SPENCER: Ariel, welcome.

ARIEL: Thanks for having me on the show.

SPENCER: Why is it that humans typically live between, let's say, 70 and 95 years, not 130 years?

ARIEL: Yeah, so it seems pretty natural for humans to live, as you say, around 80 years or so. But if you take a step back and consider humans in the context of all the other animals that live, it starts to become a bit of an odd question. Why do humans live this long, while dogs live something like 10 to 15 years? And on the other end of the spectrum, you have some whales and some turtles living almost two centuries. What's going on with the differences in lifespan between different species? When I started looking into this question while doing research for my book The Future Loves You, what became apparent quickly is that it seems to be, for each species, there's a trade-off between how quickly it reaches reproductive age and how long it lives for. For example, if you're a giant whale that, before humans invented harpoons, nothing can kill, then it makes sense to slow down reproduction a bit and only start having kids when you're in your teenage years, if, in exchange, you get to live for centuries and keep having children throughout that period. On the other hand, if you're a tiny mouse, and other mice can kill you, rats can kill you, cats can kill you, snakes, everything, then you've got to reach reproductive age very quickly, or otherwise, there's no chance you'll be able to reproduce before you die. Across the animal kingdom, this sort of trade-off seems to be happening at an evolutionary level.

SPENCER: Why would it be the case that speed of reproductive age and longevity are linked? What's the fundamental trade-off there?

ARIEL: So it's true that in an ideal case, as far as evolution is concerned, you would have animals that were both immortal and could reproduce really quickly. That would be ideal. Then, there's no chance they ever get destroyed, and they just keep pumping out kids again and again. But the problem is that that is not actually a system that's possible to design, and there fundamentally are trade-offs between these two that need to occur when a biological system is trying to do its thing. Essentially, it comes down to the fact that all species — humans, dogs, cats, everything — have a limited set of genes inside those cells, and those genes present trade-offs between whether they can favor earlier development and reach reproduction earlier, or whether they can favor living longer, for example. To pick one example, you might think that whales, being about a thousand times the size of humans and a thousand times the mass of humans, would develop cancer at much higher rates than humans do, because they have so many more cells that can become cancerous. But in reality, they seem to actually have cancer rates that are about equivalent or maybe slightly lower than humans have. And that's because the genes that govern the risk of developing cancer or suppressing cancer in whales are calibrated more towards suppressing cancer. But in turn, that can't be done without reducing the speed of development, adding in checkpoints that essentially slow down growth. You see this happening across all sorts of different trade-offs. You might have another gene that helps reach something like ovulation faster that comes with the risk of earlier menopause later in life, or any sort of equivalent trade-offs in other species and even in humans as well, as there are some examples.

SPENCER: So it sounds like you're saying that on a very low level, there's some fundamental trade-off between growth and longevity.

ARIEL: Yeah, that's pretty much it. The phenomenon is called antagonistic pleiotropy — if I can use the technical term — where pleiotropy means that a gene affects multiple traits. For example, there's a gene, OCA2, which affects your skin color, your eye color, and also actually your sleep. This is the case with most genes; most genes have effects on multiple different traits within an organism. And what happens with respect to reaching reproductive age and lifespan is that these traits sometimes have to trade-off against each other. One of the most dramatic examples with humans that I've come across while I was doing research is there's a horrible human disease called Huntington's disease. It's a neurodegenerative disease that often affects people starting in their 40s and 50s and can lead to significant deterioration and death within 10, 15 years, or so. And it can affect people who are still within child-rearing age. You'd think that natural selection (evolution) would select against it and remove it from humanity over the course of our evolutionary history. But if you look at people who are at risk or who develop Huntington's disease across their lifespan, what researchers have found is that they seem to have reduced risk of cancer earlier on in life and perhaps higher fertility. It is really the case that these trade-offs seem to be happening naturally. To just give one final laboratory experiment to validate these ideas, there has been work where scientists have taken flies, relatively short-lived animals, and either artificially increased their predation rates — so their rates of being killed by the scientists — or they've artificially lowered them and made really safe environments for these flies. What you see is flies that are in these dangerous environments start to reproduce earlier, and the flies that are in these artificially safe environments start to reproduce at later ages but also have longer lifespans.

SPENCER: Does this tell us something about how human longevity could be extended?

ARIEL: In the very long run? Yes, in the sense that if you actually look at human lifespan across our evolutionary history, you do see an expansion. For example, if you compare us to our great ape cousins, such as chimpanzees and orangutans, you see that those animals typically live for something like 40 to 50 years compared to the typical human lifespans of 70, even 80 years now. What has happened over the course of history is that human lifespans have expanded relative to our cousins. And the reason for that, probably, is that we're just less likely to die during our earlier life in our environments. If you compare rates of death in hunter-gatherers versus rates of death for chimpanzees, you see that the rates of death for chimpanzees are much higher, and that probably was the case for our ancestors as well. What that means is that gradually, over time, as humans became smarter, more capable, and more social, deaths from violence, deaths from starvation, and deaths from these sources probably reduced somewhat, in a way that favored being able to reproduce at later ages and living for longer to improve our fitness that way.

SPENCER: I just want to clarify that, because it seems there are two different effects. Obviously, if you reduce the rate of death from injury, people will live longer on average, because you simply have one fewer way to die. But that's not what you're talking about. You're talking about how, if you have a whole system where there's less death at earlier ages, let's say from injury, it enables the species to invest more and potentially have genes that let them live longer. Is that what you're saying?

ARIEL: Yeah, that's exactly right. So it's not just that the mortality — things that are killing the animals — are removed. Of course, if there are fewer external sources of death, then the population will, on average, live longer. But I mean specifically that if that source of death for animals in a species is removed, there will actually be evolutionary pressure on the genes of those animals to start enabling them to live longer and to be able to reproduce at later ages. So it's not just the direct effect. You were asking with respect to human lifespans. I guess the thing I would point out is that while that's expanded — the typical human lifespan since we diverged from our great ape cousins maybe 7 million years ago or so — what's really dramatically different for humans these days, in the past couple of hundred years, is how much we've reduced death from disease, primarily, and to a certain degree also death from homicide and violence and other sources. The problem is that evolutionary changes to our genes take hundreds of generations, thousands of years. And as far as our genes are concerned, we're still back on the ancestral savannah. So it's true that if we could really keep up what we have now for millennia, our genes would probably slowly start to change to favor longer lifespans. But that's a very slow effect compared to the natural time courses over which this happens.

