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Thursday, December 25, 2014

What about the cosmological arrow of time?

According to the theory of the Big Bang our universe had a beginning, we know well though we unknown the finer details of such origin. After it opened, it has been expanding and evolving. If we look at the arrow of time which appears to induce increasing entropy we could deduce that in the past the entropy of the universe must have been smaller than at present.


But that statement that at first glance looks innocent hides a serious problem. The problem is to understand why the hell the universe began in a state of low entropy when, if you think about it a little, that must have been a "totum revolutum" particle interactions, changing at very high energy and temperature, etc. The natural thing is that the universe was born in a state of high entropy. Or at least, there is no reason to suppose that began in a state of low entropy.
Mr. Boltzmann attempt to answer this question by saying that our universe is due to a lavish fluctuation that the entropy decreased. That's not going against any physical law, not even the law of increasing entropy. The entropy is a quantity that makes perfect sense when many particles at stake and we must accept that local fluctuations occur (in a given region) where the entropy decrease.
If our universe is a fluctuation of something previous, now to that thing we call quantum vacuum, in our past entropy was younger and now we live in that fluctuation because we are trying to recover a high entropy. This image appears consistent with what we know about cosmological inflation, a subject that you have multiple entries in this page.
But as usual, the answers generate more questions. Why is this fluctuation in a universe like ours? Why do we see many galaxies expanding when it is more likely that one will be formed? Furthermore, why do not we see only a solar system? Or a single planet? Why we exist when it is more likely that a fluctuation generates a unique brain than an universe like ours?
This chain of questions point to the problem known as the Boltzmann brains, which states that vision is Boltzmann be much more likely that fluctuation to generate a single brain that a whole universe.
Maybe it's only Physics ...
Perhaps the answer to the origin and existence of an arrow of time is due to a conspiracy of the laws of physics. Maybe one by one are not sensitive to the direction of flow of time but maybe when we have many particles or many laws while playing naturally becomes an arrow of time.
That is what we are trying to show Barbour, Koslowski and Mercati. That provides a physical arrow of time, selecting the physical states of low entropy naturally from which may evolve into higher entropy states.
His work is collected in the following article which was recently published in Physical Review Letters:
Identification of a gravitational arrow of time (and I have linked the freely publication available on arXiv at the end of this post).
This work is insultingly simple and fabulously beautiful. It may not be a definitive answer to the problem at hand but a first step on which further work.
The idea is very, very simple. Suppose we have a set of many particles that interact only through Newtonian gravity. They attract more or less depending on the mass of each particle and the involved close they are from one another. For lovers of the details I will say that these particles are simulated so that the total energy of the system and its total angular momentum is zero (relative to the center of mass).
Steps:
1. We start with all huddled particles each having a speed that is assigned randomly.
2. If we evolve the system at the end we find that the particles are grouped in pairs, orbiting each other and occupying most of the available volume.
What they have done Barbour, Koslowski and Mercati (BKM) has been started by the end. Have taken a distribution of particles in the final state above and have evolved backward in time (this is done by reversing all particle velocities in the game). They found that all (or nearly all) the initial configurations give rise to a unique state of minimum size and uniformity.
Not content with that, BKM have left the system evolve further and what they find is that this configuration evolves to give back a scattered configuration in which the particles are associated in pairs.
View images images below where:
a) only plays Newtonian gravity.
b) The condensed "initial" and homogeneous situation may extend into the future or the past likewise leading to (not identical) similar situations. This is consistent with that of the laws of physics pay little attention to the direction of the arrow of time.
c) However, yes we can say that there are states that occur naturally that if we tend to evolve more entropic configurations. That is, as simple as that can select an initial state which evolve into the future (in either direction) system. So we are giving an arrow local time.
Have you solved the problem?
No, there are still several things to do (and make them take a long time):
Extend this work to situations where gravity described by general relativity operates.
Introduce more interactions and see if the game with gravity selects an arrow of time consistent for all. For all we know radioactive nuclei always change in the same direction, electromagnetic waves are generated by the acceleration of electric charges, but it not happens that a radioactive nucleus recompose or has emission of electromagnetic waves before accelerating charges.
Sure it's interesting what BKM will discover in the future, whatever that means.
What NOT to say this work?
I read somewhere that this work shows that we live in the past of a universe explode or something. The truth I have not paid much attention.
This article shows only that can be generated with the participation of initial severity for which there is a notion of time arrow situations. For all purposes, we still remember the past and not the future. Our universe will expand and not recolapse.
What it says the work is that there are states in which one can not choose where to point the arrow of time to see only the status of the initial image, ie, so you have begun in the dense and homogeneous situation your future will be go to the diluted and structures situation (two interacting particles). For you there is an arrow of time for physics no.
Although it is fair to make an exception to this last statement and is as follows:
in 1964 first discovered a phenomenon that violates the symmetry T. This is the disintegration of a certain type of particle (called Kaon or K meson) in the image below you can see the picture. This finding represents the first microscopic process where there is a physical difference between the past and the future. Contrary to the disintegration of other particles, the kaon is the only one if we will film a movie and we passed backwards, we would see a phenomenon that can not exist in the Universe. This discovery was of great importance for the understanding of time, but obviously we are still far from reaching a clever concept.
So the laws of physics are not reversible in all cases, but more disturbing is that in the subatomic world the arrow of time is contrary to what we perceive, ie it goes from future to past, it seems that taking energy "borrowed" from the future. This is explained in more detail in the following entry (available in Spanish only)