Physics with Elliot
Physics with Elliot
  • 33
  • 5 795 524
How Feynman did quantum mechanics (and you should too)
Discover Feynman's path integral formulation of quantum mechanics! 📝 Get the notes for free here: courses.physicswithelliot.com/notes-sign-up
👨‍🏫 Enroll in my course on Lagrangian mechanics!
bit.ly/lagrangian-fundamentals
✉️ Sign up for my newsletter for additional physics lessons: www.physicswithelliot.com/sign-up
📺 My previous video about the double-slit experiment and wavefunctions:
ua-cam.com/video/Se-CpexiJLQ/v-deo.html
📺 My earlier video about the principle of least action:
ua-cam.com/video/sUk9y23FPHk/v-deo.html
🅿 Become a patron to help make videos like these possible:
www.Patreon.com/PhysicsWithElliot
🙋‍♀️ "What software did you use to make this video?" and other FAQs:
www.physicswithelliot.com/faq
⚛️ Additional links:
- Feynman's 1964 lecture on quantum mechanics:
www.feynmanlectures.caltech.edu/fml.html#6
- Feynman's PhD dissertation can be found in this book:
www.worldscientific.com/worldscibooks/10.1142/5852#t=aboutBook
- Feynman's original paper on the path integral:
authors.library.caltech.edu/records/9h858-5hv71
- Dirac's 1932 paper that inspired Feynman:
www.worldscientific.com/doi/10.1142/9789812567635_0003
- Feynman's Nobel lecture, including the origin story of the path integral:
www.nobelprize.org/prizes/physics/1965/feynman/lecture/
- Feynman and Hibbs's textbook "Quantum Mechanics and Path Integrals" from the 1960s:
books.google.com/books/about/Quantum_Mechanics_and_Path_Integrals.html?id=JkMuDAAAQBAJ
📖 Video summary:
If you've learned some quantum mechanics before, you've probably seen it described using wavefunctions, and the Schrödinger equation, and so on. That's how quantum mechanics was originally constructed by people like Schrödinger, Born, Heisenberg, and many others in the 1920s.
In the 1940s, though, a 20-something-year-old grad student named Richard Feynman discovered another approach. He found that the motion of a quantum particle can be described by taking a sum over ALL the possible trajectories that the particle could conceivably follow. That sum over all quantum paths is what's nowadays called the Feynman path integral, and it's central to our modern understanding of quantum physics.
One of the most important lessons Feynman's perspective reveals is how the usual laws of classical mechanics emerge from this more fundamental, but seemingly very different, quantum mechanical description of nature. In the video, you'll see why a single, special path emerges from the sea of all possible quantum paths when we consider the motion of a big object like a baseball--called the path of stationary action. And that of course is the path that obeys F = ma!
0:00 Introduction
3:12 Quick overview of the path integral
5:46 Review of the double-slit experiment
8:32 Intuitive idea of Feynman's sum over paths
13:27 Why exp(iS/hbar)?
15:00 How F = ma emerges from quantum mechanics
23:15 Lagrangian mechanics
24:34 Feynman's story
25:22 Next time: how to compute the path integral?
If you find the content I’m creating valuable and would like to help make it possible for me to continue sharing more, please consider supporting me! You can make a recurring contribution at www.Patreon.com/PhysicsWithElliot, or make a one time contribution at www.physicswithelliot.com/support-me. Thank you so much!
About me:
I’m Dr. Elliot Schneider. I love physics, and I want to help others learn (and learn to love) physics, too. Whether you’re a beginner just starting out with your physics studies, a more advanced student, or a lifelong learner, I hope you’ll find resources here that enable you to deepen your understanding of the laws of nature. For more cool physics stuff, visit me at www.physicswithelliot.com.
Переглядів: 419 144

