Empty is not empty
Vacuum of space
Pressure, quantum fields, virtual particles.
Industrial vacuum pumps remove most gas molecules. Remaining particles still collide, creating ~10⁻⁶ Pa pressure.
Definitions
- Absolute pressure
- Force per unit area exerted by gas molecules. Units: Pascal (Pa). 1 atm = 101,325 Pa.
- Vacuum
- A region with particles removed. No perfect vacuum exists; “vacuum” means “reduced pressure.” Industrial vacuum: 10⁻⁶ Pa. Interstellar space: 10⁻¹⁷ Pa (but not zero).
- Casimir effect
- Attractive force between two uncharged metal plates in vacuum, due to quantum field fluctuations. Magnitude ~0.1 dyne/cm² at 1 μm separation.
- Virtual particles
- Quantum fluctuations in vacuum: electron–positron (or photon) pairs constantly created and annihilated, allowed by the uncertainty principle for timescales Δt ~ ℏ/ΔE.
- Quantum vacuum
- Ground state of quantum fields. Not empty—filled with zero-point energy and virtual particle activity. Energy density nonzero.
- Zero-point energy
- Lowest possible energy of a quantum system (E = ½ℏω for a harmonic oscillator). Cannot be removed—intrinsic to quantum mechanics.
Physics
Earth's atmosphere at sea level contains ~2.5 × 10¹⁹ air molecules per cm³, exerting 1 atm (101 kPa) pressure. Pressure comes from collisions of molecules with surfaces.
A good laboratory vacuum removes ~99.9999% of molecules, reaching pressures of 10⁻⁶ Pa. Even at this extreme dilution, some molecules remain. A typical turbomolecular pump and diffusion pump achieve this. Ultrahigh vacua (10⁻¹⁰ Pa) require baking, cryogenic traps, and ion pumps.
Interstellar space is an even better vacuum: ~10⁻¹⁷ Pa, or ~100 particles per cm³. Yet it is not empty—hydrogen atoms, dust grains, and radiation fill it.
In the 1940s, Dutch physicist Hendrik Casimir predicted an unexpected effect: two uncharged metal plates held very close (nanometers to micrometers apart) experience an attractive force, even in vacuum. The reason? Quantum field fluctuations. The electromagnetic field vacuum is not truly zero; virtual photons constantly appear and disappear. Between the plates, only certain wavelengths fit, so fewer virtual photons exist there than outside. Pressure from outside pushes the plates together.
The Casimir force is ~0.1 dyne/cm² at 1 μm separation—tiny, but measurable with careful experiments. It is genuine evidence that the vacuum is not empty but seethes with quantum activity.
Quantum field theory says that every field (electron, photon, gluon, Higgs) fluctuates quantum-mechanically in its ground state. The uncertainty principle allows virtual particle–antiparticle pairs (e⁺e⁻, μ⁺μ⁻, etc.) to pop in and out of existence for times Δt ~ ℏ/(2mc²). An electron can temporarily emit a virtual photon and reabsorb it; this affects the electron's observed mass and magnetic moment. These effects are measured precisely (g−2 electron and muon anomalous magnetic moments).
However, no direct observation of virtual particles as real, free entities has been made. They are a mathematical necessity of the quantum field theory formalism, not a measurable population.
Sources
- Casimir, H. B. G. (1948). On the attraction between two perfectly conducting plates. Proceedings of the Royal Netherlands Academy, 51, 793–795.
- Peskin, M. E., & Schroeder, D. V. (1995). An Introduction to Quantum Field Theory. Addison-Wesley. (Virtual particles, vacuum energy, Casimir effect.)
- Lamoreaux, S. K. (2005). The Casimir force: background, experiments, and applications. Reports on Progress in Physics, 68(1), 201–236.