FORCES THAT FASCINATE
“Nature’s Strongest Force,” by Stanley J. Brodsky, Alexandre Deur and Craig D. Roberts, discusses new discoveries about the strong force, the most potent of the four basic forces of nature. The article describes the strong force as constant beyond a certain distance. Presumably it eventually declines in strength. Otherwise, wouldn’t it pull all matter in the universe into a really big black hole?
MIKEL D. PETTY INFORMATION TECHNOLOGY AND SYSTEMS CENTER, UNIVERSITY OF ALABAMA IN HUNTSVILLE
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
The article indicates that the strong force reaches a maximum value. Can this predict the largest possible atomic nucleus? Currently up to 118 protons can be packed into an atomic nucleus, and the highest number of neutrons is much greater.
VERNON NEMITZ VIA E-MAIL
As a young electrical engineer in the 1970s, I was fascinated to read in Scientific American about the emergence of quantum chromodynamics (QCD) theory, which describes how the strong force works. The apparent nature of the strong force just seemed unreasonable; totally different from anything in electromagnetism. So I was surprised when I saw two illustrations in the article with a curve representing data relating to the strong force from the Thomas Jefferson National Accelerator Facility. The shape bears a striking resemblance to the B-H curve describing magnetization of ferromagnetic materials. Is this similarity purely coincidental, or is there some underlying principle?
MAURI LAMPI MELBOURNE, AUSTRALIA
THE AUTHORS REPLY: Regarding Petty’s question: A phenomenon called confinement is the solution to this conundrum. As we noted in our article, quarks have a property called color, the strong force’s version of charge, and gluons have it as well. Only systems in which all colors cancel out one another can be observed in nature. Both the colored gluons and quarks are imprisoned within hadrons. Because gluons are the carriers of the strong force, their effects are active only within length scales that are bounded by a typical hadron size, which is roughly the proton radius (that is, less than one femtometer). Any leakage is carried by the color-neutral pion and kindred mesons.
The residual strong force that these mesons mediate is also hugely important. It is largely responsible for the binding of neutrons and protons into atomic nuclei. This residual intranuclear binding force is much weaker than the strong force inside the nucleon. And although the residual strong force’s reach is greater, it remains quite limited, being strongly damped at lengths greater than two femtometers.
To answer Nemitz: This is the “dream” of strong force practitioners. Today, however, it lies far beyond the bounds of computational possibility. The total number of neutrons and protons (nucleons) in a nucleus is represented by A. Calculations with a direct connection to QCD are currently being employed in the first studies of so-called light nuclei (for example, deuteron, for which A = 2, and triton and helium 3, for which A = 3). Beyond that, effective quantum equations, built with elementary degrees of freedom among nucleons and QCD-consistent potentials, are used to explain and explore the properties of light nuclei up to A = 14. Studies that desire to reach A > 14 involve additional phenomenology and insightful theoretical approximations. Therefore, while theory may be used to describe large-A nuclei, it is currently unable to predict the boundaries of nuclear stability.
“It’s critically important that kids who are rejected by their families at least feel included among their classmates.”
—Peter Mills Clermont, Fla.
In response to Lampi: The similarity between our images showing the strong force’s coupling constant (αs) at short distances and a B-H curve (the hysteresis curve in magnetism) is that they both reflect a saturation phenomenon. The saturation occurs because the original cause of the growth comes to be suppressed. For B-H curves, saturation results when all magnetic domains become aligned with the external field.
Regarding αs, it represents quantum loops involving massless gluon fields that initially cause the rise with decreasing momentum (increasing separation). Deeply locked within the physics of gluon self-interactions, however, is a mechanism that makes gluons behave as heavy particles at low momentum. Because, quite generally, it is much harder for heavy particles to make loops, the heavy gluon cannot contribute to quantum activity at momenta below the associated gluon mass scale. With nothing left to make the coupling “run,” it stops running. This is the saturation seen in our article’s figures.
EXCLUSION AMONG CHILDREN
“The Inclusive Classroom,” by Melanie Killen, concerns prejudice among young children and a strategy for combating it in schools. These kinds of interventions are important, but they also need to be more intersectional to address sexual orientation and disability status.
I am an 86-year-old gay, dyslexic man who was deathly afraid of being bullied and excluded for being different as a child. I was in the closet even to myself until I was 30 years old. That and dyslexia have framed my entire life from a very early age. I did everything in my power to fit in and not let people know I was different. I managed to hide who I was until I was 50 and finally had to come all the way out and separate from a lovely marriage. All this time I was scared that somebody would find out and that people would not accept me.
These two axes of identity, sexual orientation and disability status, are different from race and class in that children and their parents almost always share the same race and class. Queer children and disabled children, however, often have parents who are not queer or disabled. It’s critically important for these children to feel included among their peers because they might not find that kind of inclusion and support at home. In many cases that I am familiar with, friends of mine were kicked out of supposedly upstanding and religious households when they were no more than early teenagers. Many were sent to conversion therapy, which does not appear to work and traumatizes the youngsters going through it further. For many queer kids, just mentioning their sexuality at home can be dangerous.
It’s critically important that kids who are rejected by their families at least feel included among their classmates. Group exclusion doesn’t happen only among one’s peers; it appears in some homes and pervades our political and religious systems. It appears that many schools are not prepared to help classes be more inclusive of LGBTQ students and disabled students. I hope that these kinds of initiatives can be embraced and can help young students learn to be more inclusive of broad, intersectional identities.
PETER MILLS CLERMONT, FLA.
ERRATUM
“Superheavies,” by Stephanie Pappas [June], should have said that Einstein’s special theory of relativity suggests that objects moving at nearly the speed of light gain mass and get weird, not that his general theory of relativity does so.
