This Resume will provide summery for every part on that book that contains six parts such as
2. Time may have split off into two directions after the Big Bang.
3. The universe may be increasing in complexity rather than entropy.
4. The three-body problem illustrates how time splits at the Janus point.
5. According to the Janus point theory, entaxy in the universe is decreasing.
6. The Janus point occurs at a moment of total collision.
What’s in it for me?
A mind-bending exploration of time’s potential origin.
Is our universe hurtling toward chaos? Or is the cosmos, including everyone here on Earth, establishing ever greater order and structure?
For more than a century, scientists have been convinced that entropy – in other words, disorder and decay – drives everything in our world. This theory was first put forward by a brilliant physicist named Rudolf Clausius. One of his classic papers ended with this powerful statement: “The entropy of the universe tends to a maximum.”
If that statement is true, it means that everything in the universe is marching closer and closer to its own destruction.
But wait. If that’s the case, why do we continue to see such wonderful and endless variety around us? Why does everything we do, see, and know continue to build on itself? What if it’s not entropy that’s increasing in the universe, but instead order and structure?
These resumes will be your guide to these cosmic, universal questions.
In these resumes, you’ll learn
Why a diver never emerges backward out of the water;
What a game of snooker and the Big Bang have in common; and
How just three particles can turn into a ruler, clock, and compass.
The laws of nature don’t distinguish between past and future.
Picture a diver leaping off a platform. She straightens her body and splashes into the water. Spray fills the air. Now imagine the same scene played in reverse. The diver emerges out of the water, and spray appears to be sucked back into the pool. Cause and effect have switched places. Time has flowed in the wrong direction.
As this example shows, time, as we experience it, has arrows. We are used to living in a world where everything flows in the same direction: forward. Humans, animals, and stars all age and die – they never grow younger.
Clearly, the direction of time is important to life. So it must be somehow enshrined in the laws of nature, right? Actually, as it turns out, it isn’t.
The key message here is: The laws of nature don’t distinguish between past and future.
Let’s think back to those arrows of time. There are many of them, but among the most significant is what’s called equilibration. It’s a process that leads to an equilibrium, and it’s easy to see in action – just take a glass of water, stick your finger in it, and twirl it around. After you take your finger out, the water will very quickly return to its original stillness.
Note, though, that this process never happens in reverse. The water never becomes disturbed spontaneously. Our diver will never emerge backward out of the water. These phenomena are said to be time-asymmetric.
But not everything in the world is like this. Imagine a video in which two identical billiard balls collide on a perfectly smooth table. Say you showed that video to someone who’d never seen it before. You could play the original, or you could play it in reverse, and they wouldn’t be able to tell which ball actually moved first.
At the microscopic level, all the laws of nature are time-reversal symmetric. We, however, are used to asymmetry, to a clear cause and effect, to a past and a future. But why? Why doesn’t the world behave like balls on a pool table?
Well, most physicists would tell you that it’s because of the second law of thermodynamics, which states that entropy always increases. And there’s your answer: the “direction” of time results from this increase.
But the author believes that there’s another explanation.
Time may have split off into two directions after the Big Bang.
Let’s return to the pool table for a moment.
Remember that the two balls colliding was a time-reversal symmetric process. But a full game using these balls doesn’t work the same way. Let’s think about a popular billiard game called snooker, for example. If you watched the whole game in reverse, you’d witness a miracle: all the red balls jumping out of the pockets to line up in a perfect triangle atop the table, with the white ball separating itself from the pack.
That’s because a game of snooker begins with a special condition – the red balls organized in a triangle. Many physicists believe that the universe itself is a bit like this. It was, they think, in a very special condition before the Big Bang. Entropy was very low; in other words, everything was in a state of extremely high order. Right there is where the Big Bang happened – and all time arrows began.
But the author suggests a different explanation. He proposes that the Big Bang was not the birth of time, but just one very special location in time. He calls it the Janus point.
The key message here is: Time may have split off into two directions after the Big Bang.
Here’s the author’s argument: if you agree that the Big Bang happened amid some very special conditions, then you act in an arbitrary manner. To him, that betrays the very goal of science. After all, isn’t physics about describing the world in terms of inviolable laws?
