How Quantum Mechanics and Relativity can tell the same story
1. A tale of two theories
There are two widely successful theories trying to explain the universe right now: Quantum Mechanics (QM) describes the subtle interactions of quantum objects, and Relativity describes the elastic geometry of space and time. Unfortunately, they don’t seem to fit together.
QM claims that reality is probabilistic in nature, but Einstein and other realists couldn’t stand QM rejected classical concepts like determinism or locality.
Unluckily for them, tests and experiments showed QM works like clockwork. So, over the years, fixed properties were replaced by fuzzy, random probabilities. But oddly enough, this randomness follows rules: once a value is set somewhere, instant correlations can arise elsewhere, even at distances where information has had no time to arrive yet… And that’s just the tip of the iceberg.
Einstein passed away trying to reconcile both theories, with no avail. He’d have gone mad if he had realized he had the answer before his eyes.
2. Chasing a beam of light
When Einstein was young, he wondered what it would be like to chase a beam of light. Years later, this insight led him to develop Special Relativity (SR) from two simple postulates:
- The laws of physics are the same for all observers in uniform motion relative to one another.
- The speed of light in vacuum is the same for all observers, independent of the relative motion of the source (c = 299.792.458 m/s).
With those two assumptions, he changed the way we understand space and time:
- There’s a speed limit: c is the speed limit in the universe. Nothing can travel faster than c.
- Matter moves below c: Nothing with mass can reach c speed (because it would need infinite energy to accelerate).
- Spacetime warps: Objects in motion experience length contraction and time dilation. (It has to be that way if we want to keep the speed of light constant, despite observer’s motion.)
Relativity changed our ideas about absolute space and time. Concepts like distance, simultaneity or duration became flexible and dependent on motion.
But it’s kind of tragic that the same question that led him to develop SR could hold the key to understand QM.
3. A brave new world
SR replaced Galilean transformations by Lorentz transformations, the ones that include the Lorentz factor that warps space and time for moving objects. Then, Einstein should have reformulated the question he asked when young, the other way around: How does light experience our universe?
Now, we’ll see reality through the eyes of a photon. A very interesting experience:
- Always at c speed: Photons are massless quanta of light, so they always move at c in vacuum.
- No space or distance: Going at c, photons experience maximum length contraction. So much contraction they have no concept of distance. All the 3D paths we think they describe wandering the universe are compressed into a single point for them. The evolution of the material world defines those “paths”, yet photons feel just an emission/absorption point.
- No time or duration: They experience maximum time dilation. So much dilation they don’t know what time is. Their internal clock is so slow that any interval we could imagine would be instantaneous for them. Imagine a photon being emitted when the universe began, being absorbed whenever the universe ends. All those billion years would be just an instant for that photon.
- No evolution: Thus, photons can’t evolve while traversing vacuum at c, because they feel everything as a single interaction. The vast distances we see in our universe don’t exist for photons “in flight”. For them, fields and field lines describe how emission and detection points connect together. They feel emission point “glued” to detection by the field lines that lie on free space, so any medium in which they don’t travel at c would join with the next seamlessly.
This is the strange way a photon sees the universe while traversing vacuum. They feel leaving emission and arriving at detection at the same time. c is the ultimate spacetime compression factor, as nothing can experience anything faster than instantly, no matter what others could say about its speed.
4. Through the looking glass
Photons are the gauge bosons for the electromagnetic interaction, but there are other force carriers for other interactions out there. They’re the basic building blocks that hold up our universe and give us information about it.
We see photons as energy transfers between two points, propagating at c through space (being emitted somewhere and arriving somewhere else some time later). But from their own point of view, they’re not going anywhere nor taking any time to arrive there. For massless carriers, emission point blends instantly with detection point as they pop up into existence.
But does this sound familiar? Something that connects two distant spacetime points in no time… A wormhole!
