Page 18 - All About Space 68 - 2017 UK

Mirror universe
Mirror
universe
The now-decommissioned
Planck spacecraft mapped
the Big Bang's radiation
“ The arrow of time is the direction in which
© Tobias Roetsch, NASA / WMAP Science Team, ESA Newton's Laws of Motion are time
reversible – they work equally well
18 forwards and backwards.
records of the past are generated – history
books are written and fossils are formed”
Tim Koslowski
Can you remember the past, or read about Henry To see why this should be, many researchers
VIII's wives? What about recalling what you had for have turned to the point when the universe was at
breakfast tomorrow, or who was on the British throne its most ordered: the Big Bang. In the simplest Big
in 2100? Our experience of time comes with a very Bang model, time and space were both created in
distinctive direction – events flow like a relentless the cataclysmic birth of our universe. But a growing
river, from yesterday to tomorrow. Never the other band of physicists are daring to talk about events
way around. In the language of physicists, our before the Big Bang. Back in 2014, three physicists
universe has an arrow of time – one that points us – Julian Barbour, Tim Koslowski and Flavio Mercati
ever forwards. There's even a corny joke to go with it: – published research suggesting that our universe
'Time flies like an arrow, fruit flies like a banana'. emerged from a 'Janus point'. Named after the two-
But why is that? After all, physicists have known headed god from Roman mythology, they suggest
for centuries that the fundamental laws of physics that reality branched off into two separate directions
don't seem to care which direction time flows in. each with its own opposing arrow of time.
Newton's Laws of Motion work equally well with
“We weren't working on the arrow of time,” says
of a ball moving in a straight line at a constant speed de México, “we were researching quantum gravity.”
Koslowski, from the Universidad Nacional Autónoma
time running backwards as forwards. Watch a film
That's the attempt to combine quantum physics and
and there's no way to tell if the video is playing
Einstein's General Theory of Relativity, a holy grail
forwards or backwards. So the distinction between
for physicists, and a so called 'Theory of Everything'.
past and future isn't always so clear cut; except, that
is, when it comes to entropy. This notoriously tricky
Exploring models in which small particles move
about in a region where no external space or time
phenomenon is wrapped up in the equally infamous
exists, they found arrows of time emerging naturally.
Second Law of Thermodynamics. Put simply, order
of the past are generated – history books are written
lawns get scraggier, and cups of tea get colder.
tends to disorder. Bedrooms get messier, unkempt “The arrow of time is the direction in which records