With no immortality, population growth is exponential. With everyone immortal, population growth is a slightly faster exponential. It's a Problem that has to be solved no matter what, in either case, or else things get Bad. In both cases, the minimum sufficient measure is artificially capping reproduction. With immortality, the cap (as measured in children per parent) is somewhat lower.

Exponential growth is okay for awhile, but there will always come a point when it's not. There are hard limits imposed on our expansion by the speed of light, and they are only cubic, not exponential.

This is a big complex issue, which in my opinion mostly boils down to the fact that the hypothesis that immortality will cause overpopulation shouldn't be privileged over the hypothesis that it will lead to better population control. Some points to consider:

• Wanting to have kids (to the extent of being willing to cause environmental catastrophe by so doing) is plausibly in part related to the unfulfilled desire to live longer. Reproduction allows one to have an impact on human society which is meaningful over long time scales. Indefinite life extension fulfills this need without increasing the number of individuals.
• Having more than one surviving child per adult on average causes exponential growth problems even if immortality goes unsolved. The solution to this is birth control. The automated natural death cycle, aside from its inhumane aspects, is also arguably a fake solution that deludes people into thinking that contraception and conscious family planning is unnecessary.
• Humans die of accidents and suicide every so often to begin with. Thus there will be new space made for a small number of families to have new children. It will simply not be a major part of the overall culture.
• Space is big enough to mean there is a lot of time to solve the problem if we limit ourselves to natural biological reproduction. Biotech that allows gigantic family sizes with little cost to the parents (e.g. food factories and artificial wombs) is not the result of immortality but of general advancements in biotechnology.
• An engineered lifecycle wherein an individual returns to childhood after a period of elderhood is a plausible substitute for the existing status quo of all children being newly created as blank slates. Children could thus exist, but with a rich set of memories and experiences to draw upon and integrate into their lives.
• The desire to reproduce is largely a cultural artifact, as opposed to an innate drive. To the extent that it is an innate drive, it may be fulfilled by raising any kind of infant-like creature -- including reborn adults, simulated children, and animals.
• Sometimes it is acceptable to focus on near-term problems at the expense of the extremely long-term. All forms of life-saving human technology arguably contribute to human overpopulation, but given the severity of death it is a worthwhile expenditure. We would rather have a large population than have millions die of hunger or disease -- why should aging be different?
• Individuals moving to a silicon substrate would most likely not consume anywhere near the resources that natural humans do.
• Individuals could also choose to move their brain and central nervous system to a low energy-cost nourishment tank, and use robotic bodies that cost far less energy than a natural human body.
• Groups could plausibly merge into single individuals, by forming hive-minds, effectively reducing the total population.

Raemon:

If death is solved via uploads running at high speed, I'm not too concerned. (Still a little concerned, since computers still take up space, but the issue is close enough to negligible that I'm fine ignoring it).

Have you read Robin Hanson's writings on the economics of uploads? He argues very convincingly that uploads will inevitably lead to a Malthusian equilibrium, in fact much more rapidly than a biological immortality scenario.

The sense of I've gotten around here is that "exponential growth is okay, because Space is Big".

It can't possibly be okay, since sustained exponential growth must eventually catch up with the expansion of our light cone, which is (barring faster than light travel) the fundamental physical limit on how much space can be available. Exponential growth always dominates polynomial growth given enough time, and I'm sure most people here are well aware of that, so you probably got a wrong impression.

If we're dealing with physical humans taking up physical space requiring physical resources, then I'm worried. Either we're growing exponentially, or we've eliminated childbirth, or we have strict rules in place about people who have children being willing to die. (The latter might work but modifying central aspects of the human life cycle that we are hard-wired to value seems.... challenging, to say the least)

This is not true. As long as the average total number of children per person stays below 2 (and every child has at least 2 parents) population growth will be sub-linear and approach a finite limit. For example if the average is 1.99 population will never exceed 200 times the starting population, and may take quite a long time to reach that level if there is a strong social expectation of waiting with children until you have attained the necessary maturity to become parents, well after your tenth millennium. I don't think limiting people to just two children is excessively draconian.

Whenever I hear people refer to death as a tool that we can't toss aside until other problems have been fixed I want to kill them, personally, and ask them if that's what they had in mind. Well, alright, that's an exaggeration, but I want to point out that they are thinking of death as some far-away, abstract thing that happens to OTHER PEOPLE that they don't care about.

Scenario: You will die later today. Look at the clock. Before it hits midnight you will perish. Now stop looking at the clock, you're burning precious seconds.

But wait! There's a way out - yes, you already guessed it, you can cure death completely for everyone! Is it now so terribly important that all those Big Problems that you just had to hold on to Death to address be fixed before death is cured? Are you willing to die within hours because we need Death so badly to keep our Big Problems in check? Or are you ok with releasing the cure for death so you can continue breathing and get around to fixing those problems over the next few years/decades?

If you aren't willing to kill yourself today to keep the Big Problems under control then you cannot ask others to die for that reason.

it seems to me that getting rid of death BEFORE we eliminate all the other major problems facing humanity is going to make those problems much worse. Death is an enemy that should be vanquished, but it's not an enemy I'm prepared to destroy until I'm satisfied that we'll be able to handle the consequences.

I think perhaps you don't realize what "getting rid of death" means. Humanity grew up in the shadow of death, and lives every day in the shadow of death. The whole world population of 1880, and every earlier year, is completely dead. Getting rid of death means that the end never comes, for anyone, beyond a certain point in time. Rejuvenation doesn't get rid of death, it just means you're still young when you do finally die. Cryonics, uploading, and copying don't get rid of death - in fact copying multiplies death - if you live on through your copies, that means you will die many times. And physics says there will be a last time, even in that scenario.

