Richard Feynman: Electron waves are probability waves in the ocean of uncertainty.

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1 Richard Feynman: Electron waves are probability waves in the ocean of uncertainty.

2 Last Time We Solved some of the Problems with Classical Physics Discrete Spectra? Bohr Model but not complete. Blackbody Radiation? Plank Quantized Energy of Atoms: En = nhf Photoelectric Effect? Einstein Quantized energy states of light photons: E = hf Compton Effect? Light is a particle with momentum! Stability of Atoms? debroglie waves! λ = e h p

3 1924: De Broglie Waves If photons can be particles, then why can t electrons be waves? p E hf h = = = c c λ Electrons are STANDING WAVES in atomic orbitals. λ = e h p = 34 h 6.626x10 J s = λ 11 e x m

4 Electron Diffraction

5 If electron were hard bullets, there would be no interference pattern.

6 In reality, electrons do show an interference pattern, like light waves.

7 Electrons act like waves going through the slits but arrive at the detector like a particle.

8 Particle Wave Duality Interference pattern builds one electron at a time. Electrons act like waves going through the slits but arrive at the detector like a particle.

9 Results of a double-slitexperiment performed by Dr. Tonomura showing the build-up of an interference pattern of single electrons. Numbers of electrons are 11 (a), 200 (b), 6000 (c), (d), (e).

10 Double Slit is VERY IMPORTANT because it is evidence of waves. Only waves interfere like this.

(Interference) Case 2: Identical, opposite direction

11 Review on Waves: Superposition of Sinusoidal Waves Case 1: Identical, same direction, with phase difference (Interference) Case 2: Identical, opposite direction (standing waves) Case 3: Slightly different frequencies (Beats)

12 Superposition of Traveling Waves Assume two waves are traveling in the same direction, with the same frequency, wavelength and amplitude The waves differ in phase y 1 = A sin (kx - ωt) y 2 = A sin (kx - ωt + φ) y = y 1 +y 2 = 2A cos (φ/2) sin (kx - ωt + φ/2) Resultant Amplitude Depends on phase: Spatial Interference Term

13 1-D Wave Interference y = y 1 +y 2 = 2A cos (φ/2) sin (kx - ωt + φ/2) φ Resultant Amplitude: 2Acos 2 Constructive Interference (Even π): φ = 2 nπ, n= 0,1, 2,3... ( ) Destructive Interference Odd π : φ = (2n+ 1) π, n= 0,1, 2,3... Constructive Destructive Interference φ = 0 φ = π φ

14 Standing Waves: Bound Systems Superposition of two identical waves moving in opposite directions. y = A sin ( kx - ωt) y = A sin ( kx + ωt) 1 2 y = (2Asin kx)cos ωt There is no kx wt term, and therefore it is not a traveling wave! Every element in the medium oscillates in simple harmonic motion with the same frequency, w: coswt The amplitude of the simple harmonic motion depends on the location of the element within the medium: (2Asinkx)

15 Standing Waves on a String Harmonics

16 Strings & Atoms are Quantized The possible frequency and energy states of an electron in an atomic orbit or of a wave on a string are quantized. f = v n 2 l En = = nhf, n= 0,1,2,3, h x Js

Intensity of Sound Waves The intensity of a wave, the power per unit area, is the rate at which energy is being transported by the wave through a unit area A

17 Intensity of Sound Waves The intensity of a wave, the power per unit area, is the rate at which energy is being transported by the wave through a unit area A perpendicular to the direction of travel of the wave and is proportional to the amplitude of the sound wave squared! I = W 2 2 4π r m E = = t 1 ρav( ωsmax ) I = ρv( ωsmax ) 2 2

18 Power Transmitted by Waves on Strings Power Transmitted on a String is proportional to the square of the wave amplitude. : E = = t µω Av

19 Light Waves: Intensity Intensity is proportional to the square of the amplitude! E = Emax cos (kx ωt) B = Bmax cos (kx ωt) E B max max = c I E B E cb = Sav = = = 2μ 2μc 2μ 2 2 max max max max o o o I E 2 max

