What is time ?
Time is our universe's fourth dimension. It is something that is relative from an observer to another.
Time is measured in physics by the speed of light.
The speed of light in a vacuum, is constant for all observers. It is 30,000 KM/Sec.
But as time is the fouth dimension in our universe, we should ask, what is a dimension ?
A dimension, is a measurment required to define the characteristics of an object.

There are some theories, that explain dimensions as a measurment created by the brain.
What does that mean ?
These theories, state that all dimensions exist within our universe. But our brain does'nt the ability to observe all the dimensions at once. What i mean is that the universe has only one dimension, but it is our brain perception that divide this dimension into many other dimensions.
Let's have an example according to this theory:
This theory says, that all events occur in the same time, or to be more specified, there is no time. But our brain does'nt have the ability to percieve all these information at the same time, so it created the dimension of time to put the events into series.
Only less than 10% of our brain is active. And the more the technological advancment develops, the more this brain activity decreases.
So, to support that theory, we have an example that explains a lot.
Sometimes when we sleep, we dream of some events, and they really occur when we wake up.
There are two explanations for that phenomena.
First one: Some people does'nt remember the dream untill it becomes true, so they have the feeling that they saw that event before. This phenomena is called "Deja vu". It is a French phrase that means in english, "Already seen".
There are many explanations for this phenomena.
One of them, is that the left eye sees before the right eye, so it stores the information in the brain before the right eye knows it. So finally you just restore the recent stored information from your brain.
Another explanation is, as everyone knows, when we percieve some information from the outside world, our brain takes this information to proccess them and check if they are new or old information, then it gives you a sign which indicates if this is a new information for you or no.
What happens, is nothing more than an error in the processing of these information. It just gives you the sign that these information are not new. It gives you this sign by error.
Second one: Some people remember the dream, and are surprised when they find that it coming to be true.
The explanation is, the brain activates more than the 10%, so it helps us to see the future, to brake the time dimension.

Here is a question:
Is time travel possible ?!
Mental time travel, could be possible by activating more than the 10% in the brain.

Physical time travel is theoratically possible. If you moves in the speed of light, time will be ZERO. No time will pass. If you moves faster than light, you go backward in time. And if you move very close to the speed of light, you will go forward in time.
But someone would ask, how could we go forward in time ?!
Here is an explanation in a though experiment:
The twins paradox: one of two twin brothers undertakes a long space journey with a high-speed rocket at almost the speed of light, while the other twin remains on Earth. When the traveler returns to Earth, he is younger than the twin who stayed on Earth.
That's because, as getting closer than speed of light, as time will pass faster.

Some people have claimed that they are time travelers, and one of them was one of the most controversial issues in science.
John Titor:
He is a person who claimed that he is a time traveler from year 2036, and has prediected many future events to prove that he is true. To find more about John Titor, check his home page here.

There are places in the universe, that time is equal to Zero. No time passes. And this places, are the black holes.
A Black hole is an object with a gravitational field so strong that nothing can escape it — not even light.

Click Here to see a video about black holes.

Finally, time is something that is within our perception, we can control it, it exists inside our brain, but we dont know if it exists physicaly in the outside world.