Around 400 BCE, Leucippus and his pupil Democritus proposed that matter was composed of small indivisible parts called “a tomos,” better known as atoms. They argued that if we take any substance and cut it in half, and repeat this process again and again, eventually we will reach an object so small it cannot be cut in half again.
This hypothesis built upon the work of Parmenides who argued that all existence was a single, all-encompassing and unchanging mass. Democritus imagined a world of many Parmendiean entities. These entities would be made up of different arrangements of atoms, and any sensation relating to these entities was merely the byproduct of those different arrangements.
It would be 2300 years before this idea was properly expanded on. This post will explore how our understanding of the atom has evolved over time.
John Dalton (1766 – 1844)
John Dalton, an English chemist, added to our understanding by determining that elements exist as discrete combinations of matter. Dalton came to this conclusion by studying the physical properties of gases. Dalton observed that the total matter present in a system did not change when a chemical reaction occurred, even when a reaction lead to a change from a solid to a gas. Taking these observations further, Dalton wrote:
Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.
Dalton was questing why different gases dissolved in water in different amounts. The key insight he made was the idea that these gases were made up of different atoms. From this, Dalton formed his atomic theory which states:
- Elements are made of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass, and other properties.
- Atoms of a given element are different from atoms of other elements.
- Atoms cannot be subdivided, created, or destroyed.
- Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
- In chemical reactions, atoms are combined, separated, or rearranged.
Many of these assumptions would eventually be proven false, but they were still extremely important for the future of the science. The question now, was what are those atoms made up of?
Joseph John Thompson (1856 – 1940)
Joseph Thompson, and English physicist, is credited with the discovery of the electron and thus of subatomic particles. By measuring the heat produced from cathode rays, light emitted from gas-filled tubes when given a charge, Thompson was able to estimate the mass of those rays.
Thompson discovered that the mass was about 1000x lighter than hydrogen, which was the smallest bit of matter known at the time. More importantly, Thompson discovered that the mass of the cathode rays were the same no matter what atom they came from. He concluded that the rays were composed of very light, negatively charged particles and that those particles were the building blocks of all atoms.
Thompson knew that overall atoms had a neutral charge so he proposed a “plum pudding” model of the atom. This model can be described as a positive sphere of cake, within which negative plums floated around evenly. Thompson noted:
the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification
Ernest Rutherford (1871 – 1937)
The next milestone in atomic science would be discovered by accident. Earnest Rutherford, a New Zealand physicist and father of nuclear physics, devised an experiment where alpha particles would be shot at a thin film of gold. Alpha particles were produced by the decay of radium and were positively charged so Rutherford expected the particles to fly right through the gold foil. This would have been consistent with Thompson’s plum pudding model. That didn’t happen.
Instead, some of the alpha particles were deflected backwards. Rutherford concluded that the positive part of the atom, which could deflect an alpha particle, must be concentrated in a very small area. Rutherford called this area the nucleus and put forth a model of the atom that consisted of electrons floating in empty space around a positively charged nucleus. Rutherford would go on to discover the existence of protons with similar experiments. A new question arose, how do these subatomic particles behave?
Niels Bohr (1885 – 1962)
Niels Bohr, a Danish physicist, worked with Rutherford to use mathematical models to explain the results of the gold foil experiment.
Bohr observed the frequency of alpha particles being deflected and used this to calculate the most likely position of the electrons within the atom. He formulated the Bohr model which depicted the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus, held together by electrostatic forces. This is often referred to as the planetary model of the atom since it resembles the way planets orbit around the sun. Bohr’s model notes that:
- Electrons in atoms orbit the nucleus.
- The electrons can only orbit stably, without radiating, in certain orbits at certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy levels. In these orbits, the electron’s acceleration does not result in radiation and energy loss as required by classical electromagnetic.
- Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation.
The planetary model of the atom was not new, but classical mechanics predicted the electron would lose energy and rapidly spiral inwards causing the atom to collapse. The key insight that Bohr made was that an electron could drop from a higher-energy orbit to a lower one through the emission of something called quantum energy. This built on the work Max Planck was doing with quantum mechanics at the time. However, this underestimated how crazy the electron problem would turn out to be.
Werner Heisenberg (1901 – 1976)
Werner Heisenberg, a German theoretical physicist and one of the fathers of quantum mechanics, is best known for formulating the uncertainty principle. The uncertainty principle asserts that the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa.
In everyday life, we can successfully measure the position of a macroscopic object, like a car, at a definite time and use that, along with its direction and speed, to predict where the object will be at another time. We do this by assuming the trajectory of the object will not change significantly. At the atomic scale, this assumption does not hold. If we measure the position of an electron at a definite time, we can not say anything about its position before or after that moment.
For example, imagine we launch a photon at a detection screen. The more accurately we measure the position of that electron at any point along its journey, the less accurately we will be able to describe where it will hit. Accordingly, the more accurately we can measure where the electron will hit the detection screen, the less likely we can predict its position at any given moment along the way. This can visualized by adding a barrier with a small opening between the photon and the target. As the opening gets smaller, allowing us to know the photons exact position as it crosses the barrier, the area where the photon hits the detection screen becomes more diffracted.
For Heisenberg, the uncertainty principle meant that the position of electrons could no longer be described as a clean circular orbit. Instead, Heisenberg put forth a new theory that claimed electrons were neither particle nor wave and could only be described in terms of probability. Heisenberg described an atomic orbital as the region within an atom that encloses where the electron is likely to be 90% of the time. This led to our current model of the electron cloud.
This concept is hard to understand and even Einstein originally objected to it. He thought it was absurd that a measurement at one place can immediately influence a measurement at a totally different place. Mathematically, Einstein was proved wrong when John Steward Bell worked out a way to show there are no hidden variables which can explain the consequences of quantum mechanics. The subatomic world was simply uncertain.
So where does all of this leave us? After more than 2000 years working on this problem, we have come to the conclusion that reality is weird. People who believe in freewill like to point that out that if classical physics were correct, the destination of all matter and energy, since the beginning of time, would be predetermined by cause and effect.
Science often leads to philosophical conclusions. We are all made up of 99.99% nothingness, and within that nothingness, reality exists along a probability distribution. If the past teaches us anything though, it is that this understanding will also prove to be naive.