What about carbon?
From pages 69-72, The Heavenly Time
Machine
The currently accepted standard model for the
universe
posits that everything began with a so called "big bang" about 15
billion
years ago. The initial big bang universe was very small, very
dense and
very hot. It consisted of energetic matter particles mixed with
radiation
quanta in a hot mixture at equilibrium distribution. New particles
of
matter constantly sprang up out of E mc^2 type
energy-to-matter
conversions, and likewise matter was annihilated back into energy.
This
super small, dense and hot universe expanded very quickly, and it
cooled
as it expanded. Neutrons and protons started to stick around at a
temperature of ten million million (10^13) degrees on the absolute
Kelvin
scale, where the energy of radiation photons equals the rest-mass
energy
of neutrons and protons, and the environment was cool enough not
to turn
all of these back into radiation. This is the beginning of the
element
hydrogen. The energetic high temperature neutrons and protons
knocked into
each other frequently. And when the collisions were just right,
they
stayed together. We now have the formation of helium-4, consisting
of two
protons and two neutrons. In the meantime, the universe was
expanding and
cooling very quickly. In fact, this process proceeded so quickly
that the
temperature and density soon dropped below the point where higher
level
elements could form. The result is that the matter of the early
universe
consisted of about 74% hydrogen, 26% helium (by weight) and very
little of
anything else. So where does all the other stuff come from? It was
cooked
in stars and spread out in supernova explosions.
Let's take a look at what it takes to get a
supernova and
the implications of our being around to talk about it. A heavy
star gets
more compressed and hotter as it burns more and more complex
elements.
Finally, at a temperature of over a billion degrees, silicon is
transformed into iron-56, and the process ends. Soon the iron core
of the
star is compressed with enormous force, as gravity takes over
without the
counterbalance of the heat and pressure from the burning fuel. The
iron
turns into a ball of pure neutrons. But neutrons have a very much
more
compact structure than iron, and the whole core collapses in size.
A ball
about the size of the sun suddenly collapses to the size of a
large
mountain. We now have a neutron star core. The surrounding
material falls
onto the neutron star core at a speed which is an appreciable
portion of
the speed of light, and the shock wave tries to squeeze the
neutron core
even further. But the core cannot really be compressed, and it
bounces
back in an enormous explosion which is the beginning of the
supernova
process.
But now we have a problem. The reaction from
the neutron
core is not strong enough to scatter all the surrounding material,
including the life-giving carbon, into the cosmos. The reverse
shock wave
needs a boost. The boost comes from a heavy flood of neutrinos.
Neutrinos
are very strange particles. They simply don't interact with
matter. So how
come there is an interaction in this case? Everything works out
just
right, provided the weak force is of exactly the right value. The
weak and
strong forces, electromagnetism and gravity are the four basic
forces of
this universe. Change the weak interaction a tiny bit to one side
or the
other, and we would not be here. The same weak interaction is also
involved in determining the ratio of hydrogen to helium in the
early
universe. We need both supernovae and a reasonable amount of
helium, and
the range of weak interaction that permits this is very narrow. So
why is
the universe this way? I do not know.
Now comes the interesting story of that element
of life,
carbon. Helium is a very stable element. It is so stable that for
a while
physicists thought that it was a fundamental particle, and it was
named
the alpha particle. Carbon has a mass number 12, and consists of
three
helium atoms stuck together in a stable configuration.
Unfortunately, two
helium atoms, which make beryllium, are very much unstable. The
stuff
sticks around for less that 10^-16 of a second before
disintegrating. It
takes an additional neutron to make stable beryllium-9. Therein
lies our
problem. A collision of stable beryllium-9 with helium-4 will not
add up
to carbon-12. The unstable beryllium-8 does not last long enough
to permit
any reasonable level of carbon formation by interaction with
helium.
Finally, the odds of three helium atoms hitting each other
simultaneously
in just the right way to stick together as carbon is out of sight.
It
looks like there isn't any way to make carbon. And as noted,
without
carbon there is no life. Now, it is important to understand that
without
carbon there is still a universe. It does not even look very
different
from what we have, on a superficial level. The basic fuel for
stars is
here, and the stars burn very nicely. The only important
difference is
that we are not around to enjoy it. But we know that we are, in
fact,
around. And we know that there is lots of carbon around. So where
did it
come from?
In 1954, Fred Hoyle of Cambridge proposed a
solution. He
suggested that there is a resonance between helium-4, beryllium-8
and
carbon-12. A resonance describes an effect where one gets a big
result
from a relatively small effort. Pluck a string in a certain way
and you
get a big sound for a small pull; do it some other way and it goes
flat.
Taking into account the mass-energy of each nucleus, and the
calculated
kinetic energy of the moving particles based on the temperature in
the
star, Hoyle predicted a hitherto unsuspected energy level, at 7.82
million
electron volts, in the carbon-12 nucleus that would cause a
resonance for
the combined energies of the three elements. This resonance causes
three
helium-4 particles to stay together just a bit longer than usual,
and that
is long enough for these to rearrange themselves into the compact
and
stable configuration of carbon-12. The prediction was tested in
the
laboratory and found to be correct. The question to ask is, what
is it in
the basic laws of the universe that requires this resonance,
involving
three elements and the conditions inside a star, to be there? Why
not have
a universe without carbon? We do not know the answer.
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