# Common Sense Theory of Time * Pg 261 - Figure 11.2 is absolutely misleading (and actually it took me a while to see this because *my* view of time is so linked to this picture). The issue is that we want the picture to show multiple moments. We see that trajectory of the arrow over time, shown by successive snapshots. So far so good. However, the motion of the present moment is *not* shown. The illustrator tries to shoehorn it in by adding the "Motion of the present moment" label, but this doesn't fix the problem. The figure really just shows "now" at a *single moment*. But it shows the arrow at *multiple moments*. And that is the issue! The figure contains a [Contradiction](Contradiction.md). Common sense theory of time says that now "flows" through time. But it only shows now at a single moment. It looks like it pops into existence, and then immediately ceases to exist. And if that were so, it would make "now" a *fixed moment*. * Deutsch then argues that there is no single "present moment" of time, except subjectively. There is no *objective* present (pg 263) * Pg 262, 263 - relates to [Consciousness](Consciousness.md) and [Dust Theory](Dust%20Theory.md) * We do not experience time flowing, or passing. What we experience are differences between our present perceptions and our present memories of past perceptions. We interpret those differences, correctly, as evidence that the universe changes with time. We also interpret them, incorrectly, as evidence that our consciousness, or the present, or something, moves through time. * [Infinite Regress](Infinite%20Regress.md) * The origin of this sort of mistake is that we are accustomed to time being a framework exterior to any physical entity we may be considering. We are used to imagining any physical object as potentially changing, and so existing as a sequence of versions of itself at different moments. But the sequence of moments itself, in pictures like Figures 11.1—11.3, is an exceptional entity. It does not exist within the framework of time — *it is the framework of time*. Since there is no time outside it, it is [Incoherent](Incoherent.md) to imagine it changing or existing in more than one consecutive version * [Intrinsic](Intrinsic.md) vs [Extrinsic](Extrinsic.md) * could someone argue that the present isn't a single moment, but rather an attribute that can be applied to any moment? * Best summary so far this chapter of the big issue * But there is more to this problem than the difficulty of illustrating the common-sense theory of time. The theory itself contains a substantive and deep equivocation: it cannot make up its mind whether the present is, objectively, a single moment or many — and hence, for example, whether Figure 11.1 depicts one moment or many. Common sense wants the present to be a single moment so as to allow the flow of time — to allow the present to sweep through the moments from past to future. But common sense also wants time to be a sequence of moments, with all motion and change consisting of differences between versions of an entity at different moments. And that means that the moments are themselves unchanging. So a particular moment cannot become the present, or cease to be the present, for these would be changes. Therefore the present cannot, objectively, be a single moment. * Perhaps ‘paradox’ is a better word than mystery, for we have here a blatant conflict between two apparently self-evident ideas. They cannot both be true. We shall see that neither is true. * I like the idea of focusing on the "paradox of time" as a key theme of the start of this chapter * Newtonian physics treated time as an [Extrinsic](Extrinsic.md) dimension (i.e. as independent of space) * While a four-dimensional geometrical interpretation, where time is considered a fourth dimension alongside the three spatial dimensions, was possible in Newtonian physics, it wasn't essential for the theory's coherence * Einstein's Relativity and the Indispensable 4th Dimension: Einstein's theory of relativity revolutionized this understanding by demonstrating that time is not absolute but relative. The crucial insight is that observers moving at different velocities do not agree on which events are simultaneous (i.e. they do not agree which events occur within the same snapshot). This means they have different perspectives on how spacetime is "sliced up" into moments. What one observer perceives as simultaneous events, another observer might perceive as occurring at different times. This relativity of simultaneity makes the four-dimensional geometrical interpretation of time indispensable to relativity theory * Figure 11.4 depicts spacetime as a stack of snapshots, with each snapshot representing a "moment" in time as perceived by a particular observer. However, relativity tells us that different observers will slice spacetime into "moments" differently, leading to different sets of snapshots. * Figure 11.5 represents the objective reality behind Figure 11.4. It shows spacetime as a unified four-dimensional entity, without any pre-defined "now" slices. The different "moments" perceived by various observers are simply