ENTROPY

Entropy.  There is a unique connection between time and the second law of thermodynamics.  Entropy is the only quantity in physics that implies a progression - the arrow of time - that leads from now to then.  As time passes, the 2nd Law states that the entropy of an isolated system never decreases.  You could look at entropy as a kind of clock.  The 2nd Law usually implies a loss of heat, or energy, in a system.  You can't make a perpetual motion machine because of the 2nd Law.  But perhaps, if you look at that wild idea of time slowing down - then entropy is more a measure of the time dilation than a loss of energy.


The implication is that the 2nd law pulls us through time.  In Albert Einstein's theories of relativity, time dilation occurs when two observers are in relative uniform motion, one clock appears to move slower than the other.  If we are changing relative position through time, the Lorentz transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik Lorentz. It reflects the surprising fact that observers moving at different velocities may measure different distances, elapsed times, and even different orderings of events.


Not such a huge leap to see time as the movement from one relativist point to the next!  Hey, what's the probability of that?  Continuous time evolution of an isolated system that obeys Schrödinger's equation (or Dirac's equation) involves the collapse of the wave function.  Entropy, and the expansion of space, may be the relativistic expression of the 2nd Law.

WHAT IS TIME?

Time is a difficult concept to wrap your head around. We intuitively feel it passing; we can say that time flies, or crawls, or seems to stand still. It can be wasted, squandered, and it can somehow be something on our hands. But what is it? Does it flow like a river? Does it slip like the sand through an hourglass? It certainly never backs up for me. So we sense that time has an arrow, pointing from the past to the future.

However, the laws of physics do not forbid time from 'flowing' backwards. Time symmetry holds for many of the classical variables, including;

• Position of a particle in three-space 
• Acceleration of the particle 
• Force on the particle 
• Energy of the particle 
• Electric potential (voltage) 
• Electric field 
• Electric displacement 
• Density of electric charge 
• Electric polarization 

and others. As a matter of fact, the classical variables that are sensitive to time reversal are those that clearly have a time component built into them; such as, power (rate of work), velocity, and angular momentum.

Arthur Eddington, who coined the arrow of time terminology, concluded that as far as physics goes, the passage of time is merely a property of entropy (more on entropy later). So what is time? The answer is usually two parts philosophy, and one part 'it just is'. Clearly time is needed to get something from here to there, and the passage of time itself is a form of movement that carries us from now to then. From Isaac Newton's aether, to Albert Einstien's space-time continuum, time and space are necessary components of the basic laws, but are never really defined. Is time a something? Is it made out of smaller bits of fundamental goo? What is space? If it is expanding, can it be compressed?

Clearly it is important to our consciousness that time move in one direction, but is it just the part that we sense? Through history, people understood that there were limits to our senses and that sound, and vision, and indeed, the real world around us extended beyond the threshold of our limited perception. We knew that dogs could smell better, hawks and falcons see better, and cats move in the dark.

Most physicists are beginning to believe that time and space do have a finite size, with the smallest time being the Planck time, defined as the time it takes light to travel one Planck length (named after Max Planck). This means that our experience of the world is like watching a cartoon, one Planck slice at a time.

So why the arrow? Why don't the frames flip backwards?

THE EXPANDING UNIVERSE

As predicted by Alexander Alexandrovich Friedman in his solution to the general relativity field equations in 1922, and verified through red-shift observations of distant galaxies by Edwin Hubble in 1929, the universe appears to be expanding.  The Metric Expansion of Space leads to some interesting notions about our perception of reality.  But one of the most interesting concepts to come forward is from José M.M. Senovilla and his team at the University of the Basque Country in Spain.  His team has offered a solution that has time slowing down and disappearing from the universe.  While certainly not main-stream thinking, the proposal offers some insights into the reality of time.

http://www.dailygalaxy.com/my_weblog/2008/01/scientist-says.html
http://www.newscientist.com/article/mg19626354.000-is-time-slowing-down.html