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Age of the Earth/Universe

How does a scientist determine the age of the Earth and the age of the universe? It seems that the Earths age as time goes on gets exponentially older and older. I think people were saying it is some 700 million years old about 50 years ago and now it is somewhere in the 3 billions. If you could explain it in laymans terms that would be appreciated.
gpt
Earth age has been known  since pretty early in the discovery of radioactivity.  Once we understood how radioactive decay works, it was very quickly applied to the problem of the age of the Earth, which had been a subject of some debate for a while.

In 1669 Nicholas Steno realised that "geologic features did not develop all at once, but rather by a series of events that look a long time, longer than human history", and in the early-mid 1700's James Hutton developed the theory of uniformitarianism - i.e. that geological processes now are likely to be the same as those in the past, therefore the rates of rock development must be similar.

Early geologists made a concerted effort to find relative ages for things - i.e. one thing is under another and therefore must be older.  By correlating units in different places a geologic column was developed which enabled us to identify the oldest and youngest units on the earth, and the stuff in between.

In the 19th Century Lord Kelvin tried to give an absolute age to the earth, and went about this by treating earth as a cooling body.  he calculated the rate at which earth was losing heat, worked out how hot it was when it probably formed, how hot it is now, and came up with an age of a few million years.  Another approach used by geologists was to measure the thickness of the complete sediment pile on Earth, apply modern sedimentation rates to it, and calculate the minimum time it must have taken to accumulate them all.  This gave an answer of several hundred million years.

The problems with these estimates were that there were a huge number of false assumptions.  The stratigraphy method did not have a complete sediment pile, and it also ignored unconformities (breaks in sedimentation) which can represent tens or even hundreds of millions of years with no sedimentation.  These surfaces are also caused by erosion (the removal of material).  Furthermore, the sediment pile was only calculated from that which was observed at the surface - early geologists had little idea of crustal depth or structure (there was certainly no seismology back then). That meant the sediment pile thickness was waaaay off.

Lord Kelvin's calculation also had a number of bad assumptions, but chief among them was he had no idea about radioactivity, which provides a heat source within the Earth.  The planet is not a simple cooling body.

In 1895 a guy called John Perry actually re-did Kelvin's calculations and instead inferred a convecting mantle.  he came up with an earth age of 2-3 billion years.  Kelvin thought this was ridiculous, and actually ended up reducing his own estimate to 20 million years.

So, in 1896 Henri Becquerel comes along and discovers radioactivity, and by 1908 Lord Rutherford has earnt a Nobel prize for his work on radioactivity.  Bruce Boltwood and he were the first people to try dating rocks using radioactive decay.

In 1904 Rutherford dated his first rock to 40 million years and within a couple of years had dated various rocks to up to 600 million years old.  Boltwood had soon pushed the age of some rocks back to 1.3 billion years.

There was then a bit of a slow down in the science as Europe was at war for the best part of a decade.  In 1927 though, Arthur Holmes published his estimates of earth age at between 1 and 3 billion years.  Geologists were still trying to find older and older rocks, and by 1931 a committee had been set up in the US to properly investigate the age of the earth.

In the 1950's, as we had developed a better understanding of earth formation, it was realised that most meteorites formed back at the same time as the earth did, however, they had spent their lives being chemically unaltered, not effected by weathering, and provided a perfect way to get a  date for the earth formation itself.  That's when we got a planetary age of 4.55 billion years which is, to all intents and purposes, the same number we use today.

The principal of radiometric dating is that an element has several isotopes.  Some isotopes of some elements are unstable and break down.  The rate of this breakdown is fixed and doesn't change with time.  So, for example - the isotope Rubidium 87 decays to form the isotope Strontium 87. Over time, the amount of Rubidium 87 (the parent isotope) in a rock will go down, and the amount of Strontium 87 (the daughter isotope) will go up. Strontium has another isotope - Strontium 86 which is not radioactive.  It is a stable isotope and the amount of it doesn't change through time. Because Sr87 and Sr86 *should* occur at a certain ratio (i.e. about 10% Sr86, 7% Sr87, and the remaining % made up of the other Strontium isotopes Sr84 and Sr88) we can measure the ratios of Sy87 and Sr86 in a rock and look at how much the Sr87 level is different to what we would expect.  That difference is produced by radioactive decay, and is proportional to the age of formation of the rock (i.e. when the elements were locked into crystal structures and unable to move about elsewhere).

There's lots of different isotopic dating systems, all good for dating different amounts of time back in the past (some things decay very quickly so have all been used up after a few years, others decay more slowly so are good for dating older rocks).