The Subatomic World – It’s a Jungle Out There

In high school, students are typically introduced to the three basic particles that constitute atoms, that being, protons, neutrons, and electrons.  Unless you decide to take physics in college, education of the atom typically stops there.  That gives the impression that these particles are the smallest bits of matter to be found.  Both protons and neutrons consist of even smaller sub-atomic particles.  The electron cannot be broken down any further.  However, unlike the simple models taught in high school, it is not a particle that orbits the nucleus like planets orbiting stars.

Quantum mechanics dictate the properties of sub-atomic particles which behave quite differently from the large objects we can see.  As a result, their behavior can be counter-intuitive as our eyesight is not capable of resolving these particles.  In the quantum world, particles can pop in and out of existence and consequently, tunnel through barriers in a manner large objects cannot.  The Standard Model guides our understanding of this realm.  This model predicts dozens of quantum particles and configurations – the subatomic jungle.  This post will not be a comprehensive going over of that as that would require a Modern Physics course, but will serve to stretch the bounds of your knowledge beyond the simple atomic model.

Protons and neutrons make up the nucleus of an atom.  Protons have a positive electrical charge and neutrons have no charge.  Both protons and neutrons are made of quarks which have a charge that comes in thirds.  Up quarks have a charge of 2/3 while down quarks have a charge of – 1/3.  It takes three quarks to make a proton or neutron.  In the case of a proton, there are two up quarks and one down quark (the charge is 2/3 + 2/3 – 1/3 = 1).  The neutron is made of one up quark and two down quarks (the charge being 2/3 – 1/3 – 1/3 = 0).  Besides the difference in charge, there is a slight difference in mass between protons and neutrons.

Neutrons are slightly more massive than protons.  If the neutron resides in the nucleus, it is stable.  If it is a free-floating particle, the neutron eventually decays into a proton.  During this process, known as beta decay, an electron and an antineutrino is released.  Beta decay often occurs in nuclear reactors.  An antineutrino is the antimatter version of a neutrino.  Neither an antineutrino or a neutrino have electrical charge and their mass is close to zero.  Neutrinos are produced in the nuclear fusion of stars including the Sun.  In fact, each second, tens of billions of neutrinos pass through your body.  These particles interact very weakly with matter and it requires very complex instruments to detect them.

It can take many thousands, and according to some estimates, millions of years for a light photon created in the Sun’s core to reach the solar surface and begin its journey in space.  As neutrino’s interact very weakly with matter, it only takes a few seconds to reach the solar surface.  Thus, the study of solar neutrinos can provide clues pertaining to the current state of the solar core.  Of course, this same property makes it very difficult to detect neutrinos and require specialized instruments.  One such facility is the SNOLAB near Sudbury, Ontario.  The detectors are located 2,000 meters below the surface to shield it from cosmic ray noise.  This is similar to locating a telescope in a dark area to prevent noise from human made light.  Neutrinos can also give an early detection method for supernovae.  As a supernova will release neutrinos before light, detecting these neutrinos can alert astronomers to turn their telescopes to observe the moment light is released from these events.

Like neutrinos, electrons are a fundamental particle.  Unlike neutrinos, electrons have a negative charge.  In neutral atoms, the negative charge of  electrons offsets the positive charge of an equal amount of protons.  In high school, we are taught the model that electrons are point-like particles orbiting the nucleus.  This is a simplified model to start students off in understanding the atom and has provided the misconception that electrons are similar to miniaturized planets orbiting the Sun.  The reality is more complex.  Electrons are smeared into a cloud encircling the nucleus.  The cloud is a probability curve in which the electron exists in all its possible states.    Bizarre?  Welcome to the quantum world.

How would this translate to the large-scale world we can see?  Think of a dice in a box.  Shake the box, which number of the dice is facing up?  In the quantum world, all six configurations exist simultaneously in the box.  That is, until you open the box and the probability curve collapses to the configuration observed.

Helium atom with 2 protons and 2 neutrons in the core. The 2 electrons are smeared in the surrounding orbital cloud. The darker the area, the higher the probability the electron resides in the area.  Heisenberg’s uncertainty principle states that more we know about the position of a quantum particle, the less we know about its momentum (and velocity).  Also, the more we know about a quantum particle’s momentum, the more uncertainty there is about its position.  Thus, the atom is not mostly empty space as we are taught in grade school.  Credit: Wiki Commons

That’s how Niels Bohr saw it and it is referred to as the Copenhagen Interpretation.  To some, this explanation was unsatisfactory and led to Schrödinger’s cat.  Erwin Schrödinger proposed a thought experiment where a cat is placed in a box with a cyanide capsule that would be triggered when a Geiger counter detected a radioactive decay.  The decay had a 50% probability of occurring.  Thus, in the quantum world, the atom exists in both states-one where it had decayed and released radioactivity and the other where it had not.  But what about the cat?  Did it too exist in two states, one dead and one alive?  Worry not, no one has tried this experiment.  It was Schrödinger’s way of pointing out the inconsistencies between quantum mechanics of the atom and the law of relativity which governs how large objects behave.  Others, such as Hugh Everett III, sought another explanation.

In his 1957 doctoral thesis, Everett argued that the universe splits with each possible action.  Thus, in the dice example, once you shake the box, the universe splits into six different universes.  Each universe has the dice with a different number facing up.  This removes the need for an observer to collapse the probability wave.  It’s a fascinating proposal, as this would mean there exists separate universes for each course of action you could have taken in your life.  While many physicists are very enthusiastic about Everett’s work, they have not yet devised a way to test it experimentally as we are unable to observe other universes.  Unless such a way is devised, for now, we have to treat it as a very interesting hypothesis.  The same can not be said about the Higgs boson.

