Carbon dating was invented in the late 1940s (from the end
of the second World War), and so it is only about 70 years old.
It relies on carbon-14 isotopes. To understand this naming:
the common carbon-12 isotope has 12 nucleons:
6 protons, 6 neutrons; whereas the carbon-14 isotope has 14 nucleons: 6
protons, 8 neutrons. (An isotope is a different form of an element, having the
same number of protons and electrons, but with differing numbers of neutrons)
Carbon-14 is unstable, and decays radioactively through beta
decay. Scientists say that carbon-14 is constantly produced (cosmic rays (rays
from the sun) collide with atoms to produce an energetic (fast-moving) neutron,
which collides with a nitrogen-14 atom, producing carbon-14 and a proton), and
has reached an equilibrium state where the percentage of carbon-14 atoms in all
the carbon atoms is now a constant value.
Scientists say there is one carbon-14 atom in 1 trillion carbon atoms.
This would give the percentage of carbon-14 compared with
normal carbon-12 as:
0.00000000001% (US trillion (1 with 12 0s)) or,
0.00000000000000001% (UK trillion (1 with 18 0s)).
Scientists say that carbon-14 has a half-life (the time it takes for half of the atoms in a given space
to decay) of ‘about’ 5,700 years. This means that the process has been around
for 1.23% of one half-life (the
process being around for about 70 years).
Scientists say that the ratio of carbon-14 atoms to
carbon-12 atoms is constant in all living things (and in the air), and that
when a living organism dies, the carbon-14 stops being produced. Therefore, by
knowing the half-life of carbon-14 decay, the age of the remains of the living
thing can be found by back-tracking from the amount detected to the original
constant amount.
Scientists also say that carbon-14 dating is only ‘reliable’
for objects up to 60,000 years old. That’s roughly 10 half-lives.
For objects older than 60,000 years, other radioactive
dating must be used:
Uranium-235 (half-life = 704 million years)
Potassium-40 (half-life = 1.3 billion years)
Uranium-238 (half-life = 4.5 billion years)
and so on.
Carbon-14 dating is considered the most reliable since
carbon is present in almost every object, living or inanimate. Although
radioactive isotopes of other elements can be used, the amount of the
radioactive isotope present in the object is significantly less than carbon-14.
At this stage, it might be worth posing a few questions:
1. Considering
the process has only been around for about 70 years, how reliable is it to assume
that the amount of carbon-14 is constant in all
things, and that this value has been constant since the birth of the universe?
2. Every
measurement has with it an element of inaccuracy. Given the already tiny
quantities being used, what is the extra level of inaccuracy introduced due to
the equipment?
3. Given that
other forms of radioactive dating are less reliable due to even smaller
quantities of the required isotope, what inaccuracies are introduced?
4. Radioactive
dating has been around for about 70 years. As mentioned earlier, this is 1.23%
of one half-life of carbon-14. For uranium-235, this would be 0.000009943% of
one half-life. Can scientists really measure that accurately and reliably?
Note:
Carbon dating came into use in the late 1940s. Codenamed
‘Trinity,’ nuclear testing began in 1945, and within 20 years at least five
countries were performing nuclear tests. This has an impact on the radioactive
makeup of objects: more so for the immediate areas of the testing, but the
effects can be far-reaching, especially due to the wind carrying the fallout.
Hence, this begs the question of the reliability of any
method of radioactive dating.
Half-life equations are of the form:
Where: N is the number of radioactive atoms
remaining
N0 is the initial number of
radioactive atoms
t is the elapsed time since death (in
years)
T½ is the half-life of the
radioactive isotope (in years)
In order to get the percentage of radioactive atoms
remaining, we need to find:
This can be done simply by rearranging the equation. For our
example of using uranium-235 with the dinosaurs:
t is 65,000,000
T½ is 704,000,000
Therefore:
and then simplifying
and
then rearranging 
and then solving
So, for the dinosaurs to be 65 million years old, there will
be 93.8% of uranium-235 still present in the material being used to date them.
Now, assume the dinosaurs only lived about 6,500 years ago
(in-line with a literal reading of Genesis 1, the Christian creation story).
Then the result would be:
and then simplifying
and then rearranging
and then solving
Taking the difference between these values, this shows that
there is only a loss of 6.8% of the uranium-235 atoms to place the dinosaurs
from being 6,500 years old to being 65 million years old.
To better help understand how a small percentage error in
the measurement of uranium-235 can vastly change the perceived age of the
dinosaurs:
|
Percentage of
uranium-235
|
Age of dinosaurs
(in years)
|
|
100 %
|
Died this morning
|
|
99.9994 %
|
6,500
|
|
99 %
|
10,200,000
|
|
98 %
|
20,500,000
|
|
97 %
|
30,900,000
|
|
96 %
|
41,500,000
|
|
95 %
|
52,100,000
|
|
94 %
|
62,800,000
|
|
93.8 %
|
65,000,000
|
Assumptions made:
- Uranium-235 is present in a dinosaur’s body.
- Uranium-235 decays in a similar method to carbon-14.
- Uranium-235 having a half-life of 704 million years is a reliable fact.
- The tiny quantities of uranium-235 present can be measured to a high degree of accuracy.
- Nuclear testing has not altered the results in any way.
No comments:
Post a Comment