A negative squared mass in a relativistic field spectrum signals instability of the background about which the field is expanded. The bosonic string mass spectrum contains such an unexcited state.
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"Tachyon" can refer to a couple of different concepts depending on the context: 1. **Physics**: In theoretical physics, a tachyon is a hypothetical particle that travels faster than the speed of light. The concept of tachyons arises in certain interpretations of quantum field theory and relativity. If they exist, tachyons would have some counterintuitive properties, such as imaginary mass, leading to complex implications for causality and the structure of spacetime.
