After noting the time, restart the process by pressing GO to resume the simulation and press PAUSE when half of the remaining parent atoms
have become daughter atoms í in other words, when about eight billion carbon atoms remain. How does this second time interval compare to
the first interval of time, when the number of parent atoms changed from 32 billion to 16 billion?
Repeat this process once more (or as many times as you like), each time pausing
when the number of parent atoms has fallen in half. Do you see a pattern? You are
measuring the half-life of the material, which is the average time needed for half of
the parent atoms in a radioactive sample to decay. Enter your measurement for the
half-life of carbon-14 by selecting the appropriate amount of years. Press CHECK to
see if you are correct.
In the second simulation, you will use your newly acquired skills at measuring half-
lives to investigate a crime scene. You are an environmental investigator and a
criminal is once again dumping pure samples of a radioactive lead isotope,
, into a vacant lot. Holy ecological disaster!You have been unable to catch the perpetrator in the act, but a security camera
filmed three suspicious-looking characters in the vacant lot at different times. If you
can determine when the radioactive waste was dumped in the lot, you will know
which of these three suspects is guilty.
The factory that creates the waste is cooperating with you. They tell you that the
isotope was pure lead-209 samples that initially contained 192 billion atoms. When
you find the waste, it is about midnight. At midnight, 24 billion lead atoms remain,
which means 168 billion of the lead atoms have decayed into bismuth atoms. In
other words, one-eighth of the original lead-209 is left.
Your mission has three parts. First, determine how many half-lives have elapsed
since the pure lead-209 sample was dumped. If you are having trouble with this
piece of your detective work, return to the first simulation and calculate how many
half-lives it takes for seven-eighths of the carbon atoms to decay.
Second, measure the half-life of lead-209 using a technique similar to what you
used in the first simulation. You have the same tools as you had before.
Third, you can use the evidence from the security camera. The camera filmed Anna
in the lot 6.51 hours before you obtained the sample. Sara was loitering in the area
about 9.76 hours before this time, and a third suspect, Katherine, was filmed there
13.0 hours before the sample was found.
To put this all together: You use the value you determined for the half life of lead,
and multiply that by the number of half-lives that have passed since the lead was dumped. That tells you how long the lead was there, so you
can nail the suspect. To confirm your conclusion, drag the handcuffs in the simulation to the dastardly dumper. The simulation (and perhaps
the suspect’s reaction) will let you know if you are correct.
38.19 - Particle physics and GUTs
With this section, we come to the end of our discussion of the atom and the nucleus. It seems appropriate to take a peek ahead to the current
state of nuclear physics, and to discuss what you and others may be learning in the decades ahead.
Particle physics, also known as high-energy physics, is one of the largest subfields of current physics. Essentially, it tries to answer the
question “What is everything made of?” Ordinary matter is composed of the particles you have encountered so far í protons, neutrons, and
electrons í and in the early 1900s, that seemed to be all that was needed to answer the big question. However, starting in the 1930s, particle
physicists began discovering new, exotic particles that were created in an energy-to-mass conversion during collisions between the known
particles. Several hundred other particles have since been discovered, most of which are unstable. Some of the particles are stable if they are
left alone, but are composed of antimatter, which annihilates ordinary matter upon contact.
Early experimenters relied on cosmic rays (high-energy particles that permeate our galaxy) to initiate reactions. (The exact source of cosmic
rays is still an open question. Stars emit them during intermittent flare-ups, but supernova explosions, when stars die, are thought to be
responsible for much of the cosmic ray output.)
Later, more controlled experiments were carried out in particle accelerators, also known as atom-smashers, where particles are made to collide
with higher and higher energies. Physicists now recognize that many of the heavier particles are made up from smaller building blocks called
quarks. They understand that the smorgasbord of particles is due to the fact that when particles collide, newly-created quarks can combine with
those already present to form systems of bound quarks.
The ultimate goal of physicists is a theory of everything (how is that for ambition?). Historically speaking, great breakthroughs in physics have
often resulted in a simplification of our view of the universe. For example, Newton’s universal law of gravity showed that the laws governing
celestial orbits were the same as those governing the motion of objects falling under earth’s gravity. Maxwell and his generation showed how
electricity and magnetism, long thought to be unrelated, were really aspects of the same force. Physicists optimistically believe that the
universe has an underlying simplicity, and that the number of fundamental forces can be reduced further.
We have discussed gravity, the electromagnetic force, and the strong force. (There is also the weak force which we have not discussed.) Albert
Einstein spent a lot of his working life trying to interpret these forces as different aspects of a single “superforce”. Historically, electricity and
magnetism were united in the 1800s, and in the latter part of the 20th century, the weak force and the electromagnetic force were also joined
theoretically. The goal of further reduction with the ultimate prize of unification still continues today.