The mind-boggling physics of time

In this Big Think interview, theoretical physicist Sean Carroll discusses the concept of time and the mysteries surrounding its properties. He points out that while we use the word “time” a lot in everyday language, the real mysteries emerge when we think about the properties of time, such as the past, present, and future, and the fact that we can influence the future but not the past.

Carroll also discusses the concept of entropy, which is a measure of how irregular or random a system is, and the second law of thermodynamics, which states that there is a natural tendency for things in the universe to transition from a low-entropy state to a high-entropy state—in other words, from least chaotic to most chaotic. He explains that the arrow of time, or the observed difference between the past and the future, arises due to the effect of the Big Bang and the fact that the universe began in a state of low entropy.

Carroll also touches on the possibility of time travel and the concept of the multiverse.

Sean Carroll: Lexicographers will tell you that “TIME” is the most commonly used noun in the English language. We can’t go through the day without talking about time all the time. I think about how we use time, when we actually talk about it. If you say, “Meet me at 7 p.m.,” no one freaks out. Nobody says like, “Oh my God, what are you talking about with these esoteric notions around 7 p.m.?” We all know what needs to be done in practice. Time, in a sense, is just a designation for various events in the universe. The universe happens over and over again in different things we call moments, and time helps us tell the difference between one moment and the next. So what is the time, I don’t think that’s the problem. The problem – the real mysteries – comes when we talk about the properties that time possessed. We have a past, we have a present, we have a future. How are they different? Do we move through it? We have memories of the past, but no memories of the future. why is that? Where does this disparity come from? Why are we all born young? Why do we inevitably age? Why do we think we can influence the future and not the past? Can we go back to it? Anyway, there are a lot of really confusing questions about the nature of time and many we don’t know the answer to, but what is time, I don’t think it’s one of them.

One of the most remarkable features of time is that it has a direction, right? There is a difference between the past and the future. Sometimes we think of this as just an intrinsic feature of reality. Like the past already happened, it’s in the books – the future is in front of you. It hasn’t happened yet, and the present is where we live. But then, comes the physics. And what people notice about our best theories in physics is that they don’t distinguish between the past and the future. But in our daily life, nothing is more obvious. It really takes a little mental discipline to say, “Well, time can exist without an arrow.” And one way to think about that is there’s no intrinsic arrow of space, but there’s still space, okay? We live in a three-dimensional world – up, down, left, right, forward, backward – at the level of the basic laws of physics, there is no special direction in space. And how do you know that? Imagine you’re an astronaut: you’re flying around in your little spacesuit. It wouldn’t make any difference which direction you looked at. There is no experiment you can do in physics that points a direction in the universe, but space still exists. Likewise, time will still exist even if there is no arrow. But here on Earth, we have an arrow in space. If you pick up a cup of coffee and leave it, it will always fall. Obviously, there is a difference between up and down. No one is inclined to think that this is an essential feature of the universe. This is not because grief is embedded in the laws of physics. It’s because we live near an influencing body – the Earth. The time arrow is exactly the same way. In our daily lives, we are aware of the arrow of time because we are living in the aftermath of an influential event: the Big Bang.

This brings us into the realm of the concept of “entropy”. Entropy is how chaotic, how disorganized, and how random a system is. When things are nice, neat, and tidy, they have low entropy. When it is all chaotic and all around, the entropy is high. There is a natural tendency for things in the universe to go from low entropy to high entropy. This is called the “second law of thermodynamics”. The real question is: why was the universe so low in the world in the first place? Why was the universe less entropy yesterday than it is today? To be honest, the explanation is not entirely satisfactory. The explanation is as follows: Because it was lower entropy the day before yesterday. And why was the universe less entropy the day before yesterday? Because it had lower entropy the day before that. And this chain of thought goes back 14 billion years to the Big Bang, to the origin of our visible universe. It is in a dense, hot, and extremely low-entropy state, and the universe has been increasing in entropy ever since. This is called the “past hypothesis” by philosophers – David Albert, a philosopher of physics, called it that. So now we say, “If you know the world is made of atoms, and you know what the entropy is, in terms of rearranging all those atoms, and you know the previous hypothesis — that the entropy of the universe started out very low — then you can explain everything that happened after that. There’s a way.” To talk about human life and entropy, which I think is misleading, which is that we should think of life. You know, literally, living, being an organism, eating and everything, as fighting increased entropy. I think that’s wrong. I think we owe life for the truth. That the entropy is increasing, because what would it mean if the entropy did not increase? That means nothing happens. Nothing interesting is happening. Without increasing entropy, there is no memory of the past. Without increasing entropy, there is no causal effect we have on the future. It will only be in What we call “thermal equilibrium”. Everything would be the same everywhere. The universe would be very boring. But what we have as a scientific question is: “Why do complex structures appear at all?” They obviously need to increase the entropy of existence, because if they are The universe has already exceeded the limit, then there will be no complication , but this does not mean that it must come into being.

Think of a famous example there: all perfume in a small bottle. He’s in a big room. You open it, and it all floats through the room. The entropy of the fragrance increases. But if you think about it, when the perfume is all in the bottle, it’s pretty simple. Once everything is spread out in the room, it’s pretty simple too. It went from low entropy to high entropy, but it went from simple to simple. It is the journey from a simple, low-entropy starting point to a simple, high-entropy ending point, where there is a huge expanse of possibilities where things can be complex. There are more fragrances here than there. There could be vortices caused by the movement of wind in the room and so on. The universe is just like that. Our universe started from simple, low entropy. In the future, stars will die, and black holes will evaporate. It would be dark and empty, and again, simple, but high entropy. It is among those things like us — complex, complex systems that are fueled by the increasing entropy of the universe — that can come into being. We don’t know the whole story there. I think it’s a very interesting and active area of ​​scientific research: Why did complex structures like organisms emerge and exactly the way we did? What is the role of information? What real-life chemistry is going on here? What is the geology going on here? Could it happen on other planets? Very interesting questions – but one thing I do know is that if entropy isn’t increasing all the way, none of it is going to materialize.

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