Special effects wizards have been using this technique to scare and mystify audiences for centuries.
Magic is defined by secrecy. A magician never performs the same trick twice and never reveals how he or she performed it. Even if audience members say they want to know how a trick was done, they may wish they hadn't once they learn the solution.
Why? Because the solution is rarely as impressive or as clever as the effect. The classic woman levitation trick, for instance, is accomplished via forklift behind a stage curtain. No matter how ingenious, solutions demystify. Take, for example, David Copperfield's famous "flying" illusion, which was Nielsen ratings gold back in the 90's.
The solution was revealed as a series of fanned cables by creator John Gaughan, who filed a US Patent in 1993. On one hand, one can appreciate the technical know-how to create such a dynamic effect. But at the end of the day, it's just a set of wires. David Copperfield is more Cathy Rigby in Peter Pan than Harry Houdini.
The Pepper's Ghost illusion is different. The solution is nearly as wonderful as the trick itself. Halloween is an excellent time to discuss how, through the use of clever science, ghosts can walk among us.
The most famous example of Pepper's Ghost debuted at Disneyland's Haunted Mansion in 1969. Midway through the ride, an entire ballroom of see-through ghosts dances to off-kilter organ music. Another set of ghosts disappears and reappears at a dining room table, where a birthday party is being held. And above them all, two paintings come to life, depicting an eternal duel in progress.
When the ride opened, this special effect confounded all who saw it—even magicians, who should have known better, were fooled. But despite this, it wasn't a technological innovation. The core principles behind this illusion were conceived centuries ago; John Baptista de Porta first wrote about this phenomenon in his work Natural Magic in 1584:
"Let there be a chamber whereinto no other light cometh unless by the door or window where the spectator looks in; let the whole window or part of it be of glass, as we used so to do to keep out the cold; but let one part be polished, that there may be a looking-glass on both sides, whence the spectator must look in; for the rest do nothing. Let pictures be set over against this window, marble statues and such like; for what is without will seem within, and what is behind the spectator's back he will think to be in the middle of the house, as far from the glass inward, as they stand from it outwardly, and clearly and certainly that he would think he sees nothing but truth. But lest the skill should be known, let the part be made so where the ornament is, that the spectator may not see it…"
In plain English, that translates as follows: a piece of glass is placed at an angle between the spectator and a room. That angled glass reflects a second room offstage, so that when the audience looks at the first room, he sees a composite image; the room in front of him and a reflection of the second room offstage. If the mirror is lined up properly, the composition is seamless. Below are visual diagrams that depict the illusion under controlled conditions. The Red Square represents the audience member's window of vision, and the Green Square represents the sheet of glass. In the first image, the offstage room is darkened. Nothing is being reflected by the glass, and so, we see no ghost.
This next image shows what happens when the offstage room is lit. Now, the mirror catches the reflection, and creates a convincing composite.
This illusion was re-discovered in the mid-1800's by British scientists Henry Dircks and John Pepper; the former invented it, and the latter popularized the effect by making it easy to perform on a theater stage. Below, you can see an artist's rendition from Recreative Science, a science journal from the early 1860's. Imagine seeing Jacob Marley or Hamlet's father's ghost in a Victorian theater, and being enthralled.
The Disney Creative team members used Pepper's technique on a much grander scale. They installed massive sheets of glass to reflect two sets of animatronic ghosts. The first set was installed directly above the audience's heads.
And the second set of ghosts was installed directly beneath the audience's feet. Notice how in both photos, everything besides the ghosts is painted black. This ensures that the glass will only reflect the ghosts and not the machinery moving them around.
Today, Pepper's Ghost is used to create entirely different types of ghosts—digital "holograms" of dead musicians, who perform onstage next to live musicians. Digital Domain recreated the late Tupac Shakur at Coachella stage in 2012, first by projecting a high resolution image onto a sheet of glass, and then by reflecting that image onto a mylar screen. The late Michael Jackson made an appearance at the Billboard Awards in 2014 by similar means. Pulse Evolution used six high powered projectors to canvas the stage, giving virtual MJ free range to perform his signature dance moves.
This raises all sorts of ethical questions and opens the door to a bizarre cottage industry. How about a digital John Lennon? Or a digital Kurt Cobain? Or a digital Elvis Presley, embarking on a brand new global tour? Pepper's Ghost may one day live up to its namesake, ensuring that long dead people live and perform, whether they want to or not, into perpetuity.