First AI ghosts appeared on cemeteries soon after personal AI assistants got popular. Since an AI assistant collects lots of personal data of his host (including his vocabulary, voice samples, images and contacts), this machine intellect can successfully imitate his deceased owner in a voice chat or in a 3D hologram. All you need to do is to turn on "mirror mode" on a dead person's assistant, and store that AI in a safe place with Internet connection (may be, in a grave with its deceased owner). Later you can come to a cemetery and talk to a holographic “spirit of the dead” - or, if you don't have time to visit those sad places, you can just add your digital grandma to your contact list in Skype. In more distant future, brains of elderly people could be scanned in details before they die so the virtual models of your departed relatives will be even more realistic. Just don't forget to pay the bills for hosting and support.