Long Time Intervals (1960)

how how old is the earth does time extend back before the earth was formed has time always existed not so long ago Bishop user an English clergyman thought that he had determined on the basis of biblical data that the Earth is about 6,000 years old however when we look at the Earth on the basis of knowledge we have gained since then we can readily believe that it is much older than that it's not hard to imagine for instance that it has taken a very long time for the Colorado River to cut a path in the earth a mile deep and here on the top of a mountain we find rock containing the fossil of an ancient sea creature how many years has it taken for the earth to change so much that an ocean floor has become a Mountaintop how many years thousands millions billions in order to find out we are going to have to measure the extremely long intervals of time over which these events took place now when we want to measure time we usually think of a clock a clock is a mechanism that ticks at a known rate with ordinary clocks like these the time interval between picks of a given clock is always the same when we use a clock We compare the time interval we want to measure with the number of times the clock is ticked in this film we will investigate clocks for measuring long intervals of time some of these clocks take regular and others oddly enough tick irregularly we use a clock which ticks slowly and regularly to measure the age of a man for instance the clock we use is the movement of the Earth around the Sun every time it completes one revolution we count one and we call that tick a year now this clock doesn't record the number of ticks that is counted men must keep records of the number number of picks it has made in other words the written record written on paper or carved on Stone is an essential part of this clock by keeping track of the yearly ticks of the Earth's movement around the sun we are able to measure time intervals of hundreds and thousands of years dating back from now for instance we know that the Earth has gone around the Sun about 6,000 times since the first pharaoh ruled in Egypt but further back than that we get lost people didn't keep records how can we measure intervals of time longer than 6,000 years if no one was watching the clock in order to measure time intervals that are longer than the course of human history we need a clock that records its own Tes here is a clock that records its takes each ring in the crosssection of a tree trunk indicates a year of growth this clock ticks off the years and keeps its own record of the number of ticks it has made but even though this tree is one of the oldest living things the time interval of its life is only about 3,000 years since we want to measure longer time intervals than that we need a clock that ticks over longer periods of time but like the tree counts its own ticks here is something that resembles the cross-section of a tree trunk over a long period of time layer after layer of matter has been laid down one on top of the other is there a clock we can read here is there something we can measure was a new layer formed every year no yet there are things here that can be measured a hundred years ago the great geologist Sir Charles LEL measuring time by ciment layers estimated the interval of time since the age of the reptiles at 250 million years he recognized that his estimate was rough but at least he had made a measurement of long time which was based upon scientific observation scientific observation has brought about the discovery of another type of clock ticking away inside the Earth this clock is perhaps the most remarkable of all it ticks irregularly but when averaged over long periods of time it ticks with great accuracy also it counts the number of ticks it is made and it records them in such a way that they can be read this clock enables us to measure time intervals as long as the lifetime of the Earth the clock is radioactivity this material is pitch blend which is a natural uranium oxide uranium is one of several radioactive elements which are found in rocks and minerals we can demonstrate that it is Radioactive by placing it in this counter like most elements uranium consists of a mixture of Isotopes Isotopes are atoms which have essentially identical chemical behavior but with different masses the most abundant uranium isotope has a mass slightly less than 238 times that of hydrogen and we use this symbol to indicate that we are speaking of this particular isotope of uranium about 99.3% of natural uranium consists of this isotope a second uranium isotope is uranium 235 natural uranium consists of about 710 of a percent of this isotope now both of these isotopes of uranium are unstable that is they are radioactive ever so often an atom of uranium 238 changes into the atom of of another radioactive element this new atom changes into the atom of a third radioactive element which in turn changes into another and so forth such a sequence of radioactive atoms is called a Decay chain now not all radioactive atoms are members of a Decay chain but it has been shown that uranium 238 is the first atom of a Decay chain which ends with the stable product Le 206 it's also been determined that uranium 235 is the first atom of another decayed chain which ends with the