Name: Why does time go forwards not backwards? | BBC Ideas
Uploaded: 2022-03-24T06:27:15.000Z
Duration: 4 min 11 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

we can only move in one direction of time -

Why can't we ever move backwards in time?

For a long while scientists couldn't come up

work fine whether you're going forwards or backwards in time.

Their answer finally came from an unexpected place -

At the start of the industrial revolution

how to make steam engines more efficient.

In examining how all that heat and energy moved around an engine,

they developed a whole new branch of science

which they called, appropriately, thermodynamics.

could explain the behaviour of a lot more than just steam engines.

In particular, the second law of thermodynamics helps us to understand

why things happen in the order that they do.

Now, we inherently know that that process is irreversible.

Things have a way of disorganising themselves

but they're not so good at reorganising themselves

and the second law of thermodynamics tells us why.

Another way of looking at it is in terms of messiness.

the more disordered, messy and useless it is.

Here's what the second law of thermodynamics looks like.

and this 'd' is a mathematical way of representing change.

So 'dS' simply means a change in entropy.

Now if you look at this equation from left to right

what it says is that the entropy of a system

that's fine according to the second law of thermodynamics

because the entropy of those things increases.

But if you expect melted ice to reassemble

That would break the second law. That's a no-no.

The second law tells us what order things can happen in the universe.

It gives us a clear direction for the flow of what we call time -

because that would decrease entropy and break the second law.

But where is the relentless march of time taking us?

The entropy of the universe, the disorder is always increasing.

this means that our entire universe will be in a state of total disorder,

But don't worry, this bleak fate won't come upon us any time soon.

by which time of course there will be no humans left to see

how time and entropy have ravaged our universe.

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