Back in the 19th century
engineers were concerned with the efficiency of steam engines.
How hot should the fire be?
What substance should you boil in the steam engine? Should it be water?
Should it be something else?
These were profound questions and out of those questions arose the science of thermodynamics.
It's when concepts like heat and temperature and energy
enter the scientific vocabulary for the first time.
Along with that deeper understanding
emerged what is probably the most important law of physics for understanding heat,
evolution of the universe and the passage of time.
It's called the Second Law of Thermodynamics
The reason the second law of thermodynamics was so profound,
was because at its heart it contained a radically new concept.
Something physicists call entropy.
Entropy explains why, left to the mercy of the elements,
mortar crumbles, glass shatters and buildings collapse.
And a good way to understand how is to think of objects not a single things,
but as been made up of many constituent parts
like the individual grains that make up this pile of sand.
Now, entropy is a measure of how many ways I can rearrange those grains
and still keep the sand pile the same.
And there are trillions and trillions and trillions of ways of doing that.
There's pretty much anything I do to this sand pile.
If I mess the sand around and move it around then it doesn't change the shape or the structure at all.
So, in the language of entropy
this sand pile has high entropy
because there are many many ways that I can rearrange its constituents
and not change it.
Now, let me create some order in the universe.
Now there are approximately as many sand grains in this sandcastle as there are in the sand pile.
But now virtually anything I do to it will mess it up, will remove the beautiful order from this structure.
And because of that the sand castle has a low entropy. It's a much more ordered state.
So.
Many ways of rearranging the sand grains without changing the structure: high entropy.
Very few ways of rearranging the sand grains without changing the structure,
without disordering it: low entropy.
I imagine now to leave the castle in the desert all day.
Then it's obvious what's going to happen.
The desert winds are going to blow the sand around
and this castle is going to disintegrate.
It's going to become less ordered. It's going to fall to bits.
But think about what's happening on the fundamental level.
The wind is taking the sand of the castle
and blowing it over there somewhere and making a sand pile.
There's nothing fundamental in the laws of physics that says
that the wind couldn't pick up some sand from over here,
deposit it here and because of it in precisely the shape of a sandcastle.
So, in principle the wind could spontaneously
build a sandcastle and the pile of sand.
There's no reason why that couldn't happen.
It's just extremely, extremely unlikely
because there are very few ways of organizing this sand so that it looks like a castle.
It's overwhelmingly more likely
that when the wind blows the sand around, it will take the low entropy structure, the castle,
and turn it into a high entropy structure, the sand castle.
So, entropy always increases.
Why is that? Because it's overwhelmingly more likely that it will.
It seems incredible that a law that says that sandcastles don't spontaneously form on the wind
could solve one of the deepest mysteries in physics.
But by saying entropy always increases
the second law of thermodynamics is able to explain why time only runs in one direction.