Yes, quanta travelling at c are more or less like wormholes in our universe. Any photon that pops up into reality experiences all its history at once, while it seemed to be travelling through vacuum for others. Both points of view only agree after the fact, so detection point is the only common link for carriers and observers. In a sense, detection point is where/when all photon’s history takes place.
So we should picture photons as “spread” (somehow) through the spacetime we perceived between emission and detection. This doesn’t mean photons have predictive superpowers or that our actions are predestined, just that we see the same events and interactions from wildly different perspectives.
The way we see events taking place is so different to their own perspective that any physical path we imagine makes no sense for them: paths, shortcuts, bifurcations and choices have no meaning when you go at c, because life feels spaceless and instantaneous at that speed.
Imagine you compress a spring instead of folding paper to illustrate how wormholes work. You see? You don’t need extra dimensions to connect spacetime points together; things going at c do the same trick all the time! Any energy quantum traversing the universe right now is doing just that. Amazing, isn’t it?
Yet, this bubbling sea of photons and force carriers is what defines our reality. They don’t experience much of the universe, but they create it for us, shaping our notions of space and time. Massless carriers are like tiny random brush strokes on an expressionist painting: We are the ones that can make sense of the big picture, because they are meaningless one by one.
So we are lucky we move below the speed of light, because that’s the only way we can collect information and know about the universe!
5. Reality bites
Now let’s suppose we’re moving at c and start decelerating. What would happen then?
- Space and time dimensions appear: Decelerating from c to any other speed will change our experience dramatically: Going at c, we were unaware that the rest of the universe existed (although the rest of the universe is what allowed our pointlike experience to exist). Now, as we reduce speed, the universe will unfold upon us and start interacting. Our pointlike experience will expand into a 3D network of interactions, evolving around us. So space and time dimensions emerge from the ability to relate the interactions we can perceive now we move below c.
- Causality develops: As we reduce speed, we are not limited to feel everything in a single event or interaction, and we start collecting different entities that are able to reach us. We get the concepts of time and causality by arranging similar, periodic or frequent events that happen to us while we traverse our lifespan below c.
- Laws of physics emerge: Although the exchanges that pop up into our new reality are different ones each time, they keep a remarkable consistency. This recurrence (both in space and time) is what defines the rules for the evolution (the physical laws) for the universe we can perceive now we move below c.
- Distances and intervals unfold: Space expands and time slows down as we reduce speed, because we go from feeling everything instantly to having to collect events bit by bit while traversing a stretching path in space.
6. Down the rabbit hole
All the facts we’ve discussed trying to see the world through the eyes of a photon have quite massive implications for our understanding of the universe:
- Matter feels spacetime: Each piece of matter moving through the universe experiences different spacetime ratios, depending on its motion.
- Massless carriers don’t feel spacetime: Massless quanta at c don’t experience spacetime dimensions. Distance, duration, locality or causality get meaningless at light speed.
- Interactions are new each time: Each observer has the consistent illusion of an evolving universe caused by the continuous detection of carriers coming from other places. This constant influx of information composes the “frames” of reality that allow them to infer the dynamics for matter and energy in the universe. These causal rules only apply to “persistent” matter connections, where floods of similar exchanges (closely related in origin, distance, frequency and duration) keep taking place consistently, but they’re not valid for each individual interaction.
- Total energy of the universe is zero: Any energy transference we observe is an instantaneous action-reaction event for the carriers involved, so they don’t even feel the energy imbalances that set out their journeys, as their proper time is zero. The universe is a zero-sum game of energy conversions, but for each interaction supporting the play, everything appears balanced in an instant.
- Absorption is part of any exchange: Any carrier traversing any medium at c will be absorbed somewhere else. It has to be that way because, otherwise, the journey never really starts (remember, emission and absorption is the same thing for them). A carrier “in flight” is just the promise that an exchange, interaction or measurement will take place, no matter what. Once we know a carrier has left birth place, we can be sure it’ll pop up elsewhere following the c speed rule, because the contract is already settled for the carrier. If we put something trying to stop it, our actions contributed to the history it tells when it materializes on our detector.
