Lecture 34: The Wonderful Quantum World - Breakdown of Classical Mechanics

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: December 1999,   views: 62567
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

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1. Discrete Energy Levels:

Electrons orbit their atomic nucleus in well defined orbits corresponding to discrete energy levels. The electrons can jump from one energy level to a vacant energy level, but they cannot exist in between. Transitions between these energy levels gives rise to absorption and emission of light in discrete spectral lines (wavelengths). The students are encouraged to look through their diffraction gratings at helium and neon light sources to see evidence of these discrete wavelengths of emitted light.

2. Particles and Waves:

Quantum mechanics introduces some very non-intuitive concepts, e.g. light behaves as both a particle (a photon) and a wave, and a particle behaves like a wave with a wavelength inversely proportional to its momentum. Interference is a wave phenomenon, and indeed particles can interfere with each other. Both the position and momentum of a particle cannot be accurately specified at the same time (Heisenberg's uncertainty principle).

3. Diffraction by a Slit:

Diffraction of light by a narrow vertical slit is a well understood classical wave phenomenon consistent with Heisenberg's uncertainty principle. The narrower the slit, the smaller is the uncertainty in the horizontal position of the photons which have to sneak through the narrow opening, so the greater is the horizontal spread of the transmitted protons (uncertainty in their momentum). Quantum mechanics only allows you to predict positions of particles with certain probabilities. In the classical, Newtonian, world you can predict the position and movement of a particle to any degree of accuracy - NOT in the microscopic quantum world. The Newtonian picture is perfect for describing the behaviour of basketballs and planets in the macroscopic world.

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Reviews and comments:

Comment1 MARYAM, October 13, 2009 at 1:28 a.m.:

I hope the best wishes for you,because you made a situation for me to enjoy the world of physics.thank you indeed.

Comment2 suryanarayanan, November 26, 2009 at 5:39 p.m.:

good lecture. but back in INdia, 1960's when I did my M.S phys, none of my teachers had any notes nor vu graphs nor powerpoints; i was so lucky they were so good; we used to think that they were all trained in usa; but very few of them ever croosed madras city!!!

Comment3 Fred, December 16, 2009 at 8:23 a.m.:

simply amazing

Comment4 chris peters, December 26, 2009 at 1:40 a.m.:

Great teachers are true treasures and Walter Lewin should consider himself as one of them. I have watched some of his other lectures online without being a paying student at MIT. (a very gracious thank you to MIT for making such wonderful material available online)

My interest to heisenberg's uncertainty was by way of reading Stephen Jay Gould's short by delightfully readable book "Questioning the Millenium" in which he describes Pierre Simon Laplace's famous remark in the preface that if the direction and speed of every particle in the universe were known, if we had a 'snapshot" of every known particle without disturbing its direction or speed, we would know all past present and future events, down to the most arbitary spontaneous of human actions (not withstanding natural phenomena like the weather, the rate at which leaves fall from any random tree, etc) just from this one snapshot. And so heisenbergs uncertainty challenges those of us who believe causal determinism, that present and future are results of unbroken chains of events from the past that cascaded forward and are still so doing.

Heisenbergs uncertainty, keeping Laplace's comments in mind, is that the direction and velocity of all particles in the universe can be known by motion and still photography, that the image of light printed to the film on a camera is a lamda cold dark matter (http://www.youtube.com/watch?v=zO2vfY...) model that casts forward events of the future by the linguistical objective positioning of objects and elements in the frame of reference of the photograph.

How the professor uses the vertical slits to widen the photons of light through the shutter onto the screen raises questions to me about the uncertainty of a snapshot vertically captured and its horizontal uncertainty in consideration of Laplace's remarks. would, for example, a horizontally wide angle photograph have a vertical uncertainty (longitude? above or below where the picture is hung on the wall or how it is viewed on a screen later on to a briefed observer?..)

Comment5 Cosmina , July 7, 2010 at 10:17 p.m.:

Professor Lewin, at the end of 2010 you still have the greatest lectures available, thank you for making 1 year of self taught Physics doable and great fun. Sometimes your lectures felt better than in a real classroom.

Sir, have you ever taught quantum mechanics? If so is the video webcast available anywhere?

Thank you again, for all the energy and heart you put in your lectures !

Comment6 Francisco J. Vila, May 11, 2012 at 7:58 p.m.:

First of all, thanks Mr. Lewing for helping me to love physics. Dealing with this single slit topic, one question I can´t fully understand : I see you use the single slit experiment to justify the Heisemberg Principle, and of course it works mathematically. But I think someone could say: ok, I think this is not a quantum effect, it is just the diffraction of light and pure classic mechanics.
I mean, I just can not understand how you can justify this experiment in two such different ways, in some way they should contradict each other: classics vs. quantum worlds!

Comment7 Evans Ojwak, May 21, 2012 at 9:55 a.m.:

Thanks for the lecturer Prof.

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Comment9 berger, November 27, 2023 at 5:39 a.m.:

Thank you for making a year of self-study in physics both feasible and enjoyable, Professor Lewin; your lectures are still the best accessible as we near the conclusion of 2010. Having your lectures seemed more like being in a real classroom at times. https://minicrossword.io/

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