- Thank you to Curiosity Stream
for sponsoring this episode. (clock ticking) Hello, welcome to Up and Atom, I'm Jade. There's nothing else quite like time, always flowing toward the future but never the past. Why is time a one way street? We can go backwards and forwards in space, so why not time? Well, what if I told you that the direction of
time could one day change that time flows both
forwards and backwards. To understand the nature of time we need to start with the nature of space. We can learn a lot about space just by throwing a ball in the air. Now move five meters to the left and lo and behold the
exact same thing happens. We've just demonstrated a
deep truth about our universe. The laws of physics do not change depending on where you are in spare, this is called space translation symmetry. Similarly, if you throw a ball at 8:00 AM and then again at 8:00 PM,
the result is the same, this is called time translation symmetry. It tells us that the laws of physics do not change with time. Symmetries are the most
fundamental ideas in physics, they tell us about the
structure of our universe. Space translation symmetry tells us that space is the same everywhere. This means that the laws of physics cannot distinguish between whether we are in one place or another. The behavior of the ball doesn't tell us anything
about where we are. The same is true for time
translation symmetry. We can't tell which of these clips was taken earlier or later just from observing the
behavior of the ball. A surprising symmetry of our universe is time reversal symmetry,
sometimes called T-symmetry. Amazingly, there is no physical difference between backwards and forwards in time. Unlike the other symmetries, we can't physically demonstrate this because we can't reverse time, but we can simulate it by reversing the direction of velocity, this is achieved by
playing a video backwards. One of these clips is played forwards and the other backwards, just like we can't tell
where we are in space, we also can't tell whether we are going backwards
or forwards in time. According to Newton, a direction of time is not baked into the
fabric of our universe, but this isn't what we experience at all. In fact, it's usually extremely obvious when a video is being played
forwards or backwards. Newton's laws are the foundation on which much of physics relies. From them we've built
rockets to explore the cosmos or predicted the movement of the planets. So why do they say something so contrary to our experience? And if the direction of time doesn't come from Newton's laws, (pendulum ticking) Where does it come from? To answer these questions we need to go back to the 1800s. A new branch of science was
forming, thermodynamics. This was the science of heat, pressure, temperature, and volume. It was a science based on
observation and experiment rather than theories or explanations. One of the pioneers of
thermodynamics, Rudolf Clausius, notice that systems always
tend to equilibrium. If you bring a hot and
cold objects together left to their own devices their temperatures would
eventually average out. Once a system reaches equilibrium,
it stays there forever, never spontaneously going
back the way it came but no one knew why Clausius
called this tendency for a system to move toward equilibrium, an increase in entropy. You might might have
heard entropy described as a measure of disorder, but a more accurate
definition is the measure of how close a system is to equilibrium, the closer it is to equilibrium,
the higher its entropy the further away it is,
the lower its entropy. And just to be crystal clear, equilibrium is the state in which the system is perfectly
balanced and no longer changes. This most often looks like
the most disordered state. Clausius declared a physical law that the entropy of a closed system always increases or stays the same, it never decreases. We now call this the second
law of thermodynamics. At the time it was huge, for the first time in history we had a law of physics that distinguishes between the forwards and
backwards directions of time. If entropy is increasing,
we are going forwards, if entropy is decreasing,
we are going backwards, but this breakthrough came at a cost, we now have a conflict between
two major fields of physics the time symmetric laws
of Newtonian mechanics and the time asymmetric
laws of thermodynamics. Was there a different set of rules for different fields of physics? Was a direction of time baked
into our universe or not? Like us, a physicist
named Ludwig Boltzmann saw this conflict as unacceptable. He didn't believe for a second, there could be one set of laws for one major field of physics and a different set for another and made it his life's work to unify them. Boltzmann had just finished his PhD on something called the
kinetic theory of gases. The idea that gas consisted
of millions of tiny particles all bouncing around with
some kinetic energy. This provided a link between
mechanics and thermodynamics. Each particle could be
viewed as a tiny ball that obeys Newtonian mechanics while the gas as a whole
followed thermodynamics. The challenge was how to
derive a time asymmetric gas from time symmetric particles, the answer, collisions. As the particles bounce around they inevitably bump into
each other and the walls. A particle will continue
moving in a straight line until it collides with
another one or the wall. They move much further in
the direction of free space, simply because they're less likely to collide with something. However, it isn't guaranteed
just overwhelmingly likely there are still arrangements where the particles stay bunched up forever, they're just extremely unlikely, so unlikely that for a regular gas, we have to wait much longer than the age of the universe to see this. Almost any arrangement
guarantees they spread out while only very few specific arrangements have them remaining bunched up. Boltzmann had shown that the journey from order to disorder was in fact a journey from the
improbable to the a probable. The second law was not
a strict physical law but rather a statistical law. A single particle obeyed
Newton's mechanical laws while a group of particles acted
as a probabilistic process. This phenomenon is beautifully captured in this simple and elegant equation. This was brilliant, but
Clausius wasn't very happy about the demotion of
his robust physical law to a mere statistical one. It had changed from the
entropy of a closed system must always increase or stay the same to the entropy of a closed system will probably increase or stay the same. Doesn't sound as good, does it? But whether it sounded good or not, Boltzmann's theory answered
a lot of questions. It reconciled the time