- Carriers don’t have intrinsic properties: Any spatio-temporal magnitude or constant we assign to a massless carrier when detected doesn’t belong to the carrier itself. Those properties describe something about the conditions that took place during the whole journey, what made that particular carrier manifest in reality. Properties like energy, frequency or momentum are specific to each individual matter interaction, but vacuum constants like Planck constant or the speed of light tell something about the way all exchanges come into existence.
- Interactions shape the geometry of spacetime: General Relativity states that matter and energy warp the geometry of spacetime. Now, we know the interactions that transmit information also support spacetime, so we could say matter presence sets how and how many exchanges develop each time. That’s why mass (or charge, or flavour, or any other property of matter) is able to “bend” spacetime: curved spacetime is nothing more than a higher probability of having interactions on a region due to how matter is arranged (there and elsewhere). There’s even a mass/volume ratio (the Schwarzschild radius) that defines regions of space where the matter configuration is so dense that any carrier generated inside can’t escape anywhere else, as all possible paths end on a matter particle contained there (that is, black holes).
- Time travel to the past is impossible: We can compress or expand time intervals changing velocity, but not reorder the interactions. Observers can’t reach instantaneity, as they move below c. It’s true that each observer has its own subjective way of ordering events depending on speed, but there’s no way an observer could detect a carrier before it’s emitted.
- c speed = infinite speed: It doesn’t matter there’s a speed limit in the universe because, for all purposes, things that travel at c feel they’re instantaneous. They traverse any distance in no time (for themselves) so they’re, effectively, teleporting instantly at infinite speed.
- All forces are contact forces: Long time ago, humanity thought all forces were contact forces, but then some forces seemed to act at a distance. Later on, we stated all forces act at a distance by gauge boson exchanges (except gravity, maybe?). But, from the point of view of things going at c, all forces are “contact” forces because they don’t feel any spacetime taking place, they just “happen to be”.
7. Everything is relative
Now, we’ll use the insights we’ve uncovered thinking massless carriers act like wormholes to explain QM’s weirdness. Here’s the list of topics we’ll cover next. Each link takes to the Wikipedia page about each concept:
- Wave-particle duality
- Wave function collapse
- Bell’s theorem
- Faster than light communication (FTL)
- EPR paradox
- Scrhödinger’s cat experiment
- Double-slit experiment
- Wheeler’s delayed choice experiment
Two or more quantum objects are entangled when they share a common quantum state and nonlocal correlations after an event or interaction. Whenever one of those particles gets perturbed, the others will know instantly, even if they had no time to exchange signals at the speed of light.
This phenomenon is easily explained now we know the whole spacetime concept is meaningless for things that go at c: All carriers from a common emission point feel directly connected. Carriers can’t evolve going at c, so it doesn’t matter if they went lightyears apart in opposite directions, emission and detection points form one simultaneous ensemble from their point of view. When we detect a carrier related to an entanglement event, we’re not intercepting a messenger; we’re dealing with the entanglement event directly, as if no time had elapsed.
We think that a matter particle has to start sending information at c the moment we perturb it or detect it, but the entangled particles were exchanging information instantly the whole time: the carriers that shape their fields don’t experience any spacetime intervals.
So we can’t trick information carriers playing time games, because they see our time intervals as a single instant and, this way, they’ll be always ahead of our intentions. We call “spooky action at a distance” something they’d rather call “action at spooky distance” (or “action at spooky time”, for that matter). Whenever there are things connecting events at c, they’re sharing the same conditions in a way that, if they don’t tell a consistent story, then the entanglement event never happened.