SPENCER: Do you know if there's any knowledge of how gene expression affects this and whether drugs could actually change gene expression without changing our genes to tilt us more towards higher longevity?

ARIEL: I should caveat here that I'm not a geneticist, and I'm not on top of gene expression changes. I sort of come at this from a higher level. I guess where a little bit of my skepticism of maybe being able to do something like that comes from is the fact that lifespan seems to be controlled by these antagonistic pleiotropy effects — these many genes presenting multiple trade-offs between making something a little bit better for earlier life at a cost to later lifespan. What that implies to me is that probably what you'd need to do is tweak a lot of things a little bit to shift human biology to be a little bit more like whale biology, let's say, in lots and lots of different ways. That's not to say it can't be done. I'm sure there are many clever people working on ways of changing epigenetic regulation or changing expression across a broad set of targets, but I'm not specifically aware of what those targets are or should be.

SPENCER: So how does thinking about the age of humans fit into the broader genre of your work?

ARIEL: In a general sense, I'm really fascinated by the history and future of medical technology. I'm a neuroscientist working in 2025, and if I look backwards to the scientific developments over the past century or two centuries, it seems insane to me that we've gone from a world where we had nothing that could treat infections, no way of providing anesthesia to people who needed surgery. We had very primitive medicine, not even that long ago. But nowadays, we can do things like sometimes re-engineer bits of people's immune systems to fight their cancers, or in the laboratory, we can do things like control bits of brains in animal tissue by shining lasers on them to selectively turn on and turn off cell populations. There's all this cool technology we have today that we didn't used to have, and that's always made me quite curious about what's going to happen in 10 years, 20 years, 50 years, a hundred years from now. How is this technology going to develop, and how can we use that to treat diseases that currently we have no ways of treating? I've been curious about that from a science perspective, but also from a science fiction perspective, because I've always been into these sorts of games and books that explore these ideas. One of the things that I've always been somewhat fascinated by is this idea of somehow maybe being able to take people who are dying of some disease that we can't currently cure, but somehow putting them in some sort of state of stasis, being able to preserve them in an unaging, unchanging, sort of inert state for a period of time, until such time as a treatment for, let's say, their cancer or their heart disease could be developed and the person could be restored to health.

SPENCER: We see different versions of this in science fiction. One version is people are going on a long starship voyage, and then they kind of go into some stasis pod, so then they don't have to experience anything on the long voyage. Another version would be cryonics, where bodies and brains are frozen just before death. Are those right?

ARIEL: Yeah, exactly. Those are the two examples I often give when I'm talking about this. But the question I've had is, although these ideas have been explored in science fiction and popular media, I've always been kind of curious about, "Well, is this just a nonsense trope that's used to advance these stories but could never actually work, sort of similar to what I guess might be the case for time travel scenarios, or is it more the case that, in the same way that 19th-century writers talked about rocketry and flights, it's something we can't necessarily do today, but in principle, if you got it right, might be able to work?" I got really fascinated by this question and started thinking about it more and more. And specifically, over the course of the past five, ten years or so, I've seen a lot of developments in neuroscience that I think really do push the idea that this sort of approach, if done appropriately, might genuinely be able to work. In my book, what I do is explore all the neuroscience, medicine, and philosophy required to unpack that idea, and then I look into the economics and the ethics and everything else that has to go around taking such a weird idea seriously.

SPENCER: Yeah, it reminds me of, I think it was the Lord Kelvin quote that "heavier-than-air flying machines are physically impossible," right? And now it's kind of hilarious to hear that in retrospect.

ARIEL: Yeah, I actually think, in the conclusion, I was basically looking into that particular case where in 1903 he's being asked, "Oh, will we ever be able to fly over the Atlantic?" It's weird because he is a man who's discovered thermodynamic principles, and has been involved in laying submarine telegraph cables. He's on the cutting edge of what is sci-fi tech for that time. And he is like, "I don't really think it can ever be done." Then it's only a few years later when there's the first flight in the US with the Wright brothers, and then something like 15 years later before someone flies over the Atlantic. He is definitively wrong, so it's hard to judge these things sometimes.

SPENCER: Exactly. But on the flip side, you have people who speculate about, "Oh, maybe one day we'll fly faster than light" or things like this. While we can't be 100% sure that can't happen, we have good reasons to think that might be physically impossible. It may be that that can never happen. It's this interesting balancing act where there are things, on the one hand, that seem like they're never going to happen, but then they might happen much faster than you think. On the other hand, there might be things that are physically impossible, and there might be things that are possible, but it's unlikely civilization will have invented them. It might take 10 billion years to discover the technology or something like that.

ARIEL: Yeah. I mean, if someone's promising you a Dyson sphere in the next five years, then you probably need to be a bit skeptical of their claims. I agree. It's hard, particularly if you're on the outside and you're not an expert in a particular field, to assess what's possible and what's not.

SPENCER: So where do you feel we are with this idea of stasis from a scientific point of view, on the spectrum from, "It's about to happen, Lord Kelvin, making a fool of himself," to "It's theoretically possible, but we kind of don't even know the first thing about how you would do it"?

ARIEL: What makes this sort of proposal even trickier to assess is that essentially, it's a two-part proposal. The idea is to have someone who's dying and then to try and preserve them, to stop them from completely decaying, being dead in an absolute sense, never able to be revived. And then later on, there's the idea that you might be able to take them and revive them, restoring them to consciousness and back to health. Those aren't necessarily the same level of difficulty. In fact, I think that the preservation part is a much easier ask than the revival part. The problem is that if you haven't done the revival part, it's hard to validate that the preservation part has been done correctly, and you sort of have to go off the strength of theoretical arguments, which is always less satisfying than just showing that the technology works from start to finish.

SPENCER: You might preserve a brain, for example, macroscopically by freezing it, and it might look like it's preserved, but there could be detailed microstructure that was damaged that actually makes it unusable in terms of preserving a person.

ARIEL: Yeah, that's exactly right. The point being that if revival is certainly not something I think we're going to see anytime in the next few years, it's hard to then validate the preservation steps. That being said, I do think that particularly in the last 5 to 10 years or so, we have developed some very good preservation techniques that give a reasonable chance of revival being possible at some point in the future. To finally answer your question, I would say there are preservation techniques today that I think are compatible with a reasonable chance, a double-digit percentage chance, of being able to revive someone in the future, even though revival itself is still at least decades, if not longer, away.

