The search for physical law

  Physical Law: Simple patterns in nature that never change

  Compare with certain/uncertain domains of science

  When do you trust a scientist? At the frontier of our understanding there is always much uncertainty and debate. Trust science only after the debate settles down.

Applications to Astronomy/Cosmology

 Assume laws of physics the same in space/on earth

•A correct assumption? Making it is a good way to find out!

•(Also, there always are some ideas out there that propose changes to the laws of physics)

Stonehenge

Copyright © Addison Wesley Longman, Inc.

Copyright © Addison Wesley Longman, Inc.

Before the “elliptical” nature of orbits was understood, people tried to explain the planetary orbits using concentric circles or “epicycles”

  Energy is “CONSERVED”. It can flow between the different forms of energy, but the total can not change:

Energy Types

• Kinetic energy

• Potential energy (due to forces, e.g. stretched rubber band)

Thermal energy (kinetic energy of microscopic particles)

 

• Mass energy

(released for example in nuclear reactions)

21

2mv

23

3

2

1.38 10 /

ThermalKE kT

k Joules K

2E mc

1 22

m mF G

r

•Always Attractive

"inverse square law"

G = 6.67 x 10-11 nt - m2 / kg2

Gravity

Some Numbers and Illustrations

•Force is often measured in “Newtons” (nt)

•G = 6.67 x 10-11 nt - m2 / kg2 (“-” means “times” when talking about units)

•The force of gravity on a 50kg person(on earth) is 490 Newtons (The force of gravity on a 100kg person is 980 Newtons)

Some Examples and Illustrations (continued)

•The force of gravity between the Earth and the Sun is 3.5 1022 Newtons

•The force between the Earth and the Moon is 9.2 1027 Newtons

•The force between the Earth and the restof the Galaxy is 1.1 1015 Newtons (how do weknow this must be less than the above?)

Gravitational Collapse and Orbits

Gravitational Collapse, Escape, and Orbits: •A sufficiently slowly moving object will be dominated by the attractive gravitational force and will move toward the source of the force

•A sufficiently fast moving object will escape the gravitational force and move away from the source of the force

Gravitational Collapse, Escape, and Orbits (Continued):

•For a wide range of intermediate speeds the object will move in a stable orbit that is periodic (retracing the same path time after time)

•This is a special feature of the “inverse square” force law

General Comment

•The idea of motion (kinetic energy) trying to spread things out, but competing with gravity which tries to pull things together, will be a recurring theme in this course.

•The interesting objects in the Universe are the ones for which these two effects have struck some sort of balance.

Galaxies Black hole with accretion

Key points

•"Laws of Physics" are simple patterns always obeyed by nature. Scientists use laws learned in the lab to understand the cosmos.

•Laws of physics discussed:

-Energy conservation

-Newton’s Laws of motion

-Newton’s Law of gravity.

•Newton’s laws account for much of the activity we observe in the cosmos: Tension between Gravity (collapse) and motion (spreading), produces orbits, galaxies, black holes, etc.