So what’s the solution? It might come from something known as the Janus point theory. It’s named after the two-faced Roman god and imposes no special conditions whatsoever. Instead, this theory argues that the laws of the universe lead to a Janus point – a condition where the size of the universe either becomes zero or passes through a minimal value. Time approaches the Janus point as a single stream and then breaks off into two streams.
If this is correct, the direction of time we experience is dictated by which side of the Janus point we’re on. For example, on the other side, it might seem perfectly normal for that diver to emerge out of water backward.
The Janus point theory also implies that the growth of the universe is not ruled by entropy but by complexity, a measure of structure or order. We’ll explore that concept in the next part.
The universe may be increasing in complexity rather than entropy.
How will the universe end? There are several theories. The most popular is that of heat death. It has nothing to do with global warming – no, in fact it describes the direct opposite. This theory says that the universe will, eventually, reach maximal entropy. Heat will die out completely, and that will leave the universe so cold that motion – and by extension, all life as we know it – will simply cease to exist.
This destruction of the universe through entropy is certainly frightening. But is it inevitable? The author argues that it isn’t. That’s because, in his view, entropy is not actually growing. Instead, what’s increasing is complexity, or, in other words, order. One by-product of this growth is that complex subsystems – like Earth – emerge and become self-contained.
The key message here is: The universe may be increasing in complexity rather than entropy.
There is a problem with how we understand entropy. It all began centuries ago, when scientists first began to use the term. Physicists who studied thermodynamics were doing so in a very special time in history – during the Industrial Revolution. Their focus was on how to improve the steam engine. And this meant that they were exploring thermodynamics in confined conditions; essentially, they were looking at how gas disperses inside a sealed box.
They extrapolated their findings to the universe at large. But the cosmos is not at all like the firebox of a steam locomotive. The universe is not confined in a box – on the contrary, it is constantly expanding.
If the universe isn’t in a box, it won’t seek equilibrium. And this means that we aren’t actually hurtling toward heat death. Instead, what we see is more particles clustering together, or complexity.
Our planet and the heavens above contain lots of evidence for this. Billions upon billions of stars create new elements, which form new molecules, which, in turn, contribute to an ever richer structure.
And here on Earth, we also find the growth of structure everywhere. The rock that lies beneath us, with its multilayered strata, is proof of how complexity has increased over millions of years. On a more local level, the houses we build, and the towns and cities they turn into, are also records of continuously expanding complexity.
The three-body problem illustrates how time splits at the Janus point.
You’ve probably heard the famous story of Isaac Newton and the apple tree. Supposedly, Newton noticed an apple drop to the ground, and that inspired him to develop his theories of motion.
In particular, he became famous for solving the problem of how the Moon moves around the Earth. What exactly he was solving became known as the two-body problem. It’s a subset of a larger theory called the N-body problem, which can help predict the behavior of a finite number of gravity-driven points.
Newton definitely had success with the two-body problem. But he complained that the three-body problem, which describes the motion of three particles, gave him headaches. And that is exactly what we’ll be using to describe what happens in the Janus point model – so buckle up!
The key message here is: The three-body problem illustrates how time splits at the Janus point.
The three-body problem asks us to imagine that the entire universe is represented by three particles. One is called a singleton because it’s unpaired. The other two orbit around each other and are called a Kepler pair.
In the Janus point model, the singleton approached the Kepler pair at some point in the distant past. Then, the motion of the three bodies suddenly became very chaotic. After a short period of time, the system passed through the Janus point, after which it once again broke up into a singleton and a Kepler pair.
The singleton that went into the Janus point didn’t necessarily come out in exactly the same way – it may have switched places with one of the particles in the Kepler pair. This is crucial because it means that the laws of time-reversal symmetry are preserved.
To illustrate this, imagine a young man walking across a ballroom floor. He is looking for a dance partner. Suddenly, he sees an attractive young woman – but she’s already paired with someone else. As the pair approaches our young man, he suddenly swoops in and leads the lady away. All the other man can do is walk on, with his head down.
You can reverse the direction of time such that either partner can get the girl. We would never think about a bizarre reversal like this happening in the real world. But that’s only because it seems to go against everything we’re used to.
