the spacetime around a cosmic string has a conical defect.

Slereah

Sure but just the geometry of the light cone itself

John Rennie

Isn't that just matter of computing the null geodesics?

Slereah

12:50

Well sure, if you have access to the metric

But you can also just consider the cone by itself

Which gives you the causal structure

ie just the assignment of whether a vector is timelike or spacelike

Charlie

what causes light cone defects?

Slereah

Cone defects are different from light cones

Charlie

oh

So it's not a defect of the light cone itself

Slereah

The basic idea of the cone defect is to consider a cone as a spacetime

If you look at a path around the cone, the circumference will be smaller than if you were in flat space, despite the curvature being zero everywhere

A full turn isn't an angle of $2\pi$

The defect is basically that you cut out an angular slice of Minkowski space and stitched the edges together

Charlie

sounds fun

Slereah

13:04

On a physical level this can be caused by matter in the shape of a line

Which doesn't really exist because topological defects don't actually have one dimensional energy distribution :p

You can just approximate them thusly

user 726941

13:46

Congratulations on approaching the 100 K event horizon @ACuriousMind

Slereah

I hear once you reach 100,000 you get to murder someone

user 726941

perhaps, a murder of crows?

Slereah

It is the ultimate form of moderation

user 726941

No copyright infringement intended.

fqq

14:10

@JohnRennie afaik only Niels Bohr got it. There's a urban myth that he was given a house with a pipeline pumping beer directly from the brewery (Carlsberg). There was no pipeline, but he did get to live in the house and apparently got free beer delivered for the rest of his life.

user 726941

14:27

did the Nobel committee approve that?

Slereah

Maybe I should check if for a start, you can get the causal structure of a spacetime from a complete flag field, that way I don't need to worry about dimensions

fqq

@user726941 I don't think it had much to do with the nobel itself. It was like a different prize. The Carlsberg guy left his mansion to be occupied by prominent scientists after he died, and Bohr got it at some point (a few years after getting the Nobel).

user 726941

15:11

Interesting story, thanks for sharing it @fqq

Nihar Karve

16:02

Congrats to ACuriousMind for crossing 100k reputation!!

11

I believe you are now entitled to SE-branded socks

PM 2Ring

@fqq There literally was a beer tap in the kitchen. But it's since been disconnected. I read about it in Abraham Pais's biography of Bohr.

Sep 14 '19 at 15:05, by PM 2Ring

For Winning The Nobel Prize, Niels Bohr Got A House With Free Beer ... after he won the Nobel Prize in 1922, the Carlsberg brewery gave him a gift – a house located next to the brewery. And the best perk of the house? It had a direct pipeline to the brewery so that Bohr had free beer on tap whenever he wanted.

ACuriousMind

@NiharKarve lol, thanks

PM 2Ring

16:26

Niels Bohr and his brother Harald enjoyed walking around the streets of Copenhagen, talking about mathematics & physics. But they'd often get pestered by people asking for Harald's autograph, so they gave up the walks and had their discussions at the house.

fqq

18:09

@PM2Ring beerena.com/the-myth-of-niels-bohr-s-beer-pipeline/1069

bolbteppa

18:37

> His popularity as a footballer was such that when he defended his doctoral thesis the audience was reported as having more football fans than mathematicians

PM 2Ring

@fqq Oh, ok. I'm sure I first read about the beer tap in the Pais book. Abraham Pais "served as a personal assistant to Bohr at his country home in Tisvilde for a year." en.wikipedia.org/wiki/Abraham_Pais I can't remember if he also spent time in Copenhagen with Bohrs, it's been more than a decade since I read that book. And maybe I'm mis-remembering...

It's quite a good book, IMHO, and well worth reading, although not quite as detailed as his remarkable biography of Einstein, Subtle Is The Lord, which alternates between biographical info and explaining relativity (with quite a bit of mathematics, far more than what you'd expect in a book aimed at a general readership).

fqq

19:19

I think the pipeline part must be a myth, it would be very inconvenient and impossible to clean, so the beer would likely be bad after a while. On the other hand there could easily be a beer tap, with kegs delivered from the brewery next door.

PM 2Ring

That makes more sense

Emilio Pisanty

19:38

@NiharKarve cc @ACuriousMind don't hold your breath. (or your bare feet, I guess.) Mine have yet to arrive.

I'm in the backlog, probably somewhere around the middle. They did send me an email about it, in October. I replied in early December and filled in the form. I've yet to receive anything or hear back at all.

(If you want to time my 100k crossing, the standard tools won't help -- it happened when question votes were upscaled from 5 to 10.)

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