Even though our concept and understanding of it continues to change and evolve, time remains one of the most difficult properties in our Universe to pin down, as it can be alternately relative, imaginary, or real.
On some fundamental level, it doesn’t exist, at least not in a way you or I might recognise, and from one day to the next, much of what we refer to as “time” is either just memory or an anticipated future.
While ancient civilisations measured the passing of time with things like the annual flooding of the river Nile or the varying length of shadows cast by sundials, our modern understanding is built on Einstein’s general theory of relativity, in which time is just a coordinate—it doesn’t always run at the same speed, it’s not just a simple line, and it exists within a space-time field of four dimensions.
Time is not symmetrical, but rather it has an inherent asymmetry, and a one-way direction. In 1927, the term and concept of an “arrow of time” was developed by a British astronomer called Arthur Eddington, who had realised that if time were to be symmetrical, the world would be rendered quite nonsensical.
Such nonsense might not be immediately evident. For example, if a video of the planets orbiting the Sun were played in reverse, you wouldn’t be able to tell the difference between that and the same video being played forwards, and everything would appear to be in keeping with the laws of physics. But if a video of someone dropping a book to the floor were played backwards, it would look like the book is falling up—an absurdity. We remember the past, but we cannot remember the future.
The term “arrow of time” mostly refers to the thermodynamic arrow of time, which is neatly tied to the second law of thermodynamics, one of four laws discovered during the 19th Century that define relationships between heat, work, energy, and temperature.
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This law states that entropy can only increase in a closed system—the closed system being our Universe, entropy being a measure of disorder, mess. As time passes, so the entropy increases, and though we cannot measure time with entropy, we know that the energy in the Universe moves slowly but surely towards an ultimate disorder. Things cannot be made any neater, and we cannot go backwards to yesterday—the second law of thermodynamics so casually imposes a direction on time.
There are other arrows of time, which vary in their connectedness to each other, including a cosmological one, which points in the direction of the Universe’s expansion; a radiative arrow of time, involving the expanding outwards of waves from their source; a causal arrow, which has to do with cause preceding effect; and a quantum one, which speaks of the symmetry of time and ties up with the famous Schrödinger equation, though nobody quite knows how this arrow relates to the other ones. There’s also a psychological arrow: our perceived movement from a known past to an unknown future.
Culturally, the organisation of time can be quite different, and this directly affects our experience of it. In some languages, the past is referred to as behind, and the future ahead, but in others, the past is ahead and the future behind, perhaps because the past can be seen, and in order to observe something, it needs to be in front of you, not behind.
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While some languages refer to time as a distance travelled, others refer to it as a growing volume—a long day, a full day. In English we think of it in linear terms, from left to right, but Chinese speakers think of time in terms of over and under, and in Greek time can be large, small. So easily do we mistake a word for the thing or phenomenon it speaks of, that it represents.
But it’s alright, because when we gaze slightly bewildered at the night sky we are looking straight into the past; light may be traveling at 299,792,458 metres a second, but the distances mean that it doesn’t arrive until we’re ready to feel nostalgic. The top of your body ages ever so fractionally faster than your feet, because as gravity increases, time slows; the lined mountain ages faster than the ocean floor.
And whether you are down or left, to the northeast or behind me, whether you call the day after tomorrow or I’ve just forgotten that you called at all, I’m certain that when we agree to meet in between order and chaos, you will be on time.
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