Unveiling the Mystery of Nuclear Forces: "Where the Strongest Force Comes to Life"
- bpsinghamu
- Jul 21
- 8 min read
By Prof. Dr. B. P. Singh | Department of Physics, AMU, Aligarh
At the very heart of every atom lies a tiny nucleus. Inside this tiny space are packed, positively charged protons and neutral neutrons, together called nucleons. These are packed so closely that it's hard to believe they don’t fly apart. What keeps them together is not magnetism or gravity, but a different, rather much stronger interaction called the nuclear force. This invisible nuclear force is sometimes popularly called the unsung hero of the universe, that holds the matter together. It also powers stars, and enable life to exist. Understanding this force means, understanding why atoms do not fall apart and why the universe is the way it is.
The journey to explaining nuclear forces began with the pioneering experiment by Lord Ernest Rutherford in 1909. The experiment conducted was famous "gold foil experiment". Rutherford and his colleagues Geiger and Marsden bombarded a thin foil of gold with alpha particles. They observed that, while most of these alphas passed through, a few deflected at large angles, and some even bounced back to the direction from where they were coming out of radioactive source, that is at 180 degrees. "Rutherford and his team expected the alpha particles to pass through the gold foil with barely any deflection, just like tiny bullets through empty space. But a few of them bounced back! This was so shocking that Rutherford famously said: 'It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.'
This comparison shows just how unexpected the result was. It meant that atoms were not just the clouds of positive charge but there had to be something small, very dense, and powerful at the center called the nucleus."
This observation was quite shocking and led Ernest Rutherford to propose a new model of the atom as a dense, positively charged nucleus surrounded by mostly empty space. This led to revelation that laid the groundwork for modern nuclear science, on which most of the nuclear physics is based on.
As nuclear science progressed, we learned more and more about the structure of the nucleus. Now we know that, nucleus comprises of protons, which are positively charged, and neutrons, which carry no charge or are neutral. Together these are called nucleon. Neutron and protons are bound inside the atomic nucleus by the strong nuclear force.
To describe a nucleus, we use two important numbers; the atomic number (Z), which tells us the number of protons, and the atomic mass number (A), which is the total number of nucleons. The number of neutrons (N) is obtained by simply subtracting Z from A i.e., (A-Z). In general, most of the stable nuclei have even numbers of both protons and neutrons. Further, many nuclei are not perfectly spherical, they are elongated like a rugby ball, in nuclear physics terminology called "prolate" or flattened like a discus called "oblate". This shape is measured by a property called the electric quadrupole moment (Q). A positive value of Q means the nucleus is prolate; a negative value of Q means it is oblate.
Beyond shapes, nuclei also have spin, behaving like tiny spinning tops. This spinning motion gives rise to magnetic properties, much like bar magnets. Moreover, some nuclei, especially those with so-called “magic numbers” of nucleons; such as 2, 8, 20, 28, 50, 82, and 126, exhibit extraordinary stability. These numbers represent filled energy shells within the nucleus, similar to noble gases in atomic structure.
One of the biggest mysteries early on was understanding how protons, which naturally repel each other due to their like charges, manage to stay confined within such a tiny volume. The answer lies in the strong nuclear force, which is a powerful, short-range force that dominates over electrostatic repulsion within the nucleus. Unlike gravitational or electromagnetic forces, which have infinite ranges, nuclear forces act only within a few femtometers (1 fm = 10⁻¹⁵ m). This incredibly short but intense force binds nucleons together and maintains the integrity of the nucleus.
Strong evidence for the nuclear force came from experiments involving the scattering of high-energy alpha particles. When these particles were fired at target nuclei, the scattering didn’t match predictions based only on Coulomb forces. Especially at small impact parameters, the alpha particles experienced an extra pull. This was proof of an attractive force operating beyond electromagnetism. This anomaly was our first look at the nuclear force in action.
In the broader framework of nature, four fundamental forces govern all interactions: the gravity, the electromagnetism, the weak force, and the strong nuclear force. Among these, the strong nuclear force is by far the most powerful, though it acts only over very short distances (10⁻¹⁵ m). A comparison of their relative strengths shows the strong nuclear force as the benchmark (strength = 1), followed by electromagnetism (10⁻²), the weak force (10⁻¹²), and gravity (10⁻³⁹).
Table 1: Relative strength of various forces
Interaction | Strength | Quanta |
Strong | 1 | p-meson |
Electromagnetic | 10-2 | Photon |
Weak (beta decay) | 10-12 | W-boson |
Gravitational | 10-39 | Graviton |
Even though we do not experience the nuclear forces directly in our everyday lives, their effects are all around us. In fact the "Gravity" helps us walk. "Electromagnetic forces" power our devices. The "weak force" plays a role in radioactive decay and is used in various medical techniques. The "strong nuclear force", though is not seen or observed in daily routine, is very much essential. It ensures that atoms remain intact, and that stars burn and produce light, and that the matter exists in any form.
To appreciate how concentrated and intense the nucleus is, consider that it contains nearly all the atom’s mass but is about 100,000 times smaller in size. This means nucleons are crammed into an incredibly dense space.
