Wave your hand from left to right and observe the movement. What you are witnessing is movement, or motion through space. This may seem extremely obvious to you but the significance is vital. If we are able to observe these dimensions, why can’t we observe the 4th dimension, time? You may think of a clock as the symbol of time, but if a clock stops ticking, time doesn’t stop, only motion. Also, like our hand that was just moving forwards and backwards through space, can we move forwards and backwards through time? Of course not, we are all fully aware that time has only one direction; it has always flown from past to future and always will.
The strange thing is, in the laws of physics there appears to be no ‘arrow’ of time. If you apply Newton’s laws of motion to calculate where a ball will land in the future, you can just as well work out where it was thrown from in the past. So where does this mysterious arrow of time emerge from? Could it have formed by human perception? A result of our brains attempt to arrange events that happen around us? With this viewpoint, time is merely an emergent concept of motion because it becomes evident through motion and is measured by comparison with other motions such as sunrise/sunsets, night and day, the changing seasons, the aging process, etc.
A clue of where this arrow originates from is in a concept known as entropy. In a nutshell, entropy is a measure of how orderly things are and is a value assigned to any macroscopic system. It reflects the number of times a system’s particles can be rearranged without altering its overall appearance. For example, a pond of water would be a high entropy system because the water molecules can be arranged in a number of ways. On the other hand, an ice crystal would be a low entropy system due to the precision of arrangement. According to the second law of thermodynamics, high entropy states are more likely than low entropy states; therefore things will become more disorderly because there are more ways to produce them. So, if you neatly stack papers on your desk, and you walk away, you’re not surprised if they turn into a mess. However, you’d be very surprised if you walked away from a mess of papers and return to them neatly stacked. That’s entropy and the arrow of time.
However, the second law of thermodynamics doesn’t necessarily include time; it focuses on particle arrangement only. The only way to explain the arrow of time is to assume the universe started off at an incredibly low entropy state. “Time’s arrow depends on the fact that the universe started up in a very peculiar state,” says physicist Carlo Rovelli of the Center for Theoretical Physics in Marseilles, France. “Had it started up in a random state, there would be nothing to distinguish the future from the past”.
Radiation from the big bang is evidence for this theory by providing us with a snapshot of the infant universe. It shows that at the beginning, matter and radiation were spread extremely smoothly throughout the universe. You would think it was a high entropy state but when you take gravity into account, entropy changes. Gravity always tries to group matter together so in a system governed by gravity, a black hole is a far more likely state than a smooth distribution. Due to gravity, this low entropy smoothness is remarkably unlikely but cosmologists do have an explanation for this rare state. At the first fraction of a second after the big bang, there was a rapid burst of expansion known as inflation. It was inflation that resulted in this unnatural smoothness by ironing out all the creases. However, in order for inflation to occur, the inflaton field which drove this expansion from 10−35 to 10−34 seconds post-big bang, must have some incredible properties. To make the matter all the more confusing, the inflaton field has low entropy itself.
A possible explanation is that inflation didn’t happen at once. You could picture the inflaton field as a high entropy state with varying and random properties. In this case, the low entropy inflaton that gave rise to our smooth universe (and therefore our arrow of time) could just be an anomaly in the larger high entropy field. So it was down to chance. The physics of the inflaton field shows that there is enough of it to give rise to more universes, leading on to the multiverse theory. Is our universe part of a vast multiverse due to this inflaton field? The theory seems justifiable and could lead on to predict some universes having an arrow of time while others don’t.
So what viewpoint will you support on the explanation of time, psychological or scientific? If you are edging towards the psychological viewpoint, that the arrow of time is merely psychological and in reality, there is no such thing, you’re implying that the past, present and future are all happening at once. In that case, at this moment in ‘time’, a part of your mind knows what will happen in the future but due to our brain’s evolution and our perception, we don’t recognize this ability… Now that’s something to think about. Quite possibly all will be revealed in the future – that is if there is such a thing.
Source: NewScientist: Special Issue, ‘Time The Most Mysterious Dimension of All’ pp 37-53, October 8th 2011
(Huge round of applause especially to Amanda Gefter and her brilliant article!)