Philosophical Detritus VI—Time and Time Travel

Ticking away the moments that make up a dull day;
You fritter and waste the hours in an offhand way.
-- Pink Floyd.

One of the most striking things about getting older is that time seems to go by much faster. No sooner has the year started, when suddenly it seems to be over. Another striking thing is being able to remember growing up in the analogue era, without mobile phones or personal computers. Things have changed so much in 60 years. And I've been writing this blog for 20 of those years.

The question "What is time?" is both completely banal, since we all experience time passing and in general have no problems understanding the concept, and at the same time deeply puzzling in that it's difficult to define exactly what time is or how it works.

My general approach in these essays has been to start with the fact that no one has epistemic privilege. We're all in the dark when it comes to metaphysics, which seems to be why philosophy is devoid of any consensus on any metaphysical matter, at any time. No one is ever compelled or obligated to accept any particular metaphysical proposition. Whereas we are compelled to accept experiences such as having weight, or being extended, oriented and located in space.

In this essay, I explore a number of time-related themes from a pragmatic perspective. While we may never know the reality of time, the experience of it is compelling enough to argue that we cannot do without it. We experience time passing, we remember past events, and we age and these are all salient. I touch on the metaphorical nature of our concept of time and the arrow of time. I don't find time deniers to be of much interest. I deflate the notion of "timelessness", and I briefly explain why time travel is impossible (not related to paradoxes).

On Time

When you finish reading a sentence, you have to recall what you read at the beginning in order to make sense of it, but you cannot do both at once. You have to start somewhere and end somewhere else. In every situation we encounter, there are sequences of events and they occur in a specific order that is salient to our understanding of both time and causation (the latter is a separate essay, but see There is No Cause & Effect, 5 August 2016).

So deeply do we experience such sequences, and understand the order of them, that most people can instantly spot if a piece of video is being played backwards, precisely because things happen in the wrong order.

Some of the experiences we have are iterative: they happen over and over. Such experiences include our own breathing and heartbeat, as well as day-night cycles, lunar cycles, seasonal cycles, and life cycles. We call these "cycles" to emphasise their iterative nature.

Iterative experiences allow us to begin to quantify durations of time. I could say, for example, that it took so many days and nights to go from location A to location B. This is useful information. If for example, I'm crossing a desert and the next watering hole is 3 days travel away, then I need to ensure I carry 3 days supply of water. Or I will die.

The obvious downside of using iterative experiences such as heart beats is that they vary in duration. If two people wish to agree on the timing or duration of events, then they must first agree on a measurement of time. A measure like "three days from now" only works because the two people experience the same day/night cycles.

Living in cities tends to create a demand for time keeping because we do things in large groups and need to coordinate. Living a coenobitic monastic lifestyle requires time-keeping for the same reason. For example, Buddhist monks who originally lived a solitary, itinerant life for most of the year, would meet up at times linked to the phases of the moon. One they began to live communally and engage in collective practices that meant they all had to arrive on time. In 2015, after puzzling over the Pāli term yāmagaṇḍikaṃ koṭṭetvā, I wrote an essay about time keeping in early Buddhist texts. I noted various early time-keeping measures such as a sinking bowl water clock, dating back at least to the Achaemenid Persians.

We define external standard durations of time like seconds, minutes, hours by linking them to some iterative process. These days, scientists define the second in terms of oscillations of an atom of Cesium.

It is a curious fact that until rail travel was invented time was more or less always defined locally. But once you had railroads connecting cities across Europe, the fact that each city was effectively its own time zone, became a hindrance to coordination. The hegemony of the British Empire when time was being standardised meant that universal time is based on the Greenwich meridian, which passes about 10 km west of my home in Cambridge.

Coordinating actions in large groups of people, is one of the most important functions of time-keeping. Societies that don't do big communal projects don't need to keep track of time in the same way.

