The Cosmic Speed Limit
The speed of light is widely known to be the absolute pinnacle of movement. When Albert Einstein first entwined mass and energy in his Theory of Relativity, it basically established the Universe’s speed limit at 299,792 kilometers per second (186,282 miles per second).
According to Einstein, nothing in the Universe that has mass could either match, or move faster than, light.
But that doesn't mean that nothing can move faster than light. In truth, physicists have discovered a number of phenomena that have the ability to match, and actually beat (in specific respects), the speed of light. And there are several theoretical models that posit specific ways that the speed of light could be surpassed.
To be clear, these things don't prove General Relativity wrong (at all). But they do help reveal just how complex our universe really is, and they show that very few things in physics can really be boiled down to one simple phrase.
In his presentation on Big Think, physicist Michio Kaku said that "If I have two electrons close together, they can vibrate in unison, according to the quantum theory...if I jiggle one electron, the other electron 'senses' this vibration instantly, faster than the speed of light. Einstein thought that this therefore disproved the quantum theory, since nothing can go faster than light."
Einstein himself referred to it as “spooky action at a distance,” though it is actually known as quantum entanglement. It's a rather complicated bit of physics, but this video by Veritasium breaks down how it all works in rather simple terms.
However, saying that this information transfer "breaks" the speed of light is, perhaps, a bit of a cheat.
You see, the information does actually go faster than light...but it's not really traveling through space faster. To break this down a bit, say that you have a two friends. One if wearing a blue shirt and one is wearing pink. You know this, but you don't know which is wearing which. Suddenly, you run into one of them, and you see they are wearing a blue shirt. When you see this, then you will know instantaneously (re. faster than the speed of light) that the other person is wearing pink.
As Kaku notes, "Information does go faster than light, but Einstein has the last laugh. This is because the information that breaks the light barrier is random, and hence useless." It can't be used to send any other information than that.
The Luminal Boom
Like the sonic boom, a luminal boom happens when something accelerates to a point that it breaks the light barrier. As spectacular as it seems, in truth, this phenomenon happens everyday inside nuclear reactors.
Known as Cherenkov radiation, this blue glow is caused when the core of a reactor is submerged in water, where light moves at a reduced speed, while the electrons generated by the reactor move past the speed of light. This creates a sort of shock wave of light.
As the aforementioned may indicate, the key to all of this is the medium which light (and other substances) are moving through. In a vacuum, light is far faster. But in ice and other mediums, light goes a lot slower. Thus, Einstein's theories are still intact.
As Jessica Orwig notes, "Cherenkov radiation glows because the core of the Advanced Test Reactor [a nuclear reactor] is submerged in water to keep it cool. In water, light travels at 75 % the speed it would in the vacuum of outer space, but the electrons created by the reaction inside of the core travel through the water faster than the light does.
Particles, like these electrons, that surpass the speed of light in water, or some other medium such as glass, create a shock wave similar to the shock wave from a sonic boom."
If popular science fiction is to be believed, travelling through the stars will heavily rely on ships being able to move faster than light. Indeed, if you were working strictly with Einstein’s theory of special relativity, your dreams of interstellar travel would stay firmly planted on solid ground.
Thankfully, Einstein’s general theory of relativity opened another possibility by weaving together space and time. As Kaku notes, "the only viable way of breaking the light barrier [for humanity and the like] may be through general relativity and the warping of spacetime."
This is done through what are called wormholes, which are tears in the fabric of spacetime that would allow one to “pass through” to whole other regions in the Universe. The biggest problem with this idea is the energy that it would take to hold that wormhole open...and of course, all of the hazards that would come with diving into it...and also, we have no evidence that they actually exist or are anything but hypothetical and theoretical models.
But if they do exist, they would allow one to travel from A to B faster than light. It's related to the basic principles of the warp drive proposed by Miguel Alcubierre (note: the drive probably can't ever work, but you can read about all of that here).
Einstein’s theory says that nothing “with mass” could pass the speed of light, so it’s reasonable to consider that other things that don’t have mass could potentially achieve this feat. One such thing is simply empty space.
To clarify, relativity says that objects cannot travel faster than the speed of light through spacetime. It doesn’t, however, have anything to say about spacetime itself. And in fact, spacetime is expanding and pushing matter apart faster than the speed of light; however, matter is not really traveling through spacetime....spacetime is pushing it.
In this respect, every portion of space is expanding and stretching. It’s not even that the edges are flying outwards, but that spacetime itself—the area between galaxies, stars, planets, you and I—is stretching. And it is doing so faster than the speed of light.