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Time. Just what is time? It's relentless, passing whether we want it to or not. It's the most precious thing of all. We never have enough of it. It slips away like grains of sand, never to return. Poetic words aside, time is a real thing. It's the stage on which our lives unfold. It's the thing that transforms yesterday into tomorrow. It's at the core of all physics theories, and yet, a full understanding of it remains elusive. Let's see what physics knows about time. (intro music) It might seem silly to ask what time is- after all, we all know what it is. It's the thing
that ticks away, second after inexorable second, separating that first breath we take after we're born from that last one that signals that we have slipped off into that eternal sleep. It's also found prominently in all physics textbooks. It forms the basis of the laws of motion, where we work out the position, velocity and acceleration of an object all as a function of time. But what you won't find in any textbook is a strict definition of what time is. That seems just kind of odd, doesn't it? I mean, shouldn't such a central parameter of physics be
rigidly and precisely defined? It might be said that the origins of our formal understanding of time can be found in the writings of Isaac Newton. He believed in absolute time, which exists independent of any observer, and progresses at a constant pace everywhere throughout the universe. An hour on Earth is the same as an hour in the Andromeda galaxy and our now is the same now experienced by our Andromedan neighbor. This very intuitive understanding was overthrown in 1905 by that other giant of science, Albert Einstein. He showed that time is relative- that different
observers, ones that are moving compared to one another, experience time differently. This is the basis of the time dilation that you hear about that might make interstellar travel possible. I made several videos on that topic, and I don't want to revisit any of them here. The topic is kind of complicated. The links can be found in this video's description. However, there exists a simpler demonstration of the relativity of time according to Einstein. Suppose you have a long train, with a light source in the center. If you're sitting in a train, the light
will flash and the flash will hit the ends at the same time. Now, suppose that you're standing outside the train and see the train moving by. The light flashes at the center of the train and moves at the speed of light. The back of the train is moving towards the flash point and the front is moving away. Because of that, according to you, the distance from the flash point and the back of the train is getting smaller, while the distance from the flash point and the front of the train is getting longer. According to you, you see the flash hit the back of the train before it hits the
front of the train. This is only true because of one of Einstein's postulates, which is that the speed of light is the same for all observers. This postulate has been proven again and again. But it shows something super important. It shows that time isn't the same for all observers. What one observer saw as the simultaneous arrival of the flash at the front and back, the other one saw as not simultaneous at all. That's kind of a crazy business, but it clearly demonstrates that time is more complicated than you think. The solution is found in Einstein's equations,
which show that space and time are basically the same thing. Very weird. Oddities of relativity aside, there is another feature of time that is really, really, weird. Time has a direction. The past is the past, unchangeable. The future hasn't happened, with limitless possibilities, at least in principle. And now is now- it's the moment that changes the past into the future. We can move forward in time, but not backward. That seems to contradict Einstein's claim that space and time are mixed. After all, you can move forward and backward in space. Why not time?
Indeed, when you look at the laws of nature in simple systems, it looks like that time can run forward or backward. You can see it in the equations, but it's easiest to see if you bounce a ball off a wall. It looks like a normal thing if time is going in one direction, but if you run the film backward, which is the same as moving backward in time, it also looks fine. No problem with that. The situation is different in more complex scenarios. For instance, if you look at a film of someone making a break on a pool table, and then reverse it, you can very easily tell
which film moves forward in time and which one is backward. While each collision of each ball is the same as the simple case we just looked at, when we look at the totality, the direction of time is easy to see. The same is true if you drop an egg. The egg falls and breaks, which looks perfectly fine. But run the film backward and it's obvious. So this is kind of mysterious. Einstein says that space and time are the same, which implies that you should be able to move backward in time, just like you can move backward in space. And if we take a simple enough physics system, backward and
forward films both look perfectly reasonable. So what's the deal when we look at more complicated systems? Scientists have invented an idea called "entropy" that seems to be important in answering this question. People often call entropy a measure of disorder, but that can be a bit misleading. Entropy is really a way of measuring the number of ways a collection of objects can be rearranged in ways that don't look different when you step back and look at the big picture. For example, the room I'm in is filled with air molecules. There are nearly countless air molecules- in the ballpark
of ten to the 26 power individual molecules. But it doesn't matter much which molecules are where. For instance, if I pick two particular molecules, it doesn't matter if they are here, or here, or here. The air I breathe doesn't much care about the detailed locations of any particular molecules. There is a physics principle, called the Second Law of Thermodynamics, which says that in an isolated system, entropy will always increase or, rarely, stay the same. This is where the idea of disorder comes. Take a load of laundry. There is only one, or at most, a few,
ways to stack it neatly on the bed. However, there are lots and lots of different ways to toss the clothes around a teenager's room. While the exact location of the clothing changes, it all adds up to being a mess. With the egg, there is one configuration that is an unbroken egg. However, there are lots and lots of ways an egg can splat. The details of the splat are different, but those details aren't so important when you have to clean up the mess. The idea of the direction of time seems to be tied up in the claim that entropy gets bigger. Eggs break, but don't un-break.
Messes tend to get worse. The arrow of time tends towards increasing messiness. Now before you point out that it's possible to make a room neater, that only happens if you inject energy. You, for example, have to do something to make it better. And that requires that you eat, which requires food to grow, which, if you follow the chain backward far enough, means sunlight hitting the Earth. That constant influx of energy means that your room isn't an isolated system. So, what does this mean about time? It means time is mysterious. It is different for different observers and,
while the laws of physics allow for time to move forward and backward in simple systems, in complicated systems, the tendency is for entropy to increase. This all works if the entropy of the early universe was unusually low. It turns out that it's not at all obvious why the entropy of the early universe was so small. It's actually an unanswered question of physics. In a follow-on video, I'll explain why the entropy of the early universe is so surprising and at least one theory arising from a combination of particle physics and cosmology that might be the explanation.
(phasing sound) For the longest time, understanding the physical world was something that philosophers did. Indeed, the early term for science was natural philosophy, but those days are long gone. Scientists understand the behavior of nature better than philosophers ever did. But the topic of time remains a mystery, stuck in that limbo between the worlds of philosophy and physics, which makes it fun to think about. If you enjoyed learning a bit about the fundamentals of time, please like the video and subscribe to the channel. One day, we may fully understand the nature of time, which
will mean that the topic has fully transitioned from philosophy to physics. That will be a great day, because, as I hope you'll agree, that physics is, at least eventually- everything.
