So.How old is it

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(No Transcript)
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So.How old is it??
SMP March 11,2006 Dating artifacts F. Becchetti,
UM Dept. of Physics
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Lets see, is there some quantity I might measure
now that could tell me when the coffee was poured
i.e. its age ?? Suggestions??
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Yes, temperature might be a good measure of the
age. Lets see what the temperature of the
coffee is now.
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Oops..I still want to drink the coffee so I
probably shouldnt use my familys baby
thermometer ..as I dont where that has been
recently (!) So, can I measure the temperature
of the coffee without touching it??
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Yesusing some principles of physics (black body
radiation) An object at a certain temperature
will have a certain color (I.e. wavelength of
the radiation emitted) depending on its
temperature.
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Recall Blue whitevery hot Yellow hot less
hot Dull red even less hot Non-visble
red(infra-red)much less hot DEMO Light bulb
From Hyperphysics
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HyperPhysics reference
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So I have this neat device called a non-contact
i.r. thermometer that measures temperature based
on the color of the object (actually it
measures the so-called black-body color I.e.
wavelengths which can include non-visible
radiation e.g. infra-red radiation ie i.r. ).
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Thus if I know the present coffee temperature
and if I have a model (and data ) on how the
temperature decreases ( decays ) with time I
can deduce its age. This model will require
a few assumptions
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• The coffee is in a certain type cup (so
• heat loss rate is similar)
• 2)The coffee when poured
• starts at the same temperature
• 3) The coffee is black or with cream
• with the same starting amount in
• the cup
• 4) The temperature decreases at a uniform known
  rate (I.e. temperature decays uniformly with
  time)

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5) The coffee cup is not in a rocket ship
traveling near speed of light (??!!)
Recall that in the Theory of Relativity an object
ages slower when it moves near the seed of
light
This is leads to the so-called Twin
Paradox..lets demo that with some human twins.
DEMO
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One of the assumptions Does the coffee
temperature decrease uniformly with time ? I/ve
measured the decay of the temperature for this
type cup of black coffee and recorded the data
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Heres the data
Hence for the present temperature we can conclude
the age of the coffee (in this case the time
since it left coffee pot)
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The curve we see is a typical exponential decay
curve..the same type we will later see for
radioactive decay curves. The exact type of curve
isnt so critical provided its a uniform decay
with time (why??)
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So for example a curve that looked like this
would not give a unique age as there are two
solutions for the age
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• Thus cooling curves can often
• be used to date objects if a good
• model exists for the cooling.
• Lets see how this can be applied to
• estimate the age (and evolution) of the
• Earth
• Stars
• Universe

NASA photo
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Earth This was attempted prior to1930s before
radioactive dating was available (Lord Kelvin and
others ) Lord Kevin (1897) estimated an age of
40-100 million years
Lord Kelvin ca. 1900 , after whom the Kelvin
temperature scale is named.
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This conflicts with the then recent theory of
Darwin and evolution, which would require a
much older Earth. Thus T. Huxley (a supporter of
Darwin) claimed Kelvin must be wrong ..but
others obtained similar age estimates..(Seems
that no one wanted to disagree with a famous
scientist such as Kelvin ..a common problem to
this day!)
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Kelvins estimate is very model dependent I.e.
depends on the initial temperature of Earth
assumed (molten rock??) and composition of the
Earth, the type of cooling mechanism (black body
radiation??) and assumes no internal heating
sources present. The latter is not valid and his
Lordship underestimated the age of the Earth (by
4 billion years!)
PBS
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Unknown to Kelvin and only discovered in the late
1890s was nuclear radioactivity..which releases
(via Emc2) lots of energy which can be
converted to heat. Hence the Earth is
significantly heated by radioactivity (hence your
water pipes dont freeze in winter).
( Marie Curie coined the term radioactivity and
in 1890s discovered the basic forms of
radioactivity in heavy elements . In the process
she discovered several new elements , eg
radium,polonium,..She won Nobel prizes in both
physics and chemistry for her work. )
AIP
MARIE SKLODOWSKA CURIE
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Including the heating due to radioactivity in
rocksor as we shall see using the radioactivity
itself in the rocks as a measure of age yields an
age for the Earth of several billion
years. Huxley was correct..the Earth is very
old..and makes evolution plausible (to most
scientists at least).
He needs an old Earth.
Charles Darwin
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Stars and Universe We have much better models
for stars since most stars contain relatively
simple elements (hydrogen, helium,..) and the
main source of internal heating (and temperature
history) is known from known nuclear fusion
reaction rates.
