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Weak Force Definition

The weak force which is also known as the weak interaction is one of the four fundamental forces in the universe. Often it is also called the weak nuclear forces in particle physics. Everything that we observe in the entire universe all the interactions or the phenomenon that we observe starting from the subatomic level to the intergalactic distances can be broken into four fundamental forces, the gravitational force, the electromagnetic force, the strong force, and the weak force. 


Even though its name suggests that it is a weak force but in reality, it is much stronger than the gravitational force (within the nucleus limit it is about 1025 times stronger than the gravitational force). It is known as the weak force because it is comparatively weaker than the strong force or the strong nuclear force. The weak force plays an important role within the nucleus and it is responsible for the beta minus decay within the nucleus. On this page, we will have a discussion regarding the weak force, weak nuclear force, weak interaction, etc.

Weak Nuclear Force

Among all four nuclear forces, the weak nuclear force is highly interesting. The weak force or weak nuclear force is the only fundamental force that is aware of the spin of the particle and can categorise the particles into either particle or antiparticle. 


According to classical physics, the spin of the object is considerably easy, i.e., it says either you can spin the object about an axis in clockwise or counterclockwise directions. But this explanation is contradictory as the axis of rotation will be extending on either side. To overcome this difficulty, quantum mechanics brought the idea of up and down spin which can be used without any misunderstanding. The up spin is denoted by and the down spin is denoted by ↓.


Italian theoretical physicist Enrico Fermi derived a theory in 1933 to explain the process involved in beta decay, which is the process by which a neutron in a nucleus changes into a proton and expels an electron, often known as a beta particle in this context. Enrico Fermi defined a new type of force, later which was also called a weak interaction, that was considered to be responsible for decay, and whose fundamental process was transforming a neutron into a proton, an electron and a neutrino, which was later determined to be an antimatter neutrino, wrote Giulio Maltese, an Italian physics historian, in Particles of Man, an article published in 2013 in the journal Lettera Matematica. 

Weak Force Interaction

In the 1950s scientists tested the interactions between both strong force and electromagnetic forces and realised that both of them are not explaining anything regarding the spin of the object under observation. However, in 1956 two theoretical physicists, Tsung Dao Lee and Chen-Ning Yang studied these with details and experimented with whether the weak force or the weak interactions are capable of explaining the spin of the object. They used the decay of Cobalt 60 via weak nuclear force into Nickel 60, electron and electron antimatter neutrino. 

Co60 Ni60+ e- + \[{v^{-}_{e}}\]


To test whether the weak nuclear force has any explanation for the spin of the particle, they built a very strong electromagnetic field to align the spin of the cobalt nuclei, if the weak force has no effect on the spin then spin of the cobalt 60 before decay and after decay must be same. But the spin of the cobalt was 5, the spin of the Nickel was 4 and the spin of electron and electron antimatter neutrino is \[\frac{1}{2}\] respectively. This clearly indicates that there is a change in the spin of the object which is clearly explaining the fact that there is an effect of weak force on the particle. So, weak force explains the spin of the object and its weak forces involve only right-handed neutrinos.

Why is a Weak Force weak?

Back in the 1930s, as physicists were just deriving the quantum theory of the much stronger electromagnetic force, they came up with an explanation. The photon, the quantum particle that transmits electromagnetism, has no mass i.e., it is a massless element, so it is easy to make photons and transmit them over large distances, sometimes even infinite distances, in theory. 


If the weak force were transmitted by a similar particle, but the weak force involves massive particles, it would be very difficult to make just like that according to the rules of quantum field theory, this explains why the weak forces are considered to be weak and what is their weakness. In fact, there are three such boson particles that carry the weak force i.e., the W+,W- and Z0, existence of these particles were confirmed by physicists at the research Centre CERN near Geneva, Switzerland, in 1983. So, it is referred to as a weak force, it is only 0.001% times the strong nuclear force. 


Did You Know?

  • We know that while studying the weak nuclear forces the parity of the particle plays a very important role. But the weak nuclear force violates the parity and this parity violation is maximal. This was confirmed in the first Wu experiment as well as in all the experiments that involve weak nuclear forces. 

  • The fundamental weak interactions are to fully left-handed fermions or to fully right-handed anti fermions.

