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vzn
vzn
12:09 AM
was reminded of this famous event from antiquity on hearing of the tragic brazilian library fire/ destruction en.wikipedia.org/wiki/Destruction_of_the_Library_of_Alexandria
 
danielunderwood
danielunderwood
12:29 AM
Are wedge products of vectors as described in MTW ever used? What they call bi/trivectors. I've seen forms all over the place, but not really antisymmetric vector combinations
I suppose that may actually just be a cross product. I had always assumed a cross product was a 2-form though
 
ZeroTheHero
ZeroTheHero
12:56 AM
Would someone know if there is a way of including LaTeX packages (such as packages to draw Young diagrams) inside MathJax? My obvious attempts using \include failed but that could be I used the wrong synthax or there is a more specific MathJax command or the package is just not loadable without some further trickery.
 
danielunderwood
danielunderwood
1:06 AM
I've looked for a way to use tikz in mathjax a couple times, but haven't found anything so far
 
Semiclassical
Semiclassical
tfw you see $y=1-\cos x\implies y^2=1-\cos^2 x=\sin^2 x$
aaaghh
 
Cows
Cows
1:40 AM
:)
 
danielunderwood
danielunderwood
hmmm
I remember so many $(1 - \cos x)^2$s when dealing with noninertial frames. So many that it kind of hurts
Can't say they ever became $1 - \cos^2 x$ though
 
Secret
Secret
Nature have been getting more cunning and an increased instance of upsetting my plans in my Grand Scheme of Things
 
Fawad
Fawad
 
Semiclassical
Semiclassical
@danielunderwood to clarify: "when you see someone else write"
 
danielunderwood
danielunderwood
Hah I figured that part. You wouldn't have noticed if you wrote it!
 
Semiclassical
Semiclassical
1:50 AM
lol
 
Fawad
Fawad
I am getting normal force = mg and F=ma (assuming a is acceleration of rod)
 
danielunderwood
danielunderwood
Which means that some of mine may very well have become that at some point
 
Fawad
Fawad
I don’t know how to proceed
 
rob
rob
@Fawad Are there any individual/net torques acting on the rod?
 
Fawad
Fawad
Hmm taking rod end as axis and applying pseudo force at com
then equating rotational and anti rotational torque
 
Fawad
Fawad
2:41 AM
Your hat got robbed ? πŸ€”
 
invadingdingo
invadingdingo
3:19 AM
Anyone on to help with homework?
Beams of high-speed protons can be produced in "guns" using electric fields to accelerate the protons. (a) What acceleration would a proton experience if the gun's electric field were 1.88 × 104 N/C? (b) What speed would the proton attain if the field accelerated the proton through a distance of 1.00 cm?
That's my problem. I solved part a. It's correct and has an answer of 1.8e12.
Velocity should then be v = sqrt(2ax), where a is 1.8e12 and x is .01m(gotten from 1cm)
The answer to that is 1.8e5
which isn't considered right..
Wait, nevermind.
I tried entering the number without scientific notation and it worked. Nowhere did it specify whether or not to use scientific notation or not....
 
Semiclassical
Semiclassical
maybe something to do with sig figs?
though in that case I can't see why the first answer would've gone through
 
invadingdingo
invadingdingo
It was to do with the notation. They wanted the raw answer, 189736.6596, not 1.8e5
 
Semiclassical
Semiclassical
hmm
well, note that you'd typically round that to 1.9e5 not truncate it to 1.8e5
 
invadingdingo
invadingdingo
Ahh, I guess it is much more accurate that way
I'm a super noob to all of this physics stuff, haha
 
rob
rob
4:04 AM
@JohnRennie Good morning! Your arrival is usually my signal that I've stayed up too late. Good night!
 
John Rennie
John Rennie
@rob I guess it's around midnight on your side of the Atlantic :-)
 
David Z
David Z
4:26 AM
@invadingdingo Yeah if the answer to full precision is 189736.6596, it might be acceptable to write it as $1.9\times 10^5$ (or 1.9e5), but not $1.8\times 10^5$.
 
Sir Cumference
Sir Cumference
4:44 AM
Doubly special relativity
Doubly special relativity (DSR) – also called deformed special relativity or, by some, extra-special relativity – is a modified theory of special relativity in which there is not only an observer-independent maximum velocity (the speed of light), but an observer-independent maximum energy scale and minimum length scale (the Planck energy and Planck length). == History == First attempts to modify special relativity by introducing an observer-independent length were made by Pavlopoulos (1967), who estimated this length at about 10βˆ’15 metres. In the context of quantum gravity, Giovanni Amelino-Camelia...
TIL this is a thing
 
invadingdingo
invadingdingo
Are any of you guys physics professors?
 
Sir Cumference
Sir Cumference
5:20 AM
@invadingdingo @dmckee is.
 
invadingdingo
invadingdingo
Ahh, I wanted to see what the general consensus on sites like Chegg is.
 
