Black holes:
Overview: a black hole is a region of spacetime from which gravity prevents anything, including light, from escaping.
Formation: When dying stars collapse, (i.e. when they run out of fuel and the radiation pressure of the nuclear reactions no longer balances the gravitational force) they can end up in one of a few states. Blackholes are formed when the heaviest types of stars collapse. BH grow bigger by absorbing mass from their surroundings.
History: Idea first put froward by John Michell in 1783, but idea largely ignored because scientists not sure how a massless wave like light could be affected by gravity.
Theory: Predicted by Karl Schwarzschild using in Einstein’s theory of general relativity. All mass bends the space around them. The gravity we feel on earth is due to the fact it curves space a little. With blackholes this occurs to such an extent that it is wrapped around itself and even light cannot escape. Imagine an endless spiral. You would need more energy than there is in the universe to pull you out (i.e. you would need to travel faster than the speed of light which is impossible). In a BH mass collapses to a point – zero dimensions – known as a singularity – no atoms, no particles we would recognize.
The theory shows that the boundary of the region from which no escape is possible is called the event horizon.
Laws of physics of blackholes largely unknown as we need a theory that reconciles general relativity with quantum physics, so called ‘Theory of everything’. An area of active research – string theory/quantum gravity.
Blackholes can evaporate as was shown by Stephen Hawking.
Basically, general relativity posits that space and time are not absolute properties of the universe but rather they depend on the relative motion of an observer.
A stationary external observer would therefore witness time stop for an object that crosses the event horizon. From the point of view of an infallen observer time continues. Hence known as frozen stars as we observe a frozen surface.
Detection: Invisible interior, but their presence can be inferred by the behaviour of other mass and by the passage of light through space. Matter falling into a black hole gets heated by friction and form some of the brightest objects in the universe. There is a black hole 4.3 times bigger than the sun at the centre of our galaxy, the milky way.
Only three independent physical properties are observable from outside a blackhole: mass charge and angular momentum.
Magnetism:
Overview: Magnetism is the force exerted by magnets on other magnetic material.
Formation: – what makes something magnetic? Rotating/moving charged particles. Permanent and non-permanent magnetic material
History: Evidence for first use of magnetic materials 10000BC! Orsted first link between electricity and magnetism. Maxwell’s equations unifying theory for the fundamental force of electromagnetism.
Theory: Maxwell equations, predicted speed of light, but not the quantum effects of waves. Some people thought that the magnetic force was transferred via the ether, which has since been proven not to exist.
Detection: school – iron filings. Charges.
Unusual behaviour: Magnetic monopoles – do they exist? Quantum magnetism. Different types of magnetism.
Applications: birds use magneto receptors to navigate. Electromagnets. Electricity generation. Superconducting magnets. Levitation. Used at LCH CERN.