Many physical phenomena , such as electricity, magnetism, and thermal conductivity etc., involve electrons in the main role. They are said to be everywhere. However, no one has ever seen an electron. “Seeing” an electron has no physical meaning, as the wavelength of light is much larger than the “size” of the electron. This means that we simply would not be able to see the structure of electrons, even with an extremely advanced microscope. The fact that we cannot observe them directly, raises many mind-bending questions: do electrons really exist, or are they just a figment of our imagination? If they do exist, then how do we know that? Well, electrons are unobservable. We cannot experience their existence with our own human senses. Nonetheless, there is some strong evidence that they do exist and it will be presented in this essay.
In 1838, Michael Faraday found out that if he constructed a glass tube with an anode and a cathode inserted at both ends, and pumped out as much of the air as he could, an electric charge would pass across the tube and creating a fluorescent beam. During the 1870s, an English chemist and physicist Sir William Crookes by applying a magnetic field, was able to deflect the rays and deducted that the cathode ray was negatively charged. [Fig. 1] In 1879 Crookes had proposed that the cathode rays were ‘radiant matter’ ‘ negatively charged molecules. However, in 1896, a British physicist, J. J. Thomson, and his colleagues performed experiments indicating that cathode rays were actually unique particles, rather than waves, atoms or molecules as it was believed earlier. He called those particles “corpuscles”, but later they were referred to as electrons. Later he also showed they were identical with particles given off by photoelectric and radioactive materials.
Later on, by carefully measuring how the cathode rays were deflected by electric and magnetic fields, Thomson was able to determine the ratio between the electric charge (e) and the mass (m) of the rays. Thomson also estimated the mass of cathode ray particle by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested that cathode rays were more than 1,000 times lighter than the hydrogen atom. In 1906, Robert Millikan was able to determine the value of the charge on the electron in his oil drop experiment. [Fig. 2] What Millikan did in his experiment was to put a charge on a tiny drop of oil, and measure how strong the applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out what the mass of the oil drop was and then calculate the force of gravity on one drop, he could determine the electric charge that the drop must have. By changing the charge on different drops, he noticed that it was always a multiple of -1.6 x 10-19 C, the charge on a single electron. Then by using Thomson’s value of e/m, he was able to calculate the exact mass of an electron.
Even though we cannot actually see electrons there are ways to observe how they interact with the macroscopic world. For instance, in 1911 Charles Wilson used his cloud chamber to photograph the tracks of fast-moving electrons. [Fig. 3] He was able to do that because of the fact that in a cloud chamber, these particles ionize air molecules and create trails by condensing supersaturated alcohol vapor. This is similar to the way condensation trails are formed in the sky behind airplanes.
To conclude, if scientists we are able to determine the properties of electrons and there are ways to observe their interactions, there is strong evidence for the existence of electrons. However, in physics ‘ or in natural science in general ‘ we can never be 100% sure of anything. All theories and models must be tested experimentally. If the results do not match the predictions, then the theory is disproved. However, if they do match, that does not mean that the theory is ‘100%’ correct. This only indicates that the results of the experiment coincide with the predictions to a certain degree. This leaves me to say that we only believe in the existence of electrons as we can never be sure that experiments that enabled us to examine and observe electrons were not just coincidences.