Veritasium
VeritasiumDoes light take all possible paths at the same time? 🌏 Get exclusive NordVPN deal here ➵ https://NordVPN.com/veritasium It’s risk free with Nord’s 30 day money-back guarantee!✌
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: https://www.youtube.com/watch?v=2iuLwWDYC4U&t=1s
We’re 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): https://www.youtube.com/watch?v=ALEDjjAVZSY
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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 Broglie’s 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: https://ve42.co/PatreonLA2
Try Snatoms! A molecular modelling kit I invented where the atoms snap together.
https://ve42.co/SnatomsV
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References:
Armin Hermann (Nov 1974). The Genesis of Quantum Theory (1899-1913). – https://ve42.co/genquanttheor
Richard P. Feynman (1985). QED The Strange Theory of Light and Matter. – https://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. – https://ve42.co/Bloch2018
Coopersmith, J. (2017). The lazy universe: an introduction to the principle of least action. Oxford University Press. – https://ve42.co/LazyU
PBS Space Time. (Nov 2021) – https://www.youtube.com/watch?v=Q_CQDSlmboA
Sabine Hossenfelder. (May 2022) – https://www.youtube.com/watch?v=A0da8TEeaeE
Max Planck (1901). On the Law of Distribution of Energy in the Normal Spectrum. Annalen der Physik – https://ve42.co/lawdistenergy
Paul Ehrenfest (1911). Welche Züge der Lichtquantenhypothese spielen in der Theorie der Wärmestrahlung eine wesentliche Rolle?. Wiley Online Library – https://ve42.co/quantthermrad
Nils-Erik Bomark and Reidun Renstrøm (Aug 2023). The Ultraviolet myth. Proceedings of Science – https://ve42.co/uvmyth
Planck’s law. In Wikipedia – https://ve42.co/planckslaw
PBS Space Time. (Jul 2017). – https://www.youtube.com/watch?v=vSFRN-ymfgE&t=10s
Einstein and The Photoelectric Effect. (Jan 2005) via APS News – https://ve42.co/einsteinphotoelec
Louis de Broglie (1924). On the Theory of Quanta. – https://ve42.co/theoryquanta
Larry Sorensen (2012). Path Integrals 1. University of Washington – https://ve42.co/pathintegrals
Feynman’s Method of “A Particle Exploring All Possible Paths”. (Mar 2018) via Pure Dhamma – https://ve42.co/allposspaths
A great intuitive video on the Feynman path integral approach – https://www.youtube.com/watch?v=Sp5SvdDh2u8&t=1253s
Origins of Quantum Theory. via University of Pittsburgh – https://ve42.co/origquanttheor
Images & Video:
Nobistor Altona 1880 via NDR – https://ve42.co/streetlight
Edison incandescent lights by William J. Hammer via Wikimedia Commons – https://ve42.co/edinclights
The full expanded form of the Standard Model Lagrangian by Chen Ning Yang et al. via Wikipedia – https://ve42.co/stmodellang
Muon Ray. (2011). https://www.youtube.com/watch?v=-QUj2ZRUa7c
Muon Ray. (2011). https://www.youtube.com/watch?v=1C7abO7LtV0
Bohrs Atomic Model via EEEGUIDE.COM – https://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 Bean, Geeta Thakur, Emilia Gyles, Zoe Heron, Emily Zhang and Tori Brittain
Thumbnail contributions by Ben Powell, Jakub Misiek, Ren Hurley, and Peter Sheppard
Additional video/photos supplied by Getty Images and Storyblocks
Music from Epidemic Sound
source
47 Comments
@veritasium
03/07/2025 - 10:15 AMTo take all possible paths the internet has to offer, get the exclusive NordVPN deal here: https://NordVPN.com/veritasium It’s risk free with Nord’s 30 day money-back guarantee!
@happygames1841
03/07/2025 - 10:15 AMCan somebody help me understand this?
If I have a perfect mirror and shine a perfect at laser at it, I can register 100% energy at the reflected point, right?
But if I put the foil like at the end of the video, I can register additional light from a different source.
Where does the energy come from? Does the original registered light get dimmer?
Thanks in advance!
@slade-runner6773
03/07/2025 - 10:15 AMSo now my brain just started bouncing into (in my limited knowledge) does this relate to antimatter being created. I can't quite word it from where my brain is trying to conceive how is could produce antimatter in the moments that the waves are interacting. I had a moment of thinking back to you going on to how matter is also waves, and it made more sense of the many world theory (limited knowledge and understanding of if it relates) that we have the potential to be moving in multiple directions at once, but the waves interact that it cancels out to where we "are." Is it just that say that during the process of things cancelling, 2 versions of the wave had a delay or some kind of interaction that cause them to blink in and out when they finally cancel each other? Yeah I'm sorry, grasping at a little rabbit hole that opened up in my brain that I can't articulate clearly.
