Hey everyone! Hope you like the video. I can honestly say that this is one of my favorite results in physics. Also, if you didn't see already, I am considering starting a Twitch channel where I would live-stream informal physics lectures. I want to do Twitch because it would give more opportunities for interaction/questions from the audience in real time. I would also upload the videos to KZbin after the fact. Let me know your thoughts on this!
@beastlybuickv64024 жыл бұрын
Last point seriously taken. I'm flying solely on intuition. 😒🐒😄
@Darkev774 жыл бұрын
Sounds like an amazing idea! I myself was actually looking for educational streamers on twitch, but was not very successful at finding them! Hopefully you'll be the first!
@zapphysics4 жыл бұрын
@Ascolano Irl, this is a great idea. I've never really interacted much with KZbin's livestreaming, so Twitch was my first thought. This is the direction I think I will go!
This may be one of your favorite topics but you got the 100% wrong intuition here. Please take the time to read Mott's 1929 paper about the derivation of alpha ray tracks from wave mechanics. That is how nature creates macroscopic reality, by constant weak measurement. Heisenberg gave a similar example to Mott using Rydberg atoms a year or two earlier in one of his papers. Both are nearly forgotten early results of weak measurement theory. Why they were forgotten and replaced by this mathematical limit nonsense is the real question. Physicists kept observing high energy particle tracks in gas, liquid and solid detector media, but they have never observed the averaging of paths around the classical path in vacuum. Such a thing simply doesn't happen.
@PrettyMuchPhysics4 жыл бұрын
That was a great visualization of how to think about path integrals :D Also, the infinite-frequency part was really well explained!
@dor000123 жыл бұрын
As a Physics MSc, I've watched endless videos on the principle of stationary action in order to understand it. This is the first one that actually explains why it makes sense! and it came in a total surprise, thanks man!
@Tehom12 жыл бұрын
I second the recommendation of Feynman's book QED. It is entirely readable and explains QED well.
@smxnke2 жыл бұрын
One of the BEST physics/science creators on KZbin.
@schmetterling44772 жыл бұрын
Also completely wrong on this one. He got this idea from Feynman, who didn't think it through. Since then everybody is playing monkey-see-monkey-do with path integral averaging.
@smxnke2 жыл бұрын
@@schmetterling4477 shut up and calculate.
@simonO712 Жыл бұрын
@@schmetterling4477 In what way is it wrong?
@OdysseyWorks4 жыл бұрын
Wow, this is the absolute best explanation that I have ever seen of the Path Integral! You my friend just earned a cookie and a new subscriber to your channel!
@beastlybuickv64024 жыл бұрын
Come on, friend, he merited more than a mere cookie! I offer a half-dozen chocolate donuts. BTW are you perhaps Pole or Finn? I'm Hungarian.
@LukePalmer11 ай бұрын
I really appreciate these videos. They are technical enough that I don't feel like I'm being hand-waved at, but not so dense that I can still follow without getting lost in the weeds. Thank you!
@joyboricua37214 жыл бұрын
I somewhat see a chicken-egg arguments occuring twice here: 1) Explicitly @3:55 with the "backward derivation". 2) Subtly @11:17 when the stated goal was to "derive classical behavior"... But it's just a conformity of mathematical interpretation. The underlying principle, which is that of least action, is the foundational presumpted-result (thus the 'egg-chicken' argument, if you will). So what emerges is a conclusion of something already readily known; which is good... is pretty much a meromorphism. I do reckon this content is quite excellent and admire very much the endeavors of propagating some of the insights that Richard had. Kudos!
@Darkev774 жыл бұрын
I'm SO glad I found this channel! Thanks for the brilliant content
@treborg7779 ай бұрын
This helped me with Feynman’s book. I found it frustrating, and your correspondences helped.
@KaihangShi4 жыл бұрын
Very clear explanation. Professors should learn how to teach from this video!
@shivangprasad Жыл бұрын
There is a classical to Quantum transition referred as coherent superposition which is described by non zero off diagonal entries where in quantum to classical transition of a system is described by loss of these off diagonal entries from the matrix,
@rbkstudios29234 жыл бұрын
I'm sorry I can't watch it now as it's 1:30 AM here and my mom is going mad at me for using my phone But I'll surely watch it in the morning
@Univercius3 жыл бұрын
You lost me at the connection at 7:40 but this is super interesting! Currently taking an honors physics course, but I'll only get to ap physics next year where they use integrals, I'll return to this video then to see if it's my re understandable
@xanterrx97414 ай бұрын
I love your videos , thanks for spending that huge amount of time creating this videos for us viewers
@Higgsinophysics4 жыл бұрын
Whaaat those animations :o
@THEDIVINEMISCARRIAGE3 жыл бұрын
I loved the part where action was involved in the Wave equations, made me very eager to play with funny physics sinarios.
