Topic 20: Steller Evolution, Black Holes, and Time

1. Topic 20: Steller Evolution, Black Holes, and Time The macroscopic universe meets the microscopic world of quantum mechanics

2. A Universe of Experiments • We normally think that the behavior of atoms and subatomic particles will be best identified in large and expensive particle accelerators (e.g., CERN) where we can throw subatomic particles at one another and view the results. • It turns out that the macroscopic Universe has several processes active (luckily far from our Solar System) that do the same thing, but on even larger spatial scales and higher energy!

3. Stellar Evolution - I Normal Stars: • Form from gas nebula by gravitational attraction • As gas heats up and pressure from contraction reaches a special point, H+H combine to form He (fusion) • Heat and Pressure balance gravitational forces and star is stable • When H fuel is gone star contracts to white dwarf (can’t collapse more due to Pauli Exclusion Principle - can’t normally squeeze electrons into nucleus)

4. Stellar Evolution - II • If stars have mass 1-2x Sol, collapse may push e electrons into nucleus (e- + p+ = neutrons) forming neutron stars • If star mass is above Chandrasekhar Limit, (~2x Sol) stars collapse to infinite density forming a singularity. • The region surrounding a singularity is termed a Black Hole.

5. Black Holes • The singularity with infinite mass has strong enough gravitational pull to even pull light into it. • At some distance from the singularity will be a boundary termed the Event Horizon. • Anything (including light) inside the event horizon will be pulled into the singularity. • Anything outside the event horizon will go its merry way!

7. Virtual Particles • We believe that information about the four fundamental forces in the Universe are carried by massless particles called Bosons. • The Bosons are: • Photon - electromagnetic force • Graviton - gravity force • Gluon - strong force • W/Z (three particles including the Higgs Boson) - weak force • Wherever in the Universe these forces are not exactly zero, we think the Bosons (and their anti-particles) are constantly being formed as pairs. They normally recombine and annihilate each other very quickly.

8. What Happens at the Edge of Black Holes? • Hawking argues that the Boson pairs and even some Fermion pairs (electrons and their anti-particles, positrons) are forming near the event horizon of a black hole. • The black hole sucks in some of these particles, leaving the ‘mate’ to exist and travel outward from the black hole. • This would look, to an external observer, like the black hole is emitting radiation.

10. Nature of Time • Our ideas about time have changed significantly since the time of the ancient Greeks. • With the addition of the Expanding Universe, Relativity, and Quantum Mechanics paradigms in the 20th Century, we have a revolution in the nature of time. • Time is • No longer constant in rate depending on reference frame • Simply one part of something called the space/time continuum • Perhaps not even present in subatomic particle interactions that do not display causality

11. Types of Time in Our Universe • Thermodynamic time - increasing entropy • Psychological time - memories with no assurance of future • Cosmological time - expansion of universe