Your birth is out there in space-time. Your death, too, is in space-time. Every moment of your life is out there, somewhere, in space-time.
So says the block universe model of our world.
According to the block universe theory, the universe is a giant block of all the things that ever happen at any time and at any place. On this view, the past, present and future all exist — and are equally real.
How can this be?
The block has four dimensions: three spatial dimensions — say length, height and width — plus a fourth temporal dimension, or time. Or let's make it easier, by visualising the block model of our world as a three-dimensional rectangle, or cuboid.
Two of that cuboid's dimensions (let's say height and width) represent two of the universe's three spatial dimensions.
The third spatial dimension in the above diagram is left out — the length of the cuboid — and replace it with time. At one end of the cuboid is the big bang. At the other is the very last moment of the universe. Maybe it's a big crunch.
The cuboid is filled with every event that ever happens. Where these events are in the cuboid represents their location in space-time. All events, including your birth and death, and this very moment as you read these words, exist somewhere in the block.
In the block universe, time doesn't pass
It often seems as though where we are "today" is present, and "yesterday" is past, and "tomorrow" is future.
It also seems the present moment changes too — after all, tomorrow it will seem as though tomorrow is present, and yesterday it appeared yesterday was present!
So from our perspective, it appears that time flows or passes. But in the block universe model, time doesn't flow.
In other words, in a block universe, there is no specific present moment, and "past" and "future" moments are relative.
Think about the idea of "here". I am here. You, while reading this, can truly say "I am here", even though your "here" is different to mine.
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On the block universe model, talk about the "present" or "now" works just like talk of "here".
Remember last week when you said to your friend, who was late arriving for coffee, "now you're here"; or when, long ago, Caesar said, "I am now crossing the Rubicon"?
These claims are both true. That's because all it means to talk about the present, or now, is to talk about the place in time where you happen to be.
Since we are always located wherever we are (that's trivially true), everyone is located in the present, just as everyone is located at the place they call "here".
According to the block universe view, time or temporal relations of "earlier than" and "later than" exist. These relations hold regardless of where anyone is located.
So, suppose Bert the dinosaur is located earlier than Sally the dog. That relation between Bert and Sally holds, regardless of whether we are located earlier than Bert or later than Sally.
Bearing this in mind, it is possible to see how to make sense of the idea of past and future. Just as on this model "now" picks out whatever time I happen to be located at, "past" picks out any time (or events at those times) that are earlier than my location, and "future" picks out any times or events that are later than my location.
Does that mean we can travel in time?
If time is just another dimension, a lot like the spatial dimensions, does that mean we can travel in time?
The short answer is yes.
Of course, things are way more complicated than that. Travelling in time is clearly much more difficult than travelling in space. It might be very technologically costly to time travel, so perhaps it's not really something that, practically speaking, we can do.
But it's certainly possible.
We already know that travelling very fast will result in time dilation, so we know it's possible to travel into the future just by travelling very fast.
We can travel quite a way into the future if we can travel at some reasonable percentage of the speed of light. We also know how to travel into the past. We can do that by using wormholes, which are short cuts through space-time.
Wormholes — shortcuts through space-time — are, in theory, possible in a block universe.
So, if I can travel in time, can I change the past?
No. That would create a contradiction, and there are no contradictions.
Remember, on the block universe model, the past is no different than the future or the present.
Everything is relative: what is past to you, will be future to someone else.
So if I travel back to the past I'm travelling to what is someone else's future. That means the past won't be any different, in kind, to the present.
What will happen if I travel to the past? I'll get out of my time machine and start walking around. I'll breathe the air and chat to people.
Obviously, this will have effects on the time I travel to. I'll tread on ants; I'll talk to people from that time; I'll pat horses, and feed donkeys and so on.
I'll act, in the past, in the sorts of ways I act in the present. But I won't be changing the past. Just as when I eat cornflakes instead of toast tomorrow I am not changing the future, I'm just making the future the way it is, when I travel to the past I don't change it, I just make it the way it is, and always has been.
Albert Einstein's special theory of relativity states time passes at different rates for people moving relative to each other.
What I do tomorrow makes tomorrow the way it is, and the way it always has been. What I do in the past makes the past time the way it is, and always has been.
If I travel to the past, I am part of the past. Importantly, I was always part of the past.
The events in the block are there for all time: they do not change. So, as a time traveller, it's not as though I suddenly appear at a past time. It's always been the case that I am located at that past time.
Nothing a time traveller does changes anything in the block. Instead, what the traveller does at any time makes that time, and later times, the way they are.
That means that we know that some things we attempt to do in the past, fail. We know that Hitler rose to power in the 1930s, so we know that if our time travelling future selves try to prevent this from happening, they fail.
But that doesn't show that our time travelling selves don't succeed in doing lots of things in the past. For all we know, the reason the past is the way it is, is in part due to the presence of time travellers.
Associate Professor Kristie Miller is the joint director for the Centre for Time at the University of Sydney. Watch her explain the block universe theory on Catalyst on iview.
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