A View from the Back of the Envelope top

Fermi Questions
Knowledge you can work with...

In a sea of values, one chooses landmarks to navigate by.

Landmarks stake out useful or interesting points in some space,
provide stepping-stones for thought.

[A first, crufty draft.]

You build a landmark by making some value memorable. [...incomplete description...]
Another approach is to develop a `hands-on feel' by Scaling the universe to your desktop.

Why create landmarks? To have stepping-stones for thought.
For thought and imagination to move easily, one needs handholds, markers. Bits of easily accessible knowledge scattered across the landscape. Making its shape visible, and providing places to stand.

Why remember landmarks when we have them in books? To carry them with us.
Books can be a source of landmarks, and a place to keep landmarks you don't often use. Also a place to keep more detailed versions of the landmarks you carry around. But when something would be useful, it doesn't help much to know just where you left it, forgotten at home. Either it is ready at hand, or you need to use something else.

Over time we change what we carry with us. Choose knowledge which helps with our interests and current questions about the world (including fermi questions).

The choice of landmarks is often a personal one. Some values are recognized and remembered by many. But many others, like your street corner, or a friend's name, are mainly your own.

A good place to start is with things you are familiar with.
You can use them for comparisons. And some, to measure with. Knowing how big something is, you can get a feel for other things you learn about. If you know the area of, say your parking lot, then you can picture surviving on a 10-acre farm. Or [...]


Niel Brandt's Timelines and Scales of Measurement Page
Including Evolutionary / Geological Timeline, Cosmological Timeline, Scales of Measurement.
The Magnitudes of Physics
Useful Numbers in Vision Science
Computer Almanac - Numbers About Computers

Some various landmarks...

Population , Time , Length , Mass , .

Human population
ever 1011 1011, ±×2
Earth 6 billion 6 × 109, +4%, ±1%?
US 250 million 2.5 × 108, -6%, ±1%?
city 107 big, 106 mid, 105 small
Sphere's area
"1/2 that of the box it came in" -5%, ±<1%
3 -5%, ±<1%
3.14 ±<0.1%

A View from the Back of the Envelope
Comments encouraged. - Mitchell N Charity <mcharity@lcs.mit.edu>

 2003-Feb-03   Repaired links - 2 fixed, 1 left (cmu).
 2002-Apr-11   Changed a link (evacuating www.tiac.net).
 1997.Aug.01 - Format upgrade.  Content basically unchanged, still crufty.
 1997.Jun.28 - Upgraded notes into a first draft.

Mumble mumble mumble... [visible_universe page was under construction]
Visible universe: 1018 s old, 10~26 m, 10~50 kg
Universe's galaxies: 10~11, 10~49 kg
Milky Way: 1042 kg
Speed of light in vacuum (c): 3 × 108 m/s
[should add error, non-vacuum]