Stellar Alchemy - how the chemical elements were synthesised -

The Cosmos is within us, we're made of star-stuff - Carl Sagan (Astronomer, 1934-1996)

Have you ever considered that every atom in your body came from stars? Everything living and non-living thing and every part of you and me were forged in the grand life of stars. The carbon atoms in your right hand probably came from a different star than your left hand. We wouldn’t be here if stars hadn’t lived, died and exploded, and as they did so, fusing together atomic nuclei and building the elements - carbon, nitrogen, oxygen, sulphur, iron, all the things that matter for life to get started on earth. Every atom in our bodies was once part of something else. Everything is made of the same basic ingredients called the chemical elements, the building blocks of the universe. Written in every atom is the history of the universe.

Start with a Bang

Hydrogen and helium were generated in the first few seconds of the Big Bang. Hydrogen being the most basic of the 92 naturally occurring chemical elements as it consists of only one proton and one electron. The coalescing of four hydrogen atoms at enormous pressures and temperatures within stars generates a helium atom which consists of 2 protons and 2 neutrons. In the synthesis of helium from hydrogen, there is a loss in mass which is converted to energy (heat and light) and is described by Einstein’s eloquent equation, E=mc2. In a stellar fusion reaction, the nuclei of two atoms join to form a single atom of a different element. Helium is therefore the source material for the creation of heavier and heavier atomic nuclei. The sequence of building heavier elements from a helium nucleus proceeds in this sequence:

1. 2 protons + 2 neutrons = 1 helium nuclei.
2. Then the combination of 2 helium nuclei forms the element beryllium.
3. Following this, 2 beryllium nuclei + 1 helium nuclei gives carbon.
4. Then, 4 helium nuclei would give oxygen, 5 helium nuclei would give nitrogen, 6 helium nuclei would give magnesium, 7 would give silicon and 8 would give sulphur, and so on.
However, this only works for elements as heavy as iron or less. This is because creation of elements heavier than iron requires additional energy rather than the production of energy. Therefore there needs to be another mechanism to explain the generation of nuclei heavier than iron in stars.

Stellar Alchemy

When a star runs out of hydrogen it begins to die and collapses in on itself creating pressures and temperatures high enough to overcome nuclear forces which allow helium nuclei to fuse together to make atoms of heavier nuclei. Collapsing stars do not go out with a whimper, but explode in one of the most energetic events in the Universe, to produce a cloud rich in atoms of heavier atomic nuclei. These events are known as supernovae and are the final phase of element creation. The energy flux is so great during a supernova event, that all of the naturally occurring elements above iron (cobalt to uranium) are synthesised by the relentless pelting of atomic nuclei with neutrons. This highly energetic environment promotes the synthesis of elements heavier than iron, only now, nuclei are not fused together as in a fusion reaction, but the heavier nuclei undergo nuclear fission. Iron is converted to cobalt, which in turn is converted to cadmium, then, indium, tin, antimony and so on, including gold.

The stars died so that you could be here today - Lawrence Krauss (Physicist 1954 - )


The stuff we are made of

Hydrogen and helium were produced in the Big Bang, and heavier elements were created later by stars and scattered into space by stellar supernovae. There, in the spaces between the stars, these elements mixed with interstellar gas and became incorporated into subsequent generations of stars. The remnants of the stellar explosions coalesced to form the rocky and giant gas planets in our solar system. All 92 naturally occurring elements were incorporated into our planet during its accretion some 4.5 billion years ago. Water, essential to biology, is made of one atom of oxygen and two atoms of hydrogen. Next time you drink a glass of water, consider that you are imbibing hydrogen, which is 13.7 billion years old and also the most abundant element in the Universe. We inhale oxygen and iron-bearing haemoglobin carries this oxygen through the bloodstream. Chains of carbon, nitrogen, oxygen and phosphorus form the support structure for proteins, fats, and carbohydrates in our cells. Calcium strengthens our bones, while potassium and sodium ions are a conduit for impulses through the nervous system. The essential elements for biology on the early earth made it possible for life to start and for the eventual propagation of living species through Darwinian natural selection.


Our Cosmic Connection

The stars seem far detached from our everyday lives, but they are connected to us in the most profound way possible - none of us would be here if stars hadn’t been born, lived their epic lives and died in a dramatic way.



