Spies from another planet

(a science fiction story)

Arithmetic coding

• Arithmetic coding is a way to represent a text using a single real number

• Each symbol is represented using a fixed number of digits.

• e.g. A↦01, B↦02, …, space↦00 and so on
• We put all numbers together and form a large number

• Then we write 0. in front of the number and we get a fraction

• Arithmetic coding encodes the entire message into a single number, a fraction q where 0.0 ≤ q < 1.0.

“Arithmetic coding” in WikiPedia

Example

Initial text

[1] "All Wikipedia"

Same, encoded as numbers:

 [1] 33 76 76  0 55 73 75 73 80 69 68 73 65

Finally, in the form of a decimal fraction

[1] "0.3376760557375738069687365"

Physical encoding

An alien visited Earth, collected all observations in a big file—let’s say, all Wikipedia

Then the alien encoded it using arithmetic code, resulting in a value q

Finally, the alien took a 1 meter bar, and made a mark at q meters

The bar is sent back to the alien country

What is the problem with this idea?

Uncertainty

Our tools have limited “resolution”

We cannot “resolve” the difference between small variations

wpscms.pearsoncmg.com/wps/media/objects/1860/1905663/mathtutorial/rulers.gif

We need to know the resolution

We need to know the margin of error of our measurement

That is, how well we know the values

This is also called “precision”

Uncertainty must be declared

How big is the margin? How bad is the doubt?

We declare an interval: [xmin, xmax]

Most of the time we write x ± 𝚫x

Example: 20cm ± 1cm

The aim of scienceis not to open the door to infinite wisdom,but to set a limit to infinite error

Bertolt Brecht, in “Life of Galileo” (1939)

Practice

Massive Stone Ball Discovered by Bosnian

In 2016 a large stone ball was found in Podubravlje village near Zavidovici, Bosnia and Herzegovina.

Souvenirs for Tourists

We do not know how these stones were made

Similar stones have been found in Costa Rica

Nevertheless, people have made small round stones to sell as souvenirs

Density of the stone ball

I was given a stone ball from Bosnia

We want to know its density

So we need to know mass and volume

• Estimate its mass
• Estimate its volume
• Estimate its density

Our estimation of mass

Based on intuition we think that the mass is more than 100gr and less than 1Kg

Taking the geometric mean, we got 300gr

But it can be anything between 200gr and 400gr

We write 300gr ± 100gr

Our estimation of volume

Comparing with a tea cup, we guess 80ml

But it can be anything between 70ml and 90ml

We write 80ml ± 10ml

How would you estimate the density?

How can we improve our margin of error?

Summary

• Intervals can be written $$[x_{min},x_{max}]$$ or $$x_{mean}±\Delta x$$