Відео

But why wavefunctions? A practical approach to quantum mechanics
Переглядів 167 тис.9 місяців тому
Discover how the behavior of a quantum particle is described by its wavefunction! 📝 Get the notes for free here: courses.physicswithelliot.com/notes-sign-up ✉️ Sign up for my newsletter for additional physics lessons: www.physicswithelliot.com/sign-up 📺 Watch part 2 on Feynman's path integral formulation of quantum mechanics: ua-cam.com/video/Sp5SvdDh2u8/v-deo.html 📺 More quantum mechanics: ua-...
Physics Students Need to Know These 5 Methods for Differential Equations
Переглядів 976 тис.Рік тому
Differential equations are hard! But these 5 methods will enable you to solve all kinds of equations that you'll encounter throughout your physics studies. Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Sign up for my newsletter for additional physics lessons: www.physicswithelliot.com/sign-up Almost every physics problem eventually comes down to solving a differential...
To Understand the Fourier Transform, Start From Quantum Mechanics
Переглядів 452 тис.Рік тому
Develop a deep understanding of the Fourier transform by appreciating the critical role it plays in quantum mechanics! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Sign up for my newsletter for additional physics lessons: www.physicswithelliot.com/sign-up The Fourier transform has a million applications across all sorts of fields in science and math. But one of the v...
The Surprisingly Subtle Limits of General Relativity
Переглядів 79 тис.2 роки тому
Discover how Einstein's general theory of relativity spans the cosmic spectrum from planetary orbits to black holes! Sponsored by Babbel 🎉: Start speaking a new language in 3 weeks. Get up to 65% OFF your subscription ▶️ HERE: go.babbel.com/12m65-youtube-physicswithelliot-may-2022/default Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Einstein's theory of gravity gener...
Particle Physics is Founded on This Principle!
Переглядів 158 тис.2 роки тому
Take your first steps toward understanding gauge field theory, which underlies everything we know about particle physics! Sponsored by Blinkist: Start your free trial and get 25% off! www.blinkist.com/elliot Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Conservation laws, symmetries, and in particular gauge symmetries are fundamental to the construction of the standar...
The Most Important Math Formula For Understanding Physics
Переглядів 344 тис.2 роки тому
Math is the language of physics, and there's one formula that you'll encounter time and again in your study of physics: the Taylor series. Get the notes for free here: courses.physicswithelliot.com/notes-sign-up If I had to name one mathematical theorem that's the most essential for understanding physics, it would be Taylor's formula. It shows up in virtually everything we do in physics. In thi...
Field Theory Fundamentals in 20 Minutes!
Переглядів 624 тис.2 роки тому
Field theory is the mathematical language that we use to describe the deepest theories of physics. I'll teach you the basics in about 20 minutes. Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Take my full course on Lagrangian mechanics! courses.physicswithelliot.com/lagrangian-fundamentals-page The most fundamental laws of nature that human beings have understood the ...
To Master Physics, First Master the Harmonic Oscillator
Переглядів 311 тис.2 роки тому
It's hard to overstate how important the simple harmonic oscillator is in so many areas of physics. Almost every system is an oscillator near a stable equilibrium! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Of all the systems you'll study in your first physics class, the oscillations of a block attached to a spring are a subject you'll meet again and again througho...
The Symmetry at the Heart of the Canonical Commutation Relation
Переглядів 39 тис.2 роки тому
The canonical commutator is one of the most fundamental equations of quantum mechanics. But where does it come from? I'll show you how symmetry leads straight to it! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up The canonical commutation relation between the position operator and momentum operator is one of the defining equations of quantum mechanics. It's what leads ...
The Most Beautiful Result in Classical Mechanics
Переглядів 55 тис.2 роки тому
