The arrow of time

By Marcus Wilson 02/03/2015

Benjamin is now two-and-two-thirds, or near enough. As ever, his grasp of physics continues to improve.  In the last few weeks, he has been picking up the idea of time. 

We have a large (more accurately, LARGE) analogue clock on the wall of our lounge. He's watched me take it off the wall, change the battery and move the hands to a new position when it started to run slow. It's clear to Benjamin that what the hands do on the clock is related to the time of day, although just how I think is some way off.  Over the weekend he wanted me to read him a book, but didn't want me to get it for him. He shoved me away, and rather matter-of-factly said "Daddy stay there. Poppet will get a story." Then he turned to get the book, but quickly came back to me and added "I'll be back at half-past-four. See you soon." And off he went to get his favourite book (which, as ever, is about excavators and other large machines). 

That was just one of those amusing moments you get with a young child. But another 'time' incident was rather more interesting. Karen was out, and Benjamin was somewhat upset over her absence. I was trying to reassure him that she would be back. "She'll come home at about half-past-eight". Benjamin looked at the clock longingly and said "Daddy change the clock so it's time for Mummy to come home". 

Now there's a thought! Wouldn't it be great if we could just manipulate time by turning the hands of the clock? So, somehow, if we changed what the clock says, then the time of day would actually change. Does Benjamin think that this is how it actually is? Maybe. It would be useful if it were true – extended weekends, short work days; one could cut hours of a plane trip to Europe by taking the clock with you. 

While it might work for science fiction writers, unfortunately that's not how time works for us. While the clock and time are intimately linked, it is time that controls the clock, not the other way around. We are stuck with progressing through time at the rate of one second every second. 

That makes time a rather strange concept from a physics perspective. Unlike space, where we are free, more or less, to move to any point in it, we don't have that option with time. We can only move forward in it, and only move forward at the same rate – one second every second. The past is forever behind us; and the future is always unknown. Physicists call this the 'Arrow of Time'. It points one way: forward. 

Special Relativity makes it more interesting still. The way time works for you may not be the same as it works for me. If I were to get on a Really Fast Spaceship and travel close to the speed of light for a while, then return to earth, I would be noticeably younger than my identical twin brother. (I actually DO have an identical twin, by the way.)  Not only would the clock in my spaceship be telling me less time had past, I would have actually aged less. From my point of view, I might have been gone for two weeks; from yours, I might have been gone ten years. But even so, each of us will still have perceived time as travelling at one second every second. Forward. 

What about real time-travel – going back in time. Just maybe physics permits this to happen. That's in the realms of General Relativity and Quantum Gravity and involves some really big masses indeed. Matt Visser's work at Wellington might give us some pointers here. But the summary of it is: Don't expect a such a time-machine to be built in Benjamin's lifetime, even if he does prolong it indefinitely take the battery out of our lounge clock.






0 Responses to “The arrow of time”

  • Time is a subject that interests me a bit, though mostly it just frustrates me, because I can see that all current theories are inadequate, but it seems to be impossible to get anywhere in formulating a better theory. I am sure that Einstein was wrong: time ISN’T the fourth dimension. It has some attributes in common with “dimensions” like spatial dimensions, but yet it is fundamentally different. Time has two aspects: (1) an ordering of events; and (2) the distinction between past, present and future. Aspect (1) is relatively easy to understand. Aspect (2) is a complete mystery. Most current theories simply ignore aspect (2), or explain it away as some kind of perspective effect arising from aspect (1). This is unsatisfactory. Aspect (2) is CRUCIAL to understanding time. Without it, we are left with a “block universe”, where all of history is somehow present at once, from an objective perspective. Our perception of past, present and future is then explained as relative to the position in the time dimension of each of our “selves-at-a-time”. Objectively, there is nothing “dynamic” going on here. Past, present and future are fully formed and static from an objective point of view. “Now” becomes analogous to “here”, it is simply where I am located in time when I say the word “now”. But I don’t believe any of this! Rather, I suspect that the nature of the distinction between past, present and future is fundamental, but sadly, probably something that we will never understand..

    • I wouldn’t go as far as to say that Einstein was wrong about time being the fourth dimension. Not yet. Special Relativity works beautifully, with time acting as a fourth dimension, but it does get treated slightly differently, with a minus signs popping up where one might expect positive signs. But, our experience of time does seem odd – we are stuck moving in a single direction at a single rate. Why? If you work it out, do let me know…

  • Are we moving through time, or does time pass (move)? We tend to say both. If it is time that moves, then that kind of explains why we only go one way (like a river to the sea). We really don’t know anything about the metaphysics of time.