Time 2
In today’s lecture… RecapThe Girl who Leapt through Time
Recap The nature of timeNewton, Kant and EinsteinTime travelTime travel paradoxes
3 theories of time 1. Eternalism (永恆論) 2. Presentism (現在論) 3. Parallel universes (平行宇宙論)
3 theories of time ‘Eternalism’ – also known as ‘four-dimensionalism’ or ‘block universe theory’ – is the view that time is the fourth dimension, and that space and time are connected together as a single whole.
3 theories of time According to eternalism, there is no difference between time and the other 3 spatial dimensions (length, width and height). Space and time together form a four-dimensional ‘space-time block’ or the so-called ‘block universe’.
3 theories of time In the Newtonian universe, it was supposed that there was no connection between space and time. The ‘block universe’ (or ‘space-time block’), as postulated (提出) by Einstein in his theories of relativity, is a four-dimensional structure in which space and time are inseparably connected.
3 theories of time According to eternalism, all events are fixed and determined (既定的、注定的). The universe is one immutable (不變的), four-dimensional ‘space-time block’. All objects and events – past, present, future – can be seen as in different fixed positions on the space-time block.
3 theories of time As such, ‘the past’, ‘the present’ and ‘the future’ are just different parts or locations on the space-time block.All objects and events – past, present, future – are equally real. Past and future objects and events are just as real as currently existing ones.
3 theories of time The space-time block is like a movie on a videotape, and our experience of time is like that of watching a movie – although the whole movie is already there on the tape, we can only see it one frame at a time.
3 theories of time From the standpoint of eternalism, the appearance (表象) of time passing is not real (i.e. not an objective fact of the external world). There is no objective flow of time. The passage of time is a subjective illusion (假象) – it is because of our mental structure that we see time as flowing.
3 theories of time According to eternalism, we know more about the past than the future, but this is not because the future is not real. The future is already there, fixed and determined. We cannot know the future simply because of the limitation of the human consciousness – our minds cannot see into the future.
3 theories of time If time really is the fourth dimension, it should be possible to move along the axis of time in either direction (like movement along the 3 spatial dimensions). In other words, it should be theoretically possible to travel either forward or backward in time.
3 theories of time Thus, if eternalism is true, time travel is possible – it is theoretically possible to travel either to the past or to the future.However, a time traveler cannot change the past nor the future because all events in the block universe are fixed and determined.
3 theories of time Why does time always move (or seem to move) only in one direction? Is there any real difference between the past, the present and the future?
3 theories of time Opponents (反對者) to eternalism argue that unlike the 3 spatial dimensions (where movement in either direction is possible), some events are irreversible (不可逆轉的) and happen only in one direction in time.
3 theories of time For example, you can burn a piece of paper to ashes, but you cannot turn the ashes back into a piece of paper.Similarly, glasses fall and break all the time, but shards of glass do not put themselves back together and fly up on tables.
3 theories of time The fact that time has a direction is called ‘time’s arrow’; for example: [1] some processes and events are irreversible; [2] causes always precede (先於) their effects; [3] we have memories of the past, but not the future; and [4] we cannot change the past, but our actions can affect the future.
3 theories of time Another theory of time is called ‘presentism’, and it can be regarded as the common sense view of time.According to presentism, as time passes, the moment that was once the present becomes part of the past; and part of the future, in turn, becomes the new present.
3 theories of time The main argument of presentism is that reality is always changing. Time flows when events change from being future, to being present, to being past. The past has gone. The future has yet to come into being. The past and future are both unreal. Only present objects, events and experiences are real.
3 theories of time One of the implications (含義) of presentism is that time travel is impossible. Since only the present is real, there is no future or past to travel to.The main problem with presentism is that it is in conflict with (有抵觸) currently accepted scientific theories, especially the special theory of relativity.
3 theories of time The third theory of time is called ‘parallel universes’. As its name suggests, the theory states that reality consists of an infinite number of parallel universes.The theory was first proposed as an interpretation (解釋、解說) of quantum mechanics (量子力學), but the most common use of ‘parallel universes’ is in science fiction.
3 theories of time According to one interpretation of quantum theory, when we make a choice between two or more options, it will result in the creation of two or more universes or ‘timelines’ – one for each option in a given choice – through a process called ‘branching’ (分枝).
3 theories of time The parallel universes theory suggests that each of these parallel worlds is just as real as our own, and there is an alternative history for every possible outcome of every decision ever made.
3 theories of time The parallel universes theory suggests that backward time travel would take a time traveler to a parallel universe in which the course of history would begin to diverge (出現分歧) at the very moment the time traveler arrives in the past.
3 theories of time In other words, when a time traveler goes back to the past, the universe splits (分叉) at that moment into two parallel universes.The time traveler’s attempt to change the past will give rise to an alternative universe with a different history (i.e. a different ‘timeline’).
3 theories of time However, in the original universe, nothing has changed and everything remains the same. Thus, if the parallel universe theory is true, the time traveler’s actions will not have any effects on the universe from which he came.
3 theories of time Suppose a time traveler goes back in time to kill Hitler in order to prevent the World War II. His action will result in two parallel universes, one in which the war happened exactly as it did (the original universe), and an alternative universe where Hitler is killed (if he succeeds) and the war may not happen.
Think! Which theory – eternalism, presntism, or parallel universes – offers the best explanation of the nature of time?
Think! You should notice, first of all, that the 3 theories of time are in conflict or incompatible (不相容) with one another. It means they cannot all be true. But, of course, it is possible that they are all false.
Think! The 3 different theories also provide different answers to paradoxes of time travels such as the grandfather paradox.For example, from the standpoint of presentism, these paradoxes indicate that time travel, especially backward time travel, is logically impossible.
Think! According to eternalism, all events are fixed and determined. Thus, although time travel may be possible, a time traveler may not be able to interact with objects and people of the past.Even if he was able to interact with objects and people of the past, anything he did must have been part of known history.
Think! The parallel universes theory suggests that backward time travel would cause the universe to branch off into two different timelines, creating another universe with a different course of events. However, everything in the original universe (where the time traveler came from) would remain unaffected.
Think! According to the parallel universes theory, because different timelines are completely separated from one another, actions and events that take place in one parallel universe do not have any effects on the others.
Think! Thus, in one parallel universe a time traveler may kill his grandfather so that his family will cease to exist. But in the original universe, his grandfather continues to live to a ripe old age.
Read! Go to the course website, download and read the article titled ‘Aristotle and Fatalism’. (Some parts of the article may look difficult but we will look at related issues in next week’s lecture.)
The Girl who Leapt through Time
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