Apr 4, 2007 PHYS 117B.02 1 PHYS 117B.02 Lecture Apr 4 The last few lectures we’ve been switching gears from classical to quantum physics This way: The quantum leap

Apr 4, 2007 PHYS 117B.02 2 “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words ? I believe it is the atomic hypothesis ( or the atomic fact) that all things are made of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.” Richard Feynman

Apr 4, 2007PHYS 117B.023 To understand atoms we need to understand the quantum behavior. The theme of today’s lecture is particles and waves.

Apr 4, 2007 PHYS 117B.02 4 We already learned some things about particles and waves: Photoelectric effect – light behaves as a particle … but previously we have shown that it also behaves as a wave. Atomic spectra : Bohr’s model explained many features of the hydrogen atom spectra (and hydrogen-like atoms) by assuming that angular momentum is quantized and that photons are emitted and absorbed when electrons jump from one orbit to another Today we’ll talk about particle waves and quantum behavior in general One “lucky break” – electrons behave just like light! I’ll mostly follow R. Feynman, but our textbook also has nice discussion on the topic in ch 38.9 and ch

Apr 4, 2007 PHYS 117B.02 5 The Bohr atom: another perspective Angular momentum is quantized: L = hn/2 , n=1,2,3 …. You can also think of the quantization of L as requiring the electrons to form standing waves along the classical orbit: 2  r = n  ut, you need to assume that the electron is a wave ! Now if = h/p 2  r = n h/( mv) => L = mvr = n h/2 

Apr 4, 2007 PHYS 117B.02 6 Particles and waves: de Broglie’s hypothesis ( 1924) Nature is beautifully symmetric. If light behaves both as a particle and a wave, then matter ( electrons, protons, etc) should also have wave properties.  = h/p, where h is Planck’s constant and p is the momentum of the particle

Apr 4, 2007 PHYS 117B.02 7 What is the wavelength of the electron ?

Apr 4, 2007 PHYS 117B.02 8 Double slit experiment revisited: Shooting solid objects (particles) through double slit

Apr 4, 2007 PHYS 117B.02 9 Let’s analyze the distribution of bullets at the backstop The assumptions:  Machine gun shoots at constant rate  Only whole bullets arrive at the detector  Move the detector and count how many bullets are collected at each position for some fixed amount of time Define probability:  P = N (x)/N total

Apr 4, 2007 PHYS 117B The results of the bullets experiment Close slit 2 Measure P 1 Close slit 1 Measure P 2 Both slits open: Measure P = P 1 + P 2

Apr 4, 2007 PHYS 117B Do an experiment with water waves Now we measure intensity of the wave: I ~ E 2 I 12 ≠ I 1 + I 2 Interference: I 12 = I 1 + I √(I 1 I 2 ) *cos φ

Apr 4, 2007 PHYS 117B But what will happen if our bullets are microscopic particles ? Two slit experiment using electron gun: If you take PHYS225 you can do this experiment yourself !

Apr 4, 2007 PHYS 117B Electrons produce an interference pattern! Even if we shoot electrons one-by-one - we get an interference pattern ! Each electron interferes with itself ! How can this be ? How do the electrons pass through the slits ? bullets water electrons

Apr 4, 2007 PHYS 117B Watching the electrons Let’s try to observe through which slit the electron will go Shine light near the slits: photons scattered from the electron will come to our eyes – bingo ! We know which way the electron went! Hmm… it looks like we are disturbing the electrons with the light! Of course – we know that light has E and B field, carries energy, exerts pressure !

Apr 4, 2007 PHYS 117B Let’s be more sneaky Reduce the intensity of the light source or Change the wavelength

Apr 4, 2007 PHYS 117B Let’s first reduce the intensity: OK here we go:  Three groups of electrons A group that we see going through slit 1 A group that we see going through slit 2 A group that we don’t see, but we detect with our detector And the result is:  Group 1 and 2 perfectly behaved (no interference)  Group 3 produces an interference pattern!

Apr 4, 2007 PHYS 117B Let’s now try to increase the wavelength Longer wavelength means smaller momentum: = h/p, so we will disturb the electrons less Gradually change the photon wavelength ( make the light “redder” In the beginning – all looks the same. We can tell the position of the electrons at the slits and there is no interference Remember:  in order to resolve the position of the electrons, the wavelength of the photon has to be of the order of the distance between the slits  Once we get to longer wavelengths: we’ll get a fuzzy spot  The interference pattern will reappear, but we will no longer be able to tell through which slit the electron went Heisenberg’s uncertainty principle:  It is impossible to design an experiment in which we can measure which one of two possible paths was taken without destroying the interference pattern!  ∆p x ∆x ≥ ћ  ∆E ∆t ≥ ћ

Apr 4, 2007 PHYS 117B Probability in classical and quantum physics Assume we have an ideal experiment:  We know the initial conditions (electron leaves the gun headed to the slits)  There are no external influences  We can NOT predict the final state of the system exactly (don’t know where it will land)  We can only predict the odds ! This is different from classical physics We have given up the idea that the world is deterministic