Here's a sobering thought, lifted straight from Dean Buonomano's latest book, Your Brain is a Time Machine: How does the brain's architecture shape our ability to answer questions that it did not evolve to address? Among the many things the brain certainly did not evolve to understand was the brain itself. Another is the nature of time.

DW: So let's start with one of your own questions. Time: what the hell is it? Can you put it in a nutshell?

Dean Buonomano: Probably not! Or else philosophers over the past thousand years would have done so! Part of the fascination with time, the mystery of time, and the trouble we have defining it - you know, we all know what it is unless we're asked to explain what it is - is that it is more than one thing. I give this contrived example in the book, where I say, 'Einstein's talk on the nature of time started on time and seemed to last a long time.'

So we use the word time to mean many different things. We use time to mean clock time, and that's the easiest. Time is what clocks tell. But we're talking about the nature of time, the physical nature of time, what time is - is time a dimension, where the past and the future are equally real? Or is only the present real? That's the question of the nature of time, and it's a very deep question which I try to discuss.

Yes, you present this idea of two camps - in the one you have the "presentists," for whom only the present is real, and in the other you have the "eternalists."

And for the eternalist, the past, present and future are equally real. Eternalists treat time much like people treat space. For example, I know you're in Germany and you know I'm in Los Angeles, and they are different points in space. But we accept that those points in the east and the west are equally real. That's the case with time for eternalists. The past and the future are equally real. They just happen to be at an arbitrary point in what we call the present.

And where would you place yourself in that?

I think in physics there is a tendency to embrace eternalism, and that's in part due to Einstein's theory of relativity. But physics doesn't have any evidence that the universe is eternalist. There are reasons to believe it is based on interpretations of Einstein's theory of relativity. But I tend to favor presentism, in which there is something special about the present. The present is real, but the past and the future aren't real. But time is still relative, because my present is different from your present. 

How does that fit in with the idea of our brains being time machines?

Well, all animals have memories in the sense that they learn from the past. Pavlov's dog will salivate in response to the bell, because in the past, it remembers, or there is information there, that when that bell is presented food will be presented shortly after. But that doesn't mean the animal is reminiscing.

You and I, when we say remember we tend to mean some kind of conscious recollection. The terms are a bit fuzzy here, but when I say, 'Remember your last birthday or a wonderful trip you took,' you can actually revisit that in your head.

But even if other animals think about the past, the big difference is the future. The concept of saying, 'I have to get up early tomorrow because I have a phone meeting with Zulfikar in Germany,' that kind of thing, where I'm consciously preparing for the future, animals probably don't do that.

Then there's the way we learn the concepts of time as children. You describe a lovely experiment involving kids and slugs …

Yes, and I think this is a good transition, because part of the reason why other animals are unable to conceptualize the past, present and future, or have a concept of the passage of time, is because it's really complicated, and kids show this.

There are these very old experiments that have been confirmed where, if you show five year olds two little trains, or slugs, starting at the same time, and one of them travels farther than the other, but they both end at the same time, children will think that the one that traveled farther, and which was just going at a faster speed, was traveling for more time, because they don't have this clear distinction, or concept, of the amount of time. But if you think about it, you feel sorry for children because it's so confusing.

I wonder whether our using clocks in the external world is just a way of getting a handle on time, because clock time is arbitrary. We round the number of hours in a day to 24, but it's not strictly 24 hours, is it? So is this another case of humans trying to control nature in a way that perhaps we can't?

That's a good perspective in the sense that you said we use clocks to get a handle on the passage of time. It's not natural for children to pick up the idea of a clock. It's a thing we learn culturally.

And our ancestors a hundred thousand years ago, even if they had a clock, they probably had no use for it most of the time. All animals look at the sunrise and sunset, so in a sense that's a clock. But, yes, I think for humans, the concept of clocks emerged fairly late in our evolution.

So do you think we might eventually lose a sense of time as our lives get more digital or even space bound? I'm thinking about the idea that we can no longer store telephone numbers in our heads, but also with space travel, some of our concepts of space time are completely different to what we have on Earth.

No, I don't think so. You use it [time] in different ways. And we can learn to do timing better if what you do in your life, or in your interests, relies on it. But, on another hand, what you're saying is true in the sense that we rely on external time. I don't go to sleep when I'm tired - I go to sleep when the clock says it's midnight. We're overriding our internal clocks, but they might be more important than external ones. So in a sense I think we're losing touch with some of our internal clocks. There is some truth to that.

Dean Buonomano is a professor of neurobiology and psychology at UCLA and a leading theorist on the neuroscience of time. His latest book, Your Brain is a Time Machine: The Neuroscience and Physics of Time, is published by W.W. Norton (2017).