Separating the unalterable from the unknowable Part 1...
Somehow "now" sorts through the very large number of possible futures of the universe and selects the one that is to survive the passage of "now" to enjoy its instant of reality and become part of the past. I am going to be bold enough here to explore a possible mechanism by which this might be accomplished, limiting our consideration to systems large enough that we will not need quantum mechanics to describe them.
By “possible futures of the universe” I mean the possible states of the universe at a future time. The state of the universe at any instant is the total information required to reproduce the universe as it exists in that instant. That is way too much information for us to handle. Fortunately we do not have to treat the universe as a whole to consider its future states. There is enough empty space in the universe that the interactions with distant objects may be neglected even though in principle they are connected by gravity and electrostatic forces.
It seems to me there are three categories of state changes. Think back to the Simultaneity Situation page in which we used a clock-carrying satellite to demonstrate the effect on time of relativity principle. As it drifted around it orbit its position and velocity was constantly changing so from instant to instant the state of the universe was altered. This sort of state change I would call a change of the first kind. The satellite is just following Natures preferred path, a spacetime geodesic in this instance.
Now consider an event where a meteorite collides with the satellite. Each object is following its own geodesic but the geodesics intersect at a point where both objects happen to arrive at the same time. Clearly both objects will be knocked onto new geodesics, changing the state of the universe. Events of this sorts involving multiple objects I would call a state change of the second kind.
If I move my pen from the right side of the computer to the left, I have changed the state of the universe, even if most of the universe doesn’t care or even know. Clearly both the pen-right and pen-left states are possible future states. I would like to call this sort of state change a change of the third kind. State changes of the third kind involve wilful action by a living change agent. Most of the universe, most of the time, only suffers state changes of the first and second kind. I will think more about changes of the third kind later.
Changes of the first and second kind have causes originating in Nature’s preference for minimum fuss and preceding events. In principle we could follow the string of events that led up to a collision between two asteroids all the way back to the big bang. That notion is called causal closure.
Let’s handle the state changes of the first kind first. These state changes include no change at all. Think about the satellite in orbit or a stone dropped from the leaning tower of Pisa. They are both following geodesics, changing state as they go in accordance with Nature’s preference. Their future states are predetermined by their history. As "now" approaches a predetermined future time the state approaches the predetermined corresponding state. Until "now" carries an object undergoing state changes of the first kind to the predetermined future time, it risks interference by anything on an intersecting geodesic if the timing is right, precipitating a state change of the second kind. For instance the rock dropped from the leaning tower can not enjoy state changes of the first kind for more than a few seconds before the inevitable collision with the Earth. Until the arrival of "now" at the predetermined future time, the state of an object at that future time is uncertain. It is only with the arrival of "now" that the state of the universe is made real (realized).
State changes of the second kind are more difficult to explain. Here I will lift another result from Einsteinian Relativity. No object, or even information, can travel faster than the speed of light. To do so would violate the law of causality, which says no event may happen before its cause in any reference frame. The speed of an object is represented on a spacetime diagram, with time and distance in consistent units, by the slope of the curve connecting the series of events “object found here”. Such curves trace out the history of an object’s location in space and are called world lines.
Imagine that an object is proceeding along Nature’s preferred path and scheduled to arrive on the t-axis at future time tf. If it is to suffer a state change at the point (0,tf), the agent of that change, another object being swept along with the advance of "now", anything from a planet to a photon, must be close enough to the t-axis to get there at time tf. In other words it must be on the green line in the below spacetime diagram.
Spacetime Diagram – Past light-cone of future time
Now if we turn time back on and "now" for both objects creeps up the t-axis toward tf at the speed of light, the volume of spacetime from which events might impact the state of an object at (0,tf) is reduced. The green segment shrinks to zero length as "now" reaches tf. At that instant no further changes at any point in space are possible, since each point is subject to the same analysis, and the state of the universe at tf is realized.
More specifically, let’s imagine that we want to predict the state of the universe at midnight tonight. Remember that we threw out two spatial dimensions in drawing the spacetime diagram so the green segment actually represents a spherical volume in space.
As "now" approaches the appointed hour, the volume of spacetime in which events may have an impact at any particular point in the universe, changing the state of objects there, is shrinking. At one nanosecond (10-9 seconds) before midnight nothing that happens outside a sphere in space of 0.30 meter radius can alter the state in which we will find that object at midnight. At one picosecond (10-12 seconds) to midnight that radius has shrunk to 3x10-4 meters. At one femtosecond (10-15 seconds) the radius is 3x10-7 meters) and at one attosecond (10-18 seconds) the radius is at 3x10-10 meters, about the separation of atoms in a solid.
For distances about 3x10-10 meters or less, object behavior is dominated by quantum mechanical effects. Does that mean that relativity theory no longer holds at those distances? There has been some discussion that spacetime itself may become grainy at some point. Even if that is true, the scale of that effect is thought to be at the Planck Time (about 5x10-44 seconds) and Plank Length (about 2x10-35 meters). At that scale the distanced between the particles that make up the nucleus of an atom are enormous so "now" has plenty of room to carry on isolating the points in spacetime from anything that might cause a state change there.
As you can see, as "now" approaches the hour of midnight, relativity pares away the number of events that can affect the state at any point in the universe until ultimately the clock runs out on all the alternative states that were possible, leaving the universe in its midnight state. Once that state is realized by the shedding of alternatives it becomes part of the frozen past as "now" sweeps on past the midnight hour.
In summary, "now" is when what is... must be. It is the great destroyer of possibilities. If you want conditions "over there" at some future time to be different than they are "now", you best set up the conditions "over here" for that change before "now" gets too close to the chosen future time. That is what we old sailors call being forehanded. It is a pretty good policy for getting on in life as well as for not being taken by surprise by the effects of relativity.
It remains for us to address the state changes that fall under the rules of quantum mechanics. That will be the topic of another page.