According to quantum mechanics, even the most evacuated of spaces, such as a box emptied of all visible and invisible matter, is not truly empty. As Peter Milonni of the University of Rochester notes, the residual energy that remains is known as ground-state energy or zero-point energy, a phenomenon that has been studied since its introduction by Max Planck in 1911.
This unavoidable residue comes in two basic forms: one associated with fields like the electromagnetic field and the other with discrete objects such as atoms and molecules. Despite efforts to eliminate it, zero-point energy persists, a characteristic of any material structure or object that is at least partly confined.
The Quantum Conundrum
The existence of zero-point energy reveals a deeper level of quantum complexity, one that depends on the interpretation of quantum mechanics. As Milonni explains, if particles are situated in their lowest energy state and measured, they will exhibit a spread of values, appearing to jiggle despite being drained of energy. This phenomenon has been observed in experiments, such as one published in 2025 by researchers at the European X-Ray Free-Electron Laser Facility, who found that cooled iodopyridine molecules still vibrated when broken with a laser pulse.
Rebecca Boll, an experimental physicist at the facility, notes that this discovery was not the main goal of the experiment but rather an unexpected finding. The implications of zero-point energy are far-reaching, influencing our understanding of fields and particles at the quantum level.
Fields and Particles
Quantum field theorists describe fields as collections of oscillators, each with its own zero-point energy. This leads to an infinite amount of zero-point energy in a field, a concept that was initially doubted but later accepted as physicists learned to work with the infinities. However, this creates a problem when considering gravity, as an infinite amount of zero-point energy should produce a powerful gravitational field, a paradox that remains unresolved.
Sean Carroll, a physicist at Johns Hopkins University, emphasizes that all forms of energy gravitate, including vacuum energy, which cannot be ignored. The reason for this muted gravitational effect remains a mystery, highlighting the ongoing challenges in understanding the quantum vacuum.
The Potential to Be Anything
Despite the complexities, zero-point energy represents more than just a challenge or a reason why a box cannot be truly emptied. It signifies the potential for anything to emerge from the vacuum, as every field and particle is somehow represented. As Milonni notes, even in the absence of electrons, the vacuum contains electronness, a concept that underscores the inherent potential of the quantum vacuum.
This concept has been explored in various experiments and theories, including the Casimir effect, which predicts that two uncharged plates will exert a force on each other due to the difference in zero-point energy. The study of zero-point energy continues to reveal the intricacies of the quantum world, pushing our understanding of the fundamental laws of physics.
