Is time travel possible? Is time just an illusion that our brains merely believe time to be moving forward in a linear fashion? According to proponents of the block universe theory, the answer to both of these questions is, simply, yes.
The block universe theory describes 'now' as an arbitrary place in time, and states that the past, future, and present all exist simultaneously.
Much in the same way that your current location doesn't exclude the existence of other locations, the block universe theory claims that being in the present doesn't mean the past and future aren't currently taking place.
We take a look at different versions of the theory and how this static perception of spacetime means that time travel, in theory, is possible.
Time and space, and space and time
The block universe theory, as explained last year by Dr. Kristie Miller, posits that our universe might be a giant four-dimensional block of spacetime, containing all the things that ever happened and will happen in our traditional perception of time.
Dr. Miller, who is the joint director for the Centre for Time at the University of Sydney, explained the theory in a piece published by ABC Science. Miller described how all moments that exist are relative to each other within three spatial dimensions and a single time dimension.
The block universe theory is also known in some scientific circles as Eternalism, because it describes how the past, present, and future all co-exist 'now'. This is opposed to Presentism, which states that the past doesn't exist anymore and is constantly disappearing, thanks to that pesky notion of 'present' time.
Could time travel be possible?
According to Dr Miller, hypothetically speaking, yes, it is possible. But there is one big caveat. We would have to figure out how to travel at a speed close to the speed of light, allowing us to use wormholes as a shortcut to travel into another "location" in spacetime. This would be possible due to a phenomenon known as time dilation.
However, if we were to be able to create the technology to allow us to travel in time, we would not be able to affect our present by changing the past, Miller says. That's because the present exists at the same time as the past and is, therefore, inextricably linked to the past. No need to worry then that killing an insect in the past would lead to a snowballing chain of events that would set off another world war.
"If I travel to the past, I am part of the past. Importantly, I was always part of the past," Miller says. In other words, going to the past would mean that we are simply fulfilling pre-ordained actions that are already written out in the block that is spacetime.
The block universe does, of course, have its detractors, as Big Think points out. Physicist Lee Smolin, for example, wrote that "The future is not now real and there can be no definite facts of the matter about the future." He also added, at a 2017 conference, that what is real is just "the process by which future events are generated out of present events."
The idea, if true, would also lend weight to the philosophical idea of Predeterminism, which states that everything is preordained and therefore an individual has no agency over the outcome of their life and may as well just let it run its course. Not a very 21st Century idea.
A counter to the notion of Predeterminism is another theory, growing block-ism — ridiculous name, I know— which posits that the block of spacetime is actually a growing entity that can be changed. In this theory, the past and the present always exist, but the future is more of a changing entity.
So, could a predetermined life be closely linked to our ability to be able to time travel? The truth is that we are nowhere near knowing this for certain. For the moment, the block universe theory is just that, a theory. We'd need a time machine - a very tall order - to test the hypothesis.
Knowing whether all of history is happening at the same time is something that may never happen. On the other hand, it might be happening right now.