Sharing my learnings from the book, The Janus Point by Julian Barbour
The Janus Point by Julian Barbour
In The Janus Point renowned physicist Julian Barbour presents a major new solution to one of the most profound questions in physics – what is time? – with ground-breaking implications for the origin and destiny of our universe.
- 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.
- among the most significant arrow of time 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.
- this process never happens in reverse. These phenomena are said to be time-asymmetric.
- 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.
- Many physicists believe that the universe 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.
- 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. 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.
- 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.
- 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.
- 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. 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 famous story of Isaac Newton and the apple tree: 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.
- 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
- 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.
- 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.
- 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.