Arrow of time

That puts them in the same mathematical family tree as black holes, wormholes, and some of our more exotic ideas about baby universes.

They've haunted science fiction for decades, and they sit right at the crossroads of black holes, the Big Bang, the arrow of time, and the question of whether our universe might itself have been born from something else.

So today we ask, "What is a white hole? Could one exist? Could a black hole lead to one? Could the Big Bang have been something white hole-like? And if white holes are real, why have we never seen one?" Before we go hunting for the universe's strangest exit sign, please don't forget to like, subscribe, and ring the bell.

Cosmology has a bad habit of looking impossible right up until the math starts working out.

[laughs] You can map out every action physically possible inside a black hole, and they all stay in there.

For a white hole, everyone involved is leaving, and every action outside leads to staying outside.

A white hole is what you get if you take the causal structure of a black hole and reverse the arrow of time.

And that immediately makes white holes fascinating because black holes are real enough.

That's entropy increasing.

That's the second of thermodynamics and the arrow of time.

You don't shuffle a pack of cards and get it more ordered.

It always just gets more disordered.

I mentioned earlier this Loschmidt's paradox about, you know, the laws of physics equations suggest that time could move in both directions.

I agree.

argue quite strongly in my book that the arrow of time, the linearity or the direction of time, the irreversibility of time is embedded and baked into the universe.

It's not just our perception.

How do we know? We might be in the past and we don't realize it [laughs], you know? So, uh, there's, uh, Daniel Saadeq, I think is his name.

He wrote a book called Divine Relativity and the Arrow of Time.

Was a, a recent book-

Yeah

Even biology is an enormous amount of order created as entropy grows.

This connects directly to his ideas about the arrow of time.

Right, and Rovelli connects the past, memory, and planning to entropy, but he resists simple stories.

The low-entropy early universe remains a mystery.

But unlike in space, there is only one way in which we can move through time.

The reason for this apparent directionality, this arrow of time through the cosmos, is one of the universe's most perplexing mysteries, and it underpins the topic of our video today, the second part in a double header on the mysterious workings of time.

In the last episode, we covered time in its human context, but today we'll be dispensing with the mutually-agreed delusions of minutes and seconds to take on time in its purest form.

So get ready to tackle some of the universe's most profound questions.

Time's unwavering flow only really starts to become apparent when we step back from the smallest frames of reference.

On macroscopic scales, there's no mistaking which way the clock ticks.

As such, we can assume that the cause of this so-called arrow of time through the cosmos must lie within the large-scale laws of physics governing reality as a whole.

One such law is the cosmic speed limit, AKA the speed of light.

Some of the formulations of quantum mechanics, the more traditional formulations, the orthodox formulation, the formulation bestowed upon us by Paul Dirac and John von Neumann, some of the dynamical collapse approaches, the stochastic approaches, a whole variety of different approaches they don't fall under the umbrella of bilaterally deterministic theories.

And Watanabe argued, I think very persuasively, that on such theories there is a very natural fundamental arrow of time.

So I would push back a little bit on the idea that the laws of physics are obviously, that they obviously don't favor one direction of time.

If measurement collapses the wave function accounts of quantum mechanics, you and I agree are not very good.

But I guess what I'm getting at, because it's so far out there, is that this whole discussion is from the human experience.

And we know time is one arrow, arrow direction or whatever, what's the phrase?

Arrow of time.

The arrow of time.

The universe naturally progresses from order to disorder.

And this entropy gradient is supposedly the single physical law most responsible for the felt arrow of time, the sense that time moves one way and not the other.

To grasp why this is so profound, you have to look at how bizarrely time-symmetric the rest of physics is.

What I mean is, almost every other fundamental law of physics works exactly the same whether you run the clock forward or backward.