Most often it is assumed that in the present universe a delicate balance must exist between energy density from a background Higgs field (see page 1047 ) and a cosmological term in the Einstein equations (see page 1052 ).

Messages to send [to extraterrestrials] The idea of trying to send messages to extraterrestrials has existed since at least the early 1800s.

Quantum effects Over the years, many suggested effects have been thought to be characteristic of quantum systems: • Basic quantization (1913): mechanical properties of particles in effectively bounded systems are discrete; • Wave-particle duality (1923): objects like electrons and photons can be described as either waves or particles; • Spin (1925): particles can have intrinsic angular momentum even if they are of zero size; • Non-commuting measurements (1926): one can get different results doing measurements in different orders; • Complex amplitudes (1926): processes are described by complex probability amplitudes; • Probabilism (1926): outcomes are random, though probabilities for them can be computed; • Amplitude superposition (1926): there is a linear superposition principle for probability amplitudes; • State superposition (1926): quantum systems can occur in superpositions of measurable states; • Exclusion principle (1926): amplitudes cancel for fermions like electrons to go in the same state; • Interference (1927): probability amplitudes for particles can interfere, potentially destructively; • Uncertainty principle (1927): quantities like position and momenta have related measurement uncertainties; • Hilbert space (1927): states of systems are represented by vectors of amplitudes rather than individual variables; • Field quantization (1927): only discrete numbers of any particular kind of particle can in effect ever exist; • Quantum tunnelling (1928): particles have amplitudes to go where no classical motion would take them; • Virtual particles (1932): particles can occur for short times without their usual energy-momentum relation; • Spinors (1930s): fermions show rotational invariance under SU(2) rather than SO(3); • Entanglement (1935): separated parts of a system often inevitably behave in irreducibly correlated ways; • Quantum logic (1936): relations between events do not follow ordinary laws of logic; • Path integrals (1941): probabilities for behavior are obtained by summing contributions from many paths; • Imaginary time (1947): statistical mechanics is like quantum mechanics in imaginary time; • Vacuum fluctuations (1948): there are continual random field fluctuations even in the vacuum; • Aharonov–Bohm effect (1959): magnetic fields can affect particles even in regions where they have zero strength; • Bell's inequalities (1964): correlations between events can be larger than in any ordinary probabilistic system; • Anomalies (1969): virtual particles can have effects that violate the original symmetries of a system; • Delayed choice experiments (1978): whether particle or wave features are seen can be determined after an event; • Quantum computing (1980s): there is the potential for fundamental parallelism in computations.

The first idea was to represent gravity as a field that exists in flat spacetime, and by analogy with photons in quantum electrodynamics to introduce gravitons (at one point identified with neutrinos).

However, perhaps in part because of a shift towards probabilistic theories such as quantum and statistical mechanics there remained the conviction that for relevant aspects of behavior formulas should still exist.