Why Can’t I Walk Through Walls?

Hey, hey, hey, it’s the first installment of “Ask Dr. Quantum”, where I answer those questions you’ve always wondered about the subatomic world.  [Consumer Warning: I am a science fiction author, not a physicist.  I just think this stuff is cool.  If any real physicists wander by, feel free to slap me around and tell me what I said wrong.]

So here’s today’s question.  If I’m mostly made of empty space, and walls are mostly made of empty space, why can’t I walk right through them?  For that matter, what don’t I fall right through the floor?  Why do fists stop when they hit my face?  And why do people hit me so often?  (I can’t help you with that last bit.  Sounds like a personal problem.)

For one thing, the whole “mostly made of empty space” thing depends a bit on your perspective.  If you’re thinking of the size of a nucleus (in the range of a few femtometers, which science-speak for really, really small) and the size of an electron (even smaller), compared to the size of an atom (measured in picometers, which is like *way* bigger than femtometers), then yeah, you’re like 99.9999999999998% empty space.

BUT electrons aren’t really like planets orbiting a sun.  There’s a reason their orbits are referred to as an electron “cloud” or “shell”.  In the weird way of quantum dynamics, the electrons are everywhere in their orbits, with some probability, in this continuous probability waveform.  So you can also think of the atom as completely encased in the “shell” of its electrons.

It’s kind of like a spinning fan (except, you know, not really).  If you toss a ping-pong ball into a spinning fan, it’ll get knocked back at you every time.  It’s moving too slowly not to get hit by one of the blades, so the probability of it bouncing back is 100%.  On the other hand, if you fired the ping-pong ball at the fan from a fast enough gun, you would have some chance of making it through.  This is more or less what’s happening when you try to walk through a wall–the particles that make up your body aren’t moving fast enough to pass through each other.  The atoms knock up against each other and repel each other.  If you fire a particle at an atom fast enough, though, then whoosh… most of the time, it just flies right through.

Which means, in order to run through a wall, you just have to go faster!  That’s it!  Keep trying!  Just a little more speed next time…


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