Recently, there was a little media buzz about one of my papers. With Brendon Brewer at the University of Auckland, I identified the time dilation of distant quasars, the slowing of the brightening and fading of these cosmologically distant objects due to the expansion of space. There's a bit more detail about the work in an article I wrote for The Conversation.
Talking with the media about time was quite difficult, as time itself is not simple. I often had to point people to this great philosophical discussion by David Tennant's Doctor Who.
But one question I got a few times was along the times of "Did time really run slower in the early universe, or is this some sort of optical illusion?" The answer, is, well complicated.
In the old days, time was simple, an absolute that everyone could agree on. That changed in 1905 with Einstein's Special Theory of Relativity, where he showed that time was, well, relative. And yes, it's the theory that is special, not the relativity.
This relative nature of time has confused a lot of people and has led to people claiming that relativity is riddled with logical paradoxes that render it rubbish as a physical theory and that Einstein was a first-rate idiot.
A simple statement from special relativity is that moving clocks tick at a slower rate when compare to stationary clocks. How we observe the moving clocks is a little complex, but one of the consequences of all of this is that we see clocks Doppler shifted, redshifted or blueshifted, dependent upon the relative motion. But time is more complicated than this.
If a clock zooms past you at high speed it will be ticking at a slower rate than yours. But if you accelerate and catch up with the clock, so you are travelling with it, you will find it ticking normally. So perhaps this is all an illusion. But there's more.
If you decide to decelerate and head back to your starting point, you will find something strange. If you left a clock at your starting point, you will find that it has registered more time passing than a clock you took with you. This is of course the famous twin paradox.
In 1915, Einstein published his general theory of relativity bringing gravity into the picture. And in this picture, the rate of a clock tick depends on its location in a gravitational field. If you fire light rays between observers in gravitational fields they too see redshifting and blueshifting. Again, this might sound like an illusion, but if we play the same game of moving clocks around in gravitational fields, we find that clocks lose their synchronicity, demonstrating that there is no absolute time and this is not an illusion. And again, any clock you stop and examine will just appear to tick as normal. In fact, it's been suggested that flying down and spending time near a black hole can be used to time travel into the future, an idea of course popularised in a Christopher Nolan film that didn't include nuclear weapons. Here's a picture from Hartle's textbook Gravity on this but using a massive shell of matter.
There was a nice experiment that tested this where atomic clocks were flown around the world (probably in a better class than us academics get to fly in) that showed just this difference. And the global positioning system needs to correct for all of these relativistic effects to ensure you don't get lost on your journeys.
What about the cosmological case? Is that just an illusion? Well, the photons we receive from the distant universe are redshifted, just like the examples we have seen before, and distant clocks are seen to run slowly. And if we could be there in the early universe next to a clock, we would see it tick normally. So it is as real as all of the other time dilation examples we have seen. In fact, I wrote about the fact that these ideas are all really the same kind of thing here.
But there is one thing we cannot do. We can't send a clock from today into the past to spend time with an ancient clock, and then get it to return to see how much time has passed and explore whether it is no longer synchronised with a clock that just ticked in the present. But if we could, I am sure it would, because time, well, is a complicated thing.