In the block universe — which is supported by Einstein’s theory of relativity — there is no separation between the past, present and future. All exist at once in a four-dimensional continuum called space-time.
The block universe theory has been validated in experiment after experiment as well as by the observations of astrophysicists, yet there are physicists — a small minority — who reject the overwhelming evidence and persist in searching for a theory of the universe more in keeping with the way they think the universe should behave rather than the way the universe actually behaves.
As highly esteemed Professor Sean Carrol — whose fields of expertise include physics, cosmology, astrophysics and general relativity — writes:
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.
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.