Lecture 2: Experimental Facts of Life

MIT OpenCourseWare・2 minutes read

Physics explores the existence of atoms and nuclei through various experiments, debunking classical models and revealing the composite nature of particles like protons made up of quarks. Interference effects and wave-particle duality are demonstrated through experiments involving electrons and photons, challenging traditional conceptions of particle behavior.

Insights

Atoms are not structured as classical models depict with electrons orbiting nuclei due to energy loss from radiation, challenging traditional views of atomic structure.
Quarks, discovered as constituents of protons with fractional charges, have intricate interactions during collisions, forming complex shrapnel patterns and challenging existing particle behavior models.
Light exhibits wave-like behavior with interference effects, illustrated by experiments with electrons and photons, showcasing the duality of particles behaving as both waves and localized entities.

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Recent questions

What are quarks?
Quarks are constituents of protons with fractional charges, discovered through experiments by Kendall, Friedman, and Taylor.

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Summary

00:00

Unraveling Particle Physics: From Atoms to Quarks

Physics aims to understand and model basic experimental facts, developing models to predict outcomes.
Atoms are proven to exist through various experiments, including electron observation in a cathode ray tube and individual electron detection in high-energy physics experiments.
Nuclei's existence is confirmed through experiments like shooting alpha particles at atoms, leading to the discovery of their high-density cores.
The classical model of atoms as orbiting combinations of electrons and nuclei is debunked due to energy loss from radiation, making it unsustainable.
Geiger's invention of the Geiger counter, detecting charged particles like alpha particles, showcases the randomness of atomic decay and radiation.
Hard scattering experiments, like Rutherford's with alpha particles and later ones with electrons on protons, reveal the composite nature of particles like protons, made up of quarks.
Quarks, discovered to be the constituents of protons, have fractional charges and were found through experiments by Kendall, Friedman, and Taylor, leading to a Nobel Prize.
Ongoing experiments like the relativistic heavy ion collider at Brookhaven continue to explore particle interactions at high energies, revealing complex shrapnel patterns from proton collisions.
Protons are not made of point-like quarks, but interactions between their constituents during collisions create intricate particle patterns, challenging traditional models of particle behavior.

12:42

"Quantum liquids and atomic spectra experiments"

Protons overlap forming a liquid at ultra high temperature and density, known as the RHIC fireball or quark-gluon plasma, behaving like water with wave-like properties.
Time for coffee to reach thermal equilibrium after adding hot coffee is significant, taking seconds to minutes, much slower than light crossing a mug.
Liquid formed from high-energy proton collisions takes time to reach thermal equilibrium, comparable to light crossing the liquid.
The liquid formed is a quantum liquid, well-modeled by black holes, revealing insights into atoms and randomness.
Experiment involving atomic spectra includes a power plant, wires across a spark gap, and gas like H2 or neon, producing distinct patterns of light through a prism.
Specific lines in atomic spectra, like Lyman and Balmer series, remain consistent regardless of changes in the power source, prism, or hydrogen excitation method.
Balmer's formula for hydrogen spectral lines, based on numerology, led to a phenomenological fit, later improved by Rydberg and Ritz to encompass all spectral lines.
Experiment measuring kinetic energy of electrons through the photoelectric effect involves shining light on a metal piece, inducing electrons to jump across a potential difference.
By varying the voltage, a critical minimum voltage can be determined where the current goes to zero, indicating the kinetic energy of the ejected electrons.
The experiment's tunable factors include light intensity and frequency, with frequency not affecting the total energy, similar to a harmonic oscillator where energy lies in the amplitude.

25:43

"Light intensity affects electron kinetic energy"

Increasing intensity of light leads to higher kinetic energy in particles.
Higher intensity beams result in more energetic electrons being emitted.
The potential, denoted as V0, is the stopping voltage.
A more intense beam requires a larger voltage to impede electric flow.
V0 should be independent of frequency.
Experimental results show that kinetic energy of emitted electrons is independent of beam intensity.
V0 varies linearly with frequency of light.
Einstein proposed that light comes in packets with energy linearly proportional to frequency.
Kinetic energy of emitted electrons is determined by the energy of the photon minus the work function.
Planck's constant, denoted as h, is named after Max Planck and is crucial in understanding quantum mechanics.

40:18

Wave-particle duality in Young's experiment

Waves are sourced and allowed to become plane waves before encountering a barrier with two slits.
Plane waves act as sources at the slits, producing crests and troughs that create interference fringes on a distant screen.
Interference fringes result from the constructive and destructive addition of wave amplitudes.
Young's diagram illustrates the double-slit experiment, showcasing the non-localized nature of waves.
Light, as demonstrated by the experiment, behaves like a wave due to its non-localized nature.
Light's behavior as a wave is characterized by interference effects, where amplitudes add up.
Light is described as both smooth like a wave and chunky, as seen in the photoelectric effect.
Electrons, being localized, do not exhibit interference effects like waves but can still interfere with themselves.
Hitachi's experiment with electrons through a grading demonstrated interference effects with individual electrons.
Electrons do not behave strictly as waves or particles but exhibit characteristics of both, challenging traditional conceptions.

54:37

Quantum Interference Effects in Electron Behavior

Electrons exhibit interference effects when passing through a barrier with two slits, creating interference fringes.
When a single electron is shot through the experiment, it is in a superposition state, potentially going through both slits simultaneously.
Blocking one slit forces the electron to go through the other, resulting in a distribution without interference.
Attempting to detect which path the electron took using low-energy light fails due to the chunky nature of light.
The energy imparted by light is proportional to its frequency, affecting the momentum it imparts.
Gravitational wave detectors could potentially detect which path an electron took without disrupting interference effects.
Gravitational waves must come in chunks to align with quantum mechanics principles.
An experiment with quantum mechanically isolated single photons mirrors the interference effects seen with electrons.
The Davisson and Germer experiment with electrons diffracting off a crystal showed interference effects, indicating wave-like behavior.
The momentum of electrons diffracting off the crystal corresponds to a wavelength, consistent with the wave-particle duality of quantum mechanics.

01:07:53

"Buckyballs and Bell's Inequality Experiment"

Experiment with photons and electrons, but can be imagined with soccer balls, done with Buckyballs.
Buckyballs are similar in shape to soccer balls but much larger.
Experiment conducted in Zellinger's lab with Buckyballs, using thermal energy, single slit, diffraction grating, and photo ionization.
Buckyballs go through diffraction grating one by one, showing interference fringes.
Experiment can be replicated with proper isolation of the system.
Bell's Inequality experiment involves undergraduates, blondes, and Massachusetts residents.
Bell's Inequality proven using logic and integers, leading to a tautological statement.
Electrons have a third property called whimsy, measured by angular momentum along different axes.

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