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A massive thank you to Dr. Andrew Mitchell for all his help and expertise. If you want to delve further into this topic then we highly recommend his lecture on Classical physics derived from quantum mechanics:    â˘Â Classical physics derived from quantu... Â
Weâre incredibly grateful to Prof. David Kaiser, Prof. Steven Strogatz, Prof. Geraint F. Lewis, Elba Alonso-Monsalve, Prof. Christopher S. Baird, Prof. Anthony Bloch, and Prof. Stephen Bartlett for their invaluable contributions to this video.
A special thanks to Mahesh Shenoy from FloatHeadPhysics for his help with this video. Check out his excellent intuitive video on the UV Catastrophe here (Sep 2024):    â˘Â I wish I was taught the birth of Quan... Â
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0:00 What path does light travel?
2:40 Black Body Radiation
6:47 How did Planck solve the ultraviolet catastrophe?
9:42 The Quantum of Action
13:25 De Broglieâs Hypothesis
15:16 The Double Slit Experiment
20:00 How Feynman Did Quantum Mechanics
25:01 Proof That Light Takes Every Path
31:16 The Theory of Everything
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Sign up to our Patreon to submit your Q&A questions: ve42.co/PatreonLA2
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References:
Armin Hermann (Nov 1974). The Genesis of Quantum Theory (1899-1913). - ve42.co/genquanttheor
Richard P. Feynman (1985). QED The Strange Theory of Light and Matter. - ve42.co/qed
A fantastic book on the history of the Principle of Least Action: Rojo, A. and Bloch, A. The
Principle of Least Action: History and Physics. - ve42.co/Bloch2018
Coopersmith, J. (2017). The lazy universe: an introduction to the principle of least action. Oxford University Press. - ve42.co/LazyU
PBS Space Time. (Nov 2021) -    â˘Â Is ACTION The Most Fundamental Proper... Â
Sabine Hossenfelder. (May 2022) -    â˘Â The Closest We Have to a Theory of Ev... Â
Max Planck (1901). On the Law of Distribution of Energy in the Normal Spectrum. Annalen der Physik - ve42.co/lawdistenergy
Paul Ehrenfest (1911). Welche Zßge der Lichtquantenhypothese spielen in der Theorie der Wärmestrahlung eine wesentliche Rolle?. Wiley Online Library - ve42.co/quantthermrad
Nils-Erik Bomark and Reidun Renstrøm (Aug 2023). The Ultraviolet myth. Proceedings of Science - ve42.co/uvmyth
Planck's law. In Wikipedia - ve42.co/planckslaw
PBS Space Time. (Jul 2017). -    â˘Â Feynman's Infinite Quantum Paths Â
Einstein and The Photoelectric Effect. (Jan 2005) via APS News - ve42.co/einsteinphotoelec
Louis de Broglie (1924). On the Theory of Quanta. - ve42.co/theoryquanta
Larry Sorensen (2012). Path Integrals 1. University of Washington - ve42.co/pathintegrals
Feynmanâs Method of âA Particle Exploring All Possible Pathsâ. (Mar 2018) via Pure Dhamma - ve42.co/allposspaths
A great intuitive video on the Feynman path integral approach -    â˘Â How Feynman did quantum mechanics (an... Â
Origins of Quantum Theory. via University of Pittsburgh - ve42.co/origquanttheor
Images & Video:
Nobistor Altona 1880 via NDR - ve42.co/streetlight
Edison incandescent lights by William J. Hammer via Wikimedia Commons - ve42.co/edinclights
The full expanded form of the Standard Model Lagrangian by Chen Ning Yang et al. via Wikipedia - ve42.co/stmodellang
Muon Ray. (2011).    â˘Â Richard Feynman QED Lecture 2, Reflec... Â
Muon Ray. (2011).    â˘Â Richard Feynman QED Lecture 2, Reflec... Â
Bohrs Atomic Model via EEEGUIDE.COM - ve42.co/bohrmodel
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Special thanks to our Patreon supporters:
