Newtonian physics
utilizes the Cartesian Coordinate System. When the time element was
added to this system as a forth dimension, it was noted that unlike
the other three dimensions time travels in only one direction. Think of it
like this: From wherever you are, you can travel in any direction and
its opposite. You can go up, or down; left or right; forward or
backward. The ability to go either direction within each of these
dimensions gives that dimension the attribute of symmetry. Time on
the other hand, only allows you to go in one direction. That is why
we call it the arrow of time. Intuitively this is not very important,
but in mathematics or philosophy it is troubling to the extreme, and
becomes an issue of debate.
In his book The
Fabric of the Cosmos – Space Time and the Texture of Reality,
Brian Greene, breaks down the asymmetry issue into four distinct
arrows of time. These are:
I had never thought
about breaking down the issue into four distinct questions in this
way, but find it worth thinking about. He further notes the
possibility exists that “they might, in principle, acquire their
time asymmetry from completely different physical principles”
although he does not expect it to be so. For those interested, the
section in question is quoted here from Page 510, Note 18:
"Throughout this chapter, we've spoken of the arrow of time, referring to the apparent
fact that there is an asymmetry along the time axis (any observer's
time axis) of spacetime: a huge variety of sequences of events is
arrayed in one order along the time axis, but the reverse ordering of
such events seldom, if ever, occurs. Over the years, physicists and
philosophers have divided these sequences of events into
subcategories whose temporal asymmetries might, in principle, be
subject to logically independent explanations. For example, heat flows
from hot objects to cooler ones, but not from cool objects to hot
ones; electromagnetic waves emanate outward from sources like stars
and lightbulbs, but seem never to converge inward on such sources;
the universe appears to be uniformly expanding, and not contracting;
and we remember the past and not the future (these are called the
thermodynamic, electromagnetic, cosmological, and psychological
arrows of time, respectively). All of these are time-asymmetric
phenomena, but they might, in principle, acquire their time asymmetry
from completely different physical principles. My view, one that many
share (but others don’t), is that except possibly for the
cosmological arrow, these temporally asymmetric phenomena are not
fundamentally different, and ultimately are subject to the same
explanation..."
From The Fabric of
the Cosmos – Space Time and the Texture of Reality, Brian Greene,
2004, Alfred A. Knopf, P 510 (Note 18).
The issue of asymmetrical time is treated in more detail on page 13:
"We take for granted
that there is a direction to the way things unfold in time. Eggs
break, but they don’t unbreak; candles melt, but they don’t
unmelt; memories are of the past, never of the future; people age,
but they don’t unage. These asymmetries govern our lives; the
distinction between forward and backward in time is a prevailing
element of experiential reality. If forward and backward in time
exhibited the same symmetry we witness between left and right, or
back and forth, the world would be unrecognizable. Eggs would unbreak
as often as they broke; candles would unmelt as often as they melted;
we'd remember as much about the future as we do about the past;
people would unage as often as they aged. Certainly, such a
time-symmetric reality is not our reality. But where does time's
asymmetry come from? What is responsible for this most basic of all
time’s properties?"
"It turns out that
the known and accepted laws of physics show no such asymmetry: each
direction in time, forward and backward, is treated by the laws
without distinction. And that’s the origin of a huge puzzle.
Nothing in the equations of fundamental physics shows any sign of
treating one direction in time differently from the other, and that
is totally at odds with everything we experience."
Philosophers might take exception with one particular assumption in this discussion. It is the implied assumption that asymmetrical time describes the "real" world. It was put it like this, "Certainly, such a time-symmetric reality is not our reality." This implies there is something special about "our reality," but then in the very next paragraph he tells us "the known and accepted laws of physics show no such asymmetry."
In summary: As far as time is concerned, mathematical science does not conform to the reality we experience; It includes a whole other set of possible events that are completely irrational to us; All of those things like eggs unbreaking, and candles unmelting are are completely plausible in the mathematical sciences. Most of us do not understand the language of mathematics very well. That is why we are looking at these concepts in normal people language here.
Just for the record, even though these concepts may be difficult for the non-mathematical mind to embrace, it is worth noticing that religion has accepted this way of looking at things for at least five thousand years. In Egypt the Pharaoh "was born as the living Horus, becoming Osiris at death." (Dict of AE, Abrams, 1995, P 79). These two are father and son, but Osiris is the father, and Horus is his son. Thus, the Pharaoh is born as the son, and later upon death, becomes his own father. That sort of thing happens only when time goes backwards, or cause and effect are reversed.
Another modest example for the ancient Egyptian is that "the hope of the dead for survival rested in the idea of inversion. His earthly life passed from childhood to old age, from birth to death. Therefore he hoped for rejuvenation to move in reverse from death to life. In the Book of the Earth the sun was held up as an example of the miracle of rebirth, for it traveled in its bark in the opposite direction with the stern foremost from the realm of the night into new morning." (Illustrated Dict of Gods and Symbols of AE, Lurker, Thames & Hudson, 1995ed, P79) In other words, the hope, eventually of all Egyptians, was to travel the night in a boat with his god, that traveled backward in time so he could be reborn into the past.
This sort of perspective was lost when we learned to replace myth and symbol with logic and math. It is now reasserting itself as we learn more about the math, especially cosmological math. Those four distinct areas of interest mentioned by Bryon Greene were only those that had to do with Physics. Fortunately, physics has begun to use a term that enables us to clump the less exact sciences in with it. This term is "evolution."
I am not sure what the term means, but we can at least recognize that it assumes a certain flow of cause and effect, or some kind of progressive, though meaningless process that proceeds along the arrow of time. The thing we need to recognize about evolution is that it includes a complete rejection of the idea of symmetrical time. It requires the cause and effect relationship to be continuous and progressive throughout history, or at least since the big bang. For now we can take the sciences that assume the "fact" of evolution and place them in with cosmology, but not all of cosmology. Just the sort of cosmology that rejects the idea of symmetrical time.
Bryan Greene gives us a different kind of cosmology, or at least expresses a concern for this issue more than most other writers of popular physics. In the preface of his book he tells us, "Space and time capture the imagination like no other scientific subject for good reason. They form the arena of reality, the very fabric of the cosmos. Our entire existence--everything we do, think, and experience--takes place in some region of space during some interval of time....And physicists such as myself are acutely aware that the reality we observe--matter evolving on the stage of space and time--may have little to do with the reality, if any, that's out there." (Ibid Greene, Page iX)
That sort of disclaimer is what I call a healthy view, and we could use a bit more of that. It does not prevent him from presenting the findings of mathematics in terms of the intuitive time arrow, it only allows for us to realize the uncertainty involved in doing so. It is only when that uncertainty is not allowed for, that the pragmatist within initiates the warning cry of dogmatism. We need to realize that what is special about "our reality" is based on an emotional rather than a logical assumption.
My own problem is that I have almost none of that psychological gravitation toward asymmetrical reality. While it is true that, like everyone else, I live out this mortality within that sort of framework, every moment within that framework takes place in the now. Whether the past or the future has the greatest bearing upon that present now, is open to question.
Another factor is an event that occurred during the fall of 1971. That event revealed to me the possibility that time and truth may both be very personal rather than universal in nature, as most of us generally suppose. In short, symmetrical time is no longer at odds with everything I have experienced.
Having thought it over for the past forty five years, I think I may be ready to engage in a discussion on the subject. Were that to happen, the next installment would be titled something like Mainstream cosmology: a justification of duration.