What is the fourth dimension? It depends on who you talk to—some people think it’s the dimension of time, like in Donnie Darko. Others think it’s another dimension of space, like the designer of the game Miegakure. Physicists have understood at least theoretically, that there may be higher dimensions, besides our normal three. The first clue came in 1905 when Einstein developed his theory of special relativity. Of course, by dimensions we’re talking about length, width, and height. Generally speaking, when we talk about a fourth dimension, it’s considered space-time. But here, physicists mean a spatial dimension beyond the normal three, not a parallel universe, as such dimensions are mistaken for in popular sci-fi shows. Even if there are other dimensions somewhere out there in our universe or in others, should we travel to a place which includes them, scientists aren’t so sure we could even experience them. Our brains may be incapable. Mathematically, we can describe the 4th dimension but we may never experience it in the physical realm. Two teams of physicists created two separate experiments that simulated what the quantum Hall effect would look like in four dimensions by using only 3-D (and some nearly 2-D) materials. Essentially, the scientists figured out how to visualize fourth-dimensional phenomena in our lower, simpler third dimension. The applications of this are still incredibly abstract, but there may be some sci-fi levels of payoff once we wrap our heads around the fourth dimension, according to Mikael Rechtsman, one the authors of the new papers: “Maybe we can come up with new physics in the higher dimension and then design devices that take advantage the higher-dimensional physics in lower dimensions.” When a magnetic field is produced in a 90-degree line to the 2D plane it changes the behaviour of electrons which flow through it. This can be further manipulated by reducing the temperature and increasing the voltage within the environment. The greater the voltage and the larger the field, the more of a role quantum mechanics plays. The reason for this is that the magnetic field generates a force acting at right angles to the direction of motion – the Lorentz force – which deviates the electrons. But at low temperatures and very large magnetic fields, quantum mechanics starts playing a role which means the voltage no longer increases continuously but rather jumps in discrete steps. The European team supercooled atoms close to absolute zero, which were then placed in a 2d lattice created using lasers. They were then “excited’ using the additional laser to get them moving again. The US team instead beamed a laser through a block of glass to simulate the effect of an electric field on charged particles. Michael Lohse, one of the European researchers from the Ludwig-Maximilians University in Germany, said: “I think that the two experiments nicely complement each other.” The quantum Hall effect can be understood as a topological phenomenon. An example of Topology describes how many holes an object has and into what other shapes it can be transformed without cutting it. Similar laws are responsible for the quantum Hall effect for electrons’ only being able to move along topologically well-defined paths. Results have now been published in the journal Nature alongside complementary work by an American research team, who used a photonic topological pump to show how particles would move across the surface of a 4D quantum Hall system. Together, these papers provide the first experimental look into the physics of higher-dimensional topological phases of matter. An exciting next step would be to go further and create a system that actually behaves like it has four spatial dimensions in the laboratory. As proposed by Hannah Price and collaborators, this could be achieved by exploiting recent experiments that add extra “synthetic dimensions” for atoms. In this approach, the atoms would move up/down, right/left and forward/backward, but also along a fourth “synthetic dimension” corresponding to an atom being in different internal states. Thanks to these advances, scientists are beginning to find out how physics would be different if we lived in a universe with more spatial dimensions and discovering, in this science-fiction world, a wide variety of new quantum phenomena.