Age of the Earth

 

According to Ussher's reading of the biblical genealogy the world is  some six thousand years old while multiple independent strands of evidence estimates that the earth is over four billions years old. This evidence includes;

Ice Cores

Shallow layers of ice-cores in high-accumulation areas can be dated by inspecting visual layers. Each layer represents the natural annual variation of seasons. These layers may be visible or chemical reflecting the varying seasonal transport mechanisms. Deeper into the core the layers thin as ice flow and pressure increases. Depths up to 110,000 layers can be directly observed using stratigraphic_[1] methods.  

It is possible that each layer may represent a large storm cycle or a weak summer which limits the accuracy of the method however within an appropriate uncertainty these layers  offer an effective method to date atmospheric and temperature characteristics that extends well beyond Ussher's biblical estimate for the origin of the cosmos.
 

Star Light

Light travels through space at a finite speed. The further away the light source the further back in time the light we observe left the source. In effect we are looking at images from the distant past. To estimate the age of the image we only need to know the distance to the star and the speed of light over its entire history.

The primary method used to measure cosmological distance requires a precise determination of the distance from the earth to the sun (1AU). In modern times radar measurements of Venus and other planets have been used with Kepler’s law to determine the size of the earth’s orbit with ever increasing certainty.

As the earth orbits around the sun, the positions of nearby stars shift against the more distant stars. This shift creates a right angle triangle with a known 1AU side which allows the distance to these nearby stars to be calculated. This method, known as parallax, is less accurate for more distant stars because the angle between the measured star and the background stars becomes too small.

For these more distant stars standard candles are adopted. A Cepheid_[2] variable star, for example is used as a standard candle because its absolute luminosity can be determined from its observable period_[3] while the brightness of the star (from earth) can be directly measured. By substituting both these values into the inverse square law the distance to the candle and any other object embedded in the same cosmological structure can then be determined.

Cepheid’s are of spectral class F6-K2 and are up to 30,000 times more luminous than the sun. Over 700 such stars are known throughout the Milky Way. In 2008, ESO estimated with 1% precision the distance to RS Puppis using light echoes from a nebula in which it was embedded.

The other required assumption is that the speed of light has not changed over time. There are many ways to defend this assumption. For example if the fine structure constant which involves the charge of the electron, Plank’s constant and the speed of light changes by more than 0.01% then carbon production from Helium fusion would stall. Since life (which obviously exists) depends on carbon production the constancy of the speed of light post inflation is assured (unless there is an inexplicable conspiracy between the variables). Given the near constant speed of light and our ever growing confidence in cosmological distance measurements we can be assured that the age of the universe approaches  13.7 billion years, which is many magnitudes older than Ussher's estimate.

Radiometric Dating

Most isotopes in rocks are stable but there are some trace elements that do change through the process of radioactive decay. Quantum theory postulates that it is not possible to determine when an individual atom will undergo decay but since radioactive samples generally have a large number of isotopes a statistical model is accurate enough. The model is simply this; the rate of change of decay is proportional to the quantity of isotope available_[4].

So provided the original amount of parent material is known, the sample remains in a closed system and the decay rate remains constant then the age of the sample can be calculated. But in practice we do not know the initial quantity of parent material nor can we guarantee that the sample remained in a closed system for its entire history.

The isochron method addresses both limitations. By way of example consider the two isotopes of lead; 207 and 204. The two isotopes are chemically indistinguishable so as the original rock formed we can reasonably assume that both isotopes got distributed homogeneously. The rock will also contain a third isotope; say U₂₃₅.

As Dalrymple_[5] explains, the trick then ‘to the isochron diagram is the normalization of both the parent (U₂₃₅) and daughter isotope (Pb₂₀₇) to a third isotope (Pb₂₀₄). This third isotope is the non-decay product isotope of the same element as the daughter element. In the initial state, the graph of the daughter isotope to the third isotope versus parent isotope to the third isotope should result in a straight, horizontal line [since it is independent of the parent to third isotope ratio]…..As time progresses and decay occurs, the number of atoms of the parent isotope decreases, and the number of atoms of the daughter isotope increases accordingly. The amount of non-decay isotope in the sample does not change. Thus, as decay occurs, the parent ratio decreases and the daughter ratio increases. On an isochron diagram, this change in ratios shifts each measurement from the sample up and to the left at a one-to-one rate. As time progresses, the line connecting the measurements within the sample moves counter-clockwise around a point intersecting the y-axis, a point that represents the initial ratios’.

