Now That my Pun Is Out of the Way
In my previous post about time itself, I ignored a key concept about time that has been referred to as the Arrow of Time. In its simplest form, the arrow of time explains that time moves forward and never backwards. We see this in our everyday lives. A broken plate having fallen off a table and smashed on the floor does not leap back onto the table and reassemble itself, and perhaps broken hearts don’t get fully mended either. Scientifically, we can envision time sweeping out a series of minute slices through the space time continuum that continue to flow only forward. Despite science fiction writers and great CGI from the movie studios we have no real evidence to suggest that the effects we see in our daily lives aren’t what really is. Not always the case but it works for now it seems.
Why Does the Arrow of Time Seem to Hold? Thermodynamic Arrow of Time
One valid explanation for the arrow of time is becauseof entropy. Entropy is a measure of the disorder of a system, and the Second Law of Thermodynamics states that entropy as a whole always increases, never decreases. So what does that gobbledegook actually mean?
Consider a cup of coffee. Add to that cup some cream and stir it up. Two mixing have occurred. First the coffee and cream liquids have become blended, and second, the warmer temperature of the coffee has dropped while the cooler temperature of the cream has increased. Coffee and cream as separate entities had more order to them than the mixture of the two, so entropy increased with more disorder. And the Second Law mentioned above implies that heat from the coffee flowed into the cream, but you can’t take heat out of the cooler cream making it colder and heat up the coffee even more as a result. Spontaneously, the coffee and the cream won’t separate again either (evaporation or gravity or some other action is not spontaneous).
The implications of the Second Law, by the way. predict that entropy will increase throughout the entire universe to the point where everything is the same temperature, and no more heat transfer can take place, cold, dark, and essentially empty as matter will have given up all its energy. It’s gonna take a while to happen, though, so don’t give up on getting a tan.
Cosmological Arrow of Time
The universe is expanding. For a while, a looming question was whether that expansion would eventually slow down because of gravitational forces, stop, and start contracting into The Big Crunch (opposite of The Big Bang), but now it seems the expansion will continue. We are such a minute fraction of time on the scale of the universe that our observations and theories may yet to be proved wrong. I’ll be working on my tan while that plays out.
At any rate, time doesn’t seem to go in reverse from this perspective. Perhaps the cosmological arrow of time is because of the thermodynamic one, and both likely are consequences of conditions at The Big Bang and the early few nanoseconds of the universe.
Other Arrows of Time
In the scientific realm, there is radioactive decay (smaller particles resulting from decay of larger ones don’t spontaneously become the larger one again), particle physics and quantum arrows of time. Way too esoteric to say more, so I won’t
The more relevant arrows of time to us are probably cause and effect and psychological. We expect a cause to occur before its effects do. We don’t see a billiard ball, for example, go zooming off its spot on the table and then sometime later the cue ball going to that spot and rebounding from the impact with no ball there any longer.
In psychology, we age and have a memory of days and years past. We recall with dread, affection, or indifference our pasts and plan for the future, hoping our plans will affect it. And some cultures have different notions of past and future as evidenced by their language about them. But mostly, we believe that once something happens, it is done, time had moved on and another thing can happen, but that past event can only be revisited in our memories.
Hollywood Still Holds Out Hope
Not Bob Hope, but hope that the arrow of time is not so rigid. A recurring science fiction theme is time travel, that we can go back into the past. Not just from Jules Verne, but think Terminator. And that it can flow in reverse is the dream of the fashion, cosmetic, health care and so many other modern industries the creators of The Curious Case of Benjamin Button delivered to us.
So Now Do I Know What Time Is?
No, I don’t think so. It is what we all experience, what affects our modern daily lives, and our race as a whole. We know we move through it, or perhaps it moves through us. Like dark matter in the universe, we can detect it and even measure it, but we don’t know – as yet – how to define it.