What is entropy? Also, can anyone explain how it relates to matter, gravity, time, other physics terms?
In: Physics
Have you ever dropped a glass and broken it? Probably more than once right? Have you ever seen glass shards come together and form a glass by themselves? Entropy is tool we use to know what states matter can take on from a given starting state, it always moves from lower entropy to higher entropy. The glass can shatter, but it can’t unshatter.
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Also….the gradual decline of anything organized into disorder.
As when a child cleans his room, everything eventually falls out of place and becomes a mess again.
Another way to put it, more like eli5 is that all systems need energy to keep functioning. For example, to keep the temperature stable in a room you need to put energy into the system (the ac system). To keep your computer running you need electricity, to keep your car going you need fuel. Buildings need maintenance to stay up (or else they will crumble), living things need to eat… If you don’t put energy into a system, the system will eventually stop… Think a star: it just shine but no energy is going in so eventually stars die.
As energy is not infinite in the universe, eventually all energy will be spent, meaning the amount of energy in every point of the universe will be exactly the same as in every other point… The universe will be homogeneous and, as the previous person wrote, the temperature will be the same everywhere. There will no life, no movement, nothing.
Entropy is a measure of how unevenly energy is spread out or distributed through a system. This is what they mean by ‘disorder’ which is a confusing term but think of it if you have nice neat stacks of energy over here and here such as a warm spot on the floor in an otherwise cold room, then the room is “low entropy”. But if it was all one temperature that’s “maximum entropy”. well kinda…
However what the Second Law of Thermodynamics describes is that over time, in a Closed System, the entropy will become total, which will mean it will be evenly distributed through the whole system. It’s not in nice neat stacks, it’s shattered into millions of pieces and evenly placed everywhere. A Closed System is a theoretical system where no energy can get in or out,
the Universe as a whole is the only known Closed System, everything else is an ‘open system’ – for example: the earth receives heat from the sun, the sun loses heat out into the solar system etc they’re all open. It’s believed that given enough time the universe will experience ‘heat death’ where there energy will be evenly distributed and thus reach maximum entropy.
I think Feynman described it like a coffee cup, if you shatter it into lots of little pieces then you can’t use the coffee cup anymore. But when it’s neatly in the shape of a coffee cup, you can use it. That’s sort of like energy and entropy I guess. A evenly warm or cold room is the broken cup.
Now where things get confusing is that in the early 20th century a Bell Labs engineer Claude Shannon figured out this amazing theory and equation for working out how much information you could send over a channel without losing any information. He showed it to one of the smartest men in the world, Johann Von Neumann who told him that the equation was very similar to the equation to calculate entropy, and since most people didn’t really understand entropy, he told Shannon to call what he discovered ‘Entropy’ too because it would save him a lot of questions. This is really useful in Artificial Intelligence, in Cryptography, in radio signals… like there’s so many modern applications for Shannon’s “Information Theory” which basically says that you have a receiver, a channel of which there is a certain amount of ‘noise’, and a decoder. Shannon develop equations that tell you the maximum amount of information you can send on that channel before it gets fouled up by the noise.
Shannon Entropy is this counter intuitive property where the less probably an information event is, the more information it contains. For example, if you have a worker who for the last 400 work days has come to work on time at 8am, and it’s 12pm and he’s still not at work – something strange has happened. That’s Shannon Entropy – because his absence or breaking the pattern tells you more than if he arrived as per ussual.
Now this is where the ‘disorder’ thing becomes less confusing because if you have a lot of random events with no patterns, then you approach maximal entropy for a channel. As opposed to nice, neat, consistent patterns – like turning up for work everyday – which is low entropy. So you can kind of backwards apply this to Thermodynamics: Entropy is about the pattern (or lack thereof) something.
Think of a neat room, now think about what toddler would do to it. It’s easier to go to disorder, then order.
Imagine you have a pot of water at 0 energy units that you want to heat up to 10 energy units. You have a stove that can produce 8 energy units per minute. Theoretically, in a perfect system, it should take 1.25 minutes to heat the pot of water to your target energy level, right?
However, the pot itself is also at 0 energy units and wants to be the same as the water, so some of the energy produced by the stove will go into the pot, which means that the water takes longer than 1.25 minutes to reach the target energy level, the difference in time is measured and converted to energy “lost” to the pot. That “lost” energy is entropy.
Now we make it a little more complicated. Let’s say the air around the pot is also at 0 energy units. Although there is a finite amount of air, the difference between the amount of pot, water, and air is more-or-less infinite. The air also wants to be the same as the pot and the water, so a lot of the energy of the stove goes to the air, but no matter how much energy you pour into the air, it would take an infinite amount of time to make an infinite amount of “stuff” hold an infinite amount of energy (a heat sink). But the pot and the water still gain energy because they hold onto the energy better than the air does. The energy that goes into the air is also entropy.
Finally, to make things more interesting, the outlet in your house that the stove is plugged into is actually putting out 10 energy units per minute, but the wire connecting the outlet and the stove can’t transfer the energy perfectly, kind of like transferring water from a tap to a bucket through a cardboard tube: at first the tube holds up, but eventually the water seeps through the layers of paper and drips out along the way. So while every minute the stove is losing energy to the air and the pot, the energy from the outlet is lost through the wire. This is also entropy.
The important thing to remember is the First Law of Thermodynamics, energy can be neither created nor destroyed. So this energy still exists, somewhere, entropy is all relative to the boundary that you use to define your system. If your boundary is around the pot and the water, you’re gaining energy; if the boundary is expanded to include the stove, you’re losing energy.
Think of entopy as the fuel in your car, everything, atoms, particles matter runs on fuel, so if your fuel tank has enough to take you 100 kilometers, then that’s it, no more, same as atoms, every time the electron does a revolution it uses fuel and expels a tiny spark, one day it will run down, reach highest entopy, no more fuel, no more heat, no more anything, the end.
Lots of good answers here, so I want to add to those by saying: we don’t really know what entropy is all that well. Relative to other thermodynamic quantities (free energy, enthalpy, etc) entropy is not well understood. So, while many of the explanations given in this thread are good, they are just explanations of what we do understand about it. The amount we don’t know about entropy is significant.
A big reason for this is that thermodynamics was developed with equilibrium assumptions. However, many process are out of equilibrium. In out of equilibrium systems, entropy is much more relevant. Non-equilibrium thermodynamics is actually a relatively new branch of study.
As an analogy, if you put an ice cube in a cup, it melts into water and then nothing else happens after that. Nothing can happen because it’s the same temperature everywhere. It’s entropy has increased to the maximum. There is no gradient for energy to transfer.
The universe is the same way except on a much larger scale. Energy is always spreading. It has to spread in order for anything to happen. As energy spreads out in a system we say its entropy increases.
Way into the future there will be no energy left to make new stars, black holes will evaporate, atoms and protons and neutrons will decay and time will not exist. It will be the same temperature everywhere and nothing will be able to happen. Maximum entropy, the heat death of the universe.
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