Last weekend, the sci-fi film Creed, directed by Christopher Nolan, was released, and the “time-back” story is likely to be one of the most esoteric in recent years. Putting aside the movie’s viewing, the film brings to us a scientific question that physicists have explored for more than a century:
The view of physics holds that all basic rules should be symmetrical, and that the laws of microphysics, which describe the processes of change in the universe, do not distinguish between the past and the future. But in the real world, we feel that time is only moving in the direction of the future. Why is that?
Explain that the time arrow must go back to the Big Bang and explore the “prehistork history” of the universe. Our universe may be part of a much larger multi-universe. As a whole, multiple universes are time-symmetrical. In other universes, time may flow back.
Emoclew dna olleh.
“Emoclew dna olleh” – Brian Greene, a string theory expert at Columbia University, opens an academic conference at the New York Academy of Sciences in October 2007. He then explained: “If you understand that this is saying ‘Hello and welcome’ upside down, you probably don’t have to attend the meeting.” “
But none of the world’s top theoretical physicists and cosmologists left. They gathered to challenge the mysteries of time.
On the face of it, time is ordinary, like a one-way street: broken eggs don’t return to the way they are, fishtail patterns in the corners of your eyes don’t automatically disappear (provided you don’t get Botox injections to remove wrinkles), and your grandparents are never younger than you. But the basic rules of the universe seem to show time symmetry, which means that they are not affected by the flow of time. From a physical point of view, the past, present and future all happen at the same time.
For more than a century, physicists have offered countless explanations for this apparent contradiction, from psychology (the flow of time is an illusion) to physics (some unknown features of quantum modernity can reconcile this contradiction). But none of the explanations is satisfactory. In 1927, Sir Arthur Eddington of astrophysics condensed the one-way nature of time into a term, the “time’s arrow”, and linked it to entropy: as the universe grew older, it followed the second law of thermodynamics and became more and more disorderly.
But scientists can’t explain why order exists in the past and disorder in the future. About 14 billion years ago, the universe was considered orderly, with temperatures and densities far greater than the extreme environment inside stars; Some scholars believe that “multiple universes are the most reasonable explanation if we can accept that the universe we live in is only one of many possible parallel universes.” “The universes that started out more chaotic may not be able to sustain or evolve enough to produce intelligent life. Therefore, in this sense, the one-way flow of time, even our own existence, is only a coincidence.
The mystery of entropy.
Physicists incorporate the concept of time asymmetry into the famous second law of thermodynamics, which can be expressed as: the entropy of a closed system never decreases. In general, entropy measures the degree of confusion in a system. In the 19th century, the Austrian physicist Ludwig Boltzmann used the difference between the micro and macro states of objects to explain entropy. If you are asked to describe a cup of coffee, you are probably referring to its macro state, such as temperature, pressure, and other overall properties; In any macro state, there are many microscopic states corresponding to it: we can move an atom around, and no one will notice these changes on a macro scale.
Entropy is the number of different microscopic states corresponding to the same macro state. (Exactly, entropy is the number of pairs.) Therefore, it is easier for a certain number of atoms to be arranged in a high entropy state than in a low entropy state because the former is arranged in more ways. Imagine pouring milk into the coffee. The molecular scheduling of milk and coffee is incalcerable, while the way in which milk is clearly mixed with the surrounding coffee is much less. So when the two are fully mixed, the entropy of the system is higher.
From this point of view, it is not surprising that entropy tends to increase over time. Any change in the system is equivalent to re-selecting a microstate, and the number of high entropy states far exceeds the low entropy state, so the probability of randomly selecting a low entropy state is negligible, and the system almost always selects a high entropy state. That’s why milk and coffee can be mixed, but they don’t separate naturally. Although physically all milk molecules may spontaneously gather together, statistically the probability is almost zero. If you want random movements of molecules that happen to separate milk and coffee from each other, the wait is often longer than the current age of the observable universe. The time arrow is simply a trend for the system to evolve toward a longer and more natural high entropy state.
However, it is one thing to explain why the low entropy state evolved to the high entropy state, and another to explain why entropy in our universe is increasing. The question remains: Why was entropy low at the beginning of the universe? Since low entropy is very rare, this is also extremely abnormal. Even if we admit that our universe is still in the middle entropy state today, it cannot explain why entropy in the past is lower than it is now. A number of different initial states can evolve similar universes, with the high entropy initial state accounting for the vast majority, far outsuming the low entropy initial state.
