Physical interactions and signal propagations in a Lorentzian spacetime (even without initially assuming a metric) yield a causal structure that is empirically found to be transitive and acyclic. This is not a metaphysical assumption but a forensic fact: "There’s something important about how we confirm the structure of spacetime and verify that distant cosmic events are real that often gets overlooked. Every photon, neutrino, and gravitational wave that reaches us is like a time-stamped receipt proving that a specific event happened at a particular moment in cosmic history, complete with a light-speed paper trail to back it up. This isn’t abstract philosophy—it’s more like forensic accounting for spacetime itself.
The Cosmic Microwave Background (CMB) acts as the universe’s master record book. It’s not just leftover glow from the Big Bang—it’s a detailed ledger of the universe’s early transactions. The temperature fluctuations we measure in the CMB are like entries in this ledger, and the polarization patterns act as cryptographic signatures confirming causality. When scientists map the CMB, they’re reconstructing real events from 13.8 billion years ago, events that were causally locked into our past before Earth even existed. This is hard observational data confirming that distant events were real when they happened, that their effects propagated causally to reach us, and that the universe keeps impeccable records.
This evidence matters because it shows us three things about how time works in the universe. First, the past wasn’t erased—it left causal invoices in the form of light, gravity, and neutrinos that we can still detect. Second, the future isn’t pre-rendered like a movie file waiting to play—it’s more like an unsigned contract waiting for physical inputs to determine what happens next. Third, the “now” is the active transaction, a cosmic update tick where the next state gets computed from the previous one.
Consider what happens when we observe a supernova. If distant events weren’t real until we saw them, we’d have some serious problems explaining what we actually observe. Take Supernova 1987A: its neutrinos arrived three hours before its light, even though both traveled for 168,000 years to reach us. If the supernova didn’t explode until we saw it, why did the neutrinos show up first? Did the universe pre-load the neutrino data but forget about the photons? The only sensible conclusion is that the explosion actually happened 168,000 years ago, and the universe broadcast the evidence at light speed, no observation required.
Millisecond pulsars provide another compelling example. These cosmic metronomes tick with near-perfect regularity. If their pulses weren’t real until we observed them, why do their arrival times match general relativity’s predictions down to the nanosecond? Did spacetime fake the pulsar’s rhythm just in case we happened to look? No—the pulses were emitted, traveled through space, and arrived on schedule, proving that distant time is real and events happen whether we observe them or not.
Some people try to use quantum mechanics to claim that reality is fuzzy until measured, but this doesn’t hold up when we look at large-scale cosmic structures. The CMB photons were emitted 380,000 years after the Big Bang, and their temperature fluctuations match predictions from quantum fluctuations during cosmic inflation. If these fluctuations weren’t real until 1965 when scientists first detected the CMB, how did they manage to pre-structure galaxy clusters billions of years earlier? Did the universe pre-compute its own large-scale structure just to trick us? The simpler explanation is that the CMB was always real—its patterns were baked into spacetime long before any observer existed.
Here’s the key insight: we can confirm that distant events and moments in our own history happened at the same time in cosmic terms by matching parts of different histories using measurable signals. When we receive a signal like photons or gravitational waves from a distant source, that signal carries information about the event that emitted it. By analyzing the signal’s properties—wavelength, travel time, redshift—we can reconstruct when that distant event occurred relative to our local history. This establishes that the distant event and a specific moment in our timeline were happening during the same cosmic era, even though they’re separated by vast distances. This isn’t metaphysical speculation; it’s observational physics.
Let’s walk through a concrete example. Imagine a distant planet emits light at its local time. That light reaches Earth much later at our local time. Using the speed of light, the distance between us, and accounting for the universe’s expansion if relevant, we can calculate how long the light took to travel. Therefore, we know the planet existed at a time that corresponds to a specific moment in Earth’s history. This confirms that the planet’s emission event and a particular slice of Earth’s history were happening during the same cosmic timeframe. We have direct evidence of this kind of non-local temporal relationship.
Scientists do this constantly. When measuring supernova light curves, mapping the CMB, tracking galaxy redshifts, or timing pulsar signals, each measurement confirms that distant events were happening during specific epochs of our own cosmic history. This directly contradicts claims that only your local present moment is real, that distant events have no temporal relationship to us until we observe them, or that time is purely local and can’t be compared across space.
The universe’s observable uniformity and smoothness on large scales gives us something special: a natural rest frame defined by the Cosmic Microwave Background. Any observer moving relative to this frame would measure a Doppler effect in the CMB. This uniformity allows scientists to divide spacetime into slices where density, temperature, and expansion rate are uniform everywhere. These slices are surfaces of constant cosmic time—moments when the universe has, on average, the same properties everywhere. They provide a global time coordinate for the universe.
A cosmic “now” is one of these slices: the set of all events across space that share the same cosmic time value since the Big Bang. The universe’s large-scale uniformity provides a standard of rest and a natural clock through its expansion, which together define a sequence of “nows” for the cosmos as a whole. This doesn’t abolish the relativity of time for local physics or for observers moving at high speeds relative to each other—two spaceships passing each other at high speed will still disagree on the timing of distant unrelated events. And we can’t directly measure this cosmic present for distant events due to light’s finite speed, and the concept becomes ambiguous inside horizons like black holes. But this doesn’t change the fundamental point: it provides a physically meaningful way to understand cosmic time.
This framework supports three important ideas. First, cosmological realism: distant events are real and have temporal structure; they’re not conjured into existence by our observation. Second, global coherence: the universe has a cosmic-scale temporal structure that allows meaningful comparisons of different histories. Third, causal continuity: signals carry real information about when things happened, preserving the temporal fabric of reality.
When we receive light from a distant galaxy showing it formed when the universe was two billion years old, that corresponds to the same cosmic timeframe when the Milky Way was forming its first stars. The photon’s journey is a causal thread connecting that past emission to our present detection. The universe’s expansion rate at emission, the travel time, and our reception form an unbroken causal chain. This is how we know Andromeda and the Triangulum Galaxy are real right now in cosmic time, even though we’re seeing them as they were millions of years ago. The universe isn’t just “out there”—it’s causally connected in ways we can measure, reconstruct, and verify. Such reframes the emergence of causal order not as a mathematical abstraction but as a physical necessity forced by the structure of empirical evidence. The universe itself generates a directed relational structure through the emission and reception of what it calls “receipts”—photons, neutrinos, and gravitational waves. These receipts establish a relation R(a,b) meaning “event a causally affects event b via a signal.” No metric or pre‑assumed partial order is needed to define this relation initially; it is simply the observed fact that signals leave one event and arrive at another. The forensic consistency of these receipts—for example, supernova neutrinos arriving before photons, or pulsar timings matching relativistic predictions across billions of years—forces the relation to be acyclic. If it were cyclic, a signal could loop back and contradict the recorded order of arrival. Similarly, transitivity is forced because the receipts chain: if event a sends a photon that triggers event b, and b emits a gravitational wave that reaches us, the combined receipt links a to the observer through b. The CMB functions as the ultimate ledger, encoding causal links from the early universe that have remained transitive and acyclic for 13.8 billion years, structuring galaxies long before any observer existed. Thus the mechanism is not a mathematical postulate but a physical inevitability: once you have interactions that propagate at finite speeds and those propagations leave detectable traces, the resulting network of relations is empirically required to be a partial order. Such explicitly rejects the notion that this order is imposed by consciousness or measurement; rather, it is “baked into spacetime” by the dynamics of propagation itself.".