What exactly is time?

I'm confused, if time is man made and has no matter yet it contains energy time is an illusion but its not can someone clarify for me what time is. please

The Big Bang theory describes how the universe was created from the Big Bang singularity, where all matter and space is contained in a single point of infinite density. At the moment of creation of the universe - the Big Bang - all matter, space and time came into existence, before that time did not exist. Our universe could not exist without time, and time could not exist without the universe, they are different components of the one entity.

According to Einstein's theory of relativity, time is regarded as a fourth dimension, on an equal footing with the familiar three dimensions of space. Einstein says that you can imagine all of space and time represented as a four dimensional space-time map, on which all of history, the present and the future of the universe can be represented. The four dimensions of space and time are collectively referred to as the space-time continuum, which by the way, is not just an invention from the script writers of Star Trek. The problem we have is in trying to visualise these four dimensions because we can only see the three dimensions of space, we cannot 'see' time. However, even though we cannot see it, it is necessary to include time if we are required to define a precise location. We can, for example, define an object's position in a room by three simple measurements, such as how far forward, how far to the left and height above floor level. These co-ordinates will define where the object is, but only where it is now, it may be somewhere else tomorrow.

Let's take a closer look look at the implications of time existing as a dimension. Imagine an object, not a quantum particle this time, or a massless photon moving at the speed of light, but an ordinary day to day object; let's use a one ton boulder. Now this boulder is behaving itself, as only boulders can, and isn't doing anything, its just sitting there existing. It's real, solid and big, there is no doubt at all in our minds that it is actually there, we can walk up to it and kick the thing. The question is, how long is it there for? Now at this point I should mention that this is neither the time nor place for a Smart Alec geologist to come along clutching his Ph.D and inform us that it is so many million years old and will remain in that condition for so many million more years. There is an even chance he would end up under it! We want to know how long in time it is there, in that spot, in front of us. We already have a pretty good idea, we know it is less than a Femtosecond, which to remind you is one thousandth of one trillionth of a second. So after this very brief moment of existence in our time frame, what happens to it? Does it simply disappear when it slips from the present into the past? Equally where does it come from? Does it just spring into existence into our time frame? As time flows past us and the 'now' becomes history, has that moment gone forever, or does the past, and the future, actually exist independently of our own personal time frame?

Again, according to relativity, the past and the future, as well as the present, all exist equally. As I said earlier, 'you can imagine all of space and time represented as a four dimensional space-time map, on which all of history, the present and the future of the universe can be represented.' This would mean that our boulder isn't only existing in front of us in our personal time frame, it would be existing in every time frame, along with everything else of course. We can try to visualise this the following way. Imagine our boulder is resting on a glacier that over a period of 5 thousand years is slowly moving, and we want to show this movement over time. We could make a jelly cube to represent time (I bet scientists have a lot of fun). The bottom would represent the past, at the beginning of the period we are interested in, and the top would be the finish, or the present. The length of our cube could represent a north/south movement at an appropriate scale, and likewise the width could represent an east/west movement, and as stated, time is represented by height. We could slice through the cube at different heights to represent different slices of time.

To show the movement of the boulder over time we simply have to poke a rod through the jelly cube. If the boulder moved at an uneven rate we would have to go high tech and use a bendy wire instead.

We can now slice up our cube into five slices to show us where the boulder was at 1000 year intervals. We could of course keep slicing our cube into ever small fractions showing ever more precise movements over ever smaller periods of time. Viewed this way, we can see that the existence of our boulder is continuous, it exists like a 'worm' through time, it only looks like a boulder at each 'slice' of time. We can make these slices as small as we like, but we will always only see the boulder as it is at that slice, or more accurately, at that moment in time. In this manner we can see that time does not 'flow' at all, it is a fixed dimension just as like the three dimensions of space, it is us that is moving through it.

The direction that we move through the dimension of time would appear to be governed by the arrow of time, as already described. As for the speed at which we travel through time, that is governed by the speed we are moving through the dimensions of space, as described by relativity.

Time is the 4th dimension. It was created when the universe was created. We can only perceive 4 dimensions: Length (L), Width (W), Height (H), and Time (T). This is how we view the world. It is the only way we can view the world.

We can measure the 4 dimensions. L, W, H are measured with a ruler for example. The units are man made. That is, we can say one inch is this big, or one meter is this big, one mile is this big, etc. Thus humans have arbitrarily defined how to measure these 3 dimensions, but the 3 dimensions was created in at the moment of the big band. The same with time. We can measure time. We have arbitrarily defined what a unit of time is. We did not create time, the big band did. All we did was place units on it. More specficially, humans have defined time as follows: "One second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." This is very complicated but that is how humans define time. We didnt create anything, we just use units to define the 4 dimensions so we are all on the same page.

time as the 4th dimension is a hypothesis. I think of it as a 4th dimension as well but hyperspace is the realistic 4th dimension. back to the question, time can be expressed differently. what time the universe at what time is it on earth and location. for the universe time is at 13.7 billion years. time created its self. as magnus said we put units on it to stop chaos.

I'm confused, if time is man made and has no matter yet it contains energy

This makes as much sense as saying "how can the meter be French, yet contain turnips?"

More seriously, though, time is a dimension - it can't contain energy any more than 'depth' can.