- Everyday sense: the force which causes objects to fall onto the ground.
- Newton's laws: how gravity keeps the Solar System together.
- Einstein's theory of general relativity: the role of gravity in the universe.
Some physicists think gravity is caused by gravitons, but they are still unsure.
Weight vs massEdit
In everyday talk, we say things fall because the Earth's gravity pulls on them. We talk as if our weight was a "given". Actually, weight changes when the pull of gravity changes. The Moon is much smaller and the pull of gravity on the Moon is about 1/6th that of Earth. So any object on the Moon weighs 1/6th of its weight on Earth. What does not change is the amount of matter in an object. That is called its mass. On Earth, mass and weight are the same for most purposes, though a sensitive gravimeter can detect the difference. The difference can be very different on another world such as the Moon.
From this we learn two things.
- The weight of an object is variable; its mass is constant.
- The pull of gravity varies according to the mass of an object. The Earth pulls more strongly than the Moon. A person also exerts a gravitational pull, but it is so tiny it can be ignored for all practical purposes.
Gravity vs gravitationEdit
These words mean almost the same thing in everyday use. Sometimes scientists use "gravity" for the force that pulls objects towards each other, and "gravitation" for the theory about the attraction.
History of gravitational theoryEdit
According to one of this students, Galileo did a famous experiment about gravity where he dropped balls from the Tower of Pisa. He later rolled balls down inclines. With these experiments, Galileo showed that gravitation accelerates all objects at the same rate regardless of weight.
Johannes Kepler studied the motion of planets. In 1609 and 1616 he published his three laws governing the shape of their orbits and their speed along those orbits, but did not discover why they moved that way.
In 1687, English mathematician Isaac Newton wrote the Principia. In this book, he wrote about the inverse-square law of gravitation. Newton said that the closer two objects are to each other, the more gravity will affect them. His theory about gravitation was used to predict the existence of the planet Neptune based on changes in the orbit of Uranus.
Newton's theory was later used to predict the existence of another planet closer to the Sun than Mercury. When this was done, it was learned that his theory was not entirely correct. These mistakes in his theory were corrected by Albert Einstein's theory of General Relativity. Newton's theory is still commonly used for many things because it is much more simple to work with than the theory of General Relativity and is accurate enough for many uses.
Why does the Earth not fall into the Sun? The answer is simple but very important. It is because the Earth moving round the Sun is in a dynamic equilibrium. The speed of the Earth's movement creates a centrifugal force which balances the gravitational force between the Sun and the Earth. Why does the Earth continue spinning? Because there is no force to stop it.
In general relativity there is no gravitational force deflecting objects from their natural, straight paths. Instead, gravity is seen as changes in the properties of space and time. In turn, this changes the straightest-possible paths that objects will naturally follow. The curvature is, in turn, caused by the energy–momentum of matter. Spacetime tells matter how to move; matter tells spacetime how to curve.
For weak gravitational fields and slow speeds relative to the speed of light, the theory's predictions converge on those of Newton's law of universal gravitation. Newton's equations are used to plan journeys in our Solar System.
General relativity has a number of physical consequences.
Gravitational time dilation and frequency shiftEdit
Gravity influences the passage of time. Light sent down into a gravity well is blueshifted, whereas light sent in the opposite direction (i.e., climbing out of the gravity well) is redshifted; collectively, these two effects are known as the gravitational frequency shift.
Light deflection and gravitational time delayEdit
General relativity predicts that the path of light is bent in a gravitational field; light passing a massive body is deflected towards that body. This effect has been confirmed by observing the light of stars or distant quasars being deflected as it passes the Sun.
Closely related to light deflection is the gravitational time delay (or Shapiro delay), the phenomenon that light signals take longer to move through a gravitational field than they would in the absence of that field. There have been numerous successful tests of this prediction.
Gravitational waves are ripples in the curvature of spacetime. They move as a wave, travelling outward from the source. Einstein predicted them in 1915 on the basis of his theory of general relativity. In theory, gravitational waves transport energy as gravitational radiation. Sources of detectable gravitational waves might include binary star systems composed of white dwarfs, neutron stars, or black holes. In general relativity, gravitational waves cannot travel faster than the speed of light.
On February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves, originating from a pair of merging black holes using the Advanced LIGO detectors. On June 15, 2016, a second detection of gravitational waves from coalescing black holes was announced. Besides LIGO, many other gravitational-wave observatories (detectors) are under construction.
- Duncan, Tom. 1995. Advanced physics for Hong Kong: I. Mechanics & Electricity. John Murray.
- Barbour, Julian B. and Herbert Pfister 1995. Mach's principle: from Newton's bucket to quantum gravity. Birkhäuser, p. 69. ISBN 0-8176-3823-7
- Eriksson, Ingrid V. 2008. Science education in the 21st century. Nova Books, p. 194. ISBN 1-60021-951-9
- Wald, Robert M. 1992. Space, time, and gravity: the theory of the Big Bang and black holes. Chicago: University of Chicago Press, ISBN 0-226-87029-4
- At least approximately. Poisson, Eric 2004. The motion of point particles in curved spacetime. Living Rev. Relativity 7, retrieved 2007-06-13 
- Wheeler, John; Ford, Kenneth 1998, Geons, black holes, & quantum foam: a life in physics. New York: Norton. ISBN 0-393- 31991-1
- Wald, Robert M. 1984. General relativity. University of Chicago Press. ISBN 0-226-87033-2
- Rindler, Wolgang 2001. Relativity: special, general and cosmological. Oxford University Press, 24–26. ISBN 0-19-850836-0
- Misner, Charles W; Thorne, Kip. S. & Wheeler, John A. 1973. Gravitation. W.H. Freeman, ISBN 0-7167-0344-0
- Kennefick, Daniel 2005. Astronomers test general relativity: light-bending and the solar redshift. In Renn, Jürgen (ed) One hundred authors for Einstein. Wiley, 178–181. ISBN 3-527-40574-7
- Ohanian, Hans C. & Ruffini, Remo 1994. Gravitation and spacetime. Norton, table 4.4 on p. 200. ISBN 0-393-96501-5
- Stairs, Ingrid H. 2003. Testing general relativity with pulsar timing. Living Rev. Relativity 6. 
- Will, Clifford M. 2006. The confrontation between general relativity and experiment. Living Rev. Relativity. 9, 3.
- Finley, Dave. "Einstein's gravity theory passes toughest test yet: Bizarre binary star system pushes study of relativity to new limits". Phys.Org.
- Gravity Probe B experiment The Einstein website from Stanford University
- Gravity for kids (useful Q & A)
- How stuff works: How does gravity work?
- NOVA - PBS NOVA. Galileo's experiments
- Gravity - Kepler and Newton: excellent summary
- Newton's Law of Universal Gravitation on Project PHYSNET
- PhysOrg.com. Alternative theory of gravity may explain large structure formation—without dark matter