Figure 1: The Periodic Table of the Elements. Elements heavier than hydrogen are synthesised in the hearts of stars by the fusion of lighter atomic nuclei. Elements heavier than iron are made by nuclear fission in supernova eruptions. Image taken from: http://www.elementsdatabase.com/


Figure 2: The hydrogen making up the water in this glass would have been created in first few nanoseconds of the Big Bang and is therefore as old as the universe itself, 13.7 billion years.




Figure 3: The element iron is the last in the sequence of elements formed in stars. Massive deposits of iron oxide at Vergenoeg open pit mine. Picture: Bruce Cairncross


Figure 4: Every atom on Earth would have been made in the Universe's
infancy and in supernovae events. Picture: Allan Fraser


Figure 5: Gold is one of the heavier elements produced by nuclear fission in highly energetic supernovae events. This specimen of gold is 10.5 cm (9.5 oz.) and from the Vogelstruisbult mine, Witwatersrand goldfield. Picture: Bruce Cairncross.

Bibliography

1. Delsemme, A., (1994), “Our Cosmic Origins – from the Big Bang to the Emergence of life and intelligence”. Cambridge University Press.
2. McSween. H.Y., (1997) “Fanfare for Earth – the origin of our planet and life”.

Ages of Rock, Rock of Ages

Tracing Earth's Oldest Rocks

By Allan Fraser

How old is planet earth? Our Solar System formed from a vast cloud of gas and dust 4.65 billion years ago. The age of 4.65 billion years is well established from the decay rates of radioactive elements found in meteorites and rocks from the Moon as well an abundance of evidence from chemistry and physics. As Earth is a dynamic planet in which rocks are continuously being recycled by plate tectonics much of the primordial material from the time of the formation of the Earth is no longer around. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. The oldest rocks found to-date on Earth are those of the Nuvvuagittuq greenstone belt and these give an age of 4.3 billion years. It was not until relatively recently that it has been possible to measure the age of rocks. The early ideas of the age of the Earth date to the ancient Greeks and Romans and it was not until the late 1700s that scientists begun to realise that the Earth was indeed ancient. However, it was not until the discovery of radioactivity and the invention of the mass spectrometer that the quantification of isotopes of various radiometric decay schemes could be performed. In recent decades improvements in detector technology and electronics in mass spectrometers has resulted in an improvement in the precision of analytical data translating to an increased confidence in the radiometric ages. The use of hyphenated analytical techniques such as laser ablation - mass spectrometry has also allowed the analysis of small sample sizes and individual crystals.

In the Beginning

The ancient Greeks and Romans realised that long time spans were required to lay down the thick layers of sediments observed and from this they estimated that the Earth was thousands of years old. But it wasn't until the late 1700s that scientific interest in geological age began when Scottish geologist James Hutton (1726-1797), who observed that sediments built up on landscapes were indeed indicative of an old Earth (Dalrymple, 1991). Before then, the Bible had provided the only estimate for the age of the world. Bishop James Ussher (1581 – 1656) established the time of “creation” to 6000 years. Using the book of Genesis as a history book, Ussher meticulously examined the genealogy of the Bible and concluded that the date of the creation as the night of Sunday, 23 October 4004 BC (McSween, 1997). Today some biblical scholars, as well as a number of literalist evangelical Christians, believe in a literal interpretation of the Bible calling for a 6000-year-old Earth (Barr, 1984). In 1785 Hutton published ‘Theory of the Earth’ in which he concluded that “slow” processes shape the Earth, mountains arise continuously as a balance against erosion and weathering and the physical and chemical laws that govern nature are uniform. Most geological processes are extremely slow, and evidence for slow change was everywhere; rivers eroded rock and rain inexorably wore away the tops of mountains and the slow movement of glaciers carved out entire valleys. Hutton and other contemporary scientists of the time concluded that the single most important factor why the Earth looks like the way is does was due to time, and lots of it. With this Hutton established the ‘Doctrine of Uniformitarianism: "Present is key to the past" (Ward, 1995). Hutton used fossils to establish relative ages of rocks. There was however a need to determine the absolute age of the Earth. In the late 19th Century this question was first addressed by William Thompson (Lord Kelvin, 1824 - 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the then young science of thermodynamics. His calculation was based on the cooling of the Earth through conduction and radiation of heat. Kelvin’s age of Earth was calculated to be about 24-40 million years (Ward 1995). The problem with this view is that the Earth has an internal heat source from radioactive decay – a fact not known to Lord Kelvin at the time of his estimation of the Earths age. At around the same time, John Joly (1857 – 1933) calculated the rate of transfer of salt to the ocean as a means to determine the age of the Earth. The age of Earth by this method was calculated to be 90-100 million years. The main problem with this approach was there was no means to account for recycled salt, salt incorporated into clay minerals and salt deposits. Later work by other scientists used the thickness of total sedimentary record, to determine an age of 500 million years. It was not until the discovery of radioactivity by Henri Becquerel in 1896 that geologists had a tool for determining the age of rocks and ultimately the age of the Earth (Dalrymple, 1991).