SPENCER: So you're essentially saying that you think there's a pretty good chance that they preserve enough that someone's in theory revivable, which is different from saying that they have a 20% chance of actually being revived.

ARIEL: Yeah. I mean, we can get a little bit more technical about this, or maybe I'll phrase it in the way that I did to my fellow neuroscientists when I ran a survey recently. Some colleagues and I were wondering, "What does the neuroscientific community think about these sorts of ideas? Do they think that revival or preservation is forever sci-fi nonsense, or maybe something that's possible?" So we sent out a survey to some randomly selected neuroscientists from conference abstracts, not just my friends and people I knew were already sympathetic. One of the questions we asked them was, "If you preserved a human brain in very high quality (using something we might talk about later), aldehyde-stabilized cryopreservation, what probability do you think there is that, in principle, you could one day create an emulation of that brain, essentially upload that brain?" The median response we got from neuroscientists was they assigned it about a 40% probability, although there were some who were, "Absolutely not," and there were some who were, "Yeah. Definitely, it will be possible." The range was really broad and the variability was broad. 40% is quite a high answer. Personally, I think it's higher than that, but I think that's just an indication of the fact that it's an idea that people are skeptical of — appropriately — but they take seriously as well. They don't dismiss it as negligible.

SPENCER: I also think that the way you word that, being around a specific technology of emulating a computer makes it even harder to come true, rather than saying, using any possible technology, that the person could be revived, or some version of them could be revived.

ARIEL: Yeah, now it's true that we were specifically focusing on emulation, and we get into the philosophical details of, "Are emulations conscious? Are they a continuation of personal identity? Are uploads the same, a valid way of surviving, as opposed to being biologically revived?" All that sort of stuff. Different people, different philosophers, different laypeople have different ideas on those. But the key point is just that, with at least one technology people have in mind for future revival of people preserved using current techniques, there's at least some reasonable level of credence that, "Hey, maybe this is an idea worth exploring."

SPENCER: One topic this naturally raises is what it means for someone to be dead. Obviously, if someone's entire brain is pulverized, they're clearly dead. But there are a lot of other states where it may be less clear.

ARIEL: Yeah, this is a great question, and I think it's key to really exploring any of these ideas properly. Most people start with a conception that a person dies when their heart stops, or they stop breathing, or the "beep, beep, beep" noise of the monitor fades out, and then that's it, and doctors have a really good idea of when people have died and how to declare death. In reality, if you start looking into the medicine and the neuroscience of what's going on, the picture gets quite a bit murkier as to how we should be properly defining death. I think it helps to have a little bit of a historical perspective to understand it. Prior to the middle of the 20th century, it really was the case that defining death was relatively simple, because back then, if someone's heart stopped or they stopped breathing, there really wasn't anything we could do for them. But with the invention of things like mechanical ventilators to help people breathe when their lungs were failing, or cardiopulmonary bypass machines to help circulate people's blood even when their heart had stopped, people started to realize that they needed a more sophisticated definition of death.

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SPENCER: It seems that there's a distinction here that is being raised, which is there's a difference between someone being dead and someone being in a state that will lead to death that we can't stop. So someone's heart stops and we have no way to restart their heart. We know that they're on the track to death inevitably, and so we might say they're dead, but really what we mean is they're just on an inevitable path to death, which is different from saying they're actually dead at that moment.

ARIEL: I agree with you very much. But historically, there's been some back and forth about whether that is the case or not. There really were people who thought that the appropriate way to define someone as dead, historically, was for their heart to have stopped or their breathing to have stopped. But it's true that with the development of these technologies like heart and lung machines, people started to realize that, as you say, that actually seems to be more of an indicator of imminent death rather than actual death, and that actual death should be something like all of their brain functions have stopped. This was an idea that came in in the 1980s or so of defining someone as having had irreversible cessation of all brain functions. Now, that idea was an improvement over previous versions, but it still has some problems. The problems are twofold. One is that if you actually go and look at the brains of people who are declared brain dead — so they're unconscious, they're unmoving, they don't really seem to be displaying any complex behaviors anymore, but these people are kept on life support — what you'll find is that there is often some residual brain function still occurring. For example, their hypothalamus may still be controlling their body's temperature and hormone regulation. In those circumstances, it's not the case that doctors say, "Hey, this person's actually alive; they have some brain function remaining." What instead happens is they say, "Well, when we said that someone is alive if they have any brain functions, we didn't really mean those brain functions. They're not important." On the other hand, one problem for a definition of death based on irreversible loss of all brain functions is that slowly, over time, we are getting a bit better at reversing the loss of brain functions, starting with the most primitive things, like cochlear implants to restore hearing to people who've had damage to their auditory nerves or other sorts of hearing damage. Increasingly, we have things that can help compensate for strokes by giving people control of limbs again or speech again, or other devices that can help with things like spinal cord injuries or other sorts of brain damage. While these things are very primitive compared to what we can do with replacing heart and lung functions, they are getting better and better over time in a way that puts pressure on this idea of our death as just the loss of brain functions in a sort of coarse-grained sense.

SPENCER: It seems that we have to think about what type of brain function, which type of brain activity; we can't just lump it all together. To what extent do we have good ideas about which type of brain activity should count?

ARIEL: So this is a thing that doctors and philosophers argue about still, but one camp that I and many are sympathetic to is that when we look at which brain function should count for a person still being there or not, we shouldn't be so focused on control of heart rate or body posture. The things that really matter are things like someone still being conscious, someone holding onto their memories, someone still having the same personality and goals and desires and dreams, all those sorts of things that we view typically as critical to being us. If you're looking at that, you're taking the view that these psychological properties matter. How that's often termed is you have a view of survival based on the survival of personal identity, and then you have a view of death, which is that death is the loss of personal identity. So it's not when your brain stem is destroyed and you can't control your heart rate that you've died. It's when you've lost the brain structures that hold onto your memories and personality and goals and consciousness; those things are what really matter for someone to have survived versus died.

SPENCER: A potential challenge to that view is, suppose someone gets a traumatic brain injury, they have severe memory loss and their personality changes. There are cases of this reported where people have severe problems with memory and they have shifts in personality, but they're still functioning. They're still a living being that's going around and doing stuff. Would you say that the original person died, even though clearly there's someone who's still alive?