In other words, the real challenge is not to explain why the entropy of the universe is higher tomorrow than it is today, but to explain why yesterday is lower than today and the day before yesterday is lower than yesterday. We can follow this logic until the beginning of time in the observable universe. Thus, the asymmetry of time is ultimately a question that needs to be answered by cosmology.
Photo credit: Michigan State University.
The past VS future.
The universe begins in a state of extremely low entropy, with all particles evenly squeezed together. As the universe evolves, it experiences the state of entropy, which is what we observe, the distribution of stars and galaxy clusters. Eventually, the universe will reach a state of high entropy: there is almost nothing in space, and occasionally low-energy particles roam in it.
Why is the past and the future so different? It is not enough to artificially give a reason for the universe to begin in a low entropy state by proposing a theory of initial conditions. As Huw Price, a philosopher at the University of Sydney in Australia, points out, any reasoning process that applies to the initial state should apply equally to the final state, otherwise it would be presumed that we want to prove that the “past” is really special. So there are only two options left, either to regard the inexplicable asymmetry of time as an inherent feature of the universe, or to do more research into how space-time works.
Many cosmologists try to blame the asymmetry of time on the soaring universe. Soaring can beautifully explain many of the basic features of the universe. According to this idea, it is not particles (or part of the very early universe) that are in full presence in the very early universe, but another dark energy that exists for a very short period of time, with a much higher energy density than the dark energy we observe today. This so-called ultra-dense dark energy causes the universe to expand (i.e., soar) at an unexpected rate in a short period of time, then decay into matter and radiation, leaving only a little dark energy, which has only become significant again to this day. The next story, as described in the Big Bang theory, is that evenly smoothed pristine gases evolved into stars and galaxies, becoming the universe we observed.
The Big Bang Theory and the Boom Model Diagram. Photo credit: NASA/WMAP.
The boom model has been a great success in several ways. It predicts that the distribution of matter in the universe is not completely uniform and that there is a very small deviation, which is exactly in line with the increase and fall in the density of matter in the universe that we have observed. However, a growing number of cosmologists believe that explaining the asymmetry of time by soaring is in fact a stealth of concepts; Roger Penrose of the University of Oxford in the UK and others have stressed that ultra-dense dark energy must initially be in a very special state in order for the surge to happen as expected. In fact, the entropy of this dark energy must be much lower than the hot, dense gas that later decayed from it. This means that the surge does not actually solve any problem: in order to “explain” an unusually low entropy state (a hot, dense, evenly distributed gas), it must resort to another initial state with lower entropy (a small piece of space occupied by dense dark energy). We’re just moving this puzzle one step further, and the question now is: Why did the surge happen?
Many cosmologists believe that the surge can explain the asymmetry of time, one of the reasons is that the special initial state of dark energy seems to appear to be not low. At the beginning of the surge, our observable universe was less than 1 cm in diameter. Intuitively, such a small area doesn’t seem to have much microscopic state, so it doesn’t seem so surprising that the universe is randomly plunged into some kind of microstation corresponding to a surge.
Unfortunately, this intuition is wrong. Even if the early universe was only 1 cm wide, it had the same number of microscopic states as the entire observable universe today. According to the laws of quantum force, the total number of microscopic states of a system never changes. (The increase in entropy is not due to an increase in the number of microscopic states, but to the natural evolution of the system into the most likely macro state.) In fact, the early universe was the same physical system as the later universe — after all, the present universe evolved from the early universe.
The universe can be lined up in countless different microscopic states, and only a small part (almost negligiblely) corresponds to the initial macro state necessary for a cosmic boom, where ultra-dense dark energy is squeezed almost evenly into a very small space. This state is extremely special and therefore extremely low entropy. If you randomly select the microscopic state in which the universe is located, the probability of selecting this particular state is almost zero. The theory of surge itself does not explain why the early universe had low entropy, but it implied this assumption from the start.
Therefore, the surge does not help explain why the past is different from the future. A bold but simple way to do this is to think directly that a very long past may not be any different from the future. Perhaps the distant past, like the future, is in a state of high entropy. If so, the fiery, dense state we call the “early universe” is not really the beginning of the universe, but some kind of transition between different historical periods of the universe.
Some cosmologists imagine that the universe has experienced a “bounce”. Until the rebound occurs, space shrinks, but the universe will not be squeezed into an infinite density point, and new physics, including quantum gravity, supervitivity, string theory, and other strange phenomena, will save the world at the last minute and give the universe a different look, what we now call the Big Bang. Interesting as it is, the rebounding universe does not explain the time arrow. Before the rebound occurs, entropy in the universe either increases over time or decreases over time; In either case, we can’t get around the question of why the entropy of the universe is so small in what we call the early moments of the Big Bang.