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And we have a nearby typical star we can study
(our Sun)
ESA photo
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Depending on their size(mass) some stars will
explode when they become old (supernova), but
some smaller stars (white dwarfs) will simply
cool down with a certain temperature vs time
behavior I.e.a known cooling curve (like my
coffee). Heres some white dwarf stars seen by
the Hubble space telescope (HST)
NASA HST photo
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And like my coffee we can measure their color
and hence temperature but now using a telescope
equipped with a spectrometer
Palomar 200 inch telescope. CalTech photo
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We observe a wide range of temperatures for such
stars , but the oldest ones have an apparent age
(based on a reasonable cooling model) of about 12
billion years! Is this then the age of the
Universe?? Or is the Universe even older (as
might seem reasonable) ?
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Does the Universe itself have a temperature and
if so do we have a cooling model we can apply
to deduce its age??
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Yes! The Universe itself has a black body
temperature. It was discovered by accident in the
1960s at Bell labs by scientists trying to find
the source of unusual radio noise that was
interfering with satellite radio signals.
NASA
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Their apparatus(big radio antenna)
Bell Lab photo
Demo Radio noise
(Wilson and Penzias won a Nobel prize for this
discovery beating out UM grad Prof Wilkinson at
Princeton U. who was actually trying to determine
the temperature of the Universe)
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In this case the color I.e. wavelength of the
black body radiation was in the radio part of the
electromagnetic spectrum corresponding to a very
low temperature
NASA
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The temperature deduced was about 2.7 K ie -270
C (-455 F)..yet it definitely was a
black-body type of spectrum (just like the heated
light bulb )..but shifted far down into radio
wavelengths since the source (Universe) is very
cold (and hence old)
From HyperPhysics
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In fact its the best black body spectrum
known!
NASA-COBE
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Do we have a good model for the initial
temperature of the Universe and how it cools vs.
time??
We must look between bright stars etc to get T
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Yes ! We have the Big Bang model which gives
the time span ( a few minutes) when nuclear
reactions first took place to make the protons
which make up atoms. Hence we know the time and
temperature of the Universe when the primordial
nuclear reactions stopped and atoms became stable
to form stars, galaxies etc
From HyperPhysics
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So in this model we have near t0 the Universe
(actual size)
And at t 3 min or so has T of 109 K ..(I.e.
when nuclear reactions stop)
..and then the Universe expands and cools off
according to known laws of physics for an
expanding gas DEMO Hoberman sphere Cooling gas
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Using this cooling model for the Universe to
reach its present temperature of 2.74 K ..we
deduce an age of 12-14 billion years..in good
agreement with temperature of oldest stars (12
billion years) ..since we can assume the first
stars and galaxies evolved following the Big
Bang itself.
NASA HST
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But as seen earlier we dont have a good cooling
model for the Earth..so what other methods do we
have to find the age of the Earth and how does
its age compare to that of the oldest stars and
the Universe itself ??..
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Hence Is the Earth (and us) part of a first,
second, third,generation solar system?? (and
hence where is the Earth in the time scale of the
evolution of stars and planets?)
How would this impact the composition of the
Earth,,,and life itself ??
NASA
U. Oregon
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The primary method (and probably the most
reliable) for dating objects is the use of
radioactive decay. Lets briefly review nuclear
decay. We will see that the radioactive decay of
nuclei (I.e. decay curves) closely resemble the
cooling decay curve we saw with my coffee!
As declared by US Natl. Academy of Sciences in
1920s
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We can demonstrate this using a type of chemical
decay of atoms which produces light as long lived
atoms in excited states of a chemical compound
decay and in doing so emit light
(chemiluminessence). This is the basis for
light sticks etc . DEMO Light sticks
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Heres typical light output (ie number of excited
atoms decaying per sec ) vs. time.
US Air Force data
Look familiar ?? Yes, these are exponential
decay curves like the cooling curve we/ve seen
earlier.
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Here is a typical nucleus consisting of protons
and neutrons
ACS diagram
This is a stable version of a carbon nucleus,
12C. It has equal numbers of protons(6) and
neutrons(6) 12 which makes it quite stable.
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There is another version of carbon which again
will have 6 protons (as that determines that it
is the chemical element carbon ) yet it will
have 8 neutrons. This is known as an isotope ,
in this case an isotope of carbon called carbon
14 (14C, 14 6 protons 8 neutrons) Is it also
stable??