staple atom lead 207 another isotope of lead now the rate at which the average uranium 238 atom decays into lead 206 is remarkably constant no matter how the uranium 238 is combined chemically no matter if it is dissolved in acid or subjected to high temperatures or enormous pressures the uranium 238 still goes on decaying without variation at its own unchanging rate this is equally true of course for Uranium 235 now here we potentially have two excellent clocks for measuring long intervals of time clocks which tick at an irregular rate but when averaged over a large number of ticks they're extremely accurate they are not disturbed by changes in temperature and pressure and under the proper conditions they record their own ticks the record they make is the number of lead atoms produced let this block represent a sample of pure uranium 238 there's no uranium 235 present and more importantly there's no lead of any kind further let's assume that this block will remain a closed system that is nothing will enter or leave the block from now on now since we know that uranium 238 decays to produce lead 206 we can expect that in the future there will come a time when half of this block will have decayed into lead 206 at that time the ratio of the amount of lead to the amount of uranium will be 12 over 1 12 which equals 1 we can plot this information on a graph we'll call the time interval t and at the time T we have a LED uranium ratio of one the remaining uranium 238 will go on decaying and it will be found that after a second time interval equal to T half the remaining uranium 238 will have decayed leaving 1/4 of the original amount of uranium 238 together with 34 Le 206 this is a lead to uranium ratio ratio of 34 over 1/4 which equals 3 so we plot at time 2T a lead uranium ratio of three after a third time interval T half the remaining uranium 238 will have decayed and we can plot at time 3T a lead to uranium ratio of 78 over 1/8 or 7 as you may have guessed by now the time interval T is called the halflife of the radioactive material the halflife is simply the time interval required for half of any amount of radioactive material to decay in the case of uranium 238 the halflife is about 4.5 * 10 9th years if a smooth curve is drawn through the points we have plotted we can determine the lead uranium ratio at any time along the curve actually is possible to derive the equation for this curve we can also plot the curve for the lead 207 uranium 235 clock this curve is different because the halflife of uranium 235 is shorter than that of uranium 238 namely 7/10 * 10 9th years now let's reverse the process suppose that in a rock such as this there is particular mineral in which the lead uranium clocks are running by this we mean that uranium 238 and uranium 235 are inside the mineral decaying into lead 206 and Lead 207 now we have just established a relationship between the lead to uranium ratio of these two clocks and the length of time they have been running so you see if we can determin the let 206 to uranium 238 Ratio or the lead 207 to uranium 235 ratio then we can determine the length of time the clock has been running inside the mineral and therefore the age of the rock to determine these ratios we need an atom sorder that is an instrument with which we can determine the relative number of atoms of the Isotopes of lead and uranium in the Rock we have such an instrument as a matter of fact without such an atom sorder we would not be able to measure long times in this fashion a particular mineral which is useful for measuring long time by the lead uranium clocks is called ziron zircons are chemically stable and therefore like our model tend to remain closed systems that is almost nothing enters or leaves them also when they were made zircons contained very little lead therefore any lead we find in them today is almost entirely the product of radioactive decay zircons are found in several types of rock this is a ledge of granite in a western mountain range how long ago in the history of the earth were the rocks of these mountains formed [Applause] the granite is composed of crystals formed when hot liquid rock cooled within the Earth's crust fortunately much of the tiny amount of uranium in this rock went into minute crystals of zircon grown at the same time if we can determine the age of the uranium clock at work in these ziron crystals we can learn how old the granite is first we must process the rock to extract the small amount of zircon contained within it the first step in this process is a delicate one now machines take over the small bits of rock are ground a powder then the powder is sorted through sves according to particle size this machine separates the powdered minerals magnetically the separation of the sample is continued with the use of a special heavy liquid which weighs three times as much as an equal volume of water when the powder is mixed with the liquid the lower density mineral particles float and the higher density minerals sink ziron is heavier than most minerals and will be concentrated at the bottom of the flask this device further sorts the particles by delicately distinguishing between their characteristic magnetic properties ziron is less magnetic than most minerals now the particles are finally sorted under a microscope purity of the