HAVE WE FINALLY FIGURED OUT WHERE TIME CAME FROM?
We go over all of the discoveries made so far about the origins of the universe.
Fill a glass halfway with hot water. If you leave the water alone for a while, it will cool down to the temperature of the surrounding environment. When you put an ice cube in a glass, it gradually melts until the water’s temperature matches that of the surrounding environment. Thermalization is a natural and irreversible phenomenon that occurs when two systems come into contact, as both tend to what experts call thermal equilibrium.
This seemingly innocuous phenomenon introduces a fundamental asymmetry in physics: it defines a time arrow. From our daily experience, it always flows from the past to the future. “We can mix up east and west, but not yesterday and tomorrow,” Sean Carroll, a physicist at the California Institute of Technology, explained (U.S. United States). Except that in thermodynamics, forward flow is a mute companion. “The fundamental laws of physics make no distinction between the past and the future,” Carroll explained. Physics, according to the Italian Carlo Rovelli, ignores the problem: “It describes how things evolve in their times, not how they evolve in time,” Rovelli explains. In that sense, equations are symmetrical in everything from mechanics to electromagnetism and quantum theory. As a result, if we see a video of two billiard balls colliding, we won’t be able to tell whether we’re having a good time or going backwards. However, if we watch another video in which a glass of water transforms into an ice cube, we will realize that we have been given cat by rabbit. This fundamental disparity between our surroundings and the laws of physics appears to indicate that something is missing. But what exactly?
Ludwig Boltzmann, an Austrian physicist from the late 1800s, was the first to raise the issue seriously. In his time, he proposed many revolutionary ideas, such as the existence of atoms and the fact that temperature was caused by their movements and collisions. Boltzmann, who was short-sighted, saw much further than his colleagues, but he was despised by them and committed suicide in 1906.
His ideas persisted, and thermodynamics, the science that studies heat, would be impossible to comprehend without his contribution. Among his efforts was an explanation for entropy, a strange concept introduced in 1865 by one of the fathers of thermodynamics, Rudolf Clausius (1822–1888). He was introduced by this German physicist and mathematician to explain why heat flows from a warm body to a cold body. Based on this, he developed a fundamental principle, now known as the second law of thermodynamics: natural processes are those that demonstrate an increase in the entropy of the universe, and never the other way around. Clausius thus defined the meaning of the time arrow.
But what exactly is entropy, and why should it rise? These are the unanswered questions that Boltzmann solved. Boltzmann concluded that the most likely state of any system is disorder based on the assumption that the world is nothing but “atoms and emptiness,” as Democritus stated two millennia ago. For example, if we mix some cards, we will most likely end up with a deaf deck rather than one organized by sticks and numbers. The same is true for gases: we could have all of their molecules moving in the same direction; or two gases contained in the same container that are not mixed, but separated; or one compressed, without external influence, in a corner of the vessel that contains it, leaving the rest completely empty… There is no law that prevents any of these scenarios from occurring. However, it is highly unlikely, especially since a deck is organized by numbers and sticks after being mixed.
This principle must not be overlooked: there are more disordered than ordered combinations. We can now define entropy as a measure of the disorder in nature. Entropy tends to increase as order becomes more likely. This was Boltzmann’s major contribution, and it was the one that led to the problem of the arrow of time: the future differs from the past simply because the entropy of the universe has increased.
If we consider cosmological evolution and the widely accepted theory of its origin, the big bang, the universe must have been in a very low entropy — that is, very ordered — state when it was born. “The second principle of thermodynamics suggests that any system naturally evolves into a typical and more likely state,” Rovelli emphasizes. “However, we also assume that the universe began in an extremely atypical and improbable state.” “If your configuration were chosen at random from all possible configurations,” Carroll says, “it would be in a state of very high entropy.” How is this even possible? “It’s something we don’t know how to answer,” Carroll writes in his book From Eternity to Today. And it doesn’t sound very convincing that something so well-organized could emerge from a massive explosion…
Now, there is something in this entire discussion that we have overlooked. The universe is not a thermodynamic system because it is governed by an external force: gravity. Although it must affect entropy in some way, we don’t really know how, because doing so requires a subatomic-scale gravitational theory, which we don’t yet have.
All of these stumbling blocks must be overcome in order to comprehend why the universe that arose immediately after the big bang did so in a highly improbable state of order. And we’re not even sure what kind of response would suffice. Apparently, after half a century, the big bang hypothesis continues to bind our best physics theories: without a way to describe exactly how the universe came to be, we can’t explain why it had low entropy and, thus, understand the arrow of time. And here’s the million-dollar question: what is a high-entropy state like when gravity becomes a relevant component of the system?
Roger Penrose has been working on this problem for many years. This distinguished English physicist and mathematician contends that the formation of structures in the universe, such as galaxies, stars, and planets, does not imply a decrease in entropy, but rather the opposite: “Things tend to be different when it comes to gravity […]. When gravitational bodies interact, they generate a lot of entropy “are stacked together According to Carroll, “the states of greater entropy are like empty space, with most particles scattered and progressively diluting” in the presence of gravitational forces. That is, in the same way that the universe was in its early days.
