I wrote these short essays as part of my study of philosophy at the University of Sydney last year.  They are very relevant to this month's focus so I'm sharing them with you here! 


Does time flow?

Philosophers have two mains theories about whether or not time flows. 

A-theory says, yes, time does flow.

B-theory says, no, times does not flow.

Let’s define these two theories:


Time flows.  An event has a property such as presentness, futurity or pastness.  These properties are temporal properties, meaning they change or flow into one another as time passes.  In A-theory time is dynamic.  Events go from being future through to being present and then becoming past and shift in respect to these positions (although we might have an incorrect intuition that time is always ordered in this way, due only to our experiences of how we perceive time as passing and our limited perception of it). 

Problems with A-theory

Temporal properties come and go in different combinations.  What was future will eventually become past.  This could lead us to think that only the present is real (presentism).  But how long does the present moment last?  Very quickly this line of thinking becomes perplexing and absurd, since we can’t seem to clearly pin down past and future.

Another challenge for the A-theorist concerns the rate at which time flows.  If time passes, then it makes sense to ask ‘how fast does time pass?’.  But B-theorists argue that there is no coherent response to this question – the answer ‘1 second per second’ isn’t coherent.  So they conclude that the notion that time passes is incoherent.


Time is fixed in relative locations.  For example lunch happened after breakfast today.  One event happens in the context of another and this does not change.  So time is static and a moment is described in terms of its relationship to other events.  In B-theory events don’t have temporal properties rather one event happens after another event and this does not change (temporal indexical).

Problems with B-theory

B-theory doesn’t prioritize the present, but intuitively to us the present moment (in time) seems important.  For some philosophers this is a problem with B-theory. 

What do I conclude?

The lack of emphasis on the present moment in B-theory makes A-theory more compelling for me and it is my belief that time does flow, although this may be more an intuitive sense than a sound philosophical logic. 

In addition to these two theories we should note that both rely on temporal notions (in other words ideas about time) making them unhelpfully circular as tools with which to describe or define time and its qualities.  In order to discuss time without relying on temporal notions we would need a new framework for thinking about time which may allow us to develop some more sound philosophical arguments without the problems described above. 


Backwards time travel is impossible.  Discuss.

My argument is that backwards time travel is possible, but that it is only possible to participate in the past not to change it. 

Clarify question - What does ‘possible’ mean? – literal, logical possibility.  Not self-contradictory (such as a round square).  Could be an ‘outlandish possibility’ such as ‘it will snow in Sydney tonight’.

What is time travel? - again, in this context we mean literal, physical time travel.  We usually think of time travel as the ability to travel backwards and forwards along the same timeline.  So, if you travel into the past, you are moving along your own timeline (or the exact timeline that causally led to you – e.g. the same timeline as your ancestors).  This is why we don’t think that you can change the past, but only participate in it.  If you change the past, then you are no longer traveling along the same timeline, but along some other possible timeline.

External time is linear (as per our normal perception of time), measured by a clock, objective. 

Personal time is subjective, measured by an individual person aging (e.g. hair growth) but might move around in external time.  

Changing vs. participating

We may think backwards time travel involves changing the past.  Changing the past causes a contradiction, it is impossible.  So we may conclude backwards time travel is impossible.  However if we participate in the past without changing it that may be possible (and not create contradiction).  Time travel requires only participation, in other words making something happen as it did.  If we travel backwards in time we may perform actions that caused things to happen as they did but not change the past. 

Assumptions we need for time travel to be possible

Eternalist view of time – all points of time are equally real (compared with presentism which asserts only the present is real).

Fatalist view of time – we are powerless to do anything other than what we actually do at any given moment. Fatalism says that all events that have ever happened or ever will happen must have happened.

Both eternalist and fatalist views of time are essential perspectives in concluding that backwards time travel is possible.

Fatalism gives us the notion of metaphysical necessity, which makes it impossible to change the past, and hence allows us to avoid contradictions.  So eternalism and fatalism perform different important roles in discussions of time-travel.

Problems with time travel

Free will: the view that rational agents have the capacity to choose a course of action from among several alternatives. 

If all we can do is participate in the past (and not change it) what does this mean about our capacity for free will?  If participation alone negates free will then time travelers have no free will.  Note: philosophers have widely different perspectives about how much free will human beings have, regardless of the issue of time travel.   


Backwards time travel is logically possible, but you can only participate in and not change the past.  Of course the laws of physics and technology are not understood well enough at this time to make it actually possible … yet!