ARIEL: Yeah, I think the first thing I want to be very clear on is that if somebody suffered major brain damage and they're still conscious, they're still talking, they're still doing any of those sorts of behaviors, then it's obviously the case that there is still someone there. The question that's trickier is, is it the same person who was there before? Even outside of cases of brain damage, people have changes over the course of their lifespan. They change their tastes, they change their interests. There are all sorts of things that we accept are still within the same person, but differences to how they were previously. That said, people do tend to have pretty stable personalities over the course of their lifespan, to the extent that personality tests of people when they're 5, 10, 15 are very highly predictive of the sort of person that they'll be decades down the track. In some of these circumstances with traumatic brain injuries, you see much greater changes than would naturally occur during a person's lifespan. There's one famous case of a man who was working in the 19th century on a railroad who accidentally had a pole blasted through the front left side of his brain. He survived the injury in the sense that within a few days, he was back to walking around and moving. But when his friends and his family and his colleagues were asked about him, a number of them said that he's no longer the same person. He's no longer Phineas Gage compared to before. I think there's really a case that for low levels of brain damage, just a concussion, we think, "Oh, that's definitely still the same person." But if you have an extreme amount of damage, like really significant changes, I think there's an argument to be made that that really no longer is the same person who was there before the injury.

SPENCER: I think there's some controversy in that original case, Phineas Gage, about how much he really changed, what's really true in that story. I don't know if you know about any of the details there.

ARIEL: Yes. So I'm aware that there's controversy. I guess we'll establish the facts, and then there's the controversy for people who are interested. So Phineas Gage, 19th century railroad spike through the front left side of his brain. What's clear is that initially he had a significant amount of disablement from the injury, but then he recovered and was able to move and walk around. The problem is that we've got a bit of a lack of data, except for that which is provided by the doctor who was treating him and assessing his progress over time, where he notes these personality changes and changes in performance in terms of his conscientiousness compared to before. But then later on, there are records of him being successfully employed, moving around and having other sorts of interactions. I'm not a complete expert on or really not an expert at all on all the different aspects of the controversy about exactly how damaged he was, but I think there is a fair amount of work to establish that he did have at least some significant short to medium term personality changes.

SPENCER: One thing that comes to mind, if someone goes to sleep and then they wake up. Clearly, everyone would say, "Oh yeah, that's the same person." Furthermore, if you imagine that you had a sort of magic wand that you could make someone just kind of stop, and then nothing would change about them, but some time would pass, and then they would resume, just as if nothing had happened. I think even though nothing was happening in them during that time, when they were kind of frozen in time, everyone would still say they were still alive. They just kind of were paused. It seems to me that some of the technologies are trying to get at this idea of essentially pausing a person, where if someone's paused, they're not really dead, even though there may be no activity. So do you want to comment on that distinction between no activity and a pause in a person's state?

ARIEL: Yes, exactly. That is the analogy that I use in my book, where the ideal preservation technique would be a time machine, something that just takes someone and throws them into the future with nothing having changed about them in the process. Obviously, we can't do that, so the question is, what best approximates that? There are examples of things that we use to slow down time that already give, I think, some precedent for the idea that it might be capable of doing it for long durations. One technique that's used in hospitals today is something called induced hypothermia, or more specifically, deep hypothermic circulatory arrest. What happens in that circumstance is, if somebody needs an operation on the blood vessels in their brain or heart, sometimes you have to stop the person's blood circulation entirely. You can't just stop only their heart but keep their blood circulating elsewhere. What they do is they cool the person down to about 20 degrees Celsius or 18 degrees Celsius, at which point their heart stops, their brain activity stops — as recorded by things like EEG — and for all intents and purposes, they look like a corpse. But it gives surgeons something like a 45 to 60 minute operating window where they can repair blood vessels and then warm the person back up again, at which point they have quite high survival rates, something like 90% plus. When you interview the patient afterwards, while they might lose a bit of their memory around the time of the operation, they don't lose their long-term memories of who they are, who their family is, why they were even in the hospital in the first place, those sorts of things. I see that as an argument that we definitely can survive things like breaks in consciousness, breaks in brain electrical activity, so long as the structure of our brains and bodies remains intact.

SPENCER: In what way is reducing temperature similar to freezing time? Is there a true analogy there?

ARIEL: Essentially, reducing temperature reduces the rates of chemical reactions, which is essentially like just having less time for those reactions to occur or for molecules to move around. So if we think of time as the thing that lets things happen, by changing temperatures to lower temperatures, we're just allowing less stuff to happen over the same period of time. So I think it's pretty directly analogous.

SPENCER: And at a sufficiently low temperature, the rate of reactions could be so low that an arbitrary amount of time could pass without the reaction occurring. Is that right?

ARIEL: Essentially, that's the case. If you get lower and lower temperatures in a liquid, you just get reaction rates slowing down and things moving less and less. Actually, with physical systems, like human bodies or biological tissue, if you get below a certain point, something will solidify. It'll either freeze into ice, or it'll turn into what we call a glass state, which is not quite ice, but it's still solid at cold temperatures. In those states, there's essentially no chemical reactions happening anymore because the molecules aren't moving around. This is what's used successfully to store things like embryos for decades in assisted reproductive therapy situations.

SPENCER: That's interesting proof of concept that we could do that for embryos and then still use them. So they could still revive them. But you might also think that getting to that temperature where it becomes kind of a crystal or glass state might actually change its nature in some way. To what extent do we know whether brain tissue is fundamentally altered by being in such a cold state?

ARIEL: Yeah, so also a good question, and this is where I think some of the technical improvements in the last ten years or so start to strengthen the case for something like preservation being able to work. So historically, we first tried to preserve stuff by just straight freezing it or straight cooling it down to lower temperatures, which does lower reaction rates, which is good, stops decay, but comes at the cost of ice crystal formation. The problem with forming ice in biological tissues is that it expands, it forms crystals that puncture cells, and that's not really very good for their structural integrity or viability. So what was then later invented was the addition of antifreeze or cryoprotectants, which are these compounds that prevent the formation of ice and mean you can cool things down to lower temperatures, and instead of having ice form, they just turn into a glass. And that's what's currently used in most IVF-style assisted reproductive therapies for long-term storage.

SPENCER: What does the word glass mean here? Is it actually analogous to the normal word glass, or is it something different?

ARIEL: No, it's directly analogous. So in a glass means an amorphous solid, as opposed to a crystalline solid. If you look at water ice, you see that the water molecules form themselves into these ordered crystals which cause the expansion. If you look at a glass, whether it's the silica in our normal glass or biological tissue that's being preserved, you'll see the molecules don't form crystals. They're just all jumbled together in the same sort of chaotic mess as before the tissue got cooled down, so it is just directly glass.

