The concept of a void, often understood philosophically as pure nothingness, takes on a complex physical definition in science. Across various scales, from the subatomic to the cosmological, a void is rarely truly empty. Instead, it represents a state of minimal density or energy that challenges our common perception of space. Science explores voids in three distinct ways: the classical vacuum, the quantum vacuum, and the cosmic void. Understanding these forms of emptiness reveals unique properties and phenomena governed by profound physical laws.
Defining the Scientific Void: Classical Vacuum
The classical vacuum is the traditional, macroscopic view of emptiness, defined simply as a region devoid of matter and having pressure significantly below that of the atmosphere. This is the vacuum created and studied in laboratories, often referred to as a partial vacuum. While a perfect vacuum—one with absolutely no atoms or molecules—is a theoretical ideal, scientists routinely achieve ultra-high vacuum conditions in specialized chambers.
These laboratory vacuums can reach pressures far below atmospheric pressure, leaving only a handful of particles per cubic centimeter. Such environments are necessary for manufacturing semiconductors or conducting particle physics experiments. Measuring this low pressure requires specialized instruments, such as Capacitance Manometers for direct measurement or Ionization Gauges, which infer pressure by measuring the flow of ions.
The vacuum of deep space is an even higher-quality vacuum than any achievable on Earth, especially in the vast regions between galaxies. Intergalactic space holds, on average, only a few hydrogen atoms per cubic meter. However, the presence of faint electromagnetic radiation and energetic particles means the classical void of space is never entirely free of content. This macroscopic definition centers purely on the absence of atoms and molecules.
The Quantum Reality: A Void That Is Not Empty
The classical understanding of an empty void is fundamentally contradicted by the principles of quantum mechanics, which introduce the concept of the quantum vacuum. According to quantum field theory, even space completely emptied of matter and energy is not static but a seething sea of activity. This lowest possible energy state is known as the zero-point energy.
This apparent emptiness is constantly filled with “virtual particles,” which spontaneously pop into and out of existence in fleeting particle-antiparticle pairs. These temporary particles are permitted by the Heisenberg Uncertainty Principle, allowing for a momentary violation of energy conservation before the particles annihilate each other. The quantum vacuum is therefore a dynamic, energetic field.
The physical reality of this quantum void is demonstrated by the Casimir effect, first predicted in 1948. This effect describes a small attractive force between two uncharged, parallel conducting plates placed extremely close together. The plates restrict the wavelengths of the virtual particles between them, leading to a lower energy density inside the gap compared to the outside. This pressure imbalance pushes the plates together, confirming that the quantum void possesses real, measurable physical properties.
Voids on the Largest Scale: Cosmic Structures
Shifting the perspective to the largest known structures in the universe reveals the existence of cosmological voids. These are immense, relatively spherical regions of space that contain a significantly lower density of luminous matter, such as galaxies, compared to the cosmic average. They are the emptiest components of the “cosmic web,” the network of filaments and superclusters that defines the large-scale structure of the universe.
These voids typically span tens to hundreds of millions of light-years across, forming the gaps between the dense walls and filaments of galaxies. They are believed to have formed shortly after the Big Bang, where tiny fluctuations in the early universe’s density were amplified by gravity. Gravity pulled matter away from these under-dense regions, concentrating it into the surrounding filaments and clusters.
While these regions are called voids, they are not truly empty, as they are still permeated by dark matter, dark energy, and the faint, cold radiation of the Cosmic Microwave Background (CMB). The average density of matter within a typical void is only about 10% of the universe’s average density. The presence of dark energy ensures that even these regions contribute to the overall expansion of space.
Detecting and Measuring the Absence
Scientists confirm and study these varying states of emptiness using distinct observational and laboratory methods specific to each type of void. The classical vacuum is measured using a range of mechanical and electrical gauges. Pirani gauges determine pressure by measuring the heat loss from a heated wire, while Ionization gauges measure the current created by ionizing the few remaining gas particles.
The quantum vacuum’s properties are probed through high-precision experiments, most notably those measuring the Casimir effect force on microscopic plates. These experiments compare theoretical predictions of quantum field theory with empirical observations. This provides confirmation of the vacuum’s non-zero energy state.
Cosmic voids are detected and mapped using large-scale galaxy redshift surveys, which are the primary tools for charting the three-dimensional distribution of matter in the universe. By measuring the redshift—the stretching of light from distant galaxies due to the expansion—astronomers determine the distance to millions of galaxies. Plotting these locations reveals the characteristic web-like structure of the universe, clearly delineating the vast, galaxy-poor regions. Furthermore, subtle temperature variations in the Cosmic Microwave Background radiation can indicate the presence of supervoids, as the light passing through these massive under-densities loses energy.