Adam Foreman, Albert Wenger, Alex Porter, Alexander Tamas, Anton Ragin, Autodidactic Studios, Balkrishna Heroor, Bertrand Serlet, Blake Byers, Bruce, Dave Kircher, David Johnston, David Tseng, Evgeny Skvortsov, Garrett Mueller, Gnare, gpoly, Greg Scopel, Juan Benet, Keith England, KeyWestr, Kyi, Lee Redden, Marinus Kuivenhoven, Matthias Wrobel, Meekay, meg noah, Michael Krugman, Orlando Bassotto, Paul Peijzel, qiaohui wei, Richard Sundvall, Sam Lutfi, Tj Steyn, TTST, Ubiquity Ventures, wolfee
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Directed by Casper Mebius
Written by Casper Mebius and Derek Muller
Edited by Trenton Oliver and Peter Nelson
Animated by Fabio Albertelli, Mike Radjabov, Ivy Tello, Andrew Neet and Emma Wright
Illustrations by Jakub Misiek, Tommy A. Steven and Cainejan Esperanza
Filmed by Derek Muller and Casper Mebius
Produced by Casper Mebius, Derek Muller, Rob Beasley Spence, Gabe Strong, Geeta Thakur, Emilia Gyles, Zoe Heron, Emily Zhang and Tori Brittain
Thumbnail contributions by Ignat Berbeci, Ben Powell, Jakub Misiek, Ren Hurley, and Peter Sheppard
Additional video/photos supplied by Getty Images and Storyblocks
Music from Epidemic Soun
Views : 7,911,147
Genre: Education
License: Standard YouTube License
Uploaded At Mar 4, 2025 ^^
warning: returnyoutubedislikes may not be accurate, this is just an estiment ehe :3
Rating : 4.958 (2,760/258,498 LTDR)
98.94% of the users lieked the video!!
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User score: 98.41- Masterpiece Video
RYD date created : 2025-03-24T08:51:24.392173Z
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17,381 Comments
Top Comments of this video!! :3
Not gonna lie, I didn't understand 80% of this video, so the fact that it was still entertaining to me is a huge demonstration of your quality as a communicator and teacher.
24K |
I'm disappointed that Casper missed the chance to say "I have lights, camera and action."
13K |
1:18 I never realized the innocent little snell's law they teach us in class was so deep.
6.8K |
At the end of the demo a laser is used instead of a lamp to prove that light is emitted at a straight direction only. But there is also light coming from the source of the laser which is also scattered to all directions. When a laser is on you always see the tip of the laser having some light. I fear that the light points on the foil are from this scattered light of the tip. I believe the demo should somehow hide the tip of the laser to the direction of the foil and then prove this theory if again we have the light points. Please @Veritasium or anyone else explain if I think something wrong on this.
443 |
"Now although it worked spectacularly well, no-one could make sense of it." That's exactly how I feel watching every one of these videos.
681 |
No wonder my flights are ALWAYS late! Guess they're busy exploring every damn route to get to me
7.5K |
In the mid 1980s, Feynman gave a seminar on his path integral in a small classroom at the University of Alaska at Fairbanks. I attended and was amazed. He showed that the solution to a problem could be expressed as an infinite sum of probabilities, in which probabilities greater than 1 and less than zero were included. (there might have even been imaginary probabilities that included i). He agreed that many of the terms made no sense but said not to worry about them. He then showed that all the crazy terms cancelled out, leaving just a real number between zero and 1. What a shame that that seminar was not recorded.
391 |
This feels like the first time I've ever truly grasped something about quantum mechanics. Explaining it through the concepts of constructive and destructive interference really made it click in a whole new way.
48 |
Man, the computer simulating our universe must have a TON of compute power to do all those calculations.
13K |
24:57 "They constructively interfere"
This is what finally made me get it!
