We all think of time as having a direction.
Yesterday was the past, tomorrow is the
future, but what if i told you that none
of the equations that govern our
universe have anything to do with the
direction of time?
Newton's laws, gravity, even quantum
laws like Schrodinger's equation, work
just as well going backwards as forwards.
Physics says that if you break an egg in
theory you should be able to unbreak it
and you would have no mess to clean up. I know
this sounds ridiculous but what about a
simpler system like the Earth going around
the Sun? If you didn't already know the
direction of the Earth's orbit it would be
pretty difficult to tell whether it was
going forward or backward. The physical
laws of motion, velocity, acceleration,
even gravity are totally unchanged. So how
are you so sure that this movie of the
egg unscrambling itself is being played
backwards? Time's direction is given purely by
the second law of thermodynamics. It says
that the entropy of the universe is
always increasing. Entropy is the measure
of randomness, disorder, or messiness of
a system. A low entropy system would be
very ordered an organized. A high entropy
system would be kind of a mess.
The second law of thermodynamics is
saying that the universe is always
becoming messier. Here's an example. Hot
tea and cold milk mix together into a
warm tea milk but they'll never unmix
again into cold milk and hot tea. As the tea and
milk particles swirl and jostle around
together, the system is becoming more disordered
and entropy is increasing, and it
will never decrease. If you are shown a
picture of half mixed milk tea, you would
be able to predict where it's headed.
The only difference between now and then
is that entropy has increased. This is
kind of where our universe is now. One
day in the far distant future, it will
reach a point of maximum entropy where it
just can't get anymore jumbled up. At
this point, there would be no difference
between past, present, and future, and time
as we know it would cease to exist.
Believe it or not this one aspect of
ourt universe is what gives time it's direction. In
the past there was lower entropy that there is
now, and in the future there will be
higher entropy. So I know what you're thinking,
what about that one time you cleaned
your room right?
It went from a messy disordered state to
neat, ordered state. Doesn't that break the second
law of thermodynamics? Even though the
entropy of your room has decreased, the
entropy of the entireuniverse has
increased. When you move around your room you
pick up things and you place them here
and then you start to get hot, and the
heat from your body rises into the air
and makes the atoms and molecules all
vibrate. So even though it looks like the
entropy of your room has decreased the
atoms and molecules on the microscopic
scale have gone into a bit of a frenzy.
The entropy of the entire universe has
increased and the second law is safe and sound. So why is
entropy always increasing? Is it
just some random law of our universe? Well it turns
out that disorder is just more likely
than order. Imagine you have six dice
each with six different possibilities. A
very ordered, low entropy state
would be if all dice rolled a six, but the
chances of this happening are so
fantastically small, you'd have to roll the
dice 46656 times to get that
combination. But for high entropy
disordered states there is a much higher
probability of being rolled simply
because there's so many more of them.
Now imagine that every single atom in
the universe is a die except instead of
having six possible states it has
millions, billions, trillions of
possible states. Even for a simple system
like the tea and milk, there are so many more
possible ways that the particles can be
messily mixed together than neatly
separated.
It's not impossible but the chances are
so tiny you'd have to wait a lot longer
than the lifetime of the universe.
So if you ever feel like you're in this
constant battle against the universe to
keep your room clean or to keep your
life organized,
you're right. The real question is why
was entropy ever low in the first place?
I mean if order and neatness is so improbable
isn't it a bit odd that the universe
just happen to begin that way?
This question has been one of the
biggest unsolved mysteries in physics
for over 50 years but some recent
discoveries hint an explanation. Physicists
used to think that the universe had
kind of always existed the way it is.
But then astronomers realized that all
the galaxies are flying apart, so if the
universe is expanding this means that
once upon a time everything was really
close together. This tiny hot dense speck
of matter exploded into everything that
exists now. But that still doesn't
explain why the universe ever started in
a low entropy state to begin with. I mean
a huge, hot explosion sounds pretty
messy. One hypothesis is that we live in
a multiverse where baby universes are
popping into existence all the time.
Our universe is just one of many and these
baby universes could somehow start with
low entropy while the entropy of the
multiverse increases as a whole.
Kind of like when you clean your room. Part of the
universe becomes tidy while the chaoticness
of the rest of it goes up. This is
just one theory and there needs to be a
lot more evidence before it can even
begin to look promising. To describe the
early state of the universe we need to
combine quantum mechanics and general
relativity, two things which, at the
moment, just don't seem to go together. If
physicists could somehow fuse these into
an ultimate theory of everything, all
questions will be answered, we would be able
to understand the mysteries of our
universe and explain the arrow of time.
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