# Speed of light question

## Main Question or Discussion Point

simple question - Can light or anything else, travel faster than 3x10^5 km/sec?
If not, why is there a limit on max speed particles/photos can travel at?

Related Special and General Relativity News on Phys.org
From wikipedia on Special Relativity:

Special relativity reveals that c is not just the velocity of a certain phenomenon, namely the propagation of electromagnetic radiation (light) - but rather a fundamental feature of the way space and time are unified as spacetime. A consequence of this is that it is impossible for any particle that has mass to be accelerated to the speed of light.

A good way to think about it that I recently came across is that everything is moving at a constant speed. That is, every object has a 4-velocity and a 4-speed of c (speed of light). A 4-velocity is a 4-dimensional vector where the first index is time and the last three are space.

Examples:
Object at rest: v = (c,0,0,0)
Photon: v=(0,c,0,0) *or any vector where the length of the spatial part is c and time is 0

So the question isn't "why is there a maximum speed?", its really "why is everything moving through space-time at a constant speed?".

robphy
Homework Helper
Gold Member
A good way to think about it that I recently came across is that everything is moving at a constant speed. That is, every object has a 4-velocity and a 4-speed of c (speed of light). A 4-velocity is a 4-dimensional vector where the first index is time and the last three are space.

Examples:
Object at rest: v = (c,0,0,0)
Photon: v=(0,c,0,0) *or any vector where the length of the spatial part is c and time is 0

So the question isn't "why is there a maximum speed?", its really "why is everything moving through space-time at a constant speed?".
For a photon, its tangent vector has the form
Photon: v=(c,c,0,0), for example, so that $$c^2-(c^2+0^2+0^2)=0^2$$.

Here's my comment on the "speed through spacetime" idea:
Photon: v=(c,c,0,0), for example, so that $$c^2-(c^2+0^2+0^2)=0^2$$.