The Arrow of Time and other Metaphors

In some cultures, TIME IS A PATH that we travel along. We "see" the future ahead of us and move towards it. We do not see the past since it is behind us and gone from view.

In other cultures, TIME IS A RIVER that flows past and around us. We are stationary and facing downstream. The future approaches us from upstream and, being behind us, we cannot see it coming. The past is downstream and we can see it stretching away from us.

When we say things like "time is a path" or "time is a river" we are using metaphorical language. I use the convention begun by George Lakoff and Mark Johnson of stating such metaphors in small caps.

Interestingly, many of the metaphors we use for time come from the spatial domain. We can talk about a duration in terms of length, for example. "How LONG must I wait?", "Will you be LONG?", "He's doing a STRETCH in prison". These emerge from metaphorically treating time as a path.

I've already noted that humans can usually spot a video being played backwards almost instantaneously. This is because time has a direction. Whether we conceive of time as a path or a river, we know that the past in in one direction and the future is in the opposite direction. And when we see a video with the direction of time reversed artificially, it's immediately obvious.

It is common to illustrate this using examples like knocking a cup off a table and watching it break. We never see a broken cup spontaneously reassemble itself and leap back onto a table. Physicists like to say it's "theoretically possible" for this to happen, but in practice it never does.

We can artificially create some situations in which the arrow of time is ambiguous. For example, imagine that I set up a video camera looking vertically down on a snooker table. If I set a ball in motion off-camera and record it bouncing off the cushion then leaving the frame, you won't be able tell whether you are seeing the recording forwards or backwards. Similarly for a collision between two balls, when you don't see the initial impetus or the final arrangement. However, with three balls, or if I show the cue striking the cue ball, the arrow of time once again becomes clear.

That is to say, when we look at isolated, decontextualised events in which the antecedent events are hidden from view, the arrow of time can be made to seem ambiguous. However, in practice, without these elaborate attempt to hide it, the arrow of time is always completely obvious. In practice, we never experience confusion over the direction of time. Every experience we have reinforces this.

A few years ago, I was cycling along the greenway from Haslingfield to Grantchester. Near the latter village, a field of sugar beets had been dug up and the beets were piled up waiting to the trucked off to the sugar factory in Bury St Edmunds. The loader driver was standing around for the next truck to arrive, and I stopped to chat with him for a couple of minutes. Two days later, I passed by the same field going in the other direction. And to my surprise, I saw a mature crop of beets where, just two days ago, there had been bare earth. I stopped and I really got a chill, because this was the closest thing to "a glitch in the Matrix" I'd ever seen. It was unnerving to say the least. I replayed the memory of seeing the field harvested several times. I looked closely to see that it was indeed a mature crop, several months old. And yet I knew I had seen it harvested just two days ago. At length I got back on my bike and started pedaling. About 200 m further down the track, I realised that they had only harvested the bottom half of the field. I just just assumed it was the whole field.

Physicists explain the apparent direction of time—which they call the "arrow of time"—by referencing entropy. Entropy is itself often quite poorly understood by lay people. It is often wrongly associated with complexity for example. Entropy is a measure of disorder. Sean Carroll introduced an image for this that is now everywhere

• On the left we have coffee and milk, unmixed. This is low complexity, and low disorder.
• In the middle, the coffee and milk have begun to mix. Complexity is high and disorder is medium.
• On the right the coffee and milk are thoroughly mixed and now complexity is low again, but disorder is high.

And this is how our universe seems to be. We're currently somewhere in the middle, with high complexity and medium disorder. Around the time of the big bang, the universe was simple and had low entropy. And we expect the universe to become less complex again over time, but for entropy to keep on increasing.

When we see a cup fall and break we see something becoming more disordered. Seeing the video backwards we something quite disordered becoming more ordered, and we know that, practically, this doesn't happen in our world.

The second law of thermodynamics says that the entropy of a closed system always increases over time. Lay people often overlook the "closed system" part. A closed system in one in which energy cannot enter or leave. The earth for example is not a closed system. Nor is a biological cell. So living things, which persist in ordered states (low entropy), do not contradict the second law.

