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Most people (if they are inclined to do so) think of the proton as a particle found in the atomic nucleus with a +1 elementary charge and a spin of ½, and that’s about it. In fact, it is known that the proton is much more complicated than that. For several decades now, the accepted model for the proton presents it as a collection of three smaller particles called quarks (two “up,” one “down”), held together by force particles called gluons. even before any measurements were made, it was known that the force that holds these particles together must be very large because it is sufficiently large to overcome the enormous repulsion that arises from bringing two like-charged particles (the two up quarks) into very close quarters (less than 1 femtometer, or 10-15 meter). But it turns out that the proton is even more complicated than that. Recent experiments have revealed a seething quantum soup which, in addition to the expected quarks, show evidence of components of more massive quarks, such as the charm quark.
Now come the results of new experiments which measure the actual forces of pressure and shear within the proton. These measurements were first conceived in the 1960s as obtainable if it were possible to obtain a gravitational map of the proton; however, the force of gravity is so weak on the scale of elementary particles that such a measurement was judged to be impossible. But, in the past two or three decades, theorists realized that there was a way to perform such measurements using photons (the electromagnetic force) rather than gravitons (gravity). Briefly, most of the experiments probing the interior of the proton described above have involved shooting high-energy electrons at the proton, and watching them scatter. The principal interaction involves the exchange of a single photon (force particle) which will dictate how the electron is deflected. However, very rarely, the interaction between electron and proton involves two photons. A quark in the proton can absorb a photon, move a short distance within the proton, and then emit a second photon, affecting the trajectory of the scattered electron in a way that differs from the usual one-photon scattering.
After years of collecting data on these rare scattering events, and even more years of data analysis, the results of these experiments have revealed the physical forces found within the proton, as well as a better gauge of its size. The pressure at the center of the proton is approximately 10 time the pressure at the heart of a neutron star, about 100 billion trillion trillion pascals. Further, there are shear forces toward the center of the proton twisting in one direction, while in outer layers, shear forces twist in the opposite direction. Also, the size estimate of the proton has been reduced from about 0.8 femtometer to 0.6 femtometer.
These being the first measurements of these properties, they are not as precise as better established ones, but at the technique will be refined and the precision of the measurements will improve. It will be interesting to see what comes next.
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