Monthly Archives: September 2000

What is inside the earth


      As all of us have studied the earth’s interior in our school days .

Now lets look at it it again . Earlier the earth was molten . Then the lighter elements like silica , oxygen started coming to the top and the heavier elements like nickel and iron started going towards the centre .

The layers at the top cooled off . But the interior still remains hot.

The earth now has 3 layers :


1) The crust : this is where we live . It extends upto 30 Km deep .

   It is cool and brittle . It tends to break during earth quakes .


2) The Mantle : This extends from 30 kilometres to 2900 kms below the

   earth . It accounts for most of the earth’s volume . It is composed of

   Iron , Magnesium , silicon . All earthquakes , volcanoes orignate from

   this layer . It is semisolid . It has a temperature of about 1000 degrees.


3) The core : The core has two parts :


   a) The outer core : This is liquid . consists of molten iron and nickel.

      It has a temperature of about 3700 degrees . 

   b) The inner core : Because of intense pressure this is a solid ball of

      iron . It has a temperature of > 4300 degrees and is about 5200 Kms

      deep .


How do we know what is inside the earth ??


Well there are various ways to find out . When waves generated by siesmic activity ( such as earthquakes ) goes from one layer to another it bends ( like light bends on hitting a surface with different refractive index ) .


another way is to study the insides of the earth is to study the two types of siesmic waves : The compressional P waves and the shear S waves .

The compressional P waves can travel both in solids and liquids . The shear S waves can travel only in solids ( in liquids if something is pushed sideways there is very little resistance ) .


      So suppose an earthquake occurs in New York and you observe the waves on the other end of the earth . Scientists observed that till a seperation angle of 105 degrees you get both P waves and the S waves thru the earth . After a seperation angle of 105 degrees the S waves ( Shear waves) disappeared completely . That means that the waves are passing thru liquid . By some trignometrical calculations u can find the radius of the liquid inside the earth .


      The earth’s Magnetic field can also be explained by the movement of the liquid in the core . The moon and Mars have no magnetism .

To account for the magnetic field in the earth the convection has to be very rapid . The liquid iron seems to be charged and its rapid movement causes the magnetism .


      We can also learn a lot about our magnetic field by observing the field of Sun which is similar but stronger . The magnetic lines of force twist arouund the core and then they rise to the top( cooler layers) .

Here they become cyclonically twisted till a release happens in form of sun spots . The sun’s Magnetic field reverses its polarity every 22 years .

If you apply the same equations to the earth then we see that it should change its polarity ( south becomes north and vive versa ) every 250,000 years .


      Scientists when studying old rocks have observed that the orientation in which the rocks are laid/frozen does show us that about 300,000 years ago the earth’s magnetic field was indeed opposite to what it is now .


Ockham’s Razor

Today’s VERITAS is about a doctrine that guides you when you are confronted by a set of theories each one of which makes similar/same predictions .


      Whenever a set of facts requires a creation of a theory that

explains these facts then Scientists face several possibilities .

Even if unfit hypothesis are discarded there remain several possible

theories that explain the same phenomenon with different underlying

assumptions .


      To help you choose among these various theories there is a

tool called Ockham’s Razor . Ockham’s Razor is a principle stated by

a English Philosopher , William of Ockham . He said ” Pluralitis non est

ponenda sine neccesitate” which means “entities should not be multiplied

beyond neccessity “


      This means that whenever you are making a choice between a set

of theories then select the simplest one and do not assume the existence

of unnecessary entities/forces/objects .


      Lets take an example : Newton is sitting under a apple tree and

a apple falls on his head . Now he has several theories to explain this

with :


1) The earth pulls the apple . Tha apple falls .


2) A comet has passed close to the earth . While passing it generated

   atmospheric turbulence , a downward wind, which caused the apple to

   fall .


3) The apple has a brain and sometimes it just wants to be closer to the

   earth and the earth wants it too and the apple falls to be close to

   the earth .


      Now each of these theories makes similar predictions but which one

should our Newton select first for consideration ? Ockham’s razor would

cut theories no 2 and 3 because they both assume entities that may not

be needed ( comets,downward wind , brains in apples ) . The first theory

is the simplest  and  must be considered first. If the first theory fails

then we start considering theories 2 and 3 .


      Ockham’s Razor is also called “The law of Economy ” . It does not

tell you what theory is wrong or right . It however directs you to study

the simplest theories first . It does not gurantee that the simplest

theory is correct . It establishes priorities .


      Ockham’s Razor has become a very important part of scientific

thinking/philosophy . Galileo used it to make the simplest thory of

planetary orbits .


      Applying Ockham’s Razor to everyday life can remove a lot

of superstitions . Lets take an example : Suppose u got less marks

in an exam . We can explain this in various ways :


1) u didnt study hard enough .


2) The exam was on 11 November and your birth number is 4 and 11 Nov of

   that particular year is not your lucky number . And so u didnt do well .


      Apply the Ockham’s Razor and study harder for the next exam because

theory 2 is to be cut /removed . It assumes too many unneccesary things :

lucky numbers , unlucky numbers , exam papers understanding lucky numbers ,

divine intervention etc etc .


      Try and apply Ockham’s Razor to God and see if He is actually

needed . I wont comment on this further 🙂 .


  An unrelated(related?) razor is Hanlon’s Razor : “Never attribute to malice

that which can be adequately explained by stupidity ” . 🙂