Time in Classical Physics
This is a summary of the first half of ‘Time: The First Quantum Concept’ from The Fabric of Reality by David Deutsch. This half covers time in classical physics.
Time is change.
In classical physics, it consists of moments, and change is the difference between those moments.
Time is also the structure of change: Moments are arranged in a geometry that is determined by the laws of physics.
The flow of time
If something exists at a particular moment, it exists there forever. There is no universal clock that keeps track of a ‘now’, where things come into being and subsequently disappear.
There is also no such thing as the flow of time. What we experience as the passing of time is the difference between our memory of the past and our subsequent experiences.
Since the flow of time doesn’t exist, our subjective experience of time can speed up, slow down, stop, start, or go backwards.
The block universe
The whole of physical reality—past, present and future—is frozen, once and for all, in a single four-dimensional block. It consists of three space dimensions and one time dimension. This block is called spacetime or the ‘block universe’.
A snapshot of time consists of a three dimensional slice of this block that collects events happening at the same time. When the content of one slice is different from another, we call that ‘change’.
Later, we will see the block universe is not correct, because it does not consider quantum mechanics.
Relativity
Due to special relativity, observers at different velocities do not agree on the content of snapshots. This is because they don’t agree on which events are simultaneous.
However, if the observers stack up their own snapshots, both of them will end up with the same block of spacetime. They just sliced it up at different angles, to yield different snapshots of simultaneity.
These angles are part of the reason why there is a geometry to change. Time is the name of that geometry.
Timestamps
If you start with a single snapshot, the laws of classical physics will determine what happens in all the other snapshots, past or future. The laws of physics are the glue that hold these snapshots together to form a spacetime. This property that snapshots are determined by other snapshots is called ‘determinism’.
The physical content of a single snapshot and the laws of physics also determine the order of the other snapshots. The physical is effectively a timestamp, because it determines the snapshot’s position in the sequence of moments.
This is the sense in which the everyday notion of time as a clock exists in physical reality: The content of a snapshot is effectively a timestamp. There is no other measure of time and the universe is not keeping track of time with a universal clock.
Clocks
Describing the entire contents of a snapshot is one way to denote its timestamp. But it is not always necessary. Some snapshots have actual clocks that were made by humans and you can use that clock instead.
You can also use a natural clock like the arrangement of the atoms in the Sun as it changes over time. Anything that takes definite and distinct values on different snapshots can be a clock.
The Big Bang
Before the Big Bang (or after the Big Crunch), there is no spacetime. There are no moments and nothing happens or exists.
Time doesn’t start or stop at those events because time does not flow or start or stop, any more than space does.
It is only our mistaken notion of the flow of time that makes us wonder what happened ‘before’ or ‘after’ the whole of reality.
Causation
In the block universe, there is no freewill or causation because everything is already determined. We cannot make choices and even our decision to mull over a choice is determined.
Statements like “If Faraday had died in 1830, then technology development would have slowed down”, also have no meaning in classical physics because neither their premises nor their outcomes can happen. These kind of statements are called ‘counterfactual conditionals’.
By incorporating quantum mechanics into time, we can account for causation and counterfactual conditionals.
Some parts of this summary were taken verbatim from the book. I hope to summarize the second half of this chapter someday.