Noether's theorem says that a symmetry of a Lagrangian implies a conservation law. But to fully appreciate the connection we need to go to Hamiltonian mechanics and see how symmetries act on phase space! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up The connection between symmetries and conservation laws is one of the deepest relationships in physics. Noether's theore...
How to Succeed at Physics Without Really Trying
Переглядів 59 тис.2 роки тому
Units are your physics superpower! With dimensional analysis, you can get 90% of the way to the answer for many physics problems with next to no work! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up In any given physics problem, you have a certain list of parameters at your disposal: masses, charges, lengths, and so on, and fundamental constants like G and c. And you're...
Before You Start On Quantum Mechanics, Learn This
Переглядів 119 тис.2 роки тому
Quantum mechanics is mysterious but not as mysterious as it has to be. Most quantum equations have close parallels in classical mechanics, where quantum commutators are replaced by Poisson brackets. Get the notes for free here: courses.physicswithelliot.com/notes-sign-up You can't derive quantum mechanics from classical laws like F = ma, but there are close parallels between many classical and ...
Why Newton's 3rd Law is Conservation of Momentum in Disguise
Переглядів 25 тис.2 роки тому
Newton's third law says that for every action there's an equal-but-opposite reaction. Hidden inside it is a deep principle of physics: conservation of momentum! Get the notes for free here: courses.physicswithelliot.com/notes-sign-up Newton's third law looks a little strange the first time you see it: all forces between particles come in equal-but-opposite pairs. But it's really all about a mor...
Symmetries & Conservation Laws: A (Physics) Love Story
Переглядів 93 тис.2 роки тому
Symmetries & Conservation Laws: A (Physics) Love Story
The First Thing You'll Learn in a String Theory Class
Переглядів 42 тис.2 роки тому
The First Thing You'll Learn in a String Theory Class
What I Wish I'd Known As A Beginning Physics Student
Переглядів 53 тис.2 роки тому
What I Wish I'd Known As A Beginning Physics Student
How Einstein Uncovered the Path a Particle Traces Through Spacetime!
Переглядів 48 тис.2 роки тому
How Einstein Uncovered the Path a Particle Traces Through Spacetime!
Thinking Recursively: How to Crack the Infinite Resistor Ladder Puzzle!
Переглядів 32 тис.2 роки тому
Thinking Recursively: How to Crack the Infinite Resistor Ladder Puzzle!
The Special Relativistic Action, Explained
Переглядів 33 тис.2 роки тому
The Special Relativistic Action, Explained
Explaining the Principle of Least Action: Physics Mini Lesson
Переглядів 119 тис.2 роки тому
Explaining the Principle of Least Action: Physics Mini Lesson
The Melting Ice Problem!
Переглядів 19 тис.2 роки тому
The Melting Ice Problem!
How Archimedes Solved the Buoyant Force Puzzle 2000 Years Ago
Переглядів 8 тис.2 роки тому
How Archimedes Solved the Buoyant Force Puzzle 2000 Years Ago
The Shortcut that Lets You Write Down the Orbit of a Planet in One Line: Physics Mini Lesson
Переглядів 17 тис.2 роки тому
The Shortcut that Lets You Write Down the Orbit of a Planet in One Line: Physics Mini Lesson
DERIVING the Orbit of Our Home Planet from Newton's Law of Gravity: Physics Mini Lesson
Переглядів 51 тис.2 роки тому
DERIVING the Orbit of Our Home Planet from Newton's Law of Gravity: Physics Mini Lesson
The Trick that Makes Understanding Physics as Simple as Drawing a Picture: Physics Help Room
Переглядів 86 тис.2 роки тому
The Trick that Makes Understanding Physics as Simple as Drawing a Picture: Physics Help Room
What Happens When a Block Slides Down a SLIDING Ramp? Classic Physics Problem
Переглядів 11 тис.2 роки тому
What Happens When a Block Slides Down a SLIDING Ramp? Classic Physics Problem
Blocks Sliding Down Ramps, and All That: Physics Help Room
Переглядів 21 тис.2 роки тому
Blocks Sliding Down Ramps, and All That: Physics Help Room
How Do You Find The Shape of Hanging Rope? Classic Physics Problem
Переглядів 40 тис.2 роки тому
How Do You Find The Shape of Hanging Rope? Classic Physics Problem
Lagrangian and Hamiltonian Mechanics in Under 20 Minutes: Physics Mini Lesson
Переглядів 1,1 млн2 роки тому
Lagrangian and Hamiltonian Mechanics in Under 20 Minutes: Physics Mini Lesson