Sir Cumference
Sir Cumference
You could just ask your prof
 
ACuriousMind
ACuriousMind
6:23 AM
@danielunderwood Sure - those are essentially differential forms, and the wedge of two vectors is the same as their cross product in 3d through Hodge duality
 
user157860
user157860
7:00 AM
@JohnRennie, Hi John, could you help me pick the correct answer to this question?:physics.stackexchange.com/questions/176220/…, it ha 4 contrasting answers, all of which make sense, is the accepted answer the correct one or is only an ideal unreal description? Thanks
 
John Rennie
John Rennie
@user157860 I guess it depends on what factors you want to include.
If there are no dissipative forces (i.e. no friction) then energy is conserved so the KE of the door is equal to the work you put in regardless of where and how you push the door.
 
user157860
user157860
@JohnRennie, just what happens in reality, if you push/kick/ etc a door on the edge, in the middle or close to the hinge
 
John Rennie
John Rennie
I guess a real door would have dissipative forces due to friction in the hinges and aerodynamic drag as the door swings through the air.
 
user157860
user157860
so...?
 
John Rennie
John Rennie
But then the energy lost to drag depends on how fast you swing the door, so it all starts getting rather complicated.
 
user157860
user157860
7:07 AM
what about the answer by user 77427? It seems a very thorough analysis and seems to hint that even in the case of no friction there are opposing forces.
Also, shouldn't we distinguish wether there is an impulse or the force acccompanies the swinging door, rotating with it?
 
John Rennie
John Rennie
I'm not sure what user 77427 is getting at.
I suppose their point is that technically when you push on the door you are also pushing on whatever the door is connected to (i.e. the planet Earth) but this seems unecessarily pedantic to me.
 
user157860
user157860
@JohnRennie, if there was a contraption with a spring you can compress and, when released, gave same impulse , and if you applied that to the edge of the door, to the middle ot to 5 cm from the hinge, would the door swing the same angle, you think?
 
John Rennie
John Rennie
7:24 AM
Suppose you apply a force F at a distance $d$, so the torque is $Fd$. If there is friction in the hinges then this will create some opposing torque $T_f$, so the net torque is $Fd - T_f$.
 
user157860
user157860
@JohnRennie, I am not referring to math, try to kick a door near the edge, then at 10 cm from the hinge, I tried that myself
 
John Rennie
John Rennie
For a fixed force $F$ the torque $Fd$ decreases as you get nearer the hinge i.e. as $d$ decreases. If you get close enough to the hinge then the friction torque will be greater than $Fd$ and the door won't move at all no matter how long you push.
So the distance obviously makes a difference.
 
user157860
user157860
so in reality the accepted answer is wrong?
 
John Rennie
John Rennie
The accepted answer is correct if the friction is negligible.
It's wrong if the friction is significant.
 
user157860
user157860
what do you make of the answer by ja72? and user76716?
 
John Rennie
John Rennie
7:30 AM
I think ja72 is considering the possibility that the hinges might move, as user77427 does.
That seems an unnecessary complication
 
Captain Bohemian
Captain Bohemian
@SirCumference this compared to special relativity is like Born-Infeld thery compared to general relativity?
 
user157860
user157860
7:44 AM
@JohnRennie, so, in conclusion, is friction relevant in reality? will a door always swing same angle with same impulse?
 
John Rennie
John Rennie
You'd have to do the experiment to determine what the frictional and aerodynamic forces in doors are. My guess is that both are significant so it does depend on where you push.
 
user157860
user157860
@JohnRennie, if you try , you'll find there is a significant difference, thanks for your help!
 
John Rennie
John Rennie
@user157860 be cautious about just pushing on a door because human muscles use energy even when they don't move. The feeling of how much work is done can be misleading. To get a reliable result you'd need to use some mechanical device to supply the impulse.
But in any case I agree with you that for real doors it does make a difference.
 
user157860
user157860
@JohnRennie,is there any such contraption in labs? do you know of any user working in a college lab? maybe jim could help?
 
John Rennie
John Rennie
8:47 AM
@user157860 why not use luggage scales? This sort of thing:
 
Secret
Secret
Is is possible to subject to a hydrogen atom to some really high energy (and in a bath of electron neutrino of course) so that the reverse of neutron decay occurs and the hydrogen atom becomes a neutron. If so, how to calculate the energy needed to shift this equlibrium?
 
John Rennie
John Rennie
You'll have to pull on the door not push, but that's fine. You can pull through the scales and keep the force indicated by the scales constant. Then just time how long it takes to swing the door through whatever angle you want.
 
Physics Meta
Physics Meta
0
Q: How to delete a wrong answer after it has been accepted?

niels nielsenOn those occasions where I have furnished a wrong answer to a question on one of the SE sites and that answer has been accepted, I find I cannot go back and delete the answer. I instead edit the answer, deleting the original text and inserting words to the effect that the answer was wrong and inv...

support accepted-answer
 
John Rennie
John Rennie
9:00 AM
user image
2
 
Secret
Secret
hmm...
 
