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费曼物理讲义

Catalogue
  1. 1. 原子与运动
  2. 2. 基础物理介绍
  3. 3. Energy
  4. 4. Time and Distance
    1. 4.1. Time
    2. 4.2. Distance
  5. 5. Probability
  6. 6. Gravitation

原子与运动

  1. 原子论：物质是由原子构成的——历史上最重要的假设。
  2. 空气/热/压力：密度大小<->原子撞击/原子速度
  3. 水蒸气的挥发
  4. 晶体溶解

基础物理介绍

1
At first the phenomena of nature were roughly divided into classes ... find the laws behind experiment; to amalgamate these classes
  • < 1920s: 3-dimensional + Euclid + Time.
  1. Inertia. force -> atomic + gravitation. motion.
  2. Electricity -> nucleus + electrons, nucleus -> protons + neutrons
  3. force -> Electricity field
  4. electromagnetic field/wave

  • 1920s: quantum physics

  1. space-time
  2. quantum mechanics
  3. wave <-> particle
  4. quantum electrodynamics -> electromagnetic field with photon (except for nucleus) -> properties of gamma rays, positron / antiparticle
  5. require two numbers -> mass / charge
    1
    it is fundamentally impossible to make a precise prediction of exactly what will happen. We can find only an average, statistically, as to what happens
  • Necleis
  1. Discovered new particles $\mu$-meson. $\pi$-meson (muon, pion): cannot explain forces between protons and neutrons.
  2. 30 particles from cosmic rays.
  3. baryon-meson-lepton / the photon and the graviton. baryon – neucleus, meson – neuclear interaction
    1
    in the order of decreasing strength, are the nuclear force, electrical interactions, the beta-decay interaction, and gravity.

Energy

  • Energy: sum of several forms
  • gravitation/kinetic/heat/elastic/electrical/chemical/radiant/nuclear/mass

Time and Distance

Time

  • Galileo, measure time with pulse, distance proportional to square of time
  • small pieces of time, pendulum (Galileo) -> electornic occilators -> caibrate with electron-beam oscilloscope -> modern $10^{−12}$ sec. (light amplifier) -> $10^{-16}$ sec. ($\pi$-meson lifetime)
    1
    time, periordic measurement
  • open question to infinitesimal time
  • long time: use of radioactive material
    1
    > the radioactivity of a particular sample of material decreases by the same fraction for successive equal increases in its age
  • open question to time before the universe
  • standard time: earch day -> atomic clock

Distance

  • Long measurement: triangulation, baseline, brightness
  • Short measurement: electron microscope (small-scale triangulation) -> $10^{-10}$m. Neclear dimension by effective cross section -> $10^{-15}$m.
  • Errors in measurement: time-energy, distance-momentum

Probability

  • Probability – $N_A/N$, where $N_A$ is the number of (estimated) observations of A with N (imagined) experiments. based on our knowledge and ability to make estimation.
  • Binomial probability – $C^k_n p^k q^{n-k}$. binary event, n times, p/q probability for a single experiment.
  • Random walk – $E(D_n^2)=n$. Usually represennts the error of an estimation.
  • Probability density/distribution – $P(x,\Delta x)=p(x)\Delta x$ -> p(x)
  • Why probability – Large system (gas), quantum mechanics (electron probability cloud)

Gravitation

  • Gravitation. $\frac{Gmm’}{r^2}$
  • Observation -> Kepler 1st (Ellipse + Focus) -> Kepler 2nd (equal area rate) -> Kepler 3rd ($T \propto L^{3/2}$)
  • Force -> Motion change. (Galileo->Newton)
  • Kepler 2nd -> Force towards the sun.
  • Kepler 3rd -> Force inversely proportional to the square of the distance
  • Universal gravitation -> Sun-earth/falling object with the same theory. Tide
  • Cavendish’s experiment -> gravitational constant
  • mathematics -> mechanism