The fact that we see a process where they see a simultaneous event explains why we need weird spacetime mechanisms to describe their connection (the forward and backward waves used in the Wheeler-Feynman absorber theory and QM’s transactional interpretation, or the retroactive Parisian zigzag entangled particles would perform over the Einstein light-cone, as proposed by O. Costa de Beauregard), but all information carriers that create our reality right now are correlated with other carriers that bring the same consistent histories to other times and places in the universe.
A property or a state of an object is nondeterministic when it has no real value until measured. QM showed that, at the quantum level, observers can’t detect something without disturbing it, so the act of measuring is what forces a property to take a value.
As said, emissions and detections are necessary parts for any interaction, so any observer is part of the reality it detects, because it’s making it happen. An observer is merely a bunch of absorption points, so they force reality to manifest around them (they “bend” spacetime, as explained earlier). When we place a detector somewhere, we aren’t just intercepting carriers “in flight”, our operations (and the rest of the universe configuration) create the conditions that “got the journey started”. In some way, the emission point got “linked” to our detector by our actions. What we call “the future” is part of the carrier’s instantaneous experience when detected, so the future we talk about is, in fact, always in the past. In this sense, they “always knew” our detector would be there. But the “wormholes” that connect the emission point to our detector wouldn’t exist without it in place. They’d be different wormholes, with histories that fulfil the conditions to reach other absorption points in the universe.
Anything we can perceive is conformed as a spacetime process, so nothing we can see is made of “fixed values”. Fixed properties are just recurrent or structured fluctuations. Besides, we always need at least two events to define a relation, a process or an interval, so our knowledge about anything existing out there is always at least one spacetime unit old. This seems to be a small interval, but quite literally, the universe we see each time is completely brand new, made from information carriers never detected before by any other observer.
7.3. Wave-Particle duality
QM needs wave-particle duality to explain why the same quantum objects sometimes behave as waves and sometimes as particles, depending on the measurement setup.
All the “schizophrenic” behaviour appears because we apply our spacetime concepts to instantaneous pointlike events. For the massless carriers, the whole process happens in an instant, at detection time. Being just a point, they can’t decide about travelling a path or a thousand. Every possible path they could traverse at c from start to finish is “real” (much in the same way as the Feynman path integral formulation). We use wave-particle duality because we don’t know how to make sense of dimensionless interactions from our spacetime perspective.
Interactions shouldn’t be pictured as particles or waves propagating through space, but as spacetime connections “unfolding” or “decompressing” from detection time into the past, conforming each observer’s spacetime lattice. Only the complete emission-flight-detection process is fully equivalent to the carrier’s instantaneous experience. This “spacetime projection” would be equivalent to what the carrier would feel if it could decelerate. It’s also remarkably similar to the Huygens principle, a rule applied to build waves and explain propagation, diffraction and interference.
So carriers get wave behaviour when we “map” their instantaneous existence into our spacetime lattice. Trying to get an answer in the middle of this process is worthless, as we’re missing a lot of information that isn’t really available until detection time. That’s why we are forced to explain the quantum world with waves, propagation, probabilities, superposition and collapses. We can’t predict quantum behaviour better than we do, because detection point is always ahead of us in time.
7.4. Wave function collapse
In QM, every quantum object has an associated wave function that is used to keep track of the possible states it could have due to the fact that its properties are not set until measured. Once measured, wave function collapse takes the object to a certain state. In fact, every system (even the whole universe) has a wave function. The many worlds interpretation is based on the idea that the universal wave function describes everything that could possibly happen.
Wave function collapse is one of the most controversial features on the Copenhagen interpretation, because it needs an observer to collapse the wave function and this leads to all kind of discussions about what an observer is, when the collapse actually happens or how it finally takes place (measurement problem).
But there’s only one possible state for things moving at c, because they feel everything in an instant. Our idea of a superposition of states evolving in a wave of possibilities until we cause a sudden collapse in all space at the same time is absurd for the carriers, because they only see the factual realization of the instantaneous exchange. Every exchange is an agreement on the conditions that allowed the emission-detection connection to happen (much like the Relational Quantum Mechanics interpretation states). Wave functions just describe our own ignorance about an event or interaction we know “started”, but has not become real yet.