@bradbell7251
03/07/2025 - 10:15 AMDoes the path of least action depend on the position of the observer?
@Aldraz
03/07/2025 - 10:15 AMGreat, can we finally all agree that universe and all there is in this reality is likely a quantum holographic conscious neural network?
@KufLMAO
03/07/2025 - 10:15 AMI hope I look this good when I’m 42 😭
@PetrosV5
03/07/2025 - 10:15 AMI would believe it if you somehow put the magic paper BEHIND the laser torch.
@victor_anik
03/07/2025 - 10:15 AMMultiverse!
@murpheexu2046
03/07/2025 - 10:15 AMFor anyone that wants to delve further into this, read Feynman’s QED book!
@jelle191
03/07/2025 - 10:15 AMWhat if you put that kind of film on a solar panel, isnt it like it is collecting light from hundreds of suns?
@spedupbuilder4174
03/07/2025 - 10:15 AMWhere does the extra energy come from to light up that part where the laser isn’t directly shining? Does observing it affect the brightness of the main beam?
@emmanuelonyekaezeoba6346
03/07/2025 - 10:15 AMI certainly would have done better in my quantum mechanics class if I had this video then. Everything now makes a whole lot of sense.
@andrewrivera4029
03/07/2025 - 10:15 AMDude turn off the YouTube commercials! I got hit with 10 commercials over the 30 min video! THAT TOO MUCH! YOUR VIDEO ISN’T WORTH THAT AMOUNT OF TIME! BYE!
@sarahdaviscc
03/07/2025 - 10:15 AMThis delved deeper into the fundamentals of quantum mechanics than I've ever seen – I didn't understand a lot of it and it makes me wish I had have studied physics when I was young.
@thehorizontries4759
03/07/2025 - 10:15 AMCould he have gotten the idea from the discovery of the smallest amount of electrical energy-the electron?
@niaschim
03/07/2025 - 10:15 AMThere is no "every object" just the one… (jk) (or is?)
@Manullus
03/07/2025 - 10:15 AMAllow me to ask some naive questions from a non-physicist:
Does the last experiment with the laser really show that photons travel all possible paths?
My doubts arise, because the laser pointer is not "perfect", is it? Meaning it doesn't shoot all photons into the same direction, so if there are just a few that scatter, they can hit the foil that was used, no?
This should be possible to explore when pointing the laser without the black paper interfering AND the foil in place. The path of least action isn't blocked anymore then, and the beam should behave "as we want" as Feynman put it…but if the beam is imperfect, some of it should still appear on the foil.
If someone is aware of any such experiments, please point me to it.
I'm not arguing against the points made in the video, they appear convincing to me, even if not intuitive (but evolution didn't equip us with means to accurately experience reality, so we have to learn to accept that our senses and intuitive understanding of the world will have limits). I am merely looking for conclusive experiments that show these quantum effects in action. (I already know the double slit experiments, but would like to see if there is more…)
@Kai-o2b
03/07/2025 - 10:15 AMWow! The laser demonstration was mind blowing! But why do we only see the point in the reflection that the light could have travelled but not your corresponding hand with it? Like the light point is coming out of nowhere.
@ShikhaPrasadCraft
03/07/2025 - 10:15 AMRoute of least action…. Now it dawned on me, why I am lazy 😂😂
@Lordgamer_Sarthak
03/07/2025 - 10:15 AMBut why that points not appearing because of simple refractions changing the path of light❔
@benjaminforman8901
03/07/2025 - 10:15 AMFeynman. If someone's being a lovable genius a**hole somewhere on the timeline, it's Feynman. I really wish I'd met him once. On one of his more cheerful days.
@tykobray4132
03/07/2025 - 10:15 AMi had a theory. That fate and chance exist simultaneously. All paths and possibilities already exist, we're just choosing which one we take.
@davidhine8870
03/07/2025 - 10:15 AMrad explaination
@pietrov1335
03/07/2025 - 10:15 AMI just thought about something.
If (e.g.) light particle is actually travelling all those weird and much longer paths, yet it arrives at the destination in the same moment for all of them, then the shortest path (the one we think it is actually taking) is also one where it is absolutely the slowest. For a particle to travel any other path than the "default" one, it would have to travel much, much faster. The most optimal path is also the "slowest", since the "effect" is the same.
Isn't it something like nature finding the path of the least resistance and being most effective at doing what it does? Or is it just bluntly and easily to understand phrased the content of the whole video?
