The true nature of time continues to elude us. But whether it is a fundamental part of the cosmos or an illusion made in our minds has profound implications for our understanding of the universe
WE ARE BORN; we live; at some point, we die. The
notion that our existence is limited by time is fundamental to human
experience. We can’t fight it – and truth be told, we don’t know what we are
fighting against. Time is a universal whose nature we all – and physicists
especially – fail to grasp. But why is time so problematic? “If we had a really
good answer to that question,” says Astrid Eichhorn, a theoretical physicist at
the University of Southern Denmark in Odense, “then it wouldn’t be so
problematic.”
On a certain level, time is simple: it is what stops
everything happening at once. That might seem flippant, but it is at least
something people can agree on. “The causal order of things is really what time
is all about,” says Eichhorn.
Viewed this way, the existence of time can be
interpreted as a necessary precondition for the sort of universe where things
lead to other things, among them intelligent life that can ask questions, such
as “what is time?”. Beyond that, time’s essence is mysterious. For instance,
why can things only influence other things in one direction in time, but in
multiple directions in the three dimensions of space.
Most physical theories, from Isaac Newton’s laws of
motion to quantum mechanics, skirt such questions. In these theories, time is
an “independent variable” against which other things change, but which can’t be
changed by anything else. In that sense, time exists outside physics, like the
beat of a metronome outside the universe to which everything inside it plays
out.
Albert Einstein’s theories of relativity, developed in
the early 20th century, threw such ethereal notions over a barrel. In
relativity, time is a physical, dynamic thing, fused with space to form
space-time – the fabric of the universe itself. And space-time isn’t absolute,
but relative, warped by motion and gravity. If you travel fast, or if you are
in a strong gravitational field, it slows down.
The relativity of time has wide-ranging consequences.
Because there is no unique way of defining its passage, there is no unique way
of defining “now”. Einstein concluded that all “nows” – past, present and
future – must exist simultaneously, a picture known as the block universe that
is completely at odds with our intuitions.
That mismatch occurs because, in our universe, the
speed of light is finite. We can only reach certain times within a certain,
well, time, so we can never achieve that God-like block-universe view. “In
practice, causality limits what we can perceive in a very strict way, and our
experience and anything that affects us is limited strongly by causality,” says
cosmologist Katie Mack at North Carolina State University.
Time and space
The mysteries don’t stop there. By making time part of
the physical fabric of a universe that, as far as we can tell, began in a big
bang some 13.8 billion years ago, Einstein’s theory of general relativity
implies that time itself had a beginning – and perhaps an end, too. There can
be no eternal metronome ticking outside the universe as quantum theory implies,
because such a source would have to exist outside space and time itself. This
sets up a currently unbridgeable divide between relativity and quantum theory.
In attempting to cross it, researchers such as Eichhorn hope to make progress
towards a more unified picture of physics – one that would have to have a very
different conception of time.
Many “quantum gravity” theories propose that if you
could zoom in very close to the fabric of Einstein’s space-time, to a
fine-grained level known as the Planck scale, you would discover a substructure
– a kind of quantum pixelation. That would open up entirely new possibilities.
“It may very well be that the quantum structure of space and time is different in
the presence of matter than it is if you’re just thinking of sort of a universe
which contains just space and time,” says Eichhorn.
Not everyone thinks we need to go that far. Some see
an avenue to finding the nature of time in a better understanding of quantum
theory. Or perhaps time is itself a mirage. Like the colour or pattern of a
tree leaf, time might be something of no significance, says Mack, the passage
of which we invent to make sense of local patterns around us and our own lives.
After all, we never measure time itself, but rather
regular changes – be it the passage of the seasons, the swing of a pendulum or
the oscillation of a caesium atom – that we reverse-engineer into some
mysterious thing we call “time”. “It’s something that we see, and that appears
to be there,” says Mack. “It may not matter to the cosmos.”
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