1
99:59:59,999 --> 99:59:59,999
So how do we know that there is a core

2
99:59:59,999 --> 99:59:59,999
that the core is made up of a liquid outer core and a solid inner core

3
99:59:59,999 --> 99:59:59,999
and the answer there coems from the same technique that we saw use.

4
99:59:59,999 --> 99:59:59,999
in 1909 to essentially see how the behaviour or when you measure the siesmis waves

5
99:59:59,999 --> 99:59:59,999
or whether you can even measure the siesmic waves the different distances

6
99:59:59,999 --> 99:59:59,999
from and earthquake. So if there is an earthquake right here and we are calling that 0 degrees

7
99:59:59,999 --> 99:59:59,999
let's remember a couple of things right here. Let's remember that p-waves can travel

8
99:59:59,999 --> 99:59:59,999
through anything. They can travel through solids or liquid or air for that matter.

9
99:59:59,999 --> 99:59:59,999
So they can travel through anything.

10
99:59:59,999 --> 99:59:59,999
But s-waves can only (s for secondary) s can only (these are the transverse waves)

11
99:59:59,999 --> 99:59:59,999
these can only travel through solids. So turns out that if an earthquake happens at

12
99:59:59,999 --> 99:59:59,999
0 degrees and you have seismograph stations all over the world

13
99:59:59,999 --> 99:59:59,999
and these are extrememly sensitive in order to measure earthquakes that are happening

14
99:59:59,999 --> 99:59:59,999
thousands of kilometres away. It turns out that there is something called an s shadow

15
99:59:59,999 --> 99:59:59,999
and s waves shadow. You can measure them here, you can measure them here

16
99:59:59,999 --> 99:59:59,999
They can go over there, they can go over here you can measure them over here.

17
99:59:59,999 --> 99:59:59,999
You see measurement in all of these points but then suddenly

18
99:59:59,999 --> 99:59:59,999
at 105 degrees. We are measuring 0 degrees here and we are going outwards like that.

19
99:59:59,999 --> 99:59:59,999
Suddenly at 105 degrees you stop measuring s-waves

20
99:59:59,999 --> 99:59:59,999
For soem reason you think that the s-waves would get over here.

21
99:59:59,999 --> 99:59:59,999
That they would get over here, maybe a little bit weaker

22
99:59:59,999 --> 99:59:59,999
but they would be able to get all the way over here.

23
99:59:59,999 --> 99:59:59,999
But they just abruptly stop. No more s-waves

24
99:59:59,999 --> 99:59:59,999
So in this whole area right over here you get no more s-waves

25
99:59:59,999 --> 99:59:59,999
And obviously I could flip this picture over

26
99:59:59,999 --> 99:59:59,999
and you would get see a symetric thing on the other side of the globe

27
99:59:59,999 --> 99:59:59,999
All over here you would not see s-waves

28
99:59:59,999 --> 99:59:59,999
You would only see them from 105 degrees in this direction and 105 degrees in that direction

29
99:59:59,999 --> 99:59:59,999
and the only reasonable explanation that we could give

30
99:59:59,999 --> 99:59:59,999
is that there must be some material that an s-wave cannot travel thorugh

31
99:59:59,999 --> 99:59:59,999
that is would have to travel through to get to these points beyond 105 degrees

32
99:59:59,999 --> 99:59:59,999
and we know that s-waves only travel in solids.

33
99:59:59,999 --> 99:59:59,999
So the assumption there is that at some point beyond 105 degrees

34
99:59:59,999 --> 99:59:59,999
it's hitting liquid.

35
99:59:59,999 --> 99:59:59,999
So that's what tells us that this right here is probably a liquid

36
99:59:59,999 --> 99:59:59,999
So that tells us that there is a core and that at least the outer part of that core is

37
99:59:59,999 --> 99:59:59,999
liquid enough to stop s-waves.

38
99:59:59,999 --> 99:59:59,999
So the s-waves because they only travel in solids it leads to this s-wave

39
99:59:59,999 --> 99:59:59,999
shadow and this tells us that we have a core and that core at least the outer part

40
99:59:59,999 --> 99:59:59,999
is liquid

41
99:59:59,999 --> 99:59:59,999
we don't know yet whether the inner part is liquid or solid.

42
99:59:59,999 --> 99:59:59,999
Not the next point of emphasis is how do we know

43
99:59:59,999 --> 99:59:59,999
that there is an inner core?

44
99:59:59,999 --> 99:59:59,999
We could use p-waves for that

45
99:59:59,999 --> 99:59:59,999
The p-wave, a p-wave can travel through anything.

46
99:59:59,999 --> 99:59:59,999
But remember as you get denser material

47
99:59:59,999 --> 99:59:59,999
In general for the same type of material

48
99:59:59,999 --> 99:59:59,999
if you get denser material its going to move faster

49
99:59:59,999 --> 99:59:59,999
so its going to refract outwards like we see here

50
99:59:59,999 --> 99:59:59,999
but if it goes into a iquid in general sound waves or as I should say p-waves

51
99:59:59,999 --> 99:59:59,999
or seismic waves will move slower in liquids

52
99:59:59,999 --> 99:59:59,999
and so the refraction patterns that we get

53
99:59:59,999 --> 99:59:59,999
when we do measure from seismograph stations around the world