The binding energy per nucleon is a measure of how tightly nucleons are held together and is typically around 8 MeV. In contrast, the binding energy of electrons in atoms is only a few eV. This dramatic difference underscores how powerful nuclear forces are as compared to electrical forces ones.
Despite the presence of magnetic moments within nucleons, neither gravitational nor magnetic forces are nearly strong enough to account for nuclear binding. For instance, the gravitational attraction between two protons separated by a typical nuclear distance (~1 femtometer) is a much much small force of about 1.87×10⁻³⁵ newtons. To give a sense of scale, this is roughly 10⁻³⁵ times weaker than the strong nuclear force acting at the same distance. Even when separated by over 130 meters, the gravitational pull between protons would still be only about 10⁻⁶⁸ newtons. This is completely negligible in nuclear physics.
While the intrinsic spin of nucleons gives rise to magnetic interactions, these too are vastly overshadowed by the strong nuclear force. Magnetic interactions contribute only tiny corrections to nuclear structure and energy levels. This is why modern nuclear theory emphasizes quantum chromodynamics (QCD) and residual strong interactions over classical forces like magnetism or gravity when explaining the nature of nuclear binding.
One particularly informative system is the deuteron, which is the simplest bound nucleus, made of one proton and one neutron. The deuteron has spin 1, and interestingly, it’s not perfectly spherical. This asymmetry indicates the presence of the tensor component of the nuclear force, which depends on both spin orientation and spatial arrangement.
The deuteron prefers parallel spins, as the anti-parallel configuration does not result in a bound state. This preference gives us insight into the spin dependence of nuclear forces.
As atomic nuclei get heavier, they contain more protons—and more electrostatic repulsion. To balance this out, more neutrons are needed. That’s why heavy stable nuclei tend to have a surplus of neutrons. The excess of neutrons acts as a glue, increasing the attractive nuclear force without adding repulsion.
Another intriguing property is saturation. One might assume that every nucleon interacts with all others, but that’s not the case. Nuclear forces are short-ranged and saturate, meaning each nucleon effectively interacts only with its nearest neighbors. This leads to a consistent average binding energy per nucleon across medium and heavy elements.
To explain how nucleons bind so effectively, physicists introduced the concept of exchange forces. In this model, protons and neutrons exchange particles called mesons (specifically, π-mesons or pions), creating a bond between them. Think of it like playing catch with a ball—the exchange keeps them connected.
These exchanges lead to different force components, such as Heisenberg (exchange of position and spin), Majorana (exchange of position), and Bartlett (exchange of spin).
However, not all nuclear behavior can be explained by two-body forces alone. In heavier nuclei, many-body forces come into play. These involve simultaneous interactions between three or more nucleons. The complexity increases rapidly, and such interactions are difficult to model but crucial to accurately describe nuclear behavior.
Moving deeper, we now know that protons and neutrons are not elementary particles. They are made of quarks, tiny constituents bound together by gluons via the strong interaction described by quantum chromodynamics (QCD). At research centers like CERN, using the Large Hadron Collider (LHC), scientists smash protons together at near light speeds, revealing exotic states of matter like the quark-gluon plasma, the same state that likely existed microseconds after the Big Bang.
The nuclear force, then, is not just a concept in physics textbooks. It is a living, dynamic reality that governs the structure of matter. It governs the life cycle of stars, the function of nuclear reactors, and even the future of energy on Earth. From Rutherford’s gold foil to quark-gluon plasma at CERN in Switzerland, our understanding of this force continues to grow and grow. As we probe deeper into the matter with high and high energy of particles, one truth that remains is that nuclear forces are not just holding nuclei together, but they’re holding the universe itself.
The Large Hadron Collider (LHC) is the world’s most powerful particle accelerator, located at CERN, near Geneva on the border of Switzerland and France. It’s a massive circular tunnel, 27 kilometers long, buried about 100 meters underground.
Inside this ring, protons are accelerated to speeds incredibly close to the speed of light. These protons are then made to collide head-on, releasing enormous energy and recreating conditions similar to those just after the Big Bang.
Why do we do this? To explore the building blocks of the universe. The LHC allows scientists to study fundamental particles, like quarks, gluons, and the Higgs boson (discovered here only in 2012).
These discoveries help us understand what matter is made of, how forces behave, and how the universe evolved. The LHC houses gigantic (huge) detectors, such as ATLAS and CMS, which capture and analyze the debris from particle collisions. It is not just a scientific marvel, it's a window into the deepest secrets of the universe. Scientists are trying to help us answer big questions like:
Where does mass come from?
What was the early universe like?
Are there hidden dimensions?
As a scientist we can proudly sat that the LHC is humanity’s boldest attempt to uncover the ultimate truth about matter, energy, space, and time. Keep on exploring!
ATOM DOES NOT CHOSE ITS PURPOSE WE DO!
For more exciting information, keep on visiting Atomic Explorers, regularly!
Bhanu Prakash Singh is currently working as a Professor in the Department of Physics at Aligarh Muslim University, Aligarh, INDIA. He has also served as the Chairman of the department. He is involved in research on accelerator based nuclear reaction studies with a focus on light and heavy ion induced reactions. He has nearly 30 years of experience in teaching and research. Prof. Singh is actively engaged in science popularization activities. He is author of the Atomic Explorers.
Comments