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Since the ancient world people were thinking about the world they are living in. They tried to find out how everything works and the time which has passed since then.
By now scientist found some different ways for telling the age of things found on earth, of the earth itself and also of the universe.
Things which were alive once
Scientists choose among different kinds of natural clocks. For the period reaching back around 40 000 years they rely on measuring the amount of radioactive carbon to determine how long it has been dead.
Roughly said, all atoms consist of positively charged protons and (except of hydrogen) neutrally charged neutrons in their nucleus. Generally atoms have the same number of neutrons and protons. If there's a difference between them you call them isotopes.
Radioactive isotopes decay at a predictable rate. Carbon 14 (has got six protons but eight neutrons) has got a half-life of 5730 years. That means that every 5730 years half of the isotopes decay.
All living individuals absorb carbon dioxid. As long as they are alive they've a constant level of carbon. When an organism dies the carbon inside starts to decay. By comparing the amount of Carbon 14 to the total amount of carbon scienist can calculate how long it has been since the being died.
Fossils older than 40 000 years have too little Caron 14 left that scientists have to look for other ways to define the age of them.
Around Lake Victoria in Australia are high dunes, piled up over tens of thousands of years. The so-called extinction layer holds a collection of giants. One of these vanished monsters is called Genyornis. It was a 400-pound flightless bird and many of their putty coloured eggshells are found round the lake. There are two different kinds of clock available to date these shells.
The first one measures age by determining how long it has been since a mineral - for example quartz - was exposed to sunlight.
Radioactive atoms surrounding and inside such buried quartz release particles that can knock electrons out of their normal positions. The released electrons sometimes get stuck in a defect in the crystal structure of the quartz. These crystal traps gradually fill up with electrons in a regular, clocklike way. If you know the way of the trapping and if you can count the trapped electrons, you can figure out how long it has been since the quartz saw daylight. (This method is called Optically Stimulated Luminescence)
The disadvantage is, that only a few seconds of sunlight can clear out all the trapped electrons in a grain of quartz, setting the clock back to zero. To avoid this problem hollow stainless steel cylinders get hammered into the sand that holds Genyornis and quickly wrapped in black plastic. Under dim red light in a lab the grains of quartz are put in a machine that fires a beam of photons at them, releasing the trapped electrons. When the electrons settle back into their atoms they shed some energy as light. By measuring that light you can count the electrons that had been trapped and figure out the age of the shells.
The second method is examining the proteins preserved within the shells. (It's called the Amino Acid Racemization)
Amino acids can take either a left-handed or a right-handed form. For reasons still unknown nature overwhelmingly prefers left-handed amino acids.
Once a amino acid is formed it can spontaneously flip over to become right-handed. The rate at which these acids flip isn't as regular as radioactive decay because it depends on temperature - heat speeds this reaction up and cold slows it down.
To determine the age of the Genyornis shells estimated climate changes of the past 100 000 years in Australia were taken into account.
Both clocks determined the same age for the Genyornis - the bird became extinct 50 000 years ago, when there was much water and vegetation.
The reason for extinction or death of a being can't be solved by these methods. Scientist can just guess what happened.
How old is the earth?
In the 18th century scholars tried to find out the age of the world by calculating how much time had passed since the days of Eden by adding up the ages of Adam and his descendants. The result: God created the earth on October 22, 4004 B.C.
Not until the 20th century scientist were able to determine exactly the age of the earth.
Some of Earth's radioactive atoms were blasted out of neighbouring stars in supernova explosions. Because they've been with earth from the start, these radioactive atoms can tell us how old this planet is. You know that uranium 234 has got a half-life of 704 million years and that uranium 238 has one of 4,47 billion years.
Once a rock forms it's uranium starts to decay to lead. If underground water adds lead or uranium (or takes them away) the researchers will end up in the wrong age.
But nature has created a perfect clock for geologists. When Magma cools rugged little crystals known as zircons form. Within them the uranium can steadily decay for billions of years.
Now your only problem is to find such zircons.
The calculated age of the earth is 4,55 billion years.
How old is the Universe?
By now we know that the universe is expanding. That means that galaxies hurtle away from us. We know that light moves by waves. When a star hurtles away the frequency of this waves arriving us decreases and shifts toward to the red end of the spectrum. This progress is known as redshift and by measuring it - and thus the rate at which the galaxies are flying apart - it's possible to figure how long it has been since they were all contained together in one point of infinite density at the moment of the creation of the cosmos.
Today's estimate for the expansion rate indicates that the universe is 13 billion years old.
Surely people won't ever stop thinking about everything happens around them. Maybe the answers are very near but I think we'll never know everything what happens around us and what happened in the past. Maybe that's the thing that makes life fascinating.
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