The Now is the term Einstein gave to the present moment. But as he lamented, there is no such thing in the physics. As described by Rudolf Carnap, one of the best-known philosophers of the twentieth century:
Once Einstein said that the problem of the Now worried him seriously. He explained that the experience of the Now means something special for man, something essentially different from the past and the future, but that this important difference does not and cannot occur within physics. That this experience cannot be grasped by science seemed to him a matter of painful but inevitable resignation. (1963)
Physicist David Mermin explains the issue very clearly in his article in Nature:
My Now — my current state of affairs — is a special event for me while it is happening. I can tell my Now from earlier events, which I only remember, and from later events which I can only anticipate or imagine. The status of an event as my Now is transitory: it becomes a memory as subsequent Nows emerge.
Yet clear, evident and banal as this is to us all, there is no Now in the usual physical description of space and time. Physicists represent all the events experienced by a single person as a line in four-dimensional space-time, called that person’s ‘world-line’. There is nothing about any point on my world-line that singles it out as my Now. (2014)
The worldline is the four-dimensional existence of an object as has been described in The Block Universe.
This picture represents a sequence of moments in my life. Each two-dimensional image symbolises the three-dimensional physical reality at a specific moment in time. Each one is a static moment along my worldline, the four-dimensional reality of my existence.
So this is like a movie on a DVD. Just as all the frames of the movie on the DVD are there in static sequence on the disk, all the moments of the movie of life are there in static sequence in the block universe. As Tegmark states:
So life is like a movie, and space-time is like the DVD. … there’s nothing about the DVD itself that is changing in any way, even though there’s all this drama unfolding in the movie. (Kuhn, 2015)
And, just as the DVD does not change, there is nothing about the space-time world that is changing. Nothing actually happens. Yet there is all this drama unfolding in the world.
The Moving Viewpoint
The explanation is the moving viewpoint of consciousness. The drama unfolds as the experiencing consciousness sweeps through these moments, up the worldline of the body. Thus the static world comes to life as the sequence of moments is experienced. That is what Weyl is saying.
As a result there is the effect of time passing, as moment after moment is experienced in sequence. This is how consciousness gives rise to the experience of the passage of time, as he describes. The Now is the moving viewpoint of consciousness.
This has been known about for over a century. As stated by Sir Arthur Eddington, astronomer, physicist, mathematician and philosopher of science:
Events do not happen: they are just there, and we come across them [as] the observer on his voyage of exploration. (1920)
When you take a train journey, it is clear that you are passing through the landscape of the track. Obviously, the landscape is not moving. It is just your view of the landscape that is changing. But inside the train, from your moving viewpoint, you experience the passage of the landscape. Weyl is saying that the experience of the passage of time is exactly the same kind of thing. The world simply is. It does not change at all. As the Now passes along the track of the worldline, into the future in the space-time layout, there is the effect of the passage of time.
The meaning that is so hard to grasp is that all the future moments exist already. It seems I must have already done all the things I am going to do. Of course, it would be rather disconcerting to realise it is all predetermined. But it is only predetermined in terms of probabilities. This is the nature of the quantum universe.
One specific path into the future is only one of the infinity of possible paths. And with each observation made, one is selecting one branch of the possibilities. Thus we are constantly shifting and changing the definition of of the future, as one version of the path is selected. So the movie of life is already written, but there is an infinity of choice points. Which version you will witness is the great mystery, and the extraordinary opportunity.
The next section is Consciousness.