• Physics Discussion
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Oh man, I've been watching David Tong's QFT I lectures at Perimeter from back in 2009 on Youtube. I was like "This is great, he's an awesome lecturer, but the quality is so bad. Good thing he follows his LaTeXed notes almost verbatim. Then I found PIRSA: [url]http://pirsa.org/[/url] Apparently Perimeter puts basically all of their Master's level classes up as video lectures (go to 4: Collections, switch it to Course, pick a year and view). This includes the David Tong lectures, except the version on that site has the video itself very tiny, with most of the screen taken up by hi-res images of the blackboard taken after he writes something. And you can download the images as a PDF presentation. This is like, the best shit ever. Thank you, Perimeter. You are awesome.
Can someone explain to me why in Bohr's model of an atom, centrifugal force is used in the force diagram of the electron. I understand that it's supposed to allow for the forces to equal out, but I was always under the impression that centrifugal force was not an actual force. I accidentally posted this in the mathematics discussion because I didn't know there was a physics thread.
Can anyone here answer a question that's been bugging me for a while. A lot of people talk about parallel universes like some multiverse theories or many-world interpretation of qft, and it's really popular in popsci and scifi that you can talk about these parallel universes where "some things are different", but I've never quite found any real studies or work on developing a strong case towards the idea that possibility exists in this sense. It's intuitive for us to talk about this idea that "maybe something could have been different", like maybe I could have ordered a hot dog instead of a hamburger, maybe I could have been run over by a car, maybe I could have become a doctor, etc., and people put this into the idea that there can exist parallel universes where all these possibilities are real, but are these actually possibilities? In order for anything to happen as it does there needs to be a myriad of small events that set it up, which they themselves need previous smaller events to set them up, and so on ad infinitum. How could you possibly say that "something could have been different" without re-writing the entire system that has cascaded from event to event to finally create that specific moment? How do you go about arguing that there can be room for "things to be different"? Is it just careless intuition that makes a lot of people think this? I've never really considered theories on parallel universes to be anything more than a neat idea to play around with.
[QUOTE=Lilyo;48597527]Can anyone here answer a question that's been bugging me for a while. A lot of people talk about parallel universes like some multiverse theories or many-world interpretation of qft, and it's really popular in popsci and scifi that you can talk about these parallel universes where "some things are different", but I've never quite found any real studies or work on developing a strong case towards the idea that possibility exists in this sense. It's intuitive for us to talk about this idea that "maybe something could have been different", like maybe I could have ordered a hot dog instead of a hamburger, maybe I could have been run over by a car, maybe I could have become a doctor, etc., and people put this into the idea that there can exist parallel universes where all these possibilities are real, but are these actually possibilities? In order for anything to happen as it does there needs to be a myriad of small events that set it up, which they themselves need previous smaller events to set them up, and so on ad infinitum. How could you possibly say that "something could have been different" without re-writing the entire system that has cascaded from event to event to finally create that specific moment? How do you go about arguing that there can be room for "things to be different"? Is it just careless intuition that makes a lot of people think this? I've never really considered theories on parallel universes to be anything more than a neat idea to play around with.[/QUOTE] If the universe were entirely deterministic then you'd be correct; different end games would require entirely different initial conditions, and it might not even be, in principle, possible to only perturb the universe away from the one we know by a single event (hot dog instead of burger, but everything else the same). As far as we're aware, however, the universe is not entirely deterministic (unless there's some kind of hidden variable theory that we haven't worked out yet, although I do think I recall reading somewhere that some of these had been ruled out of possibility); I can't imagine why in principle the wave function of some electron collapsing in your brain somewhere couldn't wind up cascading out of control to wind up making you order a hot dog instead of a hamburger.
[QUOTE=sltungle;48598076]If the universe were entirely deterministic then you'd be correct; different end games would require entirely different initial conditions, and it might not even be, in principle, possible to only perturb the universe away from the one we know by a single event (hot dog instead of burger, but everything else the same). As far as we're aware, however, the universe is not entirely deterministic (unless there's some kind of hidden variable theory that we haven't worked out yet, although I do think I recall reading somewhere that some of these had been ruled out of possibility); I can't imagine why in principle the wave function of some electron collapsing in your brain somewhere couldn't wind up cascading out of control to wind up making you order a hot dog instead of a hamburger.[/QUOTE] Not all deterministic theories are ruled out. Superdeterminism is still a viable local hidden variable theory, AFAIK. Many-worlds is deterministic and doesn't involve hidden variables.
