a very brief tour of the history of reality concepts
Aristotle and the relativity of up and down
In the old days down was always down, and that was the direction the apple fell from a tree. Earth was considered to be flat and therefore "down" was absolute.
But then people took a closer look at moon eclipses. It was already assumed at the time that the moon only reflects the light of the sun, but doesn't produce light on its own, since the bright side of the moon always shows towards the sun. And a moon eclipse always happens when sun, earth and moon are in a perfect line. What covers the sunlight therefore must be the earth's shadow on the moon. And that shadow is round. So Aristotle came up with the idea that earth is actually a sphere. People didn't like that idea although they already knew that the horizon sometimes appears slightly curved as well. .
People found out that the sun at the same time of the day at different places was located at different angles on the firmament. This as such would be even the case on a flat earth. However knowing the distance of two locations in north-south direction gives an estimate for the distance of the sun from earth. And that distance estimate determines the angle at any other location. And these angles didn't match, when a flat earth was assumed. But they did match up when assuming earth to be spherical. And so the concept of up and down has been proven to be relative.
Galileo Galilei and the relativity of locations in space
In the old days earth (whether flat or spherical) was located in the center of the universe. Sun, moon and stars were attached to a big sphere around the center, called firmament. This was fine for fixed stars (the vast majority of objects on the firmament). But some others moved along the firmament in a very complicated way.
Copernicus and later Galilei came up with a simpler solution where all the planets, including earth, moved around the sun, while the moon moved around earth. All objects would now move on (almost) perfect circles. Despite its beauty and simplicity this idea was widely rejected.
The theory implied that all the fixed stars would have to look as if they were moving in small circles too, which in fact would be a consequence of earth moving around the sun. This could not be observed back then because the fixed stars are much further away than people thought, and therefore the movements where too small to be observed with the telescopes available at the time. This contributed to the rejection of Galilei's theory. But later this movement has been observed indeed. And so the position of earth in space was nothing special any more. This was hard to digest, and Giordano Bruno still burned on stake for assuming that there might be millions of other earth-like stars in the universe.
Albert Einstein and the relativity of concurrency
In the late 19th century physicists reliably observed that the speed of light is the same regardless of whether the observer of the light moves or not, and in what direction. This makes no sense at all in the Newtonian world. If, for example, it is about sound instead, the speed of sound does indeed depend on the movement of the observer. This is why the noise of an approaching train or car has a higher pitch than that of a train or car moving away.
Einstein came up with a completely weird concept to explain this anomaly. His idea was to make time and space relative. If the coordinate system for space and time of an observer that doesn't move (purple) would be different from that of an observer that moves (blue) then the speed of light would appear to be the same in both coordinate systems. The only thing being static or absolute is the light itself (green). The point in space-time (which is the global realm of time and space for differently defined coordinate systems altogether) though, where the light appears after, say, one second is different in both systems. Mathematically speaking this is the only consistent explanation for the invariance of the speed of light. However the practical consequences are enormous. It not only follows that matter is a form of energy. The more creepy thing here is that time and space are interchangeable. Something that is "later" "here" from my own point of view, can be "now" "elsewhere" (which is "here" for me) for somebody else (who is "here" "elsewhere" for me) due to their different movement through space-time. When things now elsewhere do exist ("now") then things here later must exist ("now") as well. Past and future exist in the same way, thus the future is already there. This is the meaning of Einstein's metaphorical quote: "God doesn't play dice".
A mathematical consequence of Einstein's theory was to expect less time to have passed for somebody who moves away from a certain point and then changes direction to come back, compared to somebody who stayed at the same place (or strictly speaking didn't change their state of movement). When this effect was confirmed by observations people should have accepted the relativity of synchrony and the fact that the future is already there. But even the existence of nuclear bombs, which also proves relativity right, hasn't changed the Newtonian world view of most people.
Ludwig Boltzmann and the relativity of the arrow of time
Many things that happen are non-reversible. A bottle that is being thrown to the ground disintegrates into many pieces, causing tiny movements in the ground and a slight warming of both the ground and the bottle pieces. But tiny movements in the ground together with a slight cooling of the ground and the glass pieces will (practically) never cause the glass pieces to jump up and bang into each other in such a way that a glass bottle will be formed. Order changes into chaos, but chaos doesn't change into order.
