I'm going to give a brief overview of Einstein's Relativity in simple non-mathematical layman's terms.

**Light**Light is a massless particle with a constant speed to all observers. It has momentum. It acts as both a wave and a particle.

To explain the problem prior to Einstein's work with light speed I will use a simple example. Lets say Alice is standing 1000 feet away from a light source, and Bob is moving at 10ft/sec towards the light source. Both of them can detect the speed of things, because they are just cool kids. Common sense tells us that light will be observed by Alice to be light speed, and light will be observed by Bob to be light speed ( c ) - 10ft/sec. However, they both observe light speed to be the same ( c ). This seems like a paradox of sorts, but Einstein shows us how this works, among other things.

The faster things travel the more massive to an outside observer they get. Furthermore the faster they travel, the slower time seems to be passing to the object compared to an outside side observer. This is known as time dilation. As things approach the speed of light, they get more massive (and thus harder to accelerate) and seem to be passing slower to outside observers. Since to reach the speed of light their mass would need to reach infinite mass, they cannot break this barrier.

This time dilation is what makes the speed of light constant. Even though Bob is moving faster towards light than Alice, time dilates and he measures the speed the same.

**Gravity**Gravity also dilates time. In fact, Einstein says that Gravity (or more accurately, gravitation) is equivalent to acceleration locally. That is to say, if you are in a closed box and feel something pulling you down there is no way to tell if you are accelerating upwards (as in an elevator) or if you are being pulled down by gravitation.

Heres where it gets messy. Remember time dilation? Well, lets look back at Alice and Bob to expand on that a bit.

Lets say Alice and Bob are in the middle of space with no stars, gravitation, or any place marks to tell ones position. Bob starts moving away from Alice at a constant speed. However, from Bobs frame of reference, he is not moving at all, and it is Alice that is moving away! So, to bring it together, since Alice is moving faster than Bob from his Frame Of Reference, Alice actually has her time moving slower and she is more massive! It is also true that from Alice's Frame of Reference Bob is slower and more massive. This may again seem like a paradox, but it is not.

So what does this all have to do with gravitation? I promise, all of this will come together at the end.

Back to the Equivalence Principle – you know, that says that gravitation is the same as acceleration. Lets say you are in a closed box moving a constant speed. This is known as an inertial frame of reference. Now, lets say you are in a closed box accelerating upwards at the rate of 1g (about how fast you accelerate to the earth if you jump.) This is known as a non-inertial frame of reference. This is because you are being acted upon by a force that is unexplained by what you know. In reality though, this is not a “force” at all. It is actually what is known as a pseudo-force. Here is how wikipedia explains it:

“When a car accelerates hard, the common human response is to feel "pushed back into the seat." In an inertial frame of reference attached to the road, there is no physical force moving the rider backward. However, in the rider's non-inertial reference frame attached to the accelerating car, there is a backward fictitious force.”

Now here is the kicker. Einstein says gravity is a pseudo-force caused by taking a non-inertial frame of reference to be an inertial one. That is to say, we think that its a force acting on us, but really we are just accelerating through space.

Now why would we accelerate for no reason towards mass? This is most likely the most often explained part of Relativity as it relates to gravitation. Energy distorts space time, much like a bowling bowl would distort a taught sheet. Remember, energy is mass! (E=mc^2). This is why accelerating also distorts space-time! In mathematical terms, the space is no longer flat (or Euclidean) but is now non-euclidean. This means two objects traveling parallel to each other may not end up being parallel further along the line and may even eventually hit each other. Straight lines are no longer “straight” but follow these distortions in space time. When objects follow these “non-straight” straight lines, they are said to follow their geodesics.

An important consequence of this is that you don't need mass to be affected by gravitation. You simply need momentum – to keep on traveling in a straight line.

So there we have it, Gravity is not a real force according to Relativity.

[Someone please read over this, I'm sure I have made mistakes]