As I write this Last Word for this October 2024 issue, it is tempting to ask where the summer went. Perhaps time flew while I was enjoying myself. It certainly ‘went,’ but where - or when - did it go to? I thought it was about time to find out.
Since the dawn of human thought, time has been viewed as absolute and independent of other natural phenomena. It is just the way that it makes sense to our brains. Sir Isaac Newton formalised this theory of Absolute Time in his landmark 1687 work Principia Mathematica, stating that time ‘flows without regard to anything external.’ We can oversimplify this as ‘time moves at one second per second (1s/s).’
The accepted view for more than 200 years, Absolute Time was superseded in the early 20th Century by Albert Einstein’s Theory of Relativity. He stated - and I’m paraphrasing here - that time passes at different rates relative to your viewpoint. Einstein didn’t mean the way that time seems to speed up as you approach a magazine deadline or how it seems to go in slow motion during an accident - both of these are due to faulty human perception. Instead, he said that time itself can warp. Specifically, as an observer travels faster, time becomes slower. For them, a clock would continue to tick at 1s/s. A resting clock would also appear to tick at 1s/s to a different observer. Both clocks look correct until we bring them together, when we will see a difference. This is due to time dilation.
Untestable during Einstein’s own lifetime, we can now carry out real-world experiments to demonstrate time dilation using synchronised atomic clocks that are accurate to the nano second (ns) (1/1,000,000,000s). The degree of time dilation depends on how long the moving clock travelled for and at what speed. After a commercial flight, the moving clock will be 20-100ns seconds behind the resting clock. This means that, if you travelled on a plane constantly for 100 years, you would be about 1/10,000 of a second younger than if you had stayed on the ground. A prolonged space flight - like the one currently being unexpectedly carried out by the Boeing Starliner crew - would make you 1-10 microseconds younger. This is still many orders of magnitude below a second.
So far we’ve just stretched time a little bit. However, closer to the speed of light, time becomes like a gooey caramel bar that’s been stretched far beyond its original length. An astronaut travelling at 95% of the speed of light for 10 years - from their perspective - would come back to Earth to find that 32 years had passed. That’s a whopping 22 years younger!
Einstein explains this by describing time as the 4th dimension, through which our three dimensional world passes. This is space-time, which are bound together in a single continuum. It’s too much to go into detail here, but - if we delve a bit deeper into close to light-speed travel - we find that different observers can actually disagree on when and where different events took place. This is because they have different ‘nows’ relative to each other. One may not even have seen the event yet.
This last point opens up some profound questions. If an observer hasn’t seen the event but the other knows that it happened, can the observer in the ‘past’ stop it? If not, does this imply that all events that take place in the Universe are actually predetermined? Einstein’s theory does imply this - that all parts of time, past, present and future, are equally real. The dinosaurs, the opening night of Romeo and Juliet, you eating breakfast on New Year’s Day 2013, the lives of our distant descendents. Everything is equally real and happening at different ‘nows.’ We can think of this like a film. Frames from earlier on still exist on the reel and the future ones are yet to come to the projector.
This predetermined view of the Universe doesn’t square up with what we perceive, not least because humans like to think that we are masters of our own destiny. Thankfully, quantum mechanics - relativity’s even more hard to understand distant cousin - appears to contradict this. This is because quantum events are inherently unpredictable and almost certainly alter future events at random. So, while the past - including the summer of 2024 - really is real, the future consists of an infinite number of potential frames that could be beamed onto the projector. We are once again masters of our own destiny. Now you’ll have to excuse me - I have to press print... or this issue won’t happen!