John Rennie
John Rennie
Particles correspond to the states of a quantum field in the non-interacting limit i.e. when the particles are so far apart they are effectively non-interacting. In a hadron the interactions are very strong, so the states of the fields inside a hadron do not correspond to particles.
You can attempt to describe the states as a superposition of the particle states, and if you do this you call the components of this superposition the virtual particles. But these are terms ina series approximation not actually particles.
 
Secret
Secret
well I just found these pictures fun to look at. There is something funny about a diagram of a ball filled with letters
 
John Rennie
John Rennie
Did you count all the quarks to check they sum to just +3? :-)
 
Secret
Secret
not really, especially there are gluons floating around, which makes things a lot messier
Btw the previous question I had can be rephrased as follows: What is the minimum energy needed to supply to a free hydrogen atom in order to trigger it to electron capture its only electron an thus producing a neutron and electron neutrino? I have tried to lookup PSE and google and all I found is that "electron spiraling into nucleus" answers which is is not what I have in mind
Specifically, we knew a free neutron will spontaneously decay according to the following:
$$n \to \bar{\nu}_{e} + p^+ + e^-$$
so how to calculate the minimum energy needed to supply to reverse this reaction
 
John Rennie
John Rennie
9:22 AM
@Secret well add 13.6 eV so you have a free electron and free proton
 
Secret
Secret
uh... I am not sure how that will help me to make a neutron
 
John Rennie
John Rennie
Then you need to send in the electron and a neutrino with an energy $m_n - m_p - m_e$. So the total energy needed is $m_n - m_p - m_e + 13.6eV$.
 
Secret
Secret
hmm...
 
John Rennie
John Rennie
Though since the interaction is mediated by the weak force the cross section will be vanishingly small. You'll be waiting a long time to get your neutron :-)
 
Secret
Secret
$= 0.782333054 MeV + 13.6 eV = 782346.654 eV$
wow that's huge
hmm... is there a reason I need to free the electron from the hydrogen bound state before it can fuse with the proton, despite the electron wavefunction in the bound state of hydrogen has a vanishingly small but nonzero probability to be found inside the proton?
o wait...
$m_n - m_p - m_e$
that's kinetic energy
nvm
 
John Rennie
John Rennie
9:29 AM
No, I'm just using the removal of the electron to infinity as a convenient way to do the calculation. In fact $m_p + m_e - 13.6eV$ is just the mass of the hydrogen atom, so my equation for the energy can also be written as $m_n - m_H$.
Just send your neutrino into a hydrogen atom with that much energy.
 
Secret
Secret
Interesting
 
John Rennie
John Rennie
But you'll be reducing the cross section even farther since the fraction of the electron near enough to the proton to interact via the weak force is tiny.
 
Secret
Secret
right so that's why we need it free
 
John Rennie
John Rennie
I was assuming you fire in your electron and neutrino from infinity directly at the proton.
 
Secret
Secret
yeah, that's a valid assumption many physics problems
hmm...
It is very interesting how the real science whose misconception lead to the pseudoscientific concept of the hydrino is a lot more interesting than the pseudoscience itself. Had the cross section of the weak interaction was much larger, we might already have neutron batteries as highly compact energy storage
(because one can imagine having a container of hydrogen, then fire electron neutrinos at it, this uphill reaction will then store the excess energy in the form of a pile of neutrons in the container... well of course there needs to be a way to shape the neutron wavefunction a bit so that its decay can be suppressed in some way...)
also wow, the mass of a free neutron is actually heavier than the mass of a hydrogen atom, which is contradictory to common sense that more stuff = heavier, but it is easy to see why, part of that mass is used to keep the electron bound
I think that kinda reminds me of a related thing where the mass of a proton is not just the mass of the 3 quarks, but the sea of gluons also
Also... 74815990.7 kJmol-1 is a lot of energy. Too bad nature does not allow us to store energy that way
 
John Rennie
John Rennie
9:43 AM
the mass of a free neutron is actually heavier than the mass of a hydrogen atom - just as well really or we wouldn't be here to notice it :-)
 
Secret
Secret
indeed, not sure if neutron degenerate matter can actually develop the capacity to replicate like a lifeform, let alone think :-)
 
 
1 hour later…
user3613025
user3613025
11:01 AM
@EmilioPisanty ok emilio can you hit me with some advice for an aspiring quantum physicist like me. Hit me with the cold hard truth(when you're free ofc) cheers!
 
