Updated: Mar 12, 2021
With our current understanding of thermodynamics heat death seems to be an unavoidable outcome of our universe given enough time. That is because it is built into our understanding of time. We all experience time giving us a fairly good understanding of how sequential events take place in our day to day lives. Time always goes forward, today came after yesterday, and tomorrow will come next. There’s no way to go back to yesterday (without some scifi time travel Magic) it will always be in the past but tomorrow and the future we just have to wait and we will be there.
[Boulder's Court House then and now the old one burned down in 1932]
Physicists call this the “arrow of time” time. Time always marches forward and therefore the arrow of time always points that direction. This makes sense to our day to day life and plans but, scientifically this arrow of time is only quantified in one equation. Looking at all of our physics equations there is only one that gives us this forward direction of time and that is the second law of thermodynamics. There are 4 laws of thermodynamics and this is how I best remembered them before my thermo test:
0 - There is a game (The ‘zeroth’ law of thermodynamics basically defines equilibrium as everything being at the same temperature. It was added later because it was seen as an oversight to assume everyone knew what ‘equilibrium’ means)
1 - You can't win. (The first law of thermodynamics says energy is always conserved “you can’t win” meaning you always get out what you put in. You cannot have a system, like a perpetual motion machine, that makes more energy than you put in)
2 - You can't break even. (The second law of thermodynamics being what we are interested in most for the purposes of this essay says that ‘entropy’ always increases. There is no large scale processes that you truly get the same amount of energy that you put in you always lose a little to friction/heat/other forms of energy)
3- Everyone must play the game (The third law of thermodynamics basically is defining the coldest something can be ‘absolute zero’ is the absence of usable energy. Luke the 0th this law was added later because it was seen as needed to define absolute zero)
So let's look at an ideal example to explore the 2nd law; if you analyze the motion of a pendulum going back and forth you start with it raised to one side. This position holds energy as a raised object will fall (if there is gravity), this is called potential energy. As it falls that potential energy is converted to speed, this speed is called kinetic energy, once the pendulum is at the bottom of its swing it is traveling the fastest speed, having the maximum kinetic energy of this swing. Then the pendulum starts trading out this kinetic energy back to potential energy as it climbs the other side of its swing, until it loses all of its kinetic energy to potential energy and begins to swing back in the opposite direction starting the process over again.
This example is a nearly perfect example of the 1st law as the potential energy you put in when you raise the pendulum is the same you get back when it swings back. The pendulum will never swing higher than it started without an extra push from another source of energy. However the second law limits this even further as you will always lose some energy to friction at the pivot of the pendulum or resistance from the air it’s swinging through. This is why you have to wind up old grandfather clocks every so often, this is actually what old clocks (and everything else) use to measure the passing of time.
Let's look at a more obvious example; a log burning in a fire pit. Burning the log you are releasing chemical energy trapped inside the molecules that make up the log turning that energy into heat energy. Looking at three frames of a film of a log burning: one with a log in a fire pit, one of a pile of ash and one with the log on fire, it’s simple for us to put these in order and say first the log was placed in the pit, then set on fire, then burned to a pile of ash (ore just the three pictures of the Boulder Court House at the start of this blog). That comes easy with human experience of seeing fires, but the only way to prove that was the order of things using physics is using the second law of thermodynamics. Entropy is like another word for chaos, disorder, or mess, seeing all of the energy piled neatly in the chemical bonds within the log is very ordered having some of that energy making light of the flame and some of it making heat of the flame is much more disordered.
It‘s impossible to reverse these processes to take the heat and light made by the flame and use it to make the ash back to a whole log again. It would be a miracle if you saw this go the opposite way. The only way the tree was able to make the log in the first place is by siphoning off a little bit of the energy released by the nuclear explosions taking place in the sun (which is of course releasing a lot more energy than what is captured as chemical energy in the log) . Every step there is a little loss that makes it non reversible. As time marches forward everything becomes more unorganized. You cannot clean/organize something without making something else dirty/unorganized, you may wipe down a surface with a cloth but then the cloth must be cleaned in water which is then cleaned by a sewage system, which needs power, usually from burning coal, which makes a mess and so on and so on....
Heat is a good way to see how unorganized a system is. In your kitchen you likely have a freezer and an oven. Both of these appliances use energy (let’s say they are both powered by electricity) to keep that space cold or hot. If you were to leave both of these appliances off for a few days eventually the temperature in the oven will be the same as in the fridge heat tends to balance out. The only thing stopping this is the electricity which is pumping energy into the system to make these two locations very different temperatures. Normally electricity is made at a power plant where they use coal/natural gas to heat water into steam that then powers a turbine. Making power this way only works because having something at higher temperature than its surroundings do work to lose that excess heat.
If we are to go along with the arrow of time indefinitely eventually energy will spread out so evenly there would be no way to do any meaningful work. Once the last sun burns out there won’t be anything to drive wind on any planet or photosynthesise to make swamps which eventually turn into coal. Everything will slowly come to a stand still and the temperature of the universe will even out like our kitchen when we cut the power.
It would be impossible for anything to live through this outcome of the universe. All life needs to use energy to survive and once all energy becomes unusable life becomes impossible. Further more time would stop. As entropy would have hit its maximum there would no longer be a way for entropy to increase meaning there would be no way of keeping time or seeing the direction of time.
Now there is a little outside chance of hope. The second law of thermodynamics is a statistical law, which makes it less of a hard ‘law’ as other laws of physics are. A more nuanced way of state the second law of thermodynamics is things TEND towards disorder. If you were to take a dump truck full of bricks and dump all of the bricks off of the truck at once you would expect those bricks to land in an unorganized pile not in a neet stack, but one could argue if you did this trillions of times you’re bound to every once and a while have all the bricks miraculously fall in an organized pile at least once. Or like the saying if you have enough monkeys bashing on typewriters for long enough one of them is bound to eventually reproduce the works of shakespeare. Simmerarly heat fluctuations will spontaneously pop up. Hotter (faster moving molecules) statistically are more likely to spread out and even out but there is a very slight probability as particles are bouncing randomly around the room they will randomly fall into an organized state of all the hotter particles on one side and all the cold ones on the other side. If you were to leave your kitchen with the oven and freezer unplugged for a limitless amount of time eventually the particles will randomly bump around to a place where all of the cold ones are in your freezer and all the hot ones are in your oven. The likeliness of this is very close to zero but not zero. One could conceive of future past-heat-death life having to track these random fluctuations of low entropy in the universe and harvest them for usable energy to continue.
Furthermore there is an indication that the second law of thermodynamics can be broken. One of the most outstanding physicists to have ever lived, James Clerk Maxwell came up with a thought experiment which seems to obey every law of physics except the 2nd law of thermo and reverse the increase of entropy. This thought experiment is called “Maxwell’s demon” and many physicists have tried to make an argument as to why this cannot exist and many have also tried to prove it can exist by making one. Neither side has undeniably proven their case and it is still a hot topic of debate in physics.