All classical analogies for explaining the quantum world consist of a “preparation” stage (using hidden variables) and a “revealing” stage, where the prepared system evolves in time. That’s why we can’t simulate the quantum world classically, because, in this case, the preparation stage IS the revealing stage. At the same time.
Take the Monty Hall problem as an analogy for wave function collapse. The probabilities for the outcomes evolve by how information is revealed in time, but nothing really changes behind the scenes. That’s how entangled carriers can beat Bell’s inequality: their reality is the backstage, and our reality is the broadcast. They appear to switch doors while we choose randomly. We are clueless players in a show where quantum entanglement sets the backstage each time.
Our spacetime descriptions always exclude part of the information the carriers have by living an instantaneous reality. We can only access information as it unfolds in time, so we model reality the same way. Collapse is needed because it reintroduces the part of evolution we left out by extrapolating behaviour only from emission point.
Massless carriers know nothing about things happening “before” or “later”, so thinking wave function collapse is a real process makes no sense. It only proves the limitations of our predictive models, as we’re trying to describe final outcomes that have not yet happened, while they’re “unfolding” in spacetime.
7.5. Bell’s theorem
John Stewart Bell’s inequality showed that there’s no way a theory with local hidden variables could reproduce QM’s predictions. So, in order to match QM’s predictions, any theory has to discard local reality. Then, according to Bell, we only have two ways to explain QM’s weirdness: faster than light communication or superdeterminism.
Bell, like Einstein, didn’t realize reality is extremely different at the speed of light. Entities that travel at c don’t need predictive superpowers, retroactive signalling or predestination to know what will happen to them because they are able to “compress” any spacetime interval into an instantaneous event. Our reality is, then, recreated each time by detecting those exchanges, so it’s no surprise we can detect hints of weird connections across space, when spacetime structure itself emerges from the immense number of correlated connections that develop each time. Spacetime behaves like a quantum error-correction algorithm, where anything that happens keeps consistency with everything else that is directly related to it.
SR showed space and time depend on motion, but it also shows determinism and locality disappear at the speed limit. We always take our material world experience (causality and locality) as the correct point of view, while we should also take into account how carriers experience existence (instantaneity and entanglement). Any interaction that happens in the universe complies with this dual nature of reality.
If time is meaningless for things that go at c, then properties can’t have specific values “before” measurement. In the same way, if distance is meaningless for things that go at c, then what’s the difference between a local or nonlocal thing? For a force carrier, our spacetime interval is localized at its point-instant, so who contains who? Is a photon from the sun really travelling through space to our eye at c? Or is that spacetime interval the only way we have to understand that a sun-eye interaction took place?
7.6. Faster than light communication (FTL)
We tend to think there’s superluminal or instantaneous communication between two parts of the universe when performing quantum experiments, because we observe strange correlations for quanta at those places.
But we don’t need to go beyond the speed of light for faster than light communication, because whenever something is seen as traversing the universe at c, it feels it’s in fact simultaneously at all the possible points of the journey. All the things we detect travelling at c experience instant correlations with other carriers from the same emission event, forming a spacetime network that spreads through the universe at c for us, but feel as one simultaneous ensemble from their point of view. These correlated quantum networks “dissolve” quickly as they mingle with matter, creating the coherent reality all observers around emission point detect.
If you are mind blown by the existence of quantum correlations and instant “communication” between distant places, imagine if a force carrier were told that its point/instant of existence can contain a complete universe for others… That’s mind-blowing! Seeing the universe this way, the idea of extra dimensions becomes almost naive.
The universe is a vast network of interconnected nodes, and each node sees parts of this network in a different configuration depending on its speed and position respect all other things moving out there. But for carriers at c, we are the “angels on the head of a pin”, dancing on extra dimensions they don’t even feel.