SPENCER: So this sounds promising. I imagine that there are still issues that need to be worked out.

ARIEL: It sounds promising. I agree. But a skeptical listener might be, "Hey, if this works so well, how come we can't do things like bank organs? How come kidneys need to be transplanted really quickly? How come we haven't rolled this out for whole human bodies as of yet ubiquitously?" The reason for those problems is that while cryoprotectant-based cooling of tissue works really well for small bits of tissue, it doesn't work as well for larger bits of tissue while maintaining biological viability with current technology. Specifically, the issue is that the cryoprotectants themselves can be a bit toxic, and they can cause dehydration of tissues. Because they're viscous, they pull the water out of tissues, and they don't penetrate well inside things like cells or biological tissue. This is already a problem for things like kidneys, but it means that if you try to introduce cryoprotectants into the body of a terminally ill patient whose brain you might want to preserve, what you'll see is that their brain will shrink in size by something like 50%. This has been a historical criticism of groups that have tried to promote cryonics or promote preservation of people using just these cryoprotectant-based preservation techniques.

SPENCER: Are there technologies on the horizon to address this? Or do we not even know to what extent these are problems that are just potential concerns, but we're not sure what the actual facts are?

ARIEL: It's true that we don't know whether a brain preserved using these traditional techniques has lost all the information inside it that encodes memories. It could be the case that even within this sort of shriveled brain, all the connections that matter are still there, all the memories are still in there, personality is still in there, but we can't tell because we don't have good ways of tracing through those tissues, trying to look for neural connections that we know should be encoding memory. So, it's a worrying proposal to have to rely on something like that, but there are alternatives. An alternative way of stopping time, rather than just using cold temperatures, is to do something called fixation. What you do is introduce preservative chemicals, these fixatives, like aldehydes and glutaraldehydes, into the body of an animal or a human. They permeate inside the body, get through into all the different tissues, pass through cell walls, and then lock everything in place. By locking everything in place, they prevent further decay processes, further molecular movements, and they lead to really high-quality tissue preservation. If you look at the brains of animals or humans that have been preserved with these sorts of techniques, you can see that the defined structure of the brain looks essentially the same as when that creature was still biologically active, and you can do it without causing any of that shrinkage or dehydration damage.

SPENCER: Would this be like the frogs that sometimes people have to dissect in biology class in seventh grade, that kind of thing, where they put them in formaldehyde?

ARIEL: It would be, yeah, a more sophisticated version of that. It's essentially the same sort of techniques that we use when we prepare animals in neuroscience studies or other studies for high-resolution brain imaging. So it does bear some resemblance.

SPENCER: Do these kinds of techniques present their own problems?

ARIEL: Yes. They come with much better structural preservation, but the trade-off is that they really put a stop to short-term ideas about being able to biologically revive things. As we talked about before, our ideal way of preserving someone would be a time machine or something that just threw them into the future, where they come out the other end and go back to moving around. We have precedent in things like the induced hypothermia case I talked about, where we cool someone down and then warm them back up again, and they go back to being conscious. The issue with these fixation-based approaches is that we don't really have any idea of how, in the medium term at least, we could reverse the fixation procedure to be able to directly biologically revive someone. So it's more an argument that what we're doing here is providing very high-quality preservation that's compatible with either: 1) the more radical option of what we think we need in order to upload someone, to give them a new sort of physical format to live through or; 2) to hold on to them, to maybe biologically revive them at some point in the future, using technology that we don't really know how to create yet. It's sort of a bit uncomfortable because I actually think the uploading version is the one that's more likely to work, even though that's not generally people's first, most preferred sort of revival technique if they got to choose.

SPENCER: So what is this uploading method? How would you describe that?

ARIEL: Basically, what it takes is, if we think it's the case that someone can chew even if they're using dentures instead of their own original teeth, or that someone can still have blood circulation if they have an artificial heart, maybe it's also the case that somebody who's received a brain implant to control parts of their motor movements is still actually controlling their limbs instead of something else. Maybe what we can do is take that to its logical conclusion, where perhaps it's the case that it's not critical to being a person, to being human, to use one's own original biological material. Maybe it would still be the same person, even if we substituted the biology for electronic components or silicon components, with precedent coming from the fact that even in our own natural biological bodies, we're constantly changing out the material that we're made of as we eat and drink and replace ourselves over time. That's the initial starting point of the idea of uploading. It's changing the physical medium of which we're made.

SPENCER: Usually, when I hear the phrase uploading, it's sort of imagining that someone is scanned with some kind of high-precision scanner, and then their brain is transferred into bits, and then the bits are uploaded on a computer, and then there's some simulated world that they now live in. But that's not necessarily what you're talking about, right?

ARIEL: That is pretty close to what I'm talking about. But I think there's a lot of ways that the final stage of reviving the person can happen. The early stage of scanning and putting that information together is pretty much what most people think about across all sorts of different approaches. To make that more precise, the idea would be that you would take someone who's been preserved in a high-quality sense, so we see good structural integrity of their brain, and then you would scan it at very high resolution, down to almost nanometer resolution, or down to the level of almost individual molecules. You would systematically collect that across the entirety of someone's brain and maybe also their spinal cord. In doing so, you would collect the information about which neurons were connected to which other neurons and with which particular receptor types, like were they glutamatergic or serotonin or dopamine, all that sort of stuff. Then you would take that full set of information and essentially recreate it in a virtual digital format, in the same way that you can take a Game Boy or other 1980s and 1990s electronics and virtually recreate it, emulate it on a modern computer. If you get that right, if you then run it in the right way — which means you also know what the properties of those now emulated neurons should be, how to enable them to change themselves over time, so that they continue to do things like learning — then, in principle, the hope is that would restore the original person to consciousness.

SPENCER: Now, of course, some people think that even if you could do that whole process, the result would not be them in a meaningful sense, that you've just made a copy.

ARIEL: Yeah. So there are two issues with the idea of uploading or emulating from a philosophical perspective. One is a concern that such a thing, even if it had behavior, wouldn't actually be conscious. And two is that, even if it is conscious, it's not the same person who was conscious before. That's the problem of personal identity. So you're asking about the second one: that wouldn't be me, that would just be a copy of me. I guess what I would say is that there's quite a lot of pressure on that idea from things we already accept, which are changes to people where we still think they're the same person. For example, over the course of time, we change out the physical material we're made of. To give an example, I think all the proteins that make up our brain cells and their connections are replaced on the order of something like every six weeks or a few months. Also, we know we can survive things like breaks in consciousness, like when we're anesthetized or asleep or in hypothermia. So the question is, what is it that makes someone the same person over time that's not just the continuation of their psychological properties — the fact that their memories are intact, their goals are intact, all those sorts of psychological attributes are intact? I think if you really drill into it, it is the fact that it's those things that continue that ensure that someone is the same from time point one to time point two to time point three. That does imply that an upload which held onto those things would indeed be the same person as before.

