Once upon a time, there was no time, until ‘nothing’ exploded and evolved into everything. Sounds confusing, right? Indeed, it is; scientists feel the same way. Over the past few decades, several different theories have been put forth to explain the first few moments before and after the universe, in its primordial state, came to fruition; ranging from a big bang, a big chill, to a big bounce (a 4D exploding star from another universe was thrown in for good measure).
However, not a single theory or hypothesis eloquently covers every base. Even the most formidable model, the big bang theory, has holes: aspects that haven’t been reconciled with modern physics yet. Not the least of which deals with time. Researchers hope that by answering our questions about the universe’s creation, we might also explain one complication, time: where it came from; what powers it; and why it flows in a certain way.
In a new paper, researchers explore the possibility that not one, but two universes took shape following the big bang. The differences would not only be mind-boggling, with time in the other universe running backwards, but this theory could fill in some of those gargantuan holes we mentioned earlier.
Specifically, they believe their research brings clarity to several issues with the big bang model: including the original state of entropy, the direction the arrow of time moves in, and how gravity and relativity fit into the equation. In said models, physicists have found it rather perplexing that we can throw the arrow of time in reverse, and the system works just as efficiently (in other words, the laws of physics do not favor one direction over the other).
Currently, entropy is one of its biggest problems, and it may also be intrinsically linked to time (rather, the arrow of time). In order to understand entropy, think of it within the framework of thermodynamics (the second law, to be precise). It essentially tells us that isolated physical systems (like the universe) grow more disordered over time, not vice versa. This is why broken eggs do not spontaneously coalesce, and why we do not get younger instead of older (Benjamin Button be damned).
WATCH: “The Arrow of Time feat. Sean Carroll”
“Increasing entropy is a cosmic certainty because there are always a great many more disordered states than orderly ones for any given system, similar to how there are many more ways to scatter papers across a desk than to stack them neatly in a single pile,” remarks Lee Billings (for Scientific American).
Basically, when it boils down to it though, the universe’s entropy was highly ordered at the dawn of time. As the universe evolved from its initial homogeneous state, into a place filled with stars, planets and other types of matter, the system went from having low entropy (disorder) to a higher (more ordered) one, which appears to contradict the second law of thermodynamics
With that said, In this scenario, the arrow of time is merely the result of changing levels of entropy. That, however, depends on whether or not the universe’s initial conditions were indeed ordered; this argument is rendered less important per a new paper, which has been published in the ‘Physical Review Letters’ journal.
In their hypothesis, the big bang birthed our universe and another with opposite characteristics: a mirror universe with a reverse arrow of time.
“Any internal observer must be in one half of the solution and will only be aware of the records of one branch and deduce a unique past and future direction from inspection of the available records,” they write. People think that time moves in one direction because they can only see one half of the universe, in other words.”
This is clearly an oversimplification (it would take half a dozen articles to do entropy justice, and even then it would still be confusing) , so here are the highlights from their paper:
Although a preferred direction of time can occur in models of physical systems, this typically happens only if one inserts very special initial conditions. Julian Barbour at the University of Oxford and his colleagues have now shown this tinkering isn’t necessary to produce an arrow of time in a system of masses interacting via Newtonian gravity. They demonstrate that the evolution of this surprisingly simple system almost always contains a unique moment of lowest “complexity,” a point they identify as a “past” from which two distinct (and more complex) “futures” emerge.
More than a century ago, Boltzmann suggested that our visible Universe might merely be a temporary, low-entropy statistical fluctuation, affecting a small portion of a much larger equilibrium system. In that case, the direction of time would simply be the one that takes us back towards equilibrium. But most contemporary physicists find this explanation unsatisfying: a random fluctuation containing “us” would have been far more likely to produce a single galaxy, a planet, or just a “brain” rather than a whole universe. Moreover, according to the “Loschmidt irreversibility paradox,” if one posits such a moment of low entropy, entropy should increase both to the future and to the past, giving two separate arrows of time
In their gravitational model, Barbour and his colleagues find a state of “low complexity” that is analogous to Boltzmann’s low-entropy fluctuation. But in their case, no rare statistical fluctuation is necessary to explain this state; instead, it arises naturally out of simple physical laws that have no explicit dependence on the direction of time.
Starting with such a dispersed system and running time backwards, one might expect that it would coalesce in the past into a state of high density. Barbour and his coauthors show analytically that this expectation is right: for almost every initial configuration of masses, there is a unique moment of minimum size and maximum uniformity. From this point, the system expands outward, approximately symmetrically in both directions of time. The system is therefore globally symmetric in time, as the equations dictate, and yet has a local arrow of time.
The idea of time proceeding in two directions, towards two futures, from a moment of minimum complexity is not itself new. It has appeared, for example, in cosmological models of eternal inflation. But the emergence of this behavior in a system as simple as the one Barbour and his colleagues consider is unexpected.
As usual, there are other ways to work this particular kink out, but many of them create more problems than they solve. The team stresses, “One possibility, of course, is that we don’t know the right laws of physics—perhaps the correct fundamental laws do determine a preferred direction of time” They continued, “Alternatively, if the laws of nature do not pick out a preferred “future,” perhaps boundary conditions do.”