Double Slit Experiments I

1. Double Slit Experiments I by Robert Nemiroff Michigan Technological University

2. Physics X: About This Course • Pronounced "Fiziks Ecks" • Reviews the coolest concepts in physics • Being taught for credit at Michigan Tech • Michigan Tech course PH4999 • Title: Extraordinary Concepts in Physics • Aimed at upper level physics majors • Light on math, heavy on concepts • Anyone anywhere is welcome • No textbook required • Wikipedia, web links, and lectures only • Find all the lectures with Google at: • "Starship Asterisk" then click on "Physics X"  or • http://bb.nightskylive.net/asterisk/viewforum.php?f=39

3. Double Slit Experiment: Importance "I think I can safely say that nobody understands quantum mechanics."  - Feynman "Any other situation in quantum mechanics, it turns out, can always be explained by saying 'You remember the case of the experiment with two holes?  It's the same thing."  - Feynman

4. Double Slit Experiment: Importance Voted by physicists reading Physics Today magazine as the most interesting experiment ever done in physics. Two Slit Experiment illuminates: • Uncertainty principle • Interference • Entanglement • Coherence and decoherence • How truly strange QM is

5. Double Slit Experiment: Basics • Most easily done with photons but, amazingly, can be done with any particle.  Idealized Geometry:                                 source      slit screen  image screen

6. Double Slit Experiment: Basics • The source is a point source and illuminates both slits equally.  Therefore, on the average, both slits will see the same mean number of photons. • The slits are identical.  They have identical width D. • The wavelength of the light is λ. • The slit screen is completely opaque except for the slits, which are completely transparent.  

7. Double Slit Experiment: Basics • Unless otherwise important to the experiment, photons should be considered to have the same wavelength λ. • The image screen records the position of photon impacts with arbitrarily high accuracy.

8. Double Slit Experiment: Basics • Practical geometry (almost anyone can set this up): • light wavelength: λ • width of slits: W • Distance between slits: D • W ~ a few λ • D ~ a few W

9. Double Slit Experiment: Basic Results Real result of Double Slit Experiment:

10. Double Slit Experiment: Basic Results • Photons traversing the two slits might interfere • interference pattern created at the image screen • geometric projection of each slit onto the image screen shows no overlap region • even in the "plane wave" approximation • therefore, without single slit diffraction, no interference can occur • at any point on the image screen, the projection of one slit's light is usually "brighter" than the other • interference minima usually not zero

11. Double Slit Experiment: Wave-Particle Duality The results are considered the definitive indication of wave-particle duality: that photons, electrons, etc. can act like BOTH waves and particles. • Photons interfere as waves • common example: water waves • Photons are recorded at the image screen as particles • These are the SAME photons! • Examples of things that are both particles and waves: • photons • all fundamental particles • everything

12. Double Slit Experiment:One Photon at a Time • The usual double-slit experiment is done except now the photon rate is reduced so that, on the average, only one photon passes the slit screen at any time. What happens? • No (two-slit) interference pattern appears. • A normal (two-slit) interference pattern eventually appears. • The image screen remains blank. • The experimenter loses interest and begins to watch TV.

13. Double Slit Experiment:One Photon at a Time 2.  A normal (two-slit) interference pattern eventually appears. Comments: • One of the most amazing results in all of physics. • Single photons appear to go through both slits. • Dirac (paraphrase): "Photons interfere only with themselves." • This assumes that nobody knows which slit individual photons went through. 

14. Double Slit Experiment:Electrons Replace Photons • The usual double-slit experiment is done except now electrons replace photons.  The electrons have mass but the same wavelength as the photons. What happens? • An interference pattern eventually appears. • No interference pattern eventually appears. • The image screen remains blank. • Monsters from the deep devour Cleveland.

15. Double Slit Experiment:Electrons Replace Photons 1.  A normal interference pattern eventually appears. Comments: • Another of the most amazing results in all of physics. • Single electrons appear to go through both slits. • Dirac: "Electrons interfere only with themselves." • This assumes that nobody knows which slit individual electrons went through. 

16. Double Slit Experiment:One Slit Blocked • The usual double-slit experiment is done except now one slit is blocked.   What happens? • An interference pattern appears. • No interference pattern appears. • The image screen remains blank. • A janitor offers to unblock the closed slit.

17. Double Slit Experiment:One Slit Blocked 2.  No interference pattern appears. Comments: Photons go through only the open slit.  A single slit pattern appears behind this slit.  The former existence of the second slit does not affect the result.