What is entropy? Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
In: Physics
“Disorder” is the ELI5 version of entropy and is honestly a pretty good summary in a single word.
The true nature of entropy isn’t necessarily about disorder, but about the nature of potential in a system. High potential means that the system can essentially either do useful work or can be biased towards it. A simple example of this is a room where all the temperature is 5000 degrees and spread the same – this has almost no potential to do work (heating or cooling). Meanwhile a room where one end is 10000 degrees and one end is 0 degrees has a huge amount of potential. Although both rooms have the same amount of “total energy”, the potential in the 2nd room is much greater and in human terms we would label it as more useful.
> Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
Entropy is so closely connected to the passage of time that some theories abstract them to be “kind of” the same thing. One of the most bulletproof laws of the universe is that with the passage of time, the entropy of a system must increase.
When we talk about matter and entropy, often we’re talking about the relative potential or nature of energy in an atom, molecule or greater group of them. This isn’t only about the “temperature” (and localised temperature – i.e. what subatomic molecules actually have the kinetic “temperature” energy and in what % its split) but also about relative placements of atoms (it’s easy to imagine that the Earth would be a very different place if all the oxygen was only in one part of the sky, or if all the air was only on one half of the planet for some reason – that’s a massive pressure difference).
On a grander scale; entropy is a comment on the ultimate past and ultimate future of the universe. We might not know everything, but what we can be certain of is that the future will have higher entropy. This means that temperatures will become more spread out. It also means that the universe itself biases towards low-energy heat radiation instead of little packages of high energy (i.e. atoms and matter itself). It’s easy to see that some objects have inherently low entropy (stars being an extremely obvious example of this; they are so much hotter and have so much more localised energy than the empty space surrounding them) – and it is inevitable that things with low entropy will eventually transform into things with higher entropy.
What is energy? Energy is a difference in potential in a system that can be exploited to do work.
Imagine an airtank full of compressed air, inside of a sealed box, attached to a motor. Initially the air inside the tank is higher pressure than the air inside the box and so if a valve is opened, the air flows from the area of higher pressure, to lower pressure and work is performed. But eventually the air pressure in the box would be equal to the air pressure in the tank, and no more work could be performed.
You could say that the initial configuration of high pressure tank, low pressure box, is unstable or chaotic. The air wants to be equally pressurized and if allowed to, it would flow out until the pressure was equal everywhere. You could say it wants to move from a chaotic situation to an ordered one. This is what entropy is, the tendency for a closed system (like a sealed box, or the universe) to move from a high energy, chaotic state, to a low energy, ordered state.
This works for any closed system. Imagine a sealed globe with tiny animals living inside of it perfectly in balance with little plants and algae living inside of it as well. The animals live, eat, and breath exactly enough to balance out the air and food the plants provide, and the plants get just what they need from the animals in order to grow as well. This is a sealed system but it’s not a perpetual motion machine.
The plants and animals only stay balanced and alive if the plants get sunlight because they convert that sunlight to chemical energy, which the plants and the animals use for food. It only works because energy is continuously pumped into the system. If you truly sealed it, including sealed it away from sunlight, then it would slowly begin to die. Even if the plants could extract energy by eating the animals, like carnivorous plants, the amount of energy they receive from doing so would be less than the energy the animal used to grow up and become a meal. Every time one organism ate another, breathed in or out, etc, there would be less total energy in the system the same way that every time that air tank let air out into the box, there was less difference in pressure.
Eventually there would be no more energy in the sealed system, all the chemical energy would be used up, all the molecules in their lowest energy state. All the temperature would be the same. All the air pressure would be the same. All the potential kinetic energy of things moving or falling, would drop to 0 and the system would be perfectly ordered, and very dead.
There is a finite amount of things in the universe, a finite amount of potential energy in the universe, and that energy is constantly being converted into work, or into other forms of energy, and every time it does, there is less remaining energy in the universe. The number never goes up, only down. It will take hundreds of billions of years for the universe to use all of that energy up, but eventually everywhere in the universe will be the same temperature, the same pressure. There will be no more potential energy to be used. The universe will be very dark, very still, and very dead. This is entropy; the heat death of the universe.
Top comment is written by Johnathan Astrophysicist or something so here’s the real ELI5
So you have a bottle of soda, but it’s room temperature. So you pour it in a glass and add some ice cubes.
The iced soda is like our universe if it’s in a closed system. Well, a closed system would be more like a vacuum flask, but stay with me. It’s like the universe because there are parts with more energy, like the warm soda, and parts with less energy, like the ice. In the universe, this could be like the Sun and Pluto instead.
So ice melts, yeah? This is because the energy in the soda that’s keeping it at room temperature is blasting the ice. Energy always wants to move to places with low energy. That’s why an open window renders your heating useless. It’s all going out the window into the low energy outside. Again, this is like our universe. You give off body heat, which is energy escaping your body to reach the low energy air around you. The Sun does the same thing into space basically, as does basically everything.
So what happens when the ice is all the way melted? The soda is still probably a little cold, but there’s a bunch of water in it and it sucks. That’s because the soda energy energized the ice, which turns it into water. That’s kind of like the heat death of the universe. Once everything that has energy disperses it across the universe equally, the universe will be too cold for life as we know it to exist and the rest of eternity will be inhabited by a bunch of corpses. Grim stuff.
E: Also important is that entropy is all about energy in a closed system. In the cup analogy energy from the room will still heat the shitty soda to room temp so it’s not perfect.
Entropy is the inevitable mixing up of your individual lego sets into one huge disorganized box that is no fun for anyone
Think of it as having a nice clean bedroom. You’ve picked up all the clothes and put them away or washed them and folded them. Your floor is vacuumed, pictures dusted, so on and so forth. As we all know, it never stays like that. Eventually your clothes end up on the floor again, you drawers need to be resorted, things need to be dusted and cleaned. This is essentially you fighting against entropy. It takes energy to clean your room, if you don’t spend that energy to clean your room, it eventually will look like a mess (ie chaos or disorder).
Things want to return to a state of least energy (if we personify them this way). Which makes sense. If you take your shirt off and just throw it in your room it won’t neatly fold itself and go into your drawer. It will just fall to the ground in whatever crumped mess you have tossed it in.
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