Figure 1: William Thomson, (Lord Kelvin) (1824 – 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the science of thermodynamics. Source: Wikipedia.org



Figure 2: James Hutton (1726-1797). A Scottish farmer and naturalist, is known as the founder of modern geology. Source: Wikipedia.org




Figure 3: Ernest Rutherford (1871-1937). A New Zealand-born British chemist and physicist who became known as the father of nuclear physics[2] In early work he discovered the concept of radioactive half-life, proved that radioactivity involved the transmutation of one chemical element to another. Source: Wikipedia.org

Nuclear Changes in Nature

The solid rock of the Earth’s lithosphere formed from molten material that cooled and hardened. And in this process a “clock” that gives the age of the rock was going. Within the molten rock there are trace amounts of uranium-238, a radioactive element. Once the rocks had cooled this element was firmly locked in the rock. The atoms of uranium-238 however, decay at a constant rate to form atoms of lead-206 which would also be sealed within the solid rock. With the passing of time the rock would have less and less atoms of uranium-238 and more atoms of lead-206. Therefore, the rock contains some of the original amount of uranium-238 and the decay product, lead-206. Since we know the rate at which uranium-238 decays to lead-206, and the amounts of each of these atoms remaining we can calculate the age of the rock (Brandwein, 1968).

In 1905, Ernst Rutherford and Bertram Boltwood used radioactive decay to measure the age of rocks and minerals. And in 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. He then published the age of a sample of urananite at 1.64 billion years which was based on Uranium-Lead dating (Ward 1995). Scientists of the time began to realise that our planet was indeed ancient and may exceed 2 billion years in age and the search for older and older rocks was on. The invention of the Mass Spectrometer in 1918 allowed isotopes of different atoms to be separated and quantified including four isotopes of lead and two isotopes of uranium (McSween, 1997). Many radioactive elements can be used as geological clocks. Each element decays at its own constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.

There is no doubt of the Earth’s antiquity. Abundant and conclusive evidence of this is found in the rock record. However, fragments of Earth’s early primordial crust are extremely rare as most of it has been melted and recycled numerous times by plate tectonics since the Earth formed (Dalrymple, 1991). If there are any of Earth's primordial rocks left in their original state, they have not yet been found. Meteorites formed at the same time as the rest of the material in the solar system. Therefore by dating meteorites, we also get the age of the Earth, Mars, the Sun and everything else in the solar system. The best age for the Earth is 4.54 billion years (4.54 Ga)which is based on radiometric dating of iron meteorites, specifically the Canyon Diablo meteorite (Ward 1995). The Moon is better preserved that the Earth because it has not been disturbed by plate tectonics or erosion and therefore its more ancient rocks are more abundant than Earth’s ancient rocks. Rocks returned to Earth by the Apollo missions show that the oldest moon rocks have ages between 4.4 and 4.5 Ga. This is an important data as it provides a minimum age for the formation of the Moon (Dalrymple, 1991). However, remnants of ancient rocks exceeding 3.5 billion years (3.5 Ga) in age are found on all of Earth's continents. In 2008 a research group from McGill University discovered an amphibolite in Northern Quebec in an area known as the Nuvvuagittuq greenstone belt which has been radiometrically dated to 4.3 Ga (ref 8.), making them the oldest rocks discovered so far on Earth. Before the McGill study, the oldest dated rocks were from a body of rock known as the Acasta Gneiss in the Northwest Territories of Canada, which are 4.03 billion years old (ref.8). Other rocks that have been studied are nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 Ga), in Swaziland (3.4-3.5 Ga), and in Western Australia (3.4-3.6 Ga) (Barton et al, 1978). Southern Africa also has a host of rocks dating to more than 3 Ga, such as the Sand River Gneisses in the Limpopo Valley of South Africa, have been dated at 3.79 billion years (Barton et al, 1978).

