What is Time and When will it End?
The Life Span of the Human Species is just a Blip Compared to the Age of the Universe



What is Time and When will it End?
The Life Span of the Human Species is just a Blip Compared to the Age of the Universe

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.

 		 
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.

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. In 5th century BC Greece, Antiphon the Sophist, in a fragment preserved from his chief work On Truth held that:

"Time is not a reality, but a concept or a measure."

Parmenides went further, maintaining that time, motion, and change were illusions, leading to the paradoxes of his follower Zeno.

Artifacts from the Palaeolithic suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000 BP. Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 346 or 364 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months.

Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years.

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today by far the one in most common use around the world.
The Universe - The End of the Universe


The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration. Once the notion that the universe started with a Big Bang became accepted by a consensus of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion.

The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1916 theory of general relativity. General relativity can be employed to describe the universe on the largest possible scale.

There are many possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe. Alexander Friedman proposed a number of such solutions in 1922 as did Georges Lemaître in 1927. In some of these the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.

In 1931, Edwin Hubble published his conclusion, based on his observations of Cepheid variable stars in distant galaxies, that the universe was expanding. From then on, the beginning of the universe and its possible end have been the subjects of serious scientific investigation.

The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will continue to expand indefinitely, resulting in the "big freeze" scenario below.

However, new understandings of the nature of dark matter also suggest its interactions with mass and gravity demonstrate the possibility of an oscillating universe.


Big Freeze or heat death

The Big Freeze is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. This scenario is currently the most commonly accepted theory within the scientific community.

A related scenario is Heat death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed, and there are no gradients — which are needed to sustain information processing, one form of which is life. The Heat Death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.


Big Rip

In the special case of phantom dark energy, which has even more negative pressure than a simple cosmological constant, the density of dark energy increases with time, causing the rate of acceleration to increase, leading to a steady increase in the Hubble constant. As a result, all material objects in the universe, starting with galaxies and eventually (in a finite time) all forms, no matter how small, will disintegrate into unbound elementary particles and radiation, ripped apart by the phantom energy force and shooting apart from each other. The end state of the universe is a singularity, as the dark energy density and expansion rate becomes infinite.


Big Crunch

The Big Crunch theory is a symmetric view of the ultimate fate of the Universe. Just as the Big Bang started a cosmological expansion, this theory postulates that the average density of the universe is enough to stop its expansion and begin contracting. The end result is unknown; a simple extrapolation would have all the matter and space-time in the universe collapse into a dimensionless singularity, but at these scales unknown quantum effects need to be considered.

This scenario allows the Big Bang to have been immediately preceded by the Big Crunch of a preceding universe. If this occurs repeatedly, we have an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, each finite universe ending with a Big Crunch that is also the Big Bang of the next universe. Theoretically, the oscillating universe could not be reconciled with the second law of thermodynamics: entropy would build up from oscillation to oscillation and cause heat death. Other measurements suggested the universe is not closed.

These arguments caused cosmologists to abandon the oscillating universe model. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death, because of an expansion of the branes that dilutes entropy accumulated in the previous cycle.


Big Bounce

The Big Bounce is a theorized scientific model related to the beginning of the known Universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe. According to one version of the Big Bang theory of cosmology, in the beginning the universe had infinite density.

Such a description seems to be at odds with everything else in physics, and especially quantum mechanics and its uncertainty principle. It is not surprising, therefore, that quantum mechanics has given rise to an alternative version of the Big Bang theory. Also, if the universe is closed, this theory would predict that once this universe collapses it will spawn another universe in an event similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes re-expansion.


Multiverse: no complete end
One multiverse hypothesis states that our uni-"verse" is merely one Big Bang among an infinite number of simultaneously expanding Big Bangs that are spread out over endless distances (open space). Each "verse" may be either matter or antimatter, with an equal number in existence at any given time. As the "verses" expand they collide and matter and antimatter annihilate, releasing energy.

Heat death of a finite universe would be predicted as entropy increases, however, the infinite size of the multiverse and the infinite number of "verses" could mean that new "verses" would be formed as old "verses" were annihilated. A chain reaction multiverse would be analogous to a fireworks display (each explosion representing a Big Bang) that starts in one neighborhood and is followed by fireworks displays in surrounding neighborhoods and then in neighborhoods further out. The chain reaction of Big Bangs would continue to expand as Big Bang fuel is consumed.

If the multiverse is open and the fuel is infinite then the chain reaction would expand forever. Of course, it is not known what the "fuel" is, but it is logical to assume that matter and energy are the product of a transformation from a real reactant, possibly the Higgs boson. The multiverse as a whole may never end completely.


False Vacuum

If the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state. This is called the vacuum metastability event. This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime.

It is also possible that all structures will be destroyed instantaneously, without any forewarning. According to the many-worlds interpretation of quantum mechanics, the universe will not end this way. Instead, each time a quantum event happens that causes the universe to decay from a false vacuum to a true vacuum state, the universe splits into several new worlds. In some of the new worlds the universe decays; in some others the universe continues as before.


Cosmic Uncertainty

Each possibility described so far is based on a very simple form for the dark energy equation of state. But as the name is meant to imply, we know almost nothing of the real physics of the dark energy. If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the big bang; but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy.

It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict. It is also possible the universe may never have an end and continue in its present state forever.


Time and the Big Bang
The Big Bang was the event which led to the formation of the universe, according to the prevailing cosmological theory of the universe's early development (known as the Big Bang theory or Big Bang model).

Stephen Hawking in particular has addressed a connection between time and the Big Bang.

In A Brief History of Time and elsewhere, Hawking says that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.

Upon occasion, Hawking has stated that time actually began with the Big Bang, and that questions about what happened before the Big Bang are meaningless.

This less-nuanced, but commonly repeated formulation has received criticisms from philosophers such as Aristotelian philosopher Mortimer J. Adler.

Scientists have come to some agreement on descriptions of events that happened 10−35 seconds after the Big Bang, but generally agree that descriptions about what happened before one Planck time (5 × 10−44 seconds) after the Big Bang are likely to remain pure speculation.


Time by Michio Kaku

We've always structured our lives based on an unchanging past and a predictable and ordered future. But atomic and cosmic discoveries have changed all that.

What is time itself? And will it ever end?
 
Time as illusion is also a common theme in Buddhist thought, and some modern philosophers have carried on with this theme. J. M. E. McTaggart's 1908 The Unreality of Time, for example, argues that time is unreal.

However, these arguments often center around what it means for something to be "real".

Modern physicists generally consider time to be as "real" as space, though others such as Julian Barbour in his book The End of Time, argue that quantum equations of the universe take their true form when expressed in the timeless configuration spacerealm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'.

Time has historically been closely related with space, the two together comprising spacetime in Einstein's special relativity and general relativity. According to these theories, the concept of time depends on the spatial reference frame of the observer, and the human perception as well as the measurement by instruments such as clocks are different for observers in relative motion.

The past is the set of events that can send light signals to the observer, the future is the set of events to which the observer can send light signals.