All the other possible paths of action cancel out due to their collective phases of probability opposing each other, summing to an amplitude of 0. The ones that do not get cancelled out in this way are the ones which require the least amount of action.
425 |
Just for some context. The most likely reason why Planck chose "h" for the constant is because it stands for "hilfsgrĂśĂe", which means something in the line of "auxiliary quantity" in German. So originally it was really just a mathematical tool. He probably didn't think that a photon's energy was actually quantized. My intuition is that he probably intended of taking the limit "h -> 0" once he obtained a formula for the temperature-dependent radiation law (at 9:31), and once he saw that taking this limit didn't make sense, and that actually h > 0 he became puzzled... and it was the beginning of quantum mechanics.
1.6K |
I still think the laser experiment may be flawed, because what the camera is seeing from the diffraction grating are scattered light from the laser device itself, in the same way that Veritassiumâs camera captured the laser light source. The same goes to the second point in the diffraction grating is coming from the scattered laser ight reflecting off Casperâs hands, thatâs we we see 2 points in the diffraction grating. Another possible source of scattering would be air itself and any dust particles suspended in it. A perfect demonstration of this would be to do this in a âperfectâ vacuum and design the laser light device in such a way that no scatterring happens at from the point source.
23 |
Rule of thumb: any video that heavily features quantum mechanics will be confusing on the first watch, then simple on a rewatch, then confusing again on a second rewatch.
4.4K |
so much of the first half of this video covered concepts that were technically part of school curriculums, but they were often reduced to mathematical derivations rather than the underlying science. Incredibly refreshing to see the nuances, historical context, and the actual evolution of understanding behind these ideas. This might just be my new favorite Veritasium video!
819 |
I'm a physics PhD student and that demo at the end blew my mind. You get to a point where all the stuff you learn seems so far beyond the "real" world and it's really amazing to see it actually working!
EDIT: did not expect more than a couple people to see this comment. Thank you to all those who have pointed that this can be explained classically with simple diffraction rules--I was a little too eager to see path integrals demo'ed and I may have fallen for some confirmation bias. I will definitely be looking into it more though.
3.9K |
Amazing video yet again. Every video you post I feel a deep regret that I didn't study the hard sciences more. I am currently doing a master's in interaction design and computer science, but once I am done I am seriously going to consider taking a degree in physics too.
27 |
I love this topic, just finished this unit in undergraduate modern physics. It's so much fun showing students the history of how these early quantum ideas arose.
One interesting detail, Planck did not introduce the quantum of light. He believed that the atoms in the blackbody constituted little oscillators that vibrated with energies nhf and none in between. Einstein's interpretation of Planck's blackbody formula as photons - light particles - was initially rejected by the entire physics community.
Contrary to what is often explained in physics courses, even Planck himself didn't believe Einstein. In his recommendation to the Prussian academy, Planck said of Einstein, "that he may sometimes have missed the target in his speculations, as, for example, in his hypothesis of *light quanta*, cannot really be held too much against him."
Then Robert Millikan tried to prove Einstein wrong and ended up doing a photoelectric experiment in 1914 so good that proved him completely correct. The history of early quantum theory is mostly people making ridiculous proposals that experts thought were so crazy that they tried to disprove them, only to give the proposals perfect confirmation.
281 |
I love the understatement at 15:04 "That's pretty good! You get a dissertation out of that."
Louis de Broglie also got a Nobel Prize out of it.
745 |
There is an issue with the experiment. At 30:22 you can see that, like a lot of pen lasers, this one has some light spillage inside the metal dome of the aperture. It bounces around in there and escapes in random directions. With a better aperture, you could remove this and make the demonstration even more compelling.
1.5K |
@veritasium
2 weeks ago
To take all possible paths the internet has to offer, get the exclusive NordVPN deal here: nordvpn.com/veritasium Itâs risk free with Nordâs 30 day money-back guarantee!
1.1K |