Introductory Astronomy Week 3: Classical Physics

Matter By early 1900s: a unified understanding through atomic theory All matter made of a hundred or so elements – types of atoms labeled by These bind to form molecular compounds Three states: solid, liquid, gas Bulk properties determined by microscopic dynamics Temperature is a measure of average random motion of atoms and molecules In ideal gas measured in pressure in Fluid will flow from a region of higher pressure to a region of lower pressure until pressures equilibrate

Heat Transfer An object hotter than environment will lose energy until temperatures equilibrate Conduction: Heat can be transferred through continuous contact. Rare in astronomy Convection: Physical motion of fluid carries energy. Works well when heating from below - Heated fluid less dense so rises Radiation: Hot objects glow losing energy to light

Luminosity and Brightness Sun is hot so radiates energy at a rate Luminosity in Brightness is flux in At a distance radiation distributed uniformly on surface of sphere

States At low temperature and sufficient pressure form (almost) incompressible liquid. In equilibrium with gravity pressure increase with depth proportional to density Density decreases with temperature: hot fluid rises In solid state positions of atoms fixed – maintain shape under external force Perturbations travel through matter as sound waves with a speed characteristic of the material We hear sound as a perturbation in pressure and density in air. Slinky Free-falling cup

Waves Periodic disturbances characterized by frequency in Traveling at speed produce periodic wave with wavelength Amplitude is value of the perturbation at maximum Energy flux in carried by wave is When two waves meet disturbances add If opposite sign - subtract! Waves PhET Interference PhET

Interference

Doppler Effect Sound from a moving source heard at higher/lower when source approaching/receding Doppler (1842) source receding L = l0 + vT = l0 – vl0/c

Another Wave? Light carries energy at a speed of Newton 1670: white light contains all colors. A stream of particles Young 1799: observes interference. Light is a wave Young 1802: we observe relative intensity of RGB Color is the frequency of light. For visible light of order . Wavelength is 400-700nm Colors demo, grating

Diffraction Grating Time delay between waves from slits determined by Extra distance is Constructive interference when D

Another Force Dominant force in most of physics: electromagnetism Coulomb force can be attractive or repulsive Opposite charges attract so most objects neutral Charge is conserved A charge creates and is affected by electric field Electric Field PhET

Magnets – and Light Moving charges create and are affected by magnetic fields (Ørsted 1820) Changing magnetic field creates electric field (Faraday 1831) Changing electric field creates magnetic field (Maxwell 1861) Leads to propagating waves with velocity Coincides with speed of light (Fizeau-Foucault 1850) Light is an electromagnetic wave! Faraday PhET

Electromagnetic Spectrum Electromagnetic waves can have any wavelength What we see is limited by our eyes which are adapted to transparency of atmosphere What the Universe produces is not. Observing the Universe in many bands produces additional data

Heat Radiation A hot object radiates For dense dark objects radiation completely characterized by temperature – blackbody radiation Hotter objects are blue Wien 1893 Hotter objects radiate more. Stefan-Boltzmann 1879 flux at object BB simulation ICLB is ~3Ks

Example: Our Sun Measure Solar constant Compute luminosity Sun radius Luminosity is so Set to find Use Wien This is green L = 4pi(1.496E11)^2*1361 = 3.83E26

When Light Meets Matter Dense objects absorb light energy or reflect it. How much absorbed can depend on wavelength – dyes. Can learn composition from reflected spectrum Light scatters off tenuous matter (Rayleigh 1871) Scattering decreases with wavelength: blue scatters more than red Sunset demo

Scattering on Earth Atmosphere scatters blue light making sky glow blue and Sun appear yellow When we get more scattering – when Sun low in sky – lose green to scattering leaving Sun red Sky blue demo

Scattering and Refraction Moon halo Rainbow

Line Spectra Fraunhofer 1814: Sun’s spectrum has gaps Kirchoff-Bunsen 1859: Tenuous gas emits line spectrum Atoms and molecules emit/absorb at characteristic wavelengths when heated or ionized Line spectrum yields chemical composition At higher pressure and density lines broadened Three views spectra suimulator Helium discovered1868

Structure and Spectra Atoms simplify physics. ~100 atoms explain all chemical compounds Line spectra provide hints to the internal structure of atoms From a broad spectrum of excitation, atom absorbs/emits at resonant frequencies determined by internal structure Electromagnetic radiation excites internal vibrations, associated to internal dynamics of charge

What’s an Atom? Mass of atom is Are Hydrogen atoms the building blocks? Thompson 1897: negative electrons inside atoms In fact electrons! Atoms bind by rearranging, sharing, or deforming their electrons Ions have missing or extra electrons Chemistry is the science of electronic rearrangement

Inside the Atom Rutherford 1909: Structure of the Atom is Keplerian Heavy nucleus of positive charge of size Orbited by light electrons of negative charge in orbits of size Atoms can bind by trading, sharing, or deforming their electrons. Chemistry is the science of electronic rearrangement Elements immutable because nucleus not affected

Problems? Electrons in an atom are accelerating so should radiate losing energy. How are atoms stable? Why are line spectra discrete? Light exhibits particle behavior (Planck 1900, Einstein 1905) Electrons exhibit interference (Davisson-Germer 1927)

Quantum Mechanics All resolved by a revolution in our understanding of Nature A physical system (say, a particle) described by a (complex) wavefunction on space of configurations Space of such functions is a vector space Physical observables associated to Hermitian linear operators Results of a measurement are eigenvalues In a state given by measure with probability

Energy eigenvalues for electron in atom Dominant electromagnetic interaction is emission/absorption of a single photon of frequency and transition between levels with Pauli (1925): Exclusion principle. At most two electrons can occupy a given state. Explains periodic table and chemistry Hydrogen Atom Phet

Nuclear Structure Atomic nucleus contains positive protons (Hydrogen nuclei, Rutherford 1917) and neutral neutrons (Chadwick 1932) Nuclei with same but different are chemically identical isotopes New simplification: Three particles

Three Particles, Two Forces Q M Spin 1 1/2 -1 .0005