Unlike the particles above that make up matter, bosons transmit the basic forces of nature.  There are four of these forces, electromagnetism, weak-nuclear, strong-nuclear, and gravity.  Photons are particles of light that transmit electromagnetic force.  W and Z bosons transmit the weak-nuclear force that causes radioactive decay.  Gluons transmits the strong nuclear force that binds atomic nuclei together.  It is this force that is released in nuclear weapons.  Gravitons are a speculative boson that would transmit gravity.  To date, we do not have a quantum theory that explains gravity on an atomic scale.  And then there is the Higgs boson, the so-called God particle.

The God particle is a misnomer.  Leon Lederman, who was awarded a Nobel in 1988, referred to the Higgs boson as the Goddamn particle as it was so difficult to detect.  Lerderman’s popular book on nuclear physics published in the early 1990’s was to be titled after the original moniker, but the publisher shortened it to The God Particle.  While the Higgs boson has no religious connection, it is crucial as it imparts the property of mass in atoms.  Mass is often confused with weight.  Mass is constant whereas weight can change.  If you travel to the Moon, your weight will be 1/6th what it is on Earth but your mass will remain the same.  Weight is a measure of the force of gravity on a body whereas mass measures the amount of “stuff” in a body.

Aerial outline of the CERN facilities. Credit: Maximilien Brice/CERN

In 2012, it was announced the Higgs boson was discovered at the CERN Large Hadron Collider (LHC).  The Higgs boson was predicted by the Standard Model and the evidence matched the prediction.  CERN is a consortium of 22 nations and operates by the Swiss-France border.  The LHC was opened in 2008 and is a 27 km ring that accelerates sub-atomic particles close to the speed of light via supercooled magnets.  Besides its many discoveries in particle physics, CERN invented the World Wide Web in 1989 to disseminate its work.  CERN allowed the World Wide Web to enter the public domain in 1993, making the internet boom of the 1990’s possible, not to mention, this blog.

The LHC is the world’s most powerful supercollider.  The Superconducting Super Collider (SSC) that was being built in Texas during the early 1990’s would have dwarfed the LHC.  The SSC would have been a 87 km ring and three times as powerful as the LHC.  Construction on the SSC was halted in 1993.  Several factors conspired to do in the SSC, among them the economy, politics, and cost overruns.  The US economy entered into a recession during the early 1990’s prompting the federal government to look into cost cutting.  Then there was Texas senator Phil Graham, who brought the bacon back to Texas but delighted in nixing projects in other states.  Cancelling SSC was a way of returning the favor.  At the time of cancellation, over $2 billion had been spent and the project was running several billion dollars over its original estimate.

The SSC under construction. Credit: Physics Today

That the SSC had cost overruns is not a surprise.  In any project where technology has to be invented to complete it, there is a large degree of uncertainty with costs.  This is true of the space program as well.  When entering the realm of the unknown, the economics of that kind of project are not really known until completion.  The SSC site now lies abandoned, with over 20 km of tunnels dug.  Had it been completed, it could have opened our knowledge of quantum physics in the same manner the Hubble did for astronomy.  The largest American supercollider, the Tevatron at Fermilab in Illinois, was shut down in 2011 in the aftermath of the global financial crisis.  The next generation of supercolliders is being built in China, set to begin construction in 2020, will be twice the size of the LHC.  To date, there is no American proposal to match these efforts.

While the traces left behind in particle accelerators allow us to deduce the properties of sub-atomic particles, we are unable to see the particles themselves as they are much smaller than lightwaves.  Using x-rays, which have shorter wavelengths, we can see atomic structure in crystallized lattices, but not the particles themselves.  This gets even more problematic when it comes to string theory which posits sub-atomic particles consisting of strings with a length of 1035 meters.  Detecting this is beyond the capability of the LHC and while string theory is impressive in its mathematical formulation, it will remain a hypothesis until a means is found to experimentally verify their existence.  For other properties of sub-atomic particles, we can look into the most extreme environments of the universe.

If a hydrogen atom were the size of Earth, the nucleus would only be a few hundred feet wide with the rest being electron orbitals.  If that’s the case, why can’t we walk through walls?  When atoms are compressed in a smaller volume, electrons are excited to higher energy states and create outward pressure.  This pressure is what prevents you from walking through walls.  It takes a lot of energy to compress atoms.  In white dwarfs, gravity compresses matter to the point where all the available energy states are taken up by electrons.  The intense gravity of a white dwarf, 100,000 times that of Earth, is offset by the outward pressure force created by the energized electrons.  Neutron stars can compress matter even more than white dwarfs.  Formed by the supernova explosion of high-mass stars, these objects crunch electrons and protons to form neutrons – hence neutron stars.  A teaspoon of this material would weigh about a billion tons compared to 5.5 tons for a white dwarf.

The Sun, in about five billion years, will shed its outer layers and form a planetary nebula with a white dwarf at the core.  In a few tens of thousands of years afterwards, the nebula will dissipate leaving the white dwarf.  There is no longer any fusion process when a star becomes a white dwarf, its luminosity is caused by the initial core temperature of 100,000 C.  It takes many billions of years for white dwarfs to cool down.  In fact, more time than the current age of the universe of 13.8 billion years.

Understanding the nature of sub-atomic particles allows us to understand the ultimate fate of the Sun.  It has also allowed us to make many technological advances.  Transistors, lasers, semi-conductors all owe their existence to our understanding of the tiniest particles of the quantum world.  The pure theoretical work of modern physicists in the first half of the 20th Century made possible the world we currently live in.

*Image on top is the remains of neutrino collision at CERN.  The particle tracks represent electron-positron pairs recorded in the particle accelerator.  Credit:  CERN