However all known physical laws have been found to be entirely reversible. On a micro level things that can happen in one direction can happen in the other direction too. So why can order move to chaos but chaos never moves to order? The reason is basic probability theory. Starting with a certain state there is always a large number of possible states at some later point in time. Only a tiny fraction of these states will be more ordered than the previous state, while the vast majority will be less ordered than the previous state. That leaves one question: Why do highly ordered states like bottles or whole planets exist at all? And how could evolution create such an enormous amount of order to build living beings that can easily build factories that easily produce thousands of bottles every day?
Only recently this question has been rephrased: How come that WE can observe all this complexity? The answer is "anthropic bias": All the planets without intelligent and conscious life are not observed by anybody. And if our universe would have happened to be less structured in the beginning (without even forming solar systems, solid matter and such), then chances for intelligent life would have been much lower too, and most likely nobody would be there to observe them. Evolution may be unlikely in terms of thermodynamics, but we can only observe it in those rare cases where it happened to be successful.
Hugh Everett III and the relativity of destiny
Laser light that goes through a double slit forms typical interference patterns on a screen behind it. This is the same principle as a double slit in water. Light, like water molecules, also consists of particles, called photons. Something weird is going on though with light. Here the interference pattern still occurs when the laser is muted down until only single photons go to the double slit one after another. Single photons form a wave, where they absolutely shouldn't (the famous wave-particle duality). They behave as if they were somehow interfering with themselves. But in the end, when we look at the scene, there is only one photon left at one well defined location. The most common interpretation of this strange behavior (the "Copenhagen interpretation" of quantum physics) proposes that a photon really is at different locations at the same time and "decides" where it "really" is, as soon as we observe it. Therefore in the micro world nothing can be predicted precisely. Everything that happens is (in part) pure coincidence. And the photon simply disappears from all the other locations where a photon also was before. The photons at the "other" locations are considered to be "phantom" particles that were somehow less real than the "real" photon remaining in the end.
The Copenhagen interpretation not only contradicts Einstein's relativity in claiming that the future will only be determined once we observe it. It also implies that one particle either really has different locations at the same time or different particles have a different quality of being "real". And it implies that passive observation changes reality. Hugh Everett proposed a radically different interpretation. He claimed that there always really are several different real particles at several locations (even where we can only see on single particle), and therefore there is no such thing as "non-locality" with particles not having a well-defined place where they are. Also every assumed particle is real in the same way. He further claimed that it isn't the particles that change through our observation, but that it rather is us that change when observing a scene. Depending on which of several particles we observe several instances of "us" become different. We become part of the so-called "super position" of the particles ourselves, rather than the super position magically disappearing (the famous collapse of the "wave function" in the Copenhagen interpretation). This logic inevitably implies a large multitude of parallel universes that partly overlap (which is when we observe a super position). This theory is strictly "local" as everything has a defined place. The only unsettling consequence is that there are always multiple instances of everything, including us. The upside is: the contradiction to relativity's determinism is solved as all the parallel universes are already there. We only don't know in which of the universes we will end up, or, more precisely speaking, what the universe is like in detail, that we are already part of.
It logically follows from quantum physics (regardless of the interpretation of reality that we derive from it) that a thing called "quantum computer" is possible. This is a machine that makes use of a large number of simultaneously "entangled" particles that seem to be at several locations at the same time (Copenhagen interpretation) or that are part of different parallel universes (many worlds interpretation). This machine uses the information of all the multiple instances of the particles. The amount of information increases exponentially with the number of particles. Since there is no theoretical limit to the size of a quantum computer (although the practical obstacles are enormous), the accessible amount of information is vast, to say the least, and vastly exceeds the size of our universe. This thought experiment doesn't prove the many worlds interpretation to be true in an empirical sense, but it is a strong epistemological argument. What is real is basically defined by the information it carries. The amount of information that a quantum computer could, in theory, access (in fact the magnitude of all parallel universes) must therefore be connected to some reality providing this information that is of the same magnitude.
Going with the many worlds interpretation also shines a new light on the question that came up with Boltzmann and statistical physics. The highly structured form that we observe in our universe (and that made evolution possible) would also be due to anthropic bias. The vast majority of unstructured universes simply remain unobserved. Also in a multiple universe the a priori chance for the complexity generated by evolution in a given solar system in a given parallel universe could be even much lower than creationists claim without making the observation at some place (where an observer happened to have emerged) any less plausible.