20 Double Slit Intensity Distribution Resultant Electric Field The magnitude of the resultant electric field comes from the superposition principle E P = E 1 + E 2 = E o [sin ωt + sin (ωt + φ)] This can also be expressed as φ φ EP = 2Eocos sin ωt E P has the same frequency as the light at the slits The amplitude at P is given by 2E o cos (φ / 2) Intensity is proportional to the square of the amplitude: 2 I = Imax cos ( φ / 2) I A 2

21 Light Intensity for the Double Slit Bright fringe: r = dsin θ = mλ, m= 0,1,2,3,... 2π Phase Difference: φ = r λ I = I cos ( φ / 2) max 2 2 sin 2 I Imax cos πd θ Imax cos πd = y λ λl Intensity is proportional to the square of the amplitude!

22 In Sum Intensity is proportional to the square of the amplitude! Next Probability is proportional to Intensity therefore: Probability is proportional to Amplitude Squared!!! I A 2

23 Spontaneous Emission Transition Probabilities Fermi s Golden Rule Transition probabilities correspond to the intensity of light emission.

24 Double Slit: Light The intensity at a point on the screen is proportional to the square of the resultant electric field magnitude at that point. I E 2 Wave Function : E Amplitude Intensity: Amplitude squared Intensity is the REALITY for light waves. What about for light particles? PROBABILITY ~ Intensity

25 Connecting the Wave and Photon Views The intensity of the light wave is correlated with the probability of detecting photons. That is, photons are more likely to be detected at those points where the wave intensity is high and less likely to be detected at those points where the wave intensity is low. The probability of detecting a photon at a particular point is directly proportional to the square of the light-wave amplitude function at that point:

26 Probability Concepts Prob(AorB) Prob(AandB) N = N P A tot A = P + = Ptot = PA + PB + PC = A PP A P B B 100

27 Suppose you roll a die 30 times. What is the expected numbers of 1 s and 6 s? A. 12 B. 10 C. 8 D. 6 E. 4 Prob(AorB) = P + P A B N = N P A tot A 1 1 N = 30( + ) =

28 Suppose you roll a die 30 times. What is the expected numbers of 1 s and 6 s? A. 12 B. 10 C. 8 D. 6 E. 4 Do Workbook 40.2 #1

29 Connecting the Wave and Photon Views The intensity of the light wave is correlated with the probability of detecting photons. That is, photons are more likely to be detected at those points where the wave intensity is high and less likely to be detected at those points where the wave intensity is low. The probability of detecting a photon at a particular point is directly proportional to the square of the light-wave amplitude function at that point:

30 The figure shows the detection of photons in an optical experiment. Rank in order, from largest to smallest, the square of the amplitude function of the electromagnetic wave at positions A, B, C, and D. (The probability of detecting a photon!) A. D > C > B > A B. A > B > C > D C. A > B = D > C D. C > B = D > A

31 The figure shows the detection of photons in an optical experiment. Rank in order, from largest to smallest, the square of the amplitude function of the electromagnetic wave at positions A, B, C, and D. (The probability of detecting a photon!) A. D > C > B > A B. A > B > C > D C. A > B = D > C D. C > B = D > A

Probability Density We can define the probability density P(x) such that the photon probability density is directly proportional to the square of the light-wave amplitude: In one dimension,

32 Probability Density We can define the probability density P(x) such that the photon probability density is directly proportional to the square of the light-wave amplitude: In one dimension, probability density has SI units of m 1. Thus the probability density multiplied by a length yields a dimensionless probability. NOTE: P(x) itself is not a probability. You must multiply the probability density by a length to find an actual probability.

33 Do Workbook 40.2 #2

34 In Sum: Light Wave model: Interference pattern is in terms of wave intensity Photon model: Interference in terms of probability The probability of detecting a photon within a narrow region of width δx at position x is directly proportional to the square of the light wave amplitude function at that point. Prob(in δx at x) A(x) δx 2 Probability Density Function: Px ( ) A(x) 2 The probability density function is independent of the width, δx, and depends only on x. SI units are m -1.

35 Double Slit for Electrons shows Wave Interference

36 Interference pattern builds one electron at a time. Electrons act like waves going through the slits but arrive at the detector like a particle.