7.7. EPR paradox
The Einstein-Podolsky-Rosen paradox is a thought experiment closely related to quantum entanglement: Two entangled particles are launched in opposite directions. From then on, the spin measured on one particle is related to the spin measured on the other instantly, even when they can’t communicate. The correlations that appear after repeating the experiment follow a distribution that doesn’t match classical predictions, but QM’s predictions. As stated by Bell’s theorem, there’s no way to explain this behaviour using local hidden variables.
It’s kind of weird that Einstein created this riddle after having found the solution, but we should be able to understand what’s happening by now: Each massless carrier detected is at the same time the history of all the spacetime conditions that spanned from emission to detection. All carriers that appear spreading at c from entanglement event never “evolved”. They still feel connected to the entanglement event, no matter when/where they get detected in the universe. All these events reveal a correlated structure that would have manifested in a different way depending on the material world configuration and evolution. They match each other like pieces in a jigsaw puzzle, even if detections had happened any other way. Detections let observers deal with the entanglement event as if they were really there and then, so the entanglement event must tell the same history than all related detections, or reality would be inconsistent for the different observers.
The moment information about something spreads at any other velocity than c, it lose correlation with the original event. If a carrier could slow down just a bit for a moment, some amount of space and time would appear. It would get “detached” from the rest of carriers that go at c for that particular event, and it can’t do anything to reach them again. From then on, it’ll tell a story of causal relations, not instant quantum correlations.
7.8. Scrhödinger’s cat experiment
The Scrhödinger’s cat experiment is closely related to wave function collapse: A cat is set in a box where there’s also a radioactive particle that can decay (or not). If the particle decays, radiation activates a mechanism that releases some poison, killing the cat. As the decay is probabilistic in nature, it’s said that, if we leave the box closed, the cat is in a superposition of quantum states, so the cat is dead/alive until we open the box and collapse its wave function.
This experiment shows all the problems that derive from the Copenhagen interpretation:
What is an observer? It seems we have one observer (ourselves) and one wave function (the cat dead/alive) but, in fact, there’s a chain of nested observers and wave functions. All problems about who collapses what appear because we use the same wave function when, in fact, each observer at each level would have different wave functions being collapsed at different moments by different interactions.
When do wave functions collapse? All wave functions for the things inside the box are enclosed by another wave function about the box’s state, so the nested ones aren’t taken into account until we open the box. The cat could be dead by now, but if we don’t open the box, it’s still in superposition for us. But even if we open the box, our friend outside the room has a wave function waiting for us. So we could go all the way up on this ladder of nested wave functions (reaching the many worlds interpretation), or we could go down and realize that the interaction between the radioactive particle and the poison mechanism (if it happens) would be a “collapsed wave function” already, because carriers can’t evolve at c. So you can see our brain’s wave functions are not that special to what happens in the universe. We’re not that different to any other particle out there, we’re just a bunch of emitters and absorbers, participating on the interactions we trigger and detect.
How does collapse happen? There’s just one event that can change the cat’s state, and that’s the particle radiating. Some other exchanges can update our knowledge about the cat’s state, but they’re not fundamental for its fate. If it were not for us, cat’s carriers would be decoded or discarded by any other observers. If we leave the box closed forever, the cat’s state will evolve almost the same, but interactions will take place between cat’s atoms and box’s atoms.
7.9. Double slit experiment
The double-slit experiment is closely related to wave-particle duality: When we launch quantum objects through a small slit, they form a continuous pattern on a screen at a distance. But if we fire them through two slits, they create an interference pattern, even when launched one at a time. It’s said that each quantum object travels as a wave (passing through both slits), but hits the screen as a particle. All the hits reveal the interference pattern of the quantum wave.