@ARBB14 жыл бұрын
Found another golden channel. Very nice.
@pablojacome4008 Жыл бұрын
What a video! Thanks. You mentioned very deep issues
@jakublizon6375 Жыл бұрын
I've always found the path integral image beautiful.
@luhuang540510 ай бұрын
"General sadness". Yes. Immediately I knew what that felt like.
@aidansgarlato93474 жыл бұрын
It's that spiral a fresnel spiral, C(t)+iS(t), that only takes form when you integrate e^(ix^2). I'm just curious on how the integral pipes out. I can't quite tell from your discription in the video
@zapphysics4 жыл бұрын
This is a fantastic observation! In the case of the free particle, the path integral is a functional integral which is exactly a complex Gaussian. In the very simplest case of a non-relativistic particle, the Lagrangian is just 1/2*m*(dq/dt)^2 where we perform our functional integrover the coordinates q. In more general QFTs, this kinetic term looks different and we can add a separate mass term, but it is always quadratic in the fields, i.e. the Lagrangian (density) looks like A.O.A where A is a field we perform our functional integral over and O is some differential operator.
@potatofries5713 жыл бұрын
This was insightful thank you! I liked how you explained the Lagrangian's very well, maybe you could the same for the Hamiltonian one day in the future? :D
@elimarburger16594 жыл бұрын
Awesome video, this is my favorite of yours.
@zapphysics4 жыл бұрын
@Eli Marburger, Thanks! This was probably one of my favorites to make, and hopefully you will see more like it very soon!
@joelcurtis5623 жыл бұрын
Having read this argument many times, I'm still confused about what exactly it is claiming about the relationship between the classical and quantum descriptions. The quantum description gives you the prob amp for the particle starting at source to be measured at some point on the screen. I get that in the path integral description that amp is gotten by doing the path integral and getting a final arrow for that point. However, the classical description gives you the total path which the particle takes (classically) between source and destination. To put it another way, a classical treatment allows us to 'fill in' the history of the particle between source and screen, whereas the quantum treatment does not. It insists that the particle actually does take all possible paths. In Feynman's treatment and this one of yours, I am missing how we go from summing the action over all paths for a given point to 'filling in' the intervening path with a specific classical trajectory.
@zapphysics3 жыл бұрын
@Joel Knoll I can definitely see how this can be a sticking point, especially because the argument is actually more of a mathematical one than an intuitive one. It relies on a mathematical concept known as the "stationary phase approximation," which you can read more about here: en.wikipedia.org/wiki/Stationary_phase_approximation. The basic idea is this: when you sum up a bunch of functions which are all oscillatory, if the frequencies of oscillations are very rapid, then all of the contributions end up cancelling except for the one which has minimum frequency. When we look at the path integral, the amplitude from each contributing path is given by exp(i/hbar S) where hbar is Planck's constant and S is the classical action along that path. This is exactly the form of an oscillating function where the "frequency" of oscillation is given by S/hbar. If hbar is large enough to be relevant, then we can't use this approximation and we have to sum up all paths to get our total amplitude. This is the case of quantum mechanics: all paths contribute and we can't say that the system took any one path. However, if we want to "turn off" quantum mechanics, this amounts to taking hbar -> 0. Here, we see that the frequency of oscillation for each individual path, given by S/hbar, gets very, very large. Using our stationary phase approximation, we know that, when we sum all of the paths in this case, all of the contributions from the paths which don't minimize S/hbar will cancel since they are completely out of phase. So, in the limit where hbar->0, we are left with the only contributing path being that which minimizes the classical action, which is, of course, just the statement of Lagrangian mechanics. What I was trying to show in this video is how these paths begin to cancel when we start taking hbar to be smaller and smaller. This is the idea of the neat-looking spiral showing the sum of all of the individual amplitudes. When we take hbar to be smaller and smaller, the swirls which come from paths far from the classical path become tighter (meaning that the individual amplitudes are getting more out of phase) and pulling in more of the arrows from paths closer to the classical path. When we take all of the infinite paths into account and take hbar -> 0, all of the paths which are not the classical paths will perfectly cancel out (i.e. the spirals will become points and contain all of the arrows which do not come from the classical path), leaving only a single non-zero contribution to the total amplitude, which is the classical path. Basically, this is saying that in the hbar -> 0 limit, there is zero probability that the particle took any path other than the one which minimizes the action. Hopefully that helps!
