It’s a fascinating and exciting field of study simply because very, very small particles behave differently than everything else. Much of quantum mechanics seems counterintuitive and quite frankly, things get a bit weird.
Although quantum mechanics doesn’t classify or look for what subatomic particles exist (that’s the role of particle physicists), basic knowledge of subatomic particles is necessary in order to explain the world of the smallest units discovered.
Atoms were first theorized by a philosopher in ancient Greece. That theory was rejected for over 2,000 years until John Dalton developed his Atomic Theory in 1803. For almost 90 years atoms were thought to be the smallest units of matter in the universe.
In 1897, J.J. Thomson discovered the first subatomic particle, the electron. These particles are extremely small compared to the other parts of an atom. Negatively charged, electron particles form a cloud around every atom.
The nucleus is a very dense region located in the center of an atom. It makes up over 99.9% of the atom’s mass, but only a trillionth of its volume. Ernest Rutherford discovered the existence of the nucleus in 1911.
Protons and neutrons
In 1911, Ernest Rutherford discovered the nucleus of a hydrogen atom, which contains a single proton. In 1932, neutrons were discovered by James Chadwick. Neutrons have no charge (they’re neutral), while protons have a positive charge.
Protons and neutrons are made up of the smallest particles known to man, called quarks. Murray Gell-Mann and George Zweig theorized the existence of quarks in 1964. Their theory was proven in 1968 with the advent of the first particle accelerator.
If we throw a ball dipped in paint through a slit in a wall, we expect it leaves a paint splat on the wall. If we send a ripple of water through a slit in a wall, we expect it leaves a wave imprint on the wall. Add 2 slits.
Throw a ball and choose a slit before it hits the wall. Waves can go through both slits at the same time and leave a distinct pattern on the wall. Throw an electron, however, and things get weird.
Throwing single electrons through 1 slit on a wall produced the expected ball on wall pattern. English physicist Thomas Young first devised the first version of this experiment in 1801 to study the interference of light waves.
Throwing electrons through 2 slits created a wave pattern on the wall even when electrons were fired one at a time. Scientists were baffled at this and set up recording equipment to see what was happening when the electrons passed through the 2 slits.
Whenever scientists observed, the electrons formed the expected ball on wall pattern. To recap, unobserved electrons passing through 2 slits created a wave pattern on the wall. Any sort of observation — no touching or manipulating, just watching – the electrons made the ball on wall pattern.
Superposition explains that electrons are in many states, even opposite states, at the same time. Once the electron is observed, superposition is lost and it chooses a state. Superposition talks about location, spin rate, spin direction, etc. Our example uses colors.
Schrodinger's cat helps explain superposition, although Schrodinger stated this thought experiment only works at a quantum level (cats don’t really behave like that). The experiment: Inside a sealed box are a cat, a bottle of poison, and a hammer that drops and smashes the bottle if radioactivity is detected.
In the colors example, one possible color an electron could be is red. In thought experiment created by Schrodinger, one possible outcome for the poor cat is that the poison has been released and it’s dead.
In our example using colors, the other possible color the electron can be is blue. For the Schrodinger thought experiment, the other possible outcome for the cat is the bottle remains whole and the cat is alive.
Superposition states that electrons exist in both states at once, until they are observed (like in the observer effect). Without observation, the electron is both red and blue. The cat in the box is both alive and dead — both options at the same time.
Once the electron is observed, superposition collapses and one state is observed. In our colors example, the electron is either red or blue. In Schrodinger's thought experiment, once someone opens the box, the cat is either alive or dead.
Entanglement is another strange quantum phenomenon. Scientists discovered that split particles pair up and the action of one particle influences the other particles when one of the particles in the pair is observed. Physicists state that these pairs are entangled.
The basic unit that makes up all light, a photon is always electrically neutral; it has no electrical charge. Photons make up electromagnetic energy, too (like microwaves and X-rays). Physicists split these individual photons into pairs of entangled photons.
Without being observed, each pair of photons remains in superposition. The pairs will have many states, even opposite states, at the same time. In our example with colors, each photon within the pair is both red and blue at the same time.
Observing just 1 of the pair instantaneously changes the state of the other. For example, if each photon was kept in separate boxes and we opened 1 box and observed a red photon, we would discover that it’s paired with a blue photon.
No matter how far apart the paired photons are, even if whole galaxies lie between them, once 1 of the photons is observed, the paired photon takes on the opposite state instantly. This frustrated Albert Einstein, who called this phenomenon “spooky action at a distance.”
The phenomenon of superposition suggests to physicists that the universe itself might not be the only one in existence. Our universe could be one of many possible universes, and in each one, there is another version of us in different states.
The bubble universe theory states that many universes exist and they are inflating like balloons. Each universe is unobservable to the other, some are bigger, some are smaller, and the rules of physics could change between them.
The idea of infinite universes states that there are infinite copies of our universe, each one showing slight variations. For example, in one universe you’re wearing a different shirt, and in another, dinosaurs still rule the Earth as the asteroid missed.