Time Denial

Hardly a week goes by without some pundit breathlessly telling us that "time is not real" or "time is an illusion". Or, maybe, "time is not fundamental". In the previous essay, on Determinism, I noted that the "it's an illusion" move doesn't explain anything.

Once again, this denial of what we experience is a dead end. Once you assert that time is not real, there is nothing left to say. There is nothing to explain. "It doesn't exist" is supposedly the explanation, but this simply leaves us hanging because as already noted, we experience the passing of time. Saying that "time doesn't exist" does not explain the experience. Writing it off as "an illusion" is also not an explanation. If all experience is an illusion, we are none the wiser, since we still have such experiences and if we try to act as if they are illusions we rapidly run into trouble.

If the government says "you must pay your taxes on or by the 1st of April", one cannot defend a failure to comply by arguing that "time is an illusion". Even if it is an illusion, it's one that we all share, and one that we are compelled by circumstances to take seriously.

The idea that time is not "fundamental" (and sometimes it is fundamental) comes from reductionism. Metaphysical reductionism equates "fundamental" with "real", and treats anything that is not fundamental as not real. Sometimes reductionists will tacitly admit to a hierarchy of reality in which the lowest level is specially marked as "fundamentally real". But this is nonsense by their own criteria. If it's not at the bottom, it is not fundamental. This only matters to true believers in reductionism. It's a doctrine, not a truth. The rest of us allow that structure is objective and that time being a structural feature makes no practical difference to us. Time is still objective.

So any philosophy that denies the passing of time rather than explaining it, is a dead end and practically useless.

Timeless

Most philosophers don't comment on the experience of timelessness. We've all had the experience of being absorbed in an activity and failing to notice time passing. Sometimes in meditation one loses touch with the world of the senses and it seems that time stands still.

Here I reference Thomas Metzinger's book The Ego Tunnel. Metzinger spends some time in his book discussing his out-of-body experiences. But he also describes how his initial intuitions, based on folklore about out-of-body experiences, were wrong. And he explains how he arrived at an objective account of them. Importantly, Metzinger describes how his objective explanation allowed him to reliably induce out-of-body experiences in experimental subjects under laboratory conditions using a variety of methods including virtual reality. I now see this as the gold standard for explanations of experience.

The fact of having experiences of timelessness, does not mean that time is not real. "Timeless" describes an experience, not a fact of the matter. Not being aware of time passing doesn't stop it passing. And we all have relevant experiences.

Every time we sleep, we spend at least some of the time in a dreamless state in which we have no awareness of time, space, self, world, or anything else. And yet, when we emerge from this state, it's apparent that time has passed. I go to bed in the evening and I wake up in the morning. The timeless state of deep sleep is not a metaphysical timelessness, it is an epistemic timelessness.

Becoming absorbed in meditation involves becoming unaware of all the sensory information we use to create our virtual maps of space and time: resulting in an experience that is spaceless and timeless. This is often interpreted as meaning that one was literally "outside space and time". Some interpret it to mean that mind is not tied to body. And so on. The fact is that time itself does not stop. Only our awareness of time passing stops. As with sleep, time continues to pass even in deepest meditation.

We might lose track of the world, but the world never loses track of us.

Time Travel

Time travel is one of the most enduring themes in speculative fiction. Both travelling to the past, with all the attendant paradoxes, and travelling to the future have been imagined many times. I grew up watching Patrick Troughton as Dr Who on a black and white television. I read H. G. Wells' The Time Machine as a teenager and have seen the classic 1960 film adaptation, produced and directed by George Pal many times.

Towards the end of the 20th century, physicists started to say things like "Time travel may be possible." If you search this phrase you can see that it regularly turns up in newspaper headlines. However, if you dig behind the headline, it usually leads to some theoretical physics paper that never really says that time travel is possible.

