> Protons are unlikely ever to be seen, I would guess, in the sense of
> images being created by a focusable relatively non-disruptive beam.
I've 'seen' single protons, electrons and antiprotons, if you give me
some latitude with the word 'seen'. My Ph.D. project was to 'catch' a
single antiproton in a Penning trap in a high magnetic field and
measure its frequency of oscillation in that field, given by f=eB/(2
\pi m) where e/m is the charge to mass ratio of the antiproton. We then
would put a single proton in the field and compare its frequency to
the antiproton's frequency. Standard theories of physics assume that
the charge-to-mass ratios should be equal (up to the sign). This
measurement therefore was a test of one of the basic theorems of
particle physics known as the CPT theorem. We measured the two
particles to have the same charge-to-mass ratios to 1 part in one
billion (that's a US billion, 10^9). The student who followed me added
an extra digit. We now know that the two particles have the same
charge-to-mass ratios at one part in 10^10.
Now back to the question at hand. The way we observed the particles
was to excite them with radio waves (The cyclotron frequency eB/(2 \pi
m) ) was about 90 MHz) and 'watch' or 'listen' to them. (We liked to
say 'listen' because we would take the 90 MHz frequency and mix it
down to less than one kHz much as you would in an FM radio receiver.)
The particles would radiate their excess electromagnetic energy as
their kinetic energy returned back to the thermal level. We could see
them as they emitted this energy and tell you where in the trap
holding them in the magnetic field they were located from this
frequency. While we needed a 'magnifying glass' to see them, there is no
doubt we were 'seeing' or 'listening to' single subatomic particles.
The space they occupied while we listened to them was much
less than one wavelength of the radiation so that we could not focus
it to see them. It turns out that we could use properties of special
relativity to determine how much energy they had. Loosely speaking, a
particle's mass increases as its energy increases. Therefore, as its
energy increases, its frequency in the field (its cyclotron frequency)
decreases. (For those who don't like the term relativistic mass
increase, we can just say that the cyclotron frequency is really eB/(m
\gamma).) Therefore by measuring the frequency shift, we could
determine the energy and thus the radius of the antiproton's orbit.
Normally, this would be a tiny effect, but due to the extreme
precision of our experiment, we thought of 100 eV of kinetic energy
(\Beta = 10^-7) as extremely relativistic. That made a fractional
shift in the frequency of 10^-7 which we needed to understand and
eliminate or correct for if we wanted an accuracy of 10^-9 or 10^-10.
For more information on this experiment, you can find pdf files of
the journal articles at
http://hussle.harvard.edu/~gabriels/. A
Scientific American article was also published by Prof. Gabrielse (my
thesis advisor) about five years ago, but I don't have the reference
with me.[/b]