So I'm reading http://digg.com/general_sciences/13_things_that_scientists_can_t_explain?t=5526450#c5526450 [Broken] (a great read), and number 3 is this: """ 3 Ultra-energetic cosmic rays FOR more than a decade, physicists in Japan have been seeing cosmic rays that should not exist. Cosmic rays are particles - mostly protons but sometimes heavy atomic nuclei - that travel through the universe at close to the speed of light. Some cosmic rays detected on Earth are produced in violent events such as supernovae, but we still don't know the origins of the highest-energy particles, which are the most energetic particles ever seen in nature. But that's not the real mystery. As cosmic-ray particles travel through space, they lose energy in collisions with the low-energy photons that pervade the universe, such as those of the cosmic microwave background radiation. Einstein's special theory of relativity dictates that any cosmic rays reaching Earth from a source outside our galaxy will have suffered so many energy-shedding collisions that their maximum possible energy is 5 × 1019 electronvolts. This is known as the Greisen-Zatsepin-Kuzmin limit. Over the past decade, however, the University of Tokyo's Akeno Giant Air Shower Array - 111 particle detectors spread out over 100 square kilometres - has detected several cosmic rays above the GZK limit. In theory, they can only have come from within our galaxy, avoiding an energy-sapping journey across the cosmos. However, astronomers can find no source for these cosmic rays in our galaxy. So what is going on? One possibility is that there is something wrong with the Akeno results. Another is that Einstein was wrong. His special theory of relativity says that space is the same in all directions, but what if particles found it easier to move in certain directions? Then the cosmic rays could retain more of their energy, allowing them to beat the GZK limit. Physicists at the Pierre Auger experiment in Mendoza, Argentina, are now working on this problem. Using 1600 detectors spread over 3000 square kilometres, Auger should be able to determine the energies of incoming cosmic rays and shed more light on the Akeno results. Alan Watson, an astronomer at the University of Leeds, UK, and spokesman for the Pierre Auger project, is already convinced there is something worth following up here. "I have no doubts that events above 1020 electronvolts exist. There are sufficient examples to convince me," he says. The question now is, what are they? How many of these particles are coming in, and what direction are they coming from? Until we get that information, there's no telling how exotic the true explanation could be. “One possibility is that there is something wrong with the Akeno results. Another is that Einstein was wrong” """ Now I'm not a physicist, i have only rudimentary knowledge of relativity and so on, but the answer seems obvious to me: Photons travel so fast that they travel only in space, not time, so in a sense they have no net velocity, because as a collective they don't move in a sense, they're just a part of the universe. i know that doesn't make much sense, but here's the simpler way of looking at it: if something's travelling through space and things are colliding with it, their net effect on its velocity will be net velocities of everything colliding with it. so if we were to *imagine* that this cosmic ray particle is travelling through a medium of slower (but still moving, in various directions) particles and hitting them, then since motion is relative, we could also look at this as though the cosmic ray particle starts out still, and the medium is ambushing it with its individual particles' speeds but also a general overall speed. this will cause the cosmic ray particle to accelerate (or, from the original point of view, to slow down). but now imagine that all the medium's particles are moving at the speed of light. since they're all moving at the fastest possible speed, they have *no* net velocity compared to the cosmic ray. Relativity isn't exactly a MPH post on the highway that caps any particles (like photons) from attaining anything above 300000 km/second. it's the fastest possible speed by the very nature of time and space, so in a sense the speed of the cosmic ray (regardless of the fact that it's going "nearly the speed of light") is *infinitely* slower than the speeds of the photons, just like the highest possible number (or "idea") on an unlimited number line is infinity. the speed of photons must be infinite in one sense (such as that you can never catch up to them, and perhaps that a "rest mass" of 0 must be accelerated infinitely in order to become something non-zero), and yet limited in another sense, to 300000 km/s. i think to put this in a more concrete form, if we look at the velocities that photons will impart onto the cosmic ray, they *can't* be dependent on speed. the reason for that is that all photons (by virtue of being "light") travel the same speed, and they all travel the same speed from *all* frames of reference velocitywise, such as the frame of reference of that cosmic ray. This leaves only *direction* to determine how much net velocity is imparted to the ray, and i would presume that the directions are evenly distributed. I.e. how can photons less frequently hit it from behind, just because it's moving away from them, if their relative speeds are not diminished by that forward motion, and thus it wouldn't take them any longer to catch up? I guess that's pretty much what I was saying from the beginning..