The concept of time has troubled philosophers as long there have been philosophers to be troubled. There are countless science fiction stories that get entangled in 'temporal anomalies' and the seeming contradictions associated with time travel. So, is time travel possible? What happens if I go back and change something? One very simple argument against time travel is that no one from the future has ever been known to have visited us. Hence, by implication, no one in the future will ever invent a time machine or find a magic portal back to us here.
As you might imagine, the problems result from an erroneous view of time. If you like, those anomalies and contradictions are nature's way of telling you that you have it all wrong. This page goes on to suggest that our macroscopic worldly view of time may also have a fundamental impact on our physics and that we need to look sideways at it.
The crux of the problem is that there are two perspectives on time: subjective time and objective time. Subjective time is our everyday perception of time. This is characterised by a feeling that time is moving past us or that we're moving through time. Objective time is the quantity usually represented in the equations of physics by the letter 't'. This has no preferred direction (with a few exceptions) and no concept of it passing by - it behaves just like a spatial distance in those same equations.
Steven Weinberg in his ‘dreams of a final theory’ remarks that ‘Duration in time is the only thing we can measure (however imperfectly) by thought alone…’. This is very astute and deserving of further analysis. All other physical quantities can be measured or counted via our senses. The passage of time, however, can be marked even in a sensory deprivation chamber. In other words, our thought processes and the apparent passage of time are intimately connected. Stephen Hawking may have hit this on the head when he suggests that any recording device, including the human mind, can only record in the direction of increasing entropy. The net effect is that we remember the past and not the future. This is a very different view than ‘the past has already happened and the future is yet to come’.
The term ‘arrow of time’, or ‘time’s arrow’, was actually coined by Arthur Eddington in 1927.
The exceptions noted above to objective time having no preferred direction are primarily:
Other suggested arrows of time, including cause-and-effect, the radiative arrow, the weak nuclear force, the quantum arrow, and the psychological arrow, can probably all be related to these primary arrows.
As we saw above, the psychological arrow (i.e. subjective time) can be linked to the direction of increasing entropy. The concept of cause-and-effect is related to our thought processes trying to make sense of what we observe, and is hence implicitly linked to the psychological arrow (see Cause and Effect). The radiative arrow is similarly linked to the psychological arrow. The asymmetry of the weak nuclear force is slightly different and may be linked more to the prevalence of matter or anti-matter in a given locality.
The section on Big Bang makes a case for the expanding universe being a geometrical phenomenon, interpreted through our concept of subjective time. If entropy can be shown to naturally increase when the same contents of the spatial universe are examined in larger and larger volumes of space then we only have one primary arrow, and it’s a geometrical one.
It’s difficult to break away from our notion of subjective time, and to consider the universe in a non-dynamic geometrical way. Our ‘laws of physics’ and the very concept of cause-and-effect then take on profoundly different roles.
Having read this section, it will be tempting to ask ‘What about fate? If the universe is static, does that mean my future is set? Should I just give up?’ Well, yes and no. It’s a vision of time that is hugely significant for physics, and our understanding on the universe, but it has virtually no impact on our day-to-day lives. Our lives are totally ruled by subjective time, and free-will is an inherent part of that. We cannot step back and look at time as a whole, and even if we could then we would no longer have the luxury of a dynamic time by which we could rationalise what we saw and use it to make decisions. In effect, those two conceptual worlds are entirely separate.