You might think the speed-of-light is the absolute limit that things can travel through the Universe, and you'd be right, “in theory”. But in practice, the limit's actually a little bit lower! Want to find out what it is, and why? (Yes, you do!!)
Ethan Siegel in Starts With A Bang!:
But you or I, no matter how hard we try, will never attain that speed. There’s a simple reason for this: we have mass. And for an object with mass, you can accelerate it all you want, but it would take an infinite amount of energy to reach c, and I’m sorry, folks, there’s only a finite amount of energy in the Universe.
But that doesn’t mean we settle for 90% of c, or 99%, or 99.9999%. We always strive for that extra fraction of speed, that extra bit of energy, that extra push ever-closer to the unattainable limit. You may be most familiar with our latest attempts to do this at CERN, where we’ve recently discovered the Higgs Boson.
By smashing two protons into one another, one moving at 299,792,447 meters per second (just 11 m/s shy of the speed of light) in one direction and the other moving at the same speed in the opposite direction, we can produce incredibly energetic particles, bounded only by the energy available via Einstein’s E=mc2. After the LHC’s upgrade is complete, that speed will increase to 299,792,455 m/s, which will make these the fastest protons ever created on Earth.
More here.