37 Double Slit: Electrons A light analogy.. There is no electron wave so we assume an analogy to the electric wave and call it the wave function, psi, : Ψ( x) The intensity at a point on the screen is proportional to the square of the wave function at that point. Px ( ) = Ψ( x) 2 The Probability Density Function is the Reality!

38 WARNING! There is no thing waving! The wave function psi is a wave-like function that oscillates between positive and negative values that can be used to make probabilistic predictions about atomic particles.

39 Probability: Electrons The probability of detecting an electron within a narrow region of width δx at position x is directly proportional to the square of the wave function at that point: Prob(in δx at x) = Ψ(x) δx 2 Probability Density Function: Px ( ) = Ψ(x) 2 The probability density function is independent of the width, δx, and depends only on x. SI units are m -1. Note: The above is an equality, not a proportionality as with photons. This is because we are defining psi this way. Also note, P(x) is unique but psi in not since psi is also a solution.

40 Where is the electron most likely to be found? Least likely? Prob(in δx at x) = Ψ(x) δx 2

41 At what value of x is the electron probability density a maximum? Prob(in δx at x) = Ψ(x) δx 2

42 This is the wave function of a neutron. At what value of x is the neutron most likely to be found? A. x = 0 B. x = x A C. x = x B D. x = x C Prob(in δx at x) = Ψ(x) δx 2

43 This is the wave function of a neutron. At what value of x is the neutron most likely to be found? A. x = 0 B. x = x A C. x = x B D. x = x C Do Workbook 39.3 Prob(in δx at x) = Ψ(x) δx 2

44 Normalization A photon or electron has to land somewhere on the detector after passing through an experimental apparatus. Consequently, the probability that it will be detected at some position is 100%. The statement that the photon or electron has to land somewhere on the x-axis is expressed mathematically as Any wave function must satisfy this normalization condition.

45 The Probability that an electron lands somewhere between x L and x R is the sum of all the probabilities that an electron lands in a narrow strip i at position xi: N N i i= 1 i= 1 Px ( ) δx= ψ( x) δx i 2

46 The Probability that an electron lands somewhere between x L and x R is: x R Prob( x x x ) = P( x) dx = ψ ( x) dx L R x L x x R L 2 Normalization P( x) dx = ψ ( x) dx = 1 Total Area must equal 1. 2

47 The value of the constant a is A. a = 0.5 mm 1/2. B. a = 1.0 mm 1/2. C. a = 2.0 mm 1/2. D. a = 1.0 mm 1. E. a = 2.0 mm 1.

48 The value of the constant a is A. a = 0.5 mm 1/2. B. a = 1.0 mm 1/2. C. a = 2.0 mm 1/2. D. a = 1.0 mm 1. E. a = 2.0 mm 1. Do Workbook 40.4

49 De Broglie Wavelength λ = h mv 34 h= 6.626x10 J s λ 11 e = 2.4x10 m

50 Electron Waves leads to Quantum Theory Waves: De Broglie: 2 L λ =, n = 1,2,3... n n h λ = p 1 p E = mv 2 = 2 2m 2 Combine: E n 2 2 hn = Energy is Quantized! 8mL 2

51 Quantum Theory Assume electrons are waves. A wave function is defined that contains all the information about the electron: position, momentum & energy. The wave density function is the square of the wave function. The probability of finding an electron in a particular state is given by the square of the wave function. All we can know are probabilities. Probability = Ψ ( xt, ) = (possibility) 2 2

Wave Packet: Making Particles out of Waves

52 Wave Packet: Making Particles out of Waves h p = c= λ f λ p = hf / c Superposition of waves to make a defined wave packet. The more waves used of different frequencies, the more localized. However, the more frequencies used, the less the momentum is known.

The more waves you use, the more defined your packet and the more defined the

53 Heisenberg Uncertainty Principle You make a wave packet by wave superposition and interference. The more waves you use, the more defined your packet and the more defined the position of the particle. However, the more waves you use of different frequencies (energy or momentum) to specify the position, the less you specify the momentum!

