6 March 2021 | New Scientist | 47elementary particles, an oddity that prevented
the complete mutual annihilation of matter
and antimatter in the early universe and
ensured that we, being made of matter,
are here today. But there is no way it can
explain the onward flow of time.
To explain this, physicists have instead
turned to a law that isn’t considered
fundamental, but which emerges from more
basic laws. The second law of thermodynamics
says that in a closed system, overall disorder,
characterised by a statistically defined
quantity called entropy, always increases.
It does so because there are many more possible
states of disorder than of order. Thus, a small ice
cube in the corner of a large box will melt and
become liquid water, spreading the molecules
out and increasing disorder. Entropy has
increased. Note the “statistically defined” bit:
the laws of physics don’t rule out this process
being reversed, but just say that event is
statistically hugely unlikely.
In this picture, the direction of time is
created by the increase in disorder. If
snapshots showing the position of the
molecules in that box were shuffled out of
order, my 4-year-old granddaughter could put
them back in order. For many scientists, this
is enough: entropy puts direction into time.
For my part, I don’t doubt the robustness
of thermodynamics. Einstein described it
as “the only physical theory of universal
content which I am convinced that, within
the framework of applicability, its basic
concepts will never be overthrown”.
I wouldn’t be so bold as to disagree.
But Einstein’s caveat is important, and
leads to a question: does the “framework
of applicability” of thermodynamics
include our universe? It doesn’t appear
to be a closed system. It might be infinite
in size and is certainly expanding, possibly
without impediment. If so, it isn’t in a box.
But a box, physical or conceptual, is crucial
for the interpretation of entropy.
That alone is reason to question the
application of thermodynamics more or
less unchanged to cosmology. But there is
another reason. With the entropic arrow of
time, physicists assume that the universe
began at the big bang with very low entropy,Double time
Why does time only move forwards? Perhaps it
doesn’t, says physicist Julian Barbour – perhaps
it goes in two directions from the big bang
nature of time. The consequences might
even reach beyond the realm of classical
physics, the world we can easily see, and
offer fresh clues to the quantum nature of
gravity – the elusive theory that marries
general relativity with quantum mechanics.
Physicists’ current ideas about time owe
much to Albert Einstein. His general theory
of relativity merged the three dimensions
of space with one of time into space-time,
the all-encompassing backdrop against which
events play out. In principle, if not always
in practice, we can move in space as we wish.
Not so in time. Time insists on a direction
of travel: we have no choice but to be swept
along from past to future.
This flow of time isn’t dictated by the
fundamental laws of nature. All but one of
these are time-symmetric: they work equally
well towards the past or future. Take the
collision of two billiard balls, governed by one
of these laws: a film of what happens doesn’t
look odd when played backwards or forwards.
The one time-asymmetric law we know of is
one that dictates the decay of certain>Features
T
IME moves forward. This is so obvious
that we take it for granted, and the rule
seems to apply everywhere we look.
Observable phenomena only ever unfold in
one temporal direction. We get older, not
younger. We remember the past, not the
future. Stars clump in galaxies rather than
dispersing, and radioactive nuclei decay
rather than assemble.
The big question is, where does this
forward-facing arrow of time come from?
The most popular explanation relates to
entropy. In this picture, the flow of time is
essentially a manifestation of the universe’s
inescapable inclination towards disorder.
I have a different idea, or rather two. The
first is that time goes both ways – that the big
bang isn’t an origin for time, but a midpoint
from which two parts of one universe play
out, running in opposite directions. We can
never see the one unfolding in the other
temporal direction, yet it is there, I suggest, as
a consequence of a fundamental law of nature.
My second idea is even more radical. It could
transform our understanding of the very