IT IS commonly assumed that speed of light is the fastest anything with non-zero mass can be accelerated to and that is the cosmic speed limit of matter.
Is this true?
At 299,792,458 m/s (or 1,079,252,848 km/h), saying the speed of light is fast is an understatement.
While it is true that it represents the fastest something with no mass can travel in a vacuum, it isn't actually the cosmic speed limit.
If we use the definition that the cosmic speed limit is the fastest that something can travel in space, then in actuality, it is not quite equivalent to the speed of light.
This is because it is impossible to actually accelerate anything to the speed of light, apart from light itself.
All particles with non-zero mass are limited by the law of relativity to remain below the speed of light.
However, a stricter speed and energy limit set by other particles in the Universe impacts the top speed of anything with mass called the GZK limit.
You see, while we commonly think of space (as in the medium between planets and stars) as being a vacuum, it actually isn't.
Space is actually filled with gas and dust, not to mention bigger stuff like stars and black holes, that gets in the way of anything you are trying to accelerate.
While it is true that the density of this matter is very slight, to the point of non-existent for those who require lungs to breathe, it does matter greatly for things travelling at relativistic speeds, in other words things travelling at almost the speed of light.
These tiny bits of matter and energy that make up the interstellar (and stellar) medium gets in the way of anything trying to accelerate, making it effectively impossible for something to be accelerated to the speed of light.
Furthermore, once you start reaching a considerable percentage of the speed of light, your hypothetical particle will start catching up to the left-over photons from the Big Bang - the Cosmic Microwave Background (CMB).
When our particle impacts on the CMB, it creates neutral pions, which is a subatomic particle made up of a quark and antiquark, and these pions will cause our particle to rapidly lose energy (and acceleration).
So according to the GZK limit, the maximum speed anything can be accelerated to 99.99999999999999999998% the speed of light.
Saying that, the speed of light can sometimes change, in a manner of speaking.
In an experiment performed in 1999 at the Rowland Institute for Science, a private, non-profit research facility in Massachusetts, US, the physicists involved managed to slow down particles of light to just over 61 km/h.
They did this by passing a laser beam through atoms packed super-closely at super-low temperatures and super-high vacuum.
While this is an extreme case it does show that light is affected by the medium it travels through.