How Old the Earth Is?
This question like many ‘good’ questions goes back to antiquity. Aristotle figured the earth had always been there. He figured this like he figured out most things: by thinking about it..and getting it wrong!
It became apparent as the science of geology progressed that there were layers of rocks and such all over the earth and that the deeper layers had to be older than the upper layers.
Suffice it to say that this sort of thing leads to all kinds of good arguments about technique and quality of samples. But, in the 19th century Benjamin Thompson (Lord Kelvin) came up with 100 million years for the age of the earth...somehow. People who were rooted in Genesis* thought this was way too old while people who understood the slow march of evolution though it was way to young.
Then early 20th century, Marie Curie discovers radioactivity and a method to put an absolute date on the oldest rocks came about. For example...let’s say the half life of Uranium was 100 years (it’s way longer than that but we’ll use 100 for an example). Half life means the time it takes for half of the sample to decay into its daughter element, say, Lead. You can’t predict when a particular nucleus is going to decay but you can confidently say how long it will take for half the sample to decay. So if you started with 50 grams of Uranium after 100 years you’d expect to find 25 grams of Uranium and 25 grams of Lead. After another 100 years 12.5 gramps of Uranium and 37.5g of Lead and so on. So knowing the ratio of Lead to Uranium in a sample will tell you how many half lives have passed since the sample was formed.
Note that usually fossilized bones themselves cannot be dated but you can date the layers and rocks around the fossils and thereby put dates on the fossils themselves. This is how we know for example when the dinosaurs roamed the earth.
This method is tried and true. The decay rates are based on quantum mechanics and do not change over time nor are they affected by local conditions. So finding the oldest rocks (and that’s another story) and even using meteorites and moon rocks which may be as old as the very formation of the solar systems consistently gives us an answer of 4.56 billion years. Notice that sometimes scientists and especially astronomers like to give huge windows to their numbers like ‘somewhere between 100 and 200 million light years! In this case this number has 3 significant figures. It’s 4.56 not 4.55 nor 4.57. The uncertainty is in the 3rd decimal place. So many different scientists doing thousands of repeated measurements (even on moon rocks) come up with a number to 3 significant digits. You should think (whether you dig half-life or not) that this is a very dependable measurement and it is.
This method along with others allows us to build a time line of the earth.
It’s not just that we came up with one number for the age of the earth in a vacuum. Continued research has allowed people to place things and events into a logical time sequence. Not unlike the challenge of wrapping your head around huge distances in the universe, it’s so hard to wrap our heads around huge times as well. Looking at this timeline and the thousands of others like it the thing I’m always amazed at is that, ‘We just got here’. On the scale above the emergence of homo sapiens is such a thin slice at the right end of the line as to be invisible.
The clock analogy is the common way to show this with the age of the earth being a 24 hour clock with humans popping up around 11:58:43. It’s amazing how much damage we’ve been able to wreak upon the earth in so short a time.
*The ‘biblical’ method counts generations (all the begetting) starting
at Adam and Eve and usually comes up with about 5000 years.
Links
Scientific American - The age of the Earth
Twenty four Hour Clock
Dating Dinosaur Fossils
It became apparent as the science of geology progressed that there were layers of rocks and such all over the earth and that the deeper layers had to be older than the upper layers.
Suffice it to say that this sort of thing leads to all kinds of good arguments about technique and quality of samples. But, in the 19th century Benjamin Thompson (Lord Kelvin) came up with 100 million years for the age of the earth...somehow. People who were rooted in Genesis* thought this was way too old while people who understood the slow march of evolution though it was way to young.
Then early 20th century, Marie Curie discovers radioactivity and a method to put an absolute date on the oldest rocks came about. For example...let’s say the half life of Uranium was 100 years (it’s way longer than that but we’ll use 100 for an example). Half life means the time it takes for half of the sample to decay into its daughter element, say, Lead. You can’t predict when a particular nucleus is going to decay but you can confidently say how long it will take for half the sample to decay. So if you started with 50 grams of Uranium after 100 years you’d expect to find 25 grams of Uranium and 25 grams of Lead. After another 100 years 12.5 gramps of Uranium and 37.5g of Lead and so on. So knowing the ratio of Lead to Uranium in a sample will tell you how many half lives have passed since the sample was formed.
Note that usually fossilized bones themselves cannot be dated but you can date the layers and rocks around the fossils and thereby put dates on the fossils themselves. This is how we know for example when the dinosaurs roamed the earth.
This method is tried and true. The decay rates are based on quantum mechanics and do not change over time nor are they affected by local conditions. So finding the oldest rocks (and that’s another story) and even using meteorites and moon rocks which may be as old as the very formation of the solar systems consistently gives us an answer of 4.56 billion years. Notice that sometimes scientists and especially astronomers like to give huge windows to their numbers like ‘somewhere between 100 and 200 million light years! In this case this number has 3 significant figures. It’s 4.56 not 4.55 nor 4.57. The uncertainty is in the 3rd decimal place. So many different scientists doing thousands of repeated measurements (even on moon rocks) come up with a number to 3 significant digits. You should think (whether you dig half-life or not) that this is a very dependable measurement and it is.
This method along with others allows us to build a time line of the earth.
The clock analogy is the common way to show this with the age of the earth being a 24 hour clock with humans popping up around 11:58:43. It’s amazing how much damage we’ve been able to wreak upon the earth in so short a time.
Links
Scientific American - The age of the Earth
Twenty four Hour Clock
Dating Dinosaur Fossils
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