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Weak Force

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Weak Force Definition

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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- + \[\overline{v_{e}}\]


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.

FAQ (Frequently Asked Questions)

Q1. Why are Weak Interactions Weak in Nature?

Ans: 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.

Q2. Why Weak Interactions are Important?

Ans: 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.