When we say light travels without any time passing are we already, automatically, saying it travels in straight lines.

Not really. Light follows a curved trajectory near massive objects. Remember the experiment during an eclipse in 1918? Time zero: all it means is that one cannot generate a reference frame relative to which light is stationary. It always moves with a speed c. Thus when using a Lorentz transformation to calculate time-rate, relative to our time-rate, within a framework traveling with light, one always obtains zero; because the equations "explode". There is, however, a question: is it realistic to calculate time within a framework traveling with light if such a framework cannot exist?

Light always travels in a straight line. Always. No exceptions.

The space through which it travels, however, is altered by gravity.

Johnny Boy is correct in that the path of light is deflected by the gravity of large objects ... but the gravity is not affecting the photons ... it affecting the space through which the photons travel.

No Dan, light does not always travel along stright lines, unless you are talking about flat spacetimes. Light travels along something called null geodesics, which despite of the fact that they are not straight for curved spacetimes, they always represent the shortest path between two events.

Hi DA. You are of course correct. Light travels along a "straight line" as described by the geometry of space-time involved; however, it is not a straight line when thinking in terms of Euclidean geometry. According to Einstein, gravity is not a "force", but an illusion generated by the curvature of space-time. I believe he is correct.

Only after pasting my last reply, did I see Pasti's. His reply is of course more scientifically worded than mine.

The path of a light ray in spacetime vacuum defines the metric. A photon is not an inertial frame of reference. Proper length and proper time are defined as the length of an object and the amount of time that passes in a comoving frame.

A time-like vector in Minkowski space-time selects a preferred frame (coordinate system) in which the t-axis points along the vector and spatial coordinates are orthogonal to it. In this special coordinate system, the t-component of the vector is called its proper length (or proper mass-energy when talking of a 4-momentum vector).

OTOH, a light-like vector points along one of the directions contained in the light cone. The light cone is unvariant under all Lorentz transformations. Thus, a light-like vector is simply unable to pick a preferred coordinate system. We cannot make any "proper" measurements of a photon's 4-momentum.

http://arXiv.org/abs/gr-qc/9909014
Amer. J. Phys. 71 770 (2003)
Phys. Rev. Lett. 92 121101 (2004)
falling light

Yes, the question is answered, and probably the following by the same explanation. I wondered about reversing the idea and if you could travel from A to B in a "truely" straight line would you be travelling at the speed of light. I wondered could the speed of light be expressed as the measure of constant change in the universe or somelike description just for the sake of grasping the idea better of what light speed means. I didn't read the link yet, but hope to over the next few days.

I'd argue that there is no such thing as a straight line ... that there is only the shortest distance between two points.

BTW: The speed of light is not a constant. The speed of light in a vacuum is a constant.

Well said, Igor.

http://groups.google.com/group/sci.physics.research/msg/94c39b228d3a7e57?dmode=source&hl=en

If you're perhaps thinking about lightyears etc. remember it's a measurement of distance, not time..

Not correct Bee. There is no difference between a measure of distance and a measure of time. They are synonymous.

We define a distance by how long it will take light to travel a known distance.

Thanks for changing the tag ... and absolutely nothing defines science. What we don't know is what we don't know today or tomorrow. It does not indicate a violation of anything other than our still developing understanding of nature.