(Intro)

We've talked about some of the largest things in the universe here on SciShow Space, from planets to galaxies to super-voids more than a billion light years across. And it's sometimes hard or impossible to actually understand these gigantic things because humans didn't evolve to think that way. But by taking advantage of the ways our brains map out our environments, we can get around those limits and maybe start to understand how far it is to the nearest star, Proxima Centauri. See, lots of animals, including humans, create mental maps of their environments. As we evolved, our mental maps included things like where resources were, where predators lived, and nearby water sources. These days, your map is probably the reason you can stumble into the bathroom in the middle of the night, instead of walking straight into a wall or something. The thing is, these mental maps are of what physicists and astronomers like to call "medium-sized" things, houses or even mountains, but not like, extreme sizes of atoms or solar systems. They also depend on structure, meaning that the number of objects or landmarks between two points changes our understanding of the distance between them. In one study, researchers asked people to look at a page with a bunch of points on it and estimate how far apart pairs of them were. They found that the more points, what the experimenters called "clutter", there were between the two points in question, the more distance people thought there were between them. Other experiments have shown that our mental maps of cities, even ones that we grew up in, work in similar ways. Our understanding of the distance between two points is affected by what's in between. This works really well in a forest or in a city but is not so good at mapping something like outer space.

In space, most objects are anything but medium sized, and they're separated by huge expanses of nothing, like, real, actual nothing. It's too big and too spread out for our brains, but there is something that we can do about it: put the huge numbers in terms of smaller, more understandable units. Take the distance to Proxima Centauri, for example, 4.2 light-years, meaning that it would take 4.2 years to get there if you were traveling at the speed of light. Most people over the age of four can probably understand how long 4.2 years is, but the speed of light, over a billion kilometers an hour, just registers in our brains as "big number".

To really figure out how big that gap is, we need to put both factors, time and speed, into more understandable terms. The International Space Station orbits the Earth at more than 27,000 km/hr, if you were going that quickly straight up from the surface of Earth, how long would it take you to get to Proxima Centauri? Well, it would take you about 13 seconds to leave Earth's atmosphere, and 13 hours later, you pass the Moon. 226 days after leaving Earth, you'd pass the sun. 31 years later, you'd pass the orbit of everyone's favorite non-planet, Pluto, even at its farthest distance from Earth, but 31 years is still a pretty understandable amount of travel time. Proxima Centauri can't be that much farther, can it? Uh, it can. And it is.

Going as fast as the International Space Station, it would take 164,000 years to get to Proxima Centauri. What was happening here on Earth 164,000 years ago? Well, it was a while back, but at least homo sapiens were around. So if you're having trouble picturing things with huge or tiny measurements,it might help to put them in terms of something more relatable, like the diameter of the observable universe, about 10^26, or a hundred trillion trillion meters, which just happens to be the number of human hairs that you would need to cover the entire surface of Earth to a very itchy depth of one meter. Numbers. You gotta love them!

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