[QUOTE=sltungle;48598076]I can't imagine why in principle the wave function of some electron collapsing in your brain somewhere couldn't wind up cascading out of control to wind up making you order a hot dog instead of a hamburger.[/QUOTE] Wave function collapse doesn't actually ever occur, it's just a phenomenon of observation (though i would say time is a more important variable here than the actual "observation") due to decoherence, otherwise I'm not quite sure what you mean. But either way that's a bit of a stretch isn't it? An electron collapsing causing a cascading of information that changes a given event? As opposed to that electron not collapsing originally? What does that even mean? Well now you have an entire new set of events preceding it that you have to explain how you end up with a different event than before that way, and in order to explain those previous events you'd have to go further back and explain previous others forever. Plus you choosing between a hot dog and a hamburger as a computational event in your brain is processed physically within certain parameters based around available information. I guess the question is can there really be such a thing as a truly random 50/50 chance for something to go one way or the other? If youre standing before a forked road with both sides being identical and you need to chose one path, is there some sort of condition where that situation could be considered entirely random, or will there still always be subliminal information, no matter how small, that will still determine which path you take? I guess a better question would be is Schrödinger’s Cat experiment truly a 50/50 chance event or will it always have to lean to one side or the other based on physical parameters of experimentation?
[QUOTE=gangstadiddle;48597090]Can someone explain to me why in Bohr's model of an atom, centrifugal force is used in the force diagram of the electron. I understand that it's supposed to allow for the forces to equal out, but I was always under the impression that centrifugal force was not an actual force. I accidentally posted this in the mathematics discussion because I didn't know there was a physics thread.[/QUOTE] If you are describing the atom from the the reference frame of the orbiting electron there is a centrifugal force that must be balanced by the electric force. If you are describing the atom in an inertial frame, the electric force [I]is[/I] the centripetal force required for the electron to maintain its circular orbit. Either way you get the same end results. Pseudoforces are quite real in non-inertial reference frames.
I suck at psychics so gg
currently a sophomore-junior in mechanical engineering, and taking thermodynamics this semester does anybody have a really good resource for some supplementary learning? my lecturer has a terrible accent, and the notes aren't particularly decipherable either. of course I read the textbook on my own, but I like to have good notes/videos as a way to cement what i'm learning. there used to be something on reddit that was a pretty good source of info for this class, but i've can't find it now. if anybody could help, i'd really appreciate it!
So I'm watching this video lecture series on quantum mechanics - I didn't learn a lot of physics during school, so I'm doing my best to follow what's going on - but the one thing I don't get so far is particle spin, it doesn't mean the particles are literally spinning in space right? How do you tell what their spin is, do you have to look at a quantum system to determine that or something? :v:
[QUOTE=MrJazzy;48638048]So I'm watching this video lecture series on quantum mechanics - I didn't learn a lot of physics during school, so I'm doing my best to follow what's going on - but the one thing I don't get so far is particle spin, it doesn't mean the particles are literally spinning in space right? How do you tell what their spin is, do you have to look at a quantum system to determine that or something? :v:[/QUOTE] To answer your last question first, yes, you have to look at the quantum system. We don't get spins of fundamental particles from classical mechanics. As for interpretation, I'm not sure how advanced the quantum you're learning is, but here goes: You'll often be told that particle spin is not really like the particle rotating where it is, and that's kind of right as it is subtle, but it is actually a rather straightforward application of angular momentum in a quantum setting. Take a scalar field and do all the usual stuff you'll do in a first quantum field theory course (the details here are not important): reinterpret degrees of freedom as operators, demand that commutation relations hold, introduce creation and annihilation operators etc. Then take the operator that comes from rotational invariance (this is just the quantum angular momentum operator, coming from the same symmetry it does classically), and let the operator act on a one-particle state of the field with zero momentum. You find that it gives you zero. If you do the same with a vector field, you get angular momentum one, etc. So it might not be quite as easy as a baseball rotating in space, but the construction that leads to it is a straightforward transition from classical to quantum, and you're measuring exactly the quantity you'd expect: the total angular momentum of the quantum field with one particle that has zero momentum.
Thanks, I definitely don't understand it but now I understand it a little better, most of the concepts you mentioned haven't been introduced yet, so I'll keep watching and see if they do and if I understand it better then. Dang, I would kinda love to work with astrophysics, but I don't think I have the dicipline to sit down and study all the details and maths, but atleast I can try to get a better understanding of physics in general while I keep considering what I should study if I want to study.
[QUOTE=MrJazzy;48638048]So I'm watching this video lecture series on quantum mechanics - I didn't learn a lot of physics during school, so I'm doing my best to follow what's going on - but the one thing I don't get so far is particle spin, it doesn't mean the particles are literally spinning in space right? How do you tell what their spin is, do you have to look at a quantum system to determine that or something? :v:[/QUOTE] See also: [URL="http://physics.mq.edu.au/~jcresser/Phys301/Chapters/Chapter6.pdf"]The Stern-Gerlach experiment[/URL].