Emilio Pisanty
Emilio Pisanty
11:26 AM
@user3613025 that's.... not how that works. If you have specific questions, I'm happy to help, but "I want advise" is too general to say something useful.
 
user3613025
user3613025
Yea I guess I'm not really asking for anything specific. Just generally what did you do in your earlier years that you think had helped you with becoming a quantum physicist. i.e. work wise how many hours did you do?did you read a hell lot around your subject area? what kind of learning attitude did you have when you were in my position(I'm a 2nd year undergrad, looking to get into quantum computation side of things in the future)
 
Emilio Pisanty
Emilio Pisanty
> did you read a hell lot around your subject area?
yes
read as much as you can.
> work wise how many hours did you do?
no idea
I studied as much as I could on the subjects that interested me
 
user3613025
user3613025
did you work through the chapters in QM textbooks that were otherwise not taught/included within your undergrad's curriculum?
 
Emilio Pisanty
Emilio Pisanty
@user3613025 I did do a significant amount of going through textbooks before they were formally required in the courses I took
but I don't think you should consider that as a requirement
as I said, just read as much as you can on the stuff that interests you
if you're reading stuff that you find boring just because you have a vague feeling that you "have to", then you're doing it wrong
 
user3613025
user3613025
ok so basically quantum physics textbooks
nah i feel like when i do quantum physics i can just keep doing it
 
Emilio Pisanty
Emilio Pisanty
11:35 AM
so, then do it
 
user3613025
user3613025
ok
i want advice but idk what to ask lol
 
Emilio Pisanty
Emilio Pisanty
the sooner you can get to a good level of competency over the core basics of QM, the more doors will be open to explore
that is, there is only so much that you can do with QM at the level of, say, Beiser's Concepts of Modern Physics
 
user3613025
user3613025
I'm currently going through "Quantum Mechanics" by Eugen Merzbacher in my own free time
 
user1732
user1732
how strong is your math background?
 
Emilio Pisanty
Emilio Pisanty
@user3613025 yeah, that's a good book. Once you're fully conversant in the language of that book (operators, states, unitary transformations, projective measurements), then you're in a position to take on a much wider gamut of applications.
If you're specifically interested in quantum computing, then I would suggest Nielsen & Chuang once you're done with Merzbacher
 
user3613025
user3613025
11:40 AM
Ok thanks! Do you see a promising future for quantum based technologies, or do they sound more like science fiction atm
 
Emilio Pisanty
Emilio Pisanty
It's not science fiction - many quantum technologies are already out in the field and working
but I do feel there is a significant degree of over-hyping which is going to hurt the field when we start being unable to deliver on time
... which is probably going to be shortly before you join the labour market
so be cognizant of that
 
user3613025
user3613025
@user1732 I usually averaged borderline a first in my maths exams, in terms of what i've done so far I did stuff like complex analysis, green's function, multi-varcal, solving pdes using SoV etc
 
Emilio Pisanty
Emilio Pisanty
all technologies go through the cycle of promise → hype → growth spurt → under-delivery → contraction. It's a natural cycle, but the contraction does mean less jobs.
 
user3613025
user3613025
ok thanks for the input, gtg to my meeting with my supervisor now
 
Emilio Pisanty
Emilio Pisanty
that doesn't mean you shouldn't go for it, though.
@user3613025 no worries.
 
user3613025
user3613025
11:44 AM
didn't know a chat existed tbf
 
user1732
user1732
cya
 
user351417
12:06 PM
Anyone knows what year a physics undergrad in the US would do a course like the series here? (It's introductory QM) Presumably at least second year, because the obvious prereqs I could infer from the content were introductory calc-based mechanics and electromagnetism... Just curious because I couldn't get hold of the problem sets to accompany it while I was going through the lectures, so I was wondering when I'd be able to do the full-scale thing.
 
user351417
Of course, I'm sure I could find a different course where the problem sets are easily available, but that guy teaches really well and I've already gone through a lot of it.
 
Semiclassical
Semiclassical
@Chair at my grad school uni, there were multiple levels of QM offered. there was an introductory course on quantum physics at the second-year level, an intro course on QM at the third-fourth year level, and then there were some grad-level courses.
 
user351417
@Semiclassical How different were they?
 
user351417
Maybe the 3rd year courses were faster and had more intricate math?
 
user351417
Like it's a bit trying for me to go through that course because I've never formally done a course in linear algebra, so all I have is the skewed picture from self-studying whenever the need arose.
 
Semiclassical
Semiclassical
12:14 PM
I think the better comparison is the books they used. The second-year course used Eisberg and Resnick, whereas the junior/senior level course used Griffiths
lemme see if I can track down the course descriptions tho
 
user351417
@Semiclassical But I thought these things aren't sorted by years, like a student from any year can sign up for most classes.
 
Semiclassical
Semiclassical
in principle, yes. but the courses come with prereqs
and if you're taking those courses at the usual pace, you'd end up doing the quantum physics course in your second year and the QM course in your third or fourth year
(in particular, the quantum-physics course is a prereq for the QM course)
I think the main difference is that the quantum-physics course was broader and shallower, with less focus on QM formalism and more on quantum phenomena
 
user351417
Ah okee. Yeah, I guess the combination of the prereqs and the specialization ends up pushing everyone of the same year into such classes.
 
