Re: High speed particle passing thro me.

Barrie,

First, a little correction to the premise in your conditional.  High
speed particles from space, in this case muons, can be observed at the
Earth's surface as tracks in a cheap desktop cloud chamber using dry
ice and supersaturated alcohol vapor.  They can also be detected as
clicks in a Geiger counter.  Both of these detectors are readily
available for the classroom.  The mines are only used to shield
sensitive neutrino experiments from these highly penetrating muons,
which are called cosmic ray secondaries.

Next, we need to know what kinds of particles are raining down on us
from space.  There are photons in every energy range from radio waves
to visible light to X-rays and gamma rays.  Only the X-rays and gamma
rays can pass through you.  Fortunately, not many are produced in the
vicinity of our planet (we don't live near a blue giant star or a
supernova) and the few that are produced are efficiently blocked by
our atmosphere.  This is why X-ray and gamma ray telescopes are put on
satellites and orbit above the air.  The extraterrestrial X-rays are
negligible compared to earthly sources of X-radiation.

Then there are cosmic ray primaries, high-energy protons and a few
other nuclei.  Again our atmosphere stops these, but the collision
between the primaries and the molecules of the air produces secondary
particles, mostly pions, which quickly decay to mostly muons and
neutrinos.  Neutrinos interact extremely weakly with matter; 100
billion neutrinos from the Sun pass through every square centimeter of
your body every second and you never notice.  Muons interact strongly
with matter; this makes them detectable with the inexpensive equipment
mentioned above.  Roughly one cosmic ray muon per second passes through
an area the size of your hand.

So let's concentrate on the effect of cosmic ray muons on your body.
Most of the muons that hit your body will pass through leaving a trail
of ionized molecules in their path.  Chemical reactions between these
ions and your DNA can damage its ability to replicate or cause
mutations.  Your body is capable of repairing cellular damage on this
level; remember that life on Earth evolved under the barrage of
natural background radiation so there are mechanisms in place to deal
with it.

The answer to your last question, "How would we know?" is "We wouldn't."
Unless you become an astronaut, that is.  See these two stories about
flashes observed by astronauts as cosmic rays pass through their eyes:
http://science.nasa.gov/headlines/y2004/22oct_cataracts.htm
http://www.space.com/scienceastronomy/mir_lights_030416.html

Here are more resources to explore:
http://science.nasa.gov/headlines/y2004/17feb_radiation.htm
http://en.wikipedia.org/wiki/Cosmic_ray
http://en.wikipedia.org/wiki/Background_radiation


--Dr. Randall J. Scalise
http://www.phys.psu.edu/~scalise/