Modern Mass Spectrometry methods are used with laser technology and this has made it possible to analyse very small samples such as zircon crystals down to single grains and achieve very high accuracy and precision (Kruger et al, 2000). An improvement in the precision of analytical measurement allows a reduction in the uncertainty of measurement of an individual zircon crystal or a population of zircons from a particular rock deposit (Allen, 1999). The knowledge of the uncertainty implies increased confidence in the analytical determination and does not imply doubt about the validity of a measurement (Fraser, 2010).

Conclusion

There is abundant evidence that our planet is indeed ancient. This has made us think in terms of deep time, which has profoundly affected the way we the way we see ourselves in the world.


A small Collection of some of Earth’s oldest Rocks



Figure 4: A 5 cm specimen of gneiss from an outcrop of the Acasta gneiss in northern Canada. This gneiss outcrop is dated at 4.02 billion years which is considered to be from one of the oldest outcrop of rocks on Earth. Specimen and photograph: A. Fraser.



Figure 5: 3.6 billion year old Morton Gneiss, Minnesota, USA. 9 cm. Specimen and photograph: A. Fraser



Figure 6: 3.2 billion year old granite specimen (8 cm) from the Klein Jukskei River, Johannesburg. Specimen and photograph: A. Fraser




Figure 7: A polished section of Mary Ellen Jasper (7 cm) from Minnesota USA, dated at 2.5 billion years. Specimen and photograph: A.Fraser




Figure 8: Greenstone schist (8 cm), Walter Sisulu Botanical Gardens, JCI Trail.
Specimen and photograph: A. Fraser


Tracing Earth's Oldest Rocks

Figure 9: Barberton Greenstone (4 cm) dated at 3.3 Ga. Specimen and photograph; A.Fraser

References:

1. Allen L.A., Georgitis S.J., (1999) “Technical Brief - High Precision Isotope Ratio Measurements by the LECO Renaissance™ TOF-ICP-MS.
2. Barr. J. (1984). "Why the World Was Created in 4004 BC: Archbishop Ussher and Biblical Chronology", Bulletin of the John Rylands University Library of Manchester 67:575–608
3. Barton, J. M., Jr., B. Ryan, & R. E. P. Fripp. (1978) “The relationship between Rb-Sr and U-Th-Pb whole-rock and zircon systems in the 3790 m.y. old Sand River gneisses, Limpopo mobile belt, Southern Africa”. In R. E. Zartman, ed. Short papers of the fourth international conference, geochronology, cosmochronology, isotope geology. U.S. Geol. Survey Open-File Report 78-701. Page 476.
4. Brandwein P.F., Stollberg R., Burnett R.W., (1968), “Matter, it’s forms and changes” Harcourt, Brace & World, Inc. Page 182
5. Dalrymple G.B., (1991). “The Age of the Earth” Stanford University Press.
6. Fraser. A.W., (2010). “Statistical Method Validation in Analytical Chemistry – a practical approach”. Training course for Samancor.
7. Kruger, F.J., Allen, L., Fraser, A.W., (2000) “Combined electron probe and LA-ICP-TOF-MS analysis of Major and trace elements in garnet, apatite and zircon” Geoanalysis 2000.
8. McGill University (2008, September 26). Oldest Known Rocks On Earth Discovered: 4.28 Billion Years Old. ScienceDaily. (Accessed April 19 2011).
9. McSween. H.Y., (1997) “Fanfare for Earth – the origin of our planet and life”. Pages 160-161.
10. Ward, P., 1995 “The End of Evolution” . Phoenix Grant Science ISBN 1-85799-368-3. Page 133.





Allan Fraser is a consulting analytical chemist and a registered Professional Natural Scientist with the South African Council for Natural Scientific Professions. His area of interest is in the minerals of the Kalahari manganese field, the Phalaborwa Carbonatite and Peru. Allan’s other areas of interest are rocks of Archean and Hadean age, meteorite impacts and their relation to extinction events (the Cretaceous-Tertiary extinction event in particular) and the geology of Mars and Earth’s moon.

Allan Fraser
PO Box 369
Fourways
2055
mineralman@telkomsa.net

2012 End of the World - Another dumb-ass Prediction

2012 proves to be an eventful year with the end of the world looming in December. Well, that’s if you are a proponent of some very bizarre speculation about Mayan astronomy. How could the Mayan civilisation, concerned with predicting what would happen in a future millennium not use its vast knowledge to save itself from self-destruction? The Mayan elite, high priests and prophets couldn't see far enough into the future to plan for and solve the human problems they faced which ultimately lead to the abandonment of their cities due to a revolt by the plebs. Moreover, the Mayans never predicted the Spanish Conquistadors butchering hundreds of thousands of Mayans in the sixteenth century. So, why should we think the Maya prophets would be any better at seeing the distant future than other failed prophecies? Well, I’ve said my bit on this for the year.

