From the perspective of sentient beings, it seems obvious that time seems to be flowing. However, as hard as physicists have tried, no one has found any convincing evidence within the laws of physics that supports this intuitive sense that time flows. In fact, a reframing of some of Einstein’s insights from the Special Theory of Relativity provides evidence that time does not flow. To understand this, we will imagine a loaf of bread being the entirety of spacetime. Therefore, the slices making up this loaf are the ‘nows’ of an observer; each slice represents certain space at a certain moment of time from the observer’s perspective. The union obtained by placing the slices together, in the order in which the observer experiences them, fills out a region of spacetime. If we take this perspective to a logical extreme and imagine that each slice depicts all of space at a given moment of time according to one observer’s viewpoint, and we include every slice from the ancient past to the distant future, the loaf will encompass the whole universe throughout the whole of time – the whole of spacetime.
Every event, regardless of when or where, is represented by some point in the loaf. This is schematically illustrated in Figure 1, however the perspective is somewhat confusing. The ‘outside’ perspective of the figure, from which we are looking at the whole universe (all of spacetime), is a fictitious vantage point, one that none of us will ever be able to seen from. We are all within spacetime. Every experience we ever have occurs at some location in space at a moment of time. And since Figure 1 is supposed to show all of spacetime, it encompasses the totality of such experiences. If you could zoom in and closely examine all the comings and goings on planet Earth, you would be able to see every little thing from the past, present and future. Given the coarse resolution of Figure 1, you cannot actually see these moments, but you can see the schematic history of the Earth. Obviously, Figure 1 is an imaginary perspective; it stands outside of spacetime – which is impossible. It is the view from nowhere and nowhen.
Even though we cannot actually step beyond spacetime and take in the full sweep of the universe, Figure 1 provides a very strong means of analysing and clarifying the basic properties of space and time. As a main example, the intuitive sense of time’s flow can be easily and vividly portrayed in this framework by a variation on the movie-projector metaphor. We can imagine a light that illuminates one time slice after another, momentarily making the slice come alive in the present – making it the momentary now – after which it lets it go instantly dark again as the light moves on to the next time slice. Right now, in this way of thinking about time, the light is illuminating the slice in which you, sitting on planet Earth, are reading this word. But again, while this image seems to match our experience, scientists have not been able to find anything in the laws of physics that embodies a moving light like this. They have not found any physical mechanism that singles out moment after moment to be momentarily real as the mechanism ‘flows’ toward and into the future. In actual fact, it is quite the contrary. While the perspective of Figure 1 is unquestionably imaginary, there is very convincing evidence that the spacetime loaf – the totality of spacetime, not slice by single slice – is real.
An implication of Einstein’s work is that special relativistic reality treats all time as equal. Although the notion of ‘now’ plays the prime and central role in our view of the universe, relativity subverts our intuition and declares our universe an egalitarian one in which every moment of time is as real as any other. Einstein argued that every part of the spacetime loaf in Figure 1 exists on the same footing as every other, suggesting that reality embraces past, present and future equally and that the flow we envision bringing one section to light as another goes dark is nothing but an illusion.