The document discusses the concept of irreversible processes and the arrow of time. It explains that many natural phenomena, like glass shattering or organisms aging, cannot go backwards due to increasing entropy. The second law of thermodynamics states that entropy in an isolated system is constantly increasing, which requires the distinction of past and future and defines the direction of time as irreversible processes move towards more disorder. Playing videos of these processes in reverse violates our intuition about entropy and the arrow of time.

1. The
Arrow
of
Time

2. If
we
let
a
glass
bo3le
go
down
to
the
ground...

3. • Can
this
process
go
backwards?
• How
do
we
know
that
>me
is
going
in
one
direc>on?
• Which
one
is
that
direc>on?

4. • There
are
processes
in
Nature
where
there
is
no
possibility
of
going
backwards.
• This
happens
in
most
of
the
biological
processes
(
for
instance
the
aging
of
living
organisms)
and
in
everyday
phenomena
that
have
to
be
considered
as
intrinsically
irreversible.

5. • Irreversibility
is
one
of
the
most
strange
concepts
in
physics
and
is
related
to
a
physic
magnitude
called
entropy.
• Entropy
can
be
considered
as
a
measurement
of
the
disorder
of
a
system.

6. • When
we
study
the
different
processes
that
happens
spontaneously
in
Nature
we
observe
that
normally
the
disorder
has
increased
in
the
systems
involved:
water
that
evaporates
when
heated,
a
piece
of
coal
that
burns
with
fire,
an
old
building
that
falls
down,...in
all
these
cases
there
is
a
tendency
towards
disorder
and
entropy
grows.

7. • When
the
bo3le
breaks
into
many
pieces
the
disorder
has
grown,
as
well
as
the
entropy.
• There’s
no
possibility
for
these
pieces
of
glass
to
go
back
and
reassamble
in
order
to
make
the
bo3le
again,...at
least
in
the
world
we
know
this
doesn’t
happen.

8. • The
Second
Law
of
Thermodynamics
in
physics
says
that
the
entropy
of
the
Universe,
considered
as
an
isolated
system,
is
increasing.
• The
irreversible
processes
are
manifesta>ons
of
the
Second
Law
of
Thermodynamics.

9. • Videos

10. • We
recognise
phenomena
that
can’t
be
undone.
These
processes
are
irreversible
and
they
only
have
one
direc>on,
the
direc=on
of
increasing
entropy.
• Entropy
requires
a
direc>on
for
>me
and
allow
us
to
dis=nguish
between
past
and
future:
this
direc>on
is
called
the
arrow
of
=me

11. • What
if
the
videos
are
played
on
reverse?

12. • There
is
no
symmetry
of
=me
for
these
irreversible
processes
à
when
the
videos
are
played
in
reverse
mode
our
intui>on
of
what
entropy
means
warns
us
that
something
is
wrong.

13. The Moving Finger writes; and having writ,!
moves on: nor all thy piety nor wit!
shall lure it back to cancel half a line,!
nor all thy tears wash out a word of it.!
Rubaiyat of Omar Khayyam
Persian astronomer, mathematician
and poet from XI Century