Gravity


About Gravity

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Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass/energy.

The most extreme example of this curvature of spacetime is a black hole, from which nothing can escape once past its event horizon, not even light.

More gravity results in gravitational time dilation, where time lapses more slowly at a lower (stronger) gravitational potential.

Gravity is the weakest of the four fundamental interactions of nature. The gravitational attraction is approximately 10^38 times weaker than the strong force, 10^36 times weaker than the electromagnetic force and 10^29 times weaker than the weak force.

As a consequence, gravity has a negligible influence on the behavior of subatomic particles, and plays no role in determining the internal properties of everyday matter (but see quantum gravity).

On the other hand, gravity is the dominant interaction at the macroscopic scale, and is the cause of the formation, shape and trajectory (orbit) of astronomical bodies.

It is responsible for various phenomena observed on Earth and throughout the Universe; for example, it causes the Earth and the other planets to orbit the Sun, the Moon to orbit the Earth, the formation of tides, the formation and evolution of the Solar System, stars and galaxies.

Anomalies and discrepancies

There are some observations that are not adequately accounted for, which may point to the need for better theories of gravity or perhaps be explained in other ways.

  • Extra-fast stars: Stars in galaxies follow a distribution of velocities where stars on the outskirts are moving faster than they should according to the observed distributions of normal matter. Galaxies within galaxy clusters show a similar pattern. Dark matter, which would interact through gravitation but not electromagnetically, would account for the discrepancy. Various modifications to Newtonian dynamics have also been proposed.

  • Flyby anomaly: Various spacecraft have experienced greater acceleration than expected during gravity assist maneuvers.

  • Accelerating expansion: The metric expansion of space seems to be speeding up. Dark energy has been proposed to explain this. A recent alternative explanation is that the geometry of space is not homogeneous (due to clusters of galaxies) and that when the data are reinterpreted to take this into account, the expansion is not speeding up after all, however this conclusion is disputed.

  • Anomalous increase of the astronomical unit: Recent measurements indicate that planetary orbits are widening faster than if this were solely through the Sun losing mass by radiating energy.

  • Extra energetic photons: Photons travelling through galaxy clusters should gain energy and then lose it again on the way out. The accelerating expansion of the Universe should stop the photons returning all the energy, but even taking this into account photons from the cosmic microwave background radiation gain twice as much energy as expected. This may indicate that gravity falls off faster than inverse-squared at certain distance scales.

  • Extra massive hydrogen clouds: The spectral lines of the Lyman-alpha forest suggest that hydrogen clouds are more clumped together at certain scales than expected and, like dark flow, may indicate that gravity falls off slower than inverse-squared at certain distance scales.

  • Power: Proposed extra dimensions could explain why the gravity force is so weak.

Gravity and quantum mechanics

Main articles: Graviton and Quantum gravity In the decades after the discovery of general relativity, it was realized that general relativity is incompatible with quantum mechanics.

It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons.

This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required.

NOTE: Above information has been taken from wikipedia and/or official websites of topics.

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