Tuesday, January 11, 2011

What is Time?

Most people are aware that we live in a Universe in which we observe four dimensions. 

The three dimensions of space allow us to see three-dimensional shapes, like cubes and spheres.  If we lived in a world of two-dimensional space, we would instead see squares and circles with zero thickness.  Stephen Hawking has said that such a two-dimensional space could not support life, as a 2D being could not both eat and defecate.  A shape with two openings could not hold itself together, and would fall apart or bleed to death (try to draw it and you’ll get the point).

Of course, the fourth dimension of our Universe is time.  Time is an essential ingredient in the space-time continuum.  Without the dimension of time, the 3D shapes we see would not change.  Everything would be steady state, and there would be no need for the word transient.  Put simply, what is time, but the entity preventing everything from occurring at once? 

I cannot imagine a universe without the dimension of time, as it would negate one of man’s most wonderful tools: memory.  No information would need to be retained, because life would be reduced to a single instant.  On the bright side, you would have a good excuse for forgetting your anniversary.

When discussing dimensions in a scientific scope, it should be mentioned that most physicists today believe that there must be more than three dimensions of space out there, as string theory does not seem to work with so few of them.  Some physicists believe that there must be at least six additional dimensions of space, yielding a 10D world, while others argue that there must be more.  We can then think of the world we observe as a subset, or cross-section of a much bigger picture.

Leaving the actual number of spatial dimensions as an open question, let us focus on the remaining dimension of time.  The concept of time is so fundamental for all of us in terms of how we live our lives.  It is sometimes hard to conceive that it was man that invented the second, the minute, the hour.  You and I use these timeline yardsticks on a regular basis to measure the passage of time.  It is easy to approximate all three quantities without the use of a watch.  Your pulse is likely to beat once every second.  One minute is the time it takes to tie your shoes.  An hour is the longest consecutive period of time I can shop at Wal-Mart before losing my mind. 

Our biological clock is much in tune with the length of time we get when we string twenty-four hours together.  Man coined the term ‘day’, but it is of course based on the approximate length of time it takes for the Earth to make one full revolution.  The Sun’s rise and set is governed by this period of time, and life on Earth is dictated by the Sun, as it provides all of its energy.  The week and month are useful collections of days, and must be carefully considered when planning ‘sick days’.

That other all-so-important unit of time, the year, is governed by two things: the mass of the Sun and the distance from the center of the Sun to the center of the Earth when they are furthest apart (also known as the semi-major axis).  A year is a useful measure of time, as the seasonal cycle repeats with the passage of each one.  It is therefore a great span of time to repeatedly recognize and celebrate the aging of a person, i.e. a Birthday.  I am not sure where the cake tradition comes from, but I am thankful for it.

Things get a bit hazy after the ‘year’.  We can string ten of them together and call it a decade.  This amount of time usually represents a stage of life as follows...

Decade 1: Grow big and strong
Decade 2: Go to school
Decade 3: More school and then work
Decade 4: Make babies and watch them grow
Decade 5: Work a lot so that you do not need to work forever
Decade 6: Empty the nest and maybe retire
Decades 7 and beyond: the rest

The decade is also of cultural significance for many of us, often described by the popular music of the time, by political movements, and far too often, by wars.  Ten decades make a century, and this amount of time is often internalized as well; we see it as our maximum life span, although some push the envelope, living on “borrowed time”.  Like decades, centuries are often described by the events that they contained, although they are maybe best described by the technological advances that occurred within them.

The nineteenth century gave rise to the industrial revolution due to the advent of the steam engine.  The twentieth is the century of travel thanks to the automobile and airplane.  I expect that the twenty-first will be one day looked back upon as a quantum shift – a century where everything became automatic and instantaneous, and when the keys to energy and DNA manipulation were unlocked.  That sounds like a lot, but one hundred years is a long time when a species becomes as advanced as ours.

A millennium is almost beyond the comprehension of a human being.  A historian may try to conglomerate events and call it a millennium, but multiple lifetimes are nearly impossible for a human being to visualize.  Some religious leaders look back two millennia to the birth of Jesus, and others look back five millennia, to the grand creation.  I see a millennium as a hick-up as far as the universe is concerned.  We are now 13.7 billion years from the big bang.  If we call that span of time the age of our universe, then a millennium is to it what 3.8 seconds is to a one hundred-year-old man.

Evolution is a very slow process.  For life, millions of years represent a useful span of time, where noticeable changes may occur in a biological sense.  For the universe, evolution is a multi-billion year long process.  This is the massive amount of time it takes for galaxies and solar systems to form.  Time is a word we throw around, but it is a hard one to truly appreciate as it began to tick so long ago; we cannot imagine it.  If you spent your whole life watching a single rock, and documented its changes in a notepad, you would leave a blank page with your passing.

The scale of time is something we are familiar with.  Most of us are not at all familiar, however, with the true nature of time.  It is incorrect to think of time as a firm measuring stick.  Time is malleable, like a stick of gum.  The speed of light is constant in a vacuum; that is a law of physics.  To ensure that this law is maintained for all observers, time actually dilates accordingly, based on the speed of the observer (this is known as special relativity).  It is a very real challenge for physics students to conceive that clocks, be they mechanical or biological, tick slower for people who are moving near the speed of light.
Fortunately, it is not something most of us need concern ourselves with, as none of us ever travel anywhere near 300,000 km/s.  Still, the slight time dilation experienced by relatively fast-moving satellites require that the onboard clocks be adjusted to tick slightly slower, so they remain synchronized with those on Earth.  Without this adjustment, GPS technology would be utterly useless, and you would arrive at the wrong destination ... late.

Time is relative in other ways as well.  A hundredth of a second may feel like an unimportant unit of time, but try telling that to an Olympic sprinter for whom victory was separated by just that amount of time.  Microseconds represent a lifetime for certain particles, whereas they go by unnoticed by us: it is the time it takes the light of a lightning strike to travel across a field and into our eyes.

All of this may make us feel at a bit of a loss.  We cannot appreciate microseconds, nor can we conceive of actual long passages of time.  Worse still, time is not a concrete thing, so even the units of time that we can appreciate are different depending whose space ship you are travelling in.

Well, it could be a whole lot worse.  If you suffer from Alzheimer’s disease, you know all too well that time is of little use without memory.  Some philosophers argue that there is in fact no time without memory, just as a tree that falls in an empty forest does not produce any sound.   Without a conscious observer to record the passage of time, there is no time.

We tend to take the passage of time for granted, asking not what it is, but rather how we should spend it.  It is sometimes nice to take a step back and see time for what it is: a dimension without which life would simply be unimaginable.  Life is a collection of moments connected in series.  Time is the entity responsible for arranging these events in chronological order.       

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