КОМЕНТАРІ

  • @fgsd3155
    @fgsd3155 7 годин тому

    great channel, great content, great presentation and explanations and graphs.... A++

  • @The_1_One_1
    @The_1_One_1 11 годин тому

    The prime requirement of physics is that you should extremely good at mathematics.....even very basic level physics may need you to use extreme mathematics 😂😂😂........and higher physics is out of the scope of the present available mathematics 😂😂😂

  • @emilyli5179
    @emilyli5179 13 годин тому

    I think there was one similar question in the past Physics Bowl exams

  • @avova
    @avova 20 годин тому

    The "parabola" examples, are, ofc, wrong.

  • @jas_mandalorian
    @jas_mandalorian День тому

    31 minutes of absolute brilliance

  • @sergiomaia3029
    @sergiomaia3029 День тому

    I think that Newton had the idea that gravity bends light through a simple thought experiment: Bob, standing next to a wall of a free falling elevator shoots particles of lights towards the opposite wall. While Bob determines that the particles moved in a straight line, Alice on the ground, does not see the same. Instead, she sees the particles moving on a curved trajectory! Therefore, they conclude that light moves on straight lines on frames free from the action of forces, but not when moving in a gravitational field. Therefore, gravity should bend light! Furthermore,they conclude that the greater the gravity, the more curved will be the trajectory.

  • @alphamf0
    @alphamf0 2 дні тому

    What app do you use to write?

  • @tomekgwozdz6037
    @tomekgwozdz6037 2 дні тому

    Rings with all nonzero elemnts invertable go brrrrrrrr

  • @mcqueen424
    @mcqueen424 3 дні тому

    Replace Laplace transform with Fourier transform and your video might actually be good

  • @kisho2679
    @kisho2679 3 дні тому

    But isn't the real challenge in finding the potential energy in a specific problem?

  • @FLORIDIANMILLIONAIRE
    @FLORIDIANMILLIONAIRE 3 дні тому

    Just do static analysis and oversizing comes with experience dont need differential equations

  • @mohammedseid1987
    @mohammedseid1987 4 дні тому

    0-0

  • @SubEthaEngineer
    @SubEthaEngineer 4 дні тому

    i love you

  • @SubEthaEngineer
    @SubEthaEngineer 4 дні тому

    beautiful! thank you for making this

  • @RickGGb1
    @RickGGb1 4 дні тому

    I wish my professors were this good at explaining

  • @kk12181
    @kk12181 4 дні тому

    Why should we take 2nd derivative??

  • @chucky428
    @chucky428 4 дні тому

    great remedy of insomnia

  • @FB0102
    @FB0102 5 днів тому

    Can you explain why the standard Lagrangian is T-U? Intuitively, why that specific form (other than 'because it works')? Thanks

  • @captaingenyus843
    @captaingenyus843 5 днів тому

    Thats just fcking great!

  • @PHY-easy-ICS
    @PHY-easy-ICS 6 днів тому

    One of the best explanations on this topic

  • @agrajyadav2951
    @agrajyadav2951 6 днів тому

    Thanks a lot, sir!

  • @al8188
    @al8188 6 днів тому

    "There's a good chance you've run into this equation before." My man, its 11PM and I'm eating a donut on my kitchen counter. Not only have I not run into that equation before, but in an hour it is going to be replaced with more math I can't do, a video about a cool rock, or documentary about a country I ain't ever gonna go to. Simple Harmonic Oscillators look like they kick all kinds of ass but I am up too late looking for answers to problems that differential equations can't solve. Do not underestimate how dumb I am.

  • @CeRz
    @CeRz 7 днів тому

    Here before this channel gets millions and millions of subscribers. Keep doing these animations, they are invaluable when you show the concepts. It really helps visualising the physics and the math.

  • @PhysicsNg
    @PhysicsNg 8 днів тому

    My teacher said I dont need to study lagrangian until I can get into APhO😂

  • @user-uc8nz8io2k
    @user-uc8nz8io2k 10 днів тому

    E=mc^2 Schwarzschild radius

  • @nychan2939
    @nychan2939 10 днів тому

    It seems I can win Nobel prize if I take your course.

