Any time you start talking about moving matter or information faster than the speed of light (superluminal), the discussion can get quickly heated.  Science fiction is full of examples of superluminals, mostly to get around the time/distance factor in space travel.  The best (and most discussed) example of superluminal travel is the warp drive engine used to power the star ships in the Star Trek franchise.  There is even a “transwarp” drive for when multiples of light speed isn’t fast enough, and so a worm hole conduit is opened in which to accelerate beyond superluminal speeds.

Visualization of a Warp Field, USS Enterprise image owned by Paramount/CBS

As interesting as it is to think about superluminals in science fiction, where do they fit into science fact?  Nowhere, right?

Right?

I’m not so sure I’d bet on that.  True, under the special theory of relativity, a subluminal (slower than light speed) particle with non-zero rest mass needs infinite energy to accelerate to the speed of light.  What about a superluminal particle?  That would be a particle that never goes below the speed of light.  It couldn’t.  This particle is called a tachyon, and you just landed in the murky waters of theoretical particle physics.

Tachyon visualization, image by Wiki user TxAlien, all rights reserved

A tachyon is constantly moving.  Remember; if it slows to below the speed of light it ceases to exist.  Since it’s traveling at superluminal speed, you cannot see it approaching.  What you would see is it appearing and departing in two different directions.  In the animation, the black line is a shock wave of Cherenkov radiation (occurring in one specific moment in time).

At this time, there is no direct evidence to prove or disprove the existence of tachyons.  They are purely theoretical, spinless particles which must be created and annihilated in pairs.  They don’t just show up in theoretical particle physics, either.  They make an appearance in quantum field theory, string theory, and non-theoretical particle physics.

Some phenomena appear to be superluminal, but are not.  They do not move energy or information superluminally,  so they do not hit the windshield of special relativity.  Some of these phenomena are (note:  c indicates speed of light):

Closing speed:  Two objects approaching the other at near c will appear to an observer to be moving faster than c.  They are not.  If I’m in a car traveling at 55mph, and you’re in a car traveling at 55mph approaching me, our closing speed is 110mph… although we are actually only traveling at 55mph.

Phase velocity above c:  When traveling through a medium, the phase velocity of an electromagnetic wave routinely exceeds c.  X-rays through glass, for example.  However, the phase velocity of an electromagnetic wave is monochromatic, so it doesn’t impart any information above c.

Universal expansion:  Distant galaxies appear to be moving away from us at speeds greater than c.  This is the same effect as you see in closing speed, except we’re in a car traveling away from another car, both cars moving 55mph.  The expansion speed is 110mph, although neither is actually traveling that speed.

There are more examples of the same phenomena, but it all boils down to one thing:  Although mass/information is appearing to move at superluminal speed, it actually is not.

We could spend days discussing superluminal/subluminal, and whether it may be possible to some day “break the barrier” of the speed of light.  That is the direction research is taking.  We never know when some previously unknown “exception” will show up to prove the rule.  Who knows what will come in under special relativity in the future?