003 : The Heat Death of the Universe

Bleak Beginnings

The heat death of the universe is a postulated end to the universe as we know it. It is when a state of maximum disorder, or entropy, is reached; where no thermodynamic processes occur and time itself becomes meaningless. How does this happen? When will this happen? How did we get to this idea? First things first, what is a thermodynamic process?

A thermodynamic process is a physical interaction from an initial state to a final state of equilibrium, usually a transfer of heat in a system; pretty straight-forward. These processes are taking place around us all the time. Take a cup of tea for example, upon adding milk to the cup, there is a transfer of heat from the higher temperature water to the lower temperature milk. This transfer, left untouched, will continue until the system is in thermal equilibrium, that is, until the temperatures are the same.

These processes occur everywhere in nature. Our nearest star, the sun, is currently converting hydrogen to helium and producing huge amounts of energy (much in the form of heat) in the process. This naturally leads to some interesting questions: What will the universe look like when the last star dies? What about when the last black hole dissolves into non-existence? That is, what happens when all thermodynamic processes cease?

To examine this further, we need to come to grips with a few things like entropy, and the second law of thermodynamics.

Total Disorder

How did we get to this idea in the first place? It helps to begin with the second law of thermodynamics. One phrasing of this law tells us that heat travels from hotter places to colder ones, and that we can harness this process and perform “work”; that is, derive useful mechanical energy from the system. An equivalent phrasing tells us that In a closed system, disorder, or entropy, always increases. If we take the entire universe as our closed system, it is increasing in disorder all the time. What this really means is that we are in an inevitable march to homogeneity; that eventually everything will be equally disordered, and thus at the same temperature. In this state, there is no energy transfer, which means no processes can take place. If no processes can take place, then time itself doesn’t really make sense. This is a truly bizarre notion.

Scales of Evaporation

There are a few different types of physical objects we should look at to convince ourselves that this hypothesis seems reasonable. Let’s go back and think about the sun. As we mentioned, the sun is converting hydrogen into helium and emitting vast amounts of energy in the process. This part of the sun’s life is expected to continue for about five billion years or so until it exhausts its hydrogen supply. It will then begin to expand and become a red giant. After many millions of years it will slowly lose its outer layers and become a white dwarf. After yet even more billions of years the white dwarf will eventually flicker out and the star will die. While all stars don’t follow this exact life cycle, every star will eventually flicker out and die.

What about the other famous celestial objects; black holes? Black holes are currently theorised to evaporate over a period proportional to their size. This process takes time of the order of trillions of years via Hawking radiation. This is a process understood with quantum field theory whereby particle-antiparticle pairs at the edge of the black hole slowly dissolve it.

What about at the other end of the scale? Subatomic particles, our sturdy accountable constituents, are an interesting one. They themselves have life spans! Decomposing into flashes of gamma rays and flickering out of existence. The proton, for example, has a mean life span of about a thousand billion billion billion (1,000,000,000,000,000,000,000,000,000,000) years! To put that into scale, the universe is only about 14 billion years old. The Higgs boson however, has a mean life span of about a tenth of a billionth of a trillionth (0.0000000000000000000001) of a second!

So we’ve hopefully convinced ourselves that on three different scales the universe is pushing to homogeneity; the sub-atomic level of finite lived particles, the day-to-day scale of tea in a mug, and the astronomical scale of stars and black holes.

Hang on a Minute...

You may be thinking: “We cool things down all the time, does that not have an affect on all this?!” You’d be correct! In fact the process of cooling things down comes from the second law of thermodynamics from earlier. Like with all things, however, there’s a convenient catch. In this case the catch is that the net effect on a system which we’ve cooled down in a particular location is that the total heat given out overall is positive. In other words, to cool down a specific section of interest, we’ve had to heat up the larger system. You can convince yourself of this by carefully touching the outside of the back of your fridge. You’ll notice that it gets quite hot, but the contents of the fridge stay cool.

The Elusive Substance

There is an element to all this that we need a lot more research on though; dark matter. This is the seemingly invisible matter that makes up about a quarter of all the stuff in our universe. The problem for our discussion is that we don’t know what it is. We have some ideas about what it could be, but nothing concrete yet. Until we have experimental observations on this elusive substance, we can’t be sure of how it will contribute to our discussion on the timescales discussed here. So we’ll have to put that aside for now.

The All-Pervasive Substance

The other ginormous missing puzzle piece to all this is dark energy. Dark energy is the hypothesized energy that weaves through all of space-time. It is literally stretching our universe apart from the inside. Fortunately for us it is very small, and only really noticeable on astronomical scales. How this fits into our discussion here is that it helps the spreading out of energy in the universe, so over the timescales discussed, dark energy in fact assists in getting us to the heat death of the universe more quickly! But similar to dark matter, we don’t really understand dark energy as of yet, we have ideas on how to model it, but no mechanism is generally accepted. So again, just like dark matter, we’ll put this aside.

The Stage is Set

So now we’ve the tools to describe our main point. It seems as though the universe tends to disorder in the form of thermodynamic processes. What does this mean for the nature of time?

There are several ways to sensibly describe and talk about time. If something has changed in time, some process has acted upon the system. Processes (thermodynamic or otherwise) use energy. So if we’ve reached a thermodynamic equilibrium, there is nothing left to act upon any system and change it in any meaningful sense. At this stage of the universe’s life, it’s not unreasonable to say that time has stopped. It’s probably better and less sensationalist to say that time is no longer a useful concept with which to describe the universe.

A truly strange idea. But, you might ask, what if I was there in my space suit floating around. Wouldn’t I experience time? The problem there is that this is at such a late stage of the universe that all the particles of which you are composed have decayed into low energy background radiation. So in this hypothetical situation, where you are in fact floating there, you would experience time passing. However the situation is not likely. If everything else in the universe has decayed, you likely have as well. Sorry.

So at this stage of the universe there would only be a low background heat energy, like a universal soup that has cooled to fractions of a degree above absolute zero.

The universe is now dead.

The Good News

We could be wrong! This is just a (very) far ahead view of how the universe may develop based on what we think we know, what we know we don’t know, and what we don’t know we don’t know. Besides, the timescales here are mind-bogglingly unfathomable, of the order of trillions of trillions of years.

Conclusion

If we go back to our cup of tea analogy, after you’ve strained the tea bag and have added the milk, there’s the complex movement of the different liquids. Thermodynamics at work, full swing right before your eyes, the two liquids of different temperatures finding an equilibrium through random motions of the constituent compound particles. Currently, our universe is that cup of tea. Stars being born, solar systems forming, galaxies colliding. It’s all happening, swirling around. Let’s enjoy it while it lasts; which is loads of time! ▢