Fundamental physics comprises the attempt to understand the nature of the stable elementary particles (leptons and quarks), the messenger particles (gauge bosons) which mediate the interactions, and the relationship of the four interactions (electromagnetism, weak, strong and gravitational) to each other. Experiments in which particles are made to collide are consistent with elementary particles being immeasurably small, structureless objects. The upper limit on the collisionally-measured size of the electron, for example, is <10^-17 cm. However modern physics theory no longer views particles as point-like objects. In place of that view, quantum field theory assumes that all of space is filled with a quantum field and interprets all stable particles and the messenger particles as excitations of this field. This has resulted in the "standard model" which can legitimately boast precision of some predicted particle properties to an amazing 13 significant figures, but requires 19 hand-adjusted parameters as basic input. There is also the central problem that quantum theory appears to be fundamentally incompatible with general relativity.

It now appears that quantum field theory may be the low energy limit of superstring theory. Superstring theory assumes that spacetime is not merely four-dimensional, but rather that there are many additional dimensions -- such as six Calabi-Yau dimensions -- which exist but differ from the ordinary space and time that we experience in everyday life by virtue of being curled up on themselves. Both the stable particles and the messenger particles are regarded as loops of string. Parallel to quantum field theory, particles are interpreted as excitations of such strings. As bizarre as superstring theory may sound to the layman, there are amazing properties, such as resolving the conflict between quantum laws and general relativity and having no necessary free parameters, that make the theory quite intriguing. An excellent overview is that of string theorist, Brian Greene, in his book The Elegant Universe. (Recent developments since 1997 indicate that superstring theory may itself be a subset of an even more comprehensive theory, M-brane theory, which adds yet another compact dimension to superstring theory.)

In both quantum field theory and superstring theory, the quantum field excitations or string representations of particles have no intrinsic inertia. We use the term "inertia" deliberately in place of "mass" because in both quantum field theory and in superstring theory there is a postulated mechanism for massless particles to acquire mass from interactions with an hypothesized Higgs field. However the mass that is acquired in this way is mass in the sense of equivalent energy, not in the sense of inertia. If one assumes that inertia is an intrinsic property of mass or its energy equivalent, a Higgs mechanism may indeed be the end of the story. However the possibility that there exists an extrinsic mechanism for generating inertia goes back at least to the work of Ernst Mach in the 19th century. As discussed in great detail in the book Concepts of Mass in Contemporary Physics and Philosophy by physicist-philosopher Max Jammer the question of why a reaction force should arise when any physical object is accelerated remains a legitimate and heretofore unanswered question.

It is suggested that inertia is indeed a fundamental property that has not been properly addressed even by superstring theory. The acquisition of mass-energy may still allow for, indeed demand, a mechanism to generate an inertial reaction force upon acceleration. Or to put it another way, even when a Higgs particle is finally detected establishing the existence of a Higgs field, one may still need a mechanism for giving that Higgs-induced mass the property of inertia. A mechanism capable of generating an inertial reaction force has been discovered using the techniques of stochastic electrodynamics (origin of inertia). Perhaps this simple yet elegant result may be pointing to a deep new insight on inertia and the principle of equivalence, and if so, how this may be unified with modern quantum field theory and superstring theory.

The empty vacuum of older physics is today replaced by an active one in which virtual particles come into and go out of existence on timescales allowed by Heisenberg's uncertainty principle. A concrete proof of this is the measurement of the distance (or energy) dependence of the fine-structure "constant". This is explained by vacuum polarization, wherein the electric charge of a (real) particle is partially screened by those of other (virtual) particles. In general, the physics of the quantum vacuum is a rich if complex subject.

A better understanding of the origin of inertia would lead to new insights into the laws of motion, perhaps with practical applications such as to spacecraft propulsion (in the far future). The laws of the quantum vacuum are not completely understood, but certainly their manifestations are frequently stochastic. Fluctuations of vacuum fields are irregular, but their averaged effects can be calculated using quantum field theory (QFT). Within the rather broad scope of the latter term, calculations agree with observations to great accuracy in processes where electrons interact with photons, i.e. quantum electrodynamics (QED). The basic formulation of QFT as a theory of quantum electrodynamics can be extended also to the theory of the strong or nuclear interaction, where under the term quantum chromodynamics (QCD) it may be a subject for study in the future. Right now, probably the best-studied consequence of QFT as applied to electrodynamics comes from measurements of the Casimir effect. This effect, wherein parallel plates in apparently empty space experience a force of attraction, clearly shows that the quantum vacuum is not passive. Useful calculations can also be done in this subject using a semiclassical approach to the interactions of charged particles with an electromagnetic field known as stochastic electrodynamics (SED). One version of the latter envisages a zero-point electromagnetic field whose quanta buffet charged particles, producing a microscopic motion whih Schroedinger dubbed "zitterbewegung". Using the techniques of SED an intriguing new theoretical approach is suggesting a deep connection between electrodynamics, the origin of inertia and the quantum wave nature of matter.

Primary Articles (see Scientific Articles for additional articles)

Gravity and the Quantum Vacuum Inertia Hypothesis
Alfonso Rueda & Bernard Haisch, Annalen der Physik, Vol. 14, No. 8, 479-498 (2005).

Review of Experimental Concepts for Studying the Quantum Vacuum Fields
E. W. Davis, V. L. Teofilo, B. Haisch, H. E. Puthoff, L. J. Nickisch, A. Rueda and D. C. Cole, Space Technology and Applications International Forum (STAIF 2006), p. 1390 (2006).

Analysis of Orbital Decay Time for the Classical Hydrogen Atom Interacting with Circularly Polarized Electromagnetic Radiation
Daniel C. Cole & Yi Zou, Physical Review E, 69, 016601, (2004).

Inertial mass and the quantum vacuum fields
Bernard Haisch, Alfonso Rueda & York Dobyns, Annalen der Physik, Vol. 10, No. 5, 393-414 (2001).

Stochastic nonrelativistic approach to gravity as originating from vacuum zero-point field van der Waals forces
Daniel C. Cole, Alfonso Rueda, Konn Danley, Physical Review A, 63, 054101, (2001).

The Case for Inertia as a Vacuum Effect: a Reply to Woodward & Mahood
Y. Dobyns, A. Rueda & B.Haisch, Foundations of Physics, Vol. 30, No. 1, 59 (2000).

On the relation between a zero-point-field-induced inertial effect and the Einstein-de Broglie formula
B. Haisch & A. Rueda, Physics Letters A, 268, 224, (2000).

Contribution to inertial mass by reaction of the vacuum to accelerated motion
A. Rueda & B. Haisch, Foundations of Physics, Vol. 28, No. 7, pp. 1057-1108 (1998).

Inertial mass as reaction of the vacuum to acccelerated motion
A. Rueda & B. Haisch, Physics Letters A, vol. 240, No. 3, pp. 115-126, (1998).

Reply to Michel's "Comment on Zero-Point Fluctuations and the Cosmological Constant"
B. Haisch & A. Rueda, Astrophysical Journal, 488, 563, (1997).

Quantum and classical statistics of the electromagnetic zero-point-field
M. Ibison & B. Haisch, Physical Review A, 54, pp. 2737-2744, (1996).

Vacuum Zero-Point Field Pressure Instability in Astrophysical Plasmas and the Formation of Cosmic Voids
A. Rueda, B. Haisch & D.C. Cole, Astrophysical Journal, Vol. 445, pp. 7-16 (1995).

Inertia as a zero-point-field Lorentz force
B. Haisch, A. Rueda & H.E. Puthoff, Physical Review A, Vol. 49, No. 2, pp. 678-694 (1994).