Read more at BigThink.com: Follow Big Think here: YouTube: goo.gl/CPTsV5 Facebook: www.facebook.com/BigThinkdotcom Twitter: twitter.com/bigthink You can determine, calculate many things about organisms, about their growth patterns, how they grow, how long they take to mature—and in particular one that concerns many of us,and that is: how long we live? What determines our longevity? And, in fact, that’s what got me into this work originally was I became very intrigued in my fifties about the phenomenon of aging and of dying that I became more and more conscious that things had been changing in my life in terms of my body and my physiology. And that already I’d had friends die. And so I became intrigued as to “what is that?” And I also became intrigued very much as a physicist not asking what is the mechanism, the systematics about aging and immortality, but the very question “What determines 100 years for the lifespan of a human being? Why is it a hundred years, not a thousand years or a million years?” And also related to that, why is it that a mouse, which is made of pretty much the same stuff as we are (I mean we’re almost identical really in some kind of coarse grained level looking at things), how come a mouse only lives two to three years? So what is determining all this? And if you have this theory of networks underlying these scaling laws, manifesting themselves as scaling laws, you first ask: is there a scaling law for lifespan? So this is work that had already been done by many people; was to look at lifespan as a function of size, for a bunch of mammals in particular but organisms in general, just as we looked at how metabolic rate scales across these animals. And what was discovered, what had been discovered was that lifespan also increases following these quarter power scaling laws—that it increased systematically. The one difference by the way, and maybe I’ll say a few words about this in a moment, is that there’s much more scatter among the data for lifespan compared to things like metabolic rate. So even though there is a kind of predictability—that is, you give me the size of a mammal, I will tell you on the average how long that mammal will live—there’s much more variance around that number than there is for saying “you tell me the size of a mammal, I will tell you what it’s metabolic rate is and what the length of its aorta is, how many children it should have” and so on, where there’s much less variance. The variance is much tighter. Lifespan has much more variance. Now where does that number come from? So you have this theory that the scaling of metabolic rate and these many other quantities—and by the way there’s probably 50 or 75 such measurable quantities—these are determined by the constraints of flows in networks such as the circulatory system. So one of the things you immediately realize about those flows is that they are what we call “dissipative,” which simply means they involve wear and tear just as, you know, outside in those streets outside this building there’s a lot of traffic going back and forth on the roads and those roads wear out. They have to be repaired. The roadways have to be repaired and the subways have to be repaired. They wear out from the traffic so to speak. And so it is the traffic through our multiple network systems produce wear and tear. And the most damaging wear and tear occurs at the terminal units, the terminal points of these networks because they’re the smallest tubes like in our capillaries or within our cells and pushing fluid, pushing blood corpuscles or whatever it is, big molecules through them—has deleterious effects of various kinds. That causes damage, and that damage is calculable because you have a theory. The theory is telling you what the flow rates and so on and all the sizes are and so on. So these are calculable. Now so you can calculate the rate at which wear and tear is occurring, and you can also calculate something else that is going on, and that is: while it’s being damaged there’s also repair going on. And we do repair ourselves. But that repair is also determined by metabolism. That’s where the energy comes from to do repairs. So you can determine all these things and then you can postulate that the system will become nonviable, that is it can no longer be sustained when a given fraction of un-repaired damages occur. So the system eventually just cannot be sustained and so that gives you a calculation of maximum lifespan. This is the, you know, if you were to do the best you possibly could this is as long as you could possibly live for a given size of mammal. And if you do that you can understand where, roughly speaking, this hundred years for a human being comes from. But more importantly or equally importantly you can determine what the parameters are, the knobs that you could conceivably turn to change that lifespan.

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