- We need some way to explain
why people only get older and not younger.
But we have this big mystery
in the foundations of physics
because if we look at the laws
for the microscopic constituents,
like elementary particles,
they work the same way forward in time
as they do backward in time.
But the problem is we think that everything around us,
us, worlds, plants and so on and so forth
is made of those particles bouncing off each other.
But we don't experience our reality
as being the same forward in time and backward in time.
Eggs break but they don't unbreak.
If we drop a pebble into water,
you see the water splash out and the pebble sinks.
People only get older but not younger.
So where does it come from
if the laws work the same forward and backward?
The answer to this question
is that it's not just the laws themselves that matter
but it's also how the individual particles are arranged.
So despite the fundamental laws of nature,
having this symmetry between forward in time
and backward in time,
we ourselves have this experience that forward in time
is very different from backward in time.
And this experience is what we call the arrow of time.
And we need an explantation for it.
And the most common explanation
that physicists put forward for it is entropy increase.
And entropy is a measure
for the number of micro states in each macro state.
What we mean by this
is that things become more and more disorderly over time.
They break but they don't unbreak.
This explains part of the reason
for why the direction forward in time looks different
than the direction backward in time
because entropy only increases in one direction of time.
Mathematically with our equation,
we can run the movie backward
but it won't look the same.
So we've said entropy always increases
and this explains why the future's different from the past.
But it brings up a new problem
because entropy can only increase if it was small
to begin with.
And indeed, the entropy must have been very small
in the beginning of the universe,
otherwise we wouldn't be here today.
So what does the arrow of time have to do with us aging?
At first sight, you could say well,
that's all about biology,
it's all about cell processes,
and there are lots of scientists
who are trying really hard
to slow down the process of aging,
and now you physicists come
and try to tell them it's not possible.
It's certainly true
that there are cell processes that determine how we age
and it's really, really complicated.
And I admit that I'm totally not competent to talk about.
I'm just a physicist.
But it is possible to fend off entropy increase
for some amount of time
in a particular part of a system.
For example, there are certain creatures,
like types of lobsters or maybe trees
that can keep entropy increase very low
for a certain amount of time
on the expense of increasing entropy elsewhere.
And some species are better at this than others.
And maybe in the future,
we'll be able to slow down aging.
Maybe we'll be able to live 100,000 years
or maybe a billion years.
But you can't do it for the entire universe.
So entropy will eventually increase
and it'll get us in the end.
So this brings up the question
is this how the universe will end?
This question has captured the imagination
of a lot of science fiction writers.
The most famous example might be Isaac Asimov.
In his short story, "The Last Question,"
Isaac Asimov envisions a man asking a computer,
today we might say an artificial intelligence
if there's any way to stop the entropy
from increasing in the universe.
And for a long time, billions of years,
the answer of this computer
and its more sophisticated later versions
is that it has incomplete information for an answer.
And at the very end,
when all the stars have burned out and the only beings
that remain are completely disembodied consciousnesses
that float through the universe,
the computer finishes the calculation,
and says, "Let there be light."
So how will it end?
For me, the answer is we don't really know.