SPENCER: One of the counterarguments that's made here is that suppose you scan someone's brain and you had all the information sitting on a hard drive of exactly how to reconstruct their brain, you could go from that and make as many copies as you wanted of them. You could make one copy, another, and another. So if it's really just them, which one of the copies is them?

ARIEL: Yeah, that's why I think it's important to make the distinction between consciousness and personhood. Someone existing at one particular point in time, in one location, having their own experience there, and somehow linking different moments of consciousness together across time into being the same person. Take me right now, sitting here. I'm talking to you right now. I'm conscious in this moment, but a few minutes ago, I might have been in a different room, and later today, I'll be somewhere else. In that circumstance, we have different moments of conscious experience in different locations, but we're quite happy to bind them together into counting as being the same person, as being me. The pressure comes from the circumstance you mentioned of having different instantiations of the same consciousness, but in different locations at the same time. That seems to break down the idea of, "How can those all be the same person?" We're okay with it being extended over time, but not happening at the same point in time. I admit, that's really weird, because normally, when we see other consciousnesses that aren't our own, where we can't directly experience them, we think about those as different people from ourselves. The question you'd have in this particular point is, is that consciousness over there on the other side of the room, this duplicate, who has the same memories as me, the same personality as me, the same drives as me, a different person, while at the same time, the person I will count as me in a week from now, that one is the same, even though they're in a different environment and have different experiences to a slightly different degree than the person who's sitting here right now. This is sort of a long-winded way of saying it's a really weird scenario to imagine, and one that does put pressure on ideas of personhood, but I really do think it would be the case that all of those entities, at least for a while, would still be the same person before their personality and memories, all those sorts of things had sufficiently diverged from each other.

SPENCER: So that basically is taking the view that there could be multiple completely independent copies of you that are all equally you. If you had to decide, for example, to have one of them tortured, and you had to pick which one was going to be tortured, you'd be completely indifferent between choosing different copies because they're all just as much you as any other copy.

ARIEL: Insofar as you know the particular copy that suffers the torture actually consciously experiences that torture, I don't think the copy under the torture scenario is going to be indifferent. Insofar as going forward, an individual who's about to be duplicated into all of these different copies, each of those copies in the future is at the same level of continua as the pre-duplication individual, I'd be indifferent in that sense. I still don't think we should be torturing particular copies or killing off branches, any of those sorts of things.

SPENCER: Sure, we want to avoid torturing, but just a thought experiment to try to get the juices flowing. Okay, so let's think about this case. Suppose that your brain is preserved, and in the distant future, technology eventually is developed that allows scanning of the brain, and a digital copy of you is made. But then even further in the future, they actually discover how to revive the original brain. Rather than scanning it and creating a digital copy, they actually revive the original physical copy. They develop a technique that reverses the preservation process, and then you, with your brain and body, come back to life. I think most people would say that that physical copy actually has a much stronger claim to being you than the scanned copy.

ARIEL: I do think that is people's original intuitions, but I would question whether it survives once you look into trying to rigorously philosophically answer how survival occurs over time and what binds someone together over time, despite changes to their body and breaks in consciousness. Obviously, we normally survive just through standard biological continuity, whereas something like uploading is very weird and foreign and different compared to what we're used to; that seems unintuitive. But at least for me and for quite a number of other philosophers, on reflection, I think the two end up being analogous to each other. Both the digital version and the biological version of me would have equal claims to being me.

SPENCER: It seems to me that we have stronger arguments for the biological version being you. For example, we can compress the time frame, and you could say, "Imagine someone injects preservatives in your brain. Your brain gets preserved, and then, five minutes later, they inject the antidote, and it gets unpreserved, and you're back to life." Obviously, this would be in the far distant future when much better technology exists. I think we have really strong reasons to think that that's you. Whereas the brain scan that then creates a version of you on a computer requires many more jumps. Not to say that we can be confident it isn't you, but I do think it actually requires further philosophical jumps to say that's you.

ARIEL: No, I guess that is fair, insofar as whenever there are unknowns, like maybe we're wrong about the philosophy, or maybe we're wrong about the science. Or maybe it's the case that something got changed in the uploading process. Or maybe we were just wrong about aspects of the philosophy. I definitely agree there's more uncertainty. And if it's a choice about whether we assign higher probability to one or the other, then I misspoke before when I said that they're equally probable to be me in the future under both of those scenarios. I more meant the case that I assign quite high credence to both the digital version and the biological version being me, based on what I understand of current neuroscience and current philosophy.

SPENCER: My take on this is that this is just too hard for humans right now. Philosophy of mind actually just kind of breaks all of our theories. And there's no coherent, I literally don't know of any coherent theory of philosophy of mind that I think works. So I just put a big question mark.

ARIEL: I would push back on that. I think it is the case that if you look at all these different scenarios where you're considering survival, whether it's does someone survive if they have significant brain damage, or do they survive if they travel through a teleporter, or if they were duplicated, or if they were uploaded, or if they were preserved? Then if you systematically work down and go through the options where it's survival based on being made of the same biology, or survival based on having the same memories, or based on continuity of consciousness, it's true that until you can actually start to do these technologies and see the results, we're not perfectly certain that survival works the same way as we are over the course of someone's natural lifespan. But I think we can have a reasonable amount of analysis and confidence in survival under one circumstance or another, just by examining the cases we're already presented with.

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SPENCER: I think there are some of these views that almost certainly fail; they're almost certainly not what matters. The body, right? It's pretty clear that you could switch bodies and still be the same person in a meaningful sense. I think almost everyone agrees, at least some parts of the body. Maybe there are parts of the spinal cord, maybe there are neurons in the stomach that are important, or something like that. But a lot of the body could be swapped out without changing you. So that view is clearly not the case. It's the body that matters. But my suspicion is that we actually just don't understand consciousness yet well enough, and therefore none of the things that we're saying are the things that matter are really the things that matter. We don't yet have the words or way of describing the actual thing that matters, and that all of our attempts to describe what matters are sort of missing the mark because of that. Basically, what I think is based on is that I think there are pretty strong theoretical counterarguments against every view that we currently have, in my opinion, so that's why I put a bunch of question marks for the time being.

