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# Quantum Theory of Gravitation

by Ron Kurtus (revised 22 January 2011)

*Quantum Gravitation* (or Quantum Gravity) is an effort to explain gravitation on the atomic and subatomic levels. While the *Theory of General Relativity* explains gravitation for large scale events and the *Law of Universal Gravitation* provides an equation for ordinary situations, neither are able to explain gravitation for extremely small sizes and masses where quantum effects take place.

This type of gravitation is based on *Quantum Mechanics*, which provides a different perspective of matter, energy and space at very small separations. There are several theories that attempt to resolve how gravitation works at the quantum level, as well as to fit within the General Relativity concepts. However, there are also some problems concerning quantum gravitation. This is due to the wide range of quantum theories and difficulties proving them for gravitation.

- What is Quantum Mechanics?
- What are the Quantum Gravitation theories?
- What are some problems with those theories?

This lesson will answer those questions. Useful tool: Units Conversion

## Quantum Mechanics

At the atomic and subatomic levels, classical or Newtonian physics loses its validity and *Quantum Mechanics* (or Quantum Physics) takes over. There are several main concepts that provide changes.

### Energy is discrete

Quantum theory states that physical interactions and exchange of energy cannot be made arbitrarily small. Energy comes in tiny packets called *quanta* (plural of quantum). This helps to explain the properties of electrons in atoms and the relationship of energy to matter.

### Fundamental forces

Another Quantum Mechanics concept is that there are three fundamental forces or interactions between particles of mass. The forces occur through the exchange of virtual particles or particles without mass. Each interaction consists of a quantum packet.

Strong nuclear force comes from the exchange of *gluon* particles between nuclear particles, creating the force of attraction. Weak nuclear force comes from the exchange of *vector bosons*. Electromagnetic force is caused by the exchange of virtual *photon* particles.

### Wave-Particle duality

A third concept that relate to quantum gravitation is that both light and matter exhibit a combination of wave and particle behaviors.

For example, subatomic particles can appear as waves. An electron can have properties of a waveform. Likewise, waves can appear as particles. An electromagnetic wave can be a photon light particle.

## Theories of quantum gravitation

Extending the Quantum Mechanics theories to gravitation is an effort to explain the force at the quantum level. Some concepts try to reconcile quantum gravitation with general relativity.

### Gravitation as fourth fundamental force

One aspect of the *Quantum Theory of Gravitation* is that gravitation is the fourth fundamental force. This would unify all the forces or interactions between particles of matter under one concept.

In such a case, gravitation is the weakest of the fundamental forces, with a strength of only 6*10^{−39} of the strength of the strong nuclear force at a sub-nuclear separation.

However, at separations beyond the sub-nuclear range, the strength of gravitation is much larger than the nuclear forces, which are essentially zero at those separations. But still, gravitation is only 8.22*10^{−37} of the electromagnetic force at the same separations.

### Graviton particles

The Quantum Theory of Gravitation provides an explanation of the mechanism of gravitation that is different from the *Law of Universal Gravitation* and *General Relativity Theory of Gravitation*. It states that gravitation is caused by an exchange of *graviton* particles or quanta between objects.

Transfer of gravitons between two molecules

Also, due to the wave-particle duality, this means that gravitation waves are possible.

### String Theory

*String Theory* is an effort to reconcile Quantum Mechanics and General Relativity into a Quantum Theory of Gravitation.

The theory states that particles of matter are one-dimensional oscillating lines or strings. The mathematics of String Theory describes the fundamental forces into a complete system.

One aspect of String Theory is that up to 12 dimensions are required to describe matter and its interactions.

### Loop Quantum Gravity Theory

*Loop Quantum Gravity* also attempts to reconcile the theories of Quantum Mechanics and General Relativity by quantizing the gravitational field. The theory suggests that space consists of movable tiny loops that can be viewed as an extremely fine fabric.

However, the theory keeps gravitation separate from other fundamental force fields. It also incorporates General Relativity without requiring String Theory's higher dimensions.

## Problems with Quantum Gravitation

There are several problems concerning the theories of Quantum Gravitation.

### Not universally accepted

One problem is that there are so many theories of Quantum Mechanics that none is universally accepted. This, of course, affects the Quantum Theory of Gravitation.

### Does not correspond with General Relativity

Another problem is that Quantum Gravitation does not correspond very well with General Relativity. String Theory and Loop Quantum Gravity Theory make an attempt to find a common ground with General Relativity. However, a major problem with String Theory is that it may be impossible to prove or even disprove. The same may be true of Loop Quantum Gravity Theory.

### Verification extremely difficult

Experimental verification of Quantum Gravitation is extremely difficult, primarily due to the small sizes and weak interactions. Thus far, the existence of gravitons or gravitational waves has never been verified.

## Summary

The Quantum Theory of Gravitation is an effort to explain gravitation on the atomic and subatomic levels. It is based on Quantum Mechanics, which provides a different view of matter, energy and space at very small separations. There are several theories that attempt to resolve how gravitation works at the quantum level, as well as to fit with the General Relativity concepts. However, there are problems in verifying quantum gravitation due to the small forces involved.

Learn more by looking closely at what you see

## Resources and references

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**Top-rated books on Gravitation**

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