>> Light travels through space at the speed of light.
> That seems a tautology isn't? Perhaps you wanted to say something else?
Very nearly, but it is exactly what I wanted to say. Light travels at a fixed speed _through space_ which we happen to call "the speed of light". A physical object which is motionless in space travels at a fixed speed _in the time dimension_ which happens to have the same size as the speed of light.
This has consequences. A physical object which has some speed in space still has a 4-velocity vector whose length is the speed of light. That means the space velocity and time velocity must combine into a vector whose length is c. Naturally, if the space velocity is non-zero, the time velocity must be less than c. This is called time dilation.
> Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame).
Not quite.
Taken to the extreme, an object with a space velocity equal to the speed of light would have no velocity at all in the time direction. This can be a confusing statement, because we must make a distinction between times as measured by an observer and times measured by the moving object. If a photon could measure events (and most cannot), it would measure no time elapsing between it's own creation and it's own destruction. It will be emitted by an electron in one place (perhaps the coronasphere of the sun) and simultaneously absorbed by another electron in another place (perhaps a rhodopsin molecule in someone's retina). An observer, such as the owner of the retina, would of course still measure some time between the emission and the absorption; the events would not appear to be simultaneous. This is called relativity of simultaneity.
I always get messed up when I try to think about the experience of the fast moving entity.
Imagine a photon that is emitted by the sun and travels through space bouncing off the moon and onto my retina. In my frame I can trace the path of that photon and see a sequence (sun->moon->eye). For the photon, all of that happened simultaneously, right? It would have no experience of having traveled from the sun to my eye because it was in both places at once. Is that right?
Very nearly, but it is exactly what I wanted to say. Light travels at a fixed speed _through space_ which we happen to call "the speed of light". A physical object which is motionless in space travels at a fixed speed _in the time dimension_ which happens to have the same size as the speed of light.
This has consequences. A physical object which has some speed in space still has a 4-velocity vector whose length is the speed of light. That means the space velocity and time velocity must combine into a vector whose length is c. Naturally, if the space velocity is non-zero, the time velocity must be less than c. This is called time dilation.
> Therefore, if I got this right, for an external observer, light does not travel trough time, only space. However, in its own reference frame, light should travel trough time at the speed of light (and doesn't move in space in that reference frame).
Not quite.
Taken to the extreme, an object with a space velocity equal to the speed of light would have no velocity at all in the time direction. This can be a confusing statement, because we must make a distinction between times as measured by an observer and times measured by the moving object. If a photon could measure events (and most cannot), it would measure no time elapsing between it's own creation and it's own destruction. It will be emitted by an electron in one place (perhaps the coronasphere of the sun) and simultaneously absorbed by another electron in another place (perhaps a rhodopsin molecule in someone's retina). An observer, such as the owner of the retina, would of course still measure some time between the emission and the absorption; the events would not appear to be simultaneous. This is called relativity of simultaneity.