ARIEL: I actually think the criticism of the consciousness stuff is much stronger than that for personal identity, where I'm quite confident in a psychological view of personal identity, one based on the continuation of memories and personality, whereas I'm quite a lot more ambiguous on any particular theory of consciousness being correct, and whether an emulation or an upload or any sort of entity that had particular functions or was built in a particular way would or would not be conscious. If you survey consciousness scientists at the moment — of which that's actually my day job, I work in a consciousness lab — they're really in broad disagreement about whether it's functions that matter, whether it's structure that matters, or whether these systems could be conscious or not conscious. Until we have coherence on what a proper theory of consciousness should be, we're not going to settle that. It could, I agree, be the case that maybe we'll get to the point where we can emulate and restore behavior in a way that holds on to the psychological aspects of identity, but where we'd still be unclear on questions of consciousness. I agree that's a big problem.

SPENCER: I think the question of consciousness ends up getting intertwined here because, for example, if you prick me with a pin, I feel it. But if you, one day while I was sleeping, made a copy of me that was sort of semi like me by doing a brain scan and making a copy, and then you pricked it with a pin, I wouldn't feel it. I think we can all agree on that. There's something about consciousness here that's fundamental; when do you feel it? We don't even really know what that means, "you" in this sentence, but there's some difference between you feeling it and not you feeling it. It doesn't feel like it's exactly the same thing as being similar. You could still have quite similar copies that you wouldn't experience the experience they are having.

ARIEL: Again, this is why I make the distinction between consciousness in the moment, in that consciousness always involves some subject in some location experiencing something or not — a rock isn't experiencing anything, probably — as distinct from who's you, who's Spencer, who's Ariel, how do we link these different moments of consciousness together over time? It's true that if we pricked you with a needle, you'd feel the pain. If we made a bad copy of your brain that wasn't a perfect emulation, it'd be unclear to me whether that thing was conscious at all, and if it was conscious, whether it felt pain the same way that Spencer felt pain. I agree that we can't be sure of that as of yet. They do get intertwined in that when we care about the continuation of personal identity, we care that the entity that will exist in the future, tomorrow, a year from now, after a preservation and revival circumstance, that that thing would be conscious the same way that we are now. That's what it means to survive. I think there's a strong argument that if you make a high-fidelity emulation of my current brain, that that thing in the future would be conscious in the same way that I am now. But it's also the case that in the absence of a good theory of consciousness with broad consensus, there's a lot of uncertainty around that working.

SPENCER: Changing topics slightly. How do you think about the possibility of whether these kinds of technologies will ever happen or ever be possible?

ARIEL: I sort of take both a historical approach and a future estimation approach. I look back at what's happened in the past century or two and the amazing progress, and in that, I see precedent for considerable more groundbreaking progress in medical science going forward. I also think, in a practical sense, about what the current trajectory of neuroscience looks like. Is it the case that it seems we'll never be able to preserve animals or create things like emulations that might count as uploads of their brain? Or does it look like we're slowly reaching the milestones on the path to being able to do something like that? When I think about that, I see things like how, at the end of last year, in October, there was the first publication of the entire map of a fly brain, all of its 150,000 neurons or so and the various connections. Alongside that, there was a low-fidelity recreation of that fly's brain in a way that captured some of the behavior of the original fly. The scientists in question stimulated the artificial neurons of the fly brain that corresponded to tasting sugary sweet compounds, and in response, they saw activation of the motor neurons that would control sticking out the fly's proboscis or sticking out its tongue. I see that as evidence for, "Huh, this is the sort of thing I'd expect to see if one day, things like emulation or uploading were likely to work." That's what I try to use to estimate whether these things will become possible and with what timeframes.

SPENCER: If you extrapolate out that curve, and not that that's easy to do, where do you see it taking us in terms of when these technologies might actually be feasible?

ARIEL: I have a great deal of uncertainty, so I'm going to defer to people I've surveyed and other people who have surveyed for that sort of estimation. In the survey I did of neuroscientists, I asked them when they think these various animals will be uploaded with something like 50% probability. I'm not going to get my own numbers right, but I think they said a mouse by the 2040s or 2050s, and maybe a human by 2125 or almost a century from now, as something they think is the most likely time when it would become possible. On the other hand, I know you've had a lot of AI researchers and a lot of AI people on your show, and I guess a lot of these people would give much shorter timelines for when this is likely to become the case. My guess is sometime within the next century or so, but I'm not really willing to be pinned down on a more accurate time point than that.

SPENCER: Do you mean the AI people would put it sooner because they think AI is going to revolutionize everything, including brain science?

ARIEL: Yeah, that's pretty much it. It's just a force multiplier for all research.

SPENCER: Now, are you signed up for cryonics?

ARIEL: I'm not signed up for the existing organizations I talked about that just do cryopreservation without fixation because I have skepticism of the quality of those procedures and also the management of people before they get to the stage where they can be preserved by those procedures. But if it turned out that I received a terminal diagnosis tomorrow and I needed to go get myself preserved, I definitely would try and get myself preserved. I would probably go to one of the companies on the West Coast that's trying to provide the sort of fixation style procedure I was talking about. So specifically, it would be a group like either Oregon Brain Preservation or there's another one that intends to be offering commercial services soon called Nectome.

SPENCER: The ones that are a little bit older, they've been around for a while, they all use kind of freezing technologies, right?

ARIEL: They tend to use the cryopreservation technologies that I talked about, so they use antifreeze, but they don't use the fixatives. There's one group in Europe called Tomorrow Biostasis that I think sometimes uses fixatives and sometimes uses cryopreservation, and I'm not entirely sure what their protocol is for when they use one versus the other.

SPENCER: Before we wrap up, I'm wondering how you think about conversations around death. A lot of people, when they talk about death, try to use reframings to feel better about it. I think most people view death as an inevitability, and you should just come to accept that you're going to die, and you shouldn't rebel against that idea. It's actually unhealthy to rebel against that idea.