different ways of slicing through this unified spacetime block, like cutting a loaf of bread at different angles. * Moments as Subjective Perceptions: This implies that the "moments" shown in Figure 11.4 are not objective features of spacetime itself. They are simply one observer's particular way of perceiving simultaneity, influenced by their relative motion. Another observer would draw the "now" slices differently, resulting in a different set of snapshots, but the underlying spacetime would remain the same * Spacetime as the Block Universe: This understanding leads to the concept of spacetime as a "block universe," where the entire history of the universe - past, present, and future - is laid out in a fixed, four-dimensional structure. Nothing "moves" through time in this view; what we perceive as motion or change is simply the difference between the contents of different spacetime slices * Big Idea * Go through sean carroll book on chapter 5 + 6 (he directly discusses this and has good images around how time is not simultaneous for different observers from different perspectives) * That is how it must be if the concept of time is to be freed of the error of invoking an overarching framework of time that is external to physical reality. In other words, it is an error to think that time is *external* to physical reality. * No External Time: The concept of a snapshot having a time stamp encoded within its physical contents is crucial for liberating the concept of time from an external framework. There's no need for an absolute, external clock to tell us the order of events. The snapshots themselves, through their physical attributes and the laws of physics governing them, provide the necessary information for their arrangement. * Time as an Intrinsic Property: This perspective underscores the idea that time is not an independent, flowing entity but an intrinsic property of the physical universe. It's encoded within the snapshots themselves, embedded in the changing states of natural clocks. * Ordering the Snapshots: Deutsch proposes a thought experiment where we could, in principle, reconstruct spacetime by analyzing the intrinsic order of snapshots. By calculating the preceding and following snapshots based on a starting snapshot and the laws of physics, we could build up the entire spacetime, ensuring the correct temporal order. [Take Theories Seriously on Their Own Terms](Take%20Theories%20Seriously%20on%20Their%20Own%20Terms.md) Misc notes on Standardizing and Calibrating Natural Clocks * The Need for Standardization and Calibration: Deutsch uses the concept of "natural clocks" to explain how time is encoded within the physical universe. Natural clocks are physical processes that change in a predictable and measurable way over time, allowing us to infer how much time has elapsed since a particular event. However, different natural clocks might operate at different rates or have different starting points. Therefore, to establish a consistent and coherent timeline across the universe, we need to standardize and calibrate these natural clocks. * Overlapping Time Ranges: Deutsch highlights the importance of overlapping time ranges for this process. When two or more natural clocks have overlapping periods of applicability, we can compare their readings during that overlap. For instance, imagine we have two natural clocks: * Clock A: Tracks the changing chemical composition of the Sun. * Clock B: Measures the decay of a specific radioactive isotope found in rocks. If there's a period where both the Sun's composition and the radioactive isotope are changing measurably, we can compare their readings during this period. * Establishing a Common Scale: By comparing the readings of Clock A and Clock B during their overlapping period, we can establish a conversion factor between them. This conversion factor allows us to express the readings of one clock in terms of the other. For example, we might determine that a certain change in the Sun's composition corresponds to a specific amount of decay in the radioactive isotope. This establishes a common scale for measuring time, even though the two clocks rely on different physical processes. * Extending the Timeline: Once we have a common scale, we can extend our timeline beyond the overlapping period. Even when Clock A is no longer applicable (e.g., if we're considering a snapshot before the Sun formed), we can still infer a time based on the readings of Clock B, using the conversion factor we established. This allows us to create a consistent chronology across vast stretches of time, piecing together information from different natural clocks. * Ensuring a Coherent Picture of Time: Standardizing and calibrating natural clocks ensures that we have a coherent and consistent picture of time across the universe. It prevents discrepancies and contradictions that might arise from relying on uncalibrated, disparate natural clocks. This process is essential for reconstructing a meaningful and unified timeline for spacetime, even in the absence of an external, absolute timekeeper. --- Date: 20241202 Links to: Tags: References: * []()