The (non?)existence and (un?)importance of time
Time. Does it exist? If so, what is it? Discuss…
University philosopher Antony Eagle and physicist Andre Luiten answer the same series of questions exploring aspects of time and its importance to their discipline.
Does time exist/is it real?
AE (philosopher): Events certainly seem to take time, and to be temporally related (earlier/later than) to each other. And I think we are entitled to take those appearances more or less at face value. That time exists is more plausible than any philosophical (or physical) argument that it does not exist, although there are famous arguments in the history of philosophy that time isn’t real.
AL (physicist): Yes, time is real. Deep within science and philosophy is the concept of “causality” – this expresses the fact that there are causes and effects in the universe and the ordering of those events is rigid with the cause always preceding the effect. Despite all the new things we have learned through relativity and quantum mechanics, causality is still a foundational belief.
If time is real, what is it?
AE: Like space, things (both objects and events) take up a volume of time (duration); things are located in time; and there is a metrical structure to time. If pressed I would say time is a dimension, and that the question of the reality of time is rather like that of the reality of space.
AL: Time is the parameter that orders events and allows us to measure the rate of change of things. It is as fundamental to the structure of the universe as is the concept of space. One interesting observation is that for small, simple microscopic systems one can run time either forwards or backwards and everything works and looks to be normal.
Perhaps as an analogy, if I were to film balls colliding on a billiard table, then I could play the film forwards or backwards and you wouldn’t be able to determine which is the “truth”. However, if I film someone diving into a swimming pool then it is very clear which ordering of the film is the “truth”. The arrow of time itself appears to pop into existence in the move from isolated, simple systems into large, complex systems. Exactly how this happens is still an active area of study in physics and is closely connected to the concept of entropy (disorder) and the way that disorder seems to increase with time.
If it isn’t real, why have we created it?
AE: I suspect the question is somewhat self-undermining. Creation is a process that happens in time – at least any creation that humans perform. We can only create if time exists.
AL: It is real! Humans have created systems of timekeeping based around seconds, hours, days, years – these are useful, but arbitrary, ways to use time in a manageable way.
Is time linear?
AE: A tricky issue! Our best physical theories of spacetime seem to make time linear, in that there is only one time dimension and, for any three events, one is between the other two. But this line is quite unlike others we are familiar with – for example, the elapsed time measured between two events can vary depending on the motion of the measurer.
AL: In practical terms, yes. However, very careful experiments show that time can bend and warp depending on the speed or environment of the observer or the observed. Einstein’s theories of relativity predicted this and all experiments to date have verified his theories perfectly.
Why is time important to your field/people?
AE: The nature of time is one of those big foundational questions that eventually leads every discipline to philosophy. Along with notions like space, and location, and existence, time features in almost every scientific theory. Time is also intimately linked to change and identity, which have been topics of philosophicalinterest for 2,500 years.
AL: Time is foundational. Almost everything we do in modern society requires precise measurements of time. Even minor inaccuracies in timekeeping will throw these systems off, which is why we strive for extreme accuracy. In my work, we spend a lot of time ensuring we can measure time exceedingly well, although (perhaps surprisingly) we spend very little time thinking on its exact nature.
Is time travel feasible? If so, only into the past?
AE: Feasible? I would say ask an engineer! But is time travel possible? – I think so. For a person to persist, philosophers say, is for that person to be located at many different times. Normally the times at which a person is located form a nice ‘block’, connected up and with the stages of our lives progressing in an orderly way.
As long as their stages are connected to each other in the right sort of way, a person could occupy a set of times that doesn’t look like this. Their life may be disconnected, e.g. occupying the years 1900–1920 and also 1980–2040. It may be out of order, with a ‘baby stage’ in 1980, a middle-aged stage in 2030, and an elderly stage in 1915. A life like that would be a time-traveller, a life where the ‘internal clock’ of the stages of growth and development and ageing is not aligned to the external calendar.
AL: One for lovers of science fiction! There is no doubt that you can move into the future if you move at relativistic speeds (i.e. close to the speed of light) – check out the “Twin Paradox” on Wikipedia for an explanation of this and the nice point that astronaut Scott
Kelly aged eight milliseconds less than his Earth-bound twin due to a year spent on the Space Station. Essentially Scott Kelly has travelled eight milliseconds into the future. Very much longer trips in time than this would be technically very tricky. It appears that there is no scientific means to travel backwards in time.
How have human concepts/understandings of time changed, over time?
AE: For most of known history, time was seen to be ‘flowing’ or ‘passing’, and only present things were taken to be real. It took until the twentieth century to see thinkers toying with the denial of these claims – to think that past and future might also be real, and that the flow of time may be an artefact of our cognitive systems.
AL: Humans started measuring and dividing time using the natural astronomical cycles that were around them – day and night, seasons, lunar phases. As society desired more timing precision than available from calendars and sundials, it drove the development of mechanical clocks that were placed high in cathedrals and town halls to provide agreed time to an entire town. The next huge leap in timekeeping was driven by the desire to navigate over the oceans with high precision. The final great leap in timekeeping has come with the ongoing development of atomic clocks since the 1950s - the best modern clocks are capable of measuring time to one picosecond per day (one thousand-billionth of a second).
Is there a bigger question I should have asked about time, pertinent to your field of expertise? If so, what is it, and how would you answer it?
AE: I’m not sure they are bigger questions, but there are two which are the focus of considerable discussion at present. First, the question of the direction of time. Is time itself ordered from earlier to later? Or is the direction of time derived from how things are in time? For example, many would say that the direction of time is grounded in a thermodynamic asymmetry – on this view, the future is just by definition whatever direction entropy tends overall to increase in.
Second, the question of the fundamentality of time. Time might be real, but it might be grounded in something more fundamental – in the same way the economy is real but may be grounded in the behaviours and dispositions of individuals and institutions. Some think a future theory of quantum gravity will explain time (and space), and yet may not itself involve spatiotemporal notions.
AL: The Global Positioning System (GPS) underpins the effective functioning of our communications, internet, logistics as well as our ability to navigate a foreign city and get a pizza delivered to our house. The benefit of GPS to our global economy has been estimated at more than 2 billion dollars per day. What might not be as well understood is that the heart of GPS is a constellation of orbiting satellites on each of which are several very high-quality atomic clocks. Further improvements in timekeeping ability could allow a next-generation GPS with increased precision which could answer a slew of economically valuable tasks.
Associate Professor Antony Eagle has a research focus of theoretical philosophy including the philosophy of physics. He has previously lectured at Oxford and has a PhD from Princeton. He is Editor of the Australasian Journal of Philosophy.
Professor Andre Luiten is Chair of Experimental Physics at the University and Chief Innovator of our Institute for Photonics and Advanced Sensing. He and his team won a Eureka Prize for the development of the world’s most precise clock – the Cryogenic Sapphire Oscillator.
Story created by Mark Douglas, Editor of Lumen. Image by Isaac Freeman, photographic editor.