Gravity (fundamental forces)
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Over the last century, the mechanism of operation behind three of the four fundamental forces, strong nuclear, weak nuclear, and electromagnetic, has been explained using the concept of messenger particles. Attempts are currently underway to combine the concept of quantum mechanical messenger particles and the general relativity theory of gravitation into a unified whole. Hence, how the force of gravitation interacts with the other three fundamental forces is an open question.
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[edit] Electromagnetic force
The gravitational attraction between protons is approximately a factor of 1036 weaker than the electromagnetic repulsion. This factor is independent of distance, because both interactions are inversely proportional to the square of the distance. Therefore on an atomic scale mutual gravity is negligible. Hence, the main interaction between everyday objects and the Earth and between celestial bodies is gravity; at this scale matter is electrically neutral.
Essentially, there is an equal number of positively charged particles in the universe to negatively charged particles. For example, there aren't any positively charged planets that zoom into negatively charged planets. Thus, gravity dominates the universe even though it is the weaker force. However, to show the delicate balance of gravity over the electromagnetic force, given two bodies if even there were a surplus or deficit of only one electron for every 1018 protons and neutrons this would already be enough to cancel gravity or in the case of a surplus in one and a deficit in the other, double the force of attraction.
Though the force of gravity dominates the visible macro universe, the main interactions such as fusion between the charged particles in cosmic plasma, of which the sun is composed and which make up over 99% of the universe by volume, are due to the nuclear forces. In terms of Planck units, the charge of a proton is 0.085, while the mass is only 8 × 10−20. From that point of view, the gravitational force is not small as such, but because masses are small.
The relative weakness of gravity can be demonstrated with a small magnet picking up pieces of iron. The small magnet is able to overwhelm the gravitational effect of the entire Earth. Even though gravity is relatively weak, the small gravitational interaction exerted by bodies of ordinary size can fairly easily be detected through experiments such as the Cavendish torsion bar experiment.
[edit] Metrics and fields
In time-dependent gravitational systems, momentum cannot be conserved without a second gravitational force field (eg., a "cogravitational field"). As first predicted by Oliver Heaviside, the cogravitational field relates to the gravitational field much as the magnetic field relates to the electric field. Oleg D. Jefimenko has worked on the generalization of Newton's gravitational theory to time-dependent systems. In his opinion, there is no objective reason for abandoning Newton's force-field gravitational theory (in favor of a metric gravitational theory).
[edit] Quantum mechanics
It is widely believed that three of the four fundamental forces, i.e. the strong nuclear force, the weak nuclear force, and the electromagnetic force, are manifestations of a single, more fundamental force. Combining gravity with these forces of quantum mechanics to create a theory of quantum gravity is currently an important topic of research amongst some physicists.
General relativity is an essentially geometric theory that requires no exchange of particles in its explanation of gravity, whereas quantum mechanics relies on interactions between particles. Scientists have theorized about the graviton, a messenger particle that transmits the force of gravity, for years but have been frustrated in their attempts to find a consistent quantum theory to describe it. Many believe that string theory holds a great deal of promise to unify general relativity and quantum mechanics, but this promise has yet to be realized.
It is notable that in general relativity, gravitational radiation, which under the rules of quantum mechanics must be composed of gravitons, is created only in situations where the curvature of spacetime is oscillating, such as is the case with co-orbiting objects. The amount of gravitational radiation emitted by the solar system is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR 1913+16. It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.
[edit] See also
[edit] External links
- Jefimenko, Oleg D., "Causality, electromagnetic induction, and gravitation : a different approach to the theory of electromagnetic and gravitational fields". Star City [West Virginia] : Electret Scientific Co., c1992. ISBN 0-917406-09-5
- Heaviside, Oliver, "A gravitational and electromagnetic analogy". The Electrician, 1893.
