Quantum Physics

Thirteenth lecture for a course on science.

Transcript

1. Quantum Physics playing dice with God George Matthews CC 2016
2. scientific determinism
3. scientific determinism “We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.” – Pierre-Simon Laplace, 1814
4. the classical universe
5. the classical universe ! causal determinism: the initial conditions of a system determine the outcome of that system according to necessary laws
6. the classical universe ! causal determinism: the initial conditions of a system determine the outcome of that system according to necessary laws ! locality: speed of light is the speed limit of everything, causal effects take time to travel
7. the classical universe ! causal determinism: the initial conditions of a system determine the outcome of that system according to necessary laws ! locality: speed of light is the speed limit of everything, causal effects take time to travel ! continuity: motion is continuous; energy can come in arbitrarily small amounts
9. “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” – Lord Kelvin, 1900
10. dark clouds
11. dark clouds ! black body radiation: hot objects glow, all objects emit radiation, but why don’t objects emit far more energy at higher frequencies than they do?
12. dark clouds ! black body radiation: hot objects glow, all objects emit radiation, but why don’t objects emit far more energy at higher frequencies than they do? ! the photo-electric effect: Why do some colors but not others cause electrons to be emitted regardless of intensity?
13. dark clouds ! black body radiation: hot objects glow, all objects emit radiation, but why don’t objects emit far more energy at higher frequencies than they do? ! the photo-electric effect: Why do some colors but not others cause electrons to be emitted regardless of intensity? ! spectra of heated gases: why does the light emitted by heated gases exhibit clear lines in only some colors, not a continuous range?
14. ultraviolet catastrophe
15. ultraviolet catastrophe
16. Planck’s suggestion
17. Planck’s suggestion ! What if energy could only come in multiples of some tiny amount – a quantum?
18. Planck’s suggestion ! What if energy could only come in multiples of some tiny amount – a quantum? ! This would prevent infinite amounts of energy being emitted by black bodies.
19. Planck’s suggestion ! What if energy could only come in multiples of some tiny amount – a quantum? ! This would prevent infinite amounts of energy being emitted by black bodies. ! Is this just “curve fitting” or an insight into the deep nature of things?
20. Planck’s suggestion ! What if energy could only come in multiples of some tiny amount – a quantum? ! This would prevent infinite amounts of energy being emitted by black bodies. ! Is this just “curve fitting” or an insight into the deep nature of things? ! Planck himself was not sure.
21. the photo-electric effect
22. the photo-electric effect
23. Einstein’s insight
24. Einstein’s insight ! If Plank’s quanta of energy are real and light comes in individual packets (photons) the photo-electric effect can be explained.
25. Einstein’s insight ! If Plank’s quanta of energy are real and light comes in individual packets (photons) the photo-electric effect can be explained. ! Long wavelength photons do not have the energy to dislodge electrons no matter how many of them there are, but short wavelength photons do.
26. Einstein’s insight ! If Plank’s quanta of energy are real and light comes in individual packets (photons) the photo-electric effect can be explained. ! Long wavelength photons do not have the energy to dislodge electrons no matter how many of them there are, but short wavelength photons do. ! Einstein did not know why only certain photons could dislodge electrons in metals.
27. atomic spectra
28. atomic spectra
29. Bohr’s model
30. Bohr’s model ! Electrons are “standing waves” so only certain frequencies will fit in particular orbits around the nucleus of an atom.
31. Bohr’s model ! Electrons are “standing waves” so only certain frequencies will fit in particular orbits around the nucleus of an atom. ! Photons of only particular frequencies add just the right amount of energy to dislodge or excite electrons.
32. Bohr’s model ! Electrons are “standing waves” so only certain frequencies will fit in particular orbits around the nucleus of an atom. ! Photons of only particular frequencies add just the right amount of energy to dislodge or excite electrons. ! This is why atomic spectra consist of narrow lines and how the photo-electric effect works.
33. Bohr’s model ! Electrons are “standing waves” so only certain frequencies will fit in particular orbits around the nucleus of an atom. ! Photons of only particular frequencies add just the right amount of energy to dislodge or excite electrons. ! This is why atomic spectra consist of narrow lines and how the photo-electric effect works. ! Bohr’s atomic model is the cornerstone of quantum physics.
34. A deep puzzle
35. A deep puzzle ! Young showed that light is a wave.
36. A deep puzzle ! Young showed that light is a wave. ! Thompson showed that electrons are particles.
37. A deep puzzle ! Young showed that light is a wave. ! Thompson showed that electrons are particles. ! So how can light behave like a particle?
38. A deep puzzle ! Young showed that light is a wave. ! Thompson showed that electrons are particles. ! So how can light behave like a particle? ! And how do electrons act like waves?
39. A deep puzzle ! Young showed that light is a wave. ! Thompson showed that electrons are particles. ! So how can light behave like a particle? ! And how do electrons act like waves? ! Do we even understand what matter and energy are?
40. the double slit experiment
41. the double slit experiment
42. the double slit experiment
43. the double slit experiment
44. the uncertainty principle
45. the uncertainty principle The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa. – Werner Heisenberg, 1927
46. Quantum mechanics
47. Quantum mechanics ! Quantization: matter and energy can only come in multiples of particular amounts.
48. Quantum mechanics ! Quantization: matter and energy can only come in multiples of particular amounts. ! Wave/particle duality: energy and matter have properties of both waves and particles.
49. Quantum mechanics ! Quantization: matter and energy can only come in multiples of particular amounts. ! Wave/particle duality: energy and matter have properties of both waves and particles. ! Uncertainty principle: with certain pairs of properties, increasing precision of one measurement decreases precision of the other.
50. Competing interpretations
51. Competing interpretations ! Copenhagen: deep down the universe is probabilistic, measurement makes it definite.
52. Competing interpretations ! Copenhagen: deep down the universe is probabilistic, measurement makes it definite. ! Many worlds: all possible quantum states exist in different real worlds.
53. Competing interpretations ! Copenhagen: deep down the universe is probabilistic, measurement makes it definite. ! Many worlds: all possible quantum states exist in different real worlds. ! Hidden variables: there is something else going on here that causes the appearance of quantum weirdness.