That energy is transferred to the electron in an unpredictable way and the motion (momentum) becomes uncertain.

54 Heisenberg Microscope Small wavelength (gamma) of light must be used to find the electron because it is too small. But small wavelength means high energy. That energy is transferred to the electron in an unpredictable way and the motion (momentum) becomes uncertain. If you use long wavelength light (infrared), the motion is not as disturbed but the position is uncertain because the wavelength is too long to see the electron. This results in the Uncertainty Principle. x ~ λ p = h/ λ x p> h

55 The possible wavelengths for an electron in a box of length L. Electron in a Box x~ L p~ p~ h/ λ = h/ L If you squeeze the walls to decrease x, you increase p! x p ~ L h/ L> h

56 Electron in a Box A wave packet in a square well (an electron in a box) changing with time.

57 Improved technology will not save us from Quantum Uncertainty! Quantum Uncertainty comes from the particle-wave nature of matter and the mathematics (wave functions) used to describe them!

58 Heisenberg Uncertainty Trying to see what slit an electron goes through destroys the interference pattern. Electrons act like waves going through the slits but arrive at the detector like a particle.

59 Which Hole Did the Electron Go Through? If you make a very dim beam of electrons you can essentially send one electron at a time. If you try to set up a way to detect which hole it goes through you destroy the wave interference pattern. Conclusions: Trying to detect the electron, destroys the interference pattern. The electron and apparatus are in a quantum superposition of states. There is no objective reality.

60 Feynman s version of the Uncertainty Principle It is impossible to design an apparatus to determine which hole the electron passes through, that will not at the same time disturb the electrons enough to destroy the interference pattern.

61 E t > h/4π x p> h/4π

62 The Heisenberg Uncertainty Principle The quantity Δx is the length or spatial extent of a wave packet. Δp x is a small range of momenta corresponding to the small range of frequencies within the wave packet. Any matter wave must obey the condition This statement about the relationship between the position and momentum of a particle was proposed by Heisenberg in Physicists often just call it the uncertainty principle.

63 The Heisenberg Uncertainty Principle If we want to know where a particle is located, we measure its position x with uncertainty Δx. If we want to know how fast the particle is going, we need to measure its velocity v x or, equivalently, its momentum p x. This measurement also has some uncertainty Δp x. You cannot measure both x and p x simultaneously with arbitrarily good precision. Any measurements you make are limited by the condition that ΔxΔp x h/2. Our knowledge about a particle is inherently uncertain.

64 EXAMPLE 40.6 The uncertainty of an electron

65 Heisenberg Uncertainty Energy and Time Particles can be created with energy E can that live for a time t. They are called virtual particles but they can become real. E t > h/4π

66 Quantum Foam Virtual Particles are constantly popping in and out of the quantum vacuum, making a Quantum Foam. In QED, virtual particles are responsible for communicating forces.

67 Making Matter out of Nothing Matter-Antimatter Pair Production Antimatter: Same mass, different charge. When matter and antimatter collide they annihilate into pure energy: light.

68 Quantum Fluctuations

69 Casimir Effect: The Zero Point Energy of the Quantum Vacuum

70 Quantum Cosmology BIG BANG! The Universe Tunneled in from Nothing

Virtual Particles pop in and out of the Quantum Vacuum in matter-antimatter

71 In Sum: The Quantum Vacuum Due to Quantum Uncertainty, the nothingness of empty space is actually full of energy! Virtual Particles pop in and out of the Quantum Vacuum in matter-antimatter pairs. If they have enough energy they can become real! The Universe came from nothing and is being accelerated by nothing!

72 In Sum..

73 Where do the Wave Functions come from??? Solutions to the time-independent Schrödinger equation: 2 2 d ψ + 2 Uψ = Eψ 2m dx 2 d ψ dx = OR 2m U ( E) 2 2 ψ

Electron in a Box. A wave packet in a square well (an electron in a box) changing with time.

Electron in a Box. A wave packet in a square well (an electron in a box) changing with time. Electron in a Box A wave packet in a square well (an electron in a box) changing with time. Last Time: Light Wave model: Interference pattern is in terms of wave intensity Photon model: Interference in