# Beyond c what happens?

1. Aug 25, 2005

### Nirav Chavda

beyond "c" what happens?!

it is said that beyond speed of light the body is not defined hence there is no question of this body to exceed c.
Yet for the purpose of arguement lets assume that we have acheived a way by which a body can be propelled to c.(lets say by zero point energy or by some negative energy creation or something smart)

then what will happen to the time passage to that body. will it become imaginary (this is concluded from einstein's equation) or will it become negative ??

2. Aug 25, 2005

Staff Emeritus
Formally, if v > c, then the relativistic frame conversion factor
$$\gamma = \frac{1}{\sqrt{1- \frac{v^2}{c^2}}}$$
becomes complex, since the quantitiy subtracted under the square root is greater than the quantity subtracted from, and you therefore have the square root of a negative quantity. Physicists invented a hypothetical class of particles called tachyons; their mass is a pure imaginary, a multiple of $$\sqrt{-1}$$, they always travelled at speeds greater than c, and the energy for them to slow down to c increased toward infinity just as it does for us to accelerate to c. In the 1980s there were papers about these hypothetical particles and it was worked out that they would see time backwards from us. Gregory Benford, a physicist and sf writer, devised a hypothetical device for talking to the past he called the tachyonic telephone, and he used it in his classic sf novel Timescape.

Then the string theorists found tachyons in their bosonic strings. Considering them unphysical they were jubilant when perturbative superstring theory came along and was able to cancel the tachyons out. But, when non-perturbative M-theory was discovered, tachyons reappeared, and by this time physicists were able to deal with them. So tachyons, particles that travel faster than light, are a part of modern string theory, though I believe they don't show up in its bottom line.

Last edited: Aug 25, 2005
3. Aug 25, 2005

### HallsofIvy

Staff Emeritus
"t" becomes negative. Exactly what negative t means is up you. Some people assert that that means moving back in time but I have never seen a good argument for that interpretation!

4. Aug 25, 2005