Elementary Particles, Forces of Nature, and Quest for the Ultimate Theory

Anand Deo

MBBS final year



Elementary Particles, in physics are the particles that cannot be broken down into any other particles. The term elementary particle is also used more loosely to include some subatomic particles that are composed of other particles. Particles that cannot be broken further are sometimes called fundamental particles to avoid confusion. These fundamental particles provide the basic units that make up all matter and energy in the universe.

Let’s take a ride to inside a matter!


Scientists and philosophers have sought to identify and study elementary particles since ancient times. Aristotle and other ancient Greek philosophers believed that all things were composed of four elementary materials: fire, water, air, and earth. People in other ancient cultures developed similar notions of basic substances. As early scientists began collecting and analyzing information about the world, they showed that these materials were not fundamental but were made of other substances. So, what were they actually made up of?

Near the end of the 18th century, two laws about chemical reactions were popular. The first was the ‘Law of Conservation of Mass’ formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant. The second was the ‘Law of Definite Proportions.’ First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance.

John Dalton studied and expanded upon this previous work and developed the ‘Law of Multiple Proportions’: if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers. For example: Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen. Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 forms a ratio of 1:2. Dalton found that an atomic theory of matter could elegantly explain this common pattern in chemistry. In the case of Proust’s tin oxides; one tin atom will combine with either one or two oxygen atoms.

Dalton also believed ‘Atomic Theory’ (1808) could explain why water absorbed different gases in different proportions – for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in mass and complexity of the gases’ respective particles. Indeed, carbon dioxide molecules are heavier and larger than nitrogen molecules.

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed or simplified any further by chemical means, they can combine to form more complex structures. This marked the first truly scientific theory of the ‘Atom’ (from the Greek word for ‘indivisible’)


In 1896, the British physicist J. J. Thomson, with his colleagues John S. Townsend and H. A. Wilson performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier. Thomson made good estimates of the charge (e) and the mass (m), finding that cathode ray particles, which he called ‘corpuscles’, had perhaps one thousandth of the mass of the least massive ion known: hydrogen. He showed that their charge to mass ratio (e/m) was independent of cathode material. He further showed that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated materials were universal. The name ‘electron’ was again proposed for these particles by the Irish physicist George F. Fitzgerald, and the name has since gained universal acceptance.


After the electron was discovered, people realized that atoms were made up of even smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet and so the ‘plum pudding model’ was put forward in 1904 by Thompson himself. In this model, the atom is made up of negative electrons that float in a ‘soup’ of positive charge (making the whole thing electrically neutral), much like plums in a pudding or raisins in a fruit cake.  In 1906, Thomson was awarded the Nobel Prize for his work in this field. However, even with the Plum Pudding Model, there was still no understanding of how these electrons in the atom were arranged.

The discovery of radioactivity was the next step along the path to building an accurate picture of atomic structure. In the early twentieth century, Marie and Pierre Curie discovered that some elements (the radioactive elements) emit particles, which are able to pass through matter in a similar way to X–rays. Ernest Rutherford, in 1911, used this discovery to revise the model of the atom.

Rutherford overturned Thomson’s model with his well-known gold foil experiment in which he used the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. He demonstrated that the atom has a tiny, heavy nucleus surrounded by negatively charged particles, the electrons while the most of the space is still empty.

There were, however, some problems with Rutherford’s model: for example it could not explain the very interesting observation that atoms only emit light at certain wavelengths or frequencies. Niels Bohr solved this problem by proposing that the electrons could only orbit the nucleus in certain special orbits at different energy levels around the nucleus and that it was the transition between the specific orbits that caused emission of light of only specific wavelengths.


The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms (which he called ‘protyles’), based on a simplistic interpretation of early values of atomic weights.

In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson.

Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table (its atomic number) is equal to its nuclear charge.

In 1917, Rutherford noticed that, when alpha particles were shot into air (mostly nitrogen), scintillation detectors showed the radiation with penetration signatures of typical hydrogen nuclei as a product. Rutherford traced the reaction to the nitrogen in air, and found that when alphas were produced into pure nitrogen gas, the effect was larger. Rutherford determined that this hydrogen could have come only from the nitrogen, and therefore nitrogen must contain hydrogen nuclei. Rutherford proved that the hydrogen nucleus is present in other nuclei. One hydrogen nucleus was being knocked off by the impact of the alpha particle, producing oxygen-17 in the process. Infact, this was the first reported (1919) nuclear reaction (14N + α → 17O + p ).