symmetry of Newtonian mechanics with the time asymmetry of thermodynamics. It also seemed to answer why we experience a direction
of time in our everyday lives. We live life at the macro scale, we experience accrued surface level world. We don't see the millions
of microscopic particles that make it up. If we only had access to the micro world, we wouldn't experience
a direction of time. It's an emergent property, isn't that crazy? Entropy is a macro phenomenon, the fact that it's
overwhelmed likely to increase is what gives our universe
its forward arrow of time. All was well in the land of physics, until, Joseph Schmidt was
Boltzmann's colleague and friend and couldn't shake the feeling
that something was off. As a side remark in one of his papers he scribbled an argument
along the lines of, you say it is more probable
for a system to evolve from a state of low probability to a state of high probability. However, for every state that
evolves toward higher entropy there is an opposite state that evolves toward lower entropy, all one needs to do is flip the direction of the velocities of all the molecules and they will all go
back the way they came. Of course, this is perfectly allowed because of the time symmetric
nature of Newtonian mechanics. This means that there are equally many states evolving toward lower entropy as there are evolving
toward higher entropy. So how can you conclude
that it's more probable for a system to evolve
one way over the other? This flipped Boltzmann's
theory on its head. Let's investigate. When you flip a coin
there's an equal chance it'll come up, heads or tails. This was the nature of Schmidt objection because of the reversibility
of Newtonian mechanics, for every state moving
toward higher entropy there existed a flip state
moving toward lower entropy. There are an equal number of each. So if we choose a state at
random, there's a 50/50 chance that it evolves toward higher
entropy or lower entropy. What Boltzmann was claiming was that it was much more probable
to get heads over tails even though they both have
equal probability of coming up, this is called the
time-reversibility paradox. Schmidt had somehow lodged
his way to the result that the universe should decrease in entropy just as often as it increases. In other words, time should go backwards just as often as it goes forwards. But again, this isn't
what we experience at all. Now this isn't just about reconciling the laws of mechanics and thermodynamics, but logic and reality. How would Boltzmann
get us out of this one? Here, he makes his most
daring insight yet. What if Schmidt is right? What if the direction of
time does go backwards just as often as it goes forwards? All of the systems we've
talked about so far follow a similar entropy evolution. For example, the universe started in a very low entropy
state near the big bang, it's been increasing ever since and one day if things keep
going the way they've been going we'll reach equilibrium. It'll fluctuate around there because that's the most probable state, but every once in a while we'll see a big decrease in entropy. Remember, this isn't impossible,
just highly improbable. There's still a tiny chance it'll happen if we wait long enough just like there's a tiny chance we'll flip 10,000 heads in a row if we flip coins long enough. These decreases are immediately followed by increases back to
comfortable equilibrium. But here's a question, why do we always start in
such a low entropy state? Equilibrium is the most likely state. This is an extremely improbable state. In fact, if we do begin at equilibrium Schmidt's argument checks out, decreases in entropy happen
just as often as increases. There is no universal direction of time, but only pockets of
backwards and forwards. To drive the point home, imagine this curve is a hiking trail with valleys scattered throughout. If you saw someone here,
you can infer to things. At some point in the past they were most likely at higher altitude, and at some point in the future they will most likely
be at higher altitude. What you can't infer is the
direction they were going. This path does not have
an inbuilt direction, you can flip it either way
and it still makes sense. The same goes for entropy. It's true that if we're
at a low entropy point in the future we'll most
likely be at higher entropy, but it's also true that in the past we were most likely at higher entropy. Entropy is higher in both directions. This curve does not have an arrow of time. The second law of thermodynamics that entropy overwhelmingly
likely to increase only holds if we assume that we started in a low entropy state but there's no logical
basis to this assumption, it's just what happened. Why did our universe start
in such an improbable state? This is now one of the biggest unsolved
questions in cosmology. I'd love to share some
possible answers with you but we're out of time. Thanks to Boltzmann and Schmidt, we now know that the direction of time is not built into our universe but is a result of the way it started. Today we covered a lot in a
very little amount of time. I'm sure you have some questions like, if entropy is always increasing why do we see so much
spontaneous order around us? How does entropy ever decrease? Does that mean time is going backwards inside my refrigerator? What's the link between
entropy and information? There's a wonderful documentary
series on Curiosity Stream which answers all of
these questions and more, it's called Order and Disorder presented by famous
physicist Jim Al-Khalili. It gives a complete
overview of the key concepts we talked about today, discussing the historical landmarks that led to innovation and diving deeper into the ideas. I was fascinated to learn what drove the scientists in the 1800s and came away feeling inspired and in awe. Curiosity Stream makes
award-winning documentaries ranging from deep space, to
dinosaurs, to ancient Egypt. They've also partnered with a bunch of us
educational YouTube creators with our own streaming platform, Nebula. Nebula is a place where
we can make content without the pressure of
the YouTube algorithm. We can be more experimental
and try new things. I've made a couple of Nebula originals which you can check out. We're both passionate about learning, so we've put together a bundle where you can get both streaming services for the price of one. They currently have a 26%
discount on at the moment too. Just sign up to Curiosity Stream with the link
curiositystream.com/upandatom or click the link in the description. From there you'll get
free access to Nebula too. Thank you so much for watching
and supporting the channel. That's it from me, and I'll see you in the next episode, bye. (upbeat music)
