Time moves forward because a property of the universe called "entropy," roughly defined as the level of disorder, only increases, and so there is no way to reverse a rise in entropy after it has occurred. The fact that entropy increases is a matter of logic: There are more disordered arrangements of particles than there are ordered arrangements, and so as things change, they tend to fall into disarray. But the underlying question here is, why was entropy so low in the past? Put differently, why was the universe so ordered at its beginning, when a huge amount of energy was crammed together in a small amount of space? [What's the Total Energy in the Universe?]
http://www.livescience.com/34052-unsolved-mysteries-physics.html
In classical mechanics, time is something that passes uniformly regardless of whatever happens in the world. For this reason Newton spoke of absolute space and absolute time. On the other hand, Einstein's Special Theory of Relativity predicted that time does not flow at a fixed rate: moving clocks appear to tick more slowly relative to their stationary counterparts. Quantum mechanics does not neglect the time either. In standard model, photon does not experience time. Some new theories suggest that time does not exist at the quantum level. The study of the quantum universe shows us that time does not exist. It shows us that time is a function of relativity only and exists relative to some arbitrary point of reference. Whatever else may be said about time, one thing is certain, it defies definition. The best we can say is that we all know what time is, intuitively. The Seventh Edition of Webster's Collegiate Dictionary tells us that time is "the measured or measurable period during which an action, process, or condition exists or continues." Of course, what the lexicographer has done here is to tell us that time is defined by its measurement and that measurement is of a period during which something occurs. He has not told us what time really is. In fact it is the definition of a clock. What is the nature of physical time, really? In this paper, I have tried to answer this question.
Article Graviton: physical time and thermodynamics
This is the question demanding thousands pages of writing... Only in the first paragraph you mention several issues. I will refer here to one aspect only. The increase of entropy is related to spreading of internal energy. If you imagine the initial distribution (of energy, particles) limited to a small region of the available phase space of a closed system, and assume that all configurations in that phase space occur with equal probabilities, then the system will be found most of the time in a more or less evenly distributed configuration because there are many many more such configurations than those with particles "in one corner" (if T>0). That would be the state of equilibrium when entropy is constant in time. In that configuration all average values are equal to the most probable values. I personally often avoid the terms "order" and "disorder" which are antropocentric and require a definition. The proper scenario is the phase space and its population. Then I would turn your question "why was the universe so ordered at its beginning, when a huge amount of energy was crammed together in a small amount of space?" perhaps into "why a huge amount of energy was crammed together in a small amount of space?" . Call it order, if you wish. It seems that this is the virtue of a "bang" that it occurs in a limited space (there are known arguments in favour).
Your question is, of course, fundamental. I would however be cautious with "why is there..." because this is us who define what the arrow of time is. And as you probably know, there are many concepts of "arrows of time". The most common one is: why to hell are we always getting older and never getting any younger?
The question is rather why processes advance in a particular direction not the opposite. From the totality of progress you have an illusion of time and its arrow. Like sitting in a car and asking why all the world moves backwards.
I do not quite understand Mohamed's comment in this context. Even if one assumes time reversal on quantum level, one may define and observe a time arrow.
Mohamed, I would intuitively also reject the arrow of time on quantum level but I am not expert in that particular field. Perhaps studying of quantum condensates can contribute to this issue? Thanks for the references
Why does a system emit heat energy? A system such as gas is made up of molecules or atoms, and atoms are not at static state in a system. They are moving or oscillating around each other. Also, atoms contain charged particles, and they absorb or repel each other. They are working on each other continuously. So, every system emits heat energy, and intensity of radiation is depends on its temperature.
So, for an outside observer of system seems there is an arrow of time.
Every system emits heat energy; in fact a system works on itself continuously that is
positive work on the environment. Let's name it the "Inherent power of system". So, every system has an inherent power that is greater than zero P > 0. If a system loses its inherent power, it is at basic-level state of thermodynamics. In other words, a system would be at inherent power, if its inherent power was zero P = 0.
When a system is at basic-level energy, its charged particles are not able to work on each other, so the system does not emit heat energy. When a system is at basic-level energy, then its temperature is absolute zero, and its time does stop.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Mechanics
- Classical Mechanics