@joelcurtis5623 жыл бұрын
@@zapphysics Alas, it does not help my confusion. It's somewhat subtle and I have a hard time articulating it. It may be that I'm overthinking. I'll try to state it clearly. The problem being solved here, it seems to me, is this: Given the source-point and a screen-point, what is the probability that a particle starting at source-point will be measured to be at screen-point some amount of time later? One way to get the answer, according to Feynman, is the sum-over-paths approach you outlined. And the idea there is that the probability amplitude for the transition in question to happen gets its largest (and in the limit hbar > 0, only non-zero) contribution from the path which corresponds to the one we would call the classical one. I.e. the one that minimizes the action. Where I struggle is in the logical step from "the final prob amp gets its only non-negligible contribution from a certain path" to "that same path is the one we classically observe." Something seems to be missing in the argument there, as far as how quantum gives rise to classical. After all, the whole point of Feynman's model is that it does NOT give us path information, because there IS no path information because the particle "takes all possible paths". How then can this quantum model containing no non-trivial path information give you an exclusive path? I suspect the answer is something like: to get a classical path, we have to do measurements all along the way. Which makes sense, since it is by making such measurements/observations that other paths get excluded, i.e. the wavefunction collapses. So we pack the space between source and screen with infinite other screens, densely packed. Then from source to screen 1, screen 1 to screen 2, screen 2 to screen 3, etc., the amplitude (as calculated from the sum-over-paths procedure) to be measured at any point not on the classical trajectory is zero because it gets canceled. So basically iterate this argument an infinite number of times to get a classical path.
@zapphysics3 жыл бұрын
@@joelcurtis562 It turns out that, to get the classical case, all you need to do is take the hbar -> 0 limit. You don't need to make a series of measurements or any of that, and you can still try summing over all paths, it just turns out that the final amplitude in this case is exactly equal to the amplitude from the classical path. So here is the bridge: when we take the hbar -> 0 limit, the result where we sum over all possible paths (quantum) and the result where we only take into account a single path (classical) exactly agree. So in this limit, if we treat the system as if it only evolves along one single path, we don't actually lose any information and we get the exact correct answer (for finite hbar, this would only give an approximation). At the end of the day, it sort of turns out that it is a matter of how we interpret things. For finite hbar, we don't have a choice. To get the correct answer, we have to use the quantum description where we sum over all paths. However, for hbar -> 0, we can instead choose to interpret the system as only following a single, classical path and still get the correct answer. This, of course, is the classical interpretation.
@joelcurtis5623 жыл бұрын
@@zapphysics wonderful! This has vexed me for some time and that is a clarifying explanation. Thank you!
@2tehnik2 жыл бұрын
@@zapphysics > You don't need to make a series of measurements or any of that, and you can still try summing over all paths, it just turns out that the final amplitude in this case is exactly equal to the amplitude from the classical path. So here is the bridge: when we take the hbar -> 0 limit, the result where we sum over all possible paths (quantum) and the result where we only take into account a single path (classical) exactly agree. So in this limit, if we treat the system as if it only evolves along one single path, we don't actually lose any information and we get the exact correct answer (for finite hbar, this would only give an approximation). Hm, I'm not sure that answers the question Joel had. I mean, is having an amplitude contribution from only the classical path the same as actually (or at least approximately traveling along side it)? Maybe I just don't know enough, since I'm not sure if the amplitude contribution from each path to a certain point also somehow marks the probability that that specific path was actually traveled along. And then how the amplitude (both of a specific path as well as a sum) relates to amplitudes for other ending points.
@jaca28993 жыл бұрын
Can the Method of Least Action deal with friction?
@2tehnik2 жыл бұрын
What does this say about energy conservation? Of the "Lagrnagian energy" anyway (en.wikipedia.org/wiki/Lagrangian_mechanics#Definition). I mean, deriving that the Lagrangian energy is constant (at least in Landau and Lifshitz' book on mechanics) requires the Euler-Lagrange equation, which presupposes stationary action, which is rejected here. So at best it seems like we could not say whether the Lagrangian energy is constant and at worst it would vary a lot depending on the path. Do you know if there's any work done on this?