In this three-body model, we see no background to give us a sense of the “correct” direction of time – the three particles are simply all that exists. And this means that the laws of time-reversal symmetry are perfectly preserved.
The three-body system can tell us about space, as well as time, as we’ll see in the next part
According to the Janus point theory, entaxy in the universe is decreasing.
Imagine a box filled with gas. What if we suddenly removed all the walls of this box? Well, the particles would soon begin to fly off into space. This means they'd begin to occupy a larger volume. It would seem that entropy was growing.
That’s not the end of the story, though. The particles would move away from that box in a very specific way – their trajectories would remain exactly the same.
Inside the box, entropy rules. When you remove the walls, order takes over; the particles’ motion is highly structured. Does this look like an increase in entropy? Not really.
Similar events play out on a cosmic scale. As particles in the universe are released from their “box” at the Janus point, they move in a way which does not suggest an increase in entropy. To fully describe what’s happening, we need a new concept. The author calls it entaxy.
The key message here is: According to the Janus point theory, entaxy in the universe is decreasing.
Entaxy can be defined as the count of all microstates that can exist within a certain macrostate. On the level of the universe, entaxy is decreasing. We see this as particles regularly cluster together and create highly complex subsystems, like stars, galaxies, and black holes. Within these subsystems is where conventional entropy increases.
The concept of entaxy even holds up in the three-body model, which we used to describe what happens after the Janus point. Here’s how.
As the Kepler pair and the singleton move away from each other, at some point they will inevitably end up in a straight line. We can think of these instances as ticks of a clock. The distance between the particles in the Kepler pair can be thought of as a ruler.
Over time, you can measure how much the distance between the two particles has increased. This lets you determine the increase in time between each “tick” of the clock. Finally, as the singleton moves further and further away from the pair, the angle between the pair and the singleton settles down and becomes fixed.
The Janus point occurs at a moment of total collision.
Let’s go back to our three-body system. In it, the particles are arranged in a sort of triangle. And what’s a triangle? A shape, of course. The three-body model is meant to represent the entire universe. By extension, that means the universe – no matter the configuration of particles – is a shape.
Our universe, of course, has many more than three particles, but the statement still holds true. Shapes in the universe coalesce into subsystems of ever greater complexity and order.
But how exactly does structure emerge out of chaos? And what, precisely, happens at the Janus point itself? The answer to that question also lies with shapes.
The key message here is: The Janus point occurs at a moment of total collision.
To understand what happens at the Janus point, we need to introduce two sets of “rules.”
The first one is Newtonian mechanics. In this model, the size of the universe at the Janus point goes to zero. In other words, its size vanishes. But the universe doesn’t. It still has a shape, something called a central configuration.
In this configuration, gravitational forces direct each of our three particles to the exact center of mass of the system as a whole. Then, a total collision happens. This means that all three particles end up at the exact same point. What follows is a total explosion out of that point. That, essentially, is the Big Bang.
The second set of “rules” is driven by general relativity. General relativity has no problems with a universe of zero size – this doesn’t contradict the theory in any way. But it does have a problem with a universe approaching zero size. Basically, as the size of the universe approaches zero, its shape begins to behave chaotically, like a ball forever bouncing off the walls of a pool table.
That means our shape never reaches a Janus point. There is no Big Bang, no universe, no us.
One theory seems to offer a solution. It relies on a type of matter that modern physics suggests may have been present at the Big Bang. It’s called a massless scalar field, and its properties are such that the number of bounces is no longer infinite. Instead, the shape eventually reaches size zero and then emerges on the other side of the Janus point.
This theory remains unproven, for now. But it sits firmly at the forefront of current scientific research – and, if the Janus point model proves true, we may finally understand time and its arrows.
Final summary
The key message in these resumes:
The current consensus in physics is that the Big Bang was a very special initial configuration of the universe, from which all the arrows of time now flow. The author proposes that instead, the Big Bang was merely a Janus point – a special location through which time passed and then branched off in two different directions. If true, it would mean that the universe is driven not by the increase of entropy, but rather by the growth of structure and order.
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