What does spacetime consist of? It consists of events. Such an “event” simply refers to their happening at a certain point in three-dimensional space and at a specific moment in time. Using four numbers, we can locate, with precision, the event in spacetime. Thus, different observers can disagree about “appearances.” Such appearances would include the difference in space between two events. However, they do agree about the separation between the events in four-dimensional spacetime. It is in this sense that it is simplistic and incorrect to say that, according to special relativity, “everything” is relative. The “distance” or “interval” between two events in four-dimensional spacetime can be represented thanks to the inclusion of the fourth term of “time.” It is by relating the 3 dimensions of space and the 4th dimension of time that observers can communicate and mediate apparent disagreements resulting from distinct frames of reference.

Einstein initially bristled at Minkowski’s belief that time was one aspect of a four-dimensional spacetime, but he eventually accepted it, issuing in the now-famous Minkowskian spacetime that was discussed in the previous article. So Russell Stannard:

“Acceptance of a four-dimensional reality is difficult because it is not something that lends itself to easy visualizing – indeed, forming a mental picture of four axes all mutually at right angles to each other is impossible. No, we must dispense with mental pictures and simply allow the mathematics to guide us. One of the disconcerting features about four-dimensional spacetime is that nothing changes. Changes occur in time. But spacetime is not in time; time is in spacetime (as one of its axes). It appears to be saying that all of time – past, present, and future – exists on an equal footing. In other words, events that we customarily think of as no longer existing because they lie in the past, do exist in spacetime. In the same way, future events which we normally think of as not yet existing, do exist in spacetime. There is nothing in this picture to select out the present instant, labelled ‘now’, as being anything special – separating past from future. We are presented with a world where it is not only true that all of space exists at each point in time, but also all of time exists at each point in space. In other words, wherever you are seated now reading this book, not only does the present instant exist, but also the moment when you began reading, and the moment when you later decide you have had enough (perhaps because all this is giving you a headache) and you get up and go off to make a cup of tea.”

Not all physicists agree about this view of reality, however. Some view it simply as a mathematical structure. They nonetheless insist that there is something to the commonsense notion of the “present” as alone being “really” real, whereas the past no longer exists, and the future has yet to exist. Stannard notes some problems with this “commonsense” view:

“It is all very well saying that all that exists is what is happening at the present instant, what exactly do you mean by that? Presumably you mean ‘me reading this book in this particular location’. Fair enough. But I imagine you would also include what is happening elsewhere (literally elsewhere) at the present instant. For example, there might be a man in New York climbing some stairs. At the present instant he has his foot on the first step. So, you will add him, with his foot on that step, to your list of existent entities. But now suppose there is an astronaut flying overhead directly above you. Because of the loss of simultaneity of separated events, he will disagree with you over what is happening simultaneously in New York while you are reading this book. As far as he is concerned, the man in New York, at the present instant, has his foot on the second step – not the first step. Moreover, a second astronaut flying in a spacecraft travelling in the opposite direction to the first arrives at a third conclusion, namely at the present instant the man in New York hasn’t even reached the flight of stairs yet. You see the problem. It is all very well saying that ‘all that exists is what is happening at the present instant’, but nobody can agree as to what is happening at the present instant. What exists in New York? A man with his foot on the first step, or a man with his foot on the second step, or one who has not yet reached the stairs? As far as the block universe idea is concerned, there is no problem: all three alternatives in New York exist. The argument is merely over which of those three events in New York one chooses to label as having the same time coordinate as the one where you are. Relative motion means one simply takes different slices through four-dimensional spacetime as representing the events given the same time coordinate, ‘now’.”

The static block universe is not without its problems, of course. Where does the phenomenological “now” come from? Why does conscious perception function as a searchlight, sliding down the axis of time? Why does it specify and single out distinct moments of physical time as being the special “now” moment? This remains a controversial question in the philosophy of space and time and physics.

The faster one travels, the more slowly time passes. Time stops completely, in fact, when you reach the speed of light. Is it possible to travel faster than the speed of light? What would happen to time at such a rate? It is impossible to travel faster than light. In Newtonian mechanics, an object, represented by “m” and a velocity “v” has momentum “p.” This is defined by the expression p =- mw. Making an object travel more quickly involves exerting force on it. According to Newton’s second law of motion, “F” equals the rate of change of an object’s momentum. This “F” refers to “force.” Consider that “m” is a constant. This would be the same as saying that force = m times the rate of change of velocity. This is what we refer to as “acceleration,” represented by “a.” Therefore, F = ma. One can conclude that if you push long and hard enough, acceleration will continue forever. This means that there is no limit to potential velocity.

There is a limit, however. This is why relativistic physics has forced us to radically rethink our view of reality. We have to completely rethink our concept, more specifically, of momentum. The relativistic expression for momentum is p = mv / v (1 – v^2 / c^2). This is quite a bit more complicated than Newton’s understanding of momentum. The equation still includes the rate of change of momentum. However, an altogether new expression is added for momentum. This means that the formulation F = ma no longer applies. While we previously dealt only with the rate of change of velocity (acceleration, in other words), now our understanding of velocity is V(1-v^2/c^2). Iv v is small, then we can remain Newtonian in our mechanics. As it approaches c (the speed o light), the expression under the square root approaches zero and the momentum becomes infinitely large. While a constant force continues to increase the object’s momentum at a constant rate, this constant now produces hardly any increase in the velocity of the object. The velocity of light, therefore, introduces a limit, meaning that nothing can move at the speed at which light moves.