OK fine — but wait, stay with me on this. Just go outside tonight, and look up. The brightest star you can see, in the constellation of Canis Major: that’s Sirius, the Dog Star. Sirius is about nine light-years away. (Though it sounds like a length of time, one light-year is the distance light travels in one year, roughly ten million million kilometers.)
Light took nine years to travel from that star to our eyes. So really, you’re looking at the star the way it was nine years ago.
Time travel, you see?
And, your response would be: That’s still cheating.
Now go away and leave me alone.
It’s quite an amazing thought, though. Wherever you gaze, you’re not just looking out into space, you’re also looking back in time. The greater the distance to a star, the longer light has taken to travel across the intervening cosmos to reach your eyes, and so the longer ago it must have begun the journey. Some of the farthest objects ever discovered are billions of light-years away—which means we are seeing them now as they looked billions of years ago! In fact, even now we can ‘see’ the faint glow of the entire universe as it looked shortly after the Big Bang.
There is another way to travel in time, but it takes a little more effort than gaping up at the stars — so I’ll need your help. Here’s what we’ll do: find a couple of stopwatches and very fast rocket ship. Start the stopwatches together, keep one for yourself, while I take the other and climb aboard the rocket. I punch in the coordinates for a round-trip to Sirius, set the cruise-control close to the speed of light (say, nine-tenths), and wave farewell as I zoom off into the void.
At this speed, roughly 270,000 kilometres a second, I hurtle across space, reaching the star in about ten years, where I then turn around and take another ten years to return. By the time I get back, you’re twenty years older, and probably getting just a bit tired of waiting.
When I step down from my trusty spacecraft, something strikes you as odd: I don’t seem to have aged nearly as much as you have. When we compare our stopwatches, yours says it took twenty years for me to complete the trip. My watch, astoundingly, measures a time of just over eight years.
How is this possible? You think maybe the watches are defective — except I certainly haven’t aged as much as you have. Maybe I’m just blessed with a young complexion.
The answer to this conundrum lies in a physical theory known as Special Relativity. Albert Einstein’s famous work, first announced to the world in 1905, leads to some very strange conclusions about our universe. It turns our everyday notions of time and space on their heads. Before Einstein, people could change where they were, but could do nothing about when they were. Most people thought of time as a river, carrying us all inexorably to the future. Time was immutable.
So was distance. If you and I each measure the length of an object, surely we expect to get the same result. Special Relativity says this is true as long as we are standing still next to each other — but as soon as one of us is moving with respect to the other, we will measure different lengths. The faster our relative speed, the less we will agree on the distance between two points.
So when I launch off at high speed, you and I measure different distances for the journey to Sirius. To you standing on the Earth, the distance is nine light-years. But for me, travelling at close to speed of light, Sirius is just a bit over three and a half light-years away. In relativity-speak, this is known as ‘length-contraction’.
If you could watch my clock as I speed away, you would notice it running slower than yours. In fact, to you, my heart rate would seem slower too, as would my breathing, my hair growth, everything! Though it all feels normal to me, to you my world would seem to be running slow. Relativity says that time, too, changes with speed—the faster I go, the slower my time seems to you, an effect called ‘time-dilation’.
While it may sound like science fiction, these effects of relativity are real, and are routinely observed in modern physics laboratories. We don’t notice them in our daily lives, because the effects are only apparent at speeds close to the speed of light.
Still unimpressed? Wow, you’re hard to please.
OK, what about the possibility of travelling back in time? What if you could change history, go back and find your parents, somehow prevent yourself from ever being born? What would Einstein say about such heresy?
That, as they say in the classics, is another story for another time ...
[NOTE: We never did get to that other story. I graduated and the column came to an end. Oh well.]