Zircon sample is necessary for successful reading of the clocks of radioactivity we have separated less than a fraction of an ounce of zircon from 50 PB of granite we can now examine the uranium lead clocks working inside the zircons to do this the zircons must be broken down chemically to extract the uranium and lead the chemical breakdown takes place in this specially designed lead free laboratory the quantities of uranium and Lead which we must must isolate are minute it is important that the laboratory in which we read our radioactive clock be entirely lead free and uranium free except for the samples we bring in we don't want to confuse the lead and uranium in our clocks with any not belonging to the sample we are investigating when the zerons are first brought into the leadfree laboratory they are washed in a dilute acid to remove any lead contamination which they may have picked up previously now the zircons are dissolved in molten borax after the sample is cooled it is dissolved in acid the uranium and Lead are separated individually by special chemical solvents with unusual affinities for these elements hydrogen sulfide is bubbled through a solution containing the lead a precipitate of lead sulfide is obtained the precipitate weighing 5 to 10 millionth of a gram is placed on this wire filament the uranium is concentrated in a solution which is placed upon another wire [Applause] [Music] [Applause] filament this instrument is our atom disorder it's called a mass spectrometer by placing the wire filaments loaded with the samples of lead and uranium inside it one at a time we can determine from this record of the operation of the mass spectrometer the amounts of the various isotopes of lead and the amount of uranium contained in the [Music] sample we calculate in the sample we have just analyzed a lead 206 uranium 238 ratio of 0.361 this ratio of 0 361 or about 4/10 corresponds to an age of about 2 * 10 9th years we calculate the lead 207 uranium 235 ratio to be six and this corresponds also to an age of about 2 * 10 9th years since the two clocks of radioactivity agree as to the age of the sample we have a high degree of confidence in the result the sample is about 2 * 10 9th or 2 billion years old of course we actually use the equations of the curves to get the precise ages with a good deal of accuracy we have read two clocks Each of which ticks slowly enough and long enough to measure a time interval of 2 billion years we use these clocks and others to date rocks and rock formations of many kinds throughout the world how can we use these clocks to measure the age of the Earth the oldest rock in the world yet measured by radioactivity is calculated to be about 3.2 * 10 9th or 3.2 billion years old since this Rock was found in the earth we can say that the Earth is at least 3.2 billion years old thus we have a minimum limit for the age of the Earth now can we find some indication for a maximum limit for the age of the Earth it so happens that we can by making special calculations on the relative abundances of u235 and u238 which were present when the elements were formed and comparing that ratio with their present relative abundances we can arrive at an outside limit of the age of the Earth of 10 * 10 9th years or or 10 billion years we can say then that the age of the Earth lies between 3.2 * 10 the 9th years the age of the oldest known Rock and 10 * 10 the 9th years we could be much more definite about the date if we knew what the isotopic composition of lead was when the earth was formed we could then calculate how long it would have taken for the original lead to have been transformed by the decay of uranium and thorium into modern lead but unfortunately when the earth was formed no one was around to measure the isotopic composition of Le can we measure it now there is a theory that some meteorites which come from outside the earth contain lead which today has the same isotopic composition as the lead that existed when the earth was formed this lead has been isolated from sever meteorites and measured on a mass spectrometer starting with lead of this isotopic composition it is possible to calculate how long it has taken the decaying uranium en thorium to change the isotopic composition of the Earth's original lead such as that found in this meteorite to that which we find in the oceans and rocks today the result of this calculation is 4 . 5 * 10 9th years and so we say that the time interval since the formation of meteorites and the Earth is 4.5 * 10 9th or 4.5 billion years we have actually been able to obtain the same number using several other radioactive clocks on meteorites so we are quite confident in the result how about still longer interval of time how about the age of the elements from which the earth was formed here we have a figure which is more than a guess about 6 billion years how about the age of the universe no figure yet indeed it may not even have an age what about the nature of time we know that we can measure time from very short intervals or short ticks to the slow takes of billions of years but we are not as expert when it comes to explaining just what time is like many Concepts which seem commonplace and simple the concept of time is both subtle and complex indeed we have a great deal to learn about the nature of time for