I guess I'm at that age where new birthdays are not always welcome. So, when friends of mine talk about having another birthday and lament that they are another year older, I sometimes respond "You know, the universe is over 13 billion years old (13.7 +/- 1% ref. WMAP), and you are worried about a few tens of years??". This doesn't always make them feel better but we do have some interesting conversations about how old things are. So, how old are things? Pretty much everything around us (save Hydrogen and Helium) was formed in a massive star who was kind enough to spread its products through the cosmos. As the cloud of newly formed material condensed, it formed our solar system - sun, planets, the whole works. So, now I ask you: How old is the Earth and our solar system? That's easy. It is 4.6 billion years old. Period. End of discussion. We know this from many different measurements of Earth rocks, moon rocks, and meteors. We know this also from understanding what powers the sun. But we have only been privileged with this knowledge for less than 100 years! So how come it took so long? This story follows the lives and thinking of some of the most prominent scientists of the 19th and 20th centuries and explores the continuing struggle between science and religion in defining our views of the universe.
Science as we now define it has only been in existence for perhaps 500 years or so. The idea that theories need to be tested against observational evidence to gain credibility is a relatively new concept for our species. Prior to 1600, many cultures had attempted to derive the age of the Earth through "logical" extrapolations of time and cycles of time. As far as we can tell, most if not all of these chronological estimates purported a young Earth whos creation dates back to between 4000 and 5000 BC give or take. Many of these estimates were based on biblical accountings of events and so the chronologies of groups like the Saxons, Irish, and British were developed by adding estimates of time between biblical events such as the great flood, birth of Moses, etc . The Mayans of South America interestingly also figured an age of the Earth of about 5000 years although their counting begins on the first day after their accounting of a great flood.
In the 1800's, the leading scientist of the time was Lord Kelvin (William Thomas) who lived from 1824 to 1907. Kelvin, a pioneer of modern physics, was a truly brilliant scientist, entering Glasgow University at the age of 10! He developed the absolute temperature scale and was instrumental in our understanding of classical thermodynamics (the study of heat) and the nature of light. He published over 600 scientific papers and filed over 70 patents. He was perhaps the Albert Einstein of the 19th century. However, he was a bit arrogant and believed only what he could measure. Not a bad trait perhaps for a scientist but it did limit his thinking. In short, imagination was not his strong suit. He is also well known for a number of interesting quotes...
"There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." Lord Kelvin, British Association for the advancement of Science, 1900
"Heavier-than-air flying machines are impossible." (1895)
"I am never content until I have constructed a mechanical model of the subject I am studying. If I succeed in making one, I understand. Otherwise, I do not." Notes of Lectures on Molecular Dynamics and the Wave Theory of Light.
"When you can measure what you are speaking about and express it in numbers, you know something about it." Lecture to the Institution of Civil Engineers, 3 May 1883
"Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherialisation of common sense." SP Thompson in Life of Lord Kelvin, 1910
Kelvin was looking for the mechanism that makes the sun shine. We know today that it is thermonuclear fusion but in the 1800's, no one had a clue. A number of theories were proposed. These included...
Kelvin was convinced that the sun derived its heat from in falling material and said in 1862:
That some form of the meteoric theory is certainly the true and complete explanation of solar heat can scarcely be doubted, when the following reasons are considered:
Kelvin was a deeply religious man and believed, as did many other scientists of the time, that the theory had to match the Biblical teaching of the age of the Earth which says (without too much interpretation) that the Earth is less than 10,000 years old (note: Biblical scholars of the 19th century came up with ages of between 5400 and 9000 years). Since it was thought (and correctly so) that the sun and Earth formed at about the same time, it was only necessary to devise a heating mechanism that could keep the sun burning for at least 10,000 years in order to be in alignment with the Bible. For example, if the sun were made of coal (the most energy dense substance of the time), and burned all its mass like a giant fireplace, it could keep burning at its current rate only for about 1500 years - not in alignment with the Bible and so chemical combustion mechanisms were rejected. Kelvin's calculations using classical physics indicated that the sun could "burn" for up to 20 million years by converting the kinetic energy (Kelvin actually invented this term) of in falling meteors into heat. The problem was only to find enough of them and it turns out we can not. In light of this small problem, he eventually backed away from this, supporting gravitational contraction theory.
