According to Gizmodo, what the Tupac fans at Coachella saw and millions across the world watched wasn't actually a hologram.
People saw a technique called "Pepper's Ghost", which has been around since the 19th century and is named after optics researcher John Pepper.
Here is how the trick works:
The trick is based on the fact that glass is both transparent and reflective, meaning it's possible, with the right angles, to bounce a picture off of it that appears to be floating in air. But it's not—it's just stuck on an expensive screen. Pac's totally 2D. It's an impressive show when done on a Coachella scale, but it's not Princess Leia action. This might explain why AV Concepts and Dr. Dre's manager both refused to speak about the tech behind Tupac's exhumation until after this weekend's Coachella performances—where presumably more "holograms" will be trotted out. Fingers crossed for a George Harrison/Big L holo-mashup.
But w-w-w-wait it gets less holographic:
For those who thought the immaculately-chiseled rendition of Tupac was based on some sort of old footage, more disappointment: Rolling Stone reports the rapper was CGI. But at least it was good good, expensive CGI, "created by the Hollywood special effects studio Digital Domain, who have previously worked on films such as The Curious Case of Benjamin Button, TRON: Legacy and X-Men: First Class."
So there you have it folks. 19th century technology has the world losing its collective fucking minds this entire week. Who needs cars, bring back horse and buggies! Did I just ruin Christmas?