Instead, we assume that the universe was initially in a high entropy state, which is the most natural state. Vacuum is a typical representation of a high entropy state. As with all other high entropy states, the trend of vacuum evolution is set in stone. So the question now is: how can this desolate and dead space-time produce the universe we observe today? The secret may be hidden in dark energy.
If there is dark energy, the vacuum is not really empty. The rise and fall of the quantum field produces a very low temperature — much lower than today’s cosmic temperature, but by no means zero. In such a universe, all quantum farms occasionally experience hot ups and downs. In other words, a vacuum is not dead; if you wait long enough, there will always be a single particle or even a mass of particles suddenly “innate”, but soon dissipate in the vacuum. (These particles are real particles, not fleeting virtual particles; virtual particles can also appear and disappear in a vacuum without dark energy, but not real particles.) )
By the same to no reson, a small gloomy dark energy may suddenly appear. If the conditions are right, the space occupied by this dark energy will experience a surge, leaving the original space-time and forming an independent infant universe. Our universe may be a descendant of some other universe.
Photo credit: JAMES PROVOST.
On the face of it, there are many similarities between the above scenario and the standard skyrocketing model. Both models assume that a random appearance of a small group of dense dark energy triggers a surge. The difference between the two is that there is a fundamental difference between the initial conditions. In the standard skyrocketing model, this dark energy appears in a universe with unusually intense random ups and downs. The problem is that the universe seems much more likely to jump over the surge stage and go straight up and down to a certain state to start a big bang. In fact, in terms of entropy, it is more likely that the universe will rise and fall directly out of the state we see today, completely bypassing the evolution of the past 14 billion years.
In our new model, the original universe does not rise or fall at will; The theory claims that the most likely way to create a universe similar to ours in such a state is to go through a surge phase rather than go straight up and down another universe. (This has yet to be proved.) In other words, our universe is indeed the result of ups and downs, but not random ups and downs.
In 2004, I co-proposed this model with Jennifer Chen of the University of Chicago, providing a tantalizing explanation for the origin of time asymmetry in the observable universe: everything we see is just a drop in the ocean, and on a larger cosmic stage, time is perfectly symmetrical. Entropy can grow without limit by creating a new baby universe.
Fortunately, the model worked whether time flowed forward or back. Imagine that we begin at a particular moment and focus on the evolution of the vacuum in both the past and the future. (The reason for considering two time directions is that we don’t artificially assume a one-way time arrow.) No matter which direction it evolves, the baby universe can be produced in ups and downs, eventually expanding into a void and creating its own baby universe. On an oversized scale, such a multi-universe is symmetrical to time – many new universes rise and fall in the past and in the future, and expand without restriction. Each universe has a time arrow, but half of the universe has the same direction of time arrow as the other half.
The idea of a universe with reverse time arrows seems shocking. If we meet uninsed visitors from that universe, will they remember the future? Fortunately, we don’t have to worry about such a “slug”. In the model we describe, the place where time seems to reflow exists only in the very distant past — even much earlier than our Big Bang. There is a vast universe between there and here; in this universe, time does not seem to flow, there is almost no matter in space, and entropy does not change. Life in a time-reversal zone does not return to old age, nor does it have “special functions” such as predicting the future. The passage of time they feel is no different from the sense of time we are familiar with. It is only when we compare their universe with ours that things become extraordinary – our past is their future, and vice versa. However, such comparisons can only be purely hypoththetic, because we can’t go there, and they can’t come here.
So far, our model is far from final. Cosmologists have spent years thinking seriously about the concept of the baby universe, but we don’t yet understand how it was born. If quantum ups and downs can create new universes, they can also create many other things, such as a complete galaxy. To explain the universe we see, a model must predict that most galaxies are formed after events like the Big Bang, rather than rising and falling alone from an otherwise empty universe. Otherwise, our universe will look very abnormal.
However, our goal is not to build a specific model to explain the space-time structure on a very large scale. The time arrow originated in the initial state of very low entropy in the early universe, and this amazing feature of the observable universe is of concern because we believe that it provides clues that help to reveal the nature of the unsoceiveable universe.
The observable universe’s impressive time asymmetry seems to provide us with a clue as to the ultimate working mechanism of space-time. Our physicist’s task is to use such clues to piece together a convincingly complete picture.