But there was trouble in Dodge City (as they say) - Kelvin also attempted to calculate the age of the Earth using new theories on heat conduction. He knew the Earth's temperature should drop by about a degree Fahrenheit for every 50 feet you came up from the core. He assumed that the Earth formed from molten rock with a temperature of around 7,000 F. Using Fourier's new theories of heat conduction, Kelvin calculated that it should have taken about 100 million years for Earth to have cooled to its present temperature. Of course, he was unaware of radioactive decay (which is now thought to be a primary source of heat in the Earth's core) since it had not been discovered yet. Still, his newly derived age for the Earth was definitely in conflict with Biblical teachings. Kelvin published his findings, "The Secular Cooling of the Earth" which set off great public and scientific debate on the Issue. Later, Kelvin revised his number down to between 20 and 40 million years. Enter Charles Darwin.
Charles Darwin had been studying the evolution of species. He was perplexed by the geographical distributions of the animals and fossils he found on his 5 year research voyage on the "Beagle". He actually developed his ideas on natural selection in 1838 but kept it to himself and only a few close colleagues for fear of being labeled a heretic. However, in 1859, he published his famous text: "On the Origin of Species (by means of natural selection)". Darwin's studies indicated that it took more than a billion years for evolution to proceed. This was in sharp conflict with Kelvin's findings that the sun and Earth were perhaps a few 10's of millions to 100 million years old. Who would you believe; the preeminent scientist of the time - or Darwin backed by many geologists and biologists (geology and biology were not even considered proper sciences by most physicists of the time)? The teams were formed. Physicists on one side and biologists and geologists on the other. The debate raged on and got personal at times, evolution was termed "higgledy-piggledy build-up of chance variations" and "crab-catching science". Evolutionists and geologists argued that mathematical theories were no better than the presumptions they began with:
"Mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness; but, nevertheless, what you get out depends upon what you put in; and as the grandest mill in the world will not extract wheat-flour from peascods, so pages of formul will not get a definite result out of loose data." - Thomas Henry Huxley (Evolutionary biologist)
Darwin, a light weight compared to the great Lord Kelvin was quite shaken by Kelvin's authoritatively delivered pronouncements, so much so that he eliminated all mention of specific time scales in the last editions of On The Origin of the Species . In 1869, Darwin wrote to his friend and co-discoverer of natural selection, Alfred Russel Wallace, saying:
"Thompson's (Kelvin's) views on the recent age of the world have been for some time one of my sorest troubles."
Of course, we now know that Darwin was right. The Earth is, in fact, billions of years old and as a result, life has had the time it needed to evolve. Radioactive decay was discovered by Ernest Rutherford in 1902 and applied to the age of the Earth problem by American chemist, Bertram Boltwood in 1907. Boltwood calculated the age of the Earth to be 2.2. Billion years. Still shy of the current number but definitely an improvement over Kelvin's calculations. Further radiometric dating of Earth and moon rocks pegs the number at 4.6 billion years.
The problem of the sun's heat required two new concepts in physics - mass-energy equivalency (E = mc2) and quantum physics. Einstein developed the first in 1905 allowing the conversion of mass to energy and stating that mass and energy are together, conserved, essentially just different states of the same stuff. Energy sits by itself on the left hand side of the equation but on the right side, the speed of light (c = 300,000,000 m/s) is squared which equals 90,000,000,000,000,000 m2/s2. This means that even a tiny bit of mass produces a huge quantity of energy. Deep in the sun's core, temperatures reach 15 million Kelvins (Yep, same guy!) and pressures reach 150 gm/cm3, which is about 15 times the density of lead. These immense pressures keep the sun from collapsing under its enormous mass. They are also responsible for the sun's energy. We now know that the sun's energy is produced through nuclear fusion.. Through nuclear fusion, four hydrogen nuclei (protons) are fused together through a series of reactions, to form a single helium nucleus. Through these reactions approximately 0.7% of the mass of the original four protons is converted into energy. Thankfully, a lot of energy because this "proton-proton reaction", turning four hydrogen nuclei (protons) into one helium nucleus (2 protons and 2 neutrons), does not occur very often. Even within the sun's core, it is very hard to push two protons together with enough force to overcome their electrical or Coulomb repulsion so only the highest speed collisions between protons result in the protons "sticking" together. The glue that finally adheres the protons together is called the strong nuclear force.
Quantum physics provided the means for this fusion, allowing two protons to "fuse" together, overcoming their electrical repulsion, an act that is impossible under the laws of classical physics but allowed occasionally under the rules of the new quantum physics. With these in place, Hans Albrecht Bethe was able to put all the puzzle pieces together to show how four protons (hydrogen nuclei) fuse into a single helium nucleus in the sun's core. This process replicated billions of times a second is how the sun shines. Bethe won the Nobel prize for his work in 1967.
