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The fall of Classical Physics

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Classical physics: Fundamental Models

Particle Model (particles, bodies) Motion in 3 dimension; for each time t, position and speed

are known (they are well-defined numbers, regardless we know them). Mass is known.

Systems and rigid objects Extension of particle model

Wave Model (light, sound, …) Generalization of the particle model: energy is transported,

which can be spread (de-localized) Interference

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Classical physics at the end of XIX Century Scientists are convinced that the particle and wave model can describe the

evolution of the Universe, when folded with Newton’s laws (dynamics) Description of forces

Maxwell’s equations Law of gravity. …

We live in a 3-d world, and motion happens in an absolute time. Time and space (distances) intervals are absolute.

The Universe is homogeneous and isotropical; time is homogeneous. Relativity

The physics entities can be described either in the particle or in the wave model.

Natura non facit saltus (the variables involved in the description are continuous).

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Something is wrongRelativity, continuity, wave/particle (I)

Maxwell equations are not relativistically covariant!

Moreover, a series of experiments seems to indicate that the speed of light is constant (Michelson-Morley, …) A speed!

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Something is wrong Relativity, continuity, wave/particle (IIa)

In the beginning of the XX century, it was known that atoms were made of a heavy nucleus, with positive charge, and by light negative electrons Electrostatics like gravity:

planetary model All orbits allowed

But: electrons, being accelerated, should radiate and eventually fall into the nucleus

s10

41

41

32

10

2

2

0

23

2

0

mre

mFa

dtdEa

ceW

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Something is wrong Relativity, continuity, wave/particle (IIb)

If atoms emit energy in the form of photons due to level transitions, and if color is a measure of energy, they should emit at all wavelengths – but they don’t

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Something is wrong Relativity, continuity, wave/particle (III)

Radiation has a particle-like behaviour, sometimes

Particles display a wave-like behaviour, sometimes

=> In summary, something wrong involving the foundations: Relativity Continuity Wave/Particle duality

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Need for a new physics A reformulation of physics was needed

This is fascinating!!! Involved philosophy, logics, contacts with civilizations far away from us…

A charming story in the evolution of mankind

But… just a moment… I leaved up to now with classical physics, and nothing bad happened to me!

Because classical physics fails at very small scales, comparable with the atom’s dimensions, 10-10 m, or at speeds comparable with the speed of light, c ~ 3 108 m/s

Under usual conditions, classical physics makes a good job.

Warning: What follows is logically correct, although sometimes historically inappropriate.

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ILight behaves like a particle,

sometimes

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Photoelectric Effect Featuresand Photon Model explanation

The experimental results contradict all four classical predictions

Einstein interpretation: All electromagnetic radiation can be considered a stream of quanta, called photons

A photon of incident light gives all its energy hƒ to a single electron in the metal

2hhfE

h is called the Planck constant, and plays a fundamental role in Quantum Physics

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The Compton Effect

Compton dealt with Einstein’s idea of photon momentum

Einstein: a photon with energy E carries a momentum of E/c = hƒ / c

According to the classical theory, electromagnetic waves of frequency ƒo incident on electrons should scatter, keeping the same frequency – they scatter the electron as well…

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Compton’s experiment showed that, at any given angle, a different frequency of radiation is observed

The graphs show the scattered x-ray for various angles

Again, treating the photon as a particle of energy hf explains the phenomenon. The shifted peak, l‘> l0, is caused by the scattering of free electrons

This is called the Compton shift equation

' 1 cosoe

hm c

l l

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Compton Effect, Explanation The results could be explained, again, by treating the

photons as point-like particles having energy hƒ momentum hƒ / c

Assume the energy and momentum of the isolated system of the colliding photon-electron are conserved Adopted a particle model for a well-known wave

The unshifted wavelength, lo, is caused by x-rays scattered from the electrons that are tightly bound to the target atoms

The shifted peak, l', is caused by x-rays scattered from free electrons in the target

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Particle-like behavior of light:now smoking guns…

The reaction

has been recorded millions of times…

e e

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Summary The wave model cannot explain the behavior of light

in certain conditions Photoelectric effect Compton effect Blackbody radiation Gamma conversion/Bremsstrahlung

Light behaves like a particle, and has to be considered in some conditions as made by single particles (photons) each with energy

h ~ 6.6 10-34 Js is called the Planck’s constantE hf

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IIParticles behave like waves,

sometimes

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Should, symmetrically, particles display radiation-like properties?

The key is a diffraction experiment: do particles show interference?

A small cloud of Ne atoms was cooled down to T~0. It was then released and fell with zero initial velocity onto a plate pierced with two parallel slits of width 2 mm, separated by a distance of d=6 mm. The plate was located H=3.5 cm below the center of the laser trap. The atoms were detected when they reached a screen located D=85 cm below the plane of the two slits. This screen registered the impacts of the atoms: each dot represents a single impact. The distance between two maxima, y, is 1mm.

The diffraction pattern is consistent with the diffraction of waves with

ph

l

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Diffraction of electrons

Davisson & Germer 1925:Electrons display diffraction patterns !!!

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de Broglie’s wavelength What is the wavelength associated to a particle?

de Broglie’s wavelength:

Explains quantitatively the diffraction by Davisson and Germer……

Note the symmetry

What is the wavelength of an electron moving at 107 m/s ?

(smaller than an atomic length; note the dependence on m)

h p kp

l

3411

31 7

6.63 10 Js7.28 10 m

9.11 10 kg 10 m/shmv

l

kpωE

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Atomic spectra Why atoms emit according to a discrete energy spectrum?

2 2

1 1 1Per l'idrogeno interi

legata "numerologicamente" a h

H

H

R m nm n

Rl

Something must

be there...

Balmer

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Electrons in atoms: a semiclassical model Similar to waves on a cord, let’s imagine that

the only possible stable waves are stationary…

2 r = n l n=1,2,3,…

2h nhp p

nr pr Ll

=> Angular momentum is quantized (Bohr postulated it…)

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2

2

2 2

2

2

22

2

ke k e

p e ek p

Emv e eF k E kr r r r

eE k E Er

eE kr

v

rm

F

NB:• In SI, ke = (1/40) ~ 9 x 109 SI units• Total energy < 0 (bound state)• <Ek> = -<Ep/2> (true in general for bound states, virial theorem)

2 22 22

2 22 22 2

e

k en

e

L n mvrk em n

m e mr rE v kr

nr rk me

Only special values are possible for the radius !

Hydrogen (Z=1)

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Energy levels

The radius can only assume values

The smallest radius (Bohr’s radius) is

Radius and energy are related:

And thus energy is quantized:

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2ne

r nk me

2

2eeE kr

22

2 20

1 13.6 eV2 2

en e

n

k eeE kr a n n

2

1 02 .0529e

r nm ak me

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Transitions An electron, passing from an orbit of energy Ei

to an orbit with Ef < Ei, emits energy [a photon such that f = (Ei-Ef)/h]

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Level transitions and energy quanta

0

2

2 2

1 12

i fe

f i

E E ef kh a h n n

0

2

2 2 2 2

1 1 1 1 12e H

f i f i

f ek Rc a hc n n n nl

We obtain Balmer’s relation!

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Limitations Semiclassical models wave-particle duality can

explain phenomena, but the thing is still insatisfactory, When do particles behave as particles, when do they behave

as waves? Why is the atom stable, contrary to Maxwell’s equations?

We need to rewrite the fundamental models, rebuilding the foundations of physics…

kpωE