Ancient Greeks philosopher Leucippus and his pupil Democritus first proposed the idea that all matter consisted of invisible "atomos," or atoms, as we know them today. They believed these atomos could be divided into smaller and smaller particles until they reached a point that they could no longer be divided. Although they couldn't see the particles, Leucippus and Democritus tapped into a fundamental truth about our existence: The universe is made up of atoms and these atoms are responsible for life on Earth.
After considerable research and experimentation, we now know that atoms can be divided into subatomic particles -- protons, neutrons and electrons. Held together by electromagnetic force, these are the building blocks of all matter. Advances in technology, namely particle accelerators, also known as atom smashers, have enabled scientists to break subatomic particles down to even smaller pieces, some in existence for mere seconds. Subatomic particles have two classifications -- elementary and composite. Lucky for us, the names of categories can go a long way in helping us understand their structure. Elementary subatomic particles, like quarks, cannot be divided into simpler particles. Composite subatomic particles, like hadrons, can. All subatomic particles share a fundamental property: They have "intrinsic angular momentum," or spin. This means they rotate in one direction, just like a planet. Oddly enough, this fundamental property is present even when the particle isn't moving. It's this spin that makes all the difference.
Take a dive into the infinite and learn a little more about the world around us.
Fermions are subatomic particles that contain a one-half integer spin. They can be elementary or composite. Electrons, protons and neutrons are all fermions. Fermions obey the Pauli exclusion principal, which is just one way they are different than bosons (another particle discussed later in this article). First proposed by Pauli Wolfgang in 1925, this principle states that no two fermions in a single atom may exist in the same time and space. Why is that important to know? Because that concept -- that no two fermions can exist at the same time in the same space -- is a fundamental property of all basic matter. Without this principal, and without fermions, electrons would collapse to the same exact energy states, resulting in all chemical elements having the same exact properties.
Quarks and leptons, along with most composite particles, also fall in to the fermion category. Let's dig a little deeper by starting with the quark.
Matter is comprised of protons and neutrons; and protons and neutrons are each made up of three quarks. So what exactly is a quark? Quarks are one type of elementary subatomic particle with a few very unique features. Quarks are the only subatomic particles to undergo all four fundamental forces: electromagnetic force, gravitational force, strong interaction and weak interaction. They also like to be part of the crowd and therefore are never found alone. Commonly called "flavors," the six types of quarks are usually regarded in pairs: up/down, charm/strange and top/bottom. Unlike protons and electrons with their positive and negative charges, respectively, quarks have a fractional electric charge. This means they have either a -1/3 or +2/3 of the charge carried by the proton. Quarks also have a color charge. Although it would be cool if they flashed brilliant arrays of reds, blues and green, this term refers to its three charge aspects -- positive, negative or neutral. As with everything in life, quarks have an opposite. For every quark, there exists an anti-quark.
Quarks combine to form hadrons, which we'll explore a little later on. In the meantime, let's take a look at leptons.
Leptons, which are also fermions, are elementary particles. They are a fundamental component of all matter. Unlike quarks, which always stay in a crowd, leptons enjoy being solitary. There are six types of leptons, and in terms of charges, they are split smack dab down the middle: Three have charges and three don't. After that straight-forward division, things get a little more complicated.
To most people, the electron is probably the most familiar of the charged leptons. The electron was the first lepton identified. That accomplishment belonged to a team of British physicists in 1897. Muon and tau are a little less well-known. Like electrons, the muon and tau leptons are charged. However, they both have quite a bit of mass. These will often decay quite rapidly or transform into something else like a lighter lepton or quark. Like quarks and their anti-quarks, each charged lepton has a charge-less antineutrino. The neutrinos are much smaller than their charged counterparts. To help them understand lepton decay, scientists created three lepton families:
- Electron and electron neutrino
- Muon and muon neutrino
- Tau and tau neutrino
One particle of a decaying lepton must go into its neutrino while the others can transform into something else like a quark or anti-quark. However, only tau have enough mass to decay into a hadron. Let's move on to discuss hadrons.
Deriving its name from the Greek word for "stout," hadrons are composite subatomic particles formed when quarks combine and are held together by a strong force. They have mass, a net integer charge and live inside the nucleus. Protons and neutrons are two commonly recognized subatomic particles that are also hadrons. With the exclusion of protons, all hadrons undergo partial decay. They transform into other elementary particles.
Hadrons are divided into two groups or classes: baryons and mesons. What distinguishes these classes from each other is their respective quark allotment. Baryons are made up of three quarks -- two up and one down. Mesons, on the other hand, consist of one quark and one anti-quark. Currently, research is being done into the possibility of a hadron composed of five quarks. Also known as the pentaquark, this new form of atomic matter lives less than one hundredth of a billionth of a billionth of a second before decaying. Suffice to say, it's hard to observe.
Speaking of the hypothetical, there is the infamous and elusive Higgs boson. Intrigued?
Bosons are groups of either elementary or composite subatomic particles with a full integer spin. Unlike fermions, bosons obey the Bose-Einstein condensation, which states that when the gas of an atom is cooled to its lowest state, the atoms gather in that lowest state, merging into a single wave. Several can occupy one quantum state at a time. To put it another way, these atoms can act like one giant superatom.
Bosons are quite famous for a hypothetical entity known as the Higgs boson. Its very existence has been at the heart of physics debates for more than 40 years. Dubbed "God's particle," it is believed to be the last missing particle -- or set of particles -- from the Standard Model. Proving its existence would answer one of life's greatest mysteries: What exactly determines mass? Many scientists believe that when a particle moves through the Higgs boson, a small distortion is created. In theory, it's the distortion that gives this passing particle its mass. The discovery of the Higgs boson could also answer a little question about gravity, namely why it behaves the way it does. In the last several years, great advances in technology have been edging researchers closer and closer to the answer. The CERN's Large Hadron Collider (i.e. atom smasher) is one tool that is being employed to ascertain once and for all if the elusive Higgs boson is real or a product of wishful speculation.
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