I read that entropy is directly related with thermodynamics but is the concept of entropy different if I want to understand the perception of time?
We know that the transformation of matter never regress. If you throw a cup of glass in the floor it will break into pieces but will never become a entire cup again on its own even if you gather the pieces together. If entropy did not existed and if we could see the broken cup rearrange into a full cup again could we perceive time going backwards?
Do entropy make things easier to kill and destroy than create them? A house take months to be built but an explosion can destroy it in seconds for example.
Another example things get dirty but cannot get clean and shiny by their own. Does that mean that things getting deteriorated and dirt with time is a sign of entropy as well?
So time ( or the perception of time) exists because of entropy which makes us perceive it always going forward but never the other way around?
In: Physics
Strictly speaking, you *would* eventually see the glass spontaneously reassemble itself. The laws of thermodynamics are probabilistic, not mechanical, and when you frame them the right way they basically just say “if you pick a random state, you’ll mostly find really common states”.
But in practice, the probability of even slight violations of thermodynamics are so low as to be effectively zero for all practical purposes. For example, there is no physical law preventing you from flipping 100 heads in a row with a coin. But if you and every single other person on Earth did it once a second for your whole lives, it’s still overwhelmingly likely none of you would ever get all 100 heads (the chance of even one success is one in a few hundred billion). And this system only has 100 degrees of freedom – real world thermodynamic systems have trillions of trillions.
Part of why it appears to be fully one-way right now is that our Universe is very far from its “most likely” (i.e., high-entropy) state. Imagine that, say, you *started* with 70 coins on heads and 30 on tails, and flipped coins at random over time – you’d see a very steady and quick decline towards 50 heads and 50 tails, and would in all probability never return to 70 heads in any sane period of time.
So one thing you should know about the universe is that it NATURALLY wants to minimize energy. I.e. the universe is lazy and wants to use as little energy as possible to do something. We don’t know WHY it does this (this WHY question is beyond the scope of physics and enters the domain of philosophy) but we can see from observation that it does.
For example, in the context of classical mechanics, a ball NATURALLY falls down. Why does a ball fall down? Because it wants to minimize its gravitational potential energy. When you raise a ball above your head, its potential energy goes like PE = m*g*h, where m = mass, g = acceleration due to gravity and h = height above the zero point (in this case the ground). So, the higher the h, the more PE the ball will have. When you let go of the ball, it will fall down, hence reducing h, hence reducing its PE.
Another example is in the context of electrodynamics. It is well known that two like charges repel. Why? Because they want to minimize their electrical potential energy. The PE of an electric charge is: PE = k*Q1*Q2/r, where k is a constant, Q1 is one of the charges, Q2 is the other charge and r is the distance between them. If both Q1 and Q2 are the same kinds of charge (i.e. BOTH positive or BOTH negative), and you force them to stay together, you are making the r term very small (since you’re decreasing the distance between the two charges). Now, if you let them go, they will NATURALLY repel each other, hence increasing r, hence causing PE to go down.
Now, in the context of thermodynamics, the universe wants to minimize energy as well. But in the context of thermodynamics, the universe wants to minimize THERMAL energy (aka heat). This is why heat NATURALLY wants to go from where there’s more of it (i.e. hot objects) to places where there’s less of it (i.e. cold objects). Now how does entropy fit in? Entropy simply measures HOW WELL the system has minimized its heat energy. I.e. entropy measures how well the heat energy at any given point is minimized. If you have a lot of energy at one single point, then entropy is very low because that one point has a lot of energy, and other areas has very little energy. If you have small amounts of energy at a lot of different places, then entropy is very high because there’s few energy at each location.
E.g. say that you have a room. Say that you have allotted 5 units of energy to this room. Now, if all 5 units of energy is located at one single point (e.g. in one corner) of the room, then the entropy is very low because there’s a localization of energy to only one single point. Hence, energy has not been minimized (i.e. distributed evenly). If instead, you have 1 unit of energy in each of the four corners of the room and 1 unit of energy in the center of the room, then entropy is high because now, that 5 units of energy has been minimized so that each point in the room has fewer energy. This is why entropy is sometimes said to measure how much energy can be extracted out of a system. Let’s go back to our room example. It would be a lot easier for us to extract all 5 units of energy from one single location in the room using one single machine, than having to make five separate machines and going to those five separate locations to extract energy. In other words, a low entropy system (the room in which all 5 units of energy are in one spot) has more energy available for extraction PER POSITION/LOCATION — I mean, in that one location in the room, there’s 5 units of energy available for extraction! It’s easier to extract more energy out of a low entropy system. On the other hand, a high entropy system (the room in which there’s 1 unit of energy in five separate locations) has less energy available for extraction per position — I mean, in any of those five locations, there’s only 1 unit of energy available for extraction! It’s harder to extract more energy out of a high entropy system.
Now of course, if we input our own energy, then we can make five separate machines and go to those five separate locations so that we could still extract 5 units of energy. But in order to extract 5 units of energy from a high entropy system, we had to put in more effort from our side. Compare that to trying to extract energy from a low entropy system, in which we only use one machine and go to one location to extract 5 units of energy.
As a side note, you might have noticed for each of the examples I gave (classical mechanics, electrodynamics and thermodynamics), I emphasize the word “NATURALLY”. This is because the universe will not try to reduce its energy if YOU are putting in energy into the system. E.g. when you hold the ball over your head, you are expending energy in doing so. When you’re holding the two charges together, you’re expending energy in doing so. It’s only when you are NOT inputting any energy into the system that the universe will minimize its energy. This is why the 2nd law of thermodynamics is only valid for an ISOLATED system — a system in which no mass or energy can interact with the system. You holding a ball above your head means that you’re inputting energy into the system; you forcing the two like charges together means you are inputting energy into the system and as a result of you inputting energy into the system, it won’t minimize its energy (hence won’t maximize its entropy).
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