Random question for physics nuts: What prevents massless particles from exceeding the speed of light?

From my understanding of it, all their mass/energy is pure energy, which could be understood to be pure-velocity. And pure velocity would be the speed of light. The reason for this is not actually because the speed of light is some arbitrary constant, though, but because it is a fundamental feature of the relativity of simultaneity, sort of. Unfortunately, I only grasp the concept enough so that it makes sense in my own head, but not enough to explain it well. But I do know that further research into relativity (special relativity should be enough for this question) will help shed light on the question.

(Suggestion: Grab one (or both) of Brian Greene's books, The Fabric of the Cosmos, or The Elegant Universe. Very good and enlightening books.)

Here's a rough attempt at an explanation (and it actually has nothing to do with conversions between mass/energy, but merely among the 4 dimensions; I was probably misthinking above):

Consider first of all that instead of three spacial dimensions, we have only 1 (to make visualizations easier), and that the 1 dimension of space and the 1 dimension of time are indistinguishable. So we merely have a total of 2 dimensions now, X and Y.

Now consider a particle that is moving in the X dimension at 10 length-units per time-unit (L/t) (some outside sense of time, not X or Y itself). So it is moving horizontally, and it's speed in the Y dimension is 0 L/t.

[o]------------------->

Now let's consider that it starts to move upwards as well as towards the right, but that it's overall speed remains the same. We get the following:

              __             _/|           _/         _/       _/     _/   _/[o]

It's overall speed is still 10 L/t, but it's speed in the X dimension has been reduced to around 7 L/t, and it's speed in the Y dimension is now also 7 L/t. SquareRoot(72 + 72) ~= 10.

What happens when it keeps going more upwards and less towards the right, while maintaining the same overall speed? Eventually, all the motion will be in the Y dimension, and none in the X dimension.

If you think about it in 3 dimensions instead of two, it's still the same. Given a fixed overall speed, the more of that speed that is in one dimenion, the less of it is going to be in the other dimensions. The same with 4 and higher dimensions as well.

Now when 3 of those dimensions of space, and 1 is time, this process looks odd to us at first, but when we stop and think about it, it makes sense (from the perspective of relativity theory). Consider a photon, moving at the speed of light. The speed of light actually happens to be that fixed "speed" that a particles travels at through the 4 dimensions. When a particle is moving at the speed of light through the 3 spacial dimensions, time for that particle is frozen. It isn't moving at all through the time dimension. Now opposite of that, consider a particle that isn't moving at all (spatially). It's full movement is in the time dimension, thus it is passing through time as quicky as any particle can.

So we can see from this that the speed of light isn't the fastest an object can move through space, but that an object is always moving at the exact same speed; it's just that most of the time, some of that movement is through the temporal dimension, and some of it is through the spatial dimensions. And it so happens that most ordinary things that we observe are travelling almost entirely through time, and very little through space, so we never a personal feeling that time is slowing down because we're moving fast.

I probably need to do more research, but I am quite sure that mass gets involved more when you get into general relativity. Special relativity is only considered with objects moving at a constant speed. General relativity tackles the issue of speeding up or slowing down. It basically equates speeding up or slowing down with gravity; they become the same thing. And since force is intimately associated with mass (force=mass*acceleration and all that jazz), general relativity would indeed be needed to understand more fully the whole massless versus non-massless object speed questions. My feeling is that the whole explanation that it takes an infinite amount of energy to accelerate a mass to the speed of light is an interpretation that can be easily misleading, making one possibly think that it takes no energy whatsoever to accelerate a particle that weighs nothing. A better understanding of general relativity would clear that whole confusion up very easily, probably.

@Agony

That's a pretty cogent explanation.

Some modern physics theories postulate the existence of extra dimensions "rolled up" in normal space. Would you say that the facts/theory you present above would predict that within these dimensions, the speed of light is slower?

To put it bluntly, Energy == Mass. There are no "massless" particles -- mass is really just a measurement of information content, and if it's a "particle," i e you can detect it, then it has information, and thus energy/mass.