All massless particles travel at the speed of light, including the photon, gluon and gravitational waves, which carry the electromagnetic, strong nuclear and gravitational interactions, respectively. Particles with mass must always travel at speeds below the speed of light, and there’s an even more restrictive cutoff in our Universe. (NASA/Sonoma State University/Aurore Simonnet)

The Universe Has A Speed Limit, And It Isn’t The Speed Of Light

Nothing can go faster than the speed of light in a vacuum. But particles in our Universe can’t even go that fast.

When it comes to speed limits, the ultimate one set by the laws of physics themselves is the speed of light. As Albert Einstein first realized, everyone looking at a light ray sees that it appears to move at the same speed, regardless of whether it’s moving towards you or away from you. No matter how fast you travel or in what direction, all light always moves at the same speed, and this is true for all observers at all times. Moreover, anything that’s made of matter can only approach, but never reach, the speed of light. If you don’t have mass, you must move at the speed of light; if you do have mass, you can never reach it.

But practically, in our Universe, there’s an even more restrictive speed limit for matter, and it’s lower than the speed of light. Here’s the scientific story of the real cosmic speed limit.

Light, in a vacuum, always appears to move at the same speed, the speed of light, regardless of the observer’s velocity. (pixabay user Melmak)

When scientists talk about the speed of light — 299,792,458 m/s — we implicitly mean “the speed of light in a vacuum.” Only in the absence of particles, fields, or a medium to travel through can we achieve this ultimate cosmic speed. Even at that, it’s only the truly massless particles and waves that can achieve this speed. This includes photons, gluons, and gravitational waves, but not anything else we know of.

Quarks, leptons, neutrinos, and even the hypothesized dark matter all have masses as a property inherent to them. Objects made out of these particles, like protons, atoms, and human beings all have mass, too. As a result, they can approach, but never reach, the speed of light in a vacuum. No matter how much energy you put into them, the speed of light, even in a vacuum, will forever be unattainable.

The hyperdrive from Star Wars appears to depict an ultra-relativistic motion through space, extremely close to the speed of light. But under the laws of relativity, you can never reach, much less exceed, the speed of light if you’re made of matter. (Jedimentat44 / flickr)

But there’s no such thing, practically, as a perfect vacuum. Even in the deepest…