Rade said:
Thank you, but do not these two statements form a contradiction... "The speed of light cannot be reached" & "For things that travel at the speed of light (e.g. photons), time and space has no meaning"
Also, if photons are not within "space-time" where are they ? Your answer defines photons as being "things" that exist so they must exist somewhere, but where if not within "space-time" ?
The situation is this:
Massive particles and physical objects cannot move at the speed of light. They must always move slower than 'c'.
Photons and other massless particles must move at exactly 'c'.
People often try to think about what a photon would experience as if it had human qualities. This is a mistake.
I would say that the issue here is that one is "anthropormorphizing" the photon -one is treating it as if it were a human being, and it's not.
See for instance
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html
I am driving my car at the speed of light and I turn on my headlights. What do I see?
Sadly this question and all others about experiences at the speed of light do not have a definitive answer. You cannot go at the speed of light so the question is hypothetical. Hypothetical questions do not have definitive answers. Only massless particles such as photons can go at the speed of light. As a massive object approaches the speed of light the amount of energy needed to accelerate it further increases so that an infinite amount would be needed to reach the speed of light.
Sometimes people persist: What would the world look like in the reference frame of a photon? What does a photon experience? Does space contract to two dimensions at the speed of light? Does time stop for a photon?. . . It is really not possible to make sense of such questions and any attempt to do so is bound to lead to paradoxes. There are no inertial reference frames in which the photon is at rest so it is hopeless to try to imagine what it would be like in one. Photons do not have experiences. There is no sense in saying that time stops when you go at the speed of light. This is not a failing of the theory of relativity. There are no inconsistencies revealed by these questions. They just don't make sense.
Despite these empty answers, nobody should feel too put down for asking such questions. They are exactly the kind of question that Einstein often asked himself from the age of 16 until he discovered special relativity ten years later.
Of course, part of Einstein's genius was to see that these questions didn't lead anywhere.
In an abstract sense, one can construct coordinate systems in which the trajectory of a photon is stationary.
These are called "null coordinates" and the transformation rules are quite simple. Using geometrized units in which c=1, the transformations to null coordinates are just:
u = x-t
v = x+t
These coordinates are perfectly valid mathematically, and are even used in General Relativity. They describe a coordinate system in whch a photon moving along the x-axis is represented by a single number (u, or v, depending on which way the photon is going).
Combined with standard 'y' and 'z' coordinates, one can construct a complete 3-d coordinate system out of these null coordinates.
This abstract mathematical description is probably as close as one can come to ascribing a "point of view" to a photon. Note that this coordinate system does not have any such thing as a "time" coordinate - instead, one has two null coordinates (one for photons moving in the +x direction, another for photons moving in the -x direction), and two spatial coordinates.
