The internet is abuzz with people talking about the hologram of Tupac Shakur that appeared at Coachella last night, during Dr. Dre and Snoop Dogg's set. It seems like the next level in technology, which on some levels it sort of almost is. But really it's just a new application of a magic trick from the 1860s.
The trick is known as Pepper's Ghost, and the basics of it involve light being bounced off of a glass set at angle. It can allow for images to appear or disappear in front of an audience. The thing that is being appeared or disappeared - the ghost - is in a hidden room at a 45 degree angle to the audience. Everything in the room, except the ghost, is black, so when the lighted image is bounced off the glass only the ghost appears.
Pepper's Ghost was first used in a production of Charles Dickens' The Haunted Man in 1863. John Henry Pepper was a chemist who figured out an easier, streamlined way to use the effect in a stage production, but he didn't discover it. That honor goes to Henry Dircks, who got left off the naming.
The trick is used all the time today. Teleprompters use it. Disney's Haunted Mansion makes extensive use of the effect. As a child I had a magic box that allowed me to make things appear and disappear; the secret was an angled mirror, essentially a small version of Pepper's Ghost. It's been used in plenty of movies, including Diamonds Are Forever, where a girl transforms into a gorilla (although in that case it's not a 'special effect' but rather an in-film use of the Pepper's Ghost trick).
What happened in Coachella was essentially the exact same thing. A CGI recreation of Tupac Shakur - basically a life sized version of one of those real world characters in the Guitar Hero games - was projected onto a Mylar curtain on stage. There was no actual hologram, which would be a three dimensional figure that you would be able to see from all sides. It's an illusion of hi def (sort of) 3D projection.
This has nothing to do with the essentially creepy, grave-robbing aspect of the whole thing... but it is fitting that Tupac's Ghost appeared because of Pepper's Ghost.
Thanks to @Maarak, who pointed me to the page of the company who created the illusion. They don't hide the technology's humble origins.