The Gap is closing

Early humans used “Gods” and the “supernatural” to explain the sun and moon, natural disasters and disease. When we started using science to elucidate the natural world the gaps where the God's dwell started to shrink. The gaps grew smaller as science revealed how insignificant we were in the Cosmos and how it so eloquently explained the evolution of species through natural selection and allowed the development of antibiotics to cure disease. Everything we know about the Universe points to it operating by absolute physical laws of cause and effect. The Universe, however, does not look like one in which an independent outside agent is intervening, nor is it a Universe in which miracles happen and physical laws are violated by an entity that is above these laws.

Reliability of Radiometric Dating

I have read several blog sites in which there is discussion on the age of the earth and inevitably the topic of the dating of rocks and the reliability of this method and it's data is questioned. Radiometric dating relies on the natural decay of radioactive isotopes. It is a reliable and accurate way to determine the ages of rocks. As with all measurement there is uncertainty in that measurement which can however be quantified. Over the last few decades the advancement in the instrumentation (mass spectrometers) has improved precision of measurement, hence reducing the uncertainty of measurement of rock age. Dates determined by one radiometric scheme can often be verified by independantly determining the age by an alternative radiometric scheme. All of this nonsense about not believing that radiometric dating is accurate stems from the proponents of a "Young Earth" and people feeling threatend by the advancement of science.

Radioactive decay is the spontaneous release of energy in the form of radioactive particles or waves. It results in a decrease over time of the original amount of the radioactive material. A quantity is said to be subject to exponentional decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as a differential equation where N is the quantity and λ is a positive number called the decay constant: ∆N = - λN/∆ t. The number of decays are represented by ∆N
The short time interval that ∆N occurs is represented by ∆t N is the number of nuclei present λ is the decay constant.



How old is planet earth? Our Solar System formed from a vast cloud of gas and dust 4.65 billion years ago. This age of 4.65 billion years is well established from the decay rates of radioactive elements found in meteorites and rocks from the Moon as well an abundance of evidence from chemistry and physics. As Earth is a dynamic planet in which rocks are continuously being recycled by plate tectonics much of the primordial material from the time of the formation of the Earth is no longer around. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. The oldest rocks found to-date on Earth are those of the Nuvvuagittuq greenstone belt and these give an age of 4.3 billion years. It was not until relatively recently that it has been possible to measure the age of rocks. The early ideas of the age of the Earth date to the ancient Greeks and Romans and it was not until the late 1700s that scientists begun to realise that the Earth was indeed ancient. However, it was not until the discovery of radioactivity and the invention of the mass spectrometer that the quantification of isotopes of various radiometric decay schemes could be performed. In recent decades improvements in detector technology and electronics in mass spectrometers has resulted in an improvement in the precision of analytical data which translates to an increased confidence in the radiometric ages. The use of hyphenated analytical techniques such as laser ablation - mass spectrometry has also allowed the analysis of small sample sizes and individual crystals.

