It now seems that our entire universe is living on borrowed time. How long it can survive depends on whether Stephen Hawking's theory checks out. Special thanks to Ivan Bridgewater for use of footage.

Time is flying by on this busy, crowded planet... as life changes and evolves from second to second.

And yet the arc of human lifespan is getting longer: 65 years is the global average ... way up from just 20 in the Stone Age.

Modern science, however, provides a humbling perspective. Our lives... indeed the life span of the human species... is just a blip compared to the age of the universe, at 13.7 billion years and counting.

It now seems that our entire universe is living on borrowed time...

And that even it may be just a blip within the grand sweep of deep time.

Scholars debate whether time is a property of the universe... or a human invention.

What's certain is that we use the ticking of all kinds of clocks... from the decay of radioactive elements to the oscillation of light beams... to chart and measure a changing universe... to understand how it works and what drives it.

Our own major reference for the passage of time is the 24-hour day... the time it takes the Earth to rotate once. Well, it's actually 23 hours, 56 minutes and 4.1 seconds... approximately... if you're judging by the stars, not the sun.

Earth acquired its spin during its birth, from the bombardment of rocks and dust that formed it.

But it's gradually losing that rotation to drag from the moon's gravity.

That's why, in the time of the dinosaurs, a year was 370 days... and why we have to add a leap second to our clocks about every 18 months.

In a few hundred million years, we'll gain a whole hour.

The day-night cycle is so reliable that it has come to regulate our internal chemistry.

The fading rays of the sun, picked up by the retinas in our eyes, set our so-called "circadian rhythms" in motion.

That's when our brains begin to secrete melatonin, a hormone that tells our bodies to get ready for sleep. Long ago, this may have been an adaptation to keep us quiet and clear of night-time predators.

Finally, in the light of morning, the flow of melatonin stops. Our blood pressure spikes... body temperature and heart rate rise as we move out into the world.

Over the days ... and years... we march to the beat of our biology.

But with our minds, we have learned to follow time's trail out to longer and longer intervals.

Philosophers have wondered... does time move like an arrow... with all the phenomena in nature pushing toward an inevitable end?

Or perhaps, it moves in cycles that endlessly repeat... and even perhaps restore what is there?

We know from precise measurements that the Earth goes around the sun once every 365.256366 days.

As the Earth orbits, with each hemisphere tilting toward and away from its parent star, the seasons bring on cycles of life... birth and reproduction... decay and death.

Only about one billionth of the Sun's energy actually hits the Earth. And much of that gets absorbed by dust and water vapor in the upper atmosphere.

What does make it down to the surface sets many planetary processes in motion.

You can see it in the annual melting and refreezing of ice at the poles... the ebb and flow of heat in the tropical oceans...

The seasonal cycles of chlorophyll production in plants on land and at sea... and in the biosphere at large.

These cycles are embedded in still longer Earth cycles.

Ocean currents, for example, are thought to make complete cycles ranging from four to around sixteen centuries.

Moving out in time, as the Earth rotates on its axis, it completes a series of interlocking wobbles called Milankovic cycles every 23 to 41,000 years.

They have been blamed for the onset of ice ages about every one hundred thousand years.

Then there's the carbon cycle. It begins with rainfall over the oceans and coastal waves that pull carbon dioxide into the sea.