Human life is inextricably linked with time. We are used to measuring the course of processes with it - and it is also necessary for our survival. But is this phenomenon a fundamental property of reality, or is time emergent?
For decades, theoretical physics has been trying to combine quantum mechanics and General Relativity into one theory of quantum gravity. But one of the main obstacles is the so-called time problem.
In quantum mechanics, time is universal and absolute: its constant course dictates entanglement between particles. At the same time, in the General Theory of Relativity - Albert Einstein's theory of gravity - time is relative and dynamic, it is a dimension inextricably intertwined with spatial dimensions, thus forming a four-dimensional fabric of space-time. This tissue bends when there is a substance on it, because of which everything that is around it - if it has a greater mass - begins to fall towards it, slowing down the passage of time relative to clocks in the distance. The same effect can be achieved by getting into a rocket and accelerating with fuel: time will slow down for you, you will not age as fast as your friends and family on Earth.
Uniting quantum mechanics and general relativity requires reconciling their absolute and relative understanding of time. Gradually, research in the field of theoretical physics seems to bring scientists to the point of unification, as well as an understanding of the true nature of time.
Many leading physicists today are inclined to believe that space-time and gravity are emergent phenomena. The curving and curving spacetime and the matter in it are akin to a hologram emanating from a network of entangled qubits (quantum bits of information), like a three-dimensional environment in a video game that is programmed in the form of classic bits on a silicon chip. Theoretical physicist Mark Van Raamsdonk of the University of British Columbia said: "I think we now understand that spacetime is, in fact, just a geometric representation of the intricate structure of these fundamental quantum systems."
Researchers have developed a mathematical apparatus with which they have shown how a hologram appears in "toy" universes with the geometry of space-time in the form of a "fish eye" - in anti-de Sitter space. In these twisted worlds, spatial increments contract more and more as they move from the center. Ultimately, the spatial dimension extending from the center contracts into nothingness, reaching its limit. Having this limit, which contains one less spatial dimension than internal spacetime, or “bulk,” aids in computation by providing a solid foundation on which to model entangled qubits projecting a hologram within such a universe. Inside the beam, according to models and calculations, time begins to bend strongly along with space.
The states of qubits evolve in accordance with universal time, as if executing sequences in computer code, while producing curved, relativistic time in a bulk of anti-de Sitter space. The only but - in our Universe, everything does not work quite like that.
Here, the fabric of spacetime has a De Sitter geometry, stretching out as you look into the distance. The tissue stretches until the Universe hits a limit that is very different from that in anti-de Sitter space - and this will be the end of time. At that moment, during an event known as the heat death of the universe, spacetime will stretch so much that everything in it will lose causation with each other. We can say that then time will collapse. As soon as this happens, nothing will happen in the Universe.
At the timeless edge of our space-time bubble, the entanglements linking qubits (and encrypting the dynamic interior of the universe) would presumably remain intact, since these quantum relationships do not require signaling. But in this case, the state of the qubits must be static and timeless. This line of reasoning suggests that somehow - just as qubits at the edge of anti-de Sitter space generate an internal region with one additional spatial dimension - qubits at the timeless edge of de Sitter space can generate a universe over time, in particular with a dynamical one. Scientists have not yet figured out exactly how to carry out these calculations in De Sitter space - there is still no clear understanding of the origin of time.
In the 1980s, physicists Don Page and William Wutters discovered a clue. Page, now at the University of Albertus, and Wutters, who worked at Williams College until 2017, discovered that a globally static entangled system can contain a subsystem that evolves, from an observer's point of view, within it. Such a system, known as a "historical state," consists of a subsystem entangled with what might be called a clock. The state of the subsystem differs depending on whether the watch is in a state in which the hour hand points to one, two, three, and so on. Nevertheless, the general state of the system with the clock does not change, since there is no time as such. This is an unchanging state. In other words, in the global sense, time does not exist, but an effective concept of time for it arises in the subsystem.
In 2013, a team of researchers from Italy experimentally demonstrated this phenomenon. Summing up their work, the scientists reported: “We show how the static entangled state of two photons can be viewed as developing from the point of view of an observer using one of the two photons as a clock to assess the temporal development of another photon. However, an outside observer can show that a globally entangled state is not developing."
Another theoretical work, also carried out in 2013 by researchers at the California Institute of Technology (Caltech), led to very similar conclusions. Geometric patterns - like an amplitudehedron - that describe the results of interactions between particles, also imply that reality arises from something timeless, completely mathematical. However, it is not yet clear how exactly the amplitudehedron and holography are related.
In The Order of Time, physicist Carlo Rovelli also describes time as an emergent phenomenon. According to him, the absolute concept of the simultaneity of any two events does not exist due to the limitations of physical laws. For example, even looking at an object, we see it at the wrong moment in which we looked at it, for at least two reasons. So, light needs to travel some distance from the object to the eye, and then the visual signal needs to reach the brain, where it will eventually be processed before we "get the picture."Rovelli argues that time is nothing more than the result of approximations and simplifications that allow us humans to perceive reality in accordance with our limitations.
From time to time, works appear, in particular on the study of quantum systems, in which independence from cause-and-effect relationships, the passage of time backward, and many other unusual phenomena are assumed. Perhaps time can indeed arise from timeless degrees of freedom through entanglement. Time will show.