dioxide.”

Or as Nobel prize-winning physiologist Albert Szent-Györgyi put it, “life is nothing

but an electron looking for a place to rest.”

While these aphorisms might not capture the “meaning of life” that most of us look

for, their point is that living organisms ultimately depend on and facilitate the universe’s

tendency to increase entropy.

That may seem counter-intuitive, since living beings are themselves highly organized, while

entropy is a measure of disorder.

But as we know, complexity is not the same thing as order.

Every organism, just by living and breathing, acts to increase the entropy of the universe.

Think of a photon arriving from the Sun, packed with useful energy.

It can be captured by a plant or microorganism that uses photosynthesis to store that energy

in the form of sugar.

But the sugar doesn’t contain quite as much useful energy as the original photon – some

of the energy ends up heating the plant and its environment.

An animal like us eats the sugar, and uses its energy to create molecules of ATP, adenosine

triphosphate.

ATP is like a little power-pack of energy that can be sent to a part of the body where

it might be helpful, but ATP doesn’t have quite as much useful energy as the sugar that

went into making it – some of that useful energy got lost pushing around all the cell

machinery that makes the ATP.

The proteins in your muscles utilize the energy in ATP to contract, so that you can lift a

barbell or a slice of pizza.

But not all of the useful energy of the ATP goes into lifting the pizza – as before,

some of it is degraded into noise and heat.

Not only that, ATP’s useful energy can also be used to repair broken-down cells or organs,

again becoming less useful in the process.

The pattern here is obvious: every step along the way, the energy in that original photon

is gradually degraded, entropy increases, and at the end all that’s left is an organized

but slightly warmer plant and cell and muscle, plus some high-entropy infrared light that

gets radiated out into the universe.

Energy transforms from useful to useless in the cause of keeping organisms like us alive.

In fact, life itself might have arisen because of entropy.

The early Earth had pockets of low-entropy conditions full of useful energy, like warm

alkaline vents on the ocean floor.

But there may have been no simple chemical reaction that could take advantage of that

energy, use up its usefulness, and allow the entropy to increase.

There were, however, more complicated chains of reactions that could do the job.

In just the right circumstances, an appropriate network of chemical reactions might find a

way to sustain itself by tapping into the useful energy in its environment.

Some networks might have become embedded in molecular membranes, the precursors of cell

walls, and broken away from their point of origin, becoming the first “living” organisms.

Maybe that's how life began: a complex combination of chemical reactions that figured out how

to tap into otherwise unavailable useful energy.

We can tell a similar story about why stars shine.

Hydrogen nuclei have a ton of useful, low-entropy nuclear energy to release -- if you can get

them to fuse together into helium.

But there’s a big barrier to getting that to happen – fusion is hard!

And yet, the cores of stars do the job marvelously, so stars, like life, also survive because

of the increase of entropy throughout the universe.

Our sun takes a low-entropy fuel source and converts it into higher entropy energy.

Life takes that higher-entropy energy as a fuel source and converts it into even higher-entropy

energy.

In a very real sense, the purpose of life is to continue the mission of the stars.

Hey, Henry here, thanks for watching.

This is the fifth video in a series about time and entropy made in collaboration with

physicist Sean Carroll.

This final video is supported by Audible.com, a leading provider of audiobooks including

fiction, non-fiction and periodicals.

The videos in this series are based off of Sean’s book “The Big Picture: On the Origins

of Life, Meaning, and the Universe Itself,” which is available, read by him, on Audible.

You can listen to “The Big Picture” or another book of your choice – but really,

check out “The Big Picture” – for free, with a free 30-day trial at Audible.com/minutephysics.

Again, that’s audible.com/minutephysics.