Semiclassical
Semiclassical
both of them discussed the Schrodinger equation. but the quantum physics course did so about two-thirds of the way through the semester, whereas the QM course would get into that within the first week or two
plus, the standard of difficulty for problems was naturally higher In the upper-division courses
 
user351417
@Semiclassical Strange, the course I linked above is called 'Quantum Physics 1', but it got to both versions of the SE way back in the 3rd or 4th lecture, and there are 25 of them.
 
user351417
12:26 PM
On the bright side, I just found the lecture notes and problem sets. Apparently I didn't look carefully enough last time around.
 
Semiclassical
Semiclassical
Then that course is probably closer to our intro QM course
Different schools have different conventions
 
user351417
@Semiclassical I guess different places have different names for things too.
 
Semiclassical
Semiclassical
Also, if you look at the syllabus you’ll find that they’ve got E&R listed but they also give textbooks which are more specifically focused on QM formalism
So that points to a course that’ll necessarily be more mathematically-minded
 
user351417
@Semiclassical Yep. There's lots of math. Which is fun because I started using it as a replacement for pop-sci
 
Semiclassical
Semiclassical
Another good indicator is when and if they go over to Dirac notation
The earlier that shows up, the higher the level of the course
 
user351417
12:31 PM
@Semiclassical That was about halfway through. Must have been lecture 8 or 9 (of 20+)
 
Semiclassical
Semiclassical
Then that's definitely closer to our QM course than our quantum-physics course
I'm not sure E&R touch Dirac notation at all
 
user351417
@Semiclassical Fun. I haven't been using any of the textbooks too actively though... I found a pdf of griffiths, but I haven't spent much time with it.
 
Semiclassical
Semiclassical
if you're coming from a math background, then Shankar might be a good choice
its first chapter is mathematical introduction focused on the requisite linear algebra background
e.g. linear vector spaces, inner product spaces, dual spaces, etc
 
user351417
I've actually been spending more time with this book on nonlinear dynamics by Strogatz; it's not really physics but it's really fun and the math isn't too tricky for me... it just needs a good familiarity with calc-based physics, matrices, introductory multivariable calc (pretty much just notation actually), and linear algebra (however advanced eigenfunctions/eigenvectors are).
 
Semiclassical
Semiclassical
nice
 
user351417
12:36 PM
The advantage of not having to do any of this for school is that I get to be as disorganized as I want.
 
Semiclassical
Semiclassical
lol, i know what you mean
 
user351417
@Semiclassical I'm probably not ready for that stuff. I tried to read about it while doing some mad school projects about quaternions and I had absolutely no idea what I was looking at.
 
user351417
At least I looked at inner products and bivectors and a few other things. Bivectors made some sense, but the rest went way over my head.
 
Semiclassical
Semiclassical
eh, take a look at the first chapter of Shankar before you jump to conclusions
the most elementary instance of kets and bras is just column vectors vs. row vectors
with the inner product in that case just being the matrix product of a row vector and a column vector
 
user351417
@Semiclassical I've gone through a bit of bra-ket with that OCW lecture series. But since I didn't solve any problems yet, it's still a bit superficial. Maybe I'll try to get hold of Shankar; if I have the math background I may be able to take a better shot at the problems.
 
Semiclassical
Semiclassical
12:43 PM
yeah, hard to judge without solving problems
 
user351417
Anyways, I'll check out the first bits of Shankar if they have the necessary math. Thanks!
 
Semiclassical
Semiclassical
np
 
Avantgarde
Avantgarde
Shankar is a nice book.
I discovered it after a long time though.
 
Semiclassical
Semiclassical
1:06 PM
pet peeve: finding an author whose POV you agree with, but whose writing style you find irritating
(case in point: I may start a sentence as "And..." occasionally. But this person seems to use it every three to four sentences! his style is exhaustingly repetitive.)
 
dmckee
dmckee
2:00 PM
@SirCumference I'm out of that business now. Working as a scientific programmer. The pay is better.
@rob I have to say that this is what experimental work on really hard problems looks like. Good teams can do good work and only come close (factoring in their uncertainty). This may be from either messing up a correction or from overestimating their own precision.
It's also why meta analyses are useful. BEcause the odds are that over a bunch of different measurements there are roughly as many high as low.
@invadingdingo My take on Chegg was that student use it to avoid learning. And they get away with it if the course is peripheral to them, but if you do that on something that is part of your foundational knowledge it will bite you in the butt.
Use at your own risk.
I also have a couple of pretty funny stories about how not understanding what they read on Chegg hurt students in the short term, too.
 
user1732
user1732
2:19 PM
WHAT! you don't teach anymore?
:-(
that only leaves rob
and semiclassical
 
