Be careful here - the economist's position about a finite world was something the physicist picked. The economist agreed to it early on, probably to try and seem agreeable, but it's not a reasonable concession to make. Space exists, we put things there today, presumably will put more things there in future. So there is no finite world.

But let's make the same assumption for a moment. For economic growth [of wealth] to stop requires two things to stop: population growth and productivity growth. As GDP is roughly population * productivity.

Clearly, populations can stop growing or shrink. We can also assume a finite population limit. So this is an argument that productivity growth is also finite. But, why should that be the case? Take the example of computers. Modern computers are much more productivity enhancing than older computers, but they are also smaller (i.e. less physical resources needed) and more energy efficient. Even if the Earth had reached carrying capacity, smarter chips would continue to be designed and the productivity boost of computers would keep increasing. That's just one example, there are many others.

But again, the finite world assumption doesn't hold. So the whole debate is a bit of ivory tower silliness anyway.

1) Space doesn't help you, it only postpones the inevitable. Eventually the ravenous maw of your exponentially growing civilization will run out of stars to harvest, trapped as it is inside a light bubble growing at 1c, its volume expanding with mere cubic growth.

2) You've just relabeled the discussion from "wealth" to "productivity". In any case, scoring some efficiency wins here and there is once again a game with physics-imposed limits.

I repeat my question: when all humans are fully fed, housed, maximally entertained and satiated, every dopamine receptor firing on all cylinders - and you can't make more humans - what exactly is there left to grow? At all, let alone exponentially?

I confess I find the notion of infinite exponential growth of anything, let alone things humans value, so patently and obviously physically unsustainable that I am deeply perturbed by these earnest efforts to defend it. I engage in the spirit one might engage Flat Eartherism, as an intellectual exercise in probing the manifestly absurd - except that this dogma apparently pervades mainstream economic thought, an observation that should terrify anyone who wants civilization to survive.

Yes, very good. If we go hard sci-fi and talk about actually running out of stars in the universe, then I can just counter with some claim that by then we'll all have become inter-dimensional beings, or we'll never exhaust the stars before the next big bang, or that we'll discover that the universe is actually infinite somehow.

When the pair of people in the article were talking about space they meant within the realm of reasonable projection. If your rebuttal to economics is that the field hasn't considered what happens when humanity runs out of galaxies to colonize they'll just chuckle and happily concede the point, because if that's your strongest criticism then for all practical purposes they've won (people will still listen to them because the point you're making is irrelevant in practice).

That's especially true if you make absurd arguments like the survival of the world being at stake because economists don't constantly caveat all their papers with footnotes saying it doesn't apply if humanity reaches the end of the universe. That's itself incredibly silly flat-earther levels of dialogue.

> You've just relabeled the discussion from "wealth" to "productivity".

No I haven't, but this whole thread is sort of proving the point that the term wealth is quite confusing to a lot of people.

We've already seen the economic definition of wealth. How can we obtain more wealth? Because wealth is just everything we do for each other summed up, the most obvious way is to add more people. But people don't work alone, we can increase our output by deploying technology and ideas. Economists call the output scaling of a person "productivity" and it's more or less a function of how much tech that person has access to. A farmer with a tractor is more productive than one with access to only a hoe. Culture and ideas are also a factor but it's mostly tech.

So wealth and productivity (and population size) are deeply related. One is the multiple of the other two:

Wealth = population x productivity

Assume a stable population. Then you are asserting there are limits to productivity due to physics. That may be so, but again, if those limits are hit in 50,000 years then nobody cares.

Oh good, you finally agree that there are, in principle, physical limits to infinite exponential growth. I was worried we'd never get there. Now we can start haggling over what those limits are.

> When the pair of people in the article were talking about space they meant within the realm of reasonable projection

Oh good. In that case the reasonable projection is that we'll never leave the solar system, probably barely leave the Earth. You were the one making wild predictions about sustaining infinite growth by colonizing the cosmos, not me.

>That may be so, but again, if those limits are hit in 50,000 years then nobody cares.

50,000? You fail to appreciate the scale of exponential growth. The whole point of the original article about tides, and the discussion between economist and the physicist, and for that matter this entire comment thread, is that exponential growth hits physical limits really quickly - hundreds of years, not tens of thousands.

You keep hopping between "unlimited physical growth is possible because SPACE" and "actually growth doesn't need to be physical to count". I hope we've knocked the first one down. As for the second, I ask for the third and hopefully final time - when everyone's fed and happy and jacked into the Matrix, what's left to grow? What even is "productivity" in that scenario?

> there are, in principle, physical limits to infinite exponential growth

You're making a different argument now, probably without realizing. The original blog post starts by saying:

> I said to him, “economic growth cannot continue indefinitely,” just to see where things would go.

Re-reading the blog post I see that it's very confusing, because the physicist isn't retelling the story clearly and probably not thinking about his argument clearly. He uses the word "exponential" in the title of the blog post, but then his recounting of how the discussion begins doesn't involve that. The first time it comes up in his retelling, it's when the physicist starts conflating wealth with energy. So whilst you are talking about exponents they appear to be irrelevant to the argument.

Phrased another way, for as long as humanity can somehow improve its collective situation in some way the economy can grow.

But even if the economist had tried to defend unlimited exponential growth, it would only affect the time window within which economic claims are valid. Remember that we're talking about wealth, which isn't dependent on physical limits. There's no theoretical upper limit to wealth just like there's no theoretical upper limit to how big numbers can get. Or rather, to argue that there is a limit, you have to start making complex and highly speculative arguments about hypothetical limits to human ingenuity, potential and preferences which is why the counter-argument goes in the direction of talking about the Matrix.

Let's put that to one side. Just like the physics guy in the article you clearly think you're winning some meaningful points here, but you aren't. Like most fields of study, economics is the study of worlds that roughly resemble our own. This is so obvious that it doesn't normally need to be stated explicitly. Perhaps it's not obvious to people who research physics due to the unusually universal nature of physical claims. So the statements it makes are grounded in a set of axioms and contexts, one of which is when they say there are no limits to wealth they mean in practice, for a world and society somewhat like this one.

That's why to argue against this point you have to keep bringing up wholly theoretical societies, where we reached the limits of the universe or became brains-in-jars. If you want to talk about the economics of the Matrix then great! Go for it! It will make for intriguing science fiction, but not more. It is A-OK if our current definitions of terms like wealth and growth fail for hard sci-fi worlds.