# The Physics Philes, lesson 75: Physics and Biology! Living Together! In Space!

In case you’re just joining us, we’ve been learning about gravity for the past few weeks. In that time, I’ve mentioned something called apparent weightlessness. But what does that mean? It’s kind of a weird concept, when you think about it. Most of us have no idea what weightlessness would even feel like? Today we’re going to look at what’s happening from a physics perspective and what effect that has on human beings.

We know that, unless we are infinitely far away from any mass whatsoever, it’s impossible to be weightless. So what’s the deal with astronauts? Why are they able to float around in space like badasses? It actually has to do with the relationship between their true weight – the precise gravitational attraction of the Earth on the person – and their apparent weight, which is the weight we normally think of. Your apparent weight is your centripetal acceleration subtracted from your true weight in a vector equation:

For a spacecraft (and anyone who happens to be in that spacecraft) the centripetal acceleration toward Earth is equal to the acceleration due to gravity at the spacecraft’s location. That means that the apparent weight is zero.

In theory, you don’t have to be in space to feel the sensation of weightlessness. A commonly used example is the falling elevator. If you stand in an elevator and it suddenly drops with the same acceleration as gravity, you’d feel weightless. (Not that I recommend riding a free-falling elevator. That could turn out very bad for you.)

However, humans did not evolve to live in space. We evolved to live on Earth. What happens to our bodies if we live in apparent weightlessness? Some not-great things, actually. About 50 percent of astronauts suffer from something called space adaptation syndrome or space sickness. It has to do with the sensitivity of the part of the inner ear that senses gravity and acceleration. If that part is especially sensitive, it can cause an asymmetry between the right and left ear that can cause motion sickness and disorientation.

But that’s not all. Living in space can cause muscle and blood loss. While we’re on Earth, we get a work out just by managing to stand upright. We have to fight gravity to function on a daily basis. But without that, our muscles start to atrophy. This is especially true to the muscles in our calves and spine. Astronauts can lose up to five percent of the muscle mass in those areas per week.

Blood loss is weirder. Normally, blood pools in our feet and our heart needs to overcome gravity to get that blood to our brain. In an environment of apparent weightlessness, blood will more evenly distribute throughout the body and that makes the body think that you have more blood than you really do. When that happens, your body works to reduce blood volume. Astronauts can lose up to 22 percent of their blood volume in just three days in space. In addition, the heart muscles atrophy, as well, because it doesn’t have to work as hard to pump blood. Blood and muscle loss are why astronauts have to do so much exercising in space.

There is another danger. While humans are not terribly well-suited for life in apparent weightlessness, some microbes thrive. For example, E. coli and Staphylococcus can reproduce more rapidly in this environment so there is an increased risk of infection during space flight.

It’s kind of awesome how seemingly unrelated physics can have such a huge effect on our health. It’s all connected, man.

Featured image credit: NASA