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?