Recap Astronomical Observations

1. Recap Astronomical Observations • What can be observed and measured through the optical telescopes ? • Why is a shirt red ? Why is it black ? Why is the sky blue ? (explain in terms of reflection/refraction) • What is the origin of the black lines in a star’s light spectrum ? • Explain the meaning of a red-shifted star spectrum. • What is an interferometer ? • Describe non-optical astronomy. • Describe fixed, movable radiotelescopes. • Describe the Hubble telescope.

2. Observing the Micro-Universe (from Chapter 1) • The microscopes • Observing atoms and beyond • Particle-antiparticle collisions • Particle accelerators • Detectors

3. The Microscope • A microscope is equivalent to an optical telescope plus the system to illuminate the observed objects. • With a microscope one can observe very small objects, down to the size of the light wavelength (thousands of atomic units).

4. Electronic Microscope • Probability waves • Electrons and all other particles in the micro-universe move with a certain speed and behave like waves because of the uncertainty principlein the quantum world. • Louis de Broglie showed in 1926 how to calculate particles probability waves. Their size is inverse proportional to their speed. • An electronic microscope uses very fast electrons instead of visible light and can therefore see deeper inside matter.

5. Observing the structure of molecules One slit experiment: - one maximum Fluorescent Screens X rays or electrons Two (or more) slit experiments: - pattern of dark and light bands – They are the result of the interference of the associated waves Diffraction of X rays or electrons can produce a clear map of the atoms in a molecule or in a crystal. X rays or electrons Waves interference

6. Observing the Structure of Atoms • Observing atoms requires probability waves of the atomic size. • The first experiment which observed the inside of atoms was performed in 1911 by E. Rutherford. • He bombarded a metal target with alpha particles. • From the distribution of the scattered particles he was able to see that atoms have a heavy positive nucleus (many particles did not change their trajectory, but a few were bounced back). • He was the first to propose a planetary model for the atom: a heavy positive nucleus with electrons moving around like planets around the Sun.

7. Observing Subnuclear Structures • Observing the structure of matter at nuclear or subnuclear scale is similar to Rutherford’s experiment except that : • The smaller the target the higher the energy of the projectiles - accelerators are needed. • The higher the impact energy the more complex impact phenomena will be. • Instead of a simple scattering process the target can break into pieces; the interpretation of the whole process becomes more complicated and computerized detectors are needed.

8. Creating New Particles • Particles can be interpreted as “frozen energy”. Einstein established the relationship between particles masses and the corresponding energy: E=mc2 • New particles can be created by transforming the kinetic energy of the projectiles into particles. Fast projectiles can be either used to hit fixed targets or other particles. • This is also the reason for expressing the weights of particles in units of energies (electron-volts).

9. Particle-Antiparticle Collisions • An antiparticle is identical with a particle except its electric charge (ex. positron/electron). • When a particle collides with an antiparticle both are annihilated and in their place nature creates gamma rays. The energy of these gamma rays is the sum of the particles kinetic energies plus the energy equivalent of their masses. • Gamma rays can also produce particle-antiparticle pairs, (all possible pairs are produced with some probability, which can be predicted theoretically ).

10. Types of Accelerators • Projectiles: electrons, protons and their antiparticles • Energies measured in GeV (billions of electron-volts) • 1) Projectiles on fixed targets • Shapes of trajectories: linear or circular (synchrotrons). • 2) Particles on Particles (Supercolliders) • Most often beams of particles and antiparticles, accelerated with the same electromagnets

11. Examples of Particle Accelerators Supercolliders p+ - p+ Linear accelerator of electrons • Brookhaven 400 GeV • CERN (ISR) 31 GeV • CERN 7,000 GeV • Stanford (SLAC) 22 GeV Proton Synchrotrons • Serpuhovo 3,000 GeV • Fermi (DSPS) 1,000 GeV • Fermi (SPS) 450 GeV • CERN (SPS) 450 GeV • Fermi 1,000 GeV p+ - p- e--e+ • CERN (LEP) 200 GeV • Hamburg(PETRA) 20 GeV • Stanford(SPEAR) 4.5 GeV 1 GeV=1 billion eV

12. Particle Accelerators at CERN

13. Observing Elementary Particles • The size of elementary particles is too small for direct observation. • The experiments can visualize only the traces created by charged particles in special conditions. The most common technique is to examine the bubbles of vapours created by charged particles in a liquid heated to a temperature just below its boiling point. The bubbles are sufficiently big to be photographed and analized.

14. Particles Detectors • The analysis of all particles produced in a collision is even more complicated due to the fact that an experiment has to monitor many simultaneous collisions. Only computerized detectors can do this job. • One of the most famous detectors is Gargamelle, built in 1976 at CERN. • This bubble chamber contains 1000 tones of liquid freon kept under pressure at a temperature just over the boiling point. • Lowering the pressure one obtains a state when charged particles created bubbles of vapors. • For each short flux of incident particles a pump lowers the pressure and cameras take pictures from all angles. • The flux of incident particles, the pump and the cameras have to work in perfect synchronization. • The pictures are then analyzed by sophisticated computer programs.