[url]https://booko.com.au/9780716703341/Gravitation[/url] Well clearly that's not correct... [URL="http://www.amazon.com/Gravitation-Charles-W-Misner/dp/0716703343/ref=mt_hardcover?_encoding=UTF8&me="]I mean, a used, hardcover copy can legitimately set you back close to a grand[/URL], but there's no way a new, hard cover copy could set one back anymore than twice that, right? ... right? Although, I can't find anywhere that seems to sell new, hard cover copies so if they've only just gone out of print that could explain a dramatic price increase,
Is it out of print? That would be a shame. [editline]16th September 2015[/editline] When Borders was going out of business near me I saw a new copy for less than $200. I should have bought it.
[QUOTE=JohnnyMo1;48698005]Is it out of print? That would be a shame. [editline]16th September 2015[/editline] When Borders was going out of business near me I saw a new copy for less than $200. I should have bought it.[/QUOTE] I don't know, but I can't find any new hard cover copies of it anywhere online, and considering it's a book that was first published in '73 I wouldn't be surprised if the hard cover copies were out of print now. Such a shame. It's a wonderful book (got the one copy the library has; had to specially request it from their offsite storage location).
Strongly considering doing General Relativity as one of my last courses. It feels like cheating to have a master's degree in physics and not knowing GR :v: the course uses Carol. I am a bit scared of the difficulty though, I struggled with particle physics and special relativity in my bachelor's degree so I don't know if I can handle GR (although my math self-confidence has risen somewhat in the past years)
Aw man, Carroll is a great text to learn GR from though. I loved taking GR, probably my favorite physics course I took. Didn't do great though, the calculations on the homeworks were crazy. [editline]20th September 2015[/editline] But funny enough the final exam was so much easier than the homeworks I though it was a joke at first.
[QUOTE=Number-41;48726289]Strongly considering doing General Relativity as one of my last courses. It feels like cheating to have a master's degree in physics and not knowing GR :v: the course uses Carol. I am a bit scared of the difficulty though, I struggled with particle physics and special relativity in my bachelor's degree so I don't know if I can handle GR (although my math self-confidence has risen somewhat in the past years)[/QUOTE] As Johnny says, Carroll is fantastic. It might just be personal, but I struggle way less with GR than I do with particle physics type stuff (assuming you mean proper QFT particle physics); differential geometry comes way more naturally to me than linear algebra does. I also recommend strongly getting a copy of Thomas A Moore's 'A General Relativity Workbook'; if you have issues with Carroll, go there, get a more simplified version of things, then go back to Carroll and it'll probably make perfect sense.
I'm curious, what do you all think of plasma cosmology? Is it pseudoscience?
2nd year physics beginneth, exactly what am i in for
[QUOTE=catchall;48798407]2nd year physics beginneth, exactly what am i in for[/QUOTE] Exactly what classes are you taking? [editline]30th September 2015[/editline] [QUOTE=shadowdude14;48796091]I'm curious, what do you all think of plasma cosmology? Is it pseudoscience?[/QUOTE] I'm not aware of any predictions made by it that would be untestable, so maybe in a technical sense it isn't pseudoscience. But it is a crackpot hypothesis according to overwhelming consensus.
classical physics electromagnetism & quantum physics programming in python experimental physics physics of music (spring) theres one or two others but i forgot what they are because they're in the spring [editline]1st October 2015[/editline] how hard is the math? we started looking at fourier series & transforms, they looked okay. how long till the other shoe drops?
[QUOTE=catchall;48798876]classical physics electromagnetism & quantum physics programming in python experimental physics physics of music (spring) theres one or two others but i forgot what they are because they're in the spring [editline]1st October 2015[/editline] how hard is the math? we started looking at fourier series & transforms, they looked okay. how long till the other shoe drops?[/QUOTE] Are these more advanced courses specific to physics majors, or the more general engineering type? If they are just for engineers, it's likely you'll only need some differential and integral calculus. If it's a higher level of any of the courses you listed, you'll be using vector calculus, solving ODEs and PDEs, and using some linear algebra. And as far as the linear algebra goes, at least at the level I'm at, you just need to be able to do basic matrix operations and solve eigenvalue problems.
it's just physics, not engineering applications
Anybody know much about QFT? I'm learning it from David Tong's lectures. I'm a little confused when he talks about Dyson's formula. Starting at equation (3.17) in the notes: [url]http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf[/url] and going on for a couple pages he has a bunch of integrals of Hamiltonians in the interaction picture. I can see how he got them by formal manipulations, but what do they mean? If we're talking single-variable QM and the Hamiltonian acts on some state, you just get a function out of it, and so the integral over that has a simple interpretation, but these Hamiltonians don't seem to be acting on anything explicitly. What does it mean to integrate over an operator like that? I can see what the time derivative of an operator is, but the integral seems a bit weirder. [editline]30th September 2015[/editline] It occurs to me that I've been integrating over operators with impunity, but it's only just bothered me. Probably because the rest of the time it was just Fourier analysis or a mode expansion where it's taken by definition because it solves the problem. [editline]30th September 2015[/editline] I guess I kinda get it. You're still getting an operator in the end, so in practice we work with it the same way as the other "integral over operator" definitions so far: if you want to get a state out of it, you have to act on it with something that will be integrated over to get a state. Still, integrating over operators does seem like a weird formal maneuver.
