Spacetime—the future is out there

Now that I have brought up the thorny subject of whether the
future is already out there, we might as well confront it head-on
and see what special relativity has taught us.
Two years after Einstein published his paper on special
relativity one of his old university lecturers, Hermann Minkowski,
suggested that all this business of time slowing down and
distances being squashed was just a matter of different
perspectives of different moving observers. But it is not the sort
of perspective we are used to in 3D space, rather it is a perspective
in four dimensions. Minkowski showed that time and space can
no longer be treated as separate entities but are unified into what
is known as spacetime. Many people, even scientists, have been
confused by the need for such a picture, and it is important to
appreciate why Minkowski came to this conclusion. If you look at a solid object, such as a cube, you see that its
dimension of depth (being the one in the direction of your line of
sight) appears shorter than the other two dimensions of breadth
and height, making the sides of the cube look squashed. Now
consider someone else looking at the same cube from the side. To
her, the dimension which you consider to be the cube’s width is
now its depth, and she sees the side facing you as squashed. The
two of you will not argue about who is viewing the cube from the
correct angle because you both understand that it is only a matter
of different perspectives. Special relativity teaches us that fast
moving observers must view the world within 4D spacetime, in
which both spatial and temporal distances become just a matter
of perspective. An observer moving at high speed relative to another
will see spacetime from a different angle. According to one
observer the dimension of time may look shorter or longer than
it does to the other, but neither observer has a right to claim that
their perspective of spacetime is more correct than any other.
Think of two separate events, saymywriting this sentence and
your reading it. The pre-Einstein (Newtonian) view would be that
these two events are separated in space and time independently.
Both the spatial distance between the place that I wrote it and the
place where you read it (let’s say 1000 kilometres) and the temporal
distance between the times of writing and reading (say two years)
are the same for all observers. Special relativity has shown how
both these quantities will vary, depending on the observer. What
is neat about 4D spacetime is that we can define within it a single
‘distance’ between the two events which is a combination of a
space part and a time part. Such a spacetime ‘interval’ has a fixed
value for all observers. So we only get back to absolute objective
distances when space and time are combined.
Minkowski’s 4D spacetime is often referred to as the block
universe model. Once time is treated like a fourth dimension of
space we can imagine the whole of space and time modelled as a
four-dimensional block.At any given time, two-dimensional space will be a slice through
the block. The Universe at earlier times is represented by the
region to the left of this slice and future times to the right. Here
we have a view of the totality of existence in which the whole of
time—past, present and future—is laid out frozen before us. Many
physicists, including Einstein later in his life, pushed this model
to its logical conclusion: in 4D spacetime, nothing ever moves.
All events which have ever happened or ever will happen exist
together in the block universe and there is no distinction between
past and future. This implies that nothing unexpected can ever happen. Not only is the future preordained but it is already out
there and is as unalterably fixed as the past.
Is this picture really necessary? After all, we can just as easily
imagine a Newtonian spacetime modelled as a 4D block. The
difference is that in that case space and time are independent of
each other, whereas in relativity the two are linked. One of the
consequences of relativity is that no two observers will be able
to agree on when ‘now’ is. By abandoning absolute time we
must also admit that the notion of a universal present moment
does not exist. For one observer, all events in the Universe that
appear to be simultaneous can be linked together to form a certain
cross sectional slice through spacetime which that observer calls
‘now’. But another observer, moving relative to the first, will have
a different slice that will cross the first. Some events that lie on
the first observer’s ‘now’ slice will be in the second observer’s
past while others will be in his future. This mind-boggling
result is known as the relativity of simultaneity, and is the reason
why many physicists have argued that since there is no absolute
division between past and future then there can be no passage of
time, since we cannot agree on where the present should be.
Worse than that, if one observer sees an eventAoccur before an
event B, then it is possible for another observer to witness B before
A4. If two observers cannot even agree on the order that things
happen, how can we ever define an objective passage of time as a
sequence of events?
Not all physicists are prepared to take such a view. Even
Einstein was forced to admit that although space and time are
fused into one continuum we must nevertheless not fall into the
trap of treating time like an extra dimension of space. After all, we
knowfromthe last chapter that the time axis has a certain direction;
an arrow of time. None of the three space axes are like this. It is
just as easy to go in either direction in space, so time and space
remain distinct in nature. So when Minkowski first presented his ideas, even Einstein was sceptical. He came round slowly to the
idea though, and it proved vital for his subsequent development
of general relativity, in which it is 4D spacetime that is affected by
gravity. In fact, spacetime can be curved, stretched, squeezed and
twisted. We shall even see later on that general relativity allows
spacetime to form some very strange shapes known to the experts
as ‘non-trivial topologies’.
Where does all this leave us? Is relativity asking too much?
We know that we must give up the idea of a universal present
moment, but arewe forced to concede that the future already exists
as well? I will put forward three reasons as to why I do not believe
this to be the case.
Firstly, the disagreement that any two observers will have
over the ordering of events will only involve those events that
are very close together in time. Imagine two flashes of light that
according to me are separated by thirty centimetres. It would take
light one billionth of a second to cover this distance and so, in
order for the light of one flash to have triggered the other one, I
would have to see them separated in time by over a billionth of
a second. With such a time gap, it would have been impossible
for any other observers to have seen the light flashes happen the
other way round, however fast they might be moving relative to
me. This would violate a sacred law of nature that states that the
cause of something must always happen before its effect. It stands
to reason that we cannot have things happening before whatever
caused them to happen in the first place. In this example it does
not matter whether or not the first flash of light caused the second,
simply that there was enough time for this to be possible.
Therefore the reordering of events for different observers is
only allowed—if it is not to violate what is known as ‘causality’
(or causes happening before their effects)—if the two events
are so close in time that no signal, not even light, could have
passed between them5. The mixing of the order of events is thus
something that ruins the objective passage of time only on a very small scale, which makes the present moment a little ‘fuzzy’, that
is all.
The second reason is that, whatever your relative state of
motion, there is still a definite ‘now’ and hence a perfectly sensible
split of events, for you, into past and future.
Thirdly, and as for the future being ‘already out there’, it is
clear that until it has ‘happened’ for us and we know the whole
of spacetime we cannot cut slices through it anyway. To us, the
future has not happened yet. It does not matter that we could,
given enough information about the present state of the Universe
(such as the positions and states of motion of all the particles in the
Universe), calculate what will happen at all future times. This is
no more than Newton’s deterministic (clockwork) universe. The
difference now is that distances, durations and the ordering of
certain events will depend on the observer.
To view the whole of spacetime (the Universe at all times) as
one 4D block requires a vantage point that is outside the Universe.
This is the same as asking what the Universe looks like from the
outside. There is no outside. So such a view is hypothetical.
These arguments have not stopped many physicists,
mathematicians and philosophers from embracing the block
universe idea, with its static time, wholeheartedly. The mathematician
Hermann Weyl describes the block universe thus:
“The objective world simply is, it does not happen. Only our
consciousness . . . is as a process that is going forward in time”. In
fact, such a view was held long before relativity and is very close
to the arguments put forth by the German philosopher Immanuel
Kant in his Critique of Pure Reason of 1787.
I have always felt there to be aninconsistency in this argument.
Weyl would have us believe that despite nothing ever changing in
4D spacetime, our consciousness still somehow moves through it,
which is how we have the feeling of an ever-changing present
moment. He claims that this feeling is illusory. But movement,
however illusory, implies change, and change requires the passage
of time. So if our consciousness experiences change then it must
exist outside static spacetime. However mysterious consciousness
is, I am not willing to attribute to it such status.