Also, you don't realize the enormity of the mass psychological inertia here. There is no popular will to "get rid of death". If you make that your goal, you will find yourself in a desert, bereft of support and understanding. Humans can be classified into those who don't yet understand the idea of death, those who are resigned to it, and those who believe in a metaphysical safety net. The people who see death as something which the human race could choose to abolish by an appropriate effort are a microscopic minority. Since the first cryonic suspension, over a billion people died, and less than a thousand were frozen; that tells you how small a minority.

The next tipping point will come when there's popular interest in rejuvenation, that is, in getting rid of "ageing". That is a far more doable thing, than getting rid of death per se, and it has an obvious appeal for anyone who is, in fact, ageing. Again, it's only strange minorities who want to take a one-way liquid-nitrogen ticket to an unknown future, or who think of getting their brains scanned so they can live on in cyberspace. But simply staying young would appeal to a lot of people - I won't say everybody, or even a majority, because apparently you can't underestimate the human will to find its existing situation satisfactory and not in need of change.

But returning to your desire to solve "all the other major problems facing humanity" first... I am going to criticize this attitude as radically out of touch with reality in multiple ways. If it helps, I had similar thoughts once. I didn't want to do the "other major problems" first, but I wanted to do them at the same time.

So, let's see what's wrong with this picture. First: what makes you think that you get a say on any of this - either the course of world events, or the rate at which the human race tries to overcome its biological limitations? Are you in a position, of influence, wealth, knowledge, ability, to make a difference to anything much outside your own life? Or are you just an atom of will slowly realizing that it was born into a universe completely at odds with the sort of reality it would like to be inhabiting, an atom of will which is able to express this preference verbally, but which is not able to act on it in any way?

That is the harshest thing that needs saying. Most people who start young with big dreams more or less completely abandon them, because it's hard to make them actually happen. A few people persist at the price of everything else, perhaps without even realizing that this is what they are doing, or understanding why life seems to be so difficult; I'd place my younger self in this category. It would be an extremely rare and fortunate person who neither compromises their ideals nor suffers because of this. The world around you is waiting for you to abandon any idea of immortality or a better world, in order to focus on more achievable things like making money and having a relationship, and it is set up to support this sort of activity, not the other sort.

Next, let's consider what's implied by solving those "other major problems". By the sound of it, here you mostly mean "unsustainability" - overpopulation, maybe resources running out - though you could also have in mind poverty, war, and who knows what else.

Just to be realistic, let's remember that these problems have been around for the whole of human history. Saying that before you get rid of X (in this case, death), you want to get rid of Y first, where Y is at least half a dozen things that have existed forever despite the intention of numerous of your predecessors to get rid of them - that sounds like a formula for never getting around to dealing with X.

Also, the human race does not inhabit one single rational polity run by philosopher-kings. It consists of about 200 sovereign states whose governments are already always in crisis management mode, because they're dealing with insurgencies, culture wars, the exigencies of the global market, and so on. Here's a tip: do you want to know how fanatical movements get created, the sort that wage wars, demonize opponents, and so on? One way they get created is that someone decides to solve all the world's problems at once, and that becomes such an overwhelmingly positive outcome that people become desperately willing to do anything in order to make it possible. The world is so far beyond the control of any individual atom of will, or even any political coalition of them, that those who insist on having an impact at the highest levels of the historical process generally have to adopt the most extreme available measures.

Another problem is that technological development is not going to wait for you to try to achieve a perhaps impossible planetary "equilibrium" in which distressing things are no longer happening in any corner of the world. The flip side of this is that peak oil, new diseases, and terrorists with WMDs are not going to stop a singularity from happening. Despite what the myths of planetary Internet mind-union say, the world does not stand together or fall together, and it especially does not advance together. The world could be having a major eco-economic crisis, but any sufficiently advanced country can at any time pull a North Korea, put up a wall, and retire from the globalization game without sacrificing its technological culture. Two-thirds of the world could go Mad Max, and the progress of the final third would still be far more than enough to drag us all into the AI/nanotech zone. So if you do choose to maintain an interest in ultra-high-tech post-human possibilities, get used to doing this even while the slums on the other side of town are on fire, metaphorically speaking. You can always choose to be a firefighter instead, but that really does mean you will no longer be very important in solving those more futuristic problems.

I had some other things to say but this is already more than enough.

I don't know how much this reflects the site consensus, but I have two responses to the overpopulation argument.

Coarse response: What's the problem with overpopulation? It's that resources get used up and the world can't support everybody. And what's the problem with that? People die. Eliminating death by aging with the result that some starve may still result in less total death.

Sophisticated response: As education improves and wealth increases, fertility rates fall. Much of the first world has less than replacement birth rate; US population would be shrinking were it not for immigration. World population is expected to peak around 9 billion around 2050 (source: The Economist Magazine). So as standard of living increases, population growth may slow to the point that it won't be too hard to legislate limits on reproduction.

Overpopulation is last millennium's meme. The birth rate in most regions of the world has now crashed below extinction level, and the remainder look likely to follow as soon as they develop their economies. Frankly, apart from the compelling moral and humanitarian reasons for opposing death, from the most dispassionate 'good of the species as a whole' perspective, a major reason we need to do so is as a stopgap against the ongoing population decline.