We get some mileage from the fact that if we make the present time t = 0 then we can put negative values of time into many physics equations and use them to retrodict the past. But we run into the problem that on the largest scale, physicists insist that t = 0 is when the big bang occurred. So the present is not t = 0.

Let us stipulate for the sake of argument that time travel is possible and that somewhere in the formalism of relativity it is possible to reverse time, or to alter the speed at which time passes.

What's not possible in any account of objective phenomena is the idea that you could move in time and not be affected by such movement. Because when you think about it, this is the key to fictional time travel. In all of the famous time travel stories, someone steps outside of time, they move along the time axis and arrive at a different time, whereupon they step back into time and are affected by it again.

For example, when Marty McFly and the Doc Brown go back 30 years to 1955, they don't become 30 years younger, but (fortunately for teenage Marty) retain their ages from 1985. No version of physics allows this.

Forget the "grandfather paradox", the real sticking point with time travel is travelling through time without being affected by time. This is what all time travel stories have in common, and yet it is precisely this that cannot be explained and cannot possibly happen in practice, even if moving arbitrarily in time is theoretically possible. This is my own insight, I don't recall ever reading any time travel stories that tackle this problem and I'm not aware of any philosophers who do either. It seems to be generally overlooked that time travel always tacitly allows moving through time without being affected by it.

Conclusion

I've briefly covered a range of topics in this essay: what time is, the direction of time, time metaphors, time denial, and time travel. I also touched on causality once or twice.

Time is a central and unavoidable aspect of our experience of the world. We cannot make sense of the world without it. Arnold J. Toynbee's facetious comment on the naive view of (chronicle) history as "just one damn thing after another" is in fact a truism about time. Time is unavoidable, it is visible in sequences of events, and measurable via quantities of iterative events. Time really is just one damn thing after another.

Time is objective, in the sense that we all experience it and it can be measured in ways that we all agree with. For example, we can predict when and where solar eclipses will happen decades hence. If time were not objective, what would it even mean to predict such events?

Time passes for everyone (all observers) at one second per second (time dilation notwithstanding). Time also has an objective direction, which is the same for all observers. And we instantly know when we see a video in which the "arrow of time" is reversed, because such a thing never happens in practice.

The experience of time passing is subjective in the sense that we may experience time passing faster or slower. But we know that this can be explained by relating it to the emotional intensity of the experience and the rate of memory formation.

However, the question of the reality of time is not one we can ever answer because no one has epistemic privilege, i.e. know one has privileged access to reality to perform comparisons with experience. We have experience and comparing notes on experience, and we have reason and imagination. That's all we have to work with. Reality, whatever it is, remains forever inaccessible to us.

This may be why time is so problematic for physicists. They cannot decide whether time is fundamental or not; whether it is real or an illusion. And so on. But no matter what they eventually decide, it won't change how we experience time.

However, we can and do infer useful information about the world. Comparing notes about sensory experience allows up to create and maintain virtual maps, which in turn allow us to navigate the world. Describing experience without time doesn't seem possible, but even if it were, it would not be useful. In our universe, everything is interacting with everything else, everything is changing, all the time. Time is integral to how we conceptualise this.

Unlike the previous example, therefore, where I concluded that no good ever came of thinking in terms of "determinism" or "free will", I believe we have to retain the concept of "time" and resist those who wish to deny it. Even if time is only an inference drawn from experience, we cannot do without it. Nonetheless, there is still no epistemic privilege so our definitions must be pragmatic rather than absolute.

~~Φ~~

See also: Time for a Change. (03 March 2017). In this essay, I'm still talking about "reality" in a careless, reified, way. However, I think the notes on metaphors and time are still useful. These essays are not merely academic. If you watch this blog—this 20 year sequence of essays—you can see my worldview changing over time. You can see understanding dawning on me. If I do my job as an writer well, you can have the same experience with considerably less effort.