One of the most difficult question one can ask a physicists or anyone for that matter is what is time because it does not have a physical presence. This may be the reasons some define it only in the abstract saying that is an invention of the human consciousness that gives us a sense of order, a before and after so to speak of the changes that occur in our environment.
What Is Time? Determinism, Quantum Physics, Consciousness, Free Will, Causality. 
However physicists are not afforded the option of an abstract definition because they have defined gravity in terms of the physical curvature in a spacetime dimension. For example, a physical curvature in spacetime is viewed by many physicists to be causality of the force of gravity.
In other words to be consistent they should be able to define it in terms of its physicality.
Yet it is possible that time may be something which cannot be defined by a what but may be an effect similar to how color is not a something but is an effect cause by how light is reflected by a something. If this is true physicist’s would have to find another way to define gravity other that one that depends on the interactions of space and time defined by Einstein.
Another question that is difficult to answer is if nothing in the universe changed would time still exist.
Answering this question may provide an answer as to what time is because if change is the causality of our perception of time then understanding what causes it in the spacetime environment that physicist’s say we live may help us to understand how it is connected to our environment.
However, as Einstein suggested in the following quote time cannot not be physically connected to the process of change because it is a rigid unchanging component of a spacetime environment defined by both his Special and General Theories of Relatively and therefore could not be responsible of the dynamic process associated with change.
"Since there exists in this four dimensional structure [spacetime] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a threedimensional existence."
In other words according to Einstein the structure of spacetime is ridge while the changes we associated with time are merely an illusion similar to the illusion of change created in a flip book when one rapidly flips through its pages containing series of pictures that vary gradually from one page to the next.
Yet this means if, as he suggested the time dimension is not responsible for the "evolution of a threedimensional existence" some other geometric property of the our universe must be physically connected to it to allow change to propagated through it.
Therefore to understand the "evolution of a threedimensional existence" one would have to explain how the change propagates through it without referring to a time dimension.
Einstein gave us the ability to do this when he defined the energy associated with the evolution of a spacetime environment in terms of the equation E=mc^2 the constant velocity of light because that provided a method of converting a unit time and redefine the energy in that environment to its equivalent in four *spatial* dimensions. Additionally because the velocity of light is constant he also defined a one to one quantitative and qualitative correspondence between his spacetime universe and one made up of four *spatial* dimensions.
In other words he tells the physical properties of a spacetime geometry are related to an observer’s interpretation similar to how the measurements of their magnitudes are related an observer’s velocity. This is because, as was show above one can reinterpret the mathematics associated with the time dimension in an environment consisting of four dimensional spacetime with a spatial one to create one in only four *spatial* dimensions with identical properties. However one must be careful not to think of this as the physical replacement of the time dimension in Einstein’s universe with a spatial one because according to his mathematics they coexist in the same geometric plain.
Additionally the fact that the equation E=mc^2 allows us to quantitatively derive energy in a spacetime environment in terms of four *spatial* dimensions is the bases for assuming as was done in the article “Defining energy” Nov 27, 2007 that all forms of change can be derived in terms of a displacement in a "surface" of a threedimensional space manifold with respect to a fourth *spatial* dimension instead of one in a spacetime manifold.
Doing would also allow physicists to define gravity and energy in terms that do not depend on time or the interactions of space and time defined by Einstein.
Additionally it would allow one to understand how the geometric properties of space interact to create the change associated with time in terms of a physical image without using it because we can "see" or perceive how a void in space created by any displacement causes change where, as was mentioned earlier we cannot with time.
For example, we can physically observe how the energy stored in the displacement of water in dam causes change in an environment when it is released or allowed to flow over it. In other words we can form a physical image of the causality of the changing level of water in a dam in terms of its movement through the spatial void between its top and bottom.
Similarly one can form a clear physical image of how and why change occurs in our threedimensional environment by assuming the energy stored in a spatial displacement in a "surface" of a threedimensional space manifold with respect to a fourth *spatial* dimension is released though the "void" that displacement creates in four dimensional space.
This suggest the change most associate with time may be an effect caused by an interaction of a fourth spatial dimension with our three dimension environment.
In other words similar to how an the color of an apple is an effect created by an interaction between light and its surface time may be the effect of a physical interaction of our threedimensional environment with a four *spatial* dimension.