So many stories of great scientific discovery have implications across many other disciplines. The story of the age of the Earth is no exception. When the Earth formed, it must have had something to form in. If the Earth is 4.6 Billion years old, we could not comfortably assert that the universe is younger than that. Evidence for a universe that is only thousands or millions of years old would certainly be cause for alarm.
Edwin Hubble (1889-1953) was an American astronomer who lived around the turn of the 20th century. Hubble spent many hours at the eyepiece of the 100" inch Hooker telescope on Mount Wilson observing faint "nebula". Today we understand these nebula to be distant galaxies, millions or billions of light years away, but the time, no one knew about galaxies outside our own. .These nebula were assumed to be within the Milky Way which was thought to be all that there was to the universe. By observing a certain kind of variable star called a "Cephied" in these nebula, Hubble was able to estimate their distances based on a known relationship between a Cephied's period of pulsation and its intrinsic brightness discovered by Henrietta Levitt . The nebula were determined to be so far away that they were actually galaxies of their own. Hubbles observations measured galactic distances of over 60 million ly. For reference, our Milky Way galaxy is about 100,000 ly in diameter. Since then, progressively more sophisticated instruments including the Hubble Space Telescope have measured galactic distances out to more than 13 billion ly with redshifts in excess of Z=10! WMAP data pretty much peg an age for the universe at 13.7 billion years (ref. WMAP).
Footnote: At the time, the redshift due to recession velocities of distant galaxies was known and had been measured by other astronomers. Hubble and his assistant, Milton Humason combined these measurements with their own galactic distance measurements to estimate the relationship between the distances of these galaxies and their speed moving away from us. Though their initial estimates were in the range of 500 km/s per Mpc (mega parsec = 1 million parsecs = 3,260,000 ly), the number has now been refined many times using other "standard candles" as well as new spacecraft measurements (ref. WMAP) and is known within a very small error to be 71 km/s per Mpc.
So, the universe is enormous and getting bigger as we speak. If light travels at just 186,000 miles per second, it must take light billions of years to travel the distance between far off galaxies and our eyes. This easily allows for a 4.6 billion year old Earth and poses major problems for anyone believing that the time of creation was around 4,000 BC.
Figure 4 - Translation: "A missionary of the Middle Ages tells that he had found the point where heaven and earth touch"
It seems there is always a small minority group of people willing to stake their reputations on disproving almost anything. Remember the Flat Earth Society? There has been enough evidence to choke a cow that the Earth is round for almost as long as there have been cows. Records of calculations of the circumference of the Earth are found on Papyrus leaves as early as 230 BCE. Eratosthenes of Cyrene figured this out in a clever way using only simple geometry and noon day shadows. But, today after umpteen million pictures of a spherical Earth have been returned from space, there is a Flat Earth Society with around 3,000 members all willing to testify to the true shape of the Earth - flat as a pancake. And so, it should come as no real surprise that there are those among us who still believe the Earth is no more than five to ten thousand years old. They cloak themselves in "scientific" arguments, all of which have major flaws, ignore important data, and generally misrepresent findings in a number of creative ways. Since many of them are religious "creationists", this seems appropriate. Some argue incorrectly that there is not enough Helium 4 in the atmosphere to justify an old Earth given known production rates due to radioactive decay. They forget to allow for erosion or loss of Helium in our atmosphere. Some estimate the rate of meteoric dust falling on the Earth each year and apply this knowledge to the surface of the moon, claiming that similar accumulations of meteoric dust on the lunar surface (even allowing for the moon's lower gravity) should have produced a dust layer 100 feet thick, which of course, was not what Apollo or any of the robotic lunar missions have found. In fact, if we use the most current and accurate measurements of meteoric dust infall, we come to a figure more like one foot of dust which matches observations nicely. Other arguments involving the decay of the Earth's magnetic field, accumulations of metals on the ocean floor, and criticisms of contemporary radiometric dating methods have similar flaws.
In science, it is important to address a problem from many angles. It is important to be cautious, critical, and even brutal at times in our assessments to ensure the very highest quality of work. Even giants in the field can and have fallen under the weight of scientific rigor. Somehow, that helps me sleep better at night.
The core of the sun is nearly as dense as lead, and has a temperature of 15 million °C.