ARIEL: It reminds me of the fact that in the 19th century, prior to the 1840s or so, if somebody needed something like a limb amputation or the removal of a tumor, they needed some surgery, they had no pain relief, no anesthesia available to them at the time. But it wasn't the case that their surgeon would come to them and say, necessarily, "Oh, that's terrible. I'm so sorry. This is going to be really painful, and that's just bad, but there's nothing we can do." At least a subset of surgeons said things like, "Well, actually, the pain is good for you. It's part of the healing process. It's required for the surgery to go well." So they had this idea that pain was part of the natural surgery process that was required for healing. Then, with the invention of things like anesthesia, even for a short time, there was opposition to its development. There were people who said, "This isn't good. This is taking away part of the procedure that's necessary." But then over the decades that followed, this faded away to the point where, if a surgeon told you today that they wanted to do your operation without anesthesia because the pain was good for you, most people would think they were insane. I see that as kind of analogous to where we are today with death and dying, where I think it's the case that, because historically, there's nothing we've been able to do about death and dying, we've come up with ways of normalizing it, or making it seem less scary, or making it seem actually good for us or good for the world that people die. I would term that as something like "palliative philosophy." It's a way of comforting us when there's nothing better that can be done. But if you actually go and talk to people who are dying, you do something like go into hospices with people who have terminal cancer, and you ask them, "Hey, how strong is your will to live? Would you like to continue living if you had the option?" What you will find is that the vast majority of them, something like 70% or so, still maintain a very strong will to live and would like to continue living, if only it were possible.

SPENCER: Probably even more than that, if they could have health conditions cured.

ARIEL: Indeed, you see that in the data. If you look at these people and you look at their will to live and survey it over a period of time, those who tend to be dying but don't feel super sick have very high will to live scores. If they become very nauseous or are in a lot of pain, then they tend to drop down. But if those are treated or alleviated with anti-nausea medication or pain medication, then their will to live scores tend to jump up again. My guess is that most people, if given the option of more time, whether they're 50 or 80 or even 100, as long as they still felt they were in good health, would never come to a point where they'd be like, "I'm done, I'm finished. I don't want to go on anymore." That option should be available for people who do make that choice; I don't think we should be imposing greater lifespan on anyone. But I think it should be an option for people who want to live longer to be able to.

SPENCER: Yeah, I agree with you. It just seems that you don't want your family to die, you don't want your friends to die, and on any given day, unless you're really depressed or in a lot of pain, you probably don't want to die either. If you kind of add that all together, it's pretty clear that we don't like death. Death is actually not a good thing. It's something we've always had to deal with. But yeah, it seems really, really bad.

ARIEL: Yeah, I agree. I'm surprised sometimes that more people don't agree straight out of the bat. It's the case that if you talk about curing cancer or curing heart disease or dementia or any of these sorts of things, people are always really on board with curing specific diseases that people die from. It's only when you talk about it in the aggregate that I think it becomes weird and scary and different and challenging to people's ideas of how human lives work that you start to get the pushback.

SPENCER: I think another thing that happens is people imagine a world where suddenly nobody dies, or they imagine a world where suddenly, let's say, rich people don't die, but everyone else dies, or something like that. You can immediately see problems that would crop up in those worlds and how that would create a lot of issues that would have to be resolved. But that's different than saying death is good. It's just saying that if one day humans didn't have to die, if they could choose whether to die instead of being forced to die, there would be other issues that that would create that we would then have to resolve.

ARIEL: Yeah, it's definitely the case that if we abolish death, stop death, pause death, there would be all sorts of issues that we would have to contend with. I spend a lot of time in my book, The Future Loves You, sort of taking these problems seriously. There'd be concerns about overpopulation, for example, if people stop dying, or concerns about ecological damage, concerns about wealth accumulating in a gerontocratic class. Those are social and environmental issues. Then at the individual level, there'd be questions of, "What gives life meaning and purpose if it just goes on forever? Would I eventually just become super bored and not want to continue?" These are issues worthy of consideration. They shouldn't just be dismissed by techno-optimists like me. But on the other hand, I think if you go through them one by one and consider possible solutions, with something like overpopulation, you look at the fact that birth rates are slowing down, but agricultural productivity is continuing to expand. If you look at things like, "Would we become bored?" Consider the fact that humans live a lot longer today than our ancestors did millions of years ago, or even thousands of years ago, and it still seems the case that the elderly today often have very meaningful lives. When you go through them, you see that these are real problems, but they're addressable problems, and to a large degree, the solutions are much more palatable than the status quo of just millions of people dying involuntarily every year.

SPENCER: Why did you give the book the title The Future Loves You? What does that mean?

ARIEL: I'm advocating for preserving people with the hope that at some point in the future it'll be possible to restore them to health. This is only going to work if we have a flourishing future where there's the capacity and the desire to actually restore people back to health. If I look today at the world around me, it still has plenty of problems. We have issues like wealth inequality, climate change, and even individual levels of disharmony and crime. But to a large degree, most people living today live much better lives than people did centuries ago, when one in two children used to die before they reached the age of 15, and people used to be much more scared of things like famine, wars, and violence. The reason why we get to, on average, live safer, healthier, happier lives today is because our ancestors worked hard to make the world better for themselves and their children and their children's children, and to improve their situation for those who are yet to come. My hope is that if we take seriously the idea that something like preservation might work, and that we really might, if we do things well and solve the challenges of our time, get to live longer lives in the future, then that gives us the impetus to really work towards ensuring that things go well, to ensure that future generations look at the luxury that they get to live in, feel grateful, and feel love towards their ancestors, and then go to the effort of saying, "Yeah, we should bring them back and give them more time." The book title The Future Loves You is both a sort of hope and an imperative to people.

SPENCER: Supposed that this technology continues to develop, and it starts to seem really plausible that we will eventually be able to bring people back who now would inevitably permanently die. How do you think this might change the current generation's thinking about the future?

ARIEL: My hope is that it would give them a much stronger connection to the future, in that they would take more seriously the fact that there really will be a 2100, there really will be a 2200, and that the actions they take now might have a real influence on those times, both for themselves and for the people who live there at that time. It always feels like the current moment is what's normal and what's pretty much always going to be the case. It feels like the future might have slightly faster cars, they might be self-driving, maybe we'll have a restoration of being able to have supersonic travel. But it seems really weird to imagine that we might one day actually colonize space or solve poverty to a large degree throughout the world, or the dystopian versions where climate change might really cause severe damage, or that we might have nuclear war or pandemics. Those things seem too weird, too distant, too far away. But my hope is that if people thought there was a good chance that they would live in 2150 or 2200, it would encourage them to take seriously the problems of today, the problems of the near term, things that might affect the future in a way that they should be trying to tackle.

SPENCER: Ariel, thank you for coming on.

ARIEL: Oh, thanks for having me. It's been a pleasure.