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user586228

1:13 AM

@JohnRennie Sir please justify this statement," solid state diffusion in metals is kinetically enabled."

@Buraian Also give a try to this..

4 hours later…

John Rennie

5:34 AM

@user586228 I have no idea what that means.

6 hours later…

Manas Dogra

11:50 AM

Why The Speed Of Light* Can't Be Measured

►

Does one-way speed of light can be infinite in some direction make any sense?

I just found a post a PSE post on this... physics.stackexchange.com/questions/590983/…

John Rennie

It is true that it's hard to design an experiment that unambiguously measures the one way speed of light. There are lots of irritating technical details that get in the way.

But we have to step back and take a common sense view of this. A speed of light that depended on direction would violate a fundamental symmetry of the universe and that's a big deal. Anything of that sort would be immediately obvious.

The fact we have never observed an violation of isotropy suggests that arguments about the one way speed of light are best left to those with too much free time.

Manas Dogra

12:20 PM

@JohnRennie Isn't this much like the interpretation of QM?

John Rennie

Interpretations of QM are also best left to people with too much free time :-)

2 hours later…

Daniel Underwood

1:59 PM

Well that would explain why I have less interest in interpretations of QM as I get busier!

Charlie

2:43 PM

Is there any sensible interpretation of the huge number of states that you have at each level in string theory? I.e. the $N=1$ state of the closed bosonic string has 576 states, do these all represent the exact same particle or is there something more to it?

Maybe a more specific question would be, does it matter which of these states we use in calculations?

ACuriousMind

@Charlie I can also write down a QFT that has 1000 fundamental particles, does that bother you, too?

no one says bosonic strings describe the real world, so why does the number of states matter?

Charlie

Oh is bosonic string theory more of a toy model to introduce the subject?

ACuriousMind

Yes

Charlie

Ah, I didn't know that, I guess it doesn't matter in that case, ty

ACuriousMind

the "real" models are the five 10d superstrings, and these still produce particle spectra for 10d SUGRA theories that you need to compactify to arrive at 4d spectra you can actually compare to the real world

Charlie

2:48 PM

oh yikes is string theory not actually useful until you've done superstrings?

useful in the experimental sense, not for the purpose of torturing prospective students of the subject

ACuriousMind

it is debatable whether it is useful experimentally at all :P

but yes, without superstrings you cannot even hope to do something meaningful because you can't produce fermions with just bosonic strings, and the real world obviously contains fermions

Ryan Unger

3:14 PM

@ACuriousMind related nitpick to active vs passive

when computing variations, it might matter whether you're computing $\delta(v^\mu)$ vs $(\delta v)^\mu$

actually that's not an example :P

but if you're expressing things relative to a frame and not coordinates

the frame might be changing

Charlie

3:33 PM

What are you actually varying in $(\delta v)^\mu$?

I've never seen varying an abstract vector itself before

ACuriousMind

@Charlie I think he's saying that if you have a moving frame (e.g. vielbein formalism) it matters in $\delta v^\mu$ where $\mu$ is w.r.t. the frame whether you're varying $v$ or varying the frame

@RyanUnger but I'm not actually sure which of the two versions is which :P

Charlie

I didn't realise until the other day that the vier in vierbein come from the german for four

Because the word furfbein came up

ACuriousMind

3:50 PM

you mean fünfbein :P

'furf' isn't a word

Charlie

oh yeah i mean funf

idk how you get your u to have eyes

I remember from school it's called an oomlau

ACuriousMind

German keyboards have a dedicated key for the umlauts :P

Charlie

which is another word I'm almost definitely not spelling correctly

ah there you go lol

ACuriousMind

note also that vielbein isn't a typo for vierbein but 'viel' means many and hence 'vielbein' covers beins of all dimensions

Charlie

I was hoping that fünfundzwanzigbein would be a thing in string theory and they called it stringbeins

ACuriousMind

3:53 PM

Charlie

but I have a feeling I'm going to be let down

ACuriousMind

I think the days when English scientists borrowed the German words for stuff are long gone

It's often easier for me to talk about physics in English than in German because I don't actually know the German words (if they even exist) for many of the more exotic terms

ZeroTheHero

4:24 PM

4 hours later…

vzn

8:02 PM

(lol) yes, in some quarters, interpretations are like the rodney dangerfield of physics, not getting any respect. oh but, huh, people who worked on "so called" interpretations of QM: Bohr, Heisenberg, Einstein, Schroedinger, De Broglie, Bell, Bohm, ... and how about the latest cutting edge experiments that address interpretations? as the old slogan goes this aint your fathers oldsmobile™

vzn

8:39 PM

oh, on 2nd thought, have (further) axe to grind here; have recently been researching the area. and more recent inquirers, how about Gisin, Minev, Carmichael? if you ask me interpretations cut to the very heart of the foundations of QM, and points to the future, and incuriosity taken to extremes is unscientific. vzn1.wordpress.com/2021/02/04/…

BioPhysicist

9:39 PM

In tutoring lately, I have been amazed at how many students cannot write out Newton's second law for an object given the force diagram of that object

Bohemian relativist

10:30 PM

what on earth are a Feynman propagator and a photon propagator?

Thomas Klimpel

Two passages that I especially enjoyed in “[28] Collapse. What else? / Gisin” is the one attacking many-worlds for being hyper-deterministic:
“Given the complexity of the (many-) worlds, it had to be encoded in some infinitesimal digits of some quantum state, possibly in the billionths of billionths decimal place. I am always astonished that some people seriously believe in that. Mathematical real numbers are undoubtedly very useful when doing our theory. But are they physically real [15]^8? (^8 recall that the assumption that real numbers are physically real implies that there could be an …

Charlie

@Bohemianrelativist Objects in QFT that are parts of Feynman diagrams

1 hour later…

ACuriousMind

11:52 PM

@Charlie the more often I read this line the harder it cracks me up and I will now imagine all umlauts with googly eyes

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