The h Bar

2:26 AM

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Q: Is there a way to retrieve one's own edit of a different participant's PSE question, if the edit was rejected within hours of being sent?

I spent a few hours this morning, attempting an edit of a PSE question (similar to many bearing on those aspects of space that are often described as representing its expansion) that I'd attempted to relate to two cosmological models: Penrose's CCC model and Poplawski's torsion-based model. Alt...

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Naman Agarwal

5:42 AM

Hi this is a very elementary question. How can I write the Time ordered product of three operators? T[H(t_1)H(t_2)H(t_3)]? I don't really understand the notations given on wikipedia. Thanks!

Nihar Karve

6:34 AM

@NamanAgarwal assuming $H$ is bosonic, then the time-ordering operator simply reorders the operators such that the associated time decreases from left to right

for instance, if $t_3 > t_1 > t_2$, then $\mathcal T\{H(t_1)H(t_2)H(t_3)\} = H(t_3)H(t_1)H(t_2)$

napstablook

6:52 AM

how does one go about writing equation of the motion w.r.t to time?

Vincent Thacker

@napstablook By using conservation of string length

napstablook

I tried conserving angular momentum and I got v as a function of theta

but I couldn't convert this further to v as a function of time

Vincent Thacker

@napstablook Angular momentum isn't conserved. Can you think of something else that is?

@napstablook The force from the string is not directed towards O

So how can angular momentum be conserved about O?

napstablook

ok how does one go about using conservation of string length?

Vincent Thacker

Hint: The force from the string is always perpendicular to the velocity of the object

napstablook

6:58 AM

work energy theorem use?

Vincent Thacker

@napstablook Yeah, energy is conserved, not angular momentum

It's a common mistake to conserve angular momentum, for example physics.stackexchange.com/questions/640606/…

napstablook

@VincentThacker I see so velocity of the particle should remain constant no? why have you written $vsin\theta$ is constant?

Vincent Thacker

@napstablook Yeah, that's right. The post I linked is the 3-dimensional version of the problem. I was just using it as an example.

napstablook

ok I have been using the proper conservation but I relate change of $\theta$ by equate torque = $I\alpha$ and and tension force since velocity is constant

@VincentThacker thanks I got it

Vincent Thacker

@napstablook you're welcome

fqq

8:57 AM

@MoreAnonymous not really, it's been commonly used in English since the middle ages, just slightly newer than the plural they. Then some grammarian invented a "rule" that generic he should be used, and other Victorian grammarians started policing everyone else's language with this and other random rules.

@NamanAgarwal if you want a single expression without if clauses you can use Heaviside theta functions

Faded Giant

9:10 AM

@fqq and now a few amateur grammarians started policing everyone else again.

That's why I am no longer a mod on this hostile site working for free for a criminal company.

ACuriousMind

9:48 AM

@Charlie I know it's one of my stock phrases, but I didn't realize I use it that often :P

Physics Meta

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Q: Why my watched tags aren't yellow?

The questions tagged with my watched tags used to be yellow, and I found that super useful. Why are they not yellow now? Is this a problem on my end, or a change of the site?