Since the isochron method is based on the slope of the line as it rotates around the vertical intercept the method does not depend on our knowing the initial quantity of parent material. Further any variance from a straight line indicates that the sample must have been contaminated. Thus the isochron method elegantly solves the two major limitations of the direct method.

In practise there are some secondary mechanisms that can affect date estimates. These include protracted fractional crystallization, partial melting, mixing isochron and apparent isochron by metamorphism. But according to Brent Dalrymple, ‘Most inaccurate ages are caught by appropriate safeguards, like standards and repetition, but some go unrecognized until long after the data is published. In short radiometric dating methods give reliable results most of the time, but not always. With sufficient cross checks, care and experience, we don’t get fooled very often and when we do it is usually not for long.’

Similar to the direct method the Isochron method also assumes the decay rate remained constant over the samples history. How is this assumption validated? One way is to analyse gamma rays from supernovas. A supernova is an exploding star that can be billions of times brighter than our sun before gradually fading from view over a period of weeks, months or even years. At its maximum brightness the star may outshine an entire galaxy. The explosion throws a large cloud of dust and gas into space. The material expelled in the explosion includes heavier isotopes, of which some are radioactive.

There are two types of supernova events. Type I supernovae occur in binary systems where one of the stars, a white dwarf draws material from its companion star. When the white dwarf reaches a threshold, it explodes. The death of a massive single star may produce a Type II supernova.

In either case the light we observe from a supernova event is historic. For example, the gamma rays observed in supernova event SN1987A happened 169,000 years ago. The frequencies and fade over rates compared to present day values. Supernova SN1991T was sixty million light years away; and again gave comparable results. 

A more pragmatic reason to accept the constancy of the decay rate is the agreement of diverse testing methods. If the rate varied we would expect different methods to be affected in different ways. This would result in disagreement across the various methods as diverse as six thousand year old tree rings from bristlecone pines found in Sierra Nevada; seasonal sedimentary layers of lakebeds evidenced by plant material build-up creating as many as thirty five thousand observable seasonal layers and ice cores with near one hundred thousand distinguishable layers. Agreement of these varying methods gives us confidence that the assumption of constant decay rate is, within uncertainty, a valid one.

Third, the method has been used to make truthful predictions. C.C. Patterson_[6], for example, used the Pb/Pb isochron radioactive dating method to date the Canyon Diablo meteorite to 4.56 billion years old. He reasoned that this date must represent the age of the earth and sun given meteorites formed in the same accreting disk. His prediction that all meteors should date to a similar age has thus far been proven correct.

Magnetic Lava

The US Navy discovered that the seafloor of the Atlantic Ocean exhibits a magnetic signature along its central ridge. The central ridge is displaced as new lava flows into the crust. Magnetic particles in molten lava are free to rotate and align to the earth’s magnetic field. Once the lava cools and solidifies the direction of the particles freeze in a direction aligned to the magnetic field. The surveys found hundreds of reversing strips around the central ridge. Since the outer strips are older than the central strips the reversing magnetic signature can only be explained by the multiple reversing of the Earth’s ancient magnetic field. (The last reversal occurred some 780,000 years ago - according to radioactive dating estimates).

Giberson concludes that ‘there are now no scientific arguments of any consequence that point to the earth being just a few thousand years old. Not one isolated piece of nontrivial data, in any form, points in this direction’_[7] . In contrast all established methods produce a coherent dating scheme that supports a big bang around 13.7 billion years ago and the birth of our solar system some 4.5 billion years ago.

(For details on the implications of an Old Earth on creation please click here)

 

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[1] Stratigraphy: a field in geology primarily concerned with studying layers and layering.

[2] Cepheid star: a member of the variable star family with a tight relation between its period and absolute luminosity.

[3] Mv = -2.81 * log10(P) – 1.43 where Mv is the absolute magnitude and P the period of oscillation (days). Feast M,  Catchpole R

[4] N=N₀e^kt where N is quantity and k the decay constant

[5] Ancient Earth, Ancient Skies. Dalrymple, Brent G. 2004. Stanford University Press

[6] Age of meteorites and the Earth Patterson C: 1956  Geochimica et Cosmochimica Acta 10: 230-237

[7] The Language of Faith and Science Giberson K.W. Collins F.S. 2011 IVP Books