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No ! It has too many neutrons so that one of the
neutrons wants to become a proton..and it can do
this by emitting and electron in a process known
as beta decay..which is a weak decay
From HyperPhysics
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In a sense this is like a nucleus that is too
hot and wants to cool down to a more stable
nucleus (in this the 14C will become 14N 7
protons 7 neutrons)
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The conversion of a neutron to a proton in 14C
and hence the conversion of 14C to 14N follows a
decay curve similar to the cooling curve weve
seen before (I.e. an exponential decay curve)
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In this case the curve represents the amount of
14C left as a function of time for a given amount
of 14C at some initial time ( say t0). We find
that half the 14C will be gone after 5730 years
(called the half life)..and then another half
gone (I.e. 1/4 of the original) in 11,460 years
(2 half lives) etc
From HyperPhysics
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Hence if we knew the original amount of
radioactive material (say 14C) in a sample and
then could determine the amount remaining at a
later time , we could deduce the age of the
sample.(We will come back to 14C dating
later). Lets look at dating the Earth,Universe
and stars using other radioactive decays.
14C dating will not work as its half-life is only
5730years..and we need nuclei that decay over
billons of years. Are there such radioactive
nuclei we can use ? (Yes) and how is this done
??
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From HyperPhysics
Rutherford in early 1900s proposed using alpha
(4He) decay of heavy nuclei (Uranium etc)
which he and M. Curie had studied and were
nuclei known to have long half lives. He tried
measuring the helium gas in rocks (the decay
product from alpha decay..see above..(Balloon
DEMO ) but this was not accurate (tho gave an
old Earth)
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So to date the Universe we might want to use
Uranium 238 (238U) decay (4.5 billion year half
life). We note that 238U and its decay products
(also radioactive but with shorter half lives)
will eventually lead to a stable isotope of lead
206Pb (82 protons and 124 neutrons)
From HyperPhysics
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Again, we understand the nuclear physics in the
Big Bang and stars that initially makes 238U and
lead..and hence measuring the ratio of
radioactive 238U to 206Pb remaining after a
certain time allows us to deduce the age of the
sample. (We will discuss later how we can measure
the amount of 238U and 206Pb in a sample)
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Using this method we date the Universe as 13/- 1
billion years old..in agreement (within
uncertainties ) with the cooling methods we used
earlier. So how old is the Earth,Moon etc using
these methods??
Earth and moon similar in age? Other planets?
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Using various radioactive clocks in rocks in
particular radioactive 87Rb and 40K seen in old
rocks found on Earth (some in UP of Michigan) we
deduce that the Earth is 4.5/- 0,1 billion years
old.
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From HyperPhysics
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We also can use these techniques to date moon
rocks, rocks from Mars (I have both !) and
meterorites (rocks left over from formation of
solar system that hit Earth)
We find the Earth, moon and planets were formed
within 100 million years or so of each other I.e.
all are about 4.5 billion years old
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Where did I get rocks from the moon and from Mars
?? Aliens selling on eBay ?? Almost, they can
be bought on eBay! They are fragments of
meteorites from the moon and Mars. Many moon
rocks are found in the Antartica and in African
deserts. A few meteorites from Mars are also
found
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Done like this
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This implies that the Solar system is relatively
young (4.5 billion years vs 13 billion years for
the Universe)..and thus the Earth (and us) are
made from recycled atoms..most from older
exploded stars (supernova)
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This is key as only in such old exploding stars
can the heavy elements needed for life (including
oxygen, sodium, calcium, iron etc) be made in
nuclear reactions as these heavy elements must
evolve from fusion of lighter elements
(hydrogen, helium etc)
Some recycled supernova atoms (with a PhD)
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Questions tho Is the physics of radioactive
decay constant with time (or do fundamental
constants change with time perhaps) ?? Can
nuclear decay rates be changed by temperature or
other variables ??
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We can look back in time at exploded stars and
watch nuclear decay happening there. It follows
same decay we see here on Earth tho happened
167,000 years ago. SN1987A
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The light decay we see is due to decay of
radioactive cobalt (56Co) decay T1/2 77 days
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This and other observations of old stars,
galaxies etc indicate that the fundamental
constants of physics do not change with time, and
hence nuclear decay rates are reliable clocks.
.but
www.engg.ksu.edu
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Certain types of rare nuclear decays (involving
atomic electrons captured by a nucleus) can be
slightly altered by temperature etc ( lt 1
change tho)..But this doesnt change to any
significant degree nuclear decays of the type
generally used for radioactive dating
(beta and alpha decay).
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Also recall that a nucleus is very small..If UM
stadium where an atom, then the nucleus of the
atom would be the size of a quarter placed at
the 50 yd line. Its decay thus would not be
influenced by other nearby atoms.
MGoBlue.com
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Likewise even after many half lives (15 shown
here..note semi-log scale vs time), the decay
curve remains exponential and can be extrapolated
for dating
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What about relativistic time dilation ?? Cant
that alter nuclear decay rates??
(Recall that a clock or anything moving near the
speed of light runs I.e. ages slower than the
object when at rest . ..a consequence of Special
Relativity as demonstrated with our twins.
