This page contains basic information and FAQs about the force we call gravity. Gravity plays an important role in our universe. Scientists still have a tough time understanding what this force is. This remains one of the greatest challenges of 21st Century science.

My book, Patterns in the Void also explains some of the exciting, and mysterious, aspects of the nature of gravity.

Written by NASA Astronomer Dr. Sten Odenwald

Astronomy Cafe

Gravity Basics

Gravity is an always-attractive force that acts between particles of matter. (It can also act between quantities of energy!) More technically, it has an infinite range through space, and weakens as the inverse-square of the distance between bodies.[MORE] All theoretical and observational studies are completely consistent with the idea that it travels no faster than the speed of light - and no slower. [MORE] Many physicists are firmly convinced that gravity is a cousin to the other forces in Nature that we know about including electromagnetism. The exact, mathematical, way to show this unity - called Superstring Theory - remains experimentally untested. According to some skeptical physicists, this theory may be permanently untestable. [MORE]

It doesn't take a rocket scientist to remind you that humans have always known about it! Its first mathematical description as a 'universal' force was by Sir Isaac Newton in 1666. Newton's description remained unchanged until Albert Einstein published his General Theory of Relativity in 1915. Ninety years later, physicists, such as Edward Witten, Steven Hawkings, Brian Greene and Lee Smolin among others, are finding ways to improve our description of it to accommodate the strange rules of quantum mechanics. Ironically, although gravity is produced by matter, General Relativity does not really describe matter in any detail - certainly not with the detail of the modern quantum theory of atomic structure.

Perhaps the most unusual thing about gravity we know about is that, unlike the other forces of nature, gravity is intimately related to space and time. In fact, space and time are viewed by physicists, and the mathematics of relativity theory, as qualities of the gravitational field of the cosmos that have no independent existence. Gravity does not exist like the frosting on a cake, embedded in some larger arena of space and time. Instead, the 'frosting' is everything, and matter is embedded and intimately and indivisibly connected to it. If you could turn off gravity, it is mathematically predicted that space and time would also vanish![MORE]

Einstein's theory of General Relativity, published in 1915, is our most detailed mathematical theory for how gravity work, With it, astronomers and physicists have explored the origin and evolution of the universe, its future destiny, and the mysterious landscape of black holes and neutron stars. General Relativity has survived many different tests, and it has made many predictions which have been confirmed. So far, after 90 years of detailed study, no error has yet been discovered in Einstein's original, simple theory. Currently, physicists are exploring two of its most fundamental predictions: The first is that gravity waves exist and behave as the theory predicts [See LISA and LIGO]; the second is that a phenomenon called 'frame-dragging' exists around rotating massive objects [see Gravity Probe-B]. Other tests focus on the laboratory-scale measurements of the force of gravity to look for signs of 'extra dimensions'. Data from any of these experimental studies will greatly improve our understanding of gravity, and will show us how to go beyond the mathematics of General Relativity to create an even-better theory.

Theoretically, gravity waves must exist in order for Einstein's theory to be correct. They are distortions in the curvature of spacetime caused by accelerating bodies, just as radio waves are produced by accelerating charged particles. Gravity waves carry energy and travel at light-speed. Observationally, they have not been directly detected yet. But indirectly, astronomical bodies such as orbiting pulsars have been discovered that are losing energy by gravity waves at exactly the predicted rates. [MORE]

Astronomers have also detected the 'frame-dragging' phenomenon in X-ray studies of distant black holes. As a black hole (or any other body) rotates, it actually 'drags' space around with it. This means that you cannot have stable orbits around a rotating body, which is something totally unexpected in Newton's theory of gravity.[MORE].


Frequently Asked Questions

 The Force of Gravity:

  1. How does a magnetic field differ from a gravitational field?
  2. Do we really know how gravity and magnetism operate?
  3. Can gravity be simulated using electromagnetic forces?
  4. How are electromagnetic fields and space-time related, and can electromagnetic fields exert a force on space- time?
  5. If the force of gravity were reduced 50 percent, how would this change stars and the universe?
  6. How much of a charge excess would be needed in the universe to produce detectable non-gravitational motions?
  7. Is anti-gravity possible?
  8. Are all the postulates in General Relativity accepted or does there exist room for alternatives to General Relativity with different outcomes?
  9. Does anyone really understand general relativity well enough to have an intuitive grasp of the universe?
  10. How do you really know that general relativity stops working inside black holes?

 Speed of Gravity and Light:

  1. If nothing can travel faster than light, why does gravity take affect instantly?
  2. Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us?
  3. Can gravity alter the speed of light?
  4. When light falls in a gravitational well, does its speed exceed 300,000 km/sec?
  5. Is light conducted by space, or does it travel through it?
  6. Can you go faster than the speed of light by altering space in some way?

Gravity and the Nature of Space:

  1. Is the empty space inside particles the same kind as there is inside atoms?
  2. Does the physical vacuum produce a pressure that can be measured?
  3. Do other dimensions exist?
  4. What are the '10 dimensions' that physicists are always talking about?
  5. What is the simplest evidence that there are more than 4 dimensions?
  6. Where does the energy come from that produces virtual particles?
  7. Is there a 'smallest possible size' to space beyond which it cannot be further divided?
  8. What is a space anomaly?

Gravity Waves:

  1. What binary star is producing gravity waves?
  2. Are there any other astronomical systems where general relativity can be tested other than the Hulse-Taylor binary pulsar?
  3. Don't objects steadily evaporate over time by emitting gravity waves?
  4. What does the equation look like that shows how gravitational radiation is lost from the binary pulsar system?
  5. Does the star DI Herculis prove that general relativity is breaking down?

 Nature of Gravity:

  1. What is the substance of gravity?
  2. Do we really know how gravity and magnetism operate?
  3. Does the physical vacuum produce a pressure that can be measured?
  4. Where does the energy come from that produces virtual particles?
  5. What exactly is a sub-atomic particles structure like?
  6. What is Grand Unification Theory?

 Odds and Ends:

  1. What prevents densities higher than the so-called Planck Limit from happening?
  2. How do you reconcile classical quantum mechanics with general relativity?
  3. Are all the postulates in General Relativity accepted or does there exist room for alternatives to General Relativity with different outcomes?