Time is a somewhat nebulous concept, for although we can measure it with fantastic accuracy, can understand its effects (e.g. the passage of time) and can deal with it mathematically in a plethora of different ways, it is very difficult to pin down what Time is.
In Minkowski’s four dimensional Space-time, length (or distance) is the magnitude of the space between two space-like points, while time is the magnitude of the separation of two time-like points.
Let us consider then, how Time is measured. Like all measurements it is a comparison with another measure of the same kind: one that is set as a standard. So Time is measured by comparison with another time; usually from a ‘timekeeper’, or clock. That is a device that has a recurring process with a reliably repeated duration.
In fact there is a clue to the essential nature of Time in that it can only be measured against another time. Time does not exist of itself; it is one of the outcomes of change, for every change has a duration.
Is Time Absolute?
Time is the heartbeat of the Universe. Every change, every process has a duration. That duration is absolute and fixed. It is how we measure that duration that can vary according to circumstance.
The passage of time is the outcome of an infinite number of changes, from those whose duration is too small to be measured to those that have lasted since the Big Bang, as long as Time itself.
It is against this background of ongoing change that we measure Time. We may use more local comparisons of different proportions, from the incredibly small to the gigantic.
As time exists only as a comparison to other times, it is, essentially, unchanging i.e. Universal or Absolute Time.
For when Time seems to quicken or to slow, it is only the duration of a change, of a process, that is dilating or contracting. More, or less, is happening within a specific time slice. Processes are progressing faster or slower against this Universal Time but they do not, cannot define time.
No clock can measure time passing faster or slower, for Time is Absolute. The universal background of changes, the duration of those changes is what makes time.
Time is often described as another dimension but is this any more than a mathematical device for making calculations?
Time as a Dimension
Time is said to be measured, in part, as an imaginary number.
If time has to be normal to all three space-like dimensions then surely it must be normal to all three simultaneously, in fact to any Space-like point.
So how does that work?
How can it work?
Can we answer that without being able to imagine 4 dimensions?
Well yes, I do believe we can! Because the answer is in the fact that the fourth, Time-like dimension is fundamentally different from our Space-like dimensions. Even as a mathematical device it is an odd one, for squared, it has a negative sign, unlike our other three space-like dimensions.
Another difference that we should consider is that a single point in time occurs on each and every space point. It has to, so that space point can, with the addition of that time point become an event.
For a single space-like dimension is a line extending the whole of that single dimension, for two dimensions it is a plane extending across the whole of that 2D space, for three it must be a solid that includes the whole of the 3D space that we know, i.e. the whole of space time.
And if that is true we must have a similar expanse for each and every Time point.
So if we were to define a particular time point by a colour, for instance, than we would see the whole of space change colour as Time progressed and we would be able to refer to simultaneous events as all being the same colour on a progressing scale.
From this it becomes evident that there is one Space-time and that synchronous events are synchronous whereever they are observed from.
It is the whole of Space where every point has the same time. Where every Space-point has a synchronous event. Think about it. It has to be!
But enough for now we will consider this further when we look at the Simultaneity of Relativity.