It should be remember Einstein’s mathematical model which defines the physical geometry of our universe tells us that an all objects must simultaneously exist in both a spacetime environment and one consisting of four spatial dimension because as was shown above one can use his mathematics to define two identical universes; one in four dimensional time and another made up of only four *spatial* dimensions. Which one we use to define our reality is dependent on how an observer interprets his mathematics.
Later Jeff
Copyright Jeffrey O’Callaghan 2016
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Understanding the connection between space and time is difficult because we cannot see time or space even though we can perceive the void in space when it contains nothing
Additionally defining or describing what time is extremely difficult. Some define it only in the abstract saying that is an invention of the human consciousness that gives us a sense of order, a before and after so to speak of the changes that occur in our environment. However one can define or describe space in terms of the void we can see between objects that it contains.
Fay Dowker Public Lecture – Spacetime Atoms and the Unity of Physics 
In other words time and space do not appear to share any physical qualities.
Even so physicists have derived the physical structure of the universe in terms of a mathematical model that combines space and time into a single interwoven spacetime continuum.
However it is difficult to understand how in a spacetime environment they can physically interact when, as was just mentioned neither of them have any common properties.
Granted using mathematics one can explain how they do in terms of abstract concepts but it does not explain how and why they do in physical terms.
Yet one can use the fact that the one of the primary functions of time is to define change to form a physical image of how it interacts with space to create it.
Einstein gave us the ability to do this when he defined the energy contained in a volume of spacetime in terms the constant velocity of light because that provided a method of converting a unit of time to its equivalent unit of space in four *spatial* dimensions. Additionally because the velocity of light is constant it also defined a one to one quantitative and qualitative correspondence between his spacetime universe and one made up of only four *spatial* dimensions.
In other words he tells the physical properties of space and time are relative to an observer’s interpretation similar to how the measurements of their magnitudes are relative to an observer’s velocity because, as was show above one can reinterpret the mathematics associated with the spacetime environment of both the General and Special Theories of Relativity purely in terms of its spatial components to create an identical one in only four *spatial* dimensions. However one must be careful not to think of this as the physical replacement of the time dimension in Einstein’s spacetime universe with a spatial one because according to his mathematics they simultaneously coexist in the same geometric plain.
However the fact that one can use Einstein’s theories to qualitatively and quantitatively define the displacement he associated with energy in a spacetime universe in terms of four *spatial* dimensions is bases for assuming as was done in the article “Defining energy” Nov 27, 2007 that all forms of energy can be derived in terms of a spatial displacement in a "surface" of a threedimensional space manifold with respect to a fourth *spatial* dimension.
Doing would also allow one to form a physical image of how the geometric properties of space or time interact to create change because as mentioned earlier we can form a clear physical image of how a void in four spatial dimensions created by that displacement would cause change.
For example we can physically observe how the energy stored in the displacement of water in dam causes change in an environment when it is released or allowed to flow over it. In other words we can form a physical image of the causality of the changing level of water in the dam in terms of its movement through the spatial void between the top and bottom of the dam.
Similarly one can form a clear physical image of how and why change would occur in our three dimensional environment by assuming the energy stored in a spatial displacement in a "surface" of a three dimensional space manifold with respect to a fourth *spatial* dimension is released though the void that displacement creates in four dimensional space.
As was mentioned earlier it is difficult to form physical image of how time can interact with space because of its abstract properties.
However one can from a very clear image of how it does when one realizes that according Einstein theories the physical properties of space and time which as was shown above are relative to an observer coexist on in the same dimensional plain. Therefore an observer who looks at his theories form a spatial perspective can easily understand how change occurs in a space timeenvironment in terms of the physical example of water flowing over a dam.
It should be remember Einstein’s mathematical model which defines the physical geometry of our universe tells us that an all objects must simultaneously exist in both a spacetime environment and one consisting of four spatial dimension because as was shown above one can use his mathematics to define two identical universes; one in four dimensional time and another made up of only four *spatial* dimensions. Which one we use to define a solution to a problem, as mentioned earlier is only dependent on how an observer interprets his mathematics.
Later Jeff
Copyright Jeffrey O’Callaghan 2016
Anthology of 
The Reality of the Fourth Spatial Dimension Paperback $9.77 Ebook $6.24 
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