[QUOTE=JohnnyMo1;48799630]Anybody know much about QFT? I'm learning it from David Tong's lectures. I'm a little confused when he talks about Dyson's formula. Starting at equation (3.17) in the notes: [URL]http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf[/URL] and going on for a couple pages he has a bunch of integrals of Hamiltonians in the interaction picture. I can see how he got them by formal manipulations, but what do they mean? If we're talking single-variable QM and the Hamiltonian acts on some state, you just get a function out of it, and so the integral over that has a simple interpretation, [B]but these Hamiltonians don't seem to be acting on anything explicitly.[/B] What does it mean to integrate over an operator like that? I can see what the time derivative of an operator is, but the integral seems a bit weirder. [editline]30th September 2015[/editline] It occurs to me that I've been integrating over operators with impunity, but it's only just bothered me. Probably because the rest of the time it was just Fourier analysis or a mode expansion where it's taken by definition because it solves the problem. [editline]30th September 2015[/editline] I guess I kinda get it. You're still getting an operator in the end, so in practice we work with it the same way as the other "integral over operator" definitions so far: if you want to get a state out of it, you have to act on it with something that will be integrated over to get a state. Still, integrating over operators does seem like a weird formal maneuver.[/QUOTE] Grab a copy of 'Quantum Field Theory for the Gifted Amateur,' by Lancaster and Blundell. It's fucking glorious; we've been using it for a short, 6 week series of lectures that one of my professors has been giving to a bunch of us more or less for shits and giggles. The chapters are the perfect length; you can sit down and lazily read through a chapter in no time at all, and if you sit down and work through the examples and whatnot it should only take you an hour or so. From memory it's due to the fact that in the interaction picture the operators also have some time dependency in them, so the overall action of the Hamiltonian on a given state over some period of time is the same as the time independent part of the Hamiltonian, Ho, acted on some state, plus a sum (or integral) over H'\ket{Psi} at different times (over which time period H', the time dependent part of the Hamiltonian, varies). Bear in mind this is all to define the unitary time evolution operator, so you have to act it on a state for it to have any meaning (which is why I think your bold comment doesn't exactly hold, although you seem to have come to that conclusion by your last edit already anyway).
[QUOTE=sltungle;48801786]Grab a copy of 'Quantum Field Theory for the Gifted Amateur,' by Lancaster and Blundell. It's fucking glorious; we've been using it for a short, 6 week series of lectures that one of my professors has been giving to a bunch of us more or less for shits and giggles. The chapters are the perfect length; you can sit down and lazily read through a chapter in no time at all, and if you sit down and work through the examples and whatnot it should only take you an hour or so. From memory it's due to the fact that in the interaction picture the operators also have some time dependency in them, so the overall action of the Hamiltonian on a given state over some period of time is the same as the time independent part of the Hamiltonian, Ho, acted on some state, plus a sum (or integral) over H'\ket{Psi} at different times (over which time period H', the time dependent part of the Hamiltonian, varies). Bear in mind this is all to define the unitary time evolution operator, so you have to act it on a state for it to have any meaning (which is why I think your bold comment doesn't exactly hold, although you seem to have come to that conclusion by your last edit already anyway).[/QUOTE] Yeah, sounds like more or less what I figured out. I'll take a look at the book (thank god there are so many QFT books), but I'm using Tong's lectures because there are accompanying video lectures! Makes for basically the whole classroom experience except for the ability to ask my own questions, but the people in the class seem to ask good ones. I was having a tough time studying from Peskin and Schroeder because it's hard to know what's critical to learn and how much I need to have a 100% firm grasp on. Makes pacing difficult on your own. Pretty sure I plugged it in an earlier post already but pirsa.org is great for this. Free video class lectures are the wave of the future. Gonna be going right on to QFT 2 when I'm done with Tong's lectures. Then I'll either see if I like their string theory lectures, or start working through Polchinski.
Carroll's section about tensors is eye-opening but intense... I keep thinking of matrices and vectors to justify certain concepts (contraction) but I don't know if that will help or impede my understanding. Only did chapter 1 though, he says chapter 2 will be much more formal.
Tensors are about the worst motivated concept in physics, but they're really nice and easy to work with once they click.
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