Quantum Theory. The most accurate and complete description of physical reality

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1 Quantum Theory The most accurate and complete description of physical reality Originating 1900 to 1930 and still under active development Set of ideas Scientist use to study microscopic world Nature is discontinuous or QUANTIZED Newtonian worldview Nature is continuous or smooth Revolution In 1900 Quantum idea originally by Max Planck slipped nearly unnoticed. In 1905, Einstein s relativity. Matter is not indestructible Made of energy, motion and fields Reality is observer dependent Quantum Theory Main principles established in Detailed description of matter and radiation Its impact is far beyond the theory of relativity Perhaps it is the most successful scientific theory Impact extends to every device Transistors, silicon chips, integrated circuits, information and communication, lasers, superconductors, nuclear power and weapons.

2 The center to entire high tech world is an elusive and highly non-newtonian particle: ELECTRON Quantum Theory is radically different from Newtonian Physics. Newtonian Physics Quantum Physics A mechanical world Implies that the randomness or chance is built into nature at the microscopic level Nature does not know what she will do next Universe is no longer a predictable machine Nature is deeply connected Contrary to Newtonian worldview Parts of nature (electron, proton and light waves) can not be separated from their surrounding without altering their character Changes in nature occur discontinuously

3 Example Nature requires an atom to vibrate at only certain specific energy levels. When atom loses energy It must do so in sudden jumps from one allowed energy level to another lower level It must release an instantaneous burst of energy Photo Electric Effect (1890) Quantum of energy Certain experimental observations made in late 19 th century were hard to explain with Newtonian physics. Michelson-Morley Experiment Explained by Special Theory of relativity When light and/or electromagnetic radiation shine on a metal surface, electrons are ejected from the surface: 1. Electrons are ejected immediately as light arrives at surface. 2. Emission occurs only at or above certain light s frequency (Threshold frequency)

4 Using standard electromagnetic wave theory of radiation BUT Photoelectric effect can be explained qualitatively When light arrives at metal in the form of continuous wave It exerts a vibrational force on all the charged particles (electrons and protons) in the metal This force shakes the particles harder and harder until the most loosely bound electrons are ejected. In this case electrons must be ejected with some delay. Why is the threshold frequency? Why there is no emission below threshold frequency no matter how high the intensity is? Max Planck in 1900 gave a new idea The energy of a vibrating object is quantized

5 In 1905 Einstein In addition to his special theory of relativity explained Photoelectric effect He used Plank s idea The vibrating charge s energy is quantized. It can change its energy only by sudden jumps from one allowed energy level to an other energy level, Energy must be emitted in burst of radiation (rather than continuous wave) Radiation always appear as tiny bundles called PHOTONS Particles with rest mass zero. They always move with speed of light. Photons are created by vibrating charged particles when they loose energy Photons simply rain down on metal surface and knock out the electrons. No delay

6 How about Threshold frequency? According to Planck s quantization Photon s energy E = hf Quantum energy h = 6.6 * J-s called Planck s constant Radiation is made of photons (Rather than wave). Photon s energy depends only on frequency of the radiation f. If f is smaller the energy of photon will be smaller than required to knockout an electron, regardless of the intensity of the radiation Einstein won the Nobel prize for this explanation Neither Planck nor Einstein had an idea Why a vibrating particle s energy must be quantized? Why Planck s constant h = 6.6 *10-34 J-s? Same as why c = 3*10 8 m/s? Einstein played an important role in development of Quantum Theory but he never accepted the theory

7 What is Radiation?? A particle or A wave Should we reject a wave theory and replace it with particle theory of radiation? Nature won t let us do so. Light interference Photoelectric effect Direct evidence of wave theory An evidence of particle theory of radiation In 1923 Louis de Broglie (de Broy) A student at that time Radiation should exhibit a wave-particle duality and so should matter. Every particle has a wave attached to it whose wavelength is = h/p In 1926 Max Born said the de Broglie s wave is a probability wave In 1961 Claus Jonsson demonstrated the electron wave Interference. In 1989 A. Tonomura found electrons making dark and bright interference pattern in double slit experiment.

8 Is the matter particles or waves? Electrons (particles) wave interference => Quantum mystery Matter is made of waves Also matter acts as if it were made of particles Like radiation, matter has a wave-particle dual nature Radiation is quantized into PHOTONS Matter is quantized into ELECTRONS, PROTONS, etc. This wave-particle duality introduces irreducible uncertainties in nature Quantum uncertainties In 1927, Heisenberg said microscopic world has an inherent uncertainties Uncertainty in position x Probability wave Psi wave x-axis Heisenberg Uncertainty Principle ( x).( p) h/(4π) Any reduction in one uncertainty expands the other

9 Perhaps the most characteristic feature of the quantum theory is its UNCERTAINTIES The identical physical situations leads to different outcomes Uncertainty does not mean lack of knowledge Uncertainty in position the particle actually has no definite position Is our future uncertain and unpredictable? Quantum Theory says so Our understanding is not deep enough. Einstein said in 1930 God does not play dice Quantum Theory says Some don t agree Although the individual events are not predictable, the overall statistics is predictable God really does play dice Copenhagen Interpretation conclusion

10 Post Newtonian Worldview Newtonian Post Newtonian Electromagnetic radiation are real Atomism The fundamental reality but not made of atoms. Solid, massy, hard, unbreakable Microscopic world (including Particles that never wear or break in pieces atoms) are made of Psi fields. Matter is a rest mass of these fields. Atom is empty and is breakable Objectivity A crucial idea Was believed to be achievable Natural phenomena could always be Every thing is caused by atoms and atomic discovered from its surroundings behavior is determined by physical laws. Reality depends on experimental surroundings Reality does not depend on surroundings Predictability Every physical system is entirely predictable Analysis Complicated phenomena can always be separated into components Leads to focus on simplest component - Atom Individual events are not predictable A microscopic system is not necessarily separable An electron changes its nature when its environment changes

11 Even after a century Newtonian metaphor continues to deeply influence our culture Mechanical world Quantum Theory Nature is closely tied to its surrounding An Organic Universe