Semiclassical
Semiclassical
eh. I've only ever been a TA i.e. no actual lecturing
and I'm not doing that this semester anyways
I may end up doing actual instructing in the future, but I'm not a simple example of such
 
user1732
user1732
kk
 
danielunderwood
danielunderwood
@Semiclassical hey I do that with "and" and "also" a lot. Fortunately I'm aware of it, but I haven't really figured out how to fix it
 
Semiclassical
Semiclassical
in informal writing I can understand it
but this is a published book
 
danielunderwood
danielunderwood
oh
 
Semiclassical
Semiclassical
2:30 PM
as such it's a bit more grating
 
danielunderwood
danielunderwood
Yeah I'd probably never get a book published because I'd keep going over it...kind of like with the blog I sort of started
 
user1732
user1732
link please
 
Semiclassical
Semiclassical
uh, no
I'm fine with slagging off a writing style, but I'm not going to name & shame a specific author
 
user1732
user1732
i meant the blog
 
Semiclassical
Semiclassical
oh, sorry
 
user1732
user1732
2:33 PM
np
 
danielunderwood
danielunderwood
Oh by sort of started, I meant that I set up a github repo for it and never put anything on it
 
user1732
user1732
lol
 
danielunderwood
danielunderwood
You can see it in all its wonderful defaultness danielunderwood.github.io
 
Semiclassical
Semiclassical
I set up a Wordpress blog a few weeks ago but I haven't actually published anything
 
danielunderwood
danielunderwood
Well you're beating me. Mine's been on my to-do list for over a year. I actually had some local changes that changed all the info and added a post, but I wasn't too happy about the post in the end
 
Semiclassical
Semiclassical
2:41 PM
I had a first post in mind, but as I worked through the details I realized I wasn't as clear on it as I wanted
i'm glad i realized that, but it was nonetheless frustrating
(turns out I don't understand actual position measurements in QM as well as I assumed)
 
ZeroTheHero
ZeroTheHero
@dmckee My sound bite on this is that Chegg etc are for those interested in getting a degree rather than getting an education.
 
Secret
Secret
nukes Chegg from orbit
 
danielunderwood
danielunderwood
Turns out that I knew nothing about measurements of operators with continuous spectra after some of the conversations in here
Kind of funny that position/momentum is one of the first things talked about in a QM course
 
Semiclassical
Semiclassical
@danielunderwood yeah, that's what I have in mind
consider two different setups:
1) you shoot a particle at a photographic plate and it leaves a mark at some point $p$
2) you place a screen with aperture at point $p$ and place a photographic plate behind the aperture, then shoot particles until you see one leave a mark on the photographic plate
(you have to do it repeatedly in the second case because the particle is more likely to be reflected than pass through)
in the first case, you've made a measurement of the particle's location, and can specify that location rather precisely. but you're no longer in a position to observe the motion of the particle from then on
in the second case, you've made a measurement of the particle's location and the particle is able to keep going. so in that sense it's a less destructive measurement
but the narrower you make the aperture (and therefore the less uncertainty for said position measurement) the more trials you'll have to run before such a measurement actually succeeds.
so the less destructive measurement is also the less convenient one...I think?
Setting that aside, here’s a notation question
 
danielunderwood
danielunderwood
2:58 PM
hmmm does having the screen there not mess with the wavefunction in a way that's more or less destructive?
 
Semiclassical
Semiclassical
Hmmm
It’s definitely destructive on any part of the wavefunction which wouldn’t pass through the slit
Which is most of the wavefunction, lol, if the aperture is narrow
 
danielunderwood
danielunderwood
But you'd also have an infinite potential in a significant region, which would affect everything about the wavefunction right?
 
Semiclassical
Semiclassical
So I guess that’s not a great way to describe it
Hmm, maybe. I’m less sure as I think through it
I guess the main difference is whether you can make subsequent measurements on the particle
If you put multiple apertures in sequence, then I think each of these would comprise a position measurement
Whereas, once it’s landed on the screen, that’s itβ€”can’t measure it further
But the only way you know it’s passed through all the apertures is if it lands on a plate placed after all of them
i dunno. I’m probably thinking about this wrong
Notation question: if I measure the spin z-component of a spin-s particle to be m (with hbar=1) then the spin state is labeled as |s,m>
How do I denote the state I get if I got this result by measuring along a different axis instead?
 
Anonymous
3:30 PM
Jul 5 at 19:22, by dmckee
@Blue Caveat emptor: I've had students turn in solution they found on Chegg that used concepts, symbols, and techniques we never covered in class.
 
Anonymous
General consensus about Chegg: Students use it to copy-paste hw solutions ;)
 
Anonymous
The one-on-one tutoring option is great though. There are some highly qualified people you can find there, if you happen to be lucky.
 
