The universe is full of fast-moving sub-atomic particles. Even a staid, middle-class star like our Sun generates a stream of energetic particles which we occasionally see interacting with the Earth’s magnetic field and atmosphere in the form of Auroræ.
Even higher-energy particles arrive from outside the Solar System, and when they crash into our atmosphere they generate a tiny flash of light. The University of Utah operates one telescope system specifically to observe these flashes, because the original particle’s path and identity can be deduced, giving the astronomers information about what is happening out in the universe.
But one night in 1991, the scope detected a flash which could only have been created by a particle with 3.2×10²° electron volts, about forty million times more powerful than the Large Hadron Collider. This particle came to bear a name based on the astronomers’ first reaction. It’s called the “Oh-My-God Particle”.
You’ll know from Special Relativity that nothing (with mass) can be accelerated to the speed of light, but by adding more and more energy, you can edge ever-closer to light speed. Well, the OMG Particle, if it was a proton, would have been doing 0.9999999999999999999999951 of light speed, or to put it another way, in the time light crossed a light year of distance, the OMG would cross a light year less 46 nanometres.
Back to Special Relativity: obviously it takes light a year to travel a light year, and likewise for the OMG (minus 46nm). But for the particle itself, time dilation occurs. Time actually slows down. From the OMG’s point of view, time is slowed so much that the travel time is much shorter. Much, much shorter: 100 microseconds.
If you could accelerate a spacecraft to OMG speeds, you could visit the centre of our galaxy in 3 seconds, or neighbouring galaxies in a few minutes. I can’t see how such speeds will ever be possible for a solid object like a spaceship, but maybe one day we’ll use more modest relativistic effects to make interstellar flight feasible.
The problem that the OMG presented the astronomers — and it still hasn’t been solved — was that when they traced the path back, there was nothing there. Nothing which could plausibly have generated such huge energy in a particle. In fact, because all of space is filled with the Cosmic Microwave Background (CMB) radiation, energetic particles interact with it and are eventually slowed down. The maximum distance that OMG could have travelled is about 50 megaparsecs (160 million light years), which implies that there is a fixed volume of space out there within which it must have originated, but when you investigate: nothing.
Since OMG, there have been about 15 additional detections of its brothers and sisters, but there is still no generally-accepted theory to explain what they are and where they come from. My own favourite is that the universe contains “topological defects”, where the actual geometry of space-time has a discontinuity or flaw. Postulated one-dimensional defects are called “cosmic strings” (nothing to do with string theory) and would be a rippling crack in space-time, the thickness of a proton, but hundreds of light-years long. If a string was to thrash around and loop back on itself, the parts would split off, with a huge release of energy, more than enough to generate OMGs.