Life by definition processes energy, and conservation of energy and the laws of thermodynamics require that this process generate waste heat. We call it "body heat" and it is the reason that we are warmer than our environments. Even "cold-blooded" animals like reptiles are warmer than their surroundings (on average) because they eat things, covert the caloric content into energy, use that energy to do work, and then radiate that energy away as heat.
We talk about "using energy," but that does not mean that the energy is consumed. If you run a computer off of a set of solar panels, then the system you are using takes energy from the Sun (which has an "effective temperature" of about 5,800 Kelvins (or about 5,500 Celcius = 10,000 degrees F) and using it to move electrons around your processor and create photons and sound waves for the monitor and speaker. Because this is done in semiconductors and not superconductors, work is done and the resistance offered by the hardware converts the useful energy the solar panels started with into ordinary heat. Your computer's temperature then goes up. This heat must leave the system or your computer will melt: a fan carries it away mechanically, and if you looked at your computer with mid-infrared goggles it would be much brighter than its surroundings because it is giving off energy in the form of infrared radiation at a rate higher than the table it sits upon (you can feel this with your hand held just beside the computer's surface).
Waste heat has less ability to do work than some other forms of energy (we say it has less "free energy", that is, energy available to do useful work in a given environment). We have to get rid of it to stay cool. Anything that does work -- computers, cars, people, sharks -- must give off waste heat. Our civilization as a whole gives off 500 exajoules of waste heat every year -- 0.01% more than the planet would if we were not here.
Life by definition uses energy, and also by definition it reproduces (in general; mules are alive, but if they were the only equines on Earth they would not last long as a species). Given all of the necessary resources, reproduction is an exponential process and so a species' population will grow exponentially until some resource is exhausted: the mold reaches the edge of the dish; the locusts eat all of the crops. A major resource limitation is the surface of the Earth: life as a whole has covered almost every square inch of the planet, but hasn't had much luck exceeding that, because spaceships are hard to build.
But we can now build spaceships. What, then, limits us in 10, 100, or 10,000 generations' time? After land, we are not constrained by any other physical resource: there is plenty of water on Earth and in the Solar System, if we are willing to expend energy to extract it. We require vitamins, minerals, calories, and a few other dietary substances but we have gotten very good at collecting, manufacturing, farming, and otherwise acquiring these in unnatural ways (the Earth could not support 6 billion humans until modern agriculture was invented). If we had the surfaces (and even interiors) of other planets, or ever space stations, we could continue to grow our population. In terms of the atoms we require for survival, we have the entire mass of all of the planets to work with. That's a lot of carbon.
Now, this seems crazy and sci-fi and all of that, but let's look at it from a physics perspective: if we wanted to grow our population exponentially (or if we otherwise were forced to) what would ultimately limit our species?
We can look to see which of our available resources we are already using up. I've argued that water and minerals aren't really a problem: we don't "consume" these because they are recycled and we can extract more into the cycle through the expenditure of energy. We could increase our access to surface area and raw materials dramatically by exploring other planets and by building massive networks of tunnels. The only thing that we truly consume and can run out of in a fixed amount of time is free energy, which is limited by the finite reserves in the Earth and the flux we get from the Sun. Our energy use is already 1/10,000 of all of the sunlight that strikes the Earth, and growing exponentially, and that doesn't even count the energy we use indirectly for things like agriculture and passive heating.
That's the answer: Humankind is the first species on Earth that is limited only by the free energy available
from the Sun in the Solar System, and ultimately by the free energy available in the Galaxy. As long as we can collect solar energy, we could use it to expand our population.
I'll explore whether this is plausible in future posts, but for now consider this: despite all of the advances in medicine, economics, and civilization generally, we are still growing as a species exponentially. We have long left Malthusian limits behind, and have shown that technology will progress faster than our resource demands. But no technology can give us more energy than hits the Earth until we start talking about spaceships (except for nuclear energy, which is an ultimately finite resource; fossil fuels are, ultimately, just stored energy from sunlight that hit plants millions of years ago, so, again, finite). We will continue to grow as a species until we hit a free energy resource limit, and then in order to further grow we will have to start collecting more solar energy. And we will have to start emitting that energy as waste heat. And that trend, too, will then have to proceed exponentially...
The consequences are interesting to contemplate. Stay tuned...
[Image credit: Wikipedia]
Update: Saurabh Jha, jumping ahead, asks why I say that the Sun is the source of the free energy we have available in the Solar System; can't we generate our own energy from fusion, etc.? In principle, yes, but the reason I focus on the Sun is that there is more energy in sunlight over the course of a star's life than there is in available in the rest of the Solar system, even with perfect mass-to-energy conversion, so the Sun is the ultimately limiting factor (we'll need the Solar System mass for our tech and our bodies, anyway). But this point won't matter for where I'm going with this.