Hey, Vsauce. Michael here. Ten centimeters - about four inches. This is how much taller on average people are today than they were 150 years ago. Better nutrition and medical care early in life has allowed us to better take advantage of the blueprints within our genes. Blueprints that carry plans for just how big a healthy human being can get given an optimal environment. In terms of height, those plans rarely exceed 7 feet 6 inches. But individuals with endocrine disorders, for instance a tumour, near the pituitary gland in the brain can experience growth that occurs more rapidly and for a longer period of time than usual. For instance, Igor Vovkovinskiy, who at 7 foot 8 inches is the tallest man currently living in America. The tallest living person anywhere on Earth is Sultan Kösen, who at 8 foot 3 inches tall also holds the Guinness world record for largest hands and feet. But the tallest person ever officially recorded was Robert Wadlow. He was the size of an average adult male when he entered kindergarten at the age of 5. When he died in 1940 at the age of 22, he was 8 foot 11 inches tall. Andre the Giant was 7' 4". And this is me holding a 12-ounce can. Here's Andre doing the same. Human size variation is fascinating, but what's the maximum, biologically how big can a human get? And more importantly, how big are you really? It turns out that today, now in history, average human height is probably quite near the genetic limit. By manipulating the very genes responsible for height, we may be able to add an extra 15 centimeters or so to that average, but beyond that we are likely to hit a ceiling. In order to regularly produce people over 8 feet tall, 2.44 meters, those people would probably need to be a different shape. Not human shaped. This is because of the square-cube law. As a shape grows, say, taller, its volume increases at a greater rate. Take a look at this cube. If we make it 10 times larger, well, sure, it's 10 times as tall, but the area covered by its faces is 100 times larger and its volume, the space within it, is 1000 times larger. Now, since weight is connected to volume, this cube only has one hundred times the cross-sectional area to support itself, but one thousand times the weight to support. So, if you were ten times larger, and still shaped like a person, that is your proportions were the same as they are now, you would need to either have a skeleton made out of something stronger than bone or bones that were monstrously thick, like way out of proportion. But even if you solved the bone and muscle strength problem, there would still be a whole host of other issues. For instance, your heart wouldn't scale up fast enough to keep blood pumping throughout a body that large. Animals can get that big, because their proportions and organs are quite different. Chris Howard from Earth Unplugged tipped me off to the giant, not human proportioned, legs of the largest land animal ever known to have existed with the certainty of a complete skeleton, the awesomely named giraffatitan. Discovered in Tanzania and now mounted in Berlin's Humboldt museum it probably weighed 20 to 30 thousand kilograms. The Bruhathkayosaurus may have been even larger, but this is controversial because we only have a few of its bones. Estimates put this guy at 140,000 kilograms. Any larger than that, and in order to survive long enough to reproduce, an animal would need more buoyancy to counteract its weight than air can provide. This is one of the reasons blue whales love the water so much. The heaviest blue whale ever measured by NMML weighed in at 177,000 kilograms, making it the heaviest animal we are aware of that has ever existed. It might be the heaviest possible, because animal size is limited by simple geometry and the gravity of our planet. Theoretically, humans born on Mars could grow a few inches taller, because gravity there is only one third of what it is on earth. The trade off of course being that their bones and muscles wouldn't grow strong enough for them to ever visit Earth and enjoy it. The point is, in order to have the same shape and proportions that we have now, we can't really get that much bigger. Some of the higher estimates of the upper limit up average human height are around 7 feet tall. A person who is more than 9 feet tall would struggle to move around. aAd up in the 12- to 15-foot range, it would be difficult to live very long at all. But what does size mean? Where do you really begin and end? So far we have been measuring people using their rigid boundaries. It's a good one to use, it's very common, but of course, when I speak I can fill an entire room and when I shout I can fill city blocks. That's huge. Of course, my voice is not a part of my physical body. It's not part of the matter that fits within my skin container. But it's relevant to the question of how big a person is. How large of an impact on their environment can a person have using what comes directly from their bodies? Well, Guy Murchie illustrated this quite well in his tome "The Seven Mysteries of Life." The little solid dogs are small, but their sound and smell extend into shapes and sizes no creature could even dream of filling up with their bodies on earth. Let's begin with sound. How far can your loudest shout travel? How much bigger are you? The volume of space within which people are aware that you exist when you shout. Well, the loudest shout a human can make is about 88 decibels from 30 centimeters away. A shout like that will die out down below the threshold of human hearing in our atmosphere, after traveling about 5 kilometres or 3 miles. A person standing downwind from you might be able to make you out a little further than that, but the point is, in space no one can hear you scream. And on earth, from 5 