@nUrnxvmhTEuU3 жыл бұрын
The links to the Feynman's QED lectures don't work anymore 😭
@clydenathaniel81173 жыл бұрын
Hey man, can I get the code that visualize the arrows and the 100 slits in this video? Is there a place where you upload these so I can mess with them? I'll credit you if I can make something with them (even tho I highly doubt that l I'm capable of doing anything new lol) Thanks! Awesome video btw, I learn a lot from it! :D
@zapphysics3 жыл бұрын
@Clyde Nathaniel Glad you liked the video! I would be more than happy to share the code! At the time being, it is in a not-so-user-friendly Mathematica notebook, but if you want to brave that, I can post it to my github. I could also relatively quickly put it into a Mathematica package which would be easier to use. Adapting it to a different language though would be tricky and definitely take some time, but probably could be done. Let me know what you prefer! (You can also feel free to email me and I can send it to you directly: zapphysics@gmail.com)
@clydenathaniel81173 жыл бұрын
@@zapphysics thank you! I'm more used to python, but feel free to throw me the mathematica package if you don't feel like wasting time translating it to another language. Just give me some guidance on how do I add more slits, check the phase, etc. would be enough for me. Do you have discord or somewhere I can reach you quick for questions? My email is dokisame@gmail.com, and my discord is doki73#9834.
@thekinghass4 жыл бұрын
Really great video
@pedrol.mammini49404 жыл бұрын
Great video, Keep it up!
@Ikbeneengeit4 жыл бұрын
Great video thank you
@pacificll87622 жыл бұрын
Great !
@Vollkornaffe4 жыл бұрын
11:14 hello there, Julia set
@Arseniy_Arseniy3 жыл бұрын
That was good even for non-English speaker)
@SampleroftheMultiverse9 ай бұрын
“U” Shape Waves This model may be related to the your topic. kzbin.info/www/bejne/raOlpKSfepWpfZYsi=waT8lY2iX-wJdjO3 Thanks for your informative and well produced video. You and your viewers might find the quantum-like analog interesting and useful. I have been trying to describe the “U” shape wave that is produced in my amateur science mechanical model in the video link. I hear if you over-lap all the waves together using Fournier Transforms, it may make a “U” shape or square wave. Can this be correct representation Feynman Path Integrals? In the model, “U” shape waves are produced as the loading increases and just before the wave-like function shifts to the next higher energy level. Your viewers might be interested in seeing the load verse deflection graph in white paper found elsewhere on my KZbin channel. Actually replicating it with a sheet of clear folder plastic and tape. Seeing it first hand is worth the effort.
@carlosomanaarredondo3 жыл бұрын
You lost me at the 37 second mark with formulas. Not the way to show the layman like me about physics.
@neuronneuron36453 жыл бұрын
The philosophical interpretation of the Lagrangian and principle of least action is Murphy's Law - anything that can happen will Minimising the difference between kinetic and potential is exactly that, allowing everything that is potential to be utilised given what is currently in motion. This gives a unique path I think using QM to explain this is a step in the wrong direction. It's a much more general, universal principle, of which physics participates in Its more of a law of logic or common sense even
@zapphysics3 жыл бұрын
@Neuron Neuron I don't know if I follow this... I'm not sure I see the connection between Murphy's law and the form of the Lagrangian. The claim is that one "allows everything that is potential to be utilized given what is currently in motion." However, the potential energy will be a fixed quantity along the path. When we vary the path, we not only change the potential energy, but also the kinetic energy, so I'm really not sure how one justifies this form of T-U based on this argument. The other thing is that the logic seems a little contradictory to me. Again, the claim is that minimizing the action allows for us to take into account all things that can happen. However, minimizing the action yields a single path, completely disregarding all other paths, so I'm really not sure how Murphy's law is at all connected with this procedure. I think that the quantum mechanical picture is much more conducive to a Murphy's law description because it truly does allow every possible path to contribute, not favoring any one over the other.
@2tehnik2 жыл бұрын
> Minimising the difference between kinetic and potential is exactly that, allowing everything that is potential to be utilised given what is currently in motion. But L=T-V, not |T-V|. So it's not necessarily the difference that's being minimized.
@rbkstudios29234 жыл бұрын
Yo First right?
@jakublizon6375 Жыл бұрын
I honestly find quantum mechanics to be easier than classical mechanics. Discrete > Continuous
@schmetterling44772 жыл бұрын
Yes, that is the usual lore and it is 100% false. That is not the mechanism by which nature creates classical mechanics from quantum mechanics. The real physics is continuous weak measurement. See e.g. Mott's 1929 paper on "The Wave Mechanics of alpha-Ray tracks".
@beastlybuickv64024 жыл бұрын
Perhaps another analogy: as Humans we will get breakthroughs in physics proportional to the chaos against which we counteract. Really is 'reward vs punishment.' The parallel universe of evil is not an option for me, anyway. There is a Spiritual Substance that holds everything in STC-perfect order, for Whom nothing is impossible. BTW loved the visuals! At age 5 I understood infinity mirrors yet here at 55 I'm incompetent as ever in math. Taoism says hope is an illusion & I must agree, LOL. 😁