What is space? In reference to
Cumming’s quip that “time is what keeps everything from happening at once,”
physicist John Wheeler joked, “Space is what prevents everything from happening
to me!”
It’s true. Pure space would be the
simple separation between things (or beings) and nothing more – no fabric, no
structure, and no curvature – just distance. But, since quantum fields fill the
universe, no pure space exists in reality.
And, there are no absolute
positions in space. Although Han Solo could accelerate forward or backward,
right or left, and up or down, the Millennium Falcon of Star Wars fame had no
way to brake to an absolute stop except with respect to something he selected,
like a landing pad! But every landing pad would have its own motion. There are
no absolute stationary reference frames in the universe. Just as space as no
absolute standstill, time has no
absolute now in relativity.
Not only is energy is a feature
of spacetime, if only vacuum energy, energy fields define spacetime. Matter and
energy restructure the causal topography in ways that change how they behave.
This is the causal geography of space and time.
If 3D space is graphed as
coordinates x, y, and z, time must be
graphed as vector t perpendicular to
them all. Math can handle it but not even a holograph can depict it, nor can the
human mind picture it. And, GR adds the requirement that every unique inertial reference
frame be graphed with different values for both space and time.
When the structure of space is
mentioned, gravity springs to mind. However, gravity is just one of many fields
that permeate the universe. Every elementary particle of matter has a
corresponding field that is omnipresent. The Higgs field fills the universe so
that every particle of matter anywhere can interact with it and experience
mass. The electromagnetic field is everywhere so that a photon produced by an
interaction anywhere can travel forever until it interacts with another field.
An electron field fills the universe, allowing electrons to pop out of various
kinds of field events.
Causal relationships and events that
take place in quantum fields constitute the history of space, the time element.
Observers in different reference frames of the causality web can see history
unfolding differently, for example, an event in one observer’s history may yet
be in another observer’s future as per the math of General Relativity.
Time and space are like supply and demand in economics. One is meaningless without the other. They dance
together in the causality web that is sometimes misnamed the “fabric of
spacetime.” But there is plenty of experimental evidence that empty space is
not an ethereal fabric.
So, what is the definition of
spacetime? It is a system of causal relationships amongst quantum fields and
their energetic interactions where space is the distance between the system’s
things and time is the rate of causality in the system’s interactions.