What is General Relativity?

Question

Now, before you start to lecture that I need to do my own homework, know this: this is out of my own curiousity! I took physics in high school, but the college I'm attending doesn't offer physics courses, so I'm left in the dark of some of the more advanced topics. I love to learn, and am fascinated by physics, and this topic has always intrigued me! I'd appreciated only serious answers.

Answer 1

General Relativity is a theory about gravity at large distance scales (typically, we're talking about hundreds of thousands of miles to light-years, which is how far a beam of light travels in a year). It was published by Albert Einstein in 1915, 10 years after he published Special Relativity.

General Relativity describes gravity not as a force, but as a manifestation of curved space and time. In this theory, spacetime is treated as a 4-dimensional manifold (or in other words a 4-dimensional geometric shape) which is curved by the presence of mass, energy, and momentum (or stress-energy) within it. The motion of objects being influenced solely by the geometry of spacetime (also called inertial motion) occurs along special paths called timelike and null geodesics of spacetime. Typically, any object falling to the ground unobstructed would be following such a path, as well as any orbital motion of planets, moons, and stars.

After Einstein published General Relativity, he and other physicists predicted the following based on the results of their calculations using Einstein's general relativity field equations:

1. Gravitational redshifting of light: The frequency of light will decrease (shifting visible light towards the red end of the spectrum) as it moves to higher gravitational potentials (out of a gravity well). Confirmed by the Pound-Rebka experiment.

2. Gravitational time dilation: Clocks will run slower at lower gravitational potentials (deeper within a gravity well). Confirmed by the Haefele-Keating experiment and GPS.

3. Gravitational time delay: Signals will take longer than expected to move through a gravitational field. Confirmed through observations of signals from spacecraft and pulsars passing behind the Sun as seen from the Earth.

4. Gravitational lensing: A distant object in front of another much more distant object can change how the more distant object is seen by curving the light from the more distant object. These effects include:

(a) Multiple views of the same object: Observations of quasars
whose light passes close to an intervening galaxy.
(b) Brightening of a star due to the focusing effects of a planet
or another star passing in front of it.
(c) Einstein rings and arcs: One object directly behind another
can make the more distant object's light appear as a ring.
When almost directly behind, the result is an arc.

5. Blackholes: objects which have gravitationally collapsed such that not even light can escape from behind its event horizon.

6. Expansion of the universe: This is predicted by cosmological solutions of the Einstein Field Equations. Its existence was confirmed by Edwin Hubble in 1929.

7. Cosmological redshift: Light from distant galaxies will be redshifted due to their movement away from the observer according to Hubble's law.

8. Big Bang: The arising of the universe from a primordial singularity. Confirmed by the discovery of the Cosmic microwave background radiation, the remnants of the primordial fireball. Discovered by Arno Penzias and Robert Wilson in 1965.

9. Dark energy: This is an energy field of unknown composition that may exist throughout the universe. Recent observations of distant supernovae indicate that the expansion of the universe is currently "accelerating". The solutions of the Einstein field equations that call for this behavior for the current universe, which may require the reintroduction of the cosmological constant, are for a stress-energy which is at least 70% dark energy.

Answer 2

Some excellent answers, and the authors are probably more knowledgeable than me. As a little simpler intro, general relativity describes the interaction between matter/energy comprising the stuff of our universe and the space-time manifold. It expands on special relativity, and preserves the notion there is no absolute frame of reference by showing that acceleration, which may seem absolute to you as you do feel the force, cannot be distinguished from the force of gravity. Further, the force of gravity is actually just a warping of the local space-time manifold, and objects moving thru such a field still move in the straightest lines possible. Earth's circular orbit around the sun is actually almost straight in 4D when the time element is included.

Einstein's first realization he was on to something was when he used the equations to accurately predict the precession of Mercury's orbit.

General relativity (big stuff) and quantum mechanics (small stuff)are the so-called pillars of our understanding of physics. The theories are very different in structure, and trying to unite them drives cutting-edge research such as in string theory.

Answer 3

I'll try to be brief. Einstein developed two theories of relativity: special and general. The special theory explains how the laws of physics will appear the same to all observers who are in constant uniform motion (no change of speed or direction). It assumes that the speed of light is constant for everyone regardless of their own speed of motion (this has been proven experimentally, even before Einstein). From this, Einstein discovered that quantities such as mass, time, and distance actually change in different ways for different observers (but the laws governing these changes remain the same).

The special theory of relativity is limited in that it ignores acceleration (changes in speed or direction, or both). The general theory applies the same principles (constant speed of light and invariance in physical laws) to observers undergoing accelerated motion.

The mathematics of special relativity are fairly simple (high school algebra). However, the mathematics of general relativity are quite complex.

There are a number of quite useful books available that do a good job of explaining this in more detail. My personal favourite is "The Fabric of the Cosmos" by Brian Greene. Books like this are difficult to find in stores (they're more interested in rubbish like astrology and New Age nonsense), but I'm sure you can order it through Amazon. Also, any decent public library should have it (except in the southern US, no doubt).

Don't take this question the wrong way, but what kind of "college" doesn't offer physics courses? I find that rather sad, actually. But not so surprising.

Answer 4

General relativity is a geometrical explanation for how things tie together. Specifically special relativity (which has to do with space and time being relative) and the force of gravity.

The layman's way of thinking of general relativity is to think of space as a rubber sheet. Things with mass distort this sheet, creating not only gravity which can affect other massive things, but a distortion in SPACE ITSELF which affects anything that moves through it. In other words, space is more dense around an object with mass, and just like any other transition from a more to a less dense medium, it causes a deflection in the paths of objects.

This theory also served to explain black holes - objects where space is so dense that it is essentially an impenetrable barrier from which nothing can escape. I say 'essentially' because there are theories about how things MIGHT escape from a black hole, but were a long way off from determining if they're true or not. We are not a long way off from finding verifiable black holes... there are a few things which have been found indisputably to be such objects.

It's an interesting field (though I think special relativity is more interesting)... and actually since most of Einstein's conclusions seemed to wacky, they are actually some of the most rigorously proven theories around. Hope that helps!

Answer 5

General relativity equates acceleration as be the same for both inertial and non inertial systems.
The Einstein field equation compares a Universal force to other forces on celestial bodies in terms of their space curvatures.

Answer 6

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