Anonymous
Although the prices are extremely high for that
 
enumaris
enumaris
4:01 PM
hmmm
 
Curio
Curio
 
David Z
David Z
Hey all, I think it's chat session time now... I'm not able to organize anything but I'm hanging around in the background this time
 
danielunderwood
danielunderwood
wooo big chat event time
@enumaris are there any common datasets for sentiment or general context? I've started poking around with spacy a bit. I'm using sentiment140 at the moment, but didn't know what else was out there. For some reason all the stanford links were down when I was looking
 
enumaris
enumaris
4:22 PM
I haven't had to do much sentiment analysis yet
Cus the dataset I'm working on doesn't really have much sentiment
I know there's datasets of like amazon reviews out there
I'm sure facebook and twitter and whatnot have made their own data sets based on likes and stuff like that, but I don't know if they release them...kinda doubt it
 
John Rennie
John Rennie
Though I have to say I don't think any of this years images are amazingly good.
 
Curio
Curio
Why isn't the electron as fast as the light despite being a wave?
 
Emilio Pisanty
Emilio Pisanty
@Curio why isn't sound as fast as light despite being a wave?
 
John Rennie
John Rennie
@Curio there is an important difference between massive and massless waves.
 
danielunderwood
danielunderwood
@enumaris yeah I haven't seen any official ones. In fact, sentiment140 seems to have limited its functionality since it was originally created. I'd guess twitter didn't like it or something
 
John Rennie
John Rennie
4:30 PM
Massless waves like light always travel at the speed of light. They can't travel at any other speed regardless of their energy.
 
Curio
Curio
What about the electron?
 
danielunderwood
danielunderwood
Looks about like one of the rooms I worked in interactions.org/sites/default/files/styles/photo_walk/public/…
 
Anonymous
lol
 
danielunderwood
danielunderwood
And probably a bit like my current office lol
 
John Rennie
John Rennie
@Curio With a massive wave like an electron we need to distinguish between two speeds - the group velocity and the phase velocity.
 
Loong
Loong
4:33 PM
I wish I could take pictures at work and show them.
 
John Rennie
John Rennie
At the risk of oversimplifying, the group velocity is the particle velocity and the phase velocity is the wave velocity. The group velocity is always less than the speed of light while the phase velocity is faster than the speed of light.
 
Anonymous
@Loong Why can't you? :P
 
Anonymous
Restricted area?
 
Loong
Loong
Yes, IT security
 
John Rennie
John Rennie
The phase velocity can be faster than the speed of light because it doesn't transmit any information.
 
danielunderwood
danielunderwood
4:34 PM
I remember seeing an email from someone who went to Los Alamos for a postdoc saying that he wanted to sent pictures but couldn't take any
 
Curio
Curio
So the velocity of the electron is the group velocity
 
John Rennie
John Rennie
Yes
 
Qmechanic
Qmechanic
@EmilioPisanty : Indeed a tragedy. I wonder how much of the collection was in storage at a (hopefully) different location?
 
Curio
Curio
And is there a correlation between them?
 
John Rennie
John Rennie
And that velocity is related to the total energy by the relativistic energy equation:
 
Emilio Pisanty
Emilio Pisanty
4:36 PM
@Qmechanic my understanding is that little of it was
if any
 
John Rennie
John Rennie
$$ E^2 = \gamma^2 m^2v^2c^2 + m^2c^4 $$
So for the massive particle the group velocity is related to the energy - higher energy means higher velocity.
 
vzn
vzn
@Loong iirc you work on a nuclear reactor? what is your job?
 
Loong
Loong
radiation protection
 
vzn
vzn
@Loong have you read about any of the new fusion research? seems to be gaining momentum last few yrs
 
John Rennie
John Rennie
@vzn I would have said that with the possible exception of the stellerator work fusion power seems to be developing at a snails pace.
ITER seems to be taking ages
 
vzn
vzn
4:41 PM
@JohnRennie there seems to be a lot of new initiatives last few yrs turning up, more privately funded... had a bunch of links... agreed the decades-long history looks very "gradual/ barely incremental" to say the least :|
 
enumaris
enumaris
fusion research super slow
I thought we were kinda close when I was an undergrad
seems like the turbulence issue isn't easily fixed lol
also NIF pretty much failed
I thought that was gonna happen 2008...10 years later...it's not even doing fusion anymore lol
 
vzn
vzn
eg this turned up recently. there seem to be major advances on controlling the plasma related to computational/ algorithmic areas dailymail.co.uk/sciencetech/article-6104161/…
 
Sir Cumference
Sir Cumference
@dmckee I see, congrats (though I'm probably late to say it)
 
Curio
Curio
@JohnRennie many thanks!
 
