The wave-particle duality tells us that everything in this Cosmos can be described in terms of particles. So, maybe we could even describe gravity using particles.
Now these particles are known to have a property called Spin. It basically tells us how the particle looks like when viewed from different directions.
A particle of spin 0 is simply a dot (it looks the same from every direction).
A particle of spin 1 is like a pencil (it looks the same if you turn it around by complete 360 degree).
A particle of spin 2 can be thought of as a pencil which has been sharpened at both ends (if you turn it by 180 degrees, it looks the same).
So basically as the spin of a particle increases, you need to turn it through smaller fractions of a circle to get the same looking particle back. Fairly straightforward.
Now there are some particles that require not only 1 but 2 complete revolutions to look the same. These amazing particles are said to have a 1/2 spin. These particles having spin 1/2 are what make up the matter particles in this cosmos. On the other hand, the particles having integer spin (0, 1, 2…) give rise to forces and interaction between the matter particles.
The matter particles do obey the Pauli’s Exclusion principle which essentially states that no 2 particles can ever have the same position and velocity. This law is too damn crucial because if it was not true, then all matter in this funny cosmos would collapse into a super dense state. The universe would have been a boiling soup of fundamental particles.
But it has been found that the force-carrying particles do not obey this law. First let me explain how matter-matter interactions can be visualized using force carrying particles…
For instance, let’s see how 2 electrons (which are matter particles) can interact. The first electron emits a force carrying particle. The recoil due to this emission causes the electron to experience a force. The force carrying particle then goes towards the second electron and is absorbed. This collision causes a change in velocity of the second particle. So, in simple terms, both electrons are repelled.
As it turns out, in this case, the force carrying particle is a virtual photon. Here’s a Feynman Diagram of electron-electron interaction that I just explained…
The force carrying particles that are exchanged between matter particles are said to be virtual particles because unlike real particles, they cannot be directly detected. Particles of spin 0,1 and 2 do exist as real particles in some circumstances (in the form of light waves,gravitational waves etc). For example in the scenario of electron-electron interaction that I explained above, real photons maybe given off when 2 electrons brush past each other. So it is sometimes possible to detect these light waves using highly sensitive photo-detectors.
As I mentioned earlier, the force carrying particles do not obey Pauli’s Exclusion Principle. This means there is no limit on the number of such particles that can be exchanged over long distances. As the mass of these exchanged particles increases, their production and exchange becomes all the more difficult over long distances. On the contrary, if these particles have no mass, they can be exchanged over long distances.
Now let me talk specifically about gravitational force. We know it is attractive in nature and can even act across long distances. There is no place in this cosmos where this force ceases to act.
From a quantum mechanical perspective, the force between 2 matter particles can be thought of as being carried by a particle of spin 2 called the Gravitons. Since the gravitational force can act over long distances, the graviton (which is a virtual particle) should have no mass of it’s own.
As said earlier, virtual particles cannot be directly detected. However, in case of real gravitons, they would make up the popular Gravitational Waves.
It’s damn hard to detect these feeble waves, but that’s where LIGO serves as the last resort.
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