kilometres away, no one can hear you scream. But could they see you scream? Really, could they see you at all? Well, on the surface of the earth, the furthest you can see another person is the horizon. If you and another person are standing on the ground, that distance is about 5 kilometers or 3 miles. Any further away than that and you will literally be hiding behind the curvature of the earth. So what about in, say, outer space, where moving away from another person doesn't mean eventually hiding behind the earth. Well, as an object moves farther and farther away, it becomes smaller and smaller. Of course, the actual size of the object doesn't change. What does change to you is its angular size. This brilliant measurement describes how much space in your visual field an object takes up. Imagine your visual field as a complete circle, 180 degrees of which go from the horizon in front of you to the horizon behind you. So, an object with an angular size of 90 degrees would have to be big enough and close enough, so as to take up all the space from the horizon to right above you. Interestingly, your thumb held at an arm's length away from your face takes up about one degree of your visual field. Its size is 1 degree. The Moon takes up about half a degree at all times. It sometimes appears larger at the horizon, but that's because of an illusion that AsapSCIENCE covered really well. Te smallest angular size we can see with the naked eye is about one arc minute, a sixtieth of a degree. But given enough contrast, we can see things like Sunspots, a mere 20 arc seconds across, a third of a sixtieth of a degree. Plugging in numbers to do the math will tell us that with perfect conditions: outer space, no air, no obstructions, a lot of contrast, because you are wearing bright white, the farthest away a person could see you with their naked eye would be about 10 to 15 kilometres. Any further away than that and they will have passed the edge of your naked eye visibility existence. But... do you smell that? It might be you. If we consider the senses of other animals, your smell, your scent, might be your largest earthly dimension. You know how animals like cats and dogs have those cute little wet noses? It's called a rhinarium. Rhinariums allow mammals to smell really really well. They don't just pick up molecules that float by, they localize them. Air cools the wet nose, allowing the animal to tell the source of the smell. It's the same as when you wet your finger and stick it in the air to tell the wind's direction. A Bloodhound can pick up and trace to you a scent trail that is days old, making your scent imprint on the earth about as large as you can move in a couple days. But the Silvertip grizzly has a sense of smell that is seven times stronger that a Bloodhound. These guys can smell things from 18 miles away, nearly 30 kilometres. That's probably the largest bubble we could draw around you and still call you, because it contains things that you omitted that can be traced back to you by other living things. But if we include everything you emit, well, fundamentally we're limited to the edges of earth's atmosphere, because with the exception of a few light molecules, hydrogen, helium, which escape into space, not much else leaves. There's no medium in space for your sound and smells to travel through. Which brings us back to light. The human body emits light, electromagnetic radiation. Most of it is infrared light, heat. But some of it lies within the visible spectrum, though it's about 1,000 times dimmer than the dimmest light the eye can see. It seems that what little visible light you do emit is tied in some way to your circadian rhythms, meaning that around 4 p.m. everyday, you are literally visibly the brightest you will be all day. But here's the thing about light, about electromagnetic radiation. It doesn't need a medium to travel through. It just keeps going out into and through space, which of course brings us to Australia. If you're not subscribed to Derek's channel Veritasium I don't know what you're doing with your life. It is amazing and he recently made a video that I have not been able to stop thinking about. The smallest possible piece of light is a photon and the human eye can kind of perceive individual photons, a bit better than chance. But some frogs can see individual photons quite well. Now, in his video he explains that, as you move away from the Sun, its angular size gets smaller and smaller and smaller, until eventually you're so far away from the Sun that you can no longer see it. But it's still there and it is still emitting light that can reach your eye, except now that light has been spread so thin it's not big packets of light you can see, it's just individual photons. The Sun is still there, it's just less frequent. Sometimes a photon hits your eye, sometimes nothing and sometimes a photon again. Well, the same thing happens to radiation emitted by you. I Skyped with Derek this afternoon and he calculated that the human body, seen at a distance of 168,000 kilometres would also be reduced to merely a few individual photons. Further than that and your size would no longer get smaller, it would just remain individual photons flying through space, they are, after all, indivisible. So, your physical size is limited by geometry and biology and gravity. Your vocal and fragrant size can be bigger but it still limited by the size of Earth. But your light, your personal glow, isn't really bounded by anything. The photons that you are emitting right now, the ones that don't get absorbed by anything, have no reason to ever really stop. And they can continue conceivably past even the observable universe, making you, in a way, completely huge and kind of immortal. And as always, thanks for watching.