Emilio Pisanty
Emilio Pisanty
@JohnRennie well, fusion has been promising "fusion is 20 years away" for many decades, but people often leave out the fact that the promise has always been "fusion is 20 years away if the research is properly funded", and proper funding has never materialized.
 
user7777777
user7777777
4:44 PM
@JohnRennie Why does the group velocity carry information instead of the phase velocity?
 
vzn
vzn
bunch of fusion advances links from last few yrs vzn1.wordpress.com/2017/04/27/…
 
John Rennie
John Rennie
@EmilioPisanty is that true? ITER seems well funded.
@user7777777 that's a bit involved for this late in the day :-)
 
Emilio Pisanty
Emilio Pisanty
@JohnRennie I don't know whether fusion researchers would qualify the current funding for the field as a whole as sufficient to get them to ignition in 20 years' time
but the historical claim is pretty bleak
 
user7777777
user7777777
@JohnRennie Alright. One last question. Can a single wave have a group velocity? Afaik the group velocity arises when 2 waves are added together
 
vzn
vzn
this turned up on reddit recently seems promising, a kind of reverse engineering/ "performance tuning" of the photosynthesis process... Scientists pioneer a new way to turn sunlight into fuel joh.cam.ac.uk/scientists-pioneer-new-way-turn-sunlight-fuel
 
Emilio Pisanty
Emilio Pisanty
4:47 PM
I'm not sure how well I could source references for it now, but it was very clearly spelled out when I visited JET
 
Terry Bollinger
Terry Bollinger
@user7777777 abstractly the phase velocity carries no information for the same reason that sweeping a flashlight beam across the night sky does not carry information between stars at greater than the speed of light.
 
John Rennie
John Rennie
Can a single wave have a group velocity? - yes. The group velocity is the velocity at which changes in the wave amplitude propagate.
 
vzn
vzn
@EmilioPisanty "ignition" seems to have been reached. (for brief periods.) the field is looking for something like "self-sustaining"... trying to remember the terminology...
 
user7777777
user7777777
Alright, I see. Thanks!
 
John Rennie
John Rennie
@EmilioPisanty I think the problem is that no-one (holding the purse strings) really believes it will ever be economically viable. I have to confess to some scepticism myself.
 
Terry Bollinger
Terry Bollinger
4:49 PM
@user7777777 that is, the phase velocity is an illusion generated by the interaction of local phases, like a billboard that flashes lights that abstractly seem to move faster than is possible given the speed of light to limit.
 
Emilio Pisanty
Emilio Pisanty
@JohnRennie yeah. Demonstration plants are one thing, but scaling up is a separate ball game.
 
John Rennie
John Rennie
@EmilioPisanty it's just that ITER is so fantastically complicated, and therefore expensive.
 
Terry Bollinger
Terry Bollinger
Fusion is amusing. It's always like trying to balance a 1M wet spaghetti noodle on its end. It's the hidden instability at the end that will always get you.
 
Loong
Loong
And in the meantime since the end of the 1980s, we have also forgotten how to build good fission reactors. :-(
 
John Rennie
John Rennie
I guess it's possible that with lessons learned from ITER the next generation will be simpler and cheaper, but that doesn't look very likely.
 
danielunderwood
danielunderwood
4:52 PM
They lost all the funding once they found out how to blow things up I suppose
 
John Rennie
John Rennie
@Loong I used to be a supporter of nuclear energy, but I've become a sceptic due to the problems of the waste disposal and reactor decommissioning.
 
Terry Bollinger
Terry Bollinger
It's also amusing that for a short time and I think the 1950s that everyone thought Fusion Energy would be much easier to get to than fission. That's because the muon at that time looked like it could do the trick. Had it been just a little more stable, it might have.
 
John Rennie
John Rennie
While I'm sure we can make cheaper safer reactors I don't see how the waste disposal problem will ever go away.
 
Terry Bollinger
Terry Bollinger
Incidentally, don't let them fool you: there is a serious waste problem with any large-scale fusion reaction design also. You have to breed that tritium.
Uh, hello everybody!
 
John Rennie
John Rennie
@TerryBollinger if you can routinely fuse DT then you can also fuse DD. Tritium is only used these days because we can't even routinely fuse DT.
 
vzn
vzn
4:56 PM
@TerryBollinger a little surprised to hear that. is there some radioactive stage? anyway its supposed to be far less radiation in theory right?
 
Terry Bollinger
Terry Bollinger
@JohnRennie that is certainly true, but the magnitudes between D D & D T Fusion should not be underestimated, considering we can't even get to the tritium version yet.
 
Loong
Loong
@JohnRennie I understand that. However, since I am in this project, my view has shifted. I drive every day past an existing repository for high-active nuclear waste, and I have seen the decommissioning working in Germany. Now I am more sceptical about the safe design and operation.
 
vzn
vzn
@Loong lol dont drive too slow :P
 
Terry Bollinger
Terry Bollinger
@vzn there was a really good old Scientific American article, decades ago, that describe the lithium clothes that would have to go around intense Neutron generation in order to generate tritium for any reactor that's plausible in The Next Century or so. It's a bit of an eye-opener. The last line casually mentioned the century her so issue.
Can all of you tell I'm voice typing today? :)
 
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