Imagine particles that can move faster than light. These are called tachyons, and they have fascinated many people. Recently, a new theory was created to explain how these super-fast particles interact with other particles. But this new theory has some big problems.

The first problem is with a tool called the Feynman propagator. This tool helps predict how particles move and interact. For tachyons, the proposed version doesn’t work right. It leads to strange and impossible results in the real world.

Another issue is with something called unitarity. In quantum mechanics, unitarity makes sure that the total probability of all possible outcomes adds up to one. This keeps things consistent. But for tachyons, unitarity is violated, meaning the calculations don't add up correctly. This shows the theory of tachyons interacting with regular particles is flawed.

There’s also a problem with the S-matrix, a tool used to predict the outcomes of particle interactions. For tachyons, the S-matrix doesn’t give clear predictions, making it hard to describe how these particles would interact.

Additionally, when tachyons interact with ordinary matter, they can cause instability. They might change the structure of the vacuum, the lowest energy state, leading to unpredictable results. This makes the theory even more questionable.

Despite the excitement about tachyons and their faster-than-light speed, the current theory trying to explain them has serious issues. These problems suggest that tachyons, as we understand them now, might not exist. The journey to understand these mysterious particles continues, but we still have a lot to learn.