Innerspace is a Steven Spielberg movie that came out in 1987 starring Dennis Quaid, Martin Short, and Meg Ryan. It’s a sort of remake/homage/rip off of a movie that came out in the sixties called Fantastic Voyage, which Isaac Asimov wrote a novelization for. Both movies center on the idea of shrinking people to microscopic sizes and then injecting them into other people to go through the body and fix diseases. This is a really neat idea, and there are some scientists who are finding ways to use microscopic robots to take the place of the humans in the movies and accomplish some of the same things. However, there are two reasons why the scientists are using robots and not Dennis Quaid. First, shrinking people is probably impossible, and second, even if it were possible people wouldn’t be able to do anything once shrunken.

I can show the how true the first point is with common sense for the most part. If humans are made up of cells, how could it be possible to shrink a human to a size smaller than a cell?

Now you could come back with “well, the cells just get smaller!” But cells have to be the size they are. Otherwise they wouldn’t be large enough to hold all the organelles that keep the cell alive and functioning the way it needs to. The organelles themselves are made up of proteins that are in specialized arrangements. A cell has to constantly maintain the numbers of ions it has inside it for example. The cell can use an organelle called an ion channel to do this, but the channel has to be a specific shape. If it is too large it will let all sorts of ions in or out and the cell won’t be able to maintain the right mix of ions. Too small and the channel won’t let anything in, and it might as well not be there. If these channels were shrunk by even five percent, they would no longer function the way they need to. If ion channels don’t work for cells, they die. If all of a person’s cells die, they die too. If a shrink ray shrinks everything equally, a person shrunk even a foot smaller would most likely die within a few moments.

And of course there’s the problem of how it could happen in the first place. In the movie Honey I Shrunk the Kids, the Rick Moranis character says that we are made up of mostly empty space and his shrink ray gets rid of that empty space. First off this idea is based off of the Bohr model of the atom, which has an electron whizzing around a nucleus like a planet orbits around a sun. This isn’t how things are. There isn’t any empty space as such. The more current electron cloud model fits better. The exact location/momentum of an electron cannot be precisely determined and so we can think of it as a sort of cloud around the nucleus. Okay but at any moment we can still say that the atom is mostly empty right? And if we could take out this empty part you could maybe shrink something?  To be fair, there is a real world situation in which this does happen. It’s called the Sun. It’s a lot more bright and ‘splody than what we see in the movie.

To be more precise, and less smart alecky, the reason why the electron is so far away from the nucleus of an atom, is due to its energy. In order to get closer to the nucleus, an electron has to lose energy. When an electron loses energy, it releases a photon. The more energy an electron loses the more energetic the photon is. Photons with a lot of energy, such as X-rays or Gamma rays, are a form of harmful radiation. Never mind that this hypothetical magic device would most likely rip someone apart rather than truly shrink them, the energy released from “removing the space” in all the atoms would be huge, and would likely kill quite a few people.

The second reason why we’ll never have a manned mission to someone’s colon is something called the Rydberg constant. The Rydberg constant is a number you get when you divide inertial forces (momentum, or how long you keep going after you stop trying to move in a direction) by drag forces (friction and viscosity, or how hard you have work to move forward in the first place). The higher the Rydberg constant, the more you are concerned about momentum and the lower, the more drag forces dominate. Generally speaking, the larger you are, higher your Rydberg constant.

We live in a world with a pretty high Rydberg constant.  We can roller skate and ride a bike, coasting almost half the time. When we swim, we pull the water back with our hands and we’re carried forward enough that we can get our hands back into position for another stroke without moving back to our previous position.  These are all situations where the Rydberg constant is high.

We can create low Rydberg constant situations for ourselves if we want though. Imagine a swimming pull full of Jello. If you try to swim in that, you are going to have some problems. For small animals though, they live in this low Rydberg constant situation all the time. An ant that wants to get a drink of water has to be very careful not to get stuck in it.

Even something as large as a cat, experiences a lower Rydberg constant. A cat can fall from many stories up and still suffer only a few broken legs due to the drag forces that act on it as it falls. The cat, being small, has a larger surface area in relation to its mass, and so drag forces come into play more quickly.

For a bacterium, or a hypothetical impossibly shrunken human, the Rydberg constant would be so low, it would be like that swimming pool full of Jello, only worse. You might imagine a vat of gravel that’s shaken up continuously while you’re inside it. Bacteria typically have some sort of flagellum that corkscrews through the stuff they’re in so they can move forward. Why don’t they just use turbines like a submarine would? Well one reason might be that they never developed such a structure in their evolutionary history. The more applicable reason is that in order to combat the drag from the surroundings, a turbine on a bacteria-sized machine would have to be so large, that the drag of the turbine itself would affect the machine’s movement. Imagine trying to use a submarine in a vat of gravel. Or even more ridiculous, an airplane. It’s just not going to work. So you’d have to have a differently shaped vehicle than in the movies. And you can just forget about leaving the vehicle.  You wouldn’t be able to swim around any more than a feather can dictate economic policy.

It often seems like size is just an arbitrary attribute. There are so many stories about shrinking and growing larger because on some level it seems possible. There are a lot of complications hidden under the surface however. An elephant is a very large animal, but it’s bones are thicker in proportion to its size to make up for that. If you shrunk an elephant down to the size of a cat, it wouldn’t be able to move it’s limbs around. If you blew up a cat to the size of an elephant, it would suffocate under its own weight.  Every time you decrease or increase size by a factor of ten, you enter a different world.

Size is everything.