Wave behaviour is always present, whenever we perform single-slit or double-slit experiments. The single-slit pattern is similar to the one macroscopic objects would create, but diffraction (a purely wave phenomena) is present in both cases. It just happens that double-slit experiments can show wave or particle behaviour much more dramatically, depending on the information we acquire:
- Getting which way information: If we determine which slit each quantum object goes through, we’re effectively doing a single-slit experiment each time, because we’re forcing some information carriers to end at the slit’s detectors, virtually closing any other open slit and preventing them from reaching the screen. We’re modifying the way information would have “propagated” undisturbed, so those carriers don’t fulfil the conditions to create the interference pattern. Although we think the detectors at the slits make no difference, revealing some information modifies the probability for other outcomes elsewhere (like on the Monty Hall problem). There’s some time delay and re-routing involved, so we switch from instant correlations to causal relations. This way, we get the superposition of two one-slit diffraction patterns, much like the pattern macroscopic objects would create.
- Without which way information: If we only measure the interference pattern at the screen, we don’t disturb the space in between trying to catch carriers “mid-flight”, so they keep delocalized the whole journey, they don’t have a “checkpoint” in between. This way quantum information can keep the correlations, as if really passing through both slits at the same time, revealing the interference pattern.
QM explains the double-slit experiment using wave-particle duality and wave function collapse: The quantum object is emitted in wave/particle superposition until an observer collapses it, setting what the quantum object was the whole time (wave or particle). Then, observers discover in amazement each quantum object acted as wave or particle the whole time, in order to match the configuration of the experiment.
There are other QM interpretations, like the De Broglie-Bohm theory (or pilot-wave theory) that don’t use superposition or collapse: Particle and wave coexist as different entities. But this theory is nonlocal, and it seems to go against SR. But we’ve seen SR allows nonlocal effects even in a universe with a speed limit.
QM’s weirdness appears because we catch carriers behaving in ways that don’t follow our spacetime rules. But there’s no fixed spacetime background, no absolute local realism. The actual events and interactions we collect each time shape our concept of a spacetime background supporting reality. So this stability or persistence of spacetime is what we should consider “weird”.
7.10. Wheeler’s delayed choice experiment
Wheeler’s delayed choice experiment is a double-slit experiment in which they change the setup while quanta traverse it, trying to understand when quanta change behaviour along the journey. Also, there’s the quantum eraser experiment, and even a delayed choice quantum eraser experiment, in which some information (like which-way information) is acquired, but gets discarded later on, with no real consequences for the final pattern detected.
Quantum objects seem to be aware of all the changes, evolutions and configurations of the measurement setup since emitted, adapting their appearance to wave or particle “retroactively”. But we should remember that all the landmarks, checkpoints and obstacles they traversed are present for them when they get detected, just as if they were omniscient of everything that happened during their journey (because they are!) The spacetime interval we see doesn’t exist for them but, at the same time, the actions we performed during that interval were part of what made them exist.
Entities sharing carriers feel connected, even when they get detected by different observers at different times and places. In a sense, any carrier detected has an associated volume of space where all events happening there “conspired” to make it exist at detection point. When this happens, it’s as if the observer was dealing with emission point directly, with no considerations of time or distance involved. That’s why two causally separated carriers can keep consistency even when they can’t communicate: observers are interacting directly with the entanglement event when they deal with the related carriers.
After an interaction, we use our common sense (based on space and time) to try to understand how they knew all rearrangements and adapted their behaviour. The answers is they knew about the whole setup at once (be it a lab or a galactic cluster) because all the conditions that made them appear as they did were met.
Measurements and detections are fundamental parts of events, interactions and exchanges. Each detected entity tells the story of the whole spacetime interval that took place. If anything happening during the emission-absorption interval had been a bit different, then the carrier wouldn’t be exactly the one perceived.
The same redundant spacetime information gets encoded once and again inside the vast number of matter interactions being detected at any given time. They all converge, “crystalizing” into a coherent image of the universe. A consistent causal reality. For each observer. Each time.