What is a hologram?

"Your appearance now is what we call residual self-image, it's the mental projection of your digital self.
What is real? How do you define real?

If you're talking about what you feel, you can smell, you can taste and see, then real is simply electrical signals interpreted by your brain[...]

Welcome to the desert of the real"




Now that you decided to dip your toes into the tricky concept of holography there's no better way than this short dialog between the two pillars of the Matrix series Morpheus and Neo.

And like not being enough of 3D paradigm cracks all of a sudden:

Boom! Elon Musk states after thinking a lot about living in a simulation that "Games are indistinguishable from reality"

With all these being said let's get to the definition of a hologram: 

A hologram is a recording in a two- or three-dimensional medium of the interference pattern formed when a point source of light (the reference beam) of fixed wavelength encounters light of the same fixed wavelength arriving from an object (the object beam). When the hologram is illuminated by the reference beam alone, the diffraction pattern recreates the wave fronts of light from the original object. Thus, the viewer sees an image indistinguishable from the original object.

There we go, understanding holograms starts with the famous double slit experiment but this is for next time.

Tachyons

Photo by Hal Gatewood 

Tachyons are a putative class of particles which able to travel faster than the speed of light. Tachyons were first proposed by physicist Arnold Sommerfeld, and named by Gerald Feinberg. 

The word tachyon derives from the Greek  (tachus), meaning "speedy." Tachyons have the strange properties that, when they lose energy, they gain speed. Consequently, when tachyons gain energy, they slow down. The slowest speed possible for tachyons is the speed of light.

Tachyons appear to violate causality (the so-called causality problem), since they could be sent to the past under the assumption that the principle of special relativity is a true law of nature, thus generating a real unavoidable time paradox (Maiorino and Rodrigues 1999). 

Therefore, it seems unavoidable that if tachyons exist, the principle of special relativity must be false, and there exists a unique time order for all observers in the universe independent of their state of motion.

Tachyons can be assigned properties of normal matter such as spin, as well as an antiparticle (the antitachyon). And amazingly, modern presentations of tachyon theory actually allow tachyons to actually have real mass (Recami 1996).

It has been proposed that tachyons could be produced from high-energy particle collisions, and tachyon searches have been undertaken in cosmic rays. Cosmic rays hit the Earth's atmosphere with high energy (some of them with speed almost 99.99% of the speed of light) making several collisions with the molecules in the atmosphere. 

The particles made by this collision interact with the air, creating even more particles in a phenomenon known as a cosmic ray shower. In 1973, using a large collection of particle detectors, Philip Crough and Roger Clay identified a putative superluminal particle in an air shower, although this result has never been reproduced.