If a car is traveling at the speed of light, why can't its light beam travel at TWICE the speed of light?

Is that possible?

No, because the correct equation for velocities composition is not the sum, but:

V = **(v**_{1} + v_{2})/(1 + v_{1}*v_{2}/c^{2}).

Note that the previous equation is correct for *all* velocities. Example:

v

_{1} = v

_{2} = 1000 m/s -->

--> V = 2*10

^{3}/(1 + 10

^{6}/299,792,458

^{2}) ≈ 2*10

^{3} -2*10

^{-8} m/s ≈ 2*10

^{3} = 2000 m/s.

So, for low speeds the (wrong) non-relativistic law: V = v

_{1} + v

_{2} is approximately correct and you can use it.

But if you use greater speeds:

v_{1} = v_{2} = 200,000,000 m/s -->

--> V = (2*2*10

^{8})/(1 + 4*10

^{16}/299,792,458

^{2}) ≈

277,000,000 m/s which is not 200,000,000 m/s + 200,000,000 m/s = 400,000,000 m/s.

Try with other values.