  • @zord1352
    @zord1352 11 днів тому

    I have just found this channel, weeks before QM exams. Your videos really helps to patch the fragments. Thank you the video!

  • @jaxetika
    @jaxetika 11 днів тому

    Anyone else still have no clue?

  • @robyn-lee-INFJ
    @robyn-lee-INFJ 11 днів тому

    Probability of finding an orgasm in Ex

  • @UdiDol
    @UdiDol 12 днів тому

    Great1

  • @liadvazina6027
    @liadvazina6027 12 днів тому

    im starting quantum theory 1 now and the next video could really help!

  • @asdf7219
    @asdf7219 12 днів тому

    Wolframalpha

  • @liltike2197
    @liltike2197 12 днів тому

    The only problem is that the hamiltonian is not always equal to the total energy.

  • @CandidDate
    @CandidDate 13 днів тому

    What are you talking about?

  • @asdf7219
    @asdf7219 13 днів тому

    Why do we use dot notation to regard n-th order differentials instead of the standard apostrophe "prime" such as f'(x)?

  • @takyon24
    @takyon24 13 днів тому

    Man this is high quality, easy some of the best physics educational content on youtube. Do you still plan on uploading any problem sets for this video? Thanks a lot for the notes btw

  • @CandidDate
    @CandidDate 13 днів тому

    You got to be some kind of big idiot to be surprised still how nature works!

  • @jordansimmons2748
    @jordansimmons2748 13 днів тому

    This just made my head hurt

  • @jiukam
    @jiukam 14 днів тому

    Im fking enlightened

  • @n_fieldgaming
    @n_fieldgaming 14 днів тому

    Thank you so much sir for this video ❤✨

  • @jayhariyani5361
    @jayhariyani5361 14 днів тому

    Why normalised wf is required??

    • @lepidoptera9337
      @lepidoptera9337 9 днів тому

      It isn't, really. The normalization is merely an expression that we are dealing with an ensemble (i.e. an infinite repetition of the same experiment). It works well for closed systems with finite volume and it doesn't work, at all, for scattering problems. It's kind of the toy version of the real theory.

  • @thestickdog3621
    @thestickdog3621 15 днів тому

    ah, yes, one of the most known engineering theorems: sin T = T

  • @MuhammadRamzan-tf6br
    @MuhammadRamzan-tf6br 15 днів тому

    Amazing, thank you so much. It was music to the ears listening to you!

  • @fikaduzawdie3677
    @fikaduzawdie3677 16 днів тому

    MashaAllah

  • @dhwang101
    @dhwang101 16 днів тому

    So... You don't need calculus unless you want to find a particle. Ok, wasted a year in school 😂😂😂

  • @olivierbarbe9269
    @olivierbarbe9269 16 днів тому

    Great video thanks, very clear and engaging

  • @ggxsky4811
    @ggxsky4811 16 днів тому

    Lagrangian is sick

  • @JeromeCallas
    @JeromeCallas 16 днів тому

    I'm gonna do u the good of telling u why I can't hang: u called Newton's interpretation of his share of facts "nonsense", as if u with the same facts could've done better. Look up the words "intellectual" and "hubris", then have a think.

  • @sensorer
    @sensorer 18 днів тому

    Hate to be a hater, but I find the explanation in this video to be lacking and circular. Okay, so we're going to pull out this factor (1/ih_bar) and this here is a quantum generator(because it just is), and now that we've pulled out of our hat that it's the quantum generator, it should be momentum. For me, Dirac's approach of getting the quantum Poisson bracket from the classical one is much more satisfying and then using the canonical commutation relations you got from that you find the translation operator and all the other stuff you want.

  • @user-ej5cd4ei2l
    @user-ej5cd4ei2l 19 днів тому

    On a xyz graph, a point is potential energy. Time is kinetic energy when going to another point. This existence is amplitude and is nested between another reality made up of shattered pieces.