Tachyons: Scientific Evolution(Mainstream)

Early Concepts & Relativity (Early 20th Century - 1950s)

1904-1905: While not explicitly proposing tachyons, Arnold Sommerfeld explores the possibility of entities traveling faster than light in the context of Max Abraham's rigid electron theory, predating Special Relativity. Albert Einstein's theory of Special Relativity establishes the speed of light in vacuum (c) as a universal speed limit for energy and information transfer, seemingly precluding faster-than-light (FTL) particles. Conventional objects require infinite energy to reach c.
Post-1905: Theoretical discussions occasionally touch upon FTL possibilities within relativity, often concluding they lead to causality violations (effects preceding causes in some reference frames).

1962: Olexa-Myron Bilaniuk, V.K. Deshpande, and E. C. George Sudarshan publish a paper ("Meta-Relativity") re-examining Special Relativity and proposing the theoretical possibility of particles that always travel faster than light. They note such particles would have imaginary rest mass and peculiar energy-momentum relationships (losing energy as they speed up).
1967: Gerald Feinberg publishes a paper ("Possibility of Faster-Than-Light Particles") independently exploring FTL particles within the framework of quantum field theory. He coins the name "tachyon" (from Greek ταχύς, tachys, meaning "swift"). Feinberg's model focuses on particles with imaginary mass in quantum field theory and addresses potential causality issues with a "reinterpretation principle," suggesting a tachyon appearing to travel backward in time is indistinguishable from an anti-tachyon traveling forward.

Late 1960s onwards: Extensive theoretical work explores the properties and consequences of tachyons. Key characteristics established include:
- Imaginary rest mass.
- Speed always greater than c.
- Losing energy as speed increases, approaching infinite speed as energy approaches zero.
- Potential to produce Cherenkov-like radiation in vacuum.
Causality Paradoxes: Intense debate focuses on the "tachyonic anti-telephone" thought experiment, illustrating how tachyons could, in principle, allow sending signals into one's own past, violating causality. The reinterpretation principle is discussed but not universally accepted as fully resolving the paradoxes within simple models.

1970s onwards: Numerous experiments search for tachyons, with none finding conclusive evidence. Methods include:
- Cosmic Rays: Searching for precursor signals or anomalous particle showers.
- Particle Accelerators: Looking for missing energy/momentum signatures or direct production in collisions.
- Astronomical Observations: Searching for astrophysical signatures.
- Neutrino Experiments: While neutrinos are not tachyons, experiments measuring neutrino speeds (like the OPERA anomaly, later found to be erroneous) are relevant to the broader search for FTL phenomena.
Consistent Null Results: Despite decades of searching, no confirmed detection of tachyonic particles has been made.

Quantum Field Theory (QFT): In QFT, a field with an imaginary mass squared (a tachyonic field) does not represent a stable FTL particle. Instead, it signals an instability in the theory's vacuum state. The field tends to "condense" – acquiring a non-zero vacuum expectation value – which shifts the vacuum to a new, stable state where the physical excitations (particles) have non-negative mass squared (and thus do not travel faster than light). This process is known as tachyon condensation.
String Theory: Tachyonic fields appear naturally in early versions of bosonic string theory and sometimes in superstring theory. Their presence indicates an instability in the spacetime configuration (e.g., unstable D-branes). Tachyon condensation plays a crucial role in resolving these instabilities and is linked to profound concepts like the decay of unstable D-branes into stable ones or even into closed string vacuum.

Hypothetical Nature: Tachyons, as actual FTL particles, remain purely hypothetical. There is no experimental evidence for their existence.
Theoretical Tool: The concept of tachyonic fields (indicating instability) is a valuable tool in theoretical physics, particularly in string theory and cosmology, to understand vacuum decay and phase transitions.
Causality Barrier: The profound implications for causality remain a major theoretical hurdle for the existence of interacting FTL particles.
Consensus: The overwhelming scientific consensus, based on theory and lack of evidence, is that fundamental particles cannot propagate faster than light through spacetime in a way that transmits information. Tachyonic fields in modern theories are interpreted as instabilities rather than actual FTL particles.