The 'Invisible' List
April 13, 2012 12:12 PM   Subscribe

Can you help me make a list of the things that pass through your body constantly? (eg. radio waves, neutrinos, micro black holes)
posted by herbplarfegan to Science & Nature (9 answers total) 13 users marked this as a favorite
Best answer: Dark matter
posted by lukemeister at 12:24 PM on April 13, 2012

I'm constantly taking air in and out of my body. Food and water seem to pass through without much problem too. I suppose there's a lot of "contaminents" in the air, water, and food I'm taking in and out as well.
posted by The 10th Regiment of Foot at 12:25 PM on April 13, 2012

Solar and terrestrial radiation
posted by jquinby at 12:35 PM on April 13, 2012

Sound and ultrasonic and subsonic vibrations.
posted by XMLicious at 12:40 PM on April 13, 2012

From time to time at least a couple photons make it through, but they're certainly not in the neutrino class as far as that goes.
posted by tommasz at 12:40 PM on April 13, 2012

Magnetic fields.
posted by XMLicious at 12:42 PM on April 13, 2012

Whatever it is that makes gravity do what it does.
posted by jedicus at 12:49 PM on April 13, 2012 [3 favorites]

We're constantly exposed to a shower of energetic particles (mostly energetic neutrons) triggered by cosmic rays striking the upper atmosphere. Although the primary particle of the cosmic ray does not generally reach the Earth's surface, it creates a shower of energetic secondary particles (protons, neutrons and pions). It is mostly the energetic neutrons that reach the Earth's surface. Having already had to pass through many kilometers of atmosphere in order to reach us, many of these neurons are not stopped by a little flesh and bone, they pass right through us.
posted by RichardP at 2:24 PM on April 13, 2012 [1 favorite]

I assume you're looking for stuff that passes through us, rather than stuff that hits us and stops. We can break things into four very basic categories:

1 - electromagnetic radiation (photons)

Any photons with enough energy to vibrate molecules and atoms comes to a stop very quickly in people. So, as you suggest, it's only the long wavelength radio waves that pass by unnoticed. You start absorbing a significant fraction of what hits you roughly in the several hundred MHz range. (Between FM radio and your cell phone.) Everything below that passes by mostly unnoticed.

In addition to all the man-made stuff, there are lots of fun astrophysical and atmospheric sources of radio that passes through you all the time.

2 - Particles we can measure.

For the most part, particles from radioactive decay and other sources stop pretty quickly in humans. The list of things that exist in significant quantities and also pass through us unnoticed is pretty short.

Neutrinos are definitely the largest flux. Of the high energy ones (that we can detect, though even then it's tough) a large fraction of them come directly from the sun, with some additional contributions from cosmic rays interacting in the atmosphere, radioactive decay, man made sources, and astrophysical events like supernova. There's also a huge quantity of very low-energy cosmological neutrinos that were generated in the hot early universe and have cooled down as the universe expands. Nobody has detected these directly yet, but there's considerable evidence of their influence on cosmological observables, and they're streaming through you all the time.

On preview, RichardP describes the neutrons from cosmic rays rather nicely.

The only other significant contribution I can think of are atmospheric muons. A muon is basically a heavy electron; they get created in large numbers in the atmosphere when energetic cosmic rays hit the air, and survive for a few microseconds and have enough energy to make to the ground. Several tens of muons pass through you every second unnoticed. (Except on rare occasions.)

3 - Particles we can't (yet) detect

Dark matter is the only thing we can be pretty sure is there but haven't yet detected directly. If we're not wrong about how gravity works in some very carefully fine-tuned ways, then there's definitely a bunch of extra matter observed at all astrophysical scales. There are pretty good reasons to believe that extra matter is non-baryonic; not only can we not see it, but the excellent agreement between cosmological models and observation only works if the stuff doesn't interact with ordinary matter except through gravitation and possibly the weak nuclear force. Whatever that stuff is, there's bound to be a whole lot of it passing through us all the time. (Exactly how much depends on how massive you think the particles are. There are some pretty broad limits based on the dynamics of the early universe.) Whether these are WIMPs or something even more exotic remains to be seen. More or less by definition, the stuff passes through us without interacting.

4 - Static fields

You might want to think of us passing through these rather than the other way around, but there are electric and magnetic fields around and inside of us constantly. The earth's magnetic field is the big obvious one, though there are plenty of transient electric and magnetic fields all over the place: magnetic fields from electric motors, electric fields due to mechanically induced static charges on surfaces, incompletely canceled fields from electrical currents in wires, etc. The atoms in your body interact with the electric fields and cancel them out to some extent, but the magnetic fields pass right through you with hardly any change. (As an aside, electric field sensors can be fiddly and usually require some additional hardware to use, but there's a good chance you've got a magnetometer built into your phone. It's great fun to play with!)

Gravity, which you can think of as a field, also exists inside us. As you walk around, the few percent variation in local gravity due to geology "passes through" you. Of course, it's also acting on you at the same time. (There are more useful ways to think of gravity, but it isn't wrong to think of it this way.)
posted by eotvos at 3:18 PM on April 13, 2012 [1 favorite]

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