Accurate twin experiment is to use a pair of
atomic clocks..one in a jet plane . Accurate to
1 sec in a million years !)
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Experiment has been done and confirms Relativity
From HyperPhysics
Nice Lab !
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Hence A radioactive cosmic ray traveling near
the speed of light will decay slower (ie live
longer) than the same cosmic ray when
stopped. This is may be responsible for much of
human evolution . How??
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As high energy cosmic rays..such as neutrons..
hit the Earths upper atmosphere they produce
high-speed radioactive sub atomic particles such
as a muon. Normally the muon would decay quickly
(a few microsecs) and not reach us on Earth.
HyperPhysics
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But since they are travelling near the speed of
light their internal clock Ie radioactive
decay, slows down (relativistic time dilation)
..enough that they can reach the Earth. So what??
From HyperPhysics
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A cosmic ray going thru DNA molecules in our body
can cause a break in the DNA (mutation).
Fortunately Nature has provided us with a double
stranded DNA that can repair a break on one
strand of the DNA.
From NASA
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But on rare occasion a cosmic ray muon may
produce a non-repairable double strand break
which can cause a genetic mutation. This is
thought to be one source for mutations
responsible for evolutionary changes in various
life forms, although most breaks do not cause a
change.
You are here
The rate of cosmic rays is fairly constant so the
rate of mutations in a given type of DNA
increases uniformly with time . This can be used
to date DNA and track evolutionary changes in
living things.
Wikepedia
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How long have humans and other life forms evolved
on Earth?
Wikepedia
Since life forms are organic they contain carbon
and we can use the decay of 14C (5730 yr half
life) to accurately trace evolutionary changes
back thousands of years and correlate this with
mutations in DNA.
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Recall we need to know the initial amount of 14C
in a living sample..or e.g. the ratio of 14C to
stable 12C. Fortunately again Nature has been
kind and provided us with a known source of 14C
from cosmic rays
From HyperPhysics
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Thus while living
And then after dying the 14C decays away
From HyperPhysics
(W. Libby received the 1960 Nobel prize in
chemistry for this )
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..so we have 14C in us and are radioactive to
some extent but again while living we have
(I.e very little 14C)
www.orau.org
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• We can measure the amount of 14C in a sample two
  ways
• Measure the betas (electrons) decaying at a given
  time as this is proportional to the amount of
  14C.This requires lots of 14C in the sample (a
  young sample or a large sample)
• Use a mass spectrometer and measure the ratio
  of 14C atoms to 12C directly (best method). Needs
  only a small sample and/or can date very old
  objects ( to gt100,000 yr)

This is a state-of-art 2 million volt mass
spectrometer for 14C dating. NEC Madison WI
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Such devices were recently used to date the cloth
in the Shroud of Torino (thought by some to be
the burial cloth of Jesus..hence ca. 2000 yrs old)
Several groups using 14C dating independently
deduced an age much less than 2000 years for the
cloth and date it to 1300s ..close to the date
when it first appeared (and maybe was made??) in
Europe (but controversy remains due to possible
sources of contamination e.g. presence of
bacteria on the cloth..which would give a younger
than true age)
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Another problem
US and USSR atmospheric nuclear bomb tests in
1950s produced radioactive nuclei in atmosphere
used for dating ( 14C etc) so must correct for
this.
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You are here
But we can accurately date human fossils and
artifacts over several million years
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Also less controversial (to most scientists at
least) is the use of 14C, other radioactive
clocks and DNA mutations to accurately date the
migration of humans and their early relatives
(via bones and artifacts) out of Africa and into
various parts of the world including North
America (1K 1000 years ago)
From Wikepedia
Note possible migrations to NA/SA
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Including their dog companions !
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Not without anomalies tho e.g. was there an
ancient migration to North America from Africa
or Europe before the Clovis site ?? ( The
latter was dated 13,500 years ago and considered
among the first occupied sites in Norh America ).

Main migration to NA
Clovis site
From Wikepedia
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(Maybe here in Michigan also?? If so where? )
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Not discussed Use of magnetic field dating to
trace the evolution of the continents and the
development of science of plate tectonics ..
We can use the magnetic field records to trace
formation (evolution again!) of the continents
Tippy Top demo and can crusher (if time)
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I hope I have demonstrated that physics,
chemistry (and the other basic sciences) can
provide techniques for accurately dating
objectsand answering that age-old question (pun
intended)
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How Old ????
You are here
X
4.5 Billion years old
And
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Thanks to SPS young twins My wife ..the old
twin Physics Dept Lecture Demo staff and you
for coming today
But shes really not very old..
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Finito