@sweswok2840
03/07/2025 - 10:15 AMCould you measure the intensity of the main reflection point of the ladder and see it drop once using the foil? That would be so amazing l.
@tshrjain
03/07/2025 - 10:15 AMUniverse is conscious 🫡
@silicon-shaman
03/07/2025 - 10:15 AMI'm not smart enough to udnerstand this lmao
@0therun1t21
03/07/2025 - 10:15 AMThank you so much for naming the symbols with underlining when you show the equations, it makes all the difference in whether I understand the math part.
Mu housemate is enjoying this too, it's very difficult for me since I didn't get this far in school but it's fun!
@kulik242
03/07/2025 - 10:15 AMMy head hurts from watching these videos.
@zachcrawford5
03/07/2025 - 10:15 AMI think that demonstration with the diffraction grating was just light leakage from the tip of the laser housing. The laser beam is wider than the aperture of the housing and is spreading slightly outward (because no real laser is actually perfectly parallel and the one that you can hold in your hand aren't even close) so light from the beam (and a lot of uncollimated light) is hitting the edges along the internal diameter of the aperture and being scattered off in all directions (like a radially emitting light source) and that is what you are seeing in the diffraction grating and not the light from the beam itself. If the inside of the aperture of the housing was vantablack and aperture of the housing bigger to prevent the actual beam from hitting the edges on the way out, I don't think you would have seen anything in diffraction grating once the dot from the main beam was blacked out.
@tryptophyl
03/07/2025 - 10:15 AMi have a really powerful laser, and the laser actually will illuminate my room from scattering,
my room gets filled with thousands of unimaginably dim dots
@splodeyferret
03/07/2025 - 10:15 AM3:34 Literally the hot new thing. They put out a lot of heat.
@philipnystromer
03/07/2025 - 10:15 AMIs there a newly released science book on the topic of Action that summarize the latest research and results on this?
@nepom1
03/07/2025 - 10:15 AMThanks!
@CNCmachiningisfun
03/07/2025 - 10:15 AMThere are some mechanics I would NEVER trust with my quantum car!
@DickShooter
03/07/2025 - 10:15 AMEverything is a singular partial? Lame. I move closer to God with every breakthrough.
@galenkehler
03/07/2025 - 10:15 AMAll this talk of standing waves reminds me of when i was a kid and contemplating how a universe could spring out of nothing, and that was the image that came to mind, of a sea of standing waves representing the nodes and anti-nodes of all the visable universe. Silly childish imaginings but its still interesting
@justindavidmathew
03/07/2025 - 10:15 AMThis is kinda mind-blowing from a classical understanding of physics!
@vaibhavgaming7875
03/07/2025 - 10:15 AMso can we see the laser if we kept the camera in the other room (since light takes all the possible paths
@AndroidAquaponicsTech
03/07/2025 - 10:15 AMThe only way light traveled through all paths to reach the destination is support for simulation theory
@faheemation
03/07/2025 - 10:15 AMThis is is the same thing that I am being taught in plus one in Kerala NCERT but I did not know it was this deep dude💀
@iangarrison9516
03/07/2025 - 10:15 AMWow… just… wow. THANK YOU!
@hellowill
03/07/2025 - 10:15 AMQuestion: if the light is split infinitely and taking infinite paths, are we able to harness some of that energy?
For example, in the demo, the laser is using all of its light/energy to focus on one point, right? So when he reveals it is indeed taking multiple paths, does that extra light shown take away from the main laser beam? Or is it additional? If it's additional, can we make use of its energy??? For example to build a perpetual motion machine?
@Frostyboy2016
03/07/2025 - 10:15 AMWith the arrow and the circle, an interesting thought is that it’s like the base clock speed of the universe. Like a CPU in a computer, moving even a small distance different will be insanely far way on that action scale
@Casimirpulaski-G2kn
03/07/2025 - 10:15 AMSo basically, “Everything's everywhere all at once?”
@putifaerio
03/07/2025 - 10:15 AMCould the experiment with the laser be repeated in a vacuum box? Since air would would reflect and refract the laser beam in every direction.
@camielplevier6422
03/07/2025 - 10:15 AMVery interesting and thanks for making such insightful videos.
Here are some additional questions that I would like to see answered:
– The illustrations show curly paths and only paths that run between A and B. If curly paths are possible, than surely also backwards paths should be considered. Can the dark side on earth be bathing in sunlight by just filtering out certain phases?
– The differences in travel time shoud cause interesting effects when switching the source. Will the indirect paths be visible longer than the direct path after switch off? Is there a momentary flash from indirect paths because some phases are off before others and they don't cancel out for a very short moment?