Why is time gone forever? To solve the mystery of the arrow of time and look forward to a major revolution in the field of physics, some baby universes time streams may follow the opposite direction, thus maintaining the time symmetry of the whole universe. ?
George santayana, a Spanish-American philosopher, has a famous exposition in his book The Realm of Truth published by 1938. He compared time to a flame burning along a fuse. The position of the flame represents the present moment. It accelerated, but never looked back, leaving only burnt-out ashes behind. He said: "The essence of the present is like a flame moving along the fuse of time. Every spark on the fuse represents the "present" that turns the future into the past. This perfect combination forms an eternal historical truth. "
In fact, this is not a perfect metaphor. The spark jumping along the fuse does not fully grasp the singularity of time, which has been puzzling physicists about relativity and quantum mechanics. However, the spark of Santayana does illustrate one of the most enduring and puzzling characteristics of time, namely the irreversibility of time.
Time runs to the future forever. You can enter the future in one breath, but the past can only stay in your memory forever. Time flies in one direction, like an arrow that leaves the string and never looks back. The water under the bridge, broken mirror is difficult to round, difficult to revive, difficult to rejuvenate.
Sean Carroll, a theoretical physicist at California Institute of Technology, said: "The difference between the past and the future is manifested in almost all scientific fields such as physics, philosophy, biology and psychology. The arrow of time is all over the way we know the universe. "
But strangely, the laws of nature that govern the universe do not recognize that time is indispensable. The equation describing the force that changes the motion state of an object applies to both backward time and forward time. A microscopic video of a bouncing molecule will need time recognition to distinguish between forward and backward. On the molecular scale, time has no direction. In the big world of bouncing basketball, although the hour hand has been running, its hands can never reverse their rotation direction.
For more than a century, physicists and philosophers have been troubled from the blank of the natural law of time to the emergence of the "arrow of time" Even today, there are many solutions to the time mystery in iPhone applications, but new explanations are still emerging one after another, just like the clockwork of an hour hand. The latest explanation of the mystery of time holds that it may be an indispensable episode in a grand drama involving the origin of the universe.
Although scientists have not reached a complete agreement on the exact source of the arrow of time, most experts believe that it is related to entropy, that is, the disorderly growth of things required by the second law of thermodynamics. Over time, disorder will increase (or at least remain the same) in any system isolated from external influences.
Unfortunately, just citing the second law of thermodynamics to explain the arrow of time cannot solve this mystery. Indeed, increasing entropy defines the direction of time, but only when everything is in balance, in technical terms, everything is chaotic. As the Austrian physicist ludwig boltzmann explained in the19th century, "chaos" is by far the most likely way of existence. The arrow of time should belong to the best luck, just like every hand can catch a flush in an all-night poker game, but as far as the entropy of the universe is concerned, it is obviously impossible to achieve such the best luck. There is no doubt that everything should be in a mess, so there should be no arrow of time.
But this is not what the universe should look like. When things are in a mess, they are not as chaotic as they should be, so the fuse of cosmic time can continue to burn. In other words, the entropy in the universe has been low enough to have more space and has become higher in the past. It is this exploration of chaos that drives the arrow of time in one direction. Explaining the arrow of time requires not only the second law of thermodynamics, but also some reasons, such as why the entropy value is so low, especially why the entropy value is at a low level when the cosmic hour hand begins to tick with the light of the cosmic fuse in the Big Bang.
Carol said at the annual meeting of the American Association for the Advancement of Science held in San Diego in February this year: "Trying to understand why you can mix cream with coffee, but you can't separate it, which brings us back to the era of BIGBANG and the origin of Hubble volume."
Before returning to BIGBANG.
From the moment BIGBANG was about 65.438+037 billion years ago, the space has been expanding. Citing this expansion theory to explain the time flow in daily life has become a standard strategy to solve the mystery of time. This practice can be traced back to half a century ago, and the arrow of time defined by the second law of thermodynamics is linked with the arrow of the universe defined by the theory of induced expansion of the Big Bang. Astronomer Thomas Gold was undoubtedly the first man. This method proves in various forms that the expanding space allows the increase of entropy, no matter how low or high it is at the beginning. Even if the entropy is high at the beginning, the expanding space allows it to become higher. Therefore, entropy continues to rise and the cosmic clock is ticking.
In his new book From Life to Life, Carol points out that this solution only assumes the existence of time direction, but does not explain why it exists. Basically, the Big Bang is defined as a point in the past and has been flowing in one direction since then. This scene does not retain the equivalence between the two time directions found in the basic equations of the universe. To find a complete explanation, it will take more time to return to BIGBANG.
"You often hear cosmologists say that the Big Bang was the beginning of space and time, and there was never such a thing before the Big Bang," Carol said. "The fact is that BIGBANG is just the moment when we understand the end. Because we don't know what happened before the big bang, but it definitely happened. "
In fact, many cosmologists today are seriously studying the possibility of what happened before the Big Bang and whether the universe it created is just one of many different bubbles in time and space, which cover the surface of the afterlife like bubbles on the surface of beer cups. This complex "multiverse" may contain countless cosmic individuals. Every cosmic individual born in his own big bang appears as a "baby" bubble, and then the umbilical wormhole connecting it with the original "emptiness" will be cut off.
