by Stephen Johnson
of 'absolute time
is an illusion.
Since Einstein posited his theory of general
relativity, we've understood that gravity has the
power to warp space and time.
This "time dilation" effect occurs even at small
Outside of physics, we experience distortions in how
we perceive time - sometimes to a startling extent.
Place one clock at the
top of a mountain. Place another on the beach.
see that each clock tells a different time.
Time moves slower as you
get closer to Earth, because, as Einstein posited in his
theory of general relativity, the gravity of a large mass, like
Earth, warps the space and time around it...
Scientists first observed this "time dilation" effect on the cosmic
scale, such as when a star passes near a black hole.
Then, in 2010,
researchers observed the same effect
on a much smaller scale, using
two extremely precise atomic clocks, one placed 33 centimeters
higher than the other. Again, time moved slower for the clock closer
The differences were tiny, but the implications were massive:
absolute time does
For each clock in the
world, and for each of us, time passes slightly differently. But
even if time is passing at ever-fluctuating speeds throughout the
universe, time is still passing in some kind of objective sense,
In his book "The Order of Time," Italian theoretical physicist
Carlo Rovelli suggests that our perception of time - our sense
that time is forever flowing forward - could be a highly subjective
After all, when you look
at reality on the smallest scale (using equations of quantum
gravity, at least), time vanishes...
"If I observe the
microscopic state of things," writes Rovelli, "then the
difference between past and future vanishes... in the elementary
grammar of things, there is no distinction between 'cause' and
So, why do we perceive
time as flowing forward?
Rovelli notes that,
although time disappears on extremely small scales, we still
obviously perceive events occur sequentially in reality.
words, we observe
Order changing into disorder; an egg
cracking and getting scrambled.
Rovelli says key aspects of time are described by the second law of
thermodynamics, which states that heat always passes from hot to
cold. This is a one-way street.
For example, an ice cube melts into
a hot cup of tea, never the reverse.
Rovelli suggests a
similar phenomenon might explain why we're only able to perceive the
past and not the future.
"Any time the future
is definitely distinguishable from the past, there is something
like heat involved," Rovelli wrote for
the Financial Times.
traces the direction of time to something called the 'low
entropy of the past', a still mysterious phenomenon on which
orients time and permits the existence of traces of the past,
and these permit the possibility of memories, which hold
together our sense of identity.
I suspect that what
we call the 'flowing' of time has to be understood by studying
the structure of our brain rather than by studying physics:
shaped our brain into a machine that feeds off memory in
order to anticipate the future.
This is what we are
listening to when we listen to the passing of time.
Understanding the 'flowing' of time is therefore something that
may pertain to neuroscience more than to fundamental physics.
Searching for the
explanation of the feeling of flow in physics might be a
Scientists still have
much to learn about how we perceive time, and why time operates
differently depending on the scale.
But what's certain is
that, outside of the realm of physics, our individual perception of
time is also surprisingly elastic.
subjectivity of time
Time moves differently atop a mountain than it does on a beach.
But you don't need to
travel any distance at all to experience strange distortions in your
perception of time. In moments of life-or-death fear, for example,
your brain would release large amounts of adrenaline, which would
speed up your internal clock, causing you to perceive the outside
world as moving slowly.
Another common distortion occurs when we focus our attention in
"If you're thinking
about how time is currently passing by, the biggest factor
influencing your time perception is attention," Aaron Sackett,
associate professor of marketing at the University of St.
"The more attention
you give to the passage of time, the slower it tends to go.
As you become
distracted from time's passing - perhaps by something
interesting happening nearby, or a good daydreaming session -
you're more likely to lose track of time, giving you the feeling
that it's slipping by more quickly than before.
'Time flies when
you're having fun,' they say, but really, it's more like
'time flies when you're thinking about other things.'
That's why time will
also often fly by when you're definitely not having fun - like
when you're having a heated argument or are terrified about an
One of the most
mysterious ways people experience time-perception distortions is
through psychedelic drugs.
In an interview with
The Guardian, Rovelli described a time he experimented with LSD.
"It was an
extraordinarily strong experience that touched me also
intellectually," he said.
"Among the strange
phenomena was the sense of time stopping. Things were happening
in my mind but the clock was not going ahead; the flow of time
was not passing any more.
It was a total
subversion of the
structure of reality."
It seems few scientists
or philosophers believe time is completely an illusion.
"What we call time is
a rich, stratified concept; it has many layers," Rovelli told
"Some of time's layers apply only at limited
scales within limited domains. This does not make
What is an illusion is
the idea that time flows at an absolute rate.
The river of time might
be flowing forever forward, but it moves at different speeds,
between people, and even within your own mind...