Can someone talk in simple terms about the theory of Relativity?

If you have a good grasp of basic algebra, you should be able to tackle the theory of Special Relativity. It deals with measurement of basic physical quantities like distance, time and speed in different frames of reference under the assumption that the speed of light is constant and that the laws of physics are the same in all inertial reference frames (i.e. they don't accelerate relative to each other).

I don't know of any websites, but wikipedia is probably a good place to start (see below). However, the best place to look is at your local library. A book that helped me was "An idea of relativity" by H. S. Perlman published by Monash University in Australia, but there should be others as well as various encyclopedias.

The theory of General Relativity is much more mathematically demanding. It deals with an explanation for gravity as well as dynamics and stuff.

Before Einstein, physicists thought there were special "privileged reference frames". If you were motionless relative to these special reference frames, you would have a special, optimized view of the universe, and of the laws of physics.

Then Einstein came along with two theories that shattered this myth:

1. General Relativity

2. Special Relativity

They are named "relativity" because they show that there are no privileged reference frames. All movements are relative to each other. There is no absolute reference frame. A "motionless" observer will deduce the same laws of physics as an observer traveling at extremely high (but constant) speed.

General relativity shows how space is curved by the presence of mass, and that the curvature affects other masses, altering their trajectories, and can even affect the path of a ray of light. It is currently the accepted theory of the nature of gravity, and it predicted the existence of black holes (regions with so much gravity that light is bent backwards and cannot escape).

Special relativity describes how physics changes when velocities become extremely large (a significant fraction of the speed of light). It predicted a number of bizarre phenomena which were later verified:

1. Time dilation - An object traveling near the speed of light experiences the passage of time more slowly than other objects.

2. Length contraction - An object traveling near the speed of light is dramatically shrunken in the direction of travel, as measured by an outside observer.

3. Mass increase - An object traveling near the speed of light increases in mass.

4. Energy and mass are interchangeable, related by E=m*c^2

Here's the theory of relativity in a nutshell:

1. The laws of physics are the same regardless of inertial frame of reference.

2. The speed of light in free space is a constant in all inertial frames of reference.

The above are known as the postulates of special relativity.

When you consider accelerating reference frames you have what is known as general relativity.

Since nothing can go faster than the speed of light in a vacuum, some strange things happen when you travel near the speed of light (i.e., time dilation, length contraction).

Time dilation refers to time slowing down when you travel close to the speed of light.

Length contraction refers to the shrinking of an object's length in the direction of travel when the object travels close to the speed of light.

The theory of relativity was able to explain the precession of the perihelion of Mercury's orbit and it explains why muons reach the Earth when they have such a short life span.

The theory of relativity states that gravity is curved space-time due to the presence of mass versus the Newtonian gravity of a force acting over a distance.

There are two main theories of relativity: General and Special.

General relativity basically relates to gravity and spacetime. It was invented be Einstein and is still used to this day (even though it's nowhere near complete).

Special Rev, also created by Einstein, has to do with our four dimensions and how they can change depending on the observer. The four dimensions (up/down, left/right, in/out, and time) are merely conceptual, and can change depending on several factors, mainly speed and gravity. For example, time will slow down for people deep in a gravitational field or people who are moving near the speed of light. Also, length will change for people who are moving near the speed of light.

Basically the theory of relativity states that how we look at things is relative. For example when driving in a car you are stationary and the ground underneath you is moving. If your friend is driving in a car next to you and traveling at the same speed, they are also stationary. This is an over simplification but should give you some basis to start with.

We measure the speed of another reference frame from our reference frame as v.

Both of us measure the speed of light whether it is approaching or receding from us as C.

That is the speed of light is independent of the speed of the observer.

If the speed of light were infinite, then the distance and time as measured by both the observers will be absolute.

Since the speed of light is definite and absolute, we have to choose different measuring units for distance and time for the one who is moving relative to us.

Length and time becomes relative to the observer and not absolute.

The one moving with a half the speed of light and we who are at rest both measure the speed of light coming toward us as C.

Our common sense says that the one who is moving must measure the speed of light as C+ C/2, since he is approaching the light.

But he is measuring it as C only. That implies that his measuring instruments have contracted to an extent so as to measure the speed as C, whichever is the direction of light.

Keeping the speed of light absolute makes all other things relative.

We select shoes according to the size of foot.

In an imaginary world, the sizes of shoes are fixed and constant. How would then the people of that world wear shoes?

One has to accept that people’s foot are adjusting and accommodating to fit for the fixed size of the shoe.

The theory of Relativity is broken into 2 distinct parts. The Special theory and the general theory. Though mathematically Special Relativity is much easier to understand than conceptually. General relativity is much easier to visualize, but much harder to get mathematically.

Special relativity all deals with the behavior of objects when they approach the speed of light from different frames of reference. The usual frames of reference are the observer and the person or object travelling at near light speeds. This leads to a couple of interesting phenomenon. Time dilation and length contraction.

Time dilation occurs when a person reaches near light speeds, the observer (stationary) will notice the with the person travelling time appears to slow down and stop as they're in a rocket flying through space the closer they approach the speed of light.

However to the moving observer the opposite will have happened, they'll observe time passing as normal, but depending on the distance they travel and come back, the stationary observer will be older than the moving observer due to the effects of time dilation. The person moving at near light speeds doesn't notice the slow down in the effect of time on them. The stationary observer goes through time at the normal speed.

This is commonly referred to as the twin paradox. 1 twin travels for several light years in a rocket at near the speed of light and comes back to greet his stationary twin, but notices that his stationary twin has aged much more than he has due to the effects of time dilation.

Length contraction is also another phenomenon of special relativity. As a moving object approaches light speed it's length appears to contract to the stationary observer. However to the moving observer the distance of objects in front of them seem to contract and they notice none of the effects of the length contraction.

General relativity is the theory about how gravity affects space and time. It predicts strange objects such as neutron stars, black holes, worm holes, etc. The best way to visualize it would be to imagine space and time as a soft sheet of rubber. The stars, galaxies, planets, etc. create dips in the sheet due to their gravitational influence.

To explain the orbits of planets General Relativity states that they orbit the sun like the sun is sunk into the sheet at a fair depth depending on how strong the sun's gravity is. The planets orbits are like rolling a marble along the side of an upright bucket, they're moving fast enough to keep their momentum going and not move up or down along the walls of the divot too far.

If they start to move down the walls that means the sun is drawing them closer and they're losing their orbital momentum, but if they start to move up the sides they're gaining orbital momentum and may escape the sun's pull and go flying off into space.

It also predicts the behavior or stars in certain circumstances, such as stars the collapse and become neutron stars and possibly black holes. It also shows how light is affected by gravity, and that gravity can visibly bend light if it's strong enough.

Light is said to travel along space and time like a golf ball along a green, if it goes near a gravity well it'll dip along the side and right itself again or change it's angle. Much like a golfball that runs just at the edge of the cup on the green but doesn't go in, how it diverts it's straight path a bit.

I realize this is a long explanation, but it's the shortest one I could come up with and still manage to cover most of the bases for both theories. I hope this helps.