Tachyons and Einstein's relativity.

Einstein's the most important work the theory of relativity whose one of the postulates is that the velocity of light is the most attainable velocity in universe. There is no need to discuss about mathematics behind it. But i was too much shocked and suprised when i heard the word "tychyns" meaning fast moving. But their velocity is greater then light according to hypothesis. But we can never increase the velocity to velocity of light so it is assumed that the particle has it's velocity grater then light from it's very birth. To make the assumtion the mass is multiplied by imaginary number. So the particle have it's mass maximum when it's velocity is nearly equal then light. And almost zero when it's velocity is infinite. This is only a hypothesis. But I'm totally attracted by it's concept of imaginary mass. When we talk about four dimension there is time is also dimension. Then one questiom arised that can we move backward in time also like other dimension. Then to satisfy this problem concept of imaginary time is induced. So another imaginary concept arises for mass. Then it may lead to discovary of fifth dimension as mass. But I don't think dimension like mass can be associated with space. Then what is the truth? Please suggest me.