What happened to Tachyons which were thought to travel faster than light?

 Tachyons remain purely hypothetical and have never been experimentally observed in nature. While the concept was mathematically formalized in the 1960s to explore the possibility of particles that always travel faster than light, mainstream physics today largely views tachyons not as physical speedsters, but as a mathematical indicator of an unstable quantum system.

Here is how the scientific understanding of tachyons has evolved:

• The Causality Paradox: According to the special theory of relativity, if a particle could travel faster than light, it would theoretically allow signals to be sent backward in time (a concept sometimes called the "tachyonic antitelephone"). Because this breaks the fundamental laws of causality—where a cause must precede its effect—most physicists dismiss the existence of physical tachyons.
  
  
• Reinterpreted as "Tachyon Condensation": In mathematical terms, tachyons are characterized as having an "imaginary rest mass" (where their mass-squared is negative). In modern quantum field theory, an imaginary mass does not denote a superluminal particle, but rather an unstable field. Like a ball perched precariously on top of a hill, a tachyonic field will inevitably "roll down" to a lower, stable energy state. This process of stabilizing is known as tachyon condensation.
  
  
• The Higgs Field: The most famous application of this mathematics is the Higgs mechanism. In the early universe, before spontaneous symmetry breaking, the Higgs field possessed an imaginary mass and was technically "tachyonic". When the universe cooled and the field stabilized (condensed), it did not produce faster-than-light particles; instead, it became a normal field with positive mass, granting mass to other fundamental particles.
  
  
• String Theory: Early versions of string theory (Bosonic string theory) inherently produced tachyonic fields in their mathematical ground states. Physicists recognized this as a sign that the theoretical model was unstable, which helped drive the development of modern Superstring theory to resolve the instability and remove the tachyons.
  
  
• Modern Speculation: Theoretical physicists still occasionally publish speculative mathematical frameworks attempting to reconcile tachyons with relativity, or suggesting tachyonic fields could explain cosmic phenomena like dark matter. However, these ideas face heavy scientific skepticism, often fail basic quantum requirements, and continue to lack any observational proof.