  • The weak interactions also violate CPT conservation.

Fundamental Force

In physics, any of the four fundamental or universal forces which are gravitational, electromagnetic, strong, and weak that regulate how things or atoms connect and how some atoms disintegrate is referred to as a fundamental force. All known natural forces may be linked back to these fundamental forces. The sorts of objects that feel the force, the relative intensity of the force, the area over which the force is effective, and the characteristics of the elements that mediate the force are the four criteria used to classify basic forces. Since their impacts on common objects are easily observable, gravity and electromagnetism were known even before the identification of the strong and weak forces.


The gravitational force, which was first defined scientifically in the 17th century by Isaac Newton, operates between all objects with mass, causing apples to fall from trees and determining the motions of the planets around the Sun. The electromagnetic force, which was first defined scientifically in the 19th century by James Clerk Maxwell, is accountable for the repelling of like and attraction of unlike electric charges, as well as the chemical behaviour of matter and light qualities.


When scientists eventually delved into the centre of the atom in the twentieth century, they found strong and weak forces. A strong force exists among quarks, the subatomic particles that makeup protons and neutrons. Despite the extreme opposition of positively bound protons to each other, the lingering impacts of the strong force bind the protons and neutrons of the atomic nucleus together. The weak force is visible in some types of radioactive decay as well as nuclear processes that power the Sun and other stars. Electrons are one of the few fundamental particles that sense both the weak and strong forces.


Standard Model

The standard model is a concept that combines two-particle physics theories into a unified approach to explain all subatomic particle interactions excluding those caused by gravity. Electroweak theory, which explains connections through the electromagnetic and weak forces, and quantum chromodynamics, the concept of the strong nuclear force, are the two aspects of the standard model. Both of these theories are gauge field theories, which characterize particle interactions through the transfer of "messenger" particles with one component of fundamental angular momentum, or rotation. 


The standard model also includes two subatomic particle families that makeup matter and have half-unit spins in addition to these force-carrying particles. The quarks and leptons are these particles, and there are six variations of each, arranged in three "generations" of rising mass in doubles.

FAQs on Weak Force

1. Why are Weak Interactions Weak in Nature?

The other forces such as the electromagnetic forces deal with the or mediate with the help of photons which are massless, no charge, etc., whereas, the weak force mediate with the W and Z bosons, both of them massive object and W boson are electrically charged and a Z boson is chargeless. Because of this, the weak forces are weaker than electromagnetic forces and the strong nuclear forces, but stronger than the gravitational forces.

2. Why Weak Interactions are Important?

Weak interactions are of high importance because they are the only fundamental forces that explain the spin of the object via the beta decay process successively. 

3. What is a weak charge?

A weak charge is a term used in nuclear and atomic physics to describe the weak correlation pairing of the vector Z boson in the Standard Model, which has an influence on symmetry breaking electron dispersion. The overall weak charge of every nuclear isotope is about 0.99 per neutron and +0.07 per proton. Weak isospin, weak hypercharge, and the vector connection of any fermion to the Z boson i.e. the bonding intensity of weak neutral flows are all terms that are occasionally used interchangeably. 

4. Which is the weakest force?

One of four universal forces is the gravitational force. It's the least effective i.e. weakest of the four. It's essentially the attracting force that results from gravitational interactions. The gravitational force among the two entities is directly proportional to the product of their weights. It is inversely proportional to the square of their distance, as per Newton's law of gravity. Gravitational pull is thought to be strong for enormous objects, such as the sun or gigantic planets because their masses are likewise large. Nevertheless, this force is regarded as weak at the atomic level. 

5. Who discovered Weak Interaction?

Enrico Fermi presented the very first weak interaction theory, known as Fermi's interaction, in the year 1933. He proposed that beta decay might be described by a four-fermion contact with a zero-range contact pressure. It's more accurately defined as a non-contact force field with a limited, though extremely small, range. Sheldon Glashow, Abdus Salam, and Steven Weinberg demonstrated that the electromagnetic force and the weak interaction are two facets of the same force, now known as the electroweak force, in the 1960s. It wasn't until 1983 that the presence of the W and Z bosons was explicitly established.

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