The arrow of time

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.

 

 

 

 

 

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