The ancient Greeks and Romans realised that long time spans were required to lay down the thick layers of sediments observed and from this they estimated that the Earth was thousands of years old. But it wasn't until the late 1700s that scientific interest in geological age began when Scottish geologist James Hutton (1726-1797), who observed that sediments built up on landscapes were indeed indicative of an old Earth. Before then, the Bible had provided the only estimate for the age of the world. Bishop James Ussher (1581 – 1656) established the time of “creation” to 6000 years. Using the book of Genesis as a history book, Ussher meticulously examined the genealogy of the Bible and concluded that the date of the creation as the night of Sunday, 23 October 4004 BC (McSween, 1997). Today some biblical scholars, as well as a number of literalist evangelical Christians, believe in a literal interpretation of the Bible calling for a 6000-year-old Earth (Barr, 1984). In 1785 Hutton published ‘Theory of the Earth’ in which he concluded that “slow” processes shape the Earth, mountains arise continuously as a balance against erosion and weathering and the physical and chemical laws that govern nature are uniform. Most geological processes are extremely slow, and evidence for slow change was everywhere; rivers eroded rock and rain inexorably wore away the tops of mountains and the slow movement of glaciers carved out entire valleys. Hutton and other contemporary scientists of the time concluded that the single most important factor why the Earth looks like the way is does was due to time, and lots of it. With this Hutton established the ‘Doctrine of Uniformitarianism: "Present is key to the past". Hutton used fossils to establish relative ages of rocks. There was however a need to determine the absolute age of the Earth. In the late 19th Century this question was first addressed by William Thompson (Lord Kelvin) (1824 - 1907). Kelvin assumed that the Earth was originally molten and calculated a date for the age of the Earth using the then young science of thermodynamics. His calculation was based on the cooling of the Earth through conduction and radiation of heat. Kelvin’s age of Earth was calculated to be about 24-40 million years (ref.). The problem with this view is that the Earth has an internal heat source from radioactive decay – a fact not known to Lord Kelvin at the time of his estimation of the Earths age. At around the same time, John Joly (1857 – 1933) calculated the rate of transfer of salt to the ocean as a means to determine the age of the Earth. The age of Earth by this method was calculated to be 90-100 million years. The main problem with this approach was there was no means to account for recycled salt, salt incorporated into clay minerals and salt deposits. Later work by other scientists used the thickness of total sedimentary record, to determine an age of 500 million years. It was not until the discovery of radioactivity by Henri Becquerel in 1896 that geologists had a tool for determining the age of rocks and ultimately the age of the Earth.

Radiometric dating is no mystery .... the truth is out there.

Human Evolution - The Evidence -

There is overwhelming evidence for human evolution. All human beings and all other life forms evolved from ancestral species. The four main lines of evidence for human evolution are:
1. Our relation to living primates;
2. Vestigal organs/structures such as the appendix;
3. The fossil record;
4. Genetics;

Hypothesis Testing

As a scientist and a rational human being, learn to question everything and strive to find answers on your own through research. 'Doubt' is the most sacred thing for a scientist and an adherent of the scientific method. This doubt keeps you on focused when you feel like surrendering or believing in something at face value without adequate scientific research. As scientists, we constantly strive to create a rational and lucid picture of the world by piecing together one fact at a time, through theorising and experimentation (Pilgrim, 2011). A Hypothesis means to “Ask a Question of Nature”. In Science we often need to test our hypothesis. A hypothesis is an educated guess, based on observation. Usually, a hypothesis can be supported or refuted through experimentation or more observation. A hypothesis can be disproven, but cannot be proven to be true (Helmenstine, 2010). To test the hypothesis we create an experiment that will yield one of two answers: Yes or No or True or False. The classical way to make statistical comparisons is to prepare a statement about a fact for which it is possible to calculate its probability of occurrence. This statement is the null hypothesis and its counterpart is the alternative hypothesis (Fraser, 2011). The null hypothesis is traditionally written as H0 and the alternative hypothesis as H1 or Ha. A statistical test measures the experimental strength of evidence against the null hypothesis. A Null Hypothesis is a statement that the difference between two values can be explained by random error. It is retained if the test for significance does not fail (H0). A null hypothesis assumes that the numerical quantities being compared are the same. The probability of the observed differences appearing as a result of random error is then calculated from statistical theory. The alternative hypothesis is therefore a statement that the difference between two values is too great to be explained by random error. The alternate hypothesis is accepted if a test for significance shows that the null hypothesis should be rejected (Ha or H1). Typical tests for significance are the F ratio test and the t-test. If we reject the null hypothesis at say, a 95% confidence level, there is a 5% probability that the null hypothesis was incorrectly rejected. An example of hypothesis testing is:

Let μ1 and μ2 be the means of two samples; If one wants to investigate the likelihood that their means are the same, then the null hypothesis is:
H0: μ1 =μ2
and the alternative hypothesis is:
H1: μ1 ≠μ2
but it could also be:
H1: μ1 >μ2
The first example of H1 is said to be two-sided or two-tailed because includes both μ1 >μ2 and μ1 <μ2; The second is said to be one-sided or one-tailed. The number of sides has implications on how to formulate the test (Fraser 2011).

References:

Fraser A.W., (2011), “Statistical Method Validation for Analytical Methods – a practical approach”
Helmestine A.M., (2010) “Scientific Hypothesis, Theory, Law Definitions” http://chemistry.about.com/od/chemistry101/a/lawtheory.htm (accessed 25 May 2011)
Pilgrim G. (2011) “Hypothesis versus Theory” http://www.buzzle.com/articles/hypothesis-vs-theory.html (accessed 25 May 2011)