MadSci Network: Physics

Re: if you can't see a neutron bullet how do you know that you are shooting it

Date: Tue Nov 17 22:52:39 1998
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Physics
ID: 910884526.Ph

Corey, you are right to be skeptical. Every scientist should be careful to
make sure that observations do indeed make good sense.

In this particular case, you are correct that we can not see atomic or
sub-atomic particles with our own built-in senses. However, we can design
tools that will sense the effects of such small particles in ways that can
be amplified or modified to make them evident to our own eyes and ears.

Let me give two examples.  Sub-atomic particles (neutrons among them) 
moving at very high speeds collide with the gas that is contained in that
chamber. If we make the chamber conditions right, then the energy 
dissipated in these collisions can cause a trail of droplets to form. This
is called a cloud chamber, and the tracks of the particles are visible
in such.

Or, I can bang such fast-moving particles into a semiconductor diode, or
a low-pressure gas tube, with a voltage across it. The incident particle
will create a swarm of electrons, etc, all knocked loose by the collisions
of its passage. This will make a low-resistance path and a pulse of current
will flow. This can be amplified and routed to a speaker. We hear a *pop!*
each time an energetic particle flies through the detector, which is 
essentially a Geiger counter type device.

Many classic experiments use such methods. A famous one is Millikan figured
out the charge on the electron. He had tiny oil drops go through an 
electric field and pick up a small charge. By then using a field to suspend 
the oil drop against gravity in a vacuum, he could balance the 
electromagnetic force against the force of gravity. By observing (through
the equivalent of an optical microscope) how much field was required to 
balance gravity for quite a number of charged oil drops, he figured out 
what the charge on the electron had to be, as his electromagnetic force was 
quantized (so many electrons on one oil drop).

Such indirect methods are not limited to small things. The recent detection
of planets around nearby stars, for instance, is done by extremely careful
measurements of the spectrum of that star, looking for a Doppler effect
caused by the wobble of the star around the common center of gravity of the
planet and star. Note that we can not, now, directly image the planet. It
is as invisible to us as the neutron, but by using classical gravitation
and Doppler effects (and a very sophisticated spectral analysis system)
we can detect its effects.

The general answer to your question, therefore, is that we use a variety of
indirect methods of measurement when working with atomic and sub-atomic
scale events. The specific method used, of course, depends on
what one is trying to detect or evaluate. There is, in fact, a whole branch
of science called metrology (the study of measurement) that worries about
some very, very fundamental things (how you measure how much time a second 
is, for example) to extremely high levels of precision.

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