====== 1. Road to reality ======

===== Astronomical numbers =====
We will deal with a lot of large or //astronomical// numbers. The best way to deal with these numbers is through **orders of magnitude**.

I can write that the age of the universe is either 14 billion (giga) years or $14\times 10^9$ years. Another way of saying $10^9$ is //10 orders of magnitude//.

A number that only tells you how many zeros to put after $1$ is in the form of orders of magnitude. Nine orders of magnitude or $10^9$ means that there are 9 zeros after 1.

Calculations become very easy and calculators are never needed when we stick to only orders of magnitude. Let us calculate the **mass of the universe** in order to demonstrate this counterintuitive fact.

There are approximately 100 billion stars in a typical galaxy. 

==== Units and prefixes ====
^ Quantity ^ Unit ^ Symbol ^
| Length | meter | m |
| Time | second | s |
| Mass | kilogram | kg |

^ Prefix ^ ^ Order of 10 ^ English word ^
| tera | T | $10^{12}$ | trillion |
| giga | G | $10^9$ | billion |
| mega | M | $10^6$ | million | 
| kilo | k | $10^3$ | thousand | 
|  |  | $10^0$ | one |  
| milli | m | $10^{-3}$ | thousandth | 
| micro | μ | $10^{-6}$ | millionth | 
| nano | n | $10^{-9}$ | billionth | 
| pico | p | $10^{-12}$ | trillionth | 

===== Space and time =====

===== Time and river =====

===== Evolution =====

===== Poetry =====

===== Cosmos and universe =====
There are two terms for 'everything that exists' in English, one from Greek, one from Latin. //Cosmos// is Greek in origin and it means order; it is related to the Sanskrit word //samsati// which has the same connotations. On the other hand, //Universe// is Latin in origin and has a complex set of meanings.

On first encounter, 'universe' seem to just mean 'one poem' (uni + verse).

===== Three possible realms =====
Book: //Road to Reality// by Roger Penrose.
{{ :courses:phy100:threeworlds.jpg?nolink |}}

===== Tree of knowledge =====
{{ :courses:phy100:foursciences.png?nolink |}}