Beyond The Standard Model: Gravitational Interaction

Most of us have heard the story of how Sir Isaac Newton realized the existence of gravity through a following apple. Though the anecdote may not be true, it nevertheless presented the notion of gravity and its effect on objects in simplified terms. Gravity affects every object is no mystery to everyone.

Gravity is carried out by a theoretical subatomic particle named graviton.

In another word, according to the theory, the exchange of graviton endows gravitational force. For instance, the reason the chair rests on the floor is that the chair exchange these massless spin-2 particles with the earth. Graviton, however, is not similar to higgs boson by all means. Briefly speaking, if the theories were true, weight is caused by the exchange of graviton whilst mass is cause by the interaction with the higgs field. Mass is seen to be an intrinsic property of an object while weight is the result of gravitational attraction.

Gravity, unlike the three interactions described in the Standard Model, seemed to have unlimited range. Gravitational force is also the weakest of all four interactions. Thus, gravity has rather limited and negligible effect upon the subatomic particles. While the general understanding of gravity is based on Newtonian physics, it has corrected some erroneous understanding from the distant past. Developed by Albert Einstein, general relativity claimed the unity of space and time. Massive celestial objects like black hole and neutron stars exert momentous gravitational force that can bend the fabric of space-time significantly. Light passing the fabric of space-time will thus be bent and causing the error due to the path the light has to take.


Since space-time can be influenced by gravitational pull, our sense of time can also be affected by gravity. The phenomenon, called the gravitational time dilation, is rather crucial to the development of modern technology. The phenomenon predicted by the theory states that time is slower when that specific frame of reference undergoes stronger gravitational force. For instance, time will be slower on earth when compared with time in space. This was confirmed by the famous Gravity Probe B. The discrepancy between GPS satellites and ground can be eliminated using Einstein’s theory and thus providing unerring results to scientists working with particle detectors.

Gravity Probe B circling the Earth within the wrapped spacetime fabric

Gravity Probe B circling the Earth within the wrapped space-time fabric


General relativity also predicted the existence of gravitational wave caused by the propagation of wrinkles in the fabric of space-time. The wrinkles, caused by mass in space-time, can radiate in the form of wave with traveling at the speed of light. Though gravitational waves becomes rather difficult to be discovered due to the distance they traveled, scientists involved within the Background Imaging of Cosmic Extragalactic Polarization (BICEPII) experiments nonetheless was able to detected the earliest gravitational waves ever found that was emitted due to cosmic inflation, the rapid expanding of universe shortly after the big bang. Their data and discoveries was released in March, 2014 and was regarded the strongest evidence that could proof the big bang theory.

BICEPII experiment at South Pole

BICEPII experiment at South Pole

Though many studies has been done about gravity, there are still many mysteries remain veiled:

  • Why does gravity only pull while all other interactions are can both attract and repel?
  • Is anti-gravity possible?
  • Why is gravity so weak?
  • Does Graviton exist?
  • How can gravity be used to reconciled relativity with Quantum Mechanics?

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