Rutherford knew hydrogen to be the simplest and lightest element and was influenced by Prout’s hypothesis that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in all other nuclei as an elementary particle, led Rutherford to give the hydrogen nucleus a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles. He named this new fundamental building block of the nucleus the ‘proton’, after the neuter singular of the Greek word for ‘first’. However, Rutherford also had in mind the word ‘protyle’ as used by Prout. Rutherford spoke at. Rutherford was asked by Oliver Lodge for a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. He initially suggested both ‘proton’ and ‘Prouton’ (after Prout). Rutherford later reported that the British Association for the Advancement of Science at its Cardiff meeting (1920) had accepted his suggestion that the hydrogen nucleus be named the ‘proton’, following Prout’s word ‘protyle’.

Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. Oh! You Rutherford, what would chemistry be without you? He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! Also, to make sure that the atom stays electrically neutral, this particle would have to be neutral itself.

In 1931, Walther Bothe and Herbert Becker in Giessen, Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. Since this radiation was not influenced by an electric field, it was thought to be gamma radiation. But, the radiation was more penetrating than any gamma rays known, and the details of experimental results were difficult to interpret. The following year Irène Joliot-Curie and Frédéric Joliot in Paris showed that if this unknown radiation fell on paraffin, or any other hydrogen-containing compound, it ejected protons of very high energy. This observation was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis. In Rome, the young physicist Ettore Majorana suggested that the manner in which the new radiation interacted with protons required a new neutral particle.

On hearing of the Paris results in 1932, neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge was convinced by the gamma ray hypothesis. Chadwick had searched for Rutherford’s predicted particle by several experiments throughout the 1920s without success. Chadwick quickly performed a series of experiments showing that the gamma ray hypothesis was untenable. He repeated the creation of the radiation using beryllium, used better approaches to detection, and aimed the radiation at paraffin following the Paris experiment. Paraffin is high in hydrogen content, hence offers a target dense with protons; since neutrons and protons have almost equal mass, protons scatter energetically from neutrons. Chadwick measured the range of these protons, and also measured how the new radiation impacted the atoms of various gases. He found that the new radiation consisted of not gamma rays, but uncharged particles with about the same mass as the proton; these particles were ‘neutrons’. Chadwick won the Nobel Prize in Physics for this discovery in 1935.

Our atomic model was taking its shape with protons and neutrons making the positive charged heavy nucleus at the center with negatively charged electrons around it while the most of space itself was empty.


Today, scientists have evidence that the proton and neutron are themselves made up of even smaller particles, called quarks. Scientists now believe that the quarks and three other types of particles—leptons, force-carrying bosons, and the Higgs boson—are truly fundamental and cannot be split into anything smaller.

Even these particles of no size or structure that are considered fundamental as of now may not be truly fundamental. They may not be zero dimension point particles as considered now and there could be some other structure inside making them up. We’ve probed as deep as 10^-19 meters into them as nothing smaller has been found. There are problems with them being pointless like the issue of infinite forces as you go near to such particles and then there are speculated ideas to solve the problems. Some of these ideas demand even smaller particle constituting them up. A promising one is strings (from String theory) – the one dimensional particle of order 10^-35m (Planck length; smallest theoretically possible length) in which some parts of the string would be closer than other parts to a test particle and the problem of infinites would be solved. But this theory is still in its initial phase and anything of certain is yet to come. The LHC and CERN guys are back in action. We hopefully can find something interesting soon. Till then, our family of fundamental particles has 4 brothers- Leptons, Quark, force carrying Bosons and Higgs boson.

In the 1960s American physicists Steven Weinberg and Sheldon Glashow and Pakistani physicist Abdus Salam developed a mathematical description of the nature and behavior of elementary particles. Their theory, known as the ‘Standard model of Particle Physics’, has greatly advanced understanding of the fundamental particles and forces in the universe. Yet some questions about particles remain unanswered by the standard model, and physicists continue to work toward a theory that would explain even more about particles.

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