There is a postulate that says that all bodies are travelling through the dimensions of Space and Time at light speed. Hence light speed is a limit, and your movement in one specifies dimension is getting faster, the speed on other dimensions slows down because the modulus of the speed must be, the light speed.
So, as much faster you get, closer to the light speed you reach but, unless you stop your movement on the other dimensions, you will never be able to reach light speed.
Tachyons are still hypothesized particles. The reason for the limit proposed was because of the mass increment formula in special relativity where in if you increase the velocity of object it gets heavier but at the speed of light it becomes (mass / zero ) so a particle cannot achieve speed of light. Many laws of physics are valid within limited region such as Newton’s laws are not valid in subatomic conditions that may be the case with this speed of light stuff.
Tachyons do not need to violate causality; however, they would be easily identified, if they existed. And they were not, in the trillions of events examined in accelerators. So, they almost certainly do not exist, and even if they exist, do not allow for "time travel". Besides, in quantum field theory formal tachyons sometimes appear, but can be reinterpreted as normal, subluminal particles in a redefined vacuum. In this way extremely important discoveries were made, like the spontaneous breakdown of symmetry, an essential component of the Standard Model of elementary particles.
The Elegant Universe, among physicists this means the following:
There is, in relativity, a 4-vector you called 4-velocity. This has 4 components which are the derivatives of x,y,z,t in with respect to the proper time, tau. Now, the invariant interval ds2=dx2+dy2+dz2-(c dt)2= (c dtau)2. If you divide both members by (dtau)2, you end up with
ux is the x-component of the 4-vector u, and ux2 is the square of this quantity.
So if we use the Murkowski metric, the squared modulus of the 4-vector u is c2, that is, the "length" of the 4-velocity is always c. This is stated by Greene as meaning that everybody moves in space-time with the light velocity. But this is not the usual velocity! Saying that the 4-velocity has always modulus c does not mean that the speed of everybody is c. The components of the speed are 3, and are given by dx/dt, dy/dt, dz/dt, whereas the spatial components of the 4-velocity are dx/(dtau), dy/(dtau), dz/(dtau).
Finally, this is not a postulate, but a consequence of the Lorentz transformation. In the Einstein presentation of relativity, the postulates are : motion is relative, and , the light velocity does not depend on the velocity of the observer (velocity here being understood as the 3-velocity, not the 4-velocity).
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