To all of this, we must add an incredible phenomenon that occurred in the very first moments of the universe’s existence: inflation. It was proposed at the end of 1979 by the American physicist Alan Guth, and it states that we will never know what happened just before the big bang because the entire universe — or at least a region of it — experienced an exponential increase in volume. Such an expansion erased any irregularities that may have existed in the beginning, resulting in a flat, curvature-free, and homogeneous cosmos. Although this amazing mechanism explains many of the mysteries left unanswered by the Great Initial Burst theory, it does not explain why the universe was born with such low entropy.
Many theoretical physicists will raise their hands and point to string theory as a solution to the problem, but this may not be the case. She was the star model of physics at the end of the last century, and her spokespeople, such as Brian Greene and Michio Kaku, raised her to the heavens, declaring that we were facing the theory of everything. However, it has gradually deflated. Some, like Indian string physicist Shiraz Minwalla, see it as a new way of doing science: “It has more to do with a mathematical theory inspired by physics than with physics in the classical style.” To all of this, we must add that, like almost any other fundamental physical theory, its equations do not make a clear distinction between the past and the future, so the arrow of time does not emerge naturally; it must be added ad hoc.
Some experts argue that we should look in a different direction, to the competing string hypothesis: loop quantum gravity. Lee Smolin, a Canadian physicist, is one of those attempting to solve the mystery. The universe, according to him and his collaborator, the Portuguese Marina Cortês, is made up of completely unique events that never repeat. Because each set of events can only influence the next, the arrow of time appears naturally. “We hope to get to the problem of the universe’s initial conditions and discover that they aren’t so special,” Cortês says.
Solving the problem of the cosmos’ initial entropy has sparked the imagination of theoretical physicists: some argue that, while known physical laws do not contemplate the existence of a temporal arrow, this is explained by the fact that they are not the true ones, but only good approximations. We’ll see how it appears naturally once we’ve discovered the fundamentals. Penrose, for example, proposes Weyl’s curvature hypothesis: there is a natural law that distinguishes between past and future space-time singularities and endows the universe with a temporal arrow.
Deep down, these are all attempts to explain the gap between physical laws and reality. But, regardless of which hypothesis is correct, or if it is a combination of both, they all leave out one fundamental question: when did time begin to flow?
This is where the research by Thomas Gasenzer and Jürgen Berges, physicists from the University of Heidelberg (Germany), published at the end of 2019 comes in. “How did the arrow of time appear if you start far from balance, as happened at the birth of the universe?” Berges wonders.
What these scientists have created is a theoretical construct to explain what happened at the subatomic level before the process of thermalization of the universe began, i.e. when entropy began to rise. Things were not as we know them at the time: the universe was a vast ocean of quantum energy that was “far from balanced,” according to Gasenzer.
According to Heidelberg physicists’ calculations, that energy field had fractal properties similar to what happens when we remove a cup of coffee. When studying turbulent fluids in 1941, one of the greatest mathematicians of the mid-twentieth century, the Russian Andrei Kolmogorov (1903–1987), described them. By removing the coffee, we create a vortex (spiral rotating flow) that causes smaller ones to appear, and so on. They feed on the energy transferred to them by the main vortex, as in a cascade, and at an exponential rate, as described by the law of powers. German scientists discovered that the same phenomenon occurs in quantum systems that are far from equilibrium. The difference between Kolmogorov’s coffee and the systems of Berges and Gasenzer is that in the first case, the energy cascade is verified in space — in different areas of the coffee-, whereas in the second case, it is also produced in time. And this detail is crucial because it means that if we could observe that ocean of primordial energy over time and at different scales, we would see that everything remained exactly the same, as frozen, in a state of no change, no time.
And it would have remained in this state indefinitely if not for a sudden phase change similar to what happens when we have supercooled water-liquid, but below zero degrees-and introduce a small impurity, such as a speck of dust: it freezes abruptly and instantly. According to Heidelberg scientists, something similar happened to the universe. A very slight disturbance ended that non-time state governed by fractal dynamics, resulting in the formation of a dense soup of quartks (protons and neutron bricks) and gluons (particles that hold the quartks together). The universe becomes subject to the second law at that point, and entropy begins to increase in an active process that will last billions of years.
The important thing to remember is that there was no time, no history before that phase change. The universe arose from an initial singularity, a large explosion, and inflation, which multiplied its size by 1026 in an infinitesimal fraction of a second. That primal energy could have remained unchanging indefinitely, with no past, present, or future. Then, all of a sudden, there was a tiny change that imposed a direction of flow on the energetic baby, an arrow of time.
Can this theory be put to the test experimentally? Markus Oberthaler, also of the University of Heidelberg, has taken a first step in this direction by employing 7000 rubidium atoms. He formed a Bose-Einstein condensate, a state of aggregation of matter below the solid, with them and after two years of work at a temperature very close to absolute zero. Surprisingly, the properties of matter in that situation are similar to those of quartk-gluon plasma at extremely high temperatures. Oberthaler explained in his September 2019 article that during the forty seconds that the condensate remained stable, they injected a lot of energy and observed its evolution. It was far from equilibrium, and they discovered that it followed Gasenzer and Berges’ fractal dynamics. “It appears to be proof of the existence of this universal scale law,” Oberthaler concluded. Meanwhile, CERN is hard at work developing an experiment that will generate primordial plasma. Then we’ll see if we’ve finally discovered the origin of time.