Carol believes that the so-called "void" is a high entropy environment, and the technical term is called De Sitter space. However, a word "empty" cannot convey a very correct description. Because in quantum mechanics, especially Heisenberg uncertainty principle, completely empty space is not allowed. The fluctuation of energy is inevitable. In extremely rare cases, such fluctuations will be large enough to burst a brand-new space-time bubble into a realistic "baby" universe. This "baby" universe will be able to expand into the kind of universe that human physicists see in bubbles and can be examined from the inside.
"Every wave tends to form a small universe, which is controlled by the energy that makes it expand faster and faster," Carol explained. "This kind of energy will stay for a period of time before it becomes ordinary matter and radiation. The whole scene looks like our BIGBANG. "
In this way, before the Big Bang, this space-time with high entropy always increased its entropy more, and the total entropy of the system (mother and baby in De Sitter space) would be higher, thus maintaining the second law of thermodynamics. Away from its "mother" space, the "baby" universe with low entropy will expand. When the entropy of the "baby" universe increases, the second law of thermodynamics will drive a time direction. Finally, the entropy of the "baby" universe reaches a maximum and becomes a universe similar to "mother" in De Sitter space. After that, it will also be able to create its own "baby" universe.
"With the development of time, the universe exploded into what it is now, and the" baby "universe was born. They expand and cool, and the arrow of time appears at this moment, "Carroll said. "That moment is a trillion years."
Best of all, this situation may also happen in two time directions, because De Sitter space can produce many space-time bubbles. Any bubble has a time arrow that only goes in one direction, but the arrow of another bubble may point in the opposite direction. Generally speaking, time symmetry will be maintained.
The owner of any particular bubble always believes that the big bang that created its own bubble exists in their past. From the perspective of a super observer who is above everything else, the movement of time in two directions is as clear as the laws of physics have always indicated.
"The point is that the whole thing, the whole multiverse, is about the symmetry of time as a whole." Carol said.
Transcending the second law of thermodynamics
Carol reminded that this idea is just an idea and needs to be strictly verified after cosmologists greatly improve their computing power. Using the arrow of time to reconcile the eternal laws of physics is just one of many ideas. Lorenzo Maccone, a physicist at the Massachusetts Institute of Technology, pointed out that the second law does not actually allow two time directions. But anything that happens in the opposite direction will not leave any traces, records or even memories. If you are scrambled eggs, time will suddenly turn around. At this time, not only will the eggs return to their original state, but also the nerve cells in your brain will return to their original state, eliminating the information that you are scrambled eggs. So, as Maccone put forward in a paper published in Physical Review Letters last year, if time can really go back, no one will realize it in any way.
However, McCann admits that even this method still can't explain why the entropy predicted by scientists in the past was much lower than today.
More complicated, solving the mystery of time by invoking the second law of thermodynamics may also depend on the effectiveness of the second law itself. When the second law is used to help explain time, if the law itself is wrong, the validity of the explanation will be greatly reduced.
Skeptics of the second law of thermodynamics were once regarded as unforgivable heresy, partly because of a famous assertion by Sir Arthur Eddington, a British astrophysicist (who accidentally coined the term arrow of time), that the second law is absolutely correct. However, in recent years, the second law has been challenged as never before.
Mathematician Stephen wolfram quoted the results of computer simulation in his new book "New Science" published in 2002, and claimed that the evidence showed that the second law was completely wrong. Other scientists also try to find loopholes in the law by examining various situations, especially the quantum effect.
German de Abramo, an Italian physicist, pointed out in the latest issue of Physical Review: Series A that the challenge to the status of the second law of thermodynamics has reached an unprecedented scale in the past 10 to 15 years. During this period, more than 50 papers in this field appeared in the periodical literature.
Regardless of the ultimate fate of the second law, on the surface, the fuse of time continues to burn in one direction, and scientists still need to make considerable efforts to explain the mystery of time. Considering that all the solutions so far can't fully explain the mystery of time, some mysteries about the direction of time may still remain unsolved. This is not shocking, because physics has spent decades trying to explain some important mysteries, but not all of them have failed.
In the 20th century, the great scientific revolution of relativity and quantum mechanics opened a huge window for understanding the complex internal operating mechanism of nature. However, many of the most profound questions remain unanswered. What is the main composition of the universe? An unknown substance and an equally mysterious form of energy exist in all spaces at unexplained density levels. Will the laws applicable on a large scale (based on gravity) eventually contradict the quantum laws of the microscopic world? No one wants to believe this, but efforts to combine gravity with quantum physics have never been very successful.
What these problems bring to some scientists is that it is only a matter of time before another revolution comparable to Einstein will occur in 2 1 century. Others believe that solving the secret of the arrow of time may require or start such a revolution. Among them, anthony leggett, a Nobel Prize winner, is a physicist at the University of Illinois. In his speech at the annual meeting of the American Association for the Advancement of Science, he pointed out, "If and when can we have a truly major revolution? Compared with relativity or quantum mechanics, at some point in the next few decades, a large number of elements brought by the physical revolution will completely correct our view of the arrow of time. "