Monika Schleier-Smith, a scientist at Stanford University in California, is working on an experimental setup to try to create Space-Time in the laboratory from nothing. Interviewed by New Scientist, Schleier-Smith said she was convinced that her research, linked to the so-called conjecture of the holographic principle, has “the potential to reveal how the behavior of entities operating on very small spatial scales causes space-time ‘emerges’”.

In search of quantum gravity

To understand Schleier-Smith’s work, we must begin by remembering the two pillars of modern physics, namely Albert Einstein’s general relativity and quantum mechanics. The first explains the behaviour of gravity in terms of a kind of deformation of space-time, while the second describes the behaviour of particles on microscopic spatial scales. Separately, quantum mechanics and general relativity work just fine, and both have been (and continue to be) experimentally verified with ever-increasing precision. The problem is that when physicists try to fit them into a single theoretical framework, to harmonize them into a general theory that also describes gravity in quantum terms, things stop working. 

Three of the four fundamental forces (electromagnetism, the strong force and the weak force) have been “unified” and “quantized”, and for each of them, a mediator has been identified, that is, a quantum particle responsible for the transmission of the force itself; this, however, has not yet happened for the other fundamental force, gravity, precisely: the search for the so-called graviton, the hypothetical particle that should mediate gravity, has so far proved fruitless – also because gravity is the weakest of all the fundamental forces (even if its range of action is enormously greater), and is therefore “hidden” by the others.

A lot of holograms

One of the possibilities explored by physicists to try to quantize gravity is the so-called string theory: each particle would be the “manifestation” of the different vibrations of one-dimensional entities called, precisely, “strings”, capable of propagating in space and time and interacting with each other. The graviton would correspond precisely to a particular mode of vibration of the strings. It must be said, however, that at the moment, string theory remains only a (beautiful) mathematical conjecture, not only never experimentally verified but for which it is even impossible, according to many experts, to even think of an experiment.

Thus, we come to the holographic principle, a conjecture proposed for the first time by Gerardus t’Hooft, developed by Leonard Susskind and connected to string theory by the young physicist Juan Maldacena in 1997. Also, in this case, it concerns a completely hypothetical mathematical conjecture, moreover applied to a “model Universe” with a geometry quite different from ours. 

Maldacena’s theories about Black Holes

Maldacena, in particular, was based on two considerations. The first concerns, precisely, string theory, and in particular, the hypothesis of the physicist Alexander Polyakov, who was among the first to realize that to make the calculations add up the hypothetical strings would have to live in a Universe with more than four dimensions (the most modern versions of the theory assume a Universe of at least ten dimensions). 

The second instead has to do with black holes, and in this case, with the theory developed by Stephen Hawking and colleagues, according to which the amount of information that can theoretically be “packaged” inside a black hole depends on the area of the horizon of the events (the “visible surface” of the black hole) and not by the volume of the black hole itself. It’s a bit like if we said that the number of sweets that can fit in a jar depends only on the opening of the jar and not on its capacity: it’s counterintuitive, but apparently, this is how black holes work.

Space-Time journey: the Universe in the laboratory

In any case, putting these two pieces of information together (the fact that our Universe is equivalent to a ten-dimensional world made of strings and that all the information contained in a three-dimensional black hole resides in its two-dimensional event horizon), Maldacena came to think that maybe, just maybe, our Universe could be the holographic projection of a lower-dimensional reality. To put it in his own words: “It’s as if the Universe was inside a box, and all its contents were engraved on the box’s surface”. As strange and abstruse as it may seem, string and general relativity theorists really liked this idea, above all because among its implications – and here we return to the starting question – there is the fact that gravity would also be a sort of hologram, which could be an answer to the problem of quantum gravity.

Even if the holographic principle were never to be confirmed or verified, in short, the effort of the Schleier-Smith team would not be useless: “Our experiments – concludes – nevertheless represent a very fruitful area of research